Radiatio heat transfer

Transcript

1. Chapter 23 Radiation Heat Transfer The mechanism of radiation heat transfer has no analogy in either momentum or mass transfer. Radiation heat transfer is extremely important in many phases of engineering design such as boilers, home heating, and spacecraft. In this chapter, we will concern ourselves first with understanding the nature of thermal radiation. Next, we will discuss properties of surfaces and consider how system geometry influences radiant heat transfer. Finally, we will illustrate some techniques for solving relatively simple problems where surfaces and some gases participate in radiant energy exchange. 23.1 NATURE OF RADIATION The transfer of energy by radiation has several unique characteristics when contrasted with conduction or convection. First, matter is not required for radiant heat transfer; indeed the presence of a medium will impede radiation transfer between surfaces. Cloud cover is observed to reduce maximum daytime temperatures and to increase minimum evening temperatures, both of which are dependent upon radiant energy transfer between earth and space. A second unique aspect of radiation is that both the amount of radiation and the quality of the radiation depend upon temperature. In conduction and convection, the amount of heat transfer was found to depend upon the temperature difference; in radiation, the amount of heat transfer depends upon both the temperature difference between two bodies and their absolute temperatures. In addition, radiation from a hot object will be different in quality than radiation from a body at a lower temperature. The color of incandescent objects is observed to change as the temperature is changed. The changing optical properties of radiation with temperature are of paramount importance in determining the radiant-energy exchange between bodies. Radiation travels at the speed of light, having both wave properties and particle-like properties. The electromagnetic spectrum shown in Figure 23.1 illustrates the tremendous range of frequency and wavelength over which radiation occurs. The unit of wavelength which we shall use in discussing radiation is the micron, symbolized m. One micron is 10À6 m or 3:94(10)À5 in: The frequency, v, of radiation is related to the wavelength l, by lv ¼ c, where c is the speed of light. Short-wavelength radiation such as gamma rays and x-rays is associated with very high energies. To produce radiation of this type we must disturb the nucleus or the inner-shell electrons of an atom. Gamma rays and x-rays also have great penetrating ability; surfaces that are opaque to visible radiation are easily traversed by gamma and x-rays. Very-long-wavelength radiation, such as radiowaves, also may pass through solids; however, the energy associated with these waves is much less than that for short-wavelength radiation. In the range from l ¼ 0:38 to 0.76 microns, radiation is sensed by the optical nerve of the eye and is what we call light. Radiation in the visible range is observed to have little penetrating power except in some liquids, plastics, and glasses. The radiation between wavelengths of 0.1 and 100 microns is 359
2. termed as thermal radiation. The thermal band of the spectrum includes a portion of the ultraviolet and all of the infrared regions. 23.2 THERMAL RADIATION Thermal radiation incident upon a surface as shown in Figure 23.2 may be either absorbed, reflected, or transmitted. If r, a, and t are the fractions of the incident radiation that are reflected, absorbed and transmitted, respectively, then r þ a þ t ¼ 1 (23-1) where r is called the reflectivity, a is called the absorptivity, and t is called transmissivity. There are two types of reflection that can occur, specular reflection and diffuse reflection. In specular reflection, the angle of incidence of the radiation is equal to the angle of reflection. The reflection shown in Figure 23.2 is specular reflection. Most bodies do not reflect in a specular manner, they reflect radiation in all directions. Diffuse reflection is sometimes likened to a situation in which the incident thermal radiation is absorbed and then reemitted from the surface, still retaining its initial wavelength. Absorption of thermal radiation in solids takes place in a very short distance, on the order of 1 mm in electrical conductors and about 0.05 in. in electrical nonconductors, the difference being caused by the different population of energy states in electrical conductors, which can absorb energy at thermal radiation frequencies. λ 10–6 10–5 10–4 10–3 10–2 10–1 100 101 102 103 104 105 106 Cosmic rays Gamma rays x-rays Infrared Blue, green Yellow Red Ultra- violet Visible spectrum 0.3 0.4 0.5 0.6 Wavelength, in microns 0.7 0.8 Radio waves Figure 23.1 The electromagnetic spectrum. Reflected radiation Incident radiation Absorbed radiation Transmitted radiation Figure 23.2 Fate of radiation incident upon a surface. 360 Chapter 23 Radiation Heat Transfer
3. For most solids, the transmissivity is zero, and thus they may be called opaque to thermal radiation. equation (23-1) becomes, for an opaque body, r þ a ¼ 1: The ideally absorbing body, for which a ¼ 1, is called a black body. A black body neither reflects nor transmits any thermal radiation. As we see reflected light (radiation), a so-called black body will appear black, no light being reflected from it. A small hole in a large cavity closely approaches a black body, regardless of the nature of the interior surface. Radiation incident to the hole has very little opportunity to be reflected back out of the hole. Black bodies may also be made of bright objects, as can be shown by looking at a stack of razor blades, sharp edge forward. The total emissive power, E, of a surface is defined as the total rate of thermal energy emitted via radiation from a surface in all directions and at all wavelengths per unit surface area. The total emissive power is also referred to elsewhere as the emittance or the total hemispheric intensity. Closely related to the total emissive power is the emissivity. The emissivity, e, is defined as the ratio of the total emissive power of a surface to the total emissive power of an ideally radiating surface at the same temperature. The ideal radiating surface is also called a black body, so we may write e ¼ E Eb (23-2) where Eb is the total emissive power of a black body. As the total emissive power includes radiant-energy contributions from all wavelengths, the monochromatic emissive power, El, may also be defined. The radiant energy El contained between wavelengths l and l þ dl is the monochromatic emissive power; thus dE ¼ El dl, or E ¼ Z 1 0 El dl The monochromatic emissivity, el, is simply el ¼ El/El,b, where El,b is the monochromatic emissive power of a black body at wavelength l at the same temperature. A monochromatic absorptivity, al, may be defined in the same manner as the monochromatic emissivity. The monochromatic absorptivity is defined as the ratio of the incident radiation of wavelength l that is absorbed by a surface to the incident radiation absorbed by a black surface. A relation between the absorptivity and the emissivity is given by Kirchhoff’s law. Kirchhoff’s law states that, for a system in thermodynamic equilibrium, the following equality holds for each surface: el ¼ al (23-3) Thermodynamic equilibrium requires that all surfaces be at the same temperature so that there is no net heat transfer. The utility of Kirchhoff’s law lies in its use for situations in which the departure from equilibrium is small. In such situations the emissivity and the absorptivity may be assumed to be equal. For radiation between bodies at greatly different temperatures, such as between Earth and the sun, Kirchhoff’s law does not apply. A frequent error in using Kirchhoff’s law arises from confusing thermal equilibrium with steady-state conditions. Steady state means that time derivatives are zero, whereas equilibrium refers to the equality of temperatures. 23.3 THE INTENSITY OF RADIATION In order to characterize the quantity of radiation that travels from a surface along a specified path, the concept of a single ray is not adequate. The amount of energy traveling in a given direction is determined from I, the intensity of radiation. With reference to Figure 23.3, we 23.3 The Intensity of Radiation 361
