HEAT TRANSFER CONDUCTION& CONVECTION

Chapter 15 Fundamentals of Heat Transfer The next nine chapters
deal with the transfer of energy. Gross quantities of heat added to or rejected from a system may be evaluated by applying the control- volume expression for the first law of thermodynamics as discussed in Chapter 6. The result of a first-law analysis is only a part of the required information necessary for the complete evaluation of a process or situation that involves energy transfer. The overriding consideration is, in many instances, the rate at which energy transfer takes place. Certainly, in designing a plant in which heat must be exchanged with the surroundings, the size of heat-transfer equipment, the materials of which it is to be constructed, and the auxiliary equipment required for its utilization are all important considerations for the engineer. Not only must the equipment accomplish its required mission but it must also be economical to purchase and to operate. Considerations of an engineering nature such as these require both a familiarity with the basic mechanisms of energy transfer and an ability to evaluate quantitatively these rates as well as the important associated quantities. Our immediate goal is to examine the basic mechanisms of energy transfer and to consider the fundamental equations for evaluating the rate of energy transfer. There are three modes of energy transfer: conduction, convection, and radiation. All heat-transfer processes involve one or more of these modes. The remainder of this chapter will be devoted to an introductory description and discussion of these types of transfer. 15.1 CONDUCTION Energy transfer by conduction is accomplished in two ways. The first mechanism is that of molecular interaction, in which the greater motion of a molecule at a higher energy level (temperature) imparts energy to adjacent molecules at lower energy levels. This type of transfer is present, to some degree, in all systems in which a temperature gradient exists and in which molecules of a solid, liquid, or gas are present. The second mechanism of conduction heat transfer is by ‘‘free’’ electrons. The free- electron mechanism is significant primarily in pure-metallic solids; the concentration offree electrons varies considerably for alloys and becomes very low for nonmetallic solids. The ability of solids to conduct heat varies directly with the concentration of free electrons, thus it is not surprising that pure metals are the best heat conductors, as our experience has indicated. As heat conduction is primarily a molecular phenomenon, we might expect the basic equation used to describe this process to be similar to the expression used in the molecular 201

transfer of momentum, equation (7-4). Such an equation was first
stated in 1822 by Fourier in the form qx A ¼ Àk dT dx (15-1) where qx is the heat-transfer rate in the x direction, in Watts or Btu/h; A is the area normal to the direction of heat flow, in m2 or ft2; dT/dx is the temperature gradient in the x direction, in K/m or F/ft; and k is the thermal conductivity, in W/ðmKÞ or Btu/h ft F. The ratio qx/A, having the dimensions of W/m2 or Btu/h ft2, is referred to as the heat flux in the x direction. A more general relation for the heat flux is equation (15-2) q A ¼ Àk=T (15-2) which expresses the heat flux as proportional to the temperature gradient. The proportionality constant is seen to be the thermal conductivity, which plays a role similar to that of the viscosity in momentum transfer. The negative sign in equation (15-2) indicates that heat flow is in the direction of a negative temperature gradient. equation (15-2) is the vector form of the Fourier rate equation, often referred to as Fourier’s first law of heat conduction. The thermal conductivity, k, which is defined by equation (15-1), is assumed independent of direction in equation (15-2); thus, this expression applies to an isotropic medium only. Most materials of engineering interest are isotropic. Wood is a good example of an anisotropic material where the thermal conductivity parallel to the grain may be greater than that normal to the grain by a factor of 2 or more. The thermal conductivity is a property of a conducting medium and, like the viscosity, is primarily a function of temperature, varying significantly with pressure only in the case of gases subjected to high pressures. 15.2 THERMAL CONDUCTIVITY As the mechanism of conduction heat transfer is one of the molecular interaction, it will be illustrative to examine the motion of gas molecules from a standpoint similar to that in Section 7.3. Considering the control volume shown in Figure 15.1, in which energy transfer in the y direction is on a molecular scale only, we may utilize the first-law analysis of Chapter 6 as follows. Mass transfer across the top of this control volume is considered to occur only on the molecular scale. This criterion is met for a gas in laminar flow. Applying equation (6-10) and considering transfer only across the top face of the element considered dQ dt À dWs dt À dWm dt ¼ Z Z c:s: e þ P r r(v n) dA þ @ @t Z Z Z c:v: er dV (6-10) ⌬x ⌬y T = T (y) y x Figure 15.1 Molecular motion at the surface of a control volume. 202 Chapter 15 Fundamentals of Heat Transfer

For Z molecules crossing the plane Dx Dz per unit
time, this equation reduces to qy ¼ X Z n¼1 mncp(Tj yÀ À Tj yþ)Dx Dz (15-3) wheremn isthemasspermolecule;cp isthemolecularheatcapacityofthegas;Zisthefrequency with which molecules will cross area Dx Dz; and Tj yÀ, À Tj yþ are the temperatures of the gas slightly below and slightly above the plane considered, respectively. The right-hand term is the summationof the energyfluxassociatedwiththe moleculescrossing the control surface. Noting now that Tj yÀ ¼ T À T/yj y0 d, where yÀ ¼ y0 À d, and that a similar expression may be written for Tj yþ, we may rewrite equation (15-3) in the form qy A ¼ À2 X Z n¼1 mncpd T y y0 (15-4) where d represents the y component of the distance between collisions. We note, as previously in Chapter 7, that d ¼ (˜Ù¯)l , where l is the mean free path of a molecule. Using this relation and summing over Z molecules, we have qy A ¼ À 4 3 rcpZl T y y0 (15-5) Comparing equation (15-5) with the y component of equation (15-6) qy A ¼ Àk T y it is apparent that the thermal conductivity, k, becomes k ¼ 4 3 rcpZl Utilizing further the results of the kinetic theory of gases, we may make the following substitutions: Z ¼ NC 4 where C is the average random molecular velocity, C ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 8kT/pm p (k being the Boltzmann constant); l ¼ 1 ffiffiffi 2 p pNd 2 where d is the molecular diameter; and cp ¼ 3 2 k N giving, finally k ¼ 1 p3/2d 2 ffiffiffiffiffiffiffiffiffiffiffiffiffi k3T/m p (15-6) This development, applying specifically to monatomic gases, is significant in that it shows the thermal conductivity of a gas to be independent of pressure, and to vary as the 1/2 power of the absolute temperature. The significance of this result should not be overlooked, even though some oversimplifications were used in its development. Some 15.2 Thermal Conductivity 203

relations for thermal conductivity of gases, based upon more sophisticated
molecular models, may be found in Bird, Stewart, and Lightfoot.1 TheChapman–Enskogtheoryusedin Chapter 7 topredict gas viscositiesatlow pressures has a heat-transfer counterpart. For a monatomic gas, the recommended equation is k ¼ 0:0829 ffiffiffiffiffiffiffiffiffiffiffiffi (T/M) p /s2Vk (15-7) where k is in W/m Á K, s is in Angstroms, M is the molecular weight, and Vk is the Lennard–Jones collision integral, identical with V as discussed in Section 7.3. Both s and Vk may be evaluated from Appendices J and K. The thermal conductivity of a liquid is not amenable to any simplified kinetic-theory development, as the molecular behavior of the liquid phase is not clearly understood and no universally accurate mathematical model presently exists. Some empirical correlations have met with reasonable success, but these are so specialized that they will not be included in this book. For a discussion of molecular theories related to the liquid phase and some empirical correlations of thermal conductivities of liquids, the reader is referred to Reid and Sherwood.2 A general observation about liquid thermal conductivities is that they vary only slightly with temperature and are relatively independent of pressure. One problem in experimentally determining values of the thermal conductivity in a liquid is making sure the liquid is free of convection currents. In the solid phase, thermal conductivity is attributed both to molecular interaction, as in other phases, and to free electrons, which are present primarily in pure metals. The solid phase is amenable to quite precise measurements of thermal conductivity, as there is no problem with convection currents. The thermal properties of most solids of engineering interest have been evaluated, and extensive tables and charts of these properties, including thermal conductivity, are available. The free-electron mechanism of heat conduction is directly analogous to the mechanism of electrical conduction. This realization led Wiedemann and Franz, in 1853, to relate the two conductivities in a crudeway; and in 1872, Lorenz3 presented the following relation, known as the Wiedemann, Franz, Lorenz equation: L ¼ k keT ¼ constant (15-8) where k is the thermal conductivity, ke is the electrical conductivity, T is the absolute temperature, and L is the Lorenz number. The numerical values of the quantities in equation (15-8) are of secondary importance at this time. The significant point to note here is the simple relation between electrical and thermal conductivities and, specifically, that those materials that are good conductors of electricity are likewise good heat conductors, and vice versa. Figure 15.2 illustrates the thermal conductivity variation with temperature of several important materials in gas, liquid, and solid phases. A more complete tabulation of thermal conductivity may be found in Appendices H and I. The following two examples illustrate the use of the Fourier rate equation in solving simple heat-conduction problems. 1 R. B. Bird, W. E. Stewart, and E. N. Lightfoot, Transport Phenomena, Wiley, New York, 1960, chap. 8. 2 Reid and Sherwood, The Properties of Gases and Liquids, McGraw-Hill Book Company, New York, 1958, chap. 7. 3 L. Lorenz, Ann. Physik und Chemie (Poggendorffs), 147, 429 (1872). 204 Chapter 15 Fundamentals of Heat Transfer

Thermal conductivity (W/mиK) Thermal conductivity (W/mиK) Temperature (K) 100 300
500 1000 Fused quartz Pyroceram 2000 4000 2 5 10 Temperature (K) 200 300 Water 400 500 0 0.2 0.4 0.6 0.8 20 50 100 200 300 400 500 1 Thermal conductivity (W/mиK) Temperature (K) 0 200 400 600 800 1000 0 0.1 0.2 0.3 Stainless steel, AISI 304 Iron Aluminum oxide Copper Gold Platinum Silver Tungsten Glycerine Engine oil Freon 12 Ammonia Freon 12 Carbon dioxide Water (steam, 1 atm) Helium Air Hydrogen Aluminum alloy 2024 Aluminum (c) Gases and vapors (b) Liquids (a) Solid materials Figure 15.2 Thermal conductivity of several materials at various temperatures. 15.2 Thermal Conductivity 205

EXAMPLE 1 A steel pipe having an inside diameter of
1.88 cm and a wall thickness of 0.391 cm is subjected to inside and outside surface temperature of 367 and 344 K, respectively (see Figure 15.3). Find the heat flow rate per meter of pipe length, and also the heat flux based on both the inside and outside surface areas. The first law of thermodynamics applied to this problem will reduce to the form dQ/dt ¼ 0, indicating that the rate of heat transfer into the control volume is equal to the rate leaving, that is Q ¼ q ¼ constant: As the heat flow will be in the radial direction, the independent variable is r, and the proper form for the Fourier rate equation is qr ¼ ÀkA dT dr Writing A ¼ 2pL, we see that the equation becomes qr ¼ Àk(2prL) dT dr where qr , is constant, which may be separated and solved as follows: qr Z ro ri dr r ¼ À2pkL Z To Ti dT ¼ 2pkL Z Ti To dT qr ln ro ri ¼ 2pkL(Ti À To) qr ¼ 2pkL ln ro/ri (Ti À To) (15-9) Substituting the given numerical values, we obtain qr ¼ 2p(42:90 W/m Á K)(367 À 344)K ln(2:66/1:88) ¼ 17 860 W/m(18 600 Btu/hr Á ft) The inside and outside surface areas per unit length of pipe are giving Ai ¼ p(1:88)(10À2)(1) ¼ 0:059 m2/m(0:194 ft2/ft) Ao ¼ p(2:662)(10À2)(1) ¼ 0:084 m2/m(0:275 ft2/ft) qr Ai ¼ 17 860 0:059 ¼ 302:7 kW/m2(95 900 Btu/hr Á ft2) qo Ai ¼ 17 860 0:084 ¼ 212:6 kW/m2(67 400 Btu/hr Á ft2) One extremely important point to be noted from the results of this example is the requirement of specifying the area upon which a heat-flux value is based. Note that for the same amount of heat flow, the fluxes based upon the inside and outside surface areas differ by approximately 42%. Ti ri r ro To Figure 15.3 Heat conduction in a radial direction with uniform surface temperatures. 206 Chapter 15 Fundamentals of Heat Transfer

EXAMPLE 2 Consider a hollow cylindrical heat-transfer medium having inside
and outside radii of ri and ro with the corresponding surface temperatures Ti and To . If the thermal-conductivity variation may be described as a linear function of temperature according to k ¼ k0(1 þ bT) calculate the steady-state heat-transfer rate in the radial direction, using the above relation for the thermal conductivity, and compare the result with that using a k value calculated at the arithmetic mean temperature. Figure 15.3 applies. The equation to be solved is now qr ¼ À[ko(1 þ bT)](2prL) dT dr which, upon separation and integration, becomes qr Z ro ri dr r ¼ À2pkoL Z ro ri (1 þ bT)dT ¼ 2pkoL Z Ti To (1 þ bT)dT qr ¼ 2pkoL ln ro/ri T þ bT2 2 !Ti To qr ¼ 2pkoL ln ro/ri 1 þ b 2 (Ti þ To) ! (Ti À To) (15-10) Noting that the arithmetic average value of k would be kavg ¼ ko 1 þ b 2 (Ti þ To) ! we see that equation (15-10) could also be written as qr ¼ 2pkavgL ln ro/ri (Ti À To) Thus, the two methods give identical results. The student may find it instructive to determine what part of the problem statement of this example is responsible for this interesting result; that is, whether a different geometrical configuration or a different thermal-conductivity expression would make the results of the two types of solutions different. 15.3 CONVECTION Heat transfer due to convection involves the energy exchange between a surface and an adjacent fluid. A distinction must be made between forced convection, wherein a fluid is made to flow past a solid surface by an external agent such as a fan or pump, and free or natural convection wherein warmer (or cooler) fluid next to the solid boundary causes circulation because of the density difference resulting from the temperature variation throughout a region of the fluid. 15.3 Convection 207

The rate equation for convective heat transfer was first expressed
by Newton in 1701, and is referred to as the Newton rate equation or Newton’s ‘‘law’’ of cooling. This equation is q/A ¼ h DT (15-11) where q is the rate of convective heat transfer, in W or Btu/h; A is the area normal to direction of heat flow, in m2 or ft2; DT is the temperature difference between surface and fluid, in K or 8F; and h is the convective heat transfer coefficient, in W/m2 Á K or Btu/h ft2 F. Equation (15-11) is not a law but a definition of the coefficient h. A substantial portion of our work in the chapters to follow will involve the determination of this coefficient. It is, in general, a function of system geometry, fluid and flow properties, and the magnitude of DT. As flow properties are so important in the evaluation of the convective heat transfer coefficient, we may expect many of the concepts and methods of analysis introduced in the preceding chapters to be of continuing importance in convective heat transfer analysis; this is indeed the case. From our previous experiencewe should also recall that evenwhen a fluid is flowing in a turbulent manner past a surface, there is still a layer, sometimes extremely thin, close to the surface where flow is laminar; also, the fluid particles next to the solid boundary are at rest. As this is always true, the mechanism of heat transfer between a solid surface and a fluid must involve conduction through the fluid layers close to the surface. This ‘‘film’’ of fluid often presents the controlling resistance to convective heat transfer, and the coefficient h is often referred to as the film coefficient. Two types of heat transfer that differ somewhat from free or forced convection but are still treated quantitatively by equation (15-11) are the phenomena of boiling and con- densation. The film coefficients associated with these two kinds of transfer are quite high. Table 15.1 represents some order-of-magnitude values of h for different convective mechanisms. It will also be necessary to distinguish between local heat transfer coefficients, that is, those that apply at a point, and total or average values of h that apply over a given surface area. We will designate the local coefficient hx , according to equation (15-11) dq ¼ hx DT dA Thus the average coefficient, h, is related to hx according to the relation q ¼ Z A hx DT dA ¼ hA DT (15-12) The values given in Table 15.1 are average convective heat-transfer coefficients. Table 15.1 Approximate values of the convective heat-transfer coefficient Mechanism h; Btu/h ft2 F h; W/(m2 Á K) Free convection, air 0001–10 0005–50 Forced convection, air 0005–50 0025–250 Forced convection, water 0050–3000 0250–15,000 Boiling water 0500–5000 2500–25,000 Condensing water vapor 1000–20,000 5000–100,000 208 Chapter 15 Fundamentals of Heat Transfer

15.4 RADIATION Radiant heat transfer between surfaces differs from conduction
and convection in that no medium is required for its propagation; indeed energy transfer by radiation is maximum when the two surfaces that are exchanging energy are separated by a perfect vacuum. The rate of energy emission from a perfect radiator or blackbody is given by q A ¼ sT4 (15-13) where q is the rate of radiant energy emission, in W or Btu/h; A is the area of the emitting surface, in m2 or ft2; T is the absolute temperature, in K or 8R; and s is the Stefan–Boltzmann constant, which is equal to 5:676 Â 10À8 W/m2 Á K4 or 0:1714 Â 10À8 Btu/h ft2 R4. The proportionally constant relating radiant-energy flux to the fourth power of the absolute temperature is named after Stefan who, from experimental observations, proposed equation (15-13) in 1879, and Boltzmann, who derived this relation theoretically in 1884. Equation (15-13) is most often referred to as the Stefan–Boltzmann law of thermal radiation. Certain modiﬁcations will be made in equation (15-13) to account for the net energy transfer between two surfaces, the degree of deviation of the emitting and receiving surfaces from blackbody behavior, and geometrical factors associated with radiant exchange between a surface and its surroundings. These considerations are discussed at length in Chapter 23. 15.5 COMBINED MECHANISMS OF HEAT TRANSFER The three modes of heat transfer have been considered separately in Section 15.4. It is rare, in actual situations, for only one mechanism to be involved in the transfer of energy. Itwill be instructive to look at some situations in which heat transfer is accomplished by a combination of these mechanisms. Consider the case depicted in Figure 15.4, that of steady-state conduction through a plane wall with its surfaces held at constant temperatures T1 and T2 . Writing the Fourier rate equation for the x direction, we have qx A ¼ Àk dT dx (15-1) Solving this equation for qx subject to the boundary conditions T ¼ T1 at x ¼ 0 and T ¼ T2 at x ¼ L, we obtain qx A Z L 0 dx ¼ Àk Z T2 T1 dT ¼ k Z T1 T2 dT or qx ¼ kA L (T1 À T2) (15-14) Equation (15-14) bears an obvious resemblance to the Newton rate equation qx ¼ hA DT (15-11) We may utilize this similarity in form in a problem in which both types of energy transfer are involved. Consider the composite plane wall constructed of three materials in layers with dimensions as shown in Figure 15.5. We wish to express the steady-state heat-transfer rate per unit area between a hot gas at temperature Th on one side of this wall and a cool gas at T2 T1 x L Figure 15.4 Steady-state conduction through a plane wall. 15.5 Combined Mechanisms of Heat Transfer 209

Tc on the other side. Temperature designations and dimensions are
as shown in the ﬁgure. The following relations for qx arise from the application of equations (15-11) and (15-14): qx ¼ hhA(Th À T1) ¼ k1A L1 (T1 À T2) ¼ k2A L2 (T2 À T3) ¼ k3A L3 (T3 À T4) ¼ hcA(T4 À Tc) Each temperature difference is expressed in terms of qx as follows: Th À T1 ¼ qx(1/hhA) T1 À T2 ¼ qx(L1/k1A) T2 À T3 ¼ qx(L2/k2A) T3 À T4 ¼ qx(L3/k3A) T4 À Tc ¼ qx(1/hcA) Adding these equations, we obtain Th À Tc ¼ qx 1 hhA þ L1 k1A þ L2 k2A þ L3 k3A þ 1 hcA and ﬁnally, solving for qx , we have qx ¼ Th À Tc 1/hhA þ L1/k1A þ L2/k2A þ L3/k3A þ 1/hcA (15-15) Th Tc T4 T3 k1 k2 k3 T2 T1 L1 L2 L3 Figure 15.5 Steady-state heat transfer through a composite wall. Note that the heat-transfer rate is expressed in terms of the overall temperature difference. If a series electrical circuit R1 R2 R3 R4 R5 V2 V1 is considered, we may write I ¼ DV R1 þ R2 þ R3 þ R4 þ R5 ¼ DV P Ri 210 Chapter 15 Fundamentals of Heat Transfer

The analogous quantities in the expressions for heat flow and
electrical current are apparent, DV ! DT I ! qx Ri ! 1/hA, L/kA and each term in the denominator of equation (15-15) may be thought of as a thermal resistance due to convection or conduction. Equation (15-15) thus becomes a heat-transfer analog to Ohm’s law, relating heat flow to the overall temperature difference divided by the total thermal resistance between the points of known temperature. Equation (15-15) may now be written simply as q ¼ DT P Rthermal (15-16) This relation applies to steady-state heat transfer in systems of other geometries as well. The thermal-resistance terms will change in form for cylindrical or spherical systems, but once evaluated, they can be utilized in the form indicated by equation (15-16). With specific reference to equation (15-9), it may be noted that the thermal resistance of a cylindrical conductor is ln(ro/ri) 2pkL Another common way of expressing the heat-transfer rate for a situation involving a composite material or combination of mechanisms is with the overall heat-transfer coefficient defined as U qx A DT (15-17) where U is the overall heat-transfer coefficient having the same units as h, in W/m2 Á K or Btu/h ft2 F. EXAMPLE 3 Saturated steam at 0.276 MPa flows inside a steel pipe having an inside diameter of 2.09 cm and an outside diameter of 2.67 cm. The convective coefficients on the inner and outer pipe surfaces may be taken as 5680 and 22:7 W/m2ÁK, respectively. The surrounding air is at 294 K. Find the heat loss per meter of bare pipe and for a pipe having a 3.8 cm thickness of 85% magnesia insulation on its outer surface. In the case of the bare pipe there are three thermal resistances to evaluate: R1 ¼ Rconvection inside ¼ 1/hiAi R2 ¼ Rconvection outside ¼ 1/hoAo R3 ¼ Rconduction ¼ ln(ro =ri)/2pkL For conditions of this problem, these resistances have the values R1 ¼ 1/[(5680 W/m2ÁK)(p)(0:0209 m)(1 m)] ¼ 0:00268 K/W 0:00141 h R Btu R2 ¼ 1/[(22:7 W/m2ÁK)(p)(0:0267 m)(1 m)] ¼ 0:525 K/W 0:277 h R Btu 15.5 Combined Mechanisms of Heat Transfer 211

and R3 ¼ ln(2:67/2:09) 2p(42:9 W/m Á K)(1 m) ¼
0:00091 K/W 0:00048 h R Btu The inside temperature is that of 0.276 MPa saturated steam, 404 K or 2678F. The heat transfer rate per meter of pipe may now be calculated as q ¼ DT P R ¼ 404 À 294 K 0:528 K/W ¼ 208 W 710 Btu h In the case of an insulated pipe, the total thermal resistance would include R1 and R3 evaluated above, plus additional resistances to account for the insulation. For the insulation R4 ¼ ln(10:27/2:67) 2p(0:0675 W/m Á K)(1 m) ¼ 3:176 K/W 1:675 h R Btu and for the outside surface of the insulation R5 ¼ 1/[(22:7 W/m2 Á K)(p)(0:1027 m)(1 m)] ¼ 0:1365 K/W 0:0720 h R Btu Thus, the heat loss for the insulated pipe becomes q ¼ DT P R ¼ DT R1 þ R2 þ R4 þ R5 ¼ 404 À 294 K 3:316 K/W ¼ 33:2 W 113 Btu h A reduction of approximately 85%! It is apparent from this example that certain parts of the heat-transfer path offer a negligible resistance. If, for instance, in the case of the bare pipe, an increased rate of heat transfer were desired, the obvious approach would be to alter the outside convective resistance, which is almost 200 times the magnitude of the next-highest thermal- resistance value. Example 3 could also have been worked by using an overall heat-transfer coefﬁcient, which would be, in general U ¼ qx A DT ¼ DT/ P R A DT ¼ 1 A P R or, for the speciﬁc case considered U ¼ 1 Af1/Aihi þ [ln(ro/ri)]/2pkL þ 1/Aoho g (15-18) 212 Chapter 15 Fundamentals of Heat Transfer

Equation (15-18) indicates that the overall heat-transfer coefficient, U, may
have a different numerical value, depending on which area it is based upon. If, for instance, U is based upon the outside surface area of the pipe, Ao , we have Uo ¼ 1 Ao/Aihi þ [Ao ln(ro/ri)]/2pkL þ 1/ho Thus, it is necessary, when specifying an overall coefficient, to relate it to a specific area. One other means of evaluating heat-transfer rates is by means of the shape factor, symbolized as S. Considering the steady-state relations developed for plane and cylindrical shapes q ¼ kA L DT (15-14) and q ¼ 2pkL ln(ro/ri) DT (15-9) if that part of each expression having to do with the geometry is separated from the remaining terms, we have, for a plane wall, q ¼ k A L DT and for a cylinder q ¼ k 2pL ln(ro/ri) DT Each of the bracketed terms is the shape factor for the applicable geometry. A general relation utilizing this form is q ¼ kS DT (15-19) Equation (15-19) offers some advantages when a given geometry is required because of space and configuration limitations. If this is the case, then the shape factor may be calculated and q determined for various materials displaying a range of values of k. 15.6 CLOSURE In this chapter, the basic modes of energy transfer—conduction, convection, and radiation—have been introduced, along with the simple relations expressing the rates of energy transfer associated therewith. The transport property, thermal conductivity, has been discussed and some consideration given to energy transfer in a monatomic gas at low pressure. The rate equations for heat transfer are as follows: Conduction: the Fourier rate equation q A ¼ Àk =T Convection: the Newton rate equation q A ¼ h DT 15.6 Closure 213