4. are interested in knowing the rate at which radiant energy is emitted from a representative portion, dA, of the surface shown in a prescribed direction. Our per- spective will be that of an observer at point P looking at dA. Standard spherical coordinates will be used, these being r, the radial coordinate; u, the zenith angle shown in Figure 23.3; and f, the azi- muthal angle, which will be discussed shortly. If a unit area of surface, dA, emits a total energy dq, then the intensity of radiation is given by I  d2q dA dV cos u (23-4) where dV is a differential solid angle, that is, a portion of space. Note that with the eye located at point P, in Figure 23.3, the apparent size of the emitting area is dA cos u. It is important to remember that the intensity of radiation is independent of direction for a diffusely radiating surface. Rearran- ging equation (23-4), we see that the relation between the total emissive power, E ¼ dq/dA, and the intensity, I, is dq dA ¼ E ¼ Z I cos u dV ¼ I Z cos u dV (23-5) The relation is seen to be purely geometric for a diffusely radiating (I 6¼ I(u)) surface. Consi- der an imaginary hemisphere of radius r cover- ing the plane surface on which dA is located. The solid angle dV intersects the shaded area on the hemisphere as shown in Figure 23.4. A solid angle is defined by V ¼ A/r2 or dV ¼ dA/r2, and thus dV ¼ (r sin u df)(r du) r2 ¼ sin u du df The total emissive power per unit area becomes E ¼ I Z cos u dV ¼ I Z 2p 0 Z p/2 0 cos u sin u du df or simply E ¼ pI (23-6) If the surface does not radiate diffusely, then E ¼ Z 2p 0 Z p/2 0 I cos u sin u du df dΩ dA P I q Figure 23.3 The intensity of radiation. dA r0 sin q r0 q df dq f Figure 23.4 Integration of intensity over solid angles. 362 Chapter 23 Radiation Heat Transfer
5. The relation between the intensity of radiation, I, and the total emissive power is an important step in determining the total emissive power. Radiation intensity is fundamental in formulating a quantitative description of radiant heat transfer but its definition, as already discussed, is cumbersome. Equation (23-6) relates intensity to emissive power that, potentially, is much easier to describe. We will now consider the means of such a description. 23.4 PLANCK’S LAW OF RADIATION Planck1 introduced the quantum concept in 1900 and with it the idea that radiation is emitted not in a continuous energy state but in discrete amounts or quanta. The intensity of radiation emitted by a black body, derived by Planck, is Ib,l ¼ 2c2hlÀ5 exp ch klT   À 1 where Ib,l is the intensity of radiation from a black body between wavelengths l and l þ dl, c is the speed of light, h is Planck’s constant, k is the Boltzmann constant, and T is the temperature. The total emissive power between wavelengths l and l þ dl is then Eb,l ¼ 2pc2hlÀ5 exp ch klT   À 1 (23-7) Figure 23.5 illustrates the spectral energy distribution of energy of a black body as given by equation (23-7). In Figure 23.5 the area under the curve of Eb,l vs. l (the total emitted energy) is seen to increase rapidly with temperature. The peak energy is also observed to occur at shorter and shorter wavelengths as the temperature is increased. For a black body at 5800 K (the effective temperature of solar radiation), a large part of the emitted energy is in the visible region. Equation (23-7) expresses, functionally, Eb,l as a function of wavelength and temperature. Dividing both sides of this equation by T5, we get Ebl T5 ¼ 2p2h(lT)À5 exp ch klT   À 1 (23-8) where the quantity Ebl/T5 is expressed as a function of the lT product, which can be treated as a single independent variable. This functional relationship is plotted in Figure 23.6, and discrete values of Ebl/sT5 are given in Table 23.1. The constant, s, will be discussed in the next section. The peak energy is observed to be emitted at lT ¼2897:6 mm K(5215:6 mmR), as can be determined by maximizing equation (23-8). The relation, lmaxT ¼ 2897 m K, is called Wien’s displacement law. Wien obtained this result in 1893, 7 years prior to Planck’s development. Weareofteninterestedinknowinghowmuchemissionoccursinaspecificportionofthetotal wavelengthspectrum.Thisisconvenientlyexpressedasafractionofthetotalemissivepower.The fraction between wavelengths l1 and l2 is designated Fl1 Àl2 and may be expressed as Fl1 Àl2 ¼ R l2 l1 Ebl dl R1 0 Ebl dl ¼ R l2 l1 Ebl dl sT4 (23-9) 1 M. Planck, Verh. d. deut. physik. Gesell., 2, 237 (1900). 23.4 Planck’s Law of Radiation 363
6. Equation (23-9) is conveniently broken into two integrals as follows: Fl1 Àl2 ¼ 1 sT4 Z l2 0 Ebl dl À Z l1 0 Ebl dl   ¼ F0Àl2 À F0Àl1 (23-10) So, at a given temperature, the fraction of emission between any two wavelengths can be determined by subtraction. This process can be simplified if the temperature is eliminated as a separate variable. This may be accomplished by using the fraction Ebl/sT5, as discussed. Equation (23-10) Hemispherical spectral emissive power Eλb (l, T), W/(m2 • µm) Wavelength l, mm 0 Violet Red Visible region (0.4–0.7mm) 2 4 5 8 10 12 100 101 102 103 Hemispherical spectral emissive power Elb (λ, T), W/m 2 • mm) 101 102 103 104 105 106 107 108 104 105 106 107 108 Blackbody temperature, T, °R; K 10,000; 5555 3000; 1667 1500; 833 1000; 555 Locus of maximum Values, elb ( lmax , T ) 5000; 2778 Figure 23.5 Spectral emissive power for a black body for several temperatures. (From R. Siegel and J. R. Howell, Thermal Radiation Heat Transfer, Third Edition, Hemisphere Publishers, Washington, 1992. By permission of the publishers.) 364 Chapter 23 Radiation Heat Transfer
7. may be modified in this manner to yield Fl1 TÀl2 T ¼ Z l2 T 0 Ebl sT5 d(lT) À Z l1 T 0 Ebl sT5 d(lT) ¼ F0Àl2 T À F0Àl1 T (23-11) Values of F0ÀlT are given as functions of the product, lT, in Table 23.1. 23.5 STEFAN–BOLTZMANN LAW Planck’s law of radiation may be integrated over wavelengths from zero to infinity to determine the total emissive power. The result is Eb ¼ Z 1 0 Eb,l dl ¼ 2p5k4T4 15c2h3 ¼ sT4 (23-12) where s is called the Stefan–Boltzmann constant and has the value s ¼ 5:676 Â 10À8 W/m2 Á K4(0:1714 Â 10À8 Btu/h ft2 R4). This constant is observed to be a combina- tion of other physical constants. The Stefan–Boltzmann relation, Eb ¼ sT4, was obtained prior to Planck’s law, via experiment by Stefan in 1879 and via a thermodynamic derivation by Boltzmann in 1884. The exact value of the Stefan–Boltzmann constant, s, and its relation to other physical constants were obtained after the presentation of Planck’s law in 1900. Elb (l, T)/T5, Btu/(h • ft2 • 8R5 • mm) Elb (l, T)/T5, W/(m2 • K5 • mm) Wavelength–temperature product lT, µm • °R Wavelength–temperature product lT, µm • K 1 2 6 8 1 2 4 6 8 10 20 40 3 103 4 6 8 1 2 4 6 3 104 0 0 2000 4000 6000 8000 10,000 12,000 14,000 × 10–15 40 80 120 160 200 240 × 10–15 lT, (µm)(°R) 2606 lT, (µm)(K) 1448 Percent emissive power below lT, F0 – λT 11,067 6148 75 41,200 22,890 99 Planck’s law Wien’s distribution Rayleigh–Jeans', distribution 7398 4107 50 5216 2898 25 Figure 23.6 Spectral energy distribution for a black body as a function of lT. (From R. Siegel and J. R. Howell, Thermal Radiation Heat Transfer, Third Edition, Hemisphere Publishers, Washington, 1992. By permission of the publishers.) 23.5 Stefan–Boltzmann Law 365
8. Table 23.1 Planck radiation functions lT(mm K) F0ÀlT Eb sT5 1 cm K   1000 0.0003 0.0372 1100 0.0009 0.0855 1200 0.0021 0.1646 1300 0.0043 0.2774 1400 0.0078 0.4222 1500 0.0128 0.5933 1600 0.0197 0.7825 1700 0.0285 0.9809 1800 0.0393 1.1797 1900 0.0521 1.3713 2000 0.0667 1.5499 2100 0.0830 1.7111 2200 0.1009 1.8521 2300 0.1200 1.9717 2400 0.1402 2.0695 2500 0.1613 2.1462 2600 0.1831 2.2028 2700 0.2053 2.2409 2800 0.2279 2.2623 2900 0.2505 2.2688 3000 0.2732 2.2624 3100 0.2958 2.2447 3200 0.3181 2.2175 3300 0.3401 2.1824 3400 0.3617 2.1408 3500 0.3829 2.0939 3600 0.4036 2.0429 3700 0.4238 1.9888 3800 0.4434 1.9324 3900 0.4624 1.8745 4000 0.4809 1.8157 4100 0.4987 1.7565 4200 0.5160 1.6974 4300 0.5327 1.6387 4400 0.5488 1.5807 4500 0.5643 1.5238 4600 0.5793 1.4679 4700 0.5937 1.4135 4800 0.6075 1.3604 4900 0.6209 1.3089 5000 0.6337 1.2590 5100 0.6461 1.2107 5200 0.6579 1.1640 5300 0.6694 1.1190 5400 0.6803 1.0756 lT(mm K) F0ÀlT Eb sT5 1 cm K   5500 0.6909 1.0339 5600 0.7010 0.9938 5700 0.7108 0.9552 5800 0.7201 0.9181 5900 0.7291 0.8826 6000 0.7378 0.8485 6100 0.7461 0.8158 6200 0.7541 0.7844 6300 0.7618 0.7543 6400 0.7692 0.7255 6500 0.7763 0.6979 6600 0.7832 0.6715 6700 0.7897 0.6462 6800 0.7961 0.6220 6900 0.8022 0.5987 7000 0.8081 0.5765 7100 0.8137 0.5552 7200 0.8192 0.5348 7300 0.8244 0.5152 7400 0.8295 0.4965 7500 0.8344 0.4786 7600 0.8391 0.4614 7700 0.8436 0.4449 7800 0.8480 0.4291 7900 0.8522 0.4140 8000 0.8562 0.3995 8100 0.8602 0.3856 8200 0.8640 0.3722 8300 0.8676 0.3594 8400 0.8712 0.3472 8500 0.8746 0.3354 8600 0.8779 0.3241 8700 0.8810 0.3132 8800 0.8841 0.3028 8900 0.8871 0.2928 9000 0.8900 0.2832 9100 0.8928 0.2739 9200 0.8955 0.2650 9300 0.8981 0.2565 9400 0.9006 0.2483 9500 0.9030 0.2404 9600 0.9054 0.2328 9700 0.9077 0.2255 9800 0.9099 0.2185 9900 0.9121 0.2117 366 Chapter 23 Radiation Heat Transfer
9. 23.6 EMISSIVITY AND ABSORPTIVITY OF SOLID SURFACES Whereas thermal conductivity, specific heat, density, and viscosity are the important physical properties of matter in heat conduction and convection, emissivity and absorptivity are the controlling properties in heat exchange by radiation. From preceding sections it is seen that, for black-body radiation, Eb ¼ sT4. For actual surfaces, E ¼ eEb, following the definition of emissivity. The emissivity of the surface, so defined, is a gross factor, as radiant energy is being sent out from a body not only in all directions but also over various wavelengths. For actual surfaces, the emissivity may vary with wavelength as well as the direction of emission. Consequently, we have to differentiate the monochromatic emissivity el and the directionalemissivity eu from the total emissivity e. Monochromatic Emissivity. By definition, the monochromatic emissivity of an actual surface is the ratio ofits monochromatic emissivepower to that of a black surface at the same temperature. Figure 23.7 represents a typical distribution of the intensity of radiation of two lT(mm K) F0ÀlT Eb sT5 1 cm K   10,000 0.9142 0.2052 11,000 0.9318 0.1518 12,000 0.9451 0.1145 13,000 0.9551 0.0878 14,000 0.9628 0.0684 15,000 0.9689 0.0540 16,000 0.9738 0.0432 17,000 0.9777 0.0349 18,000 0.9808 0.0285 19,000 0.9834 0.0235 20,000 0.9856 0.0196 21,000 0.9873 0.0164 22,000 0.9889 0.0139 23,000 0.9901 0.0118 24,000 0.9912 0.0101 25,000 0.9922 0.0087 26,000 0.9930 0.0075 27,000 0.9937 0.0065 28,000 0.9943 0.0057 29,000 0.9948 0.0050 lT(mm K) F0ÀlT Eb sT5 1 cm K   30,000 0.9953 0.0044 31,000 0.9957 0.0039 32,000 0.9961 0.0035 33,000 0.9964 0.0031 34,000 0.9967 0.0028 35,000 0.9970 0.0025 36,000 0.9972 0.0022 37,000 0.9974 0.0020 38,000 0.9976 0.0018 39,000 0.9978 0.0016 40,000 0.9979 0.0015 41,000 0.9981 0.0014 42,000 0.9982 0.0012 43,000 0.9983 0.0011 44,000 0.9984 0.0010 45,000 0.9985 0.0009 46,000 0.9986 0.0009 47,000 0.9987 0.0008 48,000 0.9988 0.0007 49,000 0.9988 0.0007 (From M. Q. Brewster, Thermal Radiative Transfer and Properties, John Wiley & Sons, New York, 1992. By permission of the publishers). P Q λ1 0 Eλ,b λ Figure 23.7 Emissivity at various wavelengths. 23.6 Emissivity and Absorptivity of Solid Surfaces 367
10. such surfaces at the same temperature over various wavelengths. The monochromatic emissivity at a certain wavelength, l1, is seen to be the ratio of two ordinates such as OQ and OP. That is el1 ¼ OQ OP which is equal to the monochromatic absorptivity al1 from radiation of a body at the same temperature. This is the direct consequence of Kirchhoff’s law. The total emissivity of the surface is given by the ratio of the shaded area shown in Figure 23.7 to that under the curve for the black-body radiation. Directional Emissivity. The cosine variation discussed previously, equation (23-5), is strictly applicable to radiation from a black surface but fulfilled only approximately by materials present in nature. This is due to the fact that the emissivity (averaged over all wavelengths) of actual surfaces is not a constant in all directions. Thevariation of emissivity of materials with the direction of emission can be conveniently represented by polar diagrams. If the cosine law is fulfilled, the distribution curves should take the form of semicircles. Most nonconductors have much smaller emissivities for emission angles in the neighbor- hood of 908 (see Figure 23.8). Deviation from the cosine law is even greater for many conductors (see Figure 23.9). The emissivity stays fairly constant in the neighborhood of the normal direction of emission; as the emission angle is increased, it first increases and then decreases as the former approaches 908. εq εq (c) (a) (g) (d) (b) (e) (f) 1.0 0.8 0.6 0.4 0.2 0 0.2 0.4 0.6 0.8 1.0 80° 60° 40° 20° 0° 20° 40° 60° 80° Figure 23.8 Emissivity variation with direction for nonconductors. (a) Wet ice. (b) Wood. (c) Glass. (d) Paper. (e) Clay. (f) Copper oxide. (g) Aluminum oxide. 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.04 0.02 0 0.06 0.08 0.10 0.12 0.14 εq εq 60° 0° 20° 40° 60° 20° 40° Ni polished Ni dull Cr Al Mn Figure 23.9 Emissivity variation with direction for conductors. 368 Chapter 23 Radiation Heat Transfer
11. The average total emissivity may be determined by using the following expression: e ¼ Z p/2 0 eu sin 2u du The emissivity, e, is, in general, different from the normal emissivity, en (emissivity in the normal direction). It has been found that for most bright metallic surfaces, the total emissivity is approximately 20% higher than en. Table 23.2 lists the ratio of e/en for a few representative bright metallic surfaces. For nonmetallic or other surfaces, the ratio e/en is slightly less than unity. Because of the inconsistency that can often be found among various sources, the normal emissivity values can be used, without appreciable error, for total emissivity (see Table 23.3). A few generalizations may be made concerning the emissivity of surfaces: (a) In general, emissivity depends on surface conditions. (b) The emissivity of highly polished metallic surfaces is very low. (c) The emissivity of all metallic surfaces increases with temperature. (d) The formation of a thick oxide layer and roughening of the surface increase the emissivity appreciably. (e) The ratio e/en is always greater than unity for bright metallic surfaces. The value 1.2 can be taken as a good average. (f) The emissivities of nonmetallic surfaces are much higher than for metallic surfaces and show a decrease as temperature increases. Table 23.2 The ratio e/en for bright metallic surfaces Aluminum, bright rolled (443 K) 0:049 0:039 ¼ 1:25 Nickel, bright matte (373 K) 0:046 0:041 ¼ 1:12 Nickel, polished (373 K) 0:053 0:045 ¼ 1:18 Manganin, bright rolled (392 K) 0:057 0:048 ¼ 1:19 Chromium, polished (423 K) 0:071 0:058 ¼ 1:22 Iron, bright etched (423 K) 0:158 0:128 ¼ 1:23 Bismuth, bright (353 K) 0:340 0:336 ¼ 1:08 Table 23.3 The ratio e/en for nonmetallic and other surfaces Copper oxide (3008F) 0.96 Fire clay (1838F) 0.99 Paper (2008F) 0.97 Plywood (1588F) 0.97 Glass (2008F) 0.93 Ice (328F) 0.95 23.6 Emissivity and Absorptivity of Solid Surfaces 369