Radiation: the Stefan–Boltzmann law for energy emitted from a black
surface q A ¼ sT 4 Combined modes of heat transfer were considered, specifically with respect to the means of calculating heat-transfer rates when several transfer modes were involved. The three ways of calculating steady-state heat-transfer rates are represented by the equations q ¼ DT P RT (15-16) where P RT is the total thermal resistance along the transfer path; q ¼ UA DT (15-17) where U is the overall heat transfer coefficient; and q ¼ kS DT (15-19) where S is the shape factor. The equations presented will be used throughout the remaining chapters dealing with energy transfer. A primary object of the chapters to follow will be the evaluation of the heat- transfer rates for special geometries or conditions of flow, or both. Note: Effects of thermal radiation are included, along with convection, in values of surface coefficients specified in the following problems. PROBLEMS 15.1 An asbestos pad is square in cross section, measuring 5 cm on a side at its small end increasing linearly to 10 cm on a side at the large end. The pad is 15 cm high. If the small end is held at 600 K and the large end at 300 K, what heat-flow rate will be obtained if the four sides are insulated? Assume one- dimensional heat conduction. The thermal conductivity of asbestos may be taken as 0:173 W/mÁK: 15.2 SolveProblem forthecaseofthelargercrosssectionexposed to the higher temperature and the smaller end held at 300 K. 15.3 Solve Problem 15.1 if, in addition to a varying cross- sectional area, the thermal conductivity varies according to k ¼ k0(1 þ bT); where k0 ¼ 0:138, b ¼ 1:95 Â 10À4, T ¼ temperature in Kelvin, and k is in W/m Á K. Compare this result to that using a k value evaluated at the arithmetic mean temperature. 15.4 Solve Problem 15.1 if the asbestos pad has a 1.905-cm steel bolt running through its center. 15.5 A sheet of insulating material, with thermal conductivity of 0:22 W/m Á K is 2 cm thick and has a surface area of 2.97 m2. If 4 kW of heat are conducted through this sheet and the outer (cooler) surface temperature is measured at 55C(328 K), what will be the temperature on the inner (hot) surface? 15.6 For the sheet of insulation specified in Problem 15.5, with a heat rate of 4 kW, evaluate the temperature at both surfaces if the coolside is exposed toair at 308Cwith a surface coefficient of 28:4 W/m2 Á K: 15.7 Plate glass, k ¼ 1:35 W/m Á K; initially at 850 K, is cooled by blowing air past both surfaces with an effective surface coefficient of 5 W/m2 Á K: It is necessary, in order that the glass does not crack, to limit the maximum temperature gradient in the glass to 15 K/mm during the cooling process. At the start of the cooling process,whatisthelowesttemperatureofthecoolingairthatcanbeused? 15.8 Solve Problem 15.7 if all specified conditions remain the same but radiant energy exchange from glass to the surroundings at the air temperature is also considered. 15.9 The heat loss from a boiler is to be held at a maximum of 900 Btu/h ft2 of wall area. What thickness of asbestos (k ¼ 0:10 Btu/h ft F) is required if the inner and outer surfaces of the insulation are to be 1600 and 5008F, respectively? 15.10 If, in the previous problem, a 3-in.-thick layer of kaolin brick (k ¼ 0:07 Btu/h ft F) is added to the outside of the asbestos, what heat flux will result if the outside surface of the kaolin is 2508F? What will be the temperature at the interface between the asbestos and kaolin for this condition? 15.11 A composite wall is to be constructed of 1/4-in. stainless steel (k ¼ 10 Btu/h ft F), 3 in. of corkboard (k ¼ 0:025 Btu/ h ft F) and 1/2 in. of plastic (k ¼ 1:5 Btu/h ft F): 214 Chapter 15 Fundamentals of Heat Transfer

a. Draw the thermal circuit for the steady-state conduction through
this wall. b. Evaluate the individual thermal resistance of each material layer. c. Determine the heat flux if the steel surface is maintained at 2508F and the plastic surface held at 808F. d. What are the temperatures on each surface of the corkboard under these conditions? 15.12 If, in the previous problem, the convective heat-transfer coefficients at the inner (steel) and outer surfaces are 40 and 5 Btu/h ft F, respectively, determine a. the heat flux if the gases are at 250 and 708F, adjacent to the inner and outer surfaces; b. the maximum temperature reached within the plastic; c. which of the individual resistances is controlling. 15.13 A 1-in.-thick steel plate measuring 10 in. in diameter is heated from below by a hot plate, its upper surface exposed to air at 808F. The heat-transfer coefficient on the upper surface is 5 Btu/h ft F and k for steel is 25 Btu/h ft F: a. How much heat must be supplied to the lower surface of the steel if its upper surface remains at 1608F? (Include radiation.) b. What are the relative amounts of energy dissipated from the upper surface of the steel by convection and radiation? 15.14 If, in Problem 15.13, the plate is made of asbestos, k ¼ 0:10 Btu/h ft F; what will be the temperature of the top of the asbestos if the hot plate is rated at 800 W? 15.15 A 0.20-m-thick brick wall (k ¼ 1:3 W/m Á K) separates the combustion zone of a furnace from its surroundings at 258C. For an outside wall surface temperature of 1008C, with a convective heat transfer coefficient of 18 W/m2 Á K; what will be the insidewall surface temperature at steady-state conditions? 15.16 Solve for the inside surface temperature of the brick wall described in Problem 15.15, but with the additional consideration of radiation from the outside surface to surroundings at 258C. 15.17 The solar radiation incident on a steel plate 2 ft square is 400 Btu/h. The plate is 1.4 in. thick and lying horizontally on an insulating surface, its upper surface being exposed to air at 908F. If the convective heat-transfer coefficient between the top surface and the surrounding air is 4 Btu/h ftF; what will be the steady-state temperature of the plate? 15.18 If in Problem 15.17, the lower surface of the plate is exposed to air with a convective heat transfer coefficient of 3 Btu/h ft F, what steady-state temperature will be reached a. if radiant emission from the plate is neglected; b. if radiant emission from the top surface of the plate is accounted for? 15.19 The freezer compartment in a conventional refrigerator can be modeled as a rectangular cavity 0.3 m high and 0.25 m widewith a depth of 0.5 m. Determine the thickness of styrofoam insulation (k ¼ 0:30 W/m Á K) needed to limit the heat loss to 400 W if the inner and outer surface temperatures are À10 and 338C, respectively. 15.20 Evaluate the required thickness of styrofoam for the freezer compartment in the previous problem when the inside wall is exposed to air at À108C through a surface coefficient of 16 W/m2 Á K and the outer wall is exposed to 338C air with a surface coefficient of 32 W/m2 Á K: Determine the surface temperatures for this situation. 15.21 The cross section of a storm window is shown in the sketch. How much heat will be lost through a window measuring 1.83 m by 3.66 m on a cold day when the inside and outside air temperatures are, respectively, 295 and 250 K? Convective coefficients on the inside and outside surfaces of the window are 20 and 15 W/m2 Á K, respectively. What temperature drop will exist across each of the glass panes? What will be the average temperature of the air between the glass panes? Window glass 0.32 cm thick Air space 0.8 cm wide 15.22 Compare the heat loss through the storm window described in Problem 15.21 with the same conditions existing except that the window is a single pane of glass 0.32 cm thick. 15.23 The outsidewalls of a house are constructed ofa 4-in.layer of brick, 1/2 in. of celotex, an air space 3 5=8 in. thick, and 1/4 in. of wood panelling. If the outside surface of the brick is at 308F and the inner surface of the panelling at 758F, what is the heat flux if a. the air space is assumed to transfer heat by conduction only? b. the equivalent conductance of the air space is 1:8 Btu/ h ft2 F? c. the air space is filled with glass wool? kbrick ¼ 0:38 Btu/h ft F kcelotex ¼ 0:028 Btu/h ft F kair ¼ 0:015 Btu/h ft F kwood ¼ 0:12 Btu/h ft F kwool ¼ 0:025 Btu/h ft F: 15.24 Solve Problem 15.23 if instead of the surface temperatures being known, the air temperatures outside and inside are 30 and 758F, and the convective heat-transfer coefficients are 7 and 2 Btu/h ft2 F; respectively. Problems 215

15.25 Determine the heat-transfer rate per square meter of wall
area for the case of a furnace with inside air at 1340 K. The furnace wall is composed of a 0.106-m layer of fireclay brick and a 0.635-cm thickness of mild steel on its outside surface. Heat transfer coefficients on inside and outside wall surfaces are 5110 and 45 W/m2 Á K, respectively; outside air is at 295 K. What will be the temperatures at each surface and at the brick-steel interface? 15.26 Given the furnace wall and other conditions as specified in Problem 15.25, what thickness of celotex (k ¼ 0:065 W/ m Á K) must be added to the furnace wall in order that the outside surface temperature of the insulation not exceed 340 K? 15.27 A 4-in.-OD pipe is to be used to transport liquid metals and will have an outside surface temperature of 14008F under operating conditions. Insulation is 6 in. thick and has a thermal conductivity expressed as k ¼ 0:08(1 À 0:003 T) where k is in Btu/h ft 8F and T is in 8F, is applied to the outside surface of the pipe. a. What thickness of insulation would be required for the outside insulation temperature to be no higher than 3008F? b. What heat-flow rate will occur under these conditions? 15.28 Waterat408Fistoflowthrougha11=2-in.schedule40steel pipe. The outside surface of the pipe is to be insulated with a 1-in.- thicklayerof85%magnesiaanda1-in.-thicklayerofpackedglass wool, k ¼ 0:022 Btu/h ft F: The surrounding air is at 1008F. a. Which material should be placed next to the pipe surface to produce the maximum insulating effect? b. What will be the heat flux on the basis of the outside pipe surface area? The convective heat-transfer coefficients for the inner and outer surfaces are 100 and 5 Btu/h ft F: respectively. 15.29 A 1-in.-nominal-diameter steel pipe with its outside surface at 4008F is located in air at 908F with the convective heat-transfer coefficient between the surface of the pipe and the air equal to 1:5 Btu/h ft F: It is proposed to add insulation having a thermal conductivity of 0:06 Btu/h ft F to the pipe to reduce the heat loss to one half that for the bare pipe. What thickness of insulation is necessary if the surface temperature of the steel pipe and ho remain constant? 15.30 If, for the conditions of Problem 15.29, ho in Btu/h ft F varies according to ho ¼ 0:575/D1/4 o , where Do is the outside diameter of the insulation in feet, determine the thickness of insulation that will reduce the heat flux to one half that of the value for the bare pipe. 15.31 Liquid nitrogen at 77 K is stored in a cylindrical container having an inside diameter of 25 cm. The cylinder is made of stainless steel and has a wall thickness of 1.2 cm. Insulation is to be added to the outside surface of the cylinder to reduce the nitrogen boil-off rate to 25% of its value without insulation. The insulation to be used has a thermal conductivity of 0:13 W/m Á K: Energy loss through the top and bottom ends of the cylinder may be presumed negligible. Neglecting radiation effects, determine the thickness of insulation when the inner surface of the cylinder is at 77 K, the convective heat-transfer coefficient at the insulation surface has a value of 12 W/m2 Á K, and the surrounding air is at 258C. 216 Chapter 15 Fundamentals of Heat Transfer

Chapter 16 Differential Equations of Heat Transfer Paralleling the treatment
of momentum transfer undertaken in Chapter 9, we shall now generate the fundamental equations for a differential control volume from a first-law-ofthermodynamics approach. The control-volume expression for the first law will provide our basic analytical tool. Additionally, certain differential equations already developed in previous sections will be applicable. 16.1 THE GENERAL DIFFERENTIAL EQUATION FOR ENERGY TRANSFER Consider the control volume having dimensions Dx, Dy, and Dz as depicted in Figure 16.1. Refer to the control-volume expression for the first law of thermodynamics dQ dt À dWs dt À dWm dt ¼ ZZ c:s: e þ P r r(v :n)dA þ @ @t ZZZ c:v: er dV (6-10) The individual terms are evaluated and their mean- ings are discussed below. The net rate of heat added to the control volume will include all conduction effects, the net release of thermal energy within the control volume due to volumetric effects such as a chemical reaction or induction heating, and the dissipation of electrical or nuclear energy. The generation effects will be included in the single term, q ˙, which is the volumetric rate of thermal energygeneration havingunits W/m3 or Btu/h ft3. Thus, the first term may be expressed as dQ dt ¼ k @T @x xþDx À k @T @x x ! Dy Dz þ k @T @y yþDy À k @T @y y ! Dx Dz þ k @T @z zþDz À k @T @z z ! Dx Dy þ _ q Dx Dy Dz ð16-1Þ The shaft work rate or power term will be taken as zero for our present pur- poses. This term is specifically related to work done by some effect within the y x z ⌬y ⌬x ⌬z Figure 16.1 A differential control volume. 217

control volume that, for the differential case, is not present.
The power term is thus evaluated as dWs dt ¼ 0 (16-2) The viscous work rate, occurring at the control surface, is formally evaluated by integrating the dot product of the viscous stress and the velocity over the control surface. As this operation is tedious, we shall express the viscous work rate as L Dx Dy Dz, where L is the viscous work rate per unit volume. The third term in equation (6-10) is thus written as dWm dt ¼ L Dx Dy Dz (16-3) The surface integral includes all energy transfer across the control surface due to fluid flow. All terms associated with the surface integral have been defined previously. The surface integral is ZZ c:s: e þ P r r(v :n)dA ¼ rvx v2 2 þ gy þ u þ P r xþDx À rvx v2 2 þ gy þ u þ P r x ! Dy Dz þ rvy v2 2 þ gy þ u þ P r yþDy À rvy v2 2 þ gy þ u þ P r y " # Dx Dz þ rvz v2 2 þ gy þ u þ P r zþDz À rvz v2 2 þ gy þ u þ P r z ! Dx Dy ð16-4Þ The energy accumulation term, relating the variation in total energy within the control volume as a function of time, is @ @t ZZZ c:v: er dV ¼ @ @t v2 2 þ gy þ u ! r Dx Dy Dz (16-5) Equations (16-1) through (16-5) may now be combined as indicated by the general first- law expression, equation (6-10). Performing this combination and dividing through by the volume of the element, we have k(@T/@x)j xþDx À k(@T/@x)j x Dx þ k(@T/@y)j yþDy À k(@T/@y)j y Dy þ k(@T/@z)j zþDz À k(@T/@z)j z Dz þ _ q þ L ¼ frvx (v2/2) þ gy þ u þ (P/r) Â Ã j xþDx À rvx (v2/2) þ gy þ u þ (P/r) Â Ã j x g Dx þ frvy (v2/2) þ gy þ u þ (P/r) Â Ã j yþDy À rvy (v2/2) þ gy þ u þ (P/r) Â Ã j y g Dy þ frvz (v2/2) þ gy þ u þ (P/r) Â Ã j zþDz À rvz (v2/2) þ gy þ u þ (P/r) Â Ã j z g Dz þ @ @t r v2 2 þ gy þ u 218 Chapter 16 Differential Equations of Heat Transfer

Evaluated in the limit as Dx, Dy, and Dz approach
zero, this equation becomes @ @x k @T @x þ @ @y k @T @y þ @ @z k @T @z þ _ q þ L ¼ @ @x rvx v2 2 þ gy þ u þ P r ! þ @ @y rvy v2 2 þ gy þ u þ P r ! þ @ @z rvz v2 2 þ gy þ u þ P r ! þ @ @t r v2 2 þ gy þ u ! ð16-6Þ Equation (16-6) is completely general in application. Introducing the substantial derivative, we may write equation (16-6) as @ @x k @T @x þ @ @y k @T @y þ @ @z k @T @z þ _ q þ L ¼ = :(Pv) þ v2 2 þ u þ gy = :rv þ @r @t þ r 2 Dv2 Dt þ r Du Dt þ r D(gy) Dt Utilizing the continuity equation, equation (9-2), we reduce this to @ @x k @T @x þ @ @y k @T @y þ @ @z k @T @z þ _ q þ L ¼ = :Pv þ r 2 Dv2 Dt þ r Du Dt þ r D(gy) Dt ð16-7Þ With the aid of equation (9-19), which is valid for incompressible flow of a fluid with constant m, the second term on the right-hand side of equation (16-7) becomes r 2 Dv2 Dt ¼ Àv :=P þ v :rg þ v :m =2v (16-8) Also, for incompressible flow, the first term on the right-hand side of equation (16-7) becomes = :Pv ¼ v :=P (16-9) Substituting equations (16-8) and (16-9) into equation (16-7), and writing the conduction terms as = :k=T, we have = :k =T þ _ q þ L ¼ r Du Dt þ r D(gy) Dt þ v :rg þ v :m =2v (16-10) It will be left as an exercise for the reader to verify that equation (16-10) reduces further to the form = :k=T þ _ q þ L ¼ rcv DT Dt þ v :m =2v (16-11) The function L may be expressed in terms of the viscous portion of the normal- and shear-stress terms in equations (7-13) and (7-14). For the case of incompressible ﬂow, it is written as L ¼ v :m=2 v þ F (16-12) 16.1 The General Differential Equation for Energy Transfer 219

where the ‘‘dissipation function,’’ F, is given by F ¼
2m @vx @x 2 þ @vy @y 2 þ @vz @z 2 " # þm @vx @y þ @vy @x 2 þ @vy @z þ @vz @y 2 þ @vz @x þ @vx @z 2 " # Substituting for L in equation (16-11), we see that the energy equation becomes = :k=T þ _ q þ F ¼ rcv DT Dt (16-13) From equation (16-12), F is seen to be a function of fluid viscosity and shear-strain rates, and is positive-definite. The effect of viscous dissipation is always to increase internal energy at theexpenseofpotentialenergyorstagnationpressure.Thedissipationfunctionisnegligiblein all cases that we will consider; its effect becomes significant in supersonic boundary layers. 16.2 SPECIAL FORMS OF THE DIFFERENTIAL ENERGY EQUATION The applicable forms of the energy equation for some commonly encountered situations follow. In every case the dissipation term is considered negligibly small. I. For an incompressible fluid without energy sources and with constant k rcv DT Dt ¼ k =2T (16-14) II. For isobaric flow without energy sources and with constant k, the energy equation is rcv DT Dt ¼ k =2T (16-15) Note that equations (16-14) and (16-15) are identical yet apply to completely different physical situations. The student may wish to satisfy himself at this point as to the reasons behind the unexpected result. III. In a situation where there is no fluid motion, all heat transfer is by conduction. If this situation exists, as it most certainly does in solids where cv ’ cp, the energy equation becomes rcp @T @t ¼ = :k =T þ _ q (16-16) Equation (16-16) applies in general to heat conduction. No assumption has been made concerning constant k. If the thermal conductivity is constant, the energy equation is @T @t ¼ a =2T þ _ q rcp (16-17) where the ratio k/rcp has been symbolized by a and is designated the thermal diffusivity. It is easily seen that a has the units, L2/t; in the SI system a is expressed in m2/s, and as ft2/h in the English system. If the conducting medium contains no heat sources, equation (16-17) reduces to the Fourier field equation @T @t ¼ a =2T (16-18) which is occasionally referred to as Fourier’s second law of heat conduction. 220 Chapter 16 Differential Equations of Heat Transfer

For a system in which heat sources are present but
there is no time variation, equation (16-17) reduces to the Poisson equation =2T þ _ q k ¼ 0 (16-19) The final form of the heat-conduction equation to be presented applies to a steady-state situation without heat sources. For this case, the temperature distribution must satisfy the Laplace equation =2T ¼ 0 (16-20) Each of equations (16-17) through (16-20) has been written in general form, thus each applies to any orthogonal coordinate system. Writing the Laplacian operator, =2, in the appropriate form will accomplish the transformation to the desired coordinate system. The Fourier field equation written in rectangular coordinates is @T @t ¼ a @2T @x2 þ @2T @y2 þ @2T @z2 ! (16-21) in cylindrical coordinates @T @t ¼ a @2T @r2 þ 1 r @T @r þ 1 r2 @2T @u2 þ @2T @z2 ! (16-22) and in spherical coordinates @T @t ¼ a 1 r2 @ @r r2 @T @r þ 1 r2 sin u @ @u sin u @T @u þ 1 r2 sin2u @2T @f2 ! (16-23) The reader is referred to Appendix B for an illustration of the variables in cylindrical and spherical coordinate systems. 16.3 COMMONLY ENCOUNTERED BOUNDARY CONDITIONS In solving one of the differential equations developed thus far, the existing physical situation will dictate the appropriate initial or boundary conditions, or both, which the final solutions must satisfy. Initial conditions refer specifically to thevalues of Tand vat the start of the time interval of interest. Initial conditions may be as simply specified as stating that Tj t¼0 ¼ T0 (a constant), or more complex if the temperature distribution at the start of time measure- ment is some function of the space variables. Boundary conditions refer to the values of T and v existing at specific positions on the boundaries of a system, that is, for givenvalues of the significant space variables. Frequently encountered boundary conditions for temperature are the case of isothermal boundaries, along which the temperature is constant, and insulated boundaries, across which no heat conduction occurs where, according to the Fourier rate equation, the temperature derivative normal to the boundary is zero. More complicated temperature functions often exist at system boundaries, and the surface temperature may also vary with time. Combinations of heat-transfer mechanisms may dictate boundary conditions as well. One situation often existing at a solid boundaryis the equality between heat transfer to the surface by conduction 16.3 Commonly Encountered Boundary Conditions 221

and that leaving the surface by convection. This condition is
illustrated in Figure 16.2. At the left-hand surface, the boundary condition is hk(Tk À Tj x¼0 ) ¼ Àk @T @x x¼0 (16-24) and at the right-hand surface, hc(Tj x¼L À Tc) ¼ Àk @T @x x¼L (16-25) It is impossible at this time to foresee all the initial and boundary conditions that will be needed. The student should be aware, however, that these conditions are dictated by the physical situation. The differential equations of energy transfer are not numerous, and a specific form applying to a given situation may be found easily. It remains for the user of these equations to choose the appropriate initial and boundary conditions to make the solution meaningful. 16.4 CLOSURE The general differential equations of energy transfer have been developed in this chapter, and some forms applying to more specific situations were presented. Some remarks concerning initial and boundary conditions have been made as well. In the chapters to follow, analyses of energy transfer will start with the applicable differential equation. Numerous solutions will be presented and still more assigned as student exercises. The tools for heat-transfer analysis have now been developed and examined. Our remaining task is to develop a familiarity with and facility in their use. qx = hc (T2 – Tc ) qx = – k dT dx qx = hh (Th – T1 ) Tc Th T2 T1 x L Figure 16.2 Conduction and convention at a system boundary. PROBLEMS 16.1 The Fourier field equation in cylindrical coordinates is @T @t ¼ a @2T @r2 þ 1 r @T @r þ 1 r2 @2T @u2 À @2T @z2 : a. What form does this equation reduce to for the case of steady-state, radial heat transfer? b. Given the boundary conditions T ¼ Ti at r ¼ ri T ¼ To at r ¼ ro c. Generate an expression for the heat flow rate, qr, using the result from part (b). 16.2 Perform the same operations as in parts (a), (b), and (c) of Problem 16.1 with respect to a spherical system. 16.3 Starting with the Fourier field equation in cylindrical coordinates, a. Reduce this equation to the applicable form for steady-state heat transfer in the u direction. 222 Chapter 16 Differential Equations of Heat Transfer

b. For the conditions depicted in the figure, that is,
T ¼ To at u ¼ 0, T ¼ Tz at u ¼ p, the radial surfaces insulated, solve for the temperature profile. c. Generate an expression for the heat flow rate, qu , using the result of part (b). d. What is the shape factor for this configuration? q ro r1 To Tp 16.4 Show that equation (16-10) reduces to the form = :k =T þ _ q þ L ¼ rcv DT Dt þ v :m=2v 16.5 Starting with equation (16-7), show that, for a fluid with constant thermal conductivity and no energy sources, equations (16-14) and (16-15) are obtained for incompressible and isobaric conditions, respectively. (Neglect viscous dissipation.) 16.6 Solve equation (16-19) for the temperature distribution in a plane wall if the internal heat generation per unit volume varies according to _ q ¼ _ q0 eÀbx/L. The boundary conditions that apply are T ¼ T0 at x ¼ 0 and T ¼ TL at x ¼ L. 16.7 Solve Problem 16.6 for the same conditions, except that the boundary condition at x ¼ L is dT/dx ¼ 0. 16.8 Solve Problem 16.6 for the same conditions, except that at x ¼ L, dT/dx ¼ j (a constant). 16.9 Use the relation T ds ¼ dh À dP/r to show that the effect of the dissipation function, F, is to increase the entropy, S. Is the effect of heat transfer the same as the dissipation function? 16.10 In a boundary layer where the velocity profile is given by vx v1 ¼ 3 2 y d À 1 2 y d 3 where d is the velocity boundary layer thickness, plot the dimensionless dissipation function, F d2/mv2 1, vs. y/d. 16.11 A spherical shell with inner and outer dimensions of ri and ro , respectively, has surface temperatures Ti (ri ) and To (ro ). Assuming constant properties and one-dimensional (radial) conduction, sketch the temperature distribution, T(r). Give reasons for the shape you have sketched. 16.12 Heat is transferred by conduction (assumed to be one- dimensional) along the axial direction through the truncated conical section shown in the figure. The two base surfaces are maintained at constant temperatures: T1 at the top, and T2 , at the bottom, where T1 > T2 : Evaluate the heat transfer rate, qx , when a. the thermal conductivity is constant. b. the thermal conductivity varies with temperature according to k ¼ ko À aT, where a is a constant. T2 , A2 T1 , A1 x L 16.13 Heat is generated in a radioactive plane wall according to the relationship _ q ¼ _ qmax 1 À X L ! where _ q is thevolumetric heat generation rate, kW/m3, L is the half thickness of the plate, and x is measured from the plate center line. X L Develop the equation that expresses the temperature difference between the plate center line and its surface. 16.14 Heat is generated in a cylindrical fuel rod in a nuclear reactor according to the relationship _ q ¼ _ qmax 1 À r ro 2 " # where _ q is the volumetric heat generation rate, kW/m3, and ro is theoutsidecylinderradius.Developtheequationthatexpressesthe temperature difference between the rod center line and its surface. 16.15 Heat is generated in a spherical fuel element according to the relationship _ q ¼ _ qmax 1 À r ro 3 " # where _ q is thevolumetric heat generation rate, kW/m3, and ro is the radius of the sphere. Develop the equation that expresses the temperature difference between the center of the sphere and its surface. Problems 223