12. (g) The emissivities of colored oxides of heavy metals like Zn, Fe, and Cr are much larger than emissivities of white oxides of light metals like Ca, Mg, and Al. Absorptivity. The absorptivity of a surface depends on the factors affecting the emissivity and, in addition, on the quality of the incident radiation. It may be remarked once again that Kirchhoff’s law holds strictly true under thermal equilibrium. That is, if a body at temperature T1 is receiving radiation from a black body also at temperature T1, then a ¼ e. For most materials, in the usual range of temperature encountered in practice (from room temperature up to about 1370 K) the simple relationship a ¼ e holds with good accuracy. However, if the incident radiation is that from a very-high-temperature source, say solar radiation ( $ 5800 K), the emissivity and absorptivity of ordinary surfaces may differ widely. White metal oxides usually exhibit an emissivity (and absorptivity) value of about 0.95 at ordinary temperature, but their absorptivity drops sharply to 0.15 if these oxides are exposed to solar radiation. Contrary to the above, freshly polished metallic surfaces have an emissivity value (and absorptivity under equilibrium conditions) of about 0.05. When exposed to solar radiation, their absorptivity increases to 0.2 or even 0.4. Under these latter circumstances a double-subscript notation, a1,2, may be employed, the first subscript referring to the temperature of the receiving surface and the second subscript to the temperature of the incident radiation. Gray surfaces. Like emissivity, the monochromatic absorptivity, al, of a surface may vary with wavelength. If al is a constant and thus independent of l, the surface is called gray. For a gray surface, the total average absorptivity will be independent of the spectral- energy distribution of the incident radiation. Consequently, the emissivity, e, may be used in place of a, even though the temperatures of the incident radiation and the receiver are not the same. Good approximations of a gray surface are slate, tar board, and dark linoleum. Table 23.4 lists emissivities, at various temperatures, for several materials. 23.7 RADIANT HEAT TRANSFER BETWEEN BLACK BODIES The exchange of energy between black bodies is dependent upon the temperature difference and the geometry with the geometry, in particular, playing a dominant role. Consider the two surfaces illustrated in Figure 23.10. The radiant energy emitted from a black surface at dA1 and received at dA2 is dq1!2 ¼ Ib1 cos u1 dV1À2 dA1 q1 q2 A1 A2 r dA1 dA2 Figure 23.10 Radiant heat transfer between two surfaces. 370 Chapter 23 Radiation Heat Transfer
13. Table 23.4 Normal total emissivity of various surfaces (Compiled by H. C. Hottel)y Surface T, 8Fz Emissivity A. Metals and their oxides Aluminum: Highly polished plate, 98.3% pure 440–1070 0.039–0.057 Commercial sheet 212 0.09 Oxidized at 11108F 390–1110 0.11–0.19 Heavily oxidized 200–940 0.20–0.31 Brass: Polished 100–600 0.10 Oxidized by heating at 11108F 390–1110 0.61–0.59 Chromium (see nickel alloys for Ni–Cr steels): Polished 100–2000 0.08–0.36 Copper Polished 212 0.052 Plate heated at 11108F 390–1110 0.57 Cuprous oxide 1470–2010 0.66–0.54 Molten copper 1970–2330 0.16–0.13 Gold: Pure, highly polished 440–1160 0.018–0.035 Iron and steel (not including stainless): Metallic surfaces (or very thin oxide layer) Iron, polished 800–1880 0.14–0.38 Cast iron, polished 392 0.21 Wrought iron, highly polished 100–480 0.28 Oxidized surfaces Iron plate, completely rusted 67 0.69 Steel plate, rough 100–700 0.94–0.97 Molten surfaces Cast iron 2370–2550 0.29 Mild steel 2910–3270 0.28 Lead: Pure (99.96%), unoxidized 260–440 0.057–0.075 Gray oxidized 75 0.28 Nickel alloys: Chromnickel 125–1894 0.64–0.76 Copper–nickel, polished 212 0.059 Nichrome wire, bright 120–1830 0.65–0.79 Nichrome wire, oxidized 120–930 0.95–0.98 Platinum: Pure, polished plate 440–1160 0.054–0.104 Strip 1700–2960 0.12–0.17 Filament 80–2240 0.036–0.192 Wire 440–2510 0.073–0.182 Silver: Polished, pure 440–1160 0.020–0.032 Polished 100–700 0.022–0.031 (Continued ) 23.7 Radiant Heat Transfer Between Black Bodies 371
14. Stainless steels: Polished 212 0.074 Type 310 (25 Cr; 20 Ni) Brown, splotched, oxidized from furnace service 420–980 0.90–0.97 Tin: Bright tinned iron 76 0.043 and 0.064 Bright 122 0.06 Commercial tin-plated sheet iron 212 0.07, 0.08 Tungsten: Filament, aged 80–6000 0.032–0.35 Filament 6000 0.39 Polished coat 212 0.066 Zinc: Commercial 99.1% pure, polished 440–620 0.045–0.053 Oxidized by heating at 7508F 750 0.11 B. Refractories, building materials, paints, and miscellaneous Asbestos: Board 74 0.96 Paper 100–700 0.93–0.94 Brick Red, rough, but no gross irregularities 70 0.93 Brick, glazed 2012 0.75 Building 1832 0.45 Fireclay 1832 0.75 Carbon: Filament 1900–2560 0.526 Lampblack-waterglass coating 209–440 0.96–0.95 Thin layer of same on iron plate 69 0.927 Glass: Smooth 72 0.94 Pyrex, lead, and soda 500–1000 0.95–0.85 Gypsum, 0.02 in. thick on smooth or blackened plate 70 0.903 Magnesite refractory brick 1832 0.38 Marble, light gray, polished 72 0.93 Oak, planed 70 0.90 Paints, lacquers, varnishes: Snow-white enamel varnish on rough iron plate 73 0.906 Black shiny lacquer, sprayed on iron 76 0.875 Black shiny shellac on tinned iron sheet 70 0.821 Black matte shellac 170–295 0.91 Black or white lacquer 100–200 0.80–0.95 Flat black lacquer 100–200 0.96–0.98 Oil paints, 16 different, all colors 212 0.92–0.96 A1 paint, after heating to 6208F 300–600 0.35 Table 23.4 Normal total emissivity of various surfaces (Compiled by H. C. Hottel)y Surface T, 8Fz Emissivity A. Metals and their oxides 372 Chapter 23 Radiation Heat Transfer
15. where dV1À2 is the solid angle subtended by dA2 as seen from dA1. Thus dV1À2 ¼ cos u2 dA2 r2 and as Ib1 ¼ Eb1 /p, the heat transfer from 1 to 2 is dq1!2 ¼ Eb1 dA1 n cos u1 cos u2 dA2 pr2 o The bracketed term is seen to depend solely upon geometry. In exactly the same manner the energy emitted by dA2 and captured by dA1, may be determined. This is dq2!1 ¼ Eb2 dA2 n cos u2 cos u1 dA1 pr2 o The net heat transfer between surfaces dA1 and dA2 is then simply dq1À2 net ¼ dq1Ð2 ¼ dq1!2 À dq2!1 or dq1Ð2 ¼ (Eb1 À Eb2 ) cos u1 cos u2 dA1 dA2 pr2 Integrating over surfaces 1 and 2, we obtain q1Ð2 ¼ (Eb1 À Eb2 ) Z A1 Z A2 cos u1 cos u2 dA2 dA1 pr2 the insertion of A1/A1 yields q1Ð2 ¼ (Eb1 À Eb2 )A1 1 A1 Z A1 Z A2 cos u1 cos u2 dA2 dA1 pr2 ! (23-13) Table 23.4 (Continued) Surface T, 8Fz Emissivity B. Refractories, building materials, paints, and miscellaneous Plaster, rough lime 50–190 0.91 Roofing paper 69 0.91 Rubber: Hard, glossy plate 74 0.94 Soft, gray, rough (reclaimed) 76 0.86 Water 32–212 0.95–0.963 y By permission from W. H. McAdams (ed.), Heat Transmission, Third Edition, McGraw-Hill Book Company, 1954. Table of normal total emissivity compiled by H. C. Hottel. z When temperatures and emissivities appear in pairs separated by dashes, they correspond and linear interpolation is permissible. The reader should note that the units of temperatures in Table 23.4 are F, in contrast to K, as has been used throughout the text thus far. Table 23.4 is presented as originally published by McAdams. 23.7 Radiant Heat Transfer Between Black Bodies 373
16. The bracketed term in the above equation is called the view factor F1À2. If we had used A2 as a reference, then the view factor would be F21 . Clearly, the net heat transfer is not affected by these operations, and thus A1F12 ¼ A2F21. This simple but extremely import- ant expression is called the reciprocity relationship. A physical interpretation of the view factor may be obtained from the following argument. As the total energy leaving surface A1 is EbA1, the amount of heat that sur- face A2 receives is Eb1 A1F12. The amount of heat lost by surface A2 is Eb2 A2, whereas the amount that reaches A1 is Eb2 A2F21. The net rate of heat transfer between A1 and A2 is the difference or Eb1 A1F12 À Eb2 A2F21.This may be arranged to yield (Eb1 À Eb2 )A1F12.Thus, the view factor F12 can be interpreted as the fraction of black-body energy leaving A1 which reaches A2 . Clearly the view factor cannot exceed unity. Before some specific view factors are examined, there are several generalizations worthy of note concerning view factors. 1. The reciprocity relation, A1F12 ¼ A2F21, is always valid. 2. The view factor is independent of temperature. It is purely geometric. 3. For an enclosure, F11 þ F12 þ F13 þ Á Á Á ¼ 1. In many cases the view factor may be determined without integration. An example of such a case follows. EXAMPLE 1 Consider the view factor between a hemisphere and a plane as shown in the figure. Determine the view factors F11 , F12 , and F21 . The view factor F21 is unity, as surface 2 sees only surface 1. For surface 1 we may write F11 þ F12 ¼ 1 and A1F12 ¼ A2F21. As F21 ¼ 1, A2 ¼ pr2 0 , and A1 ¼ 2pr2 0 , the above relations give F12 ¼ F21 A2 A1 ¼ (1) pr2 0 2pr2 0 ! ¼ 1 2 and F11 ¼ 1 À F12 ¼ 1 2 The view factor F12 can, in general, be determined by integration. As F12  1 A1 Z A1 Z A2 cos u1 cos u2 dA2 dA1 pr2 (23-14) this integration process becomes quite tedious, and the view factor for a complex geometry is seen to require numerical methods. In order to illustrate the analytical evaluation of view factors, consider the view factor between the differential area dA1 and the parallel plane A2 shown in Figure 23.11. The view factor FdA1A2 is given by FdA1 A2 ¼ 1 dA1 Z dA1 Z A2 cos u1 cos u2 dA2 dA1 pr2 Surface 1 Surface 2 r0 374 Chapter 23 Radiation Heat Transfer
17. and as A2 ) dA1 the view of dA2 from dA1 is independent of the position on dA1 , hence FdA1 A2 ¼ 1 p Z A2 cos u1cos u2 r2 dA2 Also, it may be noted that u1 ¼ u2 and cos u ¼ D/r, where r2 ¼ D2 þ x2 þ y2. The resulting integral becomes FdA1 A2 ¼ 1 p Z L1 0 Z L2 0 D2 dx dy (D2 þ x2 þ y2)2 or FdA1 A2 ¼ 1 2p ( L1 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi D2 þ L2 1 q tanÀ1 L2 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi D2 þ L2 1 q þ L2 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi D2 þ L2 2 q tanÀ1 L1 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi D2 þ L2 2 q ) (23-15) The view factor given by equation (23-15) is shown graphically in Figure 23.12. Figures 23.13–23.15 also illustrate some view factors for simple geometries. D/L2 , Dimension ratio D/L1 , Dimension ratio 0 0.5 1.0 1.5 2.0 2.5 3.0 0.03 0.04 0.05 0.06 0.08 0.10 0.12 0.16 0.24 3.5 4.0 4.5 5.0 5.5 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0.20 0.22 L1 and L2 are sides of rectangle, D is distance from dA to rectangle D dA L2 L1 FdA1 – A2 = 0.02 0.18 0.14 Figure 23.12 View factor for a surface element and a rectangular surface parallel to it. (From H. C. Hottel, ‘‘Radiant Heat Transmission,’’ Mech. Engrg., 52 (1930). By permission of the publishers.) L1 L2 A2 dA2 dA1 y x D r q2 q1 Figure 23.11 Differential area and parallel-finite area. 23.7 Radiant Heat Transfer Between Black Bodies 375
18. z y A2 A1 A1 = Area on which heat- transfer equation is based Y = y/x Z = z/x View factor F12 0 1.0 2.0 3.0 4.0 Dimension ratio, Z 10 8 6 0 0.10 0.20 0.30 0.40 0.50 Asymptotes Scale changes here Dimension ratio, Y = 0.1 0.2 0.3 0.4 0.6 0.8 1.0 1.5 3.0 8.0 2.0 4.0 6.0 Y = 0.1 x Figure 23.13 View factor for adjacent rectangles in perpendicular planes. (From H. C. Hottel, ‘‘Radiant Heat Transmission,’’ Mech. Engrg., 52 (1930). By permission of the publishers.) View factor F or F – Ratio, 0 1 2 1 2 3 4 5 6 7 8 3 4 5 6 7 0 0.2 0.4 0.6 0.8 1.0 smaller side or diameter distance between planes Radiation between parallel planes, directly opposed 1-2-3-4 Direct radiation between the planes, F; 5-6-7-8 Planes connected by nonconducting but reradiating wall, F; 1, 5 Disks; 3, 7 2:1 Rectangle; 2, 6 Squares; 4, 8 long, narrow rectangles – Figure 23.14 View factors for equal and parallel squares, rectangles, and disks. The curves labeled 5, 6, 7, and 8 allow for continuous variation in the sidewall temperatures from top to bottom. (From H. C. Hottel, ‘‘Radiant Heat Transmission,’’ Mech. Engrg., 52 (1930). By permission of the publishers.) 376 Chapter 23 Radiation Heat Transfer
19. EXAMPLE 2 Determine theview factor from a1 m square toa parallel rectangular plane10 m by12 m centered 8 m above the 1 m square. The smaller area may be considered a differential area, and Figure 23.12 may be used. The 10 m by 12 m area may be divided into four 5 m by 6 m rectangles directly over the smaller area. Thus, the total view factor is the sum of the view factors to each subdivided rectangle. Using D ¼ 8, L1 ¼ 6, L2 ¼ 5, we find that the view factor from Figure 23.12 is 0.09. The total view factor is the sum of the view factors or 0.36. View-Factor Algebra View factors between combinations of differential- and finite-size areas have been ex- pressed in equation form thus far. Some generalizations can be made that will be useful in evaluating radiant energy exchange in cases that, at first glance, seem quite difficult. In an enclosure all energy leaving one surface, designated i, will be incident on the other surfaces that it can ‘‘see.’’ If there are n surfaces in total, with j designating any surface that receives energy from i, we may write å n j¼1 Fij ¼ 1 (23-16) A general form of the reciprocity relationship may be written as AiFij ¼ AjFji (23-17) these two expressions form the basis of a technique designated view-factor algebra. A simplified notation will be introduced, using the symbol Gij , defined as Gij  AiFij D/r1 D 0.1 0.2 0.4 0.6 0.8 1 2 6 5 4 3 2 1.0 0.8 0.6 0.5 0.4 0.3 r2/O = 1.2 0.2 1.5 8 10 10 8 6 4 20 ϱ FA1 – A2 0.2 0.4 0.6 0.8 1.0 0 r1 r2 1 2 Figure 23.15 View factors for parallel opposed circular disks of unequal size. 23.7 Radiant Heat Transfer Between Black Bodies 377
20. Equations (23-16) and (23-17) may now be written as åGij ¼ Ai (23-18) Gij ¼ Gji (23-19) The quantity Gij is designated the geometric flux. Relations involving geometric fluxes are dictated by energy conservation principles. Some special symbolism will now be explained. If surface 1 ‘‘sees’’ two surfaces, designated 2 and 3, we may write G1À(2þ3) ¼ G1À2 þ G1À3 (23-20) This relation says simply that the energy leaving surface 1 and striking both surfaces 2 and 3 is the total of that striking each separately. Equation (23-20) can be reduced further to A1F1À(2þ3) ¼ A1F12 þ A1F13 or F1À(2þ3) ¼ F12 þ F13 A second expression, involving four surfaces, is reduced to G(1þ2)À(3þ4) ¼ G1À(3þ4) þ G2À(3þ4) which decomposes further to the form G(1þ2)À(3þ4) ¼ G1À3 þ G1À4 þ G2À3 þ G2À4 Examples of how view-factor algebra can be used follow. EXAMPLE 3 Determine the view factors, F1À2, for the finite areas shown. 3 3 m 2 1 3 3 m 3 m 2 m 2 m 5 m 2 m 2 4 1 2 m 2 m Inspection indicates that, in case (a), view factors F2À3 and F2Àð1þ3Þ can be read directly from Figure 23.13. The desired view factor, F1À2, can be obtained using view-factor algebra in the following steps. G2À(1þ3) ¼ G2À1 þ G2À3 Thus, G2À1 ¼ G2À(1þ3) À G2À3 378 Chapter 23 Radiation Heat Transfer