Chapter 17 Steady-State Conduction In most equipment used in transferring
heat, energy flows from one fluid to another through a solid wall. As the energy transfer through each medium is one step in the overall process, a clear understanding of the conduction mechanism of energy transfer through homogeneous solids is essential to the solutions of most heat-transfer problems. In this chapter, we shall direct our attention to steady-state heat conduction. Steady state implies that the conditions, temperature, density, and the like at all points in the conduction region are independent of time. Our analyses will parallel the approaches used for analyzing a differential fluid element in laminar flow and those that will be used in analyzing steady-state molecular diffusion. During our discussions, two types of presentations will be used: (1) The governing differential equation will be generated by means of the control-volume concept and (2) the governing differential equation will be obtained by eliminating all irrelevant terms in the general differential equation for energy transfer. 17.1 ONE-DIMENSIONAL CONDUCTION For steady-state conduction independent of any internal generation of energy, the general differential equation reduces to the Laplace equation =2T ¼ 0 (16-20) Although this equation implies that more than one space coordinate is necessary to describe the temperature field, many problems are simpler because of the geometry of the conduction region or because of symmetries in the temperature distribution. One-dimensional cases often arise. The one-dimensional, steady-state transfer of energy by conduction is the simplest process to describe as the condition imposed upon the temperature field is an ordinary differential equation. For one-dimensional conduction, equation (16-20) reduces to d dx xi dT dx ¼ 0 (17-1) where i ¼ 0 for rectangular coordinates, i ¼ 1 for cylindrical coordinates, and i ¼ 2 for spherical coordinates. One-dimensional processes occur in flat planes, such as furnace walls; in cylindrical elements, such as steam pipes; and in spherical elements, such as nuclear-reactor pressure vessels. In this section, we shall consider steady-state conduction through simple systems in which the temperature and the energy flux are functions of a single space coordinate. 224

Plane Wall. Consider the conduction of energy through a plane
wall as illustrated in Figure 17.1. The one-dimensional Laplace equation is easily solved, yielding T ¼ C1x þ C2 (17-2) The two constants are obtained by applying the boundary conditions at x ¼ 0 T ¼ T1 and at x ¼ L T ¼ T2 These constants are C2 ¼ T1 and C1 ¼ T2 À T1 L The temperature profile becomes T ¼ T2 À T1 L x þ T1 or T ¼ T1 À T1 À T2 L x (17-3) and is linear, as illustrated in Figure 17.1. The energy flux is evaluated, using the Fourier rate equation qx A ¼ Àk dT dx (15-1) The temperature gradient, dT/dx, is obtained by differentiating equation (17-3) yielding dT dx ¼ À T1 À T2 L Substituting this term into the rate equation, we obtain for a flat wall with constant thermal conductivity qx ¼ kA L (T1 À T2) (17-4) The quantity kA/L is characteristic of a flat wall or a flat plate and is designated the thermal conductance. The reciprocal of the thermal conductance, L/kA, is the thermal resistance. Composite Walls. The steady flow of energy through several walls in series is often encountered. A typical furnace design might include one wall for strength, an intermediate wall for insulation, and the third outer wall for appear- ance. This composite plane wall is illustrated in Figure 17.2. For a solution to the system shown in this figure, the reader is referred to Section 5. The following example illustrates the use of the composite-wall energy-rate equation for pre- dicting the temperature distribution in walls. T2 T1 L x T Figure 17.1 Plane wall with a one-dimensional temperature distribution. qx T4 x T3 T2 T1 L1 L2 L3 qx Figure 17.2 Temperature distribution for steady-state conduction of energy through a composite plane wall. 17.1 One-Dimensional Conduction 225

EXAMPLE 1 A furnace wall is composed of three layers,
10 cm of firebrick (k ¼ 1:560 W/mÁK), followed by 23 cm of kaolin insulating brick (k ¼ 0:073 W/mÁK), and finally 5 cm of masonry brick (k ¼ 1:0 W/mÁK). The temperature of the inner wall surface is 1370 K and the outer surface is at 360 K. What are the temperatures at the contacting surfaces? The individual material thermal resistances per m2 of area are R1 ; firebrick ¼ L1 k1A1 ¼ 0:10 m (1:560 W/m :K)(1 m2) ¼ 0:0641 K/W R2 ; kaolin ¼ L2 k2A2 ¼ 0:23 (0:073)(1) ¼ 3:15 K/W R3 ; masonry ¼ L3 k3A3 ¼ 0:05 (1:0)(1) ¼ 0:05 K/W The total resistance of the composite wall is equal to 0:0641 þ 3:15 þ 0:05 ¼ 3:26 K/W. The total temperature drop is equal to (T1 À T4) ¼ 1370 À 360 ¼ 1010 K. Using equation (15-16), the energy transfer rate is q ¼ T1 À T4 åR ¼ 1010 K 3:26 K/W ¼ 309:8 W As this is a steady-state situation, the energy transfer rate is the same for each part of the transfer path (i.e., through each wall section). The temperature at the firebrick–kaolin interface, T2 , is given by T1 À T2 ¼ q(R1) ¼ (309:8 W)(0:0641 K/W) ¼ 19:9 K giving T2 ¼ 1350:1 Similarly, T3 À T4 ¼ q(R3) ¼ (309:8 W)(0:05 K/W) ¼ 15:5 K giving T3 ¼ 375:5 K There are numerous situations in which a composite wall involves a combination of series and parallel energy-flow paths. An example of such a wall is illustrated in Figure 17.3, where steel is used as reinforce- ment for a concrete wall. The composite wall can be divided into three sections of length L1 , L2 , and L3 , and the thermal resistance for each of these lengths may be evaluated. The intermediate layer between planes 2 and 3 consists of two separate thermal paths in parallel; the effective thermal conductance is the sum of the con- ductances for the two materials. For the section of the wall of height y1 þ y2 and unit depth, the resistance is R2 ¼ 1 k1y1 L2 þ k2y2 L2 ¼ L2 1 k1y1 þ k2y2 L3 k1 y1 y2 L2 L1 k2 k2 Figure 17.3 A series-parallel composite wall. 226 Chapter 17 Steady-State Conduction

The total resistance for this wall is åRT ¼ R1
þ R2 þ R3 or åRT ¼ L1 k1(y1 þ y2) þ L2 1 k1y1 þ k2y2 þ L3 k1(y1 þ y2) The electrical circuit R1 R2 R3 is an analog to the compositewall. The rate of energy transferred from plane 1 to plane 4 is obtained by a modified form of equation (15-16). q ¼ T1 À T4 åRT ¼ T1 À T4 L1 k1(y1 þ y2) þ L2 1 k1y1 þ k2y2 þ L3 k1(y1 þ y2) (17-5) It important to recognize that this equation is only an approximation. Actually, there is a significant temperature distribution in the y direction close to the material that has the higher thermal conductivity. In our discussions of composite walls, no allowance was made for a temperature drop at the contact face between two different solids. This assumption is not always valid, has therewill often bevapor spaces caused by rough surfaces, or even oxide films on the surfaces of metals. These additional contact resistances must be accounted for in a precise energy- transfer equation. Long, Hollow Cylinder. Radial energy flow by conduction through a long, hollow cylinder is another example of one-dimensional conduction. The radial heat flow for this configuration is evaluated in Example 1 of chapter 15 as qr L ¼ 2pk ln(ro/ri) (Ti À To) (17-6) where ri is the inside radius, ro is the outside radius, Ti is the temperature on the inside surface, and To is the temperature on the outside surface. The resistance concept may again be used; the thermal resistance of the hollow cylinder is R ¼ ln(ro/ri) 2pkL (17-7) The radial temperature distribution in a long, hollow cylinder may be evaluated by using equation (17-1) in cylindrical form d dr r dT dr ¼ 0 (17-8) Solving this equation subject to the boundary conditions at r ¼ ri T ¼ Ti and at r ¼ ro T ¼ To we see that temperature profile is T(r) ¼ Ti À Ti À To ln(ro/ri) ln r ri (17-9) 17.1 One-Dimensional Conduction 227

Thus, the temperature in a long, hollow cylinder is a
logarithmic function of radius r, whereas for the plane wall the temperature distribution is linear. The following example illustrates the analysis of radial energy conduction through a long, hollow cylinder. EXAMPLE 2 A long steam pipe of outside radius r2 is covered with thermal insulation having an outside radius of r3 . The temperature of the outer surface of the pipe, T2 , and the temperature of the surrounding air, T1, are fixed. The energy loss per unit area of outside surface of the insulation is described by the Newton rate equation qr A ¼ h(T3 À T1) (15-11) Can the energy loss increase with an increase in the thickness of insulation? If possible, under what conditions will this situation arise? Figure 17.4 may be used to illustrate this composite cylinder. In Example 3 of Chapter 15, the thermal resistance of a hollow cylindrical element was shown to be R ¼ ln(ro/ri) 2pkL (17-10) In the present example, the total difference in temperature is T2 À T1 and the two resistances, due to the insulation and the surrounding air film, are R2 ¼ ln(r3/r2) 2pk2L for the insulation, and R3 ¼ 1 hA ¼ 1 h2pr3L for the air film. Substituting these terms into the radial heat flow equation and rearranging, we obtain qr ¼ 2pL(T2 À T1) ½ln(r3/r2)/k2 þ 1/hr3 (17-11) The dual effect of increasing the resistance to energy transfer by conduction and simultaneously increasing the surface area as r3 is increased suggests that, for a pipe of given size, a particular outer radius exists for which the heat loss is maximum. As the ratio r3 /r2 increases logarithmically, and the term 1/r3 decreases as r3 increases, the relative importance of each resistance term will change as the insulation thickness is varied. In this example, L,T2,T1,k2h and r2 are considered constant. Differentiating equation (17-11) with respect to r3 , we obtain dqr dr3 ¼ À 2pL(T2 À T1) 1 k2r3 À 1 hr2 3 " 1 k2 ln r3 r2 þ 1 hr3 # 2 (17-12) r2 r3 r1 Figure 17.4 A series composite hollow cylinder. 228 Chapter 17 Steady-State Conduction

The radius of insulation associated with the maximum energytransfer, the
critical radius, found by setting dqr/dr3 ¼ 0; equation (17-12) reduces to (r3)critical ¼ k2 h (17-13) In the case of 85% magnesia insulation (k ¼ 0:0692 W/mÁK) and a typical value for the heat transfer coefficient in natural convection (h ¼ 34 W/m2ÁK), the critical radius is calculated as rcrit ¼ k h ¼ 0:0692 W/mÁK 34 W/m2ÁK ¼ 0:0020 m (0:0067 ft) ¼ 0:20 cm (0:0787 in:) These very small numbers indicate that the critical radius will be exceeded in any practical problem. The question then is whether the critical radius given by equation (17-13) represents a maximum or a minimum condition for q. The evaluation of the second derivative, d2qr/dr2 3 , when r3 ¼ k/h yields a negative result, thus rcrit is a maximum condition. It now follows that qr will be decreased for any value of r3 greater than 0.0020 m. Hollow Sphere. Radial heat flow through a hollow sphere is another example of one-dimensional conduction. For constant thermal conductivity, the modified Fourier rate equation qr ¼ Àk dT dr A applies, where A ¼ area of a sphere ¼ 4pr2, giving qr ¼ À4pkr2 dT dr (17-14) This relation, when integrated between the boundary conditions at T ¼ Ti r ¼ ri and at T ¼ To r ¼ ro yields q ¼ 4pk(Ti À To) 1 ri À 1 r0 (17-15) The hyperbolic temperature distribution T ¼ Ti À Ti À To 1/ri À 1/ro 1 ri À 1 r (17-16) is obtained by using the same procedure that was followed to obtain equation (17-9). 17.1 One-Dimensional Conduction 229

Variable Thermal Conductivity. If the thermal conductivity of the medium
through which the energy is transferred varies significantly, the preceding equations in this section do not apply. As Laplace’s equation involves the assumption of constant thermal conductivity, a new differential equation must be determined from the general equation for heat transfer. For steady-state conduction in the x direction without internal generation of energy, the equation that applies is d dx k dT dx ¼ 0 (17-17) where k may be a function of T. In many cases the thermal conductivity may be a linear function of temperature over a considerable range. The equation of such a straight-line function may be expressed by k ¼ ko(1 þ bT) where ko and b are constants for a particular material. In general, for materials satisfying this relation, b is negative for good conductors and positive for good insulators. Other relations for varying k have been experimentally determined for specific materials. The evaluation of the rate of energy transfer when the material has a varying thermal conductivity is illustrated in Example 2 of chapter 15. 17.2 ONE-DIMENSIONAL CONDUCTION WITH INTERNAL GENERATION OF ENERGY In certain systems, such as electric resistance heaters and nuclear fuel rods, heat is generated within the conducting medium. As one might expect, the generation of energy within the conducting medium produces temperature profiles different than those for simple conduction. In this section, we shall consider two simple example cases: steady-state conduction in a circular cylinder with uniform or homogeneous energy generation, and steady-state conduction in a plane wall with variable energy generation. Carslaw and Jaeger1 and Jakob2 have written excellent treatises dealing with more complicated problems. Cylindrical Solid with Homogeneous Energy Generation. Consider a cylindrical solid with internal energy generation as shown in Figure 17.5. The cylinder will be considered long enough so that only radial conduction occurs. The density, r, the heat capacity, cp , and the thermal conductivity of the material will ⌬r r r Figure 17.5 Annular element in a long, circular cylinder with internal heat generation. 1 H. S. Carslaw and J. C. Jaeger, Conduction of Heat in Solids, 2nd Edition, Oxford Univ. Press, New York, 1959. 2 M. Jakob, Heat Transfer, Vol. I, Wiley, New York, 1949. 230 Chapter 17 Steady-State Conduction

be considered constant. The energy balance for the element shown
is rate of energy conduction into the element 8 > < > : 9 > = > ; þ rate of energy generation within the element 8 > < > : 9 > = > ; À rate of energy conduction out of the element 8 > < > : 9 > = > ; ¼ rate of accumulation of energy within the element 8 > < > : 9 > = > ; (17-18) Applying the Fourier rate equation and letting _ q represent the rate of energy generated per unit volume, we may express equation (17-18) by the algebraic expression Àk(2prL) @T @r r þ_ q(2prL Dr) À Àk(2prL) @T @r rþDr ! ¼ rcp @T @t (2prL Dr) Dividing each term by 2prL Dr, we obtain _ q þ k½r(@T/@r)j rþDr À r(@T/@r)j r r Dr ¼ rcp @T @t In the limit as Dr approaches zero, the following differential equation is generated: _ q þ k r @ @r r @T @r ¼ rcp @T @t (17-19) For steady-state conditions, the accumulation term is zero; when we eliminate this term from the above expression, the differential equation for a solid cylinder with homogeneous energy generation becomes _ q þ k r d dr r dT dr ¼ 0 (17-20) The variables in this equation may be separated and integrated to yield rk dT dr þ _ q r2 2 ¼ C1 or k dT dr þ _ q r 2 ¼ C1 r Because of the symmetry of the solid cylinder, a boundary condition that must be satisﬁed stipulates that the temperature gradient must be ﬁnite at the center of the cylinder, where r ¼ 0. This can be true only if C1 ¼ 0. Accordingly, the above relation reduces to k dT dr þ _ q r 2 ¼ 0 (17-21) A second integration will now yield T ¼ À _ qr2 4k þ C2 (17-22) If the temperature T is known at any radial value, such as a surface, the second constant, C2 , may be evaluated. This, of course, provides the completed expression for the temperature profile. The energy flux in the radial direction may be obtained 17.2 One-Dimensional Conduction with Internal Generation of Energy 231

from qr A ¼ Àk dT dr by substituting equation
(17-21), yielding qr A ¼ _ q r 2 or qr ¼ (2prL)_ q r 2 ¼ pr2L_ q (17-23) Plane Wall with Variable Energy Generation. The second case associated with energy generation involves a temperature-dependent, energy-generating process. This situation develops when an electric current is passed through a conducting medium possessing an electrical resistivity that varies with temperature. In our discussion, we shall assume that the energy-generation term varies linearly with temperature, and that the conducting medium is a ﬂat plate with temperature TL at both surfaces. The internal energy generation is described by _ q ¼ _ qL ½1 þ b(T À TL) (17-24) where _ qL is the generation rate at the surface and b is a constant. With this model for the generation function, and as both surfaces have the same temperature, the temperature distribution within the ﬂat plate is symmetric about the midplane. The plane wall and its coordinate system are illustrated in Figure 17.6. The symmetry of the temperature distribution requires a zero temperature gradient at x ¼ 0. With steady-state conditions, the differential equation may be obtained by eliminating the irrelevant terms in the general differential equation for heat transfer. Equation (16-19) for the case of steady-state conduction in the x direction in a stationary solid with constant thermal conductivity becomes d2T dx2 þ _ qL k ½1 þ b(T À TL) ¼ 0 The boundary conditions are at x ¼ 0 dT dx ¼ 0 and at x ¼ ÆL T ¼ TL These relations may be expressed in terms of a new variable, u ¼ T À TL, by d2u dx2 þ _ qL k (1 þ bu) ¼ 0 or d2u dx2 þ C þ su ¼ 0 where C ¼ _ qL /k and s ¼ b_ qL /k. The boundary conditions are at x ¼ 0 du dx ¼ 0 2L +L –L x Figure 17.6 Flat plate with temperature- dependent energy generation. 232 Chapter 17 Steady-State Conduction

and at x ¼ ÆL u ¼ 0 The integration
of this differential equation is simplified by a second change in variables; inserting f for C þ su into the differential equation and the boundary conditions, we obtain d2f dx2 þ sf ¼ 0 for x ¼ 0 df dx ¼ 0 and x ¼ ÆL f ¼ C The solution is f ¼ C þ su ¼ A cos(x ffiffi s p ) þ B sin(x ffiffi s p ) or u ¼ A1 cos(x ffiffi s p ) þ A2 sin(x ffiffi s p ) À C s The temperature distribution becomes T À TL ¼ 1 b cos(x ffiffi s p ) cos(L ffiffi s p ) À 1 ! (17-25) where s ¼ b_ qL /k is obtained by applying the two boundary conditions. The cylindrical and spherical examples of one-dimensional temperature-dependent generation are more complex; solutions to these may be found in the technical literature. 17.3 HEAT TRANSFER FROM EXTENDED SURFACES Avery useful application of one-dimensional heat-conduction analysis is that of describing the effect of extended surfaces. It is possible to increase the energy transfer between a surface and an adjacent fluid by increasing the amount of surface area in contact with the fluid. This increase in area is accomplished by adding extended surfaces that may be in the forms of fins or spines of various cross sections. The one-dimensional analysis of extended surfaces may be formulated in general terms by considering the situation depicted in Figure 17.7. The shaded area represents a portion of the extended surface that has variable cross- sectional area, A(x), and surface area, S(x), which are functions of x alone. For steady- state conditions, the first law of thermodynamics, equation (6-10), reduces to the simple expression dQ dt ¼ 0 ⌬x x q1 q2 q3 A(x) S(x) Figure 17.7 An extended surface of general configuration. 17.3 Heat Transfer from Extended Surfaces 233

Thus, in terms of the heat flow rates designated in
the figure, we may write q1 ¼ q2 þ q3 (17-26) The quantities q1 and q2 are conduction terms, while q3 is a convective heat-flow rate. Evaluating each of these in the appropriate way and substituting into equation (17-26), we obtain kA dT dx xþDx À kA dT dx x À hS(T À T1) ¼ 0 (17-27) where T1 is the fluid temperature. Expressing the surface area, S(x), in terms of the width, Dx, times the perimeter, P(x), and dividing through by Dx, we obtain kA(dT/dx)j xþDx À kA(dT/dx)j x Dx À hP(T À T1) ¼ 0 Evaluating this equation in the limit as Dx ! 0, we obtain the differential equation d dx kA dT dx À hPðT À T1Þ ¼ 0 (17-28) One should note, at this point, that the temperature gradient, dT/dx, and the surface temperature, T, are expressed such that Tis a function of x only. This treatment assumes the temperature to be ‘‘lumped’’ in the transverse direction. This is physically realistic when the cross section is thin or when the material thermal conductivity is large. Both of these conditions apply in the case of fins. More will be said about the ‘‘lumped parameter’’ approach in Chapter 18. This approximation in the present case leads to equation (17-28), an ordinary differential equation. If we did not make this simplifying analysis, we would have a distributed parameter problem that would require solving a partial differential equation. A wide range of possible forms exist when equation (17-28) is applied to specific geometries. Three possible applications and the resulting equations are described in the following paragraphs. (1) Fins or Spines of Uniform Cross Section. For either of the cases shown in Figure 17.8, the following are true: A(x) ¼ A, and P(x) ¼ P, both constants. If, additionally, both k and h are taken to be constant, equation (17-28) reduces to d2T dx2 À hP kA (T À T1) ¼ 0 (17-29) x x Figure 17.8 Two examples of extended surfaces with constant cross section. 234 Chapter 17 Steady-State Conduction

(2) Straight Surfaces with Linearly Varying Cross Section. Two configurations
for which A and P are not constant are shown in Figure 17.9. If the area and perimeter both vary in a linear manner from the primary surface, x ¼ 0, to some lesser value at the end, x ¼ L, both A and P may be expressed as A ¼ A0 À (A0 À AL) x L (17-30) and P ¼ P0 À (P0 À PL) x L (17-31) In the case of the rectangular fin shown in Figure 17.9(b), the appropriate values of A and P are A0 ¼ 2t0W AL ¼ 2tLW P0 ¼ 2½2t0 þ W PL ¼ 2½2tL þ W where t0 and tL represent the semithickness of the fin evaluated at x ¼ 0 and x ¼ L, respectively, and W is the total depth of the fin. For constant h and k, equation (17-28) applied to extended surfaces with cross-sectional area varying linearly becomes A0 À (A0 À AL) x L h i d2T dx2 À A0 À AL L dT dx À h k P0 À (P0 À PL) x L h i (T À T1) ¼ 0 (17-32) (3) Curved Surfaces of Uniform Thickness. A common type of extended surface is that of the circular fin of constant thickness as depicted in Figure 17.10. The appropriate expressions for A and P, in this case, are and A ¼ 4prt P ¼ 4pr ) r0 r rL When these expressions are substituted into equation (17-28), the applicable differential equation, considering k and h constant, is d2T dr2 þ 1 r dT dr À h kt (T À T1) ¼ 0 (17-33) 2tL (b) (a) x L x L t0 t0 Figure 17.9 Two examples of straight extended surfaces with variable cross section. rL 2t r0 r Figure 17.10 A curved fin of constant thickness. 17.3 Heat Transfer from Extended Surfaces 235

Equation (17-33) is a form of Bessel’s equation of zero
order. The solution is in terms of Bessel functions of the first kind. The description and use of these functions are beyond the mathematical scope of this text. The interested reader may consult the work of Kraus et al.3 for a complete discussion of Bessel functions and their use. In each of the cases considered, the thermal conductivity and convective heat-transfer coefficient were assumed constant. When the variable nature of these quantities is considered, the resulting differential equations become still more complex than those developed thus far. Solutions for the temperature profile in the case of the straight fin of constant cross section will now be considered; equation (17-29) applies. The general solution to equation (17-29) may be written u ¼ c1emx þ c2eÀmx (17-34) or u ¼ A cosh mx þ B sinh mx (17-35) where m2 ¼ hP/kA and u ¼ T À T1. The evaluation of the constants of integration requires that two boundary conditions be known. The three sets of boundary conditions that we shall consider are as follows: (a) T ¼ T0 at x ¼ 0 T ¼ TL at x ¼ L (b) T ¼ T0 at x ¼ 0 dT dx ¼ 0 at x ¼ L and (c) T ¼ T0 at x ¼ 0 Àk dT dx ¼ h(T À T1) at x ¼ L The first boundary condition of each set is the same and stipulates that the temperature at the base of the extended surface is equal to that of the primary surface. The second boundary condition relates the situation at a distance L from the base. In set (a) the condition is that of a known temperature at x ¼ L. In set (b) the temperaturegradient is zero at x ¼ L. In set (c) the requirement is that heat flow to the end of an extended surface by conduction be equal to that leaving this position by convection. The temperature profile, associated with the first set of boundary conditions, is u u0 ¼ T À T1 T0 À T1 ¼ uL u0 À eÀmL emx À eÀmx emL À eÀmL þ eÀmx (17-36) A special case of this solution applies when L becomes very large, that is, L ! 1, for which equation (17-36) reduces to u u0 ¼ T À T1 T0 À T1 ¼ eÀmx (17-37) The constants, c1 and c2 , obtained by applying set (b), yield, for the temperature profile, u u0 ¼ T À T1 T0 À T1 ¼ emx 1 þ e2mL þ eÀmx 1 þ eÀ2mL (17-38) 3 A. D. Kraus, A. Aziz, and J. R. Welty, Extended Surface Heat Transfer, Wiley-Interscience, New York, 2001. 236 Chapter 17 Steady-State Conduction

An equivalent expression to equation (17-38) but in a more
compact form is u u0 ¼ T À T1 T0 À T1 ¼ cosh½m(L À x) cosh mL (17-39) Note that, in either equation (17-38) or (17-39), as L ! 1 the temperature profile approaches that expressed in equation (17-37). The application of set (c) of the boundary conditions yields, for the temperature profile, u u0 ¼ T À T1 T0 À T1 ¼ cosh½m(L À x) þ (h/mk)sinh½m(L À x) cosh mL þ (h/mk)sinh mL (17-40) It may be noted that this expression reduces to equation (17-39) if du/dx ¼ 0 at x ¼ L and to equation (17-37) if T ¼ T1 at L ¼ 1: The expressions for T(x) that have been obtained are particularly useful in evaluating the total heat transfer from an extended surface. This total heat transfer may be determined by either of two approaches. The first is to integrate the convective heat-transfer expression over the surface according to q ¼ Z S h½T(x) À T1dS ¼ Z S hu dS (17-41) The second method involves evaluating the energy conducted into the extended surface at the base as expressed by q ¼ ÀkA dT dx x¼0 (17-42) The latter of these two expressions is easier to evaluate; accordingly, we will use this equation in the following development. Using equation (17-36), we find that the heat transfer rate, when set (a) of the boundary conditions applies, is q ¼ kAmu0 1 À 2 uL/u0 À eÀmL emL À eÀmL ! (17-43) If the length L is very long, this expression becomes q ¼ kAmu0 ¼ kAm(T0 À T1) (17-44) Substituting equation (17-39) [obtained by using set (b) of the boundary conditions] into equation (17-42), we obtain q ¼ kAmu0 tanh mL (17-45) Equation (17-40), utilized in equation (17-42), yields for q the expression q ¼ kAmu0 sinh mL þ (h/mk)cosh mL cosh mL þ (h/mk)sinh mL (17-46) The equations for the temperature profile and total heat transfer for extended surfaces of more involved configuration have not been considered. Certain of these cases will be left as exercises for the reader. Aquestionthatislogicallyaskedatthispointis,‘‘Whatbenefitisaccruedbytheadditionof extended surfaces?’’ A term that aids in answering this question is the fin efficiency, symbolized as hf , defined as the ratio of the actual heat transfer from an extended surface to the maximum possible heat transfer from the surface. The maximum heat transfer would occur if the temperature of the extended surface were equal to the base temperature, T0 , at all points. 17.3 Heat Transfer from Extended Surfaces 237

Figure 17.11 is a plot of hf as a function
of a significant parameter for both straight and circular fins of constant thickness (when fin thickness is small, t ( rL À r0). The total heat transfer from a finned surface is qtotal ¼ qprimary surface þ qfin ¼ A0h(T0 À T1) þ Af h(T À T1) (17-47) The second term in equation (17-47) is the actual heat transfer from the fin surface in terms of the variable surface temperature. This may be written in terms of the fin efficiency, yielding qtotal ¼ A0h(T0 À T1) þ Af hhf (T0 À T1) or qtotal ¼ h(A0 þ Af hf )(T0 À T1) (17-48) In this expression A0 represents the exposed area of the primary surface, Af is the total fin surface area, and the heat transfer coefficient, h, is assumed constant. The application of equation (17-48) as well as an idea of the effectiveness of fins is illustrated in Example 3. EXAMPLE 3 Water and air are separated by a mild-steel plane wall. It is proposed to increase the heat-transfer rate between these fluids by adding straight rectangular fins of 1.27-mm thickness and 2.5-cm length, spaced 1.27 cm apart. The air-side and water-side heat-transfer coefficients may be assumed constant with values of 11.4 and 256 W/m2ÁK respectively. Determine the percent change in total heat transfer when fins are placed on (a) the water side, (b) the air side, and (c) both sides. 1.8 1.6 4.0 nf (rL /r0 ) h/kt 0 1.0 2.0 3.0 4.0 5.0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 2t rL r0 3.0 rL /r0 = 1.0 (straight fin) For a straight fin rL – r0 = L 2.0 1.4 Figure 17.11 Fin efficiency for straight and circular fins of constant thickness. 238 Chapter 17 Steady-State Conduction

For a 1 m2 section of the wall, the areas
of the primary surface and of the fins are Ao ¼ 1 m2 À 79 fins (1 m) 0:00127 m fin ! ¼ 0:90 m2 A f ¼ 79 fins (1 m)½(2)(0:025 m) þ 0:10 m2 ¼ 4:05 m2 Values of fin efficiency can now be determined from Figure 17.11. For the air side L ffiffiffiffiffiffiffiffi h/kt p ¼ 0:025 m 11:4 W/m2ÁK (42:9 W/mÁK)(0:00127 m) !1=2 ¼ 0:362 and for the water side L ffiffiffiffiffiffiffiffiffi h/kT p ¼ 0:025 m 256 W/m2ÁK (42:9 W/mÁK)(0:00127 m) !1=2 ¼ 1:71 The fin efficiencies are then read from the figure as hair ffi 0:95 hwater ffi 0:55 The total heat transfer rates can now be evaluated. For fins on the air side q ¼ haD Ta ½Ao þ hfa Af ¼ 11:4 DTa ½0:90 þ 0:95(4:05) ¼ 54:1 DTa and on the water side q ¼ hw DTw ½Ao þ hfw Af ¼ 256DTw ½0:90 þ 0:55(4:05) ¼ 801 DTw The quantities DTa and DTw represent the temperature differences between the steel surface at temperature To and the fluids. The reciprocals of the coefficients are the thermal resistances of the finned surfaces. Without fins the heat-transfer rate in terms of the overall temperature difference, DT ¼ Tw À Ta, neglecting the conductive resistance of the steel wall, is q ¼ DT 1 11:4 þ 1 256 ¼ 10:91 DT With fins on the air side alone q ¼ DT 1 54:1 þ 1 256 ¼ 44:67 DT an increase of 310% compared with the bare-wall case. With fins on the water side alone q ¼ DT 1 11:4 þ 1 801 ¼ 11:24 DT an increase of 3.0%. 17.3 Heat Transfer from Extended Surfaces 239

With fins on both sides the heat-flow rate is q
¼ DT 1 54:1 þ 1 801 ¼ 50:68 DT an increase of 365%. This result indicates that adding fins is particularly beneficial where the convection coefficient has a relatively small value. 17.4 TWO- AND THREE-DIMENSIONAL SYSTEMS In Sections 17.2 and 17.3, we discussed systems in which the temperature and the energy transfer were functions of a single-space variable. Although many problems fall into this category, there are many other systems involving complicated geometry or temperature boundary conditions, or both, for which two or even three spatial coordinates are necessary to describe the temperature field. In this section, we shall review some of the methods for analyzing heat transfer by conduction in two- and three-dimensional systems. The problems will mainly involve two- dimensional systems, as they are less cumbersome to solve yet illustrate the techniques of analysis. Analytical Solution. An analytical solution to any transfer problem must satisfy the differential equation describing the process as well as the prescribed boundary conditions. Many mathematical techniques have been used to obtain solutions for particular energy conduction situations in which a partial differential equation describes the temperature field. Carslaw and Jaeger4 and Boelter et al.5 have written excellent treatises that deal with the mathematical solutions for many of the more complex conduction problems. As most of this material is too specialized for an introductory course, a solution will be obtained to one of the first cases analyzed by Fourier6 in the classical treatise that established the theory of energy transfer by conduction. This solution of a two-dimensional conduction medium employs the mathematical method of separation of variables. Consider a thin, infinitely longrectangular plate that is free of heat sources, as illustrated in Figure 17.12. For a thin plate @T/@z is negligible, and the temperature is a function of x and y only. The solution will be obtained for the case in which the two edges of the plate are maintained at zero temperature and the bottom is maintained at T1 as shown. The steady- state temperature distribution in the plate of constant thermal conductivity must satisfy the differential equation @2T @x2 þ @2T @y2 ¼ 0 (17-49) T = 0 T = 0 T = T1 y x L Figure 17.12 Model for two-dimensional conduction analysis. 4 H. S. Carslaw and J. C. Jaeger, Conduction of Heat in Solids, 2nd Edition, Oxford Univ. Press, New York, 1959. 5 L. M. K. Boelter, V. H. Cherry, H. A. Johnson, and R. C. Martinelli, Heat Transfer Notes, McGraw-Hill Book Company, New York, 1965. 6 J. B. J. Fourier, Theorie Analytique de la Chaleur, Gauthier-Villars, Paris, 1822. 240 Chapter 17 Steady-State Conduction

and the boundary conditions T ¼ 0 at x ¼
0 for all values of y T ¼ 0 at x ¼ L for all values of y T ¼ T1 at y ¼ 0 for 0 x L and T ¼ 0 at y ¼ 1 for 0 x L Equation (17-49) is a linear, homogeneous partial differential equation. This type of equation usually can be integrated by assuming that the temperature distribution, T(x, y), is of the form T(x; y) ¼ X(x)Y(y) (17-50) where X(x) is a function of x only and Y(y) is a function of y only. Substituting this equation into equation (17-49), we obtain an expression in which the variables are separated À 1 X d2X dx2 ¼ 1 Y d2Y dy2 (17-51) As the left-hand side of equation (17-51) is independent of y and the equivalent right- hand side is independent of x, it follows that both must be independent of x and y, and hence must be equal to a constant. If we designate this constant l2, two ordinary differential equations result d2X dx2 þ l2X ¼ 0 (17-52) and d2Y dy2 À l2Y ¼ 0 (17-53) These differential equations may be integrated, yielding X ¼ A cos lx þ B sin lx and Y ¼ Cely þ DeÀly According to equation (17-50), the temperature distribution is deﬁned by the relation T(x, y) ¼ XY ¼ (A cos lx þ B sin lx)(Cely þ DeÀly) (17-54) where A, B, C, and D are constants to be evaluated from the four boundary conditions. The condition that T ¼ 0 at x ¼ 0 requires that A ¼ 0: Similarly, sin lx must be zero at x ¼ L; accordingly, lL must be an integral multiple of p or l ¼ np/L: Equation (17-54) is now reduced to T(x, y) ¼ B sin npx L Cenpy/L þ DeÀnpy/L (17-55) The requirement that T ¼ 0 at y ¼ 1 stipulates that C must be zero. A combination of B and D into the single constant E reduces equation (17-55) to T(x, y) ¼ EeÀnpy/L sin npx L 17.4 Two- and Three-Dimensional Systems 241

This expression satisfies the differential equation for any integer n
greater than or equal to zero. The general solution is obtained by summing all possible solutions, giving T ¼ å 1 n¼1 EneÀnpy/L sin npx L (17-56) The last boundary condition, T ¼ T1 at y ¼ 0; is used to evaluate En according to the expression T1 ¼ å 1 n¼1 En sin npx L for 0 x L The constants En are the Fourier coefficients for such an expansion and are given by En ¼ 4T1 np for n ¼ 1, 3, 5, . . . and En ¼ 0 for n ¼ 2, 4, 6, . . . The solution to this two-dimensional conduction problem is T ¼ 4T1 p å 1 n¼0 e½À(2nþ1)py/L 2n þ 1 sin (2n þ 1)px L (17-57) The isotherms and energy flow lines are plotted in Figure 17.13. The isotherms are shown in the figure as solid lines, and the dotted lines, which are orthogonal to the isotherms, are energy-flow lines. Note the similarity to the lines of constant velocity potential and stream function as discussed in momentum transfer. The separation of variables method can be extended to three-dimensional cases by assuming T to be equal to the product X(x)Y(y)Z(z) and substituting this expression for T into the applicable differential equation. When the variables are separated, three second- order ordinary differential equations are obtained, which may be integrated subject to the given boundary conditions. Analytical solutions are useful when theycan be obtained. There are, however, practical problems with complicated geometry and boundary conditions, which cannot be solved analytically. As an alternative approach, one must turn to numerical methods. Shape Factors for Common Configurations The shape factor, S, is defined and discussed briefly in Chapter 15. When a geometric case of interest involves conduction between a source and a sink, both with isothermal boundaries, a knowledge of the shape factor makes the determination of heat flow a simple calculation. Table 17.1 lists expressions for shape factors of five configurations. In every case depicted, it is presumed that the heat transfer problem is two dimensional, that is, the dimension normal to the plane shown in very large. Numerical Solutions Each of the solution techniques discussed thus far for multidimensional conduction has considerable utility when conditions permit its use. Analytical solutions require relatively simple functions and geometries; the use of shape factors requires isothermal boundaries. When the situation of interest becomes sufficiently complex or when T = 0 T = 0 T = T1 Figure 17.13 Isotherms and energy flow lines for the rectangular plate in Figure 17.12. 242 Chapter 17 Steady-State Conduction

boundary conditions preclude the use of simple solution techniques, one
must turn to numerical solutions. With the presence of digital computers to accomplish the large number of manip- ulations inherent in numerical solutions rapidly and accurately, this approach is now very common. In this section we shall introducethe conceptsofnumericalproblem formulation and solution. A more complete and detailed discussion of numerical solutions to heat conduction problems may be found in Carnahan et al.7 and in Welty.8 Shown in Figure 17.14 is a two- dimensional representation of an element within a conducting medium. The element Table 17.1 Conduction shape factors. Shape Shape factor, S q/L ¼ kS(Ti À To) rb ri Ti Tb Concentric circular cylinders 2p ln(ro/ri) rb e ri Ti Tb Eccentric circular cylinders 2p coshÀ1 1 þ r2 À e2 2r r ri/ro ; e e/ro rb ri Ti Tb Circular cylinder in a hexagonal cylinder 2p ln(ro/ri) À 0:10669 rb ri Ti Tb Circular cylinder in a square cylinder 2p ln(ro/ri) À 0:27079 r Ti Tb r Inﬁnite cylinder buried in semi-inﬁnite medium 2p coshÀ1(r/r) ⌬y ⌬x i + 1, j i – 1, j i, j + 1 i, j – 1 i, j Figure 17.14 Two-dimensional volume element in a conducting medium. 7 B. Carnahan, H. A. Luther, and J. O. Wilkes, Applied Numerical Methods, Wiley, New York, 1969. 8 J. R. Welty, Engineering Heat Transfer, Wiley, New York, 1974. 17.4 Two- and Three-Dimensional Systems 243

or ‘‘node’’ i, j is centered in the figure along
with its adjacent nodes. The designation, i, j, implies a general location in a two-dimensional system where i is a general index in the x direction and j is they index. Adjacent node indices are shown in Figure 17.14. The grid is set up with constant node width, Dx; and constant height, Dy: It may be convenient to make the grid ‘‘square,’’ that is, Dx ¼ Dy; but for now we will allow these dimensions to be different. A direct application of equation (6-10) to node i, j yields dQ dt ¼ @ @t ZZZ c:v: er dV (17-58) The heat input term, dQ/dt, may be evaluated allowing for conduction into node i, j from the adjacent nodes and by energy generation within the medium. Evaluating dQ/dt in this manner, we obtain dQ dt ¼ k Dy Dx (TiÀ1,j À Ti, j) þ k Dy Dx (Tiþ1,j À Ti,j) þ k Dx Dy (Ti,jÀ1 À Ti,j) þ k Dx Dy (Ti,jþ1 À Ti,j) þ _ q Dx Dy ð17-59Þ The first two terms in this expression relate conduction in the x direction, the third and fourth express y-directional conduction, and the last is the generation term. All of these terms are positive; heat transfer is assumed positive. The rate of energy increase within node i, j may be written simply as @ @t ZZZ c:v: er dV ¼ rcTj tþDt À rcTj t Dt ! Dx Dy (17-60) Equation (17-58) indicates that the expressions given by equations (17-59) and (17-60) may be equated. Setting these expressions equal to each other and simplifying, we have k Dy Dx ½TiÀ1,j þ Tiþ1,j À 2Ti,j þ k Dx Dy ½Ti,jÀ1 þ Ti,jþ1 À 2Ti,j þ _ q Dx Dy ¼ rcTi,j j tþDt À rcTi,j j t Dt ! Dx Dy ð17-61Þ This expression has been considered in a more complete form in the next chapter. For the present we will not consider time-variant terms; moreover, we will consider the nodes to be square, that is, Dx ¼ Dy: With these simplifications equation (17-61) becomes TiÀ1,j þ Tiþ1,j þ Ti,jÀ1 þ Ti,jþ1 À 4Ti,j þ _ q Dx2 k ¼ 0 (17-62) In the absence of internal generation, equation, (17-62) may be solved for Tij to yield Ti,j ¼ TiÀ1,j þ Tiþ1,j þ Ti,jÀ1 þ Ti,jþ1 4 (17-63) or, the temperature of node i, j is the arithmetic mean of the temperatures of its adjacent nodes. A simple example showing the use of equation (17-63) in solving a two-dimensional heat conduction problem follows. EXAMPLE 4 A hollow square duct of the configuration shown (left) has its surfaces maintained at 200 and 100 K, respectively. Determine the steady-state heat transfer rate between the hot and cold surfaces of this duct. The wall material has a thermal conductivity of 1:21 W/mÁK: We may take advantage of the eightfold symmetry of this figure to lay out the simple square grid shown below (right). 244 Chapter 17 Steady-State Conduction

3 m 3 m 1 m 1 m 100 200
200 100 100 1' 2' 3 2 1 100 The grid chosen is square with Dx ¼ Dy ¼ 1/2m: Three interior node points are thus identified; their temperatures may be determined by proper application of equation (17-63). Writing the proper expressions for T1 , T2 , and T3 using equation (17-68) as a guide, we have T1 ¼ 200 þ 100 þ 2T2 4 T2 ¼ 200 þ 100 þ T1 þ T3 4 T3 ¼ 100 þ 100 þ 2T2 4 This set of three equations and three unknowns may be solved quite easily to yield the following: T1 ¼ 145:83 K, T2 ¼ 141:67 K, T3 ¼ 120:83 K: The temperatures just obtained may now be used to find heat transfer. Implicit in the procedure of laying out a grid of the sort, we have specified is the assumption that heat flows in the x and y directions between nodes. Onthis basis heat transfer occurs fromthe hotsurface tothe interioronly to nodes 1 and 2; heat transfer occurs to the cooler surface from nodes 1, 2, and 3. We should also recall that the section of duct that has been analyzed is one-eighth of the total thus, of the heat transfer to and from node 1, only one half should be properly considered as part of the element analyzed. We now solve for the heat transfer rate from the hotter surface, and write q ¼ k(200 À T1 ) 2 þ k(200 À T2 ) ¼ k 200 À 145:83 2 þ (200 À 141:67) ! ¼ 85:415k (q in W/m, k in W/mÁK) A similar accounting for the heat flow from nodes 1, 2, and 3 to the cooler surface is written as q ¼ k(T1 À 100) 2 þ k(T2 À 100) þ k(T3 À 100) ¼ k 145:83 À 100 2 þ (141:67 À 100) þ (120:83 À 100) ! ¼ 85:415k (q in W/m, k in W/mÁK) Observe that these two different means of solving for q yield identical results. This is obviously a requirement of the analysis and serves as a check on the formulation and numerical work. The example may now be concluded. The total heat transfer per meter of duct is calculated as q ¼ 8(8:415 K)(1:21 W/mÁK) ¼ 826:8 W/m 17.4 Two- and Three-Dimensional Systems 245

Example 4 has illustrated, in simple fashion, the numerical approach
to solving two-dimensional steady-state conduction problems. It is apparent that any added complex- ity in the form of more involved geometry, other types of boundary conditions such as convection, radiation, specified heat flux, among others, or simply a greater number of interior nodes, will render a problem too complex for hand calculation. Techniques for formulating such problems and some solution techniques are described by Welty. In this section, we have considered techniques for solving two- and three-dimensional steady-state conduction problems. Each of these approaches has certain requirements that limit their use. The analytical solution is recommended for problems of simple geometrical shapes and simple boundary conditions. Numerical techniques may be used to solve complex problems involving nonuniform boundary conditions and variable physical properties. 17.5 CLOSURE In this chapter, we have considered solutions to steady-state conduction problems. The defining differential equations were frequently established by generating the equation through the use of the control-volume expression for the conservation of energy as well as by using the general differential equation for energy transfer. It is hoped that this approach will provide the student with an insight into the various terms contained in the general differential equation and thus enable one to decide, for each solution, which terms are relevant. One-dimensional systems with and without internal generation of energy were considered. PROBLEMS 17.1 One-dimensional steady-state conduction, with no internal heatgeneration,occursacrossaplanewallhavingaconstantthermal conductivity of 30 W/mÁK: The material is 30 cm thick. For each case listed in the table below, determine the unknown quantities. Show a sketch of the temperature distribution for each case. 17.2 The steady-state expression for heat conduction through a plane wall is q ¼ (kA/L)DT as given by equation (17-4). For steady-state heat conduction through a hollow cylinder, an expression similar to equation (17-4) is q ¼ kA ro À ri DT where A is the ‘‘log-mean’’ area defined as A ¼ 2p ro À ri ln(ro/ri) a. Show that A as defined above satisfies the equations for steady-state radial heat transfer in a hollow cylindrical element. b. If the arithmetic mean area, p(ro À ri ), is used rather than the logarithmic mean, calculate the resulting percent error for values of ro /ri of 1.5, 3, and 5. 17.3 Evaluate the appropriate ‘‘mean’’ area for steady-state heat conduction in a hollow sphere that satisfies an equation of the form q ¼ kA ro À ri DT Repeat part (b) of Problem 17.2 for the spherical case. 17.4 It is desired to transport liquid metal through a pipe embedded in a wall at a point where the temperature is 650 K. A 1.2-m-thick wall constructed of a material having a thermal conductivity varying with temperature according to dT/dx qx Case T1 T2 (K/m) (W/m2) 1 350 K 275 K 2 300 K À2000 3 350 K À300 4 250 K À200 246 Chapter 17 Steady-State Conduction

k ¼ 0:0073(1 þ 0:0054 T), where T is in
K and k is in W/mÁK, has its inside surface maintained at 925 K. The outside surface is exposed to air at 300 K with a convective heat-transfer coefficient of 23 W/m2ÁK: How far from the hot surface should the pipe be located? What is the heat flux for the wall? 17.5 The temperature at the inner and outer surfaces of a plane wall of thickness L are held at the constant values T0 and TL , respectively, where T0 > TL : The wall material has a thermal conductivity that varies linearly according to k ¼ k0(1 þ bT), k0 and b being constants.Atwhatpointwilltheactualtemperature profile differ most from that which would exist in the case of constant thermal conductivity? 17.6 Solve Problem 17.5 for the case of a hollow cylinder with boundary conditions T ¼ T0 at r ¼ R0 and T ¼ TL at r ¼ R0 þ L: 17.7 A double-pane insulated window unit consists of two 1- cm-thick pieces of glass separated by a 1.8-cm layer of air. The unit measures 4 m in width and is 3 m wide. Under conditions where the extreme outside temperature of the glass is at À10C and air, at 27C, is adjacent to the inside glass surface, with hi ¼ 12 W/m2ÁK determine a. the inside glass surface temperature. b. the rate of heat transfer through the window unit. The air gap between glass panes may be treated as a purely conductive layer with k ¼ 0:0262 W/mÁK: Thermal radiation is to be neglected. 17.8 A furnace wall is to be designed to transmit a maximum heat flux of 200 Btu/h ft2 of wall area. The inside and outsidewall temperatures are to be 2000 and 3008F, respectively. Determine the most economical arrangement of bricks measuring 9 by 4 1/2 by 3 in. if they are made from two materials, one with a k of 0.44 Btu/h ft 8F and a maximum usable temperature of 15008F and other with a k of 0.94 Btu/h ft 8F and a maximum usable temperature of 2200 8F. Bricks made of each material cost the same amount and may be laid in any manner. 17.9 A furnace wall consisting of 0.25 m of fire clay brick, 0.20 m of kaolin, and a 0.10-m outer layer of masonry brick is exposed to furnace gas at 1370 K with air at 300 K adjacent to the outside wall. The inside and outside convective heat transfer coefficients are 115 and 23 W/m2ÁK, respectively. Determine the heat loss per square foot of wall and the temperature of the outside wall surface under these conditions. 17.10 Given the conditions of Problem 17.9, except that the outside temperature of the masonry brick cannot exceed 325 K, by how much must the thickness of kaolin be adjusted to satisfy this requirement? 17.11 A heater composed of Nichrome wire wound back and forth and closely spaced is covered on both sides with 1/8 -in. thickness of asbestos (k ¼ 0:15 Btu/h ft F) and then with a 1/8 -in. thickness of stainless steel (k ¼ 10 Btu/h ft F): If the center temperature of this sandwich construction is considered constant at 10008F and the outside convective heat-transfer coefficient is 3 Btu/h ft2 F, how much energy must be supplied in W/ft2 to the heater? What will be the outside temperature of the stainless steel? 17.12 Determine the percent in heat flux if, in addition to the conditions specified in Problem 17.8, there are two 3/4 -in.- diameter steel bolts extending through the wall per square foot of wall area (k for steel ¼ 22 But/h ft F): 17.13 A 2.5-cm-thick sheet of plastic (k ¼ 2:42 W/mÁK) is to be bonded to a 5-cm-thick aluminum plate. The glue that will accomplish the bonding is to be held at a temperature of 325 K to achieve the best adherence, and the heat to accomplish this bonding is to be provided by a radiant source. The convective heat-transfer coefficient on the outside surfaces of both the plastic and aluminum is 12 W/m2ÁK, and the surrounding air is at 295 K. What is the required heat flux if it is applied to the surface of (a) the plastic? (b) the aluminum? 17.14 A composite wall is to be constructed of 1/4 in. of stainless steel (k ¼ 10 Btu/h ft F), 3 in. of corkboard (k ¼ 0:025 Btu/h ft F), and 1/2 in. of plastic (k ¼ 1:5 Btu/h ft F): Determine the thermal resistance of this wall if it is bolted together by 1/2 -in.-diameter bolts on 6-in. centers made of a. stainless steel; b. aluminum (k ¼ 120 Btu/h ft F): 17.15 A cross section of a typical home ceiling is depicted below. Given the properties listed for the materials of construction, determine how much heat is transferred through the insulation and through the studs. Toutside ¼ À10C ho ¼ 20 W/m2ÁK Tinside ¼ 25C hi ¼ 10 W/m2ÁK kfiberglass ¼ 0:035 W/m2ÁK kplaster ¼ 0:814 W/m2ÁK kwood ¼ 0:15 W/m2ÁK Loose fiberglass insulation Pine studs Plaster 6 cm 30 cm 2 cm 15 cm 17.16 A copper bus bar measuring 5 cm by 10 cm by 2.5 m long is in a room in which the air is maintained at 300 K. The bus bar is supported by two plastic pedestals to which it is attached by an adhesive. The pedestals are square in cross section, measuring 8 cm on a side. The pedestals are mounted on a wall whose temperature is 300 K. If 1 kW of energy is dissipated in the copper bar, what will be its equilibrium Problems 247