21. Finally, by reciprocity, we may solve for F1À2 according to G1À2 ¼ G2À1 ¼ G2À(1þ3) À G2À3 A1F1À2 ¼ A2F2À(1þ3)À A2F2À3 F1À2 ¼ A2 A1 ½F2À(1þ3) À F2À3 Š From Figure 23.13 we read F2À(1þ3) ¼ 0:15 F2À3 ¼ 0:10 Thus, for configuration (a), we obtain F1À2 ¼ 5 2 (0:15 À 0:10) ¼ 0:125 Now, for case (b), the solution steps are G1À2 ¼ G1À(2þ4) À G1À4 which may be written as F1À2 ¼ F1À(2þ4) À F1À4 The result from part (a) can now be utilized to write F1À(2þ4) ¼ A2 þ A4 A1 ½F(2þ4)À(1þ3) À F(2þ4)À3 Š F1À4 ¼ A4 A1 ½F4À(1þ3) À F4À3 Š Each of the view factors on the right side of these two expressions may be evaluated from Figure 23.13; the appropriate values are F(2þ4)À(1þ3) ¼ 0:15 F4À(1þ3) ¼ 0:22 F(2þ4)À3 ¼ 0:10 F4À3 ¼ 0:165 Making these substitutions, we have F1À(2þ4) ¼ 5 2 (0:15 À 0:10) ¼ 0:125 F1À4 ¼ 3 2 (0:22 À 0:165) ¼ 0:0825 The solution to case (b) now becomes F1À2 ¼ 0:125 À 0:0825 ¼ 0:0425 23.8 RADIANT EXCHANGE IN BLACK ENCLOSURES As pointed out earlier, a surface that views n other surfaces may be described according to F11 þ F12 þ Á Á Á þ F1i þ Á Á Á þ F1n ¼ 1 or å n i¼1 F1i ¼ 1 (23-21) 23.8 Radiant Exchange in Black Enclosures 379
22. Obviously, the inclusion of A1 with equation (23-12) yields å n i¼1 A1F1i ¼ A1 (23-22) Between any two black surfaces the radiant heat exchange rate is given by q12 ¼ A1F12(Eb1 À Eb2 ) ¼ A2F21(Eb1 À Eb2 ) (23-23) For surface 1 and any other surface, designated i, in a black enclosure the radiant exchange is given as q1i ¼ A1F1i(Eb1 À Ebi ) (23-24) For an enclosure where surface 1 views n other surfaces, we may write for the net heat transfer with 1, q1Àothers ¼ å n i¼1 q1i ¼ å n i¼1 A1F1i(Eb1 À Ebi ) (23-25) Equation (23-25) can be thought of as an analog to Ohm’s law where the quantity of transfer, q; the potential driving force, Eb1 À Ebi ; and the thermal resistance, 1/A1F1i; have electrical counterparts I, DV, and R, respectively. Figure 23.16 depicts the analogous electrical circuits for enclosures with three and four surfaces, respectively. The solution to a three-surface problem, that is, to find q12 , q13 , q23 , although somewhat tedious, can be accomplished in reasonable time. When analyzing enclosures with four or more surfaces, an analytical solution becomes impractical. In such situations one would resort to numerical methods. 23.9 RADIANT EXCHANGE WITH RERADIATING SURFACES PRESENT The circuit diagrams shown in Figure 23.16 show a path to ground at each of the junctions. The thermal analog is a surface that has some external influence whereby its temperature is maintained at a certain level by the addition or rejection of energy. Such a surface is in contact with its surroundings and will conduct heat by virtue of an imposed temperature difference across it. 1 A2F2 – 3 Eb3 Eb3 Eb2 Eb1 Eb2 Eb4 Eb1 1 A2F2 – 3 1 A1F1 – 3 1 A1F1 – 2 1 A1F1 – 4 1 A1F1 – 3 1 A2F2 – 4 1 A1F1 – 2 1 A3F3 – 4 Figure 23.16 Radiation analogs. 380 Chapter 23 Radiation Heat Transfer
23. In radiation applications, we encounter surfaces that effectively are insulated from the surroundings. Such a surface will reemit all radiant energy that is absorbed— usually in a diffuse fashion. These surfaces thus act as reflectors and their temperatures ‘‘float’’ at some value that is required for the system to be in equilibrium. Figure 23.17 shows a physical situation and the corresponding electric analog for a three-surface enclosure with one being a nonabsorbing reradiating surface. Evaluating the net heat transfer between the two black surfaces, q1À2, we have q12 ¼ Eb1 À Eb2 Requiv ¼ A1F12 þ 1 1/A1F13 þ 1/A2F23 ! (Eb1 À Eb2 ) ¼ A1 F12 þ 1 1/F13 þ A1/A2F23 ! (Eb1 À Eb2 ) ¼ A1F12(Eb1 À Eb2 ) (23-26) The resulting expression, equation (23-62), contains a new term, F12, the reradiating view factor. This new factor, F12, is seen equivalent to the square-bracketed term in the previousexpression,whichincludesdirectexchange betweensurfaces1and2,F12 , plusterms that account for the energy that is exchanged between these surfaces via the intervening reradiating surface. It is apparent that F12 will always begreater than F12 . Figure 23.14 allows reradiating view factors to be read directly for some simple geometries. In other situations where curves such as in this figure are not available, the electrical analog may be used by the simple modification that no path to ground exists at the reradiating surface. 23.10 RADIANT HEAT TRANSFER BETWEEN GRAY SURFACES In the case of surfaces that are not black, determination of heat transfer becomes more involved. For gray bodies, that is, surfaces for which the absorptivity and emissivity are independent of wavelength, considerable simplifications can be made. The net heat transfer from the surface shown in Figure 23.18 is determined by the difference between the radiation leav- ing the surface and the radiation incident upon the surface. The radiosity, J, is defined as the rate at which radiation leaves a given surface per unit area. The irradia- tion, G, is defined as the rate at which radiation is incident on a surface per unit area. For a gray body, the radiosity, irradiation, and the total emissive power are related by J ¼ rG þ eEb (23-27) Eb2 Eb1 1 A2 F2 – 3 1 A1 F1 – 3 1 A1 F1 – 2 3 Figure 23.17 Incident radiation, G Radiation leaving surface, J Figure 23.18 Heat transfer at a surface. 23.10 Radiant Heat Transfer Between Gray Surfaces 381
24. where r is the reflectivity and e is the emissivity. The net heat transfer from a surface is qnet A ¼ J À G ¼ eEb þ rG À G ¼ eEb À (1 À r)G (23-28) In most cases it is useful to eliminate G from equation (23-28). This yields qnet A ¼ eEb À (1 À r) (J À eEb) r As a þ r ¼ 1 for an opaque surface qnet A ¼ eEb r À aJ r (23-29) When the emissivity and absorptivity can be considered equal, an important simpli- fication may be made in equation (23-29). Setting a ¼ e, we obtain qnet ¼ Ae r (Eb À J) (23-30) which suggests an analogy with Ohm’s law, V ¼ IR, where the net heat leaving a surface can be thought of in terms of a current, the difference Eb À J may be likened to a potential difference, and the quotient r/eA may be termed a resistance. Figure 23.19 illustrates this analogy. Now the net exchange of heat via radiation between two surfaces will depend upon their radiosities and their relative ‘‘views’’ of each other. From equation (23-17) we may write q1Ð2 ¼ A1F12(J1 À J2) ¼ A2F21(J1 À J2) We may now write the net heat exchange in terms of the different ‘‘resistances’’ offered by each part of the heat transfer path as follows: If surfaces 1 and 2 view each other and no others then each of the qs in the previous equations is equivalent. In such a case an additional expression for q can be written in terms of the overall driving force, Eb1 À Eb2 . Such an expression is q ¼ Eb1 À Eb2 r1 /A1e1 þ 1/A1F12 þ r2 /A2e2 (23-31) where the terms in the denominator are the equivalent resistances due to the characteristics of surface 1, geometry, and the characteristics of surface 2, respectively. The electrical analog to this equation is portrayed in Figure 23.20. Eb J (Eb – J) r/εA qnet = r/εA Figure 23.19 Electrical analogy for radiation from a surface. Rate of heat leaving surface 1: q ¼ A1e1 r1 (Eb1 À J1) Rate of heat exchange between surfaces 1 and 2: q ¼ A1F12(J1 À J2) Rate of heat received at surface 2: q ¼ A2e2 r2 (J2 À Eb2 ) 382 Chapter 23 Radiation Heat Transfer