temperature? The convective heat-transfer coefficient for all surfaces may be
taken as 23 W/m2ÁK: The thermal conductivity of the plastic is 2:6 W/mÁK: Neglect thermal radiation. 15 cm 17.17 Solve Problem17.16 if each plastic pedestal hasa 1.9-cm steel bolt running through the center. 17.18 A 2-in. schedule-40 steel pipe carries saturated steam at 60 psi through a laboratory that is 60 ft long. The pipe is insulated with 1.5 in. of 85% magnesia that costs $0.75 per foot. How long must the steam line be in service to justify the insulation cost if the heating cost for the steam is $0:68 per 105 Btu? The outside- surface convective heat-transfer coefficient may be taken as 5 Btu/h ft2 F: 17.19 Saturated steam at 40 psia flows at 5 fps through a schedule-40, 11/2 -in. steel pipe. The convective heat-transfer coefficient by condensing steam on the inside surface may be taken as 1500 Btu/h ft2 F: The surrounding air is at 80F, and the outside surface coefficient is 3 Btu/ft2 F: Determine the following: a. The heat loss per 10 ft of bare pipe. b. The heat loss per 10 ft of pipe insulated with 2 in. of 85% magnesia. c. The mass of steam condensed in 10 ft of bare pipe. 17.20 A 10-kW heater using Nichrome wire is to be designed. The surface of the Nichrome is to be limited to a maximum temperature of 1650 K. Other design criteria for the heater are minimum convective heat-transfer coefficient: 850 W/m2ÁK minimum temperature of the surrounding medium (air): 370 K The resistivity of Nichrome is 110 mV-cm and the power to the heater is available at 12 V. a. What size wire is required if the heater is to be in one piece 0.6 m long? b. What length of 14-gage wire is necessary to satisfy these design criteria? c. How will the answers to parts (a) and (b) change if h ¼ 1150 W/m2ÁK? 17.21 Copper wire having a diameter of 3/16 in. is insulated with a 4-in. layer of material having a thermal conductivity of 0:14 Btu/h ft F: The outer surface of the insulation is maintained at 70F: How much current may pass through the wire if the insulation temperature is limited to a maximum of 120F? The resistivity of copper is 1:72 Â 10À6 ohm-cm. 17.22 What would be the result of Problem 17.21 if the fluid surrounding the insulated wire was maintained at 70F with a convective heat-transfer coefficient between the insulation and the fluid of 4 Btu/h ft2 F? What would be the surface temperature of the insulation under these conditions? 17.23 Work Problem 17.21 for the case of aluminum rather than copper. The resistivity of aluminum is 2:83 Â 10À6 ohm-cm. 17.24 A thin slab of material is subjected to microwave radiation that causes volumetric heating to vary according to _ q(x) ¼ _ qo ½1 À (x/L) where _ qo has a constant value of 180 kW/m3 and the slab thickness, L, is 0.06 m. The thermal conductivity of the slab material is 0.6 W/m Á K. The boundary at x ¼ L is perfectly insulated, while the surface at x ¼ 0 is maintained at a constant temperature of 320 K. a. Determine an expression for T(x) in terms of X; L; k; _ qo , and To : b. Where, in the slab, will the maximum temperature occur? c. What is the value of Tmax? 17.25 Radioactivewaste (k ¼ 20 W/mÁK) is stored in a cylindrical stainless steel (k ¼ 15 W/mÁK) container with inner and outer diameters of 1.0 and 1.2 m, respectively. Thermal energy is generated uniformly within the waste material at a volumetric rate of 2 Â 105 W/m3: The outer container surface is exposed to water at 25C, with a surface coefficient of 1000 W/m2ÁK: The ends of the cylindrical assembly are insulated so that all heat transfer occurs in the radial direction. For this situation determine a. the steady-state temperatures at the inner and outer surfaces of the stainless steel. b. the steady-state temperature at the center of the waste material. 17.26 A cylindrical nuclear fuel element is 10.16 cm long and 10.77 cm in diameter. The fuel generates heat uniformly at a rate of 51:7 Â 103 kJ/sÁm3: The fuel is placed in an environment having a temperature of 360 K with a surface coefficient of 4540 W/m2ÁK. The fuel material has k ¼ 33:9 W/mÁK. For the situation described evaluate the following at steady state: a. The temperature profile as a function of radial position; b. The maximum fuel temperature; c. The surface temperature. End effects may be neglected. 248 Chapter 17 Steady-State Conduction

17.27 Liquid nitrogen at 77 K is stored in an
insulated spherical container that is vented to the atmosphere. The container is made of a thin-walled material with an outside diameter of 0.5 m; 25 mm of insulation (k ¼ 0:002 W/mÁK) covers its outside surface. The latent heat of nitrogen is 200 kJ/kg; its density, in the liquid phase, is 804 kg/m3. For surroundings at 25C and with a convective coefficient of 18 W/m2ÁK at the outside surface of the insulation, what will be the rate of liquid nitrogen boil-off? 17.28 What additional thickness of insulation will be necessary to reduce the boil-off rate of liquid nitrogen to one-half of the rate corresponding to Problem 17.27? All values and dimensions in Problem 17.27 apply. 17.29 A 1-in.-OD steel tube has its outside wall surface maintained at 250F. It is proposed to increase the rate of heat transfer by adding fins of 3/32-in. thickness and 3/4 in. long to the outside tube surface. Compare the increase in heat transfer achieved by adding 12 longitudinal straight fins or circular fins with the same total surface area as the 12 longitudinal fins. The surrounding air is at 80F, and the convective heat-transfer coefficient is 6 Btu/h ft2 F. 17.30 Solve Problem 17.29 if the convective heat-transfer coefficient is increased to 60 Btu/h ft2 F by forcing air past the tube surface. 17.31 A cylindrical rod 3 cm in diameter is partially inserted into a furnacewith one end exposed to the surrounding air, which is at 300 K. The temperatures measured at two points 7.6 cm apart are 399 and 365 K, respectively. If the convective heat- transfer coefficient is 17 W/m2ÁK, determine the thermal conductivity of the rod material. 17.32 Heat is to be transferred from water to air through an aluminum wall. It is proposed to add rectangular fins 0.05 in. thick and 3/4 in. long spaced 0.08 in. apart to the aluminum surface to aid in transferring heat. The heat-transfer coefficients on the air and water sides are 3 and 25 Btu/h ft2 F, respectively. Evaluate the percent increase in heat transfer if these fins are added to (a) the air side, (b) the water side, (c) and both sides. What conclusions may be reached regarding this result? 17.33 A semiconductor material with k ¼ 2 W/mÁK and electrical resistivity, r ¼ 2 Â 10À5 V-m, is used to fabricate a cylindrical rod 40 mm long with a diameter of 10 mm. The longitudinal surface of the rod is well insulated and may be considered adiabatic while the ends are maintained at temperatures of 100 and 0C, respectively. If the rod carries a current of 10 amps, what will be its midpoint temperature? What will be the rate of heat transfer through both ends? 17.34 An iron bar used for a chimney support is exposed to hot gases at 625 K with the associated convective heat-transfer coefficient of 740 W/m2ÁK. The bar is attached to two opposing chimney walls, which are at 480 K. The bar is 1.9 cm in diameter and 45 cm long. Determine the maximum temperature in the bar. 17.35 A copper rod 1/4 in. in diameter and 3 ft long runs between two bus bars, which are at 60F. The surrounding air is at 60F, and the convective heat-transfer coefficient is Btu/h ft2 F. Assuming the electrical resistivity of copper to be constant at 1:72 Â 10À6 ohm-cm, determine the maximum current the copper may carry ifits temperature is to remain below 150 F. 17.36 A 13 cm by 13 cm steel anglewith the dimensions shown is attached to a wall with a surface temperature of 600 K. The surrounding air is at 300 K, and the convective heat-transfer coefficient between the angle surface and the air is 45 W/m2ÁK. a. Plot the temperature profile in the angle, assuming a negligible temperature drop through the side of the angle attached to the wall. b. Determine the heat loss from the sides of the angle project- ing out from the wall. 13 cm Thickness = 1.59 cm 17.37 A steel I-beam with a cross-sectional area as shown has its lower and upper surfaces maintained at 700 and 370 K, respectively. a. Assuming a negligible temperature change through both flanges, develop an expression for the temperature variation in the web as a function of the distance from the upper flange. b. Plot the temperature profile in the web if the convective heat- transfer coefficient between the steel surface and the surrounding air is 57 W/m2ÁK. The air temperature is 300 K. c. What is the net heat transfer at the upper and lower ends of the web? 0.76 cm y 28 cm 17.38 Repeat Problem 17.37 for the case of an aluminum beam. Problems 249

17.39 Circular fins are employed around the cylinder of a
lawn mower engine to dissipate heat. The fins are made of aluminum, they are 0.3 m thick, and extend 2 cm from base to tip. The outside diameter of the engine cylinder is 0.3 m. Design operating conditions are T1 ¼ 30C and h ¼ 12 W/m2ÁK: The maximum allowable cylinder temperature is 300C: Estimate the amount of heat transfer from a single fin. How many fins are required to cool a 3-kWengine, operating at 30% thermal efficiency, if 50% of the total heat given off is transferred by the fins? 17.40 Heat from a flat wall is to be enhanced by adding straight fins, of constant thickness, made of stainless steel. The following specifications apply: h ¼ 60 W/m2 K Tb(base) ¼ 120C T1(air) ¼ 20C Fin base thickness; t ¼ 6 mm Fin length; L ¼ 20 mm Determine the fin efficiency and heat loss per unit width for the finned surface. 17.41 A 2-in.-OD stainless-steel tube has 16 longitudinal fins spaced around its outside surface as shown. The fins are 1/16 in. thick and extend 1 in. from the outside surface of the tube. a. If the outside surface of the tube wall is at 250F, the surrounding air is at 80F, and the convective heat-transfer coefficient is 8 Btu/h ft2 F, determine the percent increase in heat transfer for the finned pipe over that for the unfinned pipe. b. Determine the same information as in part (a) for values of h of 2, 5, 15, 50, and 100 Btu/h ft2 F: Plot the percent increase in q vs. h. What conclusions can be reached concerning this plot? 17.42 Repeat Problem 17.41 for the case of an aluminum pipe- and-fin arrangement. 17.43 Water flows in the channels between two aluminum plates as shown in the sketch. The ribs that form the channels are also made of aluminum and are 8 mm thick. The effective surface coefficient between all surfaces and water is 300 W/m2ÁK. For these conditions, how much heat is transferred at each end of each rib? How far from the lower plate is the rib temperature a minimum? What is this minimum value? 100 cm 8 mm T = 160°C T = 400°C T∞ = 25°C 17.44 Find the rate of heat transfer from a 3-in.-OD pipe placed eccentrically inside a 6-in.-ID cylinder with the axis of the smaller pipe displaced 1 in. from the axis of the large cylinder. The space between the cylindrical surfaces is filled with rock wool (k ¼ 0:023 Btu/h ft F). The surface temperatures at the inside and outside surfaces are 400 and 100 F, respectively. 17.45 A cylindrical tunnel with a diameter of 2 m is dug in permafrost (k ¼ À0:341 W/m2ÁK) with its axis parallel to the permafrost surface at the depth of 2.5 m. Determine the rate of heat loss from the cylinder walls, at 280 K, to the permafrost surface at 220 K. 17.46 Determine the heat flow per foot for the configuration shown, using the numerical procedure for a grid size of 1 1/2 ft. The material has a thermal conductivity of 0:15 Btu/h ft F: The inside and outside temperatures are at the uniform values of 200 and 0 F; respectively. 9 ft 9 ft 3 ft 3 ft 17.47 Repeat the previous problem, using a grid size of 1 ft. 17.48 A 5-in. standard-steel angle is attached to a wall with a surface temperature of 600 F: The angle supports a 4.375-in. by 4.375-in. section of building bring whose mean thermal conductivity may be taken as 0:38 Btu/h ft F: The convective heat- transfer coefficient between all surfaces and the surrounding air is 8 Btu/h ft2 F: The air temperature is 808F. Using numerical methods, determine a. the total heat loss to the surrounding air; 250 Chapter 17 Steady-State Conduction

b. the location and value of the minimum temperature in
the brick. 5 in. 5 in. 17.49 Saturated steam at 400 F is transported through the 1-ft pipe shown in the ﬁgure, which may be assumed to be at the steam temperature. The pipe is centered in the 2-ft-square duct, whose surface is at 100 F: If the space between the pipe and duct is ﬁlled with powdered 85% magnesia insulation, how much steam will condense in a 50-ft length of pipe? 2 ft 2 ft 1 ft 17.50 A 32.4-cm-OD pipe, 145 cm long, is buried with its centerline 1.2 m below the surface of the ground. The ground surface is at 280 K and the mean thermal conductivity of the soil is 0:66 W/mÁK. If the pipe surface is at 370 K, what is the heat loss per day from the pipe? Problems 251

Chapter 18 Unsteady-State Conduction Transient processes, in which the temperature
at a given point varies with time, will be considered in this chapter. As the transfer of energy is directly related to the temperature gradient, these processes involve an unsteady-state flux of energy. Transient conduction processes are commonly encountered in engineering design. These design problems generally fall into two categories: the process that ultimately reaches steady-state conditions, and the process that is operated a relatively short time in a continually changing temperature environment. Examples of this second category would include metal stock or ingots undergoing heat treatment, missile components during reentry into Earth’s atmosphere, or the thermal response of a thin laminate being bonded using a laser source. In this chapter, we shall consider problems and their solutions that deal with unsteady-state heat transfer within systems both with and without internal energy sources. 18.1 ANALYTICAL SOLUTIONS The solution of an unsteady-state conduction problem is, in general, more difficult than that for a steady-state problem because of the dependence of temperature on both time and position. The solution is approached by establishing the defining differential equation and the boundary conditions. In addition, the initial temperature distribution in the conducting medium must be known. By finding the solution to the partial differential equation that satisfies the initial and boundary conditions, the variation in the temperature distribution with time is established, and the flux of energy at a specific time can then be evaluated. In heating or cooling a conducting medium, the rate of energy transfer is dependent upon both the internal and surface resistances, the limiting cases being represented either by negligible internal resistance or by negligible surface resistance. Both of these cases will be considered, as well as the more general case in which both resistances are important. Lumped Parameter Analysis—Systems with Negligible Internal Resistance Equation (16-17) will be the starting point for transient conduction analysis. It is repeated below for reference. @T @t ¼ a =2T þ _ q rcp (16-17) Recall that, in the derivation of this expression, thermal properties were taken to be independentof positionandtime;however, the rate ofinternalgeneration, _ q, canvary in both. 252

It is frequently the case that temperature within a medium
varies significantly in fewer than all three space variables. A circular cylinder, heated at one end with a fixed boundary condition, will show a temperaturevariation in the axial and radial directions as well as time. If the cylinder has a length that is large compared to its diameter or, if it is composed of a material with high thermal conductivity, temperature will vary with axial position and time only. If a metallic specimen, initially with uniform temperature, is suddenly exposed to surroundings at a different temperature, it may be that size, shape, and thermal conductivity may combine in such a way that the temperature within the material varies with time only, that is, is not a significant function of position. These conditions are characteristic of a ‘‘lumped’’ system, where the temperature of a body varies only with time; this case is the easiest of all to analyze. Because of this we will consider, as our first transient conduction case, that of a completely lumped-parameter system. Shown in Figure 18.1, we have a spherical metallic specimen, initially at uniform temperature T0 after it has been immersed in a hot oil bath at temperature T1 for a period of time t. It is presumed that the temperature of the metallic sphere is uniform at any given time. A first-law analysis using equation (6-10), applied to a spherical control volume coinciding with the specimen in question will reduce to dQ dt ¼ @ @t ZZZ c:v: er dV (18-1) The rate of heat addition to the control volume, dQ/dt, is due to convection from the oil and is written as dQ dt ¼ hA(T1 À T) (18-2) The rate of energy increase within the specimen, @/@t RRR c:v: er dV, with constant properties, may be expressed as @ @t ZZZ c:v: er dV ¼ rVcp dT dt (18-3) Equating these expressions as indicated by equation (18-1) we have, with slight rearrangement dT dt ¼ hA(T1 À T) rVcp (18-4) We may now obtain a solution for the temperature variation with time by solving equation (18-4) subject to the initial condition, T(0) ¼ T0, and obtain T À T1 T0 À T1 ¼ eÀhAt/rcp V (18-5) The exponent is observed to be dimensionless. A rearrangement of terms in the exponent may be accomplished as follows: hAt rcpV ¼ hV kA A2k rV2cp t ¼ hV/A k ! at (V/A)2 ! (18-6) Each of the bracketed terms in equation (18-6) is dimensionless. The ratio, V/A, having units of length, is also seen to be a part of each of these new parametric forms. The first of Spherical specimen T(0) = T0 (uniform) T(t) = T T∞ Figure 18.1 18.1 Analytical Solutions 253

the new nondimensional parameters formed is the Biot modulus, abbreviated
Bi Bi ¼ hV/A k (18-7) By analogy with the concepts of thermal resistance, discussed at length earlier, the Biot modulusisseentobetheratioof(V/A)/k,theconductive(internal)resistancetoheattransfer,to 1/h, the convective (external) resistance to heat transfer. The magnitude of Bi thus has some physical signiﬁcance in relating where the greater resistance to heat transfer occurs. A large value of Bi indicates that the conductive resistance controls, that is, there is more capacity for heat to leave the surface by convection than to reach it by conduction. A small value for Bi represents the case where internal resistance is negligibly small and there is more capacity to transfer heat by conduction than there is by convection. In this latter case, the controlling heat transfer phenomenon is convection, and temperature gradients within the medium are quite small. An extremely small internal temperature gradient is the basic assumption in a lumped- parameter analysis. A natural conclusion to the foregoing discussion is that the magnitude of the Biot modulus is a reasonable measure of the likely accuracy of a lumped-parameter analysis. A commonly used rule of thumb is that the error inherent in a lumped-parameter analysis will be less than 5% for avalue of Bi lessthan 0.1. The evaluation of the Biot modulus shouldthus be the ﬁrst thing done when analyzing an unsteady-state conduction situation. The other bracketed term in equation (18-6) is the Fourier modulus, abbreviated Fo, where Fo ¼ at (V/A)2 (18-8) The Fourier modulus is frequently used as a nondimensional time parameter. The lumped-parameter solution for transient conduction may now be written as T À T1 T0 À T1 ¼ eÀBiFo (18-9) Equation (18-9) is portrayed graphically in Figure 18.2. The use of equation (18-9) is illustrated in the following example. BiFo 0 1 2 3 4 5 6 7 0.001 0.002 0.004 0.006 0.008 0.01 0.02 0.04 0.06 0.08 T – T∞ T0 – T∞ 0.1 0.2 0.4 0.6 0.8 1.0 Figure 18.2 Time-temperature history of a body at initial temperature, T0 , exposed to an environment at T1; lumped- parameter case. 254 Chapter 18 Unsteady-State Conduction

EXAMPLE 1 A long copper wire, 0.635 cm in diameter,
is exposed to an air stream at a temperature, T1, of 310 K. After 30 s the average temperature of the wire increased from 280 to 297 K. Using this information, estimate the average surface conductance, h. In order to determine if equation () is valid for this problem, the value of Bi must be determined. The Biot number is expressed as Bi ¼ hV/A k ¼ h pD2L 4pDL 386 W/m Á K ¼ h 0:00635 m 4 386 W/m Á K ¼ 4:11 Â 10À6h Setting Bi ¼ 0.1, which is the limiting value of Bi for a lumped-parameter analysis to be valid, and solving for h, we obtain h ¼ 0:1/4:11 Â 10À6 ¼ 24300 W/m2 Á K We may conclude that a lumped-parameter solution is valid if h < 24; 300 W/m2 Á K, which is a near certainty. Proceeding, we can apply equation () to yield h ¼ rcpV tA ln T0 À T1 T À T1 ¼ (8890 kg/m3)(385 J/kg Á K) pD2L 4pDL (30 s) ln 280 À 310 297 À 310 ¼ 51 W/m2 Á K This result is much less than the limiting value of h indicating that a lumped-parameter solution is probably very accurate. Heating a Body Under Conditions of Negligible Surface Resistance. A second class of time-dependent energy-transfer processes is encountered when the surface resistance is small relative to the overall resistance, that is, Bi is ) 0:1. For this limiting case, the temperature of the surface, Ts is constant for all time, t > 0, and its value is essentially equal to the ambient temperature, T1. To illustrate the analytical method of solving this class of transient heat-conduction problems, consider a large ﬂat plate of uniform thickness L. The initial temperature distribution through the plate will be assumed to be an arbitrary function of x. The solution for the temperature history must satisfy the Fourier ﬁeld equation @T @t ¼ a=2T (16-18) For one-directional energy flow @T @t ¼ a @2T @x2 (18-10) with initial and boundary conditions T ¼ T0(x) at t ¼ 0 for 0 x L T ¼ Ts at x ¼ 0 for t > 0 18.1 Analytical Solutions 255

and T ¼ Ts at x ¼ L for t
> 0 For convenience, let Y ¼ (T À Ts)/(T0 À Ts), where T0 is an arbitrarily chosen reference temperature; the partial differential equation may be rewritten in terms of the new temperature variable as @Y @t ¼ a @2Y @x2 (18-11) and the initial and boundary conditions become Y ¼ Y0(x) at t ¼ 0 for 0 x L Y ¼ 0 at x ¼ 0 for t > 0 and Y ¼ 0 at x ¼ L for t > 0 Solving equation (18-11) by the method of separation of variables leads to product solutions of the form Y ¼ (C1 cos lx þ C2 sin lx)eÀal2t The constants C1 and C2 and the parameter l are obtained by applying the initial and boundary conditions. The complete solution is Y ¼ 2 L å 1 n¼1 sin np L eÀ(np=2)2 Fo Z L 0 Y0(x) sin np L x dx (18-12) where Fo ¼ at/(L/2)2. Equation (18-12) points out the necessity for knowing the initial temperature distribution in the conducting medium, Y0(x), before the complete temperature history may be evaluated. Consider the special case in which the conducting body has a uniform initial temperature, Y0(x) ¼ Y0. With this temperature distribution, equation (18-12) reduces to T À Ts T0 À Ts ¼ 4 p å 1 n¼1 1 n sin np L x eÀðnp/2Þ2Fo n ¼ 1, 3, 5, . . . (18-13) The temperature history at the center of the infinite plane, as well as the central temperature history in other solids, is illustrated in Figure 18.3. The central temperature history for the plane wall, infinite cylinder, and sphere is presented in Appendix F, in ‘‘Heissler charts.’’ These charts cover a much greater range in the Fourier modulus than Figure 18.3. The heat rate, q, at any plane in the conducting medium may be evaluated by qx ¼ ÀkA @T @x (18-14) In the case of the infinite flat plate with an initial uniform temperature distribution of T0 , the heat rate at any time t is qx ¼ 4 kA L (Ts À T0) å 1 n¼1 cos np L x eÀ(np/2)2 Fo n ¼ 1, 3, 5, . . . (18-15) 256 Chapter 18 Unsteady-State Conduction

In the following example, the use of the central temperature-history
figure will be illustrated. EXAMPLE 2 A concrete cylinder, 0.1 m in length and 0.1 m in diameter, is initially at room temperature, 292 K. It is suspended in a steam environment where water vapor at 373 K condenses on all surfaces with an effective film coefficient, h, of 8500 W/m2 Á K. Determine the time required for the center of this stubby cylinder to reach 310 K. If the cylinder were sufficiently long so that it could be considered infinite, how long would it take? For the first case, the finite cylinder, the Biot number is evaluated as Bi ¼ h(V/A) k ¼ h pD2L 4 k pDL þ pD2 2 ¼ h(DL/4) k(L þ D/2) ¼ (8500 W/m2 Á K)(0:1 m)(0:1 m)/4 1:21 W/m Á K(0:1 þ 0:1/2) m ¼ 117 at/x2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0.001 0.01 0.1 1.0 (T – Ts )/(T0 – Ts ) 1 x1 x1 x1 x1 2x1 x1 x1 Figure 18.3 Central temperature history of various solids with initial uniform temperature, T0 , and constant surface temperature, Ts (From P. J. Schneider, Conduction Heat Transfer, Addison- Wesley Publishing Co., Inc., Reading Mass., 1955, p. 249. By permission of the publishers.) 18.1 Analytical Solutions 257

For this largevalue, Figure 18.3 may be used. The second
line from the bottom in this figure applies to a cylinder with height equal to diameter, as in this case. The ordinate is T À Ts T0 À Ts ¼ 310 À 373 292 À 373 ¼ 0:778 and the corresponding abscissa value is approximately 0.11. The time required may now be determined as at x2 1 ¼ 0:11 Thus, t ¼ 0:11 (0:05 m)2 5:95 Â 10À7 m2/s ¼ 462 s ¼ 7:7 min In the case of an infinitely long cylinder, the fourth line from the bottom applies. The Biot number in this case is Bi ¼ h(V/A) k ¼ h pD2L 4 k(pDL) ¼ h D 4 k ¼ (8500 W/m2 Á K)(0:1 m)/4 1:21 W/m Á K ¼ 176 which is evenlarger than the finite cylinder case. Figure 18.3 will again be used. The ordinatevalue of 0.778 yields, for the abscissa, a value of approximately 0.13. The required time, in this case, is t ¼ 0:13(0:05 m)2 5:95 Â 10À7 m2/s ¼ 546 s ¼ 9:1 min Heating a Body with Finite Surface and Internal Resistances. The most general cases of transient heat-conduction processes involve significant values of internal and surface resistances. The solution for the temperature history without internal generation must satisfy the Fourier field equation, which may be expressed for one-dimensional heat flow by @T @t ¼ a @2T @x2 (18-7) A case of considerable practical interest is one in which a body having a uniform temperature is placed in a new fluid environment with its surfaces suddenly and simultaneously exposed to the fluid at temperature T1. In this case, the temperature history must satisfy the initial, symmetry, and convective boundary conditions T ¼ T0 at t ¼ 0 @T @x ¼ 0 at the centerline of the body and À @T @x ¼ h k (T À T1) at the surface 258 Chapter 18 Unsteady-State Conduction