25. The assumptions required to use the electrical analog approach to solve radiation problems are the following: 1. Each surface must be gray, 2. Each surface must be isothermal, 3. Kirchhoff’s law must apply, that is, a ¼ e, 4. There is no heat absorbing medium between the participating surfaces. Examples 4 and 5, which follow, illustrate features of gray-body problem solutions. EXAMPLE 4 Two parallel gray surfaces maintained at temperatures T1 and T2 view each other. Each surface is sufficiently large that theymay be considered infinite. Generate an expression forthe net heat transfer between these surfaces. A simple series electrical circuit is useful in solving this problem. The circuit and important quantities are shown here. J1 J2 Eb1 = sT 1 4 R1 = r 1 /A1 e 1 R3 = r 2 /A2 e 2 R2 = 1/A1 F12 Eb2 = sT2 4 Utilizing Ohm’s law, we obtain the expression q12 ¼ Eb1 À Eb2 å R ¼ s À T 4 1 À T 4 2 Á r1 A1e1 þ 1 A1F12 þ r2 A2e2 Now, noting that for infinite parallel planes A1 ¼ A2 ¼ A and F12 ¼ F21 ¼ 1 and writing r1 ¼ 1 À e1 and r2 ¼ 1 À e2, we obtain the result q12 ¼ AsðT4 1 À T4 2 Þ 1 À e1 e1 þ 1 þ 1 À e2 e2 ¼ AsðT4 1 À T4 2 Þ 1 e1 þ 1 e2 À1 EXAMPLE 5 Two parallel planes measuring 2 m by 2 m are situated 2 m apart. Plate 1 is maintained at a temperature of 1100 K and plate 2 is maintained at 550 K. Determine the net heat transfer from the high temperature surface under the following conditions: (a) the plates are black and the surroundings are at 0 K and totally absorbing; (b) the plates are black and the walls connecting the plates are reradiating; Eb2 Eb1 J1 J2 r2 e2A2 R = r2 A1F12 R = r1 e1A1 R = Figure 23.20 Equivalent network for gray-body relations between two surfaces. 23.10 Radiant Heat Transfer Between Gray Surfaces 383
26. (c) the plates are gray with emissivities of 0.4 and 0.8, respectively, with black surroundings at 0 K. Analog electrical circuits for parts (a), (b), and (c) are shown in Figure 23.21. Heat flux evaluations will require evaluating the quantities F12 ; F1R, and F12. The appropriate values are F12 ¼ 0:20 from Figure 23.14 F12 ¼ 0:54 from Figure 23.14 and F1R ¼ 1 À F12 ¼ 0:80 Part (a). The net rate of heat leaving plate 1 is q1 net ¼ q12 þ q1R ¼ A1F12(Eb1 À Eb2) þ A1F1REb1 ¼ (4 m2)(0:2)(5:676 Â 10À8 W/m2 : K4)(11004 À 5504)K4 þ (4 m2)(0:8)(5:676 Â 10À8 W=m2 : K4)(1100 K)4 ¼ 62; 300 W þ 266; 000 W ¼ 328:3 kW J1 J1 Eb2 Eb1 1 A2 F2R (a) Eb2 Eb2 Eb1 ER Eb1 F1R = 1 – F12 = 0.80 EbR = 0 R = 1 A1 F1R R = 1 AR FR2 R = 1 A1 F1R R = 1 AR FR1 = 1 A2 F2R = 1 A1 F12 R = 1 A2 F21 = 1 A2 F2R r2 A2 e2 (c) R = 1 A1 F1R R = 1 A1 F12 R = R = r1 A1 e1 R = 1 A2 F21 = (b) 1 A1 F12 R = 1 A2 F21 = Figure 23.21 Equivalent circuits for example 3. 384 Chapter 23 Radiation Heat Transfer
27. Part (b). When reradiating walls are present the heat flux becomes q12 ¼ (Eb1 À Eb2) A1F12 þ 1 1 A1F1R þ 1 A2F2R 2 6 6 4 3 7 7 5 and, since A1 ¼ A2 and F1R = F2R q12 ¼ (Eb1 À Eb2)A1 F12 þ F1R 2 ! Since F12 þ F1R ¼ 1, the bracketed term is evaluated as F12 þ F1R 2 ¼ 0:2 þ 0:8 2 ¼ 0:6 and, finally, the heat flux is q12 ¼ (4 m4)(5:678 Â 10À4 W/m2 : K4)(11004 À 5504)K4(0:6) ¼ 187 kW We should note that an equivalent expression for the heat flux is q12 ¼ A1F12(Eb1 À Eb2) and, using the value F12 ¼ 0:54, from Figure 23.14, the result would be q12 ¼ 168:3 kW This alternate result is the more accurate in that the values of F12 plotted in Figure 23.14 allow for the temperatures along the reradiating walls to vary from T1 to T2. The use of the analog circuit considers the radiating surface to be a constant temperature. Such an assumption, in this example, leads to an error of approximately 11%. Part (c). An evaluation of the circuit shown in Figure 23.2(C) yields q1;out ¼ 131:3 kW. The concepts related to the quantities, radiosity, and irradiation are particularly useful in generalizing the analysis of radiant heat exchange in an enclosure containing any number of surfaces. The formalism to be developed in this section is directly applicable for solution by numerical methods. For a representative surface having area, Ai, in an enclosure bounded by n surfaces, equations (20–28) and (20–30) can be written as qi ¼ Ebi À Ji ri /Aiei ¼ Ai(Ji À Gi) (23-32) where qi is the net rate of heat transfer leaving surface i. The irradiation, Gi , can be expressed as AiGi ¼ å n j¼1 JjAjFji (23-33) or, using reciprocity, as AiGi ¼ Ai å n j¼1 JjFij (23-34) 23.10 Radiant Heat Transfer Between Gray Surfaces 385
28. Combining equations (23-32) and (23-34) we obtain qi ¼ Ai Ji À å n j¼1 FijJj ! (23-35) ¼ Aiei ri Ebi À Aiei ri Ji (23-36) We can now write the two basic expressions for a general surface in an enclosure. If the surface heat flux is known, equation (23-35) can be expressed in the form Ji À å n j¼1 FijJj ¼ qi Ai or Ji(1 À Fii) À å n j¼1 j 6¼ i FijJj ¼ qi Ai (23-37) and, if the temperature at surface i is known, equations (23-35) and (23-36) yield Aiei ri (Ebi À Ji) ¼ Ai Ji À å n j¼1 FijJj ! ¼ Ai Ji(1 À Fii) À å n j¼1 j 6¼ i FijJj 2 4 3 5 and, finally 1 À Fii þ ei ri   Ji À å n j¼1 j 6¼ i FijJj ¼ ei ri Ebi (23-38) Equations (23-37) and (23-38) comprise the algorithm for evaluating quantities of interest in a many-surface enclosure. The former applies to a surface of known heat flux; the latter is written when the surface temperature is specified. In these two equations the terms involving the view factor, Fii, have been separated out of the summation. This quantity, Fii will have a nonzero value in those cases when surface i ‘‘sees’’; itself, i.e., it is concave. In most cases Fii will be 0. When writing equation (23-37) or (23-38) for each surface in an enclosure a series of n simultaneous equations is generated, involvingthe unknowns Ji.This set of equations can be represented in matrix form as [A][J] ¼ [B] (23-39) where [A] is the coefficient matrix, [B] is a column matrix involving the right-hand sides of equations (23-37) and (23-38), and [J] is a column matrix of the unknowns, Ji . The solution for the Ji then proceeds according to [J] ¼ [C][B] (23-40) where [C] ¼ [A]À1 (23-41) is the inverse of the coefficient matrix. Example 6 illustrates the application of this approach. 386 Chapter 23 Radiation Heat Transfer
29. EXAMPLE 6 Solve the problem posed in example 5 using the methods developed in this section. For this case n ¼ 3 and the problem formulation will involve 3 equations—one for each surface. Part (a). Each of the surfaces is at a known temperature in this case, thus equation (23-38) applies. The following conditions are known: T1 ¼ 1100 K T2 ¼ 550 K T3 ¼ 0 K F11 ¼ 0 F21 ¼ 0:2 F31 ¼ 0:2 F12 ¼ 0:2 F23 ¼ 0 F32 ¼ 0:2 F13 ¼ 0:8 F23 ¼ 0:8 F33 ¼ 0:6 e1 ¼ 1 e2 ¼ 1 e3 ¼ 1 We can write the following: 1 þ e1 r1   J1 À ½F12J2 þ F13J3 Š ¼ e1 r1 Eb1 1 þ e2 r2   J2 À ½F21J1 þ F23J3 Š ¼ e2 r2 Eb2 1 À F33 þ e3 r3   J3 À ½F31J1 þ F32J2 Š ¼ e3 r3 Eb3 which, for the given conditions, reduce to J1 ¼ Eb1 ¼ sT4 1 J2 ¼ Eb2 ¼ sT4 2 J3 ¼ 0 The net heat leaving plate 1 is thus, according to equation (23-37), equal to q1 ¼ A1[J1 À F12J2] ¼ A1[sT4 1 À 0:2sT4 2 ] ¼ 4 m2(5:676  10À8 W/m Á K4)[11004 À 0:2(550)4]K4 ¼ 328:3 kW Part (b). Values of Ti and Fij remain the same. The only change from part (a) is that e3 ¼ 0. The set of equations, applying to the three surfaces are again 1 þ e1 r1   J1 À ½F12J2 þ F13J3 Š ¼ e1 r1 Eb1 1 þ e2 r2   J2 À ½F21J1 þ F23J3 Š ¼ e2 r2 Eb2 1 À F33 þ e3 r3   J3 À ½F31J1 þ F32J2 Š ¼ e3 r3 Eb3 as before. Substituting values for Ti, Fij, and ei, we have J1 ¼ Eb1 ¼ sT4 1 J2 ¼ Eb2 ¼ sT4 2 (1 À F33)J3 À F31J1 À F32J2 ¼ 0 23.10 Radiant Heat Transfer Between Gray Surfaces 387