One method of solution for this class of problems involves
separation of variables, which results in product solutions as previously encountered when only the internal resistance was involved. Solutions to this case of time-dependent energy-transfer processes have been obtained for many geometries. Excellent treatises discussing these solutions have been written by Carslaw and Jaeger1 and by Ingersoll, Zobel, and Ingersoll2. If we reconsider the infinite flat plate of thickness, 2x1 , when inserted into a medium at constant temperature, T1, but now include a constant surface conductance, h, the following solution is obtained T À T1 T0 À T1 ¼ 2 å 1 n¼1 sin dn cos (dnx/x1) dn þ sin dn cos dn eÀd2 n Fo (18-16) where dn is defined by the relation dn tan dn ¼ hx1 k (18-17) The temperature history for this relatively simple geometrical shape is a function of three dimensionless quantities: at/x2 1 , hx1/k, and the relative distance, x/x1 . The complex nature of equation (18-17) has led to a number of graphical solutions for the case of one-dimensional transient conduction. The resulting plots, with dimensionless temperature as a function of other dimensionless parameters as listed above, are discussed in Section 18.2. Heat Transfer to a Semi-Infinite Wall. An analytical solution to the one-dimensional heat-conduction equation for the case of the semi-infinite wall has some utility as a limiting case in engineering computations. Con- sider the situation illustrated in Figure 18.4. A large plane wall initially at a constant temperature T0 is subjected to a surface temperature Ts , where Ts > T0 . The differential equation to be solved is @T @t ¼ a @2T @x2 (18-10) and the initial and boundary conditions are T ¼ T0 at t ¼ 0 for all x T ¼ Ts at x ¼ 0 for all t and T ! T0 as x ! 1 for all t The solution to this problem may be accomplished in a variety of ways, among which are the Laplace transformation and the Fourier transformation. We shall use an alternative procedure, which is less involved mathematically. The variables in equation (18-10) Ts T0 x Figure 18.4 Temperature distribution in a semi-infinite wall at time t. 1 H. S. Carslaw and J. C. Jaeger, Conduction of Heat in Solids, Oxford University Press, 1947. 2 L. R. Ingersoll, O. J. Zobel, and A. C. Ingersoll, Heat Conduction (With Engineering and Geological Applications), McGraw-Hill Book Company, New York, 1948. 18.1 Analytical Solutions 259

may be expressed in dimensionless form by analogy with the
previous case. Thus, we may write T À T0 Ts À T0 ¼ f x x1 , at x2 1 However, in this problem there is no finite characteristic dimension, x1 , and thus (T À T0)/ (Ts À T0) ¼ f(at/x2), or with equal validity, (T À T0)/(Ts À T0) ¼ f(x/ ffiffiffiffiffi at p ). If h ¼ x/2 ffiffiffiffiffi at p is selected as the independent variable and the dependent variable Y ¼ (T À T0)/ (Ts À T0) is used, substitution into equation (18-10) yields the ordinary differential equation d2Y/dh2 þ 2h dY/dh ¼ 0 (18-18) with the transformed boundary and initial conditions Y ! 0 as h ! 1 and Y ¼ 1 at h ¼ 0 The first condition above is the same as the initial condition T ¼ T0 at t ¼ 0, and the boundary condition T ! T0 as x ! 1. Equation (18-18) may be integrated once to yield ln dY dh ¼ c1 À h2 or dY dh ¼ c2eÀh2 and integrated once more to yield Y ¼ c3 þ c2 Z eÀh2 dh (18-19) The integral is related to a frequently encountered form, the error function, designated ‘‘erf,’’ where erf f 2 ffiffiffi p p Z f 0 eÀh2 dh and erf (0) ¼ 0, erf (1) ¼ 1, A short table of erf f is given in Appendix L. Applying the boundary conditions to equation (18-19), we obtain Y ¼ 1 À erf x 2 ffiffiffiffiffi at p or T À T0 Ts À T0 ¼ 1 À erf x 2 ffiffiffiffiffi at p or Ts À T Ts À T0 ¼ erf x 2 ffiffiffiffiffi at p (18-20) 260 Chapter 18 Unsteady-State Conduction

This equation is extremely simple to use and quite valuable.
Consider a finite wall of thickness L subject to the surface temperature Ts . Until the temperature change at x ¼ L exceeds some nominal amount, say (T À T0)/(Ts À T0) equal to 0.5%, the solution for the finite and infinite walls will be the same. The value of L/(2 ffiffiffiffiffi at p ) corresponding to a 0.5% change in (T À T0)/(Ts À T0) is L/(2 ffiffiffiffiffi at p ) ’ 2, so for L/(2 ffiffiffiffiffi at p ) > 2, equation (18-20) may be used for finite geometry with little or no error. For the case of finite surface resistance, the solution to equation (18-10) for a semi- infinite wall is T1 À T T1 À T0 ¼ erf x 2 ffiffiffiffiffi at p þ exp hx k þ h2at k2 1 À erf h ffiffiffiffiffi at p k þ x 2 ffiffiffiffiffi at p ! (18-21) This equation may be used to determine the temperature distribution in finite bodies for small times in the same manner as equation (18-20). The surface temperature is particularly easy to obtain from the above equation, if we let x ¼ 0, and the heat transfer rate may be determined from q A ¼ h(Ts À T1) 18.2 TEMPERATURE–TIME CHARTS FOR SIMPLE GEOMETRIC SHAPES For unsteady-state energy transfer in several simple shapes with certain restrictive boundary conditions, the equations describing temperature profiles have been solved3 and the results have been presented in a wide variety of charts to facilitate their use. Two forms of these charts are available in Appendix F. Solutions are presented in Appendix F for the flat plate, sphere, and long cylinder in terms of four dimensionless ratios: Y, unaccomplished temperature change ¼ T1 À T T1 À T0 X, relative time ¼ at x2 1 n, relative position ¼ x x1 and m, relative resistance ¼ k hx1 where x1 is the radius or semithickness of the conducting medium. These charts may be used to evaluate temperature profiles for cases involving transport of energy into or out of the conducting medium if the following conditions are met: (a) Fourier’s field equation describes the process; i.e., constant thermal diffusivity and no internal heat source. (b) The conducting medium has a uniform initial temperature, T0 . (c) The temperature of the boundary or the adjacent fluid is changed to a new value, T1, for t ! 0. 3 Equation (18-16) pertains to a plane wall of thickness, L, and boundary conditions T(x, 0) ¼ T0 and dT/dx(0, t) ¼ 0. 18.2 Temperature–Time Charts for Simple Geometric Shapes 261

For flat plates where the transport takes place from only
one of the faces, the relative time, position, and resistance are evaluated as if the thickness were twice the true value. Although the charts were drawn for one-dimensional transport, they may be combined to yield solutions for two- and three-dimensional problems. The following is a summary of these combined solutions: 1. For transport in a rectangular bar with insulated ends Ybar ¼ YaYb (18-22) where Ya is evaluated with width x1 ¼ a, and Yb is evaluated with thickness x1 ¼ b. 2. For transport in a rectangular parallelepiped Yparallelepiped ¼ YaYbYc (18-23) where Ya is evaluated with width x1 ¼ a, Yb is evaluated with thickness x1 ¼ b, and Yc is evaluated with depth x1 ¼ c. 3. For transport in a cylinder, including both ends Ycylinder plus ends ¼ YcylinderYa (18-24) where Ya is evaluated by using the flat-plate chart, and thickness x1 ¼ a. The use of temperature–time charts is demonstrated in the following examples. EXAMPLE 3 A flat wall of fire-clay brick, 0.5 m thick and originally at 200 K, has one of its faces suddenly exposed toa hot gas at 1200K.Ifthe heat-transfer coefficientonthehotside is7:38 W/m2 Á K and the otherface of the wall is insulated so that no heat passes out of that face, determine (a) the time necessary to raise thecenterofthewallto600K;(b)thetemperatureoftheinsulatedwallfaceatthetimeevaluatedin(a). From the table of physical properties given in Appendix H, the following values are listed: k ¼ 1:125 W/m Á K cp ¼ 919 J/kg Á K r ¼ 2310 kg/m3 and a ¼ 5:30 Â 10À7 m2/s The insulated face limits the energy transfer into the conducting medium to only one direction. This is equivalent to heat transfer from a 1-m-thick wall, where x is then measured from the line of symmetry, the insulated face. The relative position, x/x1 , is 1/2. The relative resistance, k/hx1 , is 1.125/[(7.38)(0.5)] or 0.305. The dimensionless temperature, Y ¼ (T1 À T)/(T1 À T0), is equal to (1200 À 600)/(1200 À 200), or 0.6. From Figure F.7, in Appendix F, the abscissa, at/x2 1 , is 0.35 under these conditions. The time required to raise the centerline to 6008F is t ¼ 0:35x2 1 a ¼ 0:35(0:5)2 5:30 Â 10À7 ¼ 1:651 Â 105 s or 45:9 h The relative resistance and the relative time for (b) will be the same as in part (a). The relative position, x/x1 , will be 0. Using these values and Figure F.1 in Appendix F, we find the dimensionless temperature, Y, to be 0.74. Using this value, the desired temperature can be evaluated by Ts À T Ts À T0 ¼ 1200 À T 1200 À 200 ¼ 0:74 or T ¼ 460 K (368F) 262 Chapter 18 Unsteady-State Conduction

EXAMPLE 4 A billet of steel 30.5 cm in diameter,
61 cm long, initially at 645 K, is immersed in an oil bath that is maintained at 310 K. if the surface conductance is 34 W/m2 Á K determine the center temperature of the billet after 1 h. From Appendix H, the following average physical properties will be used: k ¼ 49:2 W/m Á K cp ¼ 473 J/kg Á K r ¼ 7820 kg/m3 a ¼ 1:16 Â 10À5 m2/s Equation (18-22) applies. To evaluate Ya the following dimensionless parameters apply X ¼ at x2 1 ¼ (1:16 Â 10À5 m2/s)(3600 s) (0:305 m)2 ¼ 0:449 n ¼ x/x1 ¼ 0 m ¼ k/hx1 ¼ 42:9 W/m Á K (34 W/m2 Á K)(0:305 m) ¼ 4:14 Using these values with Figure F.7 in Appendix F, the corresponding values of dimensionless temperature, Ya , is approximately 0.95. For the cylindrical surface the appropriate values are X ¼ at x2 1 ¼ (1:16 Â 10À5 m2/s)(3600 s) (0:1525 m)2 ¼ 1:80 n ¼ x x1 ¼ 0 m ¼ k/hx1 ¼ 42:9/(34)(0:1525) ¼ 8:27 and, from Figure F.8 in Appendix F, we obtain YCL ¼ T À T1 T0 À T1 cyl ﬃ 0:7 Now, for heat transfer across the cylindrical surface and both ends Yj total ¼ TCL À T1 T0 À T1 ¼ YaYCL ¼ (0:95)(0:7) ¼ 0:665 The desired center temperature is now calculated as TCL ¼ T1 þ 0:665(T0 À T1) ¼ 310 K þ 0:665(645 À 310) K ¼ 533 K (499 F) 18.3 NUMERICAL METHODS FOR TRANSIENT CONDUCTION ANALYSIS In many time-dependent or unsteady-state conduction processes, actual initial and/or boundary conditions do not correspond to those mentioned earlier with regard to analytical solutions. An initial temperature distribution may be nonuniform in nature; ambient temperature, surface conductance, or system geometry may be variable or quite irregular. For such complex cases, numerical techniques offer the best means to achieve solutions. 18.3 Numerical Methods for Transient Conduction Analysis 263

More recently, with sophisticated computing codes available, numerical solutions are
being obtained for heat transfer problems of all types, and this trend will doubtlessly continue. It is likely that many users of this book will be involved in code development for such analysis. Some numerical work is introduced in Chapter 17, dealing with two-dimensional, steady-state conduction. In this section, we will consider variation in time as well as position. To begin our discussion, the reader is referred to equation (17-61) and the development leading up to it. For the case of no internal generation of energy, equation (17-61) reduces to k Dy Dx (TiÀ1, j þ Tiþ1, j À 2Ti, j) þ k Dx Dy (Ti, jÀ1 þ Ti, jþ1 À 2Ti, j) ¼ rcpTi, j j tþDt À rcpTi, j j t Dt Dx Dy (18-25) This expression applies to two dimensions; however, it can be extended easily to three dimensions. The time-dependent term on the right of equation (18-25) is written such that the temperature at node i, j is presumed known at time t; this equation can then be solved to ﬁnd Tij at the end of time interval Dt. As Ti jjtþDt appears only once in this equation, it can be evaluated quite easily. This means of evaluating Tij at the end of a time increment is designated an ‘‘explicit’’ technique. A more thorough discussion of explicit solutions is given by Carnahan.4 Equation (18-25) may be solved to evaluate the temperature at node i, j for all values of i, j that comprise the region of interest. For large numbers of nodes, it is clear that a great number of calculations are needed and that much information must be stored for use in subsequent computation. Digital computers obviously provide the only feasible way to accomplish solutions. We will next consider the one-dimensional form of equation (18-25). For a space increment Dx, the simpliﬁed expression becomes k Dx (TiÀ1 j t þ Tiþ1 j t À 2Ti j t ) ¼ rcpTi j tþDt À rcpTi j t Dt Dx (18-26) where the j notation has been dropped. The absence of variation in the y direction allows several terms to be deleted. We next consider properties to be constant and represent the ratio k=rcp as a. Solving for TijtþDt, we obtain Ti j tþDt ¼ a Dt (Dx)2 (Tiþ1 j t þ TiÀ1 j t ) þ 1 À 2a Dt ðDxÞ2 ! Ti jt (18-27) The ratio, a Dt/(Dx)2, a form resembling the Fourier modulus, is seen to arise naturally in this development. This grouping relates the time step, Dt, to the space increment, Dx. The magnitude of this grouping will, quite obviously, have an effect on the solution. It has been determined that equation (18-27) is numerically ‘‘stable’’ when a Dt (Dx)2 1 2 (18-28) For a discussion of numerical stability the reader is referred to Carnahan et al.4 4 B. Carnahan, H. A. Luther, and J. O. Wilkes, Applied Numerical Methods, Wiley, New York, 1969. 264 Chapter 18 Unsteady-State Conduction

The choice of a time step involves a trade-off between
solution accuracy-a smaller time step will produce greater accuracy, and computation time-a solution will be achieved more rapidly for larger values of Dt. When computing is done by machine, a small time step will likely be used without major difficulty. An examination of equation (18-27) indicates considerable simplification to be achieved if the equality in equation (18-28) is used. For the case with a Dt/(Dx2) ¼ 1/2, equation (18-27) becomes TijtþDt ¼ Tiþ1jt þ TiÀ1jt 2 (18-29) EXAMPLE 5 A brick wall (a ¼ 4:72 Â 10À7 m2/s) with a thickness of 0.5 m is initially at a uniform temperature of 300 K. Determine the length of time required for its center temperature to reach 425 K if its surfaces are raised to, and maintained at 425 and 600 K, respectively. Although relatively simple, this one-dimensional problem is not amenable to a solution using the charts because there is no axis of symmetry. Either analytical or numerical methods must therefore be employed. An analytical solution using Laplace transform or separation-of-variables methodology is relatively straightforward. However, the solution is in terms of infinite series involving eigenvalues and the determination of a final answer is cumbersome. The simplest approach is thus numerical and we will proceed with the ideas introduced in this section. The illustration below depicts the wall divided into 10 increments. Each of the nodes within the wall is at the center of a subvolume, having a width, Dx. The shaded subvolume at node 4 is considered to have uniform properties averaged at its center, i.e., the location of node 4. This same idea prevails for all 11 nodes; this includes the surface nodes, 0 and 10. 0 1 2 3 4 5 6 7 8 9 10 ∆x An energy balance for any internal node, having width Dx, will yield equation (18-27) as a result. This relationship includes the dimensionless ratio, aDt/Dx2, which relates the time increment, Dt, to the space increment Dx. In this example we have specified Dx ¼ 0.05 m. The quantity, aDt/Dx2, can have any value equal to or less than 0.5, which is the limit for a stable solution. If the limiting value is chosen, equation (18-27) reduces to a simple algorithmic form Ti,tþ1 ¼ TiÀ1,t þ Tiþ1,t 2 (18-29) This expression is valid for i ¼ 1 to 9; however, as nodes 0 and 10 are at constant temperature for all time, the algorithms for nodes 1 and 9 can be written as T1,tþ1 ¼ T0,t þ T2,t 2 ¼ 425 þ T2,t 2 T9,tþ1 ¼ T8,t þ T10,t 2 ¼ T8,t þ 600 2 (18-30) The problem solution now proceeds as equations (18-29) and (18-30) and are solved at succeeding times to update nodal temperatures until the desired result, Ts ¼ 425 K, is achieved. 18.3 Numerical Methods for Transient Conduction Analysis 265

Equations (18-29) and (18-30) are quite simple and easily programed
to achieve a solution. In this case, a spreadsheet approach could also be used. The table below summarizes the form of the results for Ti;t. The desired center temperature is reached between time increments 22 and 23; an interpolated value is n ¼ 22.6 time increments. As discussed earlier, the increment Dt is related to a and Dx according to the ratio a Dt Dx2 ¼ 1/2 or Dt ¼ 1 2 Dx2 a ¼ 1 2 (0:05 m)2 4:72 Â 10À7 m2/s ¼ 2648 s ¼ 0:736 h The answer for total time elapsed is thus t ¼ 22:6(0:736) ¼ 16:6 h 18.4 AN INTEGRAL METHOD FOR ONE-DIMENSIONAL UNSTEADY CONDUCTION The von Ka ´rma ń momentum integral approach to the hydrodynamic boundary layer has a counterpart in conduction. Figure 18.5 shows a portion of a semi-infinite wall, originally at uniform temperature T0 , exposed to a fluid at temperature T1, with the surface of the wall at any time at temperature Ts . T0 T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 t ¼ 0 425 300 300 300 300 300 300 300 300 300 600 . . . t ¼ 10 425 394.8 372.1 349.4 347.9 346.4 367.1 405.7 466.1 526.4 600 . . . t ¼ 20 425 411.4 403.3 395.3 402.3 409.4 436.2 463.1 506.5 550.0 600 . . . t ¼ 22 425 414.2 408.5 402.8 411.0 419.3 445.3 471.4 512.3 553.3 600 t ¼ 23 425 416.8 408.5 409.8 411.0 428.2 445.3 478.8 512.3 556.2 600 Ts L d T∞ T0 x Figure 18.5 A portion of a semi-infinite wall used in integral analysis. 266 Chapter 18 Unsteady-State Conduction

Atanytimet,heattransferfromthefluidtothewallaffectsthetemperatureprofilewithin thewall.The‘‘penetrationdistance,’’designatedasd,isthedistancefrom thesurfacewherein this effect is manifested. At distance d,
the temperature gradient, @T/@x, is taken as zero. Applyingthe firstlaw ofthermodynamics,equation(6-10),toa controlvolume extending from x ¼ 0 to x ¼ L, where L > d, we have dQ dt À dWs dt À dWm dt ¼ ZZ c:s: e þ P r r(v Á n) dA þ @ @t ZZZ c:v: er dV (6-10) with dWs dt ¼ dWm dt ¼ ZZ c:s: e þ P r r(v n) dA ¼ 0 The applicable form of the first law is now dQ dt ¼ @ @t ZZZ c:v: er dV Considering all variables to be functions of x alone, we may express the heat flux as qx A ¼ d dt Z L 0 ru dx ¼ d dt Z L 0 rcpT dx (18-32) The interval from 0 to L will now be divided into two increments, giving qx A ¼ d dt Z d 0 rcpT dx þ Z L d rcpT0 dx ! and, since T0 is constant, this becomes qx A ¼ d dt Z d 0 rcpT dx þ rcpT0(L À d) ! The integral equation to be solved is now qx A ¼ d dt Z d 0 rcpT dx À rcpT0 dd dt (18-33) If a temperature profile of the form T ¼ T(x; d) is assumed, equation (18-33) will produce a differential equation in d(t), which may be solved, and one may use this result to express the temperature profile as T(x, t). The solution of equation (18-33) is subject to three different boundary conditions at the wall, x ¼ 0, in the sections to follow. Case 1. Constant wall temperature Thewall, initially at uniform temperature T0 , has its surface maintained at temperature Ts for t > 0. The temperature profile at two different times is illustrated in Figure 18.6. Assuming the temperature profile to be parabolic of the form T ¼ A þ Bx þ Cx2 and requiring that the following boundary conditions: T ¼ Ts at x ¼ 0 T ¼ T0 at x ¼ d 18.4 An Integral Method for One-Dimensional Unsteady Conduction 267

and @T @x ¼ 0 at x ¼ d be
satisfied, we see that the expression for T(x) becomes T À T0 Ts À T0 ¼ 1 À x d 2 (18-34) The heat flux at the wall may now be evaluated as qx A ¼ Àk @T @x x¼0 ¼ 2 k d (Ts À T0) (18-35) which may be substituted into the integral expression along with equation (18-33), yielding 2 k d (Ts À T0) ¼ d dt Z d 0 rcp T0 þ (Ts À T0) 1 À x d 2 ! dx À rcp T0 dd dt and, after dividing through by rcp , both quantities being considered constant, we have 2 a d (Ts À T0) ¼ d dt Z d 0 T0 þ (Ts À T0) 1 À x d 2 ! dx À T0 dd dt (18-36) After integration, equation (18-36) becomes 2a d (Ts À T0) ¼ d dt (Ts À T0) d 3 ! and cancelling (Ts À T0), we obtain 6a ¼ d dd dt (18-37) and thus the penetration depth becomes d ¼ ffiffiffiffiffiffiffiffiffi 12at p (18-38) The corresponding temperature profile may be obtained from equation (18-34) as T À T0 Ts À T0 ¼ 1 À x ffiffiffi 3 p (2 ffiffiffiffiffi at p ) ! 2 (18-39) x Ts T t2 t1 d (t1 ) d (t2 ) Figure 18.6 Temperature profiles at two times after the surface temperature is raised to Ts . 268 Chapter 18 Unsteady-State Conduction

which compares reasonably well with the exact result T À
T0 Ts À T0 ¼ 1 À erf x 2 ffiffiffiffiffi at p (18-40) Figure 18.7 shows a comparison of these two results. Case 2. A specified heat flux at the wall In this case the appropriate boundary conditions are T ¼ T0 at x ¼ d @T @x ¼ 0 at x ¼ d and @T @x ¼ À F(t) k at x ¼ 0 where the heat flux at the wall is expressed as the general function F(t). If the parabolic temperature profile is used, the above boundary conditions yield T À T0 ¼ ½F(t)(d À x)2 2kd (18-41) which, when substituted into equation (18-38), yields d dt F(t)d2 6k ¼ aF(t) k (18-42) and d(t) ¼ ffiffiffiffiffiffi 6a p 1 F(t) Z t 0 F(t) dt !1/2 (18-43) For a constant heat flux of magnitude q0 /A the resulting expression for Ts is Ts À T0 ¼ q0 Ak ffiffiffiffiffiffiffiffi 3 2 at r (18-44) which differs by approximately 8% from the exact expression Ts À T0 ¼ 1:13q0 Ak ffiffiffiffiffi at p (18-45) (T – T0 )/(Ts – T0 ) 0 0.5 1.0 1.5 x/2 at 2.0 2.5 0 0.2 Exact 0.4 0.6 0.8 Approximate Figure 18.7 A comparison of exact and approximate results for one-dimensional conduction with a constant wall temperature. 18.4 An Integral Method for One-Dimensional Unsteady Conduction 269

Case 3. Convection at the surface The wall temperature is
a variable in this case; however, it may be easily determined. If the temperature variation within the medium is expressed generally as T À T0 Ts À T0 ¼ f x d (18-46) we note that the temperature gradient at the surface becomes @T @x x¼0 ¼ À Ts À T0 d N (18-47) where N is a constant depending upon the form of f(x/d). At the surface we may write q A x¼0 ¼ Àk @T @x x¼0 ¼ h(T1 À Ts) which becomes, upon substituting equation (18-47) Ts À T0 ¼ hd Nk (T1 À T0) (18-48) or Ts ¼ T0 þ (hd/Nk)T1 1 þ hd/Nk (18-49) We may now write Ts À T0 T1 À T0 ¼ hd/Nk 1 þ hd/Nk (18-50) and T1 À Ts T1 À T0 ¼ 1 1 þ hd/Nk (18-51) The appropriate substitutions into the integral equation and subsequent solution follow the same procedures as in cases (a) and (b); the details of this solution are left as a student exercise. The student should recognize the marked utility of the integral solution for solving one- dimensional unsteady-state conduction problems. Temperature profile expressions more complex than a parabolic form may be assumed. However, additional boundary conditions areneededin suchcasestoevaluate the constants. Thesimilaritybetweenthepenetration depth andtheboundary-layer thickness from the integral analysis ofChapter 12should alsobenoted. 18.5 CLOSURE In this chapter, some of the techniques for solving transient or unsteady-state heat- conduction problems have been presented and discussed. Situations considered included cases of negligible internal resistance, negligible surface resistance, and those for which both resistances were significant. For flat slabs, cylinders, and spheres, with a uniform initial temperature, whose surfaces are suddenly exposed to surroundings at a different temperature, charts are available for evaluating the temperature at any position and time. Numerical and integral methods were also introduced. 270 Chapter 18 Unsteady-State Conduction

PROBLEMS 18.1 A household iron has a stainless-steel sole plate
that weights 3 lb and has a surface area of 0.5 ft2. The iron is rated at 500 W. If the surroundings are at a temperature of 808F, and the convective heat-transfer coefficient between the sole plate and surroundings is 3 Btu/h ft2 8F, how long will it take for the iron to reach 2408F after it is plugged in? 18.2 An electrical system employs fuses that are cylindrical in shape and have lengths of 0.5 cm and diameters of 0.1 mm. Air, at 308C, surrounds a fuse with a surface coefficient of 10 W/m2 Á K. The fuse material melts at 9008C. Assuming all heat transfer to be from the fuse surface, estimate the time it will take for the fuse to blow after a current of 3 A flows through it. Pertinent properties of the fuse material are Resistance ¼ 0:2 V k ¼ 20 W/m Á K a ¼ 5 Â 10À5 m2/s 18.3 Aluminum wire, having a diameter of 0.794 mm, is immersed in an oil bath that is at 258C. Aluminum wire of this size has an electrical resistance of 0.0572 V/m. For conditions where an electric current of 100 A is flowing through the wire and the surface coefficient between the wire and oil bath is 550 W/ m2 Á K, determine the steady state temperature of the wire. How long, after the current is supplied, will it take for the wire to reach a temperature within 58C of its steady-state value? 18.4 If a rectangular block of rubber (see Problem 18.12 for properties) is set out in air at 297 K to cool after being heated to a uniform temperature of 420 K, how long will it take for the rubber surface to reach 320 K? The dimensions of the block are 0.6 m high by 0.3 m long by 0.45 m wide. The block sits on one of the 0.3-m by 0.45-m bases; the adjacent surface may be considered an insulator. The effective heat-transfer coefficient at all exposed surface is 6.0 W/m2 Á K. What will the maximum temperature within the rubber block be at this time? 18.5 Cast-iron cannonballs used in the War of 1812 were occasionally heated for some extended time so that, when fired athousesorships, they would setthemafire.Ifoneofthesetheso- called ‘‘hot shot’’ were at a uniform temperature of 20008F, how long after being exposed to air at 08F with an outside convective heat-transfer coefficient of 16 But/h ft2 8F, would be required for the surface temperature to drop to 6008F? What would be the center temperature at this time? The ball diameter is 6 in. The following properties of cast iron may be used: k ¼ 23 Btu/h ft F cp ¼ 0:10 Btu/lbm F r ¼ 460 lbm/ft3: 18.6 It is known that oranges can be exposed to freezing temperatures for short periods of time without sustaining serious damage. As a representative case, consider a 0.10-m-diameter orange, originally at a uniform temperature of 58C, suddenly exposed to surrounding air at À58C. For a surface coefficient, between the air and orange surface, of 15 W/m2 Á K, how long will it take for the surface of the orange to reach 08C? Properties of the orange are the following: r ¼ 940 kg/m3 k ¼ 0:47 W/m Á K cp ¼ 3:8 kJ/kg Á K: 18.7 A copper cylinder with a diameter of 3 in. is initially at a uniform temperature of 708F. How long after being placed in a medium at 10008F with an associated convective heat-transfer coefficient of 4 Btu/h ft2 8F will the temperature at the center of the cylinder reach 5008F, if the height of the cylinder is (a) 3 in.? (b) 6 in.? (c) 12 in.? (d) 24 in.? (e) 5 ft? 18.8 A cylinder 2 ft high with a diameter of 3 in. is initially at the uniform temperature of 708F. How long after the cylinder is placed in a medium at 10008F, with associated convective heat- transfer coefficient of 4 Btu/h ft2 8F, will the center temperature reach 5008F if the cylinder is made from a. copper, k ¼ 212 Btu/h ft F? b. aluminum, k ¼ 130 Btu/h ft F? c. zinc, k ¼ 60 Btu/h ft F? d. mild steel, k ¼ 25 Btu/h ft F? e. stainless steel, k ¼ 10:5 Btu/h ft F? f. asbestos, k ¼ 0:087 Btu/h ft F? 18.9 Water, initially at 408F, is contained within a thin- walled cylindrical vessel having a diameter of 18 in. Plot the temperature of the water vs. time up to 1 h if the water and container are immersed in an oil bath at a constant temperature of 3008F. Assume that the water is well stirred and that the convective heat-transfer coefficient between the oil and cylindrical surface is 40 Btu/h ft2 8F. The cylinder is immersed to a depth of 2 ft. 18.10 A short aluminum cylinder 0.6 m in diameter and 0.6 m long is initially at 475 K. It is suddenly exposed to a convective environment at 345 K with h ¼ 85 W/m2 Á K. Determine the temperature in the cylinder at a radial position of 10 cm and a distance of 10 cm from one end of the cylinder after being exposed to this environment for 1 h. 18.11 A type-304 stainless-steel billet, 6 in. in diameter, is passing through a 20-ft-long heat-treating furnace. The initial billet temperature is 2008F, and it must be raised to a minimum temperature of 15008F before working. The heat-transfer coefficient between the furnace gases and the billet surface is 15 Btu/h ft2 8F, and the furnace gases are at 23008F. At what minimum velocity must the billet travel through the furnace to satisfy these conditions? 18.12 In the curing of rubber tires, the ‘‘vulcanization’’ process requires that a tire carcass, originally at 295 K, be Problems 271

heated so that its central layer reaches a minimum tempera-
ture of 410 K. This heatingisaccomplishedby introducingsteam at 435 K to both sides. Determine the time required, after introducing steam, for a 3-cm-thick tire carcass to reach the specified central temperature condition. Properties of rubber that may be used are the following: k ¼ 0:151 W/m Á K, cp ¼ 200 J/kg Á K, r ¼ 1201 kg/m3, a ¼ 6:19 Â 10À8 m2/s. 18.13 Buckshot, 0.2 in. in diameter, is quenched in 908F oil from an initial temperature of 4008F. The buckshot is made of lead and takes 15 s to fall from the oil surface to the bottom of the quenching bath. If the convective heat-transfer coefficient between the lead and oil is 40 Btu/h ft2 8F, what will be the temperature of the shot as it reaches the bottom of the bath? 18.14 It is common practice to treat wooden telephone poles with tar-like materials to prevent damage by water and insects. These tars are cured into the wood at elevated temperatures and pressures. Consider the case of a 0.3-m-diameter pole, originally at 258C,placedinapressurizedoven.Itwillberemovedwhenthetar has penetrated to a depth of 10 cm. It is known that a 10-cm depth ofpenetrationwilloccurwhenatemperatureof1008Cisachieved. For an oven temperature of 3808C and h ¼ 140 W/m2 Á K, determine the time required for the pole to remain in the oven. Properties of the wooden pole are k ¼ 0:20 W/m Á K a ¼ 1:1 Â 10À7 m2/s 18.15 For an asbestos cylinder with both height and diameter of 13 cm initially at a uniform temperature of 295 K placed in a medium at 810 K with an associated convective heat-transfer coefficient of 22.8 W/m2 Á K, determine the time required for the center of the cylinder to reach 530 K if end effects are neglected. 18.16 A copper bus bar is initially at 4008F. The bar measures 0.2 ft by 0.5 ft and is 10 ft long. If the edges are suddenly all reduced to 1008F, how long will it take for the center to reach a temperature of 2508F? 18.17 Rework Problem 18.4 for the case when air is blown by the surfaces of the rubber block with an effective surface coefficient of 230 W/m2 Á K. 18.18 Consider a hot dog to have the following dimensions and properties: diameter ¼ 20 mm, cp ¼ 3:35 kJ/kg Á K, r ¼ 880 kg/m3, and k ¼ 0:5 W/m Á K. For the hot dog initially at 58C, exposedto boiling water at 1008C, with a surface coefficient of 90 W/m2 Á K, what will be the cooking time if the required condition is for the center temperature to reach 808C? 18.19 This problem involves using heat transfer principles as a guide for cooking a pork roast. The roast is to be modeled as a cylinder, having its length and diameter equal to each other, with properties being those of water. The roast weights 2.25 kg. Properly cooked, every portion of the meat should attain a minimum temperature of 958C. If the meat is initially at 58C and the oven temperature is 1908C, with a surface coefficient of 15 W/m2 Á K, what is the minimum cooking time required? 18.20 Given the cylinder in Problem 18.15, construct a plot of the time for the midpoint temperature to reach 530 K as a function of H/D, where H and D are the height and diameter of the cylinder, respectively. 18.21 A rocket-engine nozzle is coated with a ceramic material having the following properties: k ¼ 1:73 Btu/h ft F, a ¼ 0:35 ft2/h. The convective heat-transfer coefficient between the nozzle and the gases, which are at 30008F, is 200 Btu/h ft2 8F. Howlongafterstartupwillittake for thetemperatureatthe ceramic surfacetoreach27008F?Whatwillbethetemperatureatapoint1/2 in. from the surface at this time? The nozzle is initially at 08F. 18.22 One estimate of the original temperature of Earth is 70008F. Using this value and the following properties for Earth’s crust, Lord Kelvin obtained an estimate of 9.8Â107 years for the Earth’s age: a ¼ 0:0456 ft2/h T2 ¼ 0F @T @y y¼0 ¼ 0:02F/ft, (measured) Comment on Lord Kelvin’s result by considering the exact expression for unsteady-state conduction in one dimension T À Ts T0 À Ts ¼ erf x 2 ffiffiffiffiffi at p 18.23 After a fire starts in a room the walls are exposed to combustion products at 9508C. If the interior wall surface is made of oak, how long after exposure to the fire will the wood surface reach its combustion temperature of 4008C? Pertinent data are the following: h ¼ 30 W/m2 Á K Ti(initial) ¼ 21C For oak: r ¼ 545 kg/m3 k ¼ 0:17 W/m Á K cp ¼ 2:385 kJ/kg Á K 18.24 Determine an expression for the depth below the surface of a semi-infinite solid at which the rate of cooling is maximum. Substitute the information given in Problem 18.22 to estimate how far below Earth’s surface this maximum cooling rate is achieved. 18.25 Soil, having a thermal diffusivity of 5:16 Â 10À7 m2/s, has its surface temperature suddenly raised and maintained at 1100 K from its initial uniform value of 280 K. Determine the temperature at a depth of 0.25 m after a period of 5 h has elapsed at this surface condition. 18.26 The convective heat-transfer coefficient between a large brick wall and air at 1008F is expressed as h ¼ 0:44 272 Chapter 18 Unsteady-State Conduction

(T À T1)1/3 Btu/h ft2 F. If the wall is
initially at a uniform temperature of 10008F, estimate the temperature of the surface after 1, 6, and 24 h. 18.27 A thick wall of oak, initially at a uniform temperature of 258C, is suddenly exposed to combustion exhaust at 8008C. Determine the time of exposure required for the surface to reach its ignition temperature of 4008C, when the surface coefficient between the wall and combustion gas is 20 W/m2 Á K. 18.28 Air at 658F is blown against a pane of glass 1/8 in. thick. If the glass is initially at 308F, and has frost on the outside, estimate the length of time required for the frost to begin to melt. 18.29 How long will a 1-ft-thick concrete wall subject to a surface temperature of 15008F on one side maintain the other side below 1308F? The wall is initially at 708F. 18.30 A stainless-steel bar is initially at a temperature of 258C. Its upper surface is suddenly exposed to an air stream at 2008C, with a corresponding convective coefficient of 22 W/m2 Á K. If the bar is considered semi-infinite, how long will it take for the temperature at a distance of 50 mm from the surface to reach 1008C? 18.31 A thick plate made of stainless steel is initially at a uniform temperature of 3008C. The surface is suddenly exposed to a coolant at 208C with a convective surface coefficient of 110 W/m2 Á K.Evaluatethetemperatureafter 3minofelapsedtimeat a. the surface; b. a depth of 50 mm. Work this problem both analytically and numerically. 18.32 If the heat flux into a solid is given as F(t), show that the penetration depth d for a semi-infinite solid is of the form d ¼ (constant) ffiffiffi a p R t 0 F(t)dt F(t) " # 1/2 18.33 If the temperature profile through the ground is linear, increasing from 358F at the surface by 0.58F per foot of depth, how long will it take for a pipe buried 10 ft below the surface to reach 328F if the outside air temperature is suddenly dropped to 08F. The thermal diffusivity of soil may be taken as 0.02 ft2/h, its thermal conductivity is 0.8 Btu/h ft 8F, and the convective heat- transfer coefficient between the soil and the surrounding air is 1.5 Btu/h ft2 8F. 18.34 A brick wall (a ¼ 0.016 ft2/h) with a thickness of 1½ ft is initially at a uniform temperature of 808F. How long, after the wall surfaces are raised to 300 and 6008F, respectively, will it take for the temperature at the center of the wall to reach 3008F? 18.35 A masonry brick wall 0.45 m thick has a temperature distribution at time, t ¼ 0 which may be approximated by the expression T(K) ¼ 520 þ 330 sin p(x/L) where L is the wall width and x is the distance from either surface. How long after bothsurfacesofthiswallareexposedtoairat280Kwillthecenter temperature of the wall be 360 K? The convective coefficient at both surface of the wall may be taken as 14 W/m2 Á K. What will the surface temperature be at this time? Problems 273

Chapter 19 Convective Heat Transfer Heat transfer by convection is
associated with energy exchange between a surface and an adjacent fluid. There are very few energy-transfer situations of practical importance in which fluid motion is not in some way involved. This effect has been eliminated as much as possible in the preceding chapters, but will now be considered in some depth. The rate equation for convection has been expressed previously as q A ¼ h DT (15-11) where the heat flux, q/A, occurs by virtue of a temperature difference. This simple equation is the defining relation for h, the convective heat-transfer coefficient. The determination of the coefficient h is, however, not at all a simple undertaking. It is related to the mechanism of fluid flow, the properties of the fluid, and the geometry of the specific system of interest. In light of the intimate involvement between the convective heat-transfer coefficient and fluid motion, we may expect many of the considerations from the momentum transfer to be of interest. In the analyses to follow, much use will be made of the developments and concepts of Chapters 4 through 14. 19.1 FUNDAMENTAL CONSIDERATIONS IN CONVECTIVE HEAT TRANSFER As mentioned in Chapter 12, the fluid particles immediately adjacent to a solid boundary are stationary, and a thin layer of fluid close to the surface will be in laminar flow regardless of the nature of the free stream. Thus, molecular energy exchange or conduction effects will always be present, and play a major role in any convection process. If fluid flow is laminar, then all energy transfer between a surface and contacting fluid or between adjacent fluid layers is by molecular means. If, on the contrary, flow is turbulent, then there is bulk mixing of fluid particles between regions at different temperatures, and the heat transfer rate is increased. The distinction between laminar and turbulent flow will thus be a major consideration in any convective situation. There are two main classifications of convective heat transfer. These have to do with the driving force causing fluid to flow. Natural or free convection designates the type of process wherein fluid motion results from the heat transfer. When a fluid is heated or cooled, the associated density change and buoyant effect produce a natural circulation in which the affected fluid moves of its own accord past the solid surface, the fluid that replaces it is similarly affected by the energy transfer, and the process is repeated. Forced convection is the classification used to describe those convection situations in which fluid circulation is produced by an external agency such as a fan or a pump. 274

The hydrodynamic boundary layer, analyzed in Chapter 12, plays a
major role in convective heat transfer, as one would expect. Additionally, we shall define and analyze the thermal boundary layer, which will also be vital to the analysis of a convective energy- transfer process. There are four methods of evaluating the convective heat-transfer coefficient that will be discussed in this book. These are as follows: (a) dimensional analysis, which to be useful requires experimental results; (b) exact analysis of the boundary layer; (c) approximate integral analysis of the boundary layer; and (d) analogy between energy and momentum transfer. 19.2 SIGNIFICANT PARAMETERS IN CONVECTIVE HEAT TRANSFER Certain parameters will be found useful in the correlation of convective data and in the functional relations for the convective heat-transfer coefficients. Some parameters of this type have been encountered earlier; these include the Reynolds and the Euler numbers. Several of the new parameters to be encountered in energy transfer will arise in such a manner that their physical meaning is unclear. For thisreason, we shall devotea short section to the physical interpretation of two such terms. The molecular diffusivities of momentum and energy have been defined previously as momentum diffusivity : n m r and thermal diffusivity : a k rcp That these two are designated similarly would indicate that they must also play similar roles in their specific transfer modes. This is indeed the case, as we shall see several times in the developmentsto follow.Forthe moment weshouldnote that both have the same dimensions, those of L2/t; thus their ratio must be dimensionless. This ratio, that of the molecular diffusivity of momentum to the molecular diffusivity of heat, is designated the Prandtl number. Pr n a ¼ mcp k (19-1) The Prandtl number is observed to be a combination of fluid properties; thus Pr itself may be thought of as a property. The Prandtl number is primarily a function of temperature and is tabulated in Appendix I, at various temperatures for each fluid listed. The temperature profile for a fluid flowing past a surface is depicted in Figure 19.1. In the figure, the surface is at a higher temperature than the fluid. The temperature profile that exists is due to the energy exchange resulting from this temperature difference. For such a case the heat-transfer rate between the surface and the fluid may be written as qy ¼ hA(Ts À T1) (19-2) and, because heat transfer at the surface is by conduction qy ¼ ÀkA @ @y (T À Ts)j y¼0 (19-3) 19.2 Significant Parameters in Convective Heat Transfer 275

These two terms must be equal; thus h(Ts À T1)
¼ Àk @ @y (T À Ts)j y¼0 which may be rearranged to give h k ¼ @(Ts À T)/@yj y¼0 Ts À T1 (19-4) Equation (19-4) may be made dimensionless if a length parameter is introduced. Multiplying both sides by a representative length, L, we have hL k ¼ @(Ts À T)/@yj y¼0 (Ts À T1)/L (19-5) The right-hand side of equation (19-5) is now the ratio of the temperature gradient at the surface to an overall or reference temperature gradient. The left-hand side of this equation is written in a manner similar to that for the Biot modulus encountered in Chapter 18. It may be considered a ratio of conductive thermal resistance to the convective thermal resistance of the fluid. This ratio is referred to as the Nusselt number Nu hL k (19-6) where the thermal conductivity is that of the fluid as opposed to that of the solid, which was the case in the evaluation of the Biot modulus. These two parameters, Pr and Nu, will be encountered many times in the work to follow. 19.3 DIMENSIONAL ANALYSIS OF CONVECTIVE ENERGY TRANSFER Forced Convection. The specific forced-convection situation, which we shall now consider, is that offluid flowing in a closed conduit at some averagevelocity, v, with a temperature difference existing between the fluid and the tube wall. The important variables, their symbols, and dimensional representations are listed below. It is necessary to include two more dimensions—Q, heat, and T, temperature—to the fundamental group considered in Chapter 11; thus all variables must be expressed dimensionally as some combination of M, L, t, Q, and T. The above variables include terms descriptive of the system geometry, thermal and flow properties of the fluid, and the quantity of primary interest, h. Ts Ts – T∞ Ts – T v∞ vx y Figure 19.1 Temperature and velocity profiles for a fluid flowing past a heated plate. 276 Chapter 19 Convective Heat Transfer

Utilizing the Buckingham method of grouping the variables as presented
in Chapter 11, the required number of dimensionless groups is found to be 3. Note that the rank of the dimensional matrix is 4, one more than the total number of fundamental dimensions. Choosing D, k, m, and v as the four variables comprising the core, we ﬁnd that the three p groups to be formed are p1 ¼ Dakbmcvdr p2 ¼ Dekfmgvhcp and p3 ¼ Dikjmkvlh Writing p1 in dimensional form 1 ¼ ðLÞa Q LtT b M Lt c L t d M L3 and equating the exponents of the fundamental dimensions on both sides of this equation, we have for L: 0 ¼ a À b À c þ d À 3 Q: 0 ¼ b t: 0 ¼ Àb À c À d T: 0 ¼ Àb and M : 0 ¼ c þ 1 Solving these equations for the four unknowns yields a ¼ 1 c ¼ À1 b ¼ 0 d ¼ 1 and p1 becomes p1 ¼ Dvr m which is the Reynolds number. Solving for p2 and p3 in the same way will give p2 ¼ mcp k ¼ Pr and p3 ¼ hD k ¼ Nu Variable Symbol Dimensions Tube diameter D L Fluid density r M/L3 Fluid viscosity m M/Lt Fluid heat capacity cp Q/MT Fluid thermal conductivity k Q/tLT Velocity v L/t Heat-transfer coefﬁcient h Q/tL2T 19.3 Dimensional Analysis of Convective Energy Transfer 277

The result of a dimensional analysis of forced-convection heat transfer
in a circular conduit indicates that a possible relation correlating the important variables is of the form Nu ¼ f1(Re, Pr) (19-7) If, in the preceding case, the core group had been chosen to include r, m, cp , and v, the analysis would have yielded the groups Dvr/m, mcp/k, and h/rvcp : The first two of these we recognize as Re and Pr. The third is the Stanton number. St h rvcp (19-8) This parameter could also have been formed by taking the ratio Nu/ðRe PrÞ. An alternative correlating relation for forced convection in a closed conduit is thus St ¼ f2(Re, Pr) (19-9) Natural Convection. In the case of natural-convection heat transfer from a vertical plane wall to an adjacent fluid, the variables will differ significantly from those used in the preceding case. The velocity no longer belongs in the group of variables, as it is a result of other effects associated with the energy transfer. New variables to be included in the analysis are those accounting for fluid circulation. They may be found by considering the relation for buoyant force in terms of the density difference due to the energy exchange. The coefficient of thermal expansion, b, is given by r ¼ r0 (1 À b DT ) (19-10) where r0 is the bulk fluid density, r is the fluid density inside the heated layer, and DT is the temperature difference between the heated fluid and the bulk value. The buoyant force per unit volume, Fbuoyant , is Fbuoyant ¼ (r0 À r)g which becomes, upon substituting equation (19-10) Fbuoyant ¼ bgr0 DT (19-11) Equation (19-11) suggests the inclusion of the variables b, g, and DT into the list of those important to the natural convection situation. The list of variables for the problem under consideration is given below. Variable Symbol Dimensions Significant length L L Fluid density r M/L3 Fluid viscosity m M/Lt Fluid heat capacity cp Q/MT Fluid thermal conductivity k Q/LtT Fluid coefficient of thermal expansion b 1/T Gravitational acceleration g L/t2 Temperature difference DT T Heat-transfer coefficient h Q/L2tT 278 Chapter 19 Convective Heat Transfer

The Buckingham p theorem indicates that the number of independent
dimensionless parameters applicable to this problem is 9 À 5 ¼ 4. Choosing L, m, k, g, and b as the core group, we see that the p groups to be formed are p1 ¼ Lambkcbdgecp p2 ¼ Lf mgkhbigjr p3 ¼ Lkmlkmbngo DT and p4 ¼ Lpmqkrbsgth Solving for the exponents in the usual way, we obtain p1 ¼ mcp k ¼ Pr p3 ¼ b DT p2 ¼ L3gr2 m2 and p4 ¼ hL k ¼ Nu The product of p2 and p3, which must be dimensionless, is (bgr2L3 DT )/m2. This parameter, used in correlating natural-convection data, is the Grashof number. Gr bgr2L3 DT m2 (19-12) From the preceding brief dimensional-analyses considerations, we have obtained the following possible forms for correlating convection data: (a) Forced convection Nu ¼ f1(Re, Pr) (19-7) or St ¼ f2(Re, Pr) (19-9) (b) Natural convection Nu ¼ f3(Gr, Pr) (19-13) The similarity between the correlations of equations (19-7) and (19-13) is apparent. In equation (19-13), Gr has replaced Re in the correlation indicated by equation (19-7). It should be noted that the Stanton number can be used only in correlating forced-convection data. This becomes obvious when we observe the velocity, v, contained in the expression for St. 19.4 EXACT ANALYSIS OF THE LAMINAR BOUNDARY LAYER An exact solution for a special case of the hydrodynamic boundary layer is discussed in Section 12.5. Blasius’s solution for the laminar boundary layer on a ﬂat plate may be extended to include the convective heat-transfer problem for the same geometry and laminar ﬂow. The boundary-layer equations considered previously include the two-dimensional, incompressible continuity equation @vx @x þ @vy @y ¼ 0 (12-10) 19.4 Exact Analysis of the Laminar Boundary Layer 279

and the equation of motion in the x direction @vx
@t þ vx @vx @x þ vy @vx @y ¼ v1 dv1 dx þ n @2vx @y2 (12-9) Recall that the y-directional equation of motion gave the result of constant pressure through the boundary layer. The proper form of the energy equation will thus be equation (16-14), for isobaric flow, written in two-dimensional form as @T @t þ vx @T @x þ vy @T @y ¼ a @2T @x2 þ @2T @y2 (19-14) With respect to the thermal boundary layer depicted in Figure 19.2, @2T/@x2 is much smaller in magnitude than @2T/@y2: In steady, incompressible, two-dimensional, isobaric flow the energy equation that applies is now vx @T @x þ vy @T @y ¼ a @2T @y2 (19-15) From Chapter 12, the applicable equation of motion with uniform free-stream velocity is vx @vx @x þ vy @vx @y ¼ n @2vx @y2 (12-11a) and the continuity equation @vx @x þ @vy @y ¼ 0 (12-11b) The latter two of the above equations were originally solved by Blasius to give the results discussed in Chapter 12. The solution was based upon the boundary conditions vx v1 ¼ vy v1 ¼ 0 at y ¼ 0 and vx v1 ¼ 1 at y ¼ 1 T∞ T = T(y) y x Ts Edge of thermal boundary layer Figure 19.2 The thermal boundary layer for laminar flow past a flat surface. 280 Chapter 19 Convective Heat Transfer

The similarity in form between equations (19-15) and (12-11a) is
obvious. This situation suggests the possibility of applying the Blasius solution to the energy equation. In order that this be possible, the following conditions must be satisfied: (1) The coefficients of the second-order terms must be equal. This requires that n ¼ a or that Pr ¼ 1. (2) The boundary conditions for temperature must be compatible with those for the velocity. This may be accomplished by changing the dependent variable from T to (T À Ts)/(T1 À Ts). The boundary conditions now are vx v1 ¼ vy v1 ¼ T À Ts T1 À Ts ¼ 0 at y ¼ 0 vx v1 ¼ T À Ts T1 À Ts ¼ 1 at y ¼ 1 Imposing these conditions upon the set of equations (19-15) and (12-11a), we may now write the results obtained by Blasius for the energy-transfer case. Using the nomenclature of Chapter 12, f0 ¼ 2 vx v1 ¼ 2 T À Ts T1 À Ts (19-16) h ¼ y 2 ffiffiffiffiffiffiffi v1 nx r ¼ y 2x ffiffiffiffiffiffiffiffiffi xv1 n r ¼ y 2x ffiffiffiffiffiffiffiffi Rex p (19-17) and applying the Blasius result, we obtain df0 dh y¼0 ¼ f00(0) ¼ d½2(vx/v1) d½(y/2x) ffiffiffiffiffiffiffiffi Rex p y¼0 ¼ df2½(T À Ts)/(T1 À Ts)g d½(y/2x) ffiffiffiffiffiffiffiffi Rex p y¼0 ¼ 1:328 ð19-18Þ It should be noted that according to equation (19-16), the dimensionless velocity profile in the laminar boundary layer is identical with the dimensionless temperature profile. This is a consequence of having Pr ¼ 1: A logical consequence of this situation is that the hydrodynamic and thermal boundary layers are of equal thickness. It is significant that the Prandtl numbers for most gases are sufficiently close to unity that the hydrodynamic and thermal boundary layers are of similar extent. We may now obtain the temperature gradient at the surface @T @y y¼0 ¼ (T1 À Ts) 0:332 x Re1/2 x ! (19-19) Application of the Newton and Fourier rate equations now yields qy A ¼ hx(Ts À T1) ¼ Àk @T @y y¼0 from which hx ¼ À k Ts À T1 @T @y y¼0 ¼ 0:332k x Re1/2 x (19-20) 19.4 Exact Analysis of the Laminar Boundary Layer 281

or hxx k ¼ Nux ¼ 0:332 Re1/2 x (19-21)
Pohlhausen1 considered the same problem with the additional effect of a Prandtl number other than unity. He was able to show the relation between the thermal and hydrodynamic boundary layers in laminar flow to be approximately given by d dt ¼ Pr1/3 (19-22) The additional factor of Pr1/3 multiplied by h allows the solution to the thermal boundary layer to be extended to Pr values other than unity. A plot of the dimensionless temperature vs. h Pr1/3 is shown in Figure 19.3. The temperature variation given in this form leads to an expression for the convective heat-transfer coefficient similar to equation (19-20). At y ¼ 0, the gradient is @T @y y¼0 ¼ (T1 À Ts) 0:332 x Re1/2 x Pr1/3 ! (19-23) which, when used with the Fourier and Newton rate equations, yields hx ¼ 0:332 k x Re1/2 x Pr1/3 (19-24) or hxx k ¼ Nux ¼ 0:332 Re1/2 x Pr1/3 (19-25) The inclusion of the factor Pr1/3 in these equations extends the range of application of equations (19-20) and (19-21) to situations in which the Prandtl number differs considerably from 1. Slope = 1.328 f' = 2 (T – Ts ) / (T∞ – Ts ) (y/2x) Rex Pr1/3 Figure 19.3 Temperature variation for laminar flow over a flat plate. 1 E. Pohlhausen, ZAMM, 1, 115 (1921). 282 Chapter 19 Convective Heat Transfer

The mean heat-transfer coefficient applying over a plate of width
w and length L may be obtained by integration. For a plate of these dimensions qy ¼ hA(Ts À T1) ¼ Z A hx(Ts À T1) dA h(wL)(Ts À T1) ¼ 0:332kw Pr1/3(Ts À T1) Z L 0 Re1/2 x x dx hL ¼ 0:332k Pr1/3 v1r m 1/2 Z L 0 xÀ1/2dx ¼ 0:664k Pr1/3 v1r m 1/2 L1/2 ¼ 0:664k Pr1/3 Re1/2 L The mean Nusselt number becomes NuL ¼ hL k ¼ 0:664 Pr1/3 Re1/2 L (19-26) and it is seen that NuL ¼ 2 Nux at x ¼ L (19-27) In applying the results of the foregoing analysis it is customary to evaluate all fluid properties at the film temperature, which is defined as Tf ¼ Ts þ T1 2 (19-28) the arithmetic mean between the wall and bulk fluid temperatures. 19.5 APPROXIMATE INTEGRAL ANALYSIS OF THE THERMAL BOUNDARY LAYER The application of the Blasius solution to the thermal boundary layer in Section 19.4 was convenient although very limited in scope. For flow other than laminar or for a configuration other than a flat surface, another method must be utilized to estimate the convective heat- transfer coefficient. An approximate method for analysis of the thermal boundary layer employs the integral analysis as used by von Ka ´rma ń for the hydrodynamic boundary layer. This approach is discussed in Chapter 12. Consider the control volume designated by the dashed lines in Figure 19.4, applying to flow parallel to a flat surfacewith nopressuregradient, having width Dx; a height equal to the q3 q1 q2 q4 y x ∆x δth Figure 19.4 Control volume for integral energy analysis. 19.5 Approximate Integral Analysis of the Thermal Boundary Layer 283

thickness of the thermal boundary layer, dt, and a unit
depth. An application of the first law of thermodynamics in integral form dQ dt À dWs dt À dWm dt ¼ Z Z c:s: (e þ P/r)r(v :n) dA þ @ @t Z Z Z c:v: er dV (6-10) yields the following under steady-state conditions: dQ dt ¼ Àk Dx @T @y y¼0 dWs dt ¼ dWm dt ¼ 0 Z Z c:s: (e þ P/r)r(v :n) dA ¼ Z dt 0 v2 x 2 þ gy þ u þ P r rvx dy xþDx À Z dt 0 v2 x 2 þ gy þ u þ P r rvx dy x À d dx Z dt 0 rvx v2 x 2 þ gy þ u þ P r dt ! dy Dx and @ @t Z Z Z c:v: er dV ¼ 0 In the absence of significant gravitational effects, the convective-energy-flux terms become v2 x 2 þ u þ P r ¼ h0 ’ cpT0 where h0 is the stagnation enthalpy and cp is the constant-pressure heat capacity. The stagnation temperature will now be written merely as T (without subscript) to avoid confusion. The complete energy expression is now Àk Dx @T @y y¼0 ¼ Z dt 0 rvx cpT dy xþDx À Z dt 0 rvx cpT dy x À rcp Dx d dx Z dt 0 vxT1 dy ð19-29Þ Equation (19-29) can also be written as q4 ¼ q2 À q1 À q3 where these quantities are shown in Figure 19.4. In equation (19-29), T1 represents the free-stream stagnation temperature. If flow is incompressible, and an average value of cp is used, the product rcp may be taken outside the integral terms in this equation. Dividing both sides of equation (19-29) by Dx and evaluating the result in the limit as by Dx approaches zero, we obtain k rcp @T @y y¼0 ¼ d dx Z dt 0 vx(T1 À T) dy (19-30) Equation (19-30) is analogous to the momentum integral relation, equation (12-37), with the momentum terms replaced by their appropriate energy counterparts. This equation may be solved if both velocity and temperature profile are known. Thus, for the energy equation both the variation in vx and in T with y must be assumed. This contrasts slightly with the momentum integral solution in which the velocity profile alone was assumed. 284 Chapter 19 Convective Heat Transfer

An assumed temperature profile must satisfy the boundary conditions (1)
T À Ts ¼ 0 at y ¼ 0 (2) T À Ts ¼ T1 À Ts at y ¼ dt (3) @ @y (T À Ts) ¼ 0 at y ¼ dt (4) @2 @y2 (T À Ts) ¼ 0 at y ¼ 0 ½see equation (19-15) If a power-series expression for the temperature variation is assumed in the form T À Ts ¼ a þ by þ cy2 þ dy3 the application of the boundary conditions will result in the expression for T À Ts T À Ts T1 À Ts ¼ 3 2 y dt À 1 2 y dt 3 (19-31) If the velocity profile is assumed in the same form, then the resulting expression, as obtained in Chapter 12, is v v1 ¼ 3 2 y d À 1 2 y d 3 (12-40) Substituting equations (19-31) and (12-40) into the integral expression and solving, we obtain the result Nux ¼ 0:36 Re1/2 x Pr1/3 (19-32) which is approximately 8% larger than the exact result expressed in equation (19-25). This result, although inexact, is sufficiently close to the knownvalue to indicate that the integral method may be used with confidence in situations in which an exact solution is not known. It is interesting to note that equation (19-32) again involves the parameters predicted from dimensional analysis. A condition of considerable importance is that of an unheated starting length. Problem 19.17attheendofthechapterdealswiththissituationwherethewalltemperature,Ts ,isrelated to the distance from the leading edge, x, and the unheated starting, length X, according to Ts ¼ T1 for 0 < x < X and Ts > T1 for X < x The integral technique, as presented in this section, has proved effective in generating a modified solution for this situation. The result for Ts ¼ constant, and assuming both the hydrodynamic and temperature profiles to be cubic, is Nux ffi 0:33 Pr 1 À (X/x)3/4 " #1 3 Rex (19-33) Note that this expression reduces to equations (19-25) for X = 0 19.6 ENERGY- AND MOMENTUM-TRANSFER ANALOGIES Many times in our consideration of heat transfer thus far we have noted the similarities to momentum transfer both in the transfer mechanism itself and in the manner of its quantitative description. This section will deal with these analogies and use them to develop relations to describe energy transfer. 19.6 Energy- and Momentum-Transfer Analogies 285

Osborne Reynolds first noted the similarities in mechanism between energy
and momentum transfer in 1874.2 In 1883, he presented3 the results of his work on frictional resistance to fluid flow in conduits, thus making possible the quantitative analogy between the two transport phenomena. As we have noted in the previous sections, for flow past a solid surface with a Prandtl number of unity, the dimensionless velocity and temperature gradients are related as follows: d dy vx v1 y¼0 ¼ d dy T À Ts T1 À Ts y¼0 (19-34) For Pr ¼ mcp/k ¼ 1; we have mcp/k and we may write equation (19-34) as mcp d dy vx v1 y¼0 ¼ k d dy T À Ts T1 À Ts y¼0 which may be transformed to the form mcp v1 dvx dy y¼0 ¼ À k Ts À T1 d dy (T À Ts) y¼0 (19-35) Recalling a previous relation for the convective heat-transfer coefficient h k ¼ d dy (Ts À T) (Ts À T1) ! y¼0 (19-4) it is seen that the entire right-hand side of equation (19-34) may be replaced by h, giving h ¼ mcp v1 dvx dy y¼0 (19-36) Introducing next the coefficient of skin friction Cf ffi t0 rv2 1/2 ¼ 2m rv2 1 dvx dy y¼0 we may write equation (19-36) as h ¼ Cf 2 (rv1cp) which, in dimensionless form, becomes h rv1cp St ¼ Cf 2 (19-37) Equation (19-37) is the Reynolds analogy and is an excellent example of the similar nature of energy and momentum transfer. For those situations satisfying the basis for the development of equation (19-37), a knowledge of the coefficient of frictional drag will enable the convective heat-transfer coefficient to be readily evaluated. The restrictions on the use of the Reynolds analogy should be kept in mind; they are (1) Pr ¼ 1 and (2) no form drag. The former of these was the starting point in the preceding development and obviously must be satisfied. The latter is sensible when one considers that, in relating two transfer mechanisms, the manner of expressing them quantitatively must 2 O. Reynolds, Proc. Manchester Lit. Phil. Soc., 14:7 (1874). 3 O. Reynolds, Trans. Roy. Soc. (London, 174A, 935 (1883). 286 Chapter 19 Convective Heat Transfer

remain consistent. Obviously the description of drag in terms of
the coefficient of skin friction requires that the drag be wholly viscous in nature. Thus, equation (19-37) is applicable only for those situations in which form drag is not present. Some possible areas of application would be flow parallel to plane surfaces or flow in conduits. The coefficient of skin friction for conduit flow has already been shown to be equivalent to the Fanning fiction factor, which may be evaluated by using Figure 14.1. The restriction that Pr ¼ 1 makes the Reynolds analogy of limited use. Colburn4 has suggested a simple variation of the Reynolds analogy form that allows its application to situations where the Prandtl number is other than unity. The Colburn analogy expression is St Pr2/3 ¼ Cf 2 (19-38) which obviously reduces to the Reynolds analogy when Pr ¼ 1. Colburn applied this expression to a wide range of data for flow and geometries of different types and found it to be quite accurate for conditions where (1) no form drag exists, and (2) 0:5 < Pr < 50. The Prandtl number range is extended to include gases, water, and several other liquids of interest. The Colburn analogy is particularly helpful for evaluating heat transfer in internal forced flows. It can be easily shown that the exact expression for a laminar boundary layer on a flat plate reduces to equation (19-38). The Colburn analogy is often written as jH ¼ C f 2 (19-39) where jH ¼ St Pr2/3 (19-40) is designated the Colburn j factor for heat transfer. A mass-transfer j factor, is discussed in Chapter 28. Note that forPr ¼ 1,theColburnandReynoldsanalogiesarethesame.Equation (19-38) is thus an extension of the Reynolds analogy for fluids having Prandtl numbers other than unity, withintherange 0.5– 50 asspecified above. Highandlow Prandtl numberfluids falling outside this range would be heavy oils at one extreme and liquid metals at the other. 19.7 TURBULENT FLOW CONSIDERATIONS The effect of the turbulent flow on energy transfer is directly analogous to the similar effects on momentum transfer as discussed in Chapter 12. Consider the temperature profile variation in Figure 19.5 to exist in turbulent flow. The distance moved by a fluid ‘‘packet’’ in the y direction, which is normal to the direction of bulk flow, is denoted by L, the Prandtl mixing length. The packet of fluid moving through the distance L retains the mean temperature from its point of origin, and upon reaching its destination, the packet will differ in temperature from that of the adjacent fluid by an amount Tj yÆL À Tj y : The mixing length is assumed small enough to permit the temperature difference to be written as Tj yÆL À Tj y ¼ ÆL dt dy y (19-41) We now define the quantity T0 as the fluctuating temperature, synonymous with the fluctuating velocity component, v0 x , described in Chapter 12. The instantaneous 4 A. P. Colburn, Trans. A.I.Ch.E., 29, 174 (1933). 19.7 Turbulent Flow Considerations 287

temperature is the sum of the mean and fluctuating values,
as indicated in Figure 19.5(b), or, in equation form T ¼ T þ T0 (19-42) Any significant amount of energy transfer in the y direction, for bulk flow occurring in the x direction, is accomplished because of the fluctuating temperature, T0; thus, it is apparent from equations (19-41) and (19-42) that T0 ¼ ÆL dT dy (19-43) The energy flux in the y direction may now be written as qy A y ¼ rcpTv0 y (19-44) where v0 y may be either positive or negative. Substituting for T its equivalent, according to equation (19-42) qy A y ¼ rcpv0 y (T þ T0) and taking the time average, we obtain, for the y-directional energy flux due to turbulent effects qy A turb ¼ rcp(v0 y T0) (19-45) or, with T0 in terms of the mixing length qy A turb ¼ rcpv0 y L dT dy (19-46) The total energy flux due to both microscopic and turbulent contributions may be written as qy A ¼ Àrcp ½a þ jv0 y Lj dT dy (19-47) As a is the molecular diffusivity of heat, the quantity jv0 y Lj is the eddy diffusivity of heat, designated as eH. This quantity is exactly analogous to the eddy diffusivity of momentum, eM ; as defined in equation (12-52). In a region of turbulent flow, eH ) a for all fluids except liquid metals. T T y (a) L t T (b) T T' Figure 19.5 Turbulent-flow temperature variation. 288 Chapter 19 Convective Heat Transfer

As the Prandtl number is the ratio of the molecular
diffusivities of momentum and heat, an analogous term, the turbulent Prandtl number, can be formed by the ratio e M/ e H. Utilizing equations (19-47) and (12-55), we have Prturb ¼ eM eH ¼ L2jdvx/dyj jLv0 y j ¼ L2jdvx/dyj L2jdvx/dyj ¼ 1 (19-48) Thus, in a region of fully turbulent flow the effective Prandtl number is unity, and the Reynolds analogy applies in the absence of form drag. In terms of the eddy diffusivity of heat, the heat flux can be expressed as qy A turb ¼ Àrcp eH DT dy (19-49) The total heat flux, including both molecular and turbulent contributions, thus becomes qy A ¼ Àrcp(a þ eH) dT dy (19-50) Equation (19-50) applies both to the region wherein flow is laminar, for which a ) e H, and to that for which flow is turbulent and eH ) a. It is in this latter region that the Reynolds analogy applies. Prandtl5 achieved a solution that includes the influences of both the laminar sublayer and the turbulent core. In his analysis solutions were obtained in each region and then joined at y ¼ j, the hypothetical distance from the wall that is assumed to be the boundary separating the two regions. Within the laminar sublayer the momentum and heat flux equations reduce to t ¼ rv dvx dy (a constant) and qy A ¼ Àrcpa dT dy Separating variables and integrating between y ¼ 0 and y ¼ j, we have, for the momentum expression Z vx j j 0 dvx ¼ t rv Z j 0 dy and for the heat flux Z Tj Ts dT ¼ À qy Arcpa Z j 0 dy Solving for the velocity and temperature profiles in the laminar sublayer yields vx j j ¼ tj rv (19-51) and Ts À Tj ¼ qyj Arcpa (19-52) 5 L. Prandtl, Zeit. Physik., 11, 1072 (1910). 19.7 Turbulent Flow Considerations 289

Eliminating the distance j between these two expressions gives rvvx
j j t ¼ rAcpa qy (Ts À Tj) (19-53) Directing our attention now to the turbulent core where the Reynolds analogy applies, we may write equation (19-37) h rcp(v1 À vx j j ) ¼ Cf 2 (19-37) and, expressing h and Cf in terms of their defining relations, we obtain qy/A rcp(v1 À vx j j )(Tj À T1) ¼ t r(v1 À vx j j )2 Simplifying and rearranging this expression, we have r(v1 À vx j j ) t ¼ rAcp (Tj À T1) qy (19-54) which is a modified form of the Reynolds analogy applying from y ¼ j to y ¼ ymax : Eliminating Tj between equations (19-53) and (19-54), we have r t v1 þ vx j j v a À 1 h i ¼ rAcp qy (Ts À T1) (19-55) Introducing the coefficient of skin friction Cf ¼ t rv2 1/2 and the convective heat-transfer coefficient h ¼ qy A(Ts À T1) we may reduce equation (19-54) to v1 þ vx j j (v/a À 1) v2 1Cf /2 ¼ rcp h Inverting both sides of this expression and making it dimensionless, we obtain h rcpv1 St ¼ Cf /2 1 þ (vx j j /v1)½(v/a) À 1 (19-56) This equation involves the ratio v/a, which has been defined previously as the Prandtl number. For a value of Pr ¼ 1, equation (19-56) reduces to the Reynolds analogy. For Pr ¼ 1, the Stanton number is a function of Cf , Pr, and the ratio vx j j /v1. It would be convenient to eliminate the velocity ratio; this may be accomplished by recalling some results from Chapter 12. At the edge of the laminar sublayer vþ ¼ yþ ¼ 5 and by definition vþ ¼ vx/( ffiffiffiffiffiffiffi t/r p ). Thus for the case at hand vþ ¼ vx j j /( ffiffiffiffiffiffiffi t/r p ) ¼ 5 290 Chapter 19 Convective Heat Transfer

Again introducing the coefficient of skin friction in the form
Cf ¼ t rv2 1/2 we may write ffiffiffi t r r ¼ v1 ffiffiffiffiffi Cf 2 r which, when combined with the previous expression given for the velocity ratio, gives vx j j v1 ¼ 5 ffiffiffiffiffi Cf 2 r (19-57) Substitution of equation (19-57) into (19-56) gives St ¼ Cf /2 1 þ 5 ffiffiffiffiffiffiffiffiffi Cf /2 p (Pr À 1) (19-58) which is known as the Prandtl analogy. This equation is written entirely in terms of measurable quantities. von Ka ´rma ń6 extended Prandtl’s work to include the effect of the transition or buffer layer in addition to the laminar sublayer and turbulent core. His result, the von Ka ´rma ń analogy, is expressed as St ¼ Cf /2 1 þ 5 ffiffiffiffiffiffiffiffiffi Cf /2 p fPr À 1 þ ln½1 þ 5 6 (Pr À 1)g (19-59) Note that, just as for the Prandtl analogy, equation (19-59) reduces to the Reynolds analogy for a Prandtl number of unity. The application of the Prandtl and vonKa ´rma ń analogies is, quite logically, restricted to those cases in which there is negligible form drag. These equations yield the most accurate results for Prandtl numbers greater than unity. An illustration of the use of the four relations developed in this section is given in the example below. EXAMPLE 1 Water at 50F enters a heat-exchanger tube having an inside diameter of 1 in. and a length of 10 ft. The water flows at 20 gal/min. For a constant wall temperature of 210F, estimate the exit temperature of the water using (a) the Reynolds analogy, (b) the Colburn analogy, (c) the Prandtl analogy, and (d) the von Ka ´rma ń analogy. Entrance effects are to be neglected, and the properties of water may be evaluated at the arithmetic-mean bulk temperature. Considering a portion of the heat-exchanger tube shown in Figure 19.6, we see that an application of the first law of thermodynamics to the control volume indicated will yield the result that rate of heat transfer into c:v: by fluid flow 8 < : 9 = ; þ rate of heat transfer into c:v: by convection 8 < : 9 = ; ¼ rate of heat transfer out of c:v: by fluid flow 8 < : 9 = ; 6 T. von Ka ´rma ń, Trans. ASME, 61, 705 (1939). 19.7 Turbulent Flow Considerations 291

If these heat-transfer rates are designated as q1 , q2
, and q3 , they may be evaluated as follows: q1 ¼ r pD2 4 vxcpTj x q2 ¼ hpD Dx(Ts À T) and q3 ¼ r pD2 4 vxcpTj xþDx The substitution of these quantities into the energy balance expression gives r pD2 4 vxcp ½Tj xþDx À Tj x À hpD Dx(Ts À T) ¼ 0 which may be simplified and rearranged into the form D 4 Tj xþDx À Tj x Dx þ h rvxcp (T À Ts) ¼ 0 (19-60) Evaluated in the limit as Dx ! 0, equation (19-59) reduces to dT dx þ h rvxcp 4 D (T À Ts) ¼ 0 (19-61) Separating the variables, we have dT T À Ts þ h rvxcp 4 D dx ¼ 0 and integrating between the limits indicated, we obtain Z TL T0 dT T À Ts þ h rvxcp 4 D Z L 0 dx ¼ 0 ln TL À Ts T0 À Ts þ h rvxcp 4L D ¼ 0 (19-62) Equation (19-62) may now be solved for the exit temperature TL . Observe that the coefficient of the right-hand term, h/rvxcp, is the Stanton number. This parameter has been achieved quite naturally from our analysis. The coefficient of skin friction may be evaluated with the aid of Figure 14.1. The velocity is calculated as vx ¼ 20 gal/min (ft3/7:48 gal)½144/(p/4)(12)ft2(min/60 s) ¼ 8:17 fps Initially, we will assume the mean bulk temperature to be 908F. The film temperature will then be 1508F, at which n ¼ 0:474 Â 10À5 ft2/s. The Reynolds number is Re ¼ Dvx v ¼ (1/12 ft)(8:17 ft/s) 0:474 Â 10À5 ft2/s ¼ 144,000 At this value of Re, the friction factor, ff , assuming smooth tubing, is 0.0042. For each of the four analogies, the Stanton number is evaluated as follows: D q1 q2 q3 Ts Flow ∆x Figure 19.6 Analog analysis of water flowing in a circular tube. 292 Chapter 19 Convective Heat Transfer

(a) Reynolds analogy St ¼ Cf 2 ¼ 0:0021 (b)
Colburn analogy St ¼ Cf 2 PrÀ2/3 ¼ 0:0021(2:72)À2/3 ¼ 0:00108 (c) Prandtl analogy St ¼ Cf /2 1 þ 5 ffiffiffiffiffiffiffiffiffi Cf /2 p (Pr À 1) ¼ 0:0021 1 þ 5 ffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0:0021 p (1:72) ¼ 0:00151 (d) von Ka ´rma ń analogy St ¼ Cf /2 1 þ 5 ffiffiffiffiffiffiffiffiffi Cf /2 p Pr À 1 þ ln 1 þ 5 6 (Pr À 1) Â Ã È É ¼ 0:0021 1 þ 5 ffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0:0021 p 2:72 À 1 þ ln 1 þ 5 6 (2:72 À 1) Â Ã È É ¼ 0:00131 Substituting these results into equation (19-62), we obtain, for TL , the following results: (a) TL ¼ 152F (b) TL ¼ 115F (c) TL ¼ 132F (d) TL ¼ 125F Some fine tuning of these results may be necessary to adjust the physical property values for the calculated film temperatures. In none of these cases is the assumed film temperature different than the calculated one by more than 68F, so the results are not going to change much. The Reynolds analogy value is much different from the other results obtained. This is not surprising, as the Prandtl number was considerably above a value of one. The last three analogies yielded quite consistent results. The Colburnanalogy is the simplest to use and is preferable from that standpoint. 19.8 CLOSURE The fundamental concepts of convection heat transfer have been introduced in this chapter. New parameters pertinent to convection are the Prandtl, Nusselt, Stanton, and Grashof numbers. 19.8 Closure 293

HEAT TRANSFER CONDUCTION& CONVECTION

HEAT TRANSFER CONDUCTION& CONVECTION

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