30. The expression for qi is q1 ¼ A1[J1 À F12J2 À F13J3] ¼ A1 J1 À F12J2 À F13 1 À F33 (F31J1 þ F32J2) ! ¼ A1 J1 1 À F13F31 1 À F33   À J2 F12 þ F13F32 1 À F33   ! and, with numerical values inserted, we obtain q1 ¼ 4(5:676 Â 10À8) (1100)4 1 À (0:8)(0:2) 1 À 0:6 ! À (550)4 0:2 þ (0:8)(0:2) 1 À 0:6 ! & ' ¼ 187:0 kW Part (c). Values of Ti and Fij remain the same. Emissivities are e1 ¼ 0:4 e2 ¼ 0:8 e3 ¼ 1 Equations for the three surfaces are, again 1 þ e1 r1   J1 À [F12J2 þ F13J3] ¼ e1 r1 Eb1 1 þ e2 r2   J2 À [F21J1 þ F23J3] ¼ e2 r2 Eb2 1 À F33 þ e3 r3   J3 À [F31J1 þ F32J2] ¼ e3 r3 Eb3 which become 1 þ 0:4 0:6   J1 À (F12J2 þ F13J3) ¼ 0:4 0:6 Eb1 1 þ 0:8 0:2   J2 À (F21J1 þ F23J3) ¼ 0:8 0:2 Eb2 J3 ¼ 0 We now have 1:67J1 À 0:2J2 ¼ 0:67Eb1 5J2 À 0:2J1 ¼ 4Eb2 Solving these two equations simultaneously for J1 and J2 we get J1 ¼ 33 900 W/m2 J2 ¼ 5510 W/m2 and the value for qi is evaluated as q1 ¼ 33 900 À 5510 5 ! 4 ¼ 131:2 kW 23.11 RADIATION FROM GASES So far, the interaction of radiation with gases has been neglected. Gases emit and absorb radiation in discrete energy bands dictated by the allowed energy states within the molecule. As theenergyassociatedwith,say,thevibrationalorrotationalmotionofamoleculemayhaveonly certainvalues,itfollowsthattheamountofenergyemittedorabsorbedbyamoleculewillhavea 388 Chapter 23 Radiation Heat Transfer
31. frequency, n ¼ DE/h, corresponding to the difference in energy DE between allowed states. Thus, while the energy emitted by a solid will comprise a continuous spectrum, the radiation emitted and absorbed by a gas will be restricted to bands. Figure 23.22 illustrates the emission bands of carbon dioxide and water vapor relative to black-body radiation at 15008F. The emission of radiation for these gases is seen to occur in the infrared region of the spectrum. For nonluminous gases, the inert gases and diatomic gases of symmetrical composition such as O2 , N2 , and H2 may be considered transparent to thermal radiation. Important types of media that absorb and emit radiations are polyatomic gases such as CO2 and H2 O and unsymmetrical molecules such as CO. These gases are also associated with the products of combustion of hydrocarbons. The determination of the absorption and emission of radiation is very difficult, as it involves the temperature, composition, density, and geometry of the gas. There are several simplifications that allow estimation of radiation in gases to be made in a straightforward manner. These idealizations are as follows: 1. The gas is in thermodynamic equilibrium. The state of the gas may therefore be characterized locally by a single temperature. 2. The gas may be considered gray. This simplification allows the absorption and emission of radiation to be characterized by one parameter as a ¼ e for a gray body. In the range of temperatures associated with the products of hydrocarbon combustion, the gray gas emissivities of H2 O and CO2 may be obtained from the results of Hottel. A hemispherical mass of gas at 1 atm pressure was used by Hottel to evaluate the emissivity. While the graphs apply strictly only to a hemispherical gas mass of radius L, other shapes can be treated by consideration of a mean beam length L as given in Table 23.5. For geometries not covered in the table, the mean beam length may be approximated by the relation L ¼ 3:4 (volume)/(surface area). Figure 23.23 gives the emissivity of a hemispherical mass of water vapor at 1 atm total pressure and near-zero partial pressure as a function of temperature and the product pwL, where pw is the partial pressure of the water vapor. For pressures other than atmospheric, Figure 23.24 gives the correction factor, Cw, which is the ratio of the emissivity at total pressure P to the emissivity at a total pressure of 1 atm. Figures 23.25 and 23.26 give the corresponding data for CO2 . From Figure 23.22, it may be seen that the emission bands of CO2 and H2 O overlap. When both carbon dioxide and water vapor are present, the total emissivity may be determined from the relation etotal ¼ eH2 O þ eCO2 À De where De is given in Figure 23.27. Intensity Wavelength, in microns 0 2 4 6 8 10 14 16 18 Carbon dioxide Water vapor 2000°R 12 Figure 23.22 Emission bands of CO2 and H2 O. 23.11 Radiation From Gases 389
32. Table 23.5 Mean beam length, L, for various geometriesy Shape L Sphere 2 3 Â diameter Infinite cylinder 1 Â diameter Space between infinite parallel planes 1:8 Â distance between planes Cube 2 3 Â side Space outside infinite bank of tubes with centers on equilateral triangles; tube diameter equals clearance 2:8 Â clearance Same as preceding except tube diameter equals one-half clearance 3:8 Â clearance yFrom H. C. Hottel, ‘‘Radiation,’’ Chap. IV in W. H. McAdams (ed.), Heat Transmission, Third Edition, McGraw-Hill Book Company, New York, 1964. By permission of the publishers. Absolute temperature, in °R Cas emissivity, 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 0.007 0.008 0.009 0.010 0.012 0.015 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.12 0.15 0.2 0.3 0.4 0.5 0.6 0.7 Pw L = 20 atm ft 0.2 0.3 0.5 0.8 1.2 0.4 0.6 1.0 1.5 10 0.005 0.007 0.010 0.012 0.015 0.025 0.035 0.02 0.03 0.04 0.05 0.06 0.08 0.12 0.07 0.10 0.15 0.25 2 3 5 Figure 23.23 Emissivity of water vapor at one atmosphere total pressure and near-zero partial pressure. 390 Chapter 23 Radiation Heat Transfer
33. Cw (P + pw ), in atm 0 0.2 0.4 0.6 0.8 1.0 1.2 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 1 2 pw L = 10.0 atm ft pw L = 10.0 atm ft 0.25 0.50 1.0 2.5 5.0 Figure 23.24 Correction factor for converting emissivity of H2 O at one atmosphere total pressure to emissivity at P atmospheres total pressure. Absolute temperature, in °R 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.010 0.015 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.10 0.15 0.2 0.3 0.09 0.001 0.002 0.003 0.004 0.006 0.010 0.015 0.03 0.06 0.10 0.2 0.4 0.8 1.5 3.0 0.005 0.008 0.02 0.04 0.08 0.15 0.3 0.6 1.0 2.0 pC L = 5.0 atm ft Figure 23.25 Emissivity of CO2 at one atmosphere total pressure and near-zero partial pressure. 23.11 Radiation From Gases 391
34. The results presented here for the gray gas are gross simplifications. For a more complete treatment, textbooks by Siegel and Howell,2 Modest,3 and Brewster4 present the fundamentals of nongray-gas radiation, along with extensive bibliographies. 23.12 THE RADIATION HEAT-TRANSFER COEFFICIENT Frequently in engineering analysis, convection and radiation occur simultaneously rather than as isolated phenomena. An important approximation in such cases is the linearization of the radiation contribution so that htotal ¼ hconvection þ hradiation (23-42) pw pc + pw 0 0.2 0.4 0.6 0.8 1.0 pw pc + pw 0 0.2 0.4 0.6 0.8 1.0 pw pc + pw 0 0.2 0.4 0.6 0.8 1.0 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.20 0.30 3.0 2.0 1.5 1.0 0.2 0.3 0.5 0.75 1.0 1.5 2.0 3.0 0.75 0.50 0.30 0.20 0.50 0.75 1.0 1.5 2 3 1700°F and above pc L + pw L = 5 atm ft pc L + pw L = 5 atm ft pc L + pw L = 5 atm ft 1000°F 260°F Figure 23.27 Correction to gas emissivity due to spectral overlap of H2 O and CO2 . Cc Total pressure P, in atm 0.05 0.1 0.2 0.3 0.5 0.8 1.0 2.0 5.0 3.0 0.08 0.3 0.4 0.5 0.6 0.8 1.0 2.0 1.5 0.12 0.5 2.5 0.05 0.25 1.0 pc L = 0-0.02 pc L = 2.5 atm ft 0.5 0.25 0.12 0-0.02 0.05 1.0 Figure 23.26 Correction factor for converting emissivity of CO2 at one atmosphere total pressure to emissivity at P atmospheres total pressure. 2 R. Siegel and J. R. Howell, Thermal Radiation Heat Transfer, 3rd Edition, Hemisphere Publishing Corp., Washington, 1992. 3 M. F. Modest, Radiative Heat Transfer, McGraw-Hill, New York, 1993. 4 M. Q. Brewster, Thermal Radiative Transfer and Properties, J. Wiley and Sons, New York, 1992. 392 Chapter 23 Radiation Heat Transfer
35. where hr  qr/A1 (T À TR) ¼ F1À2 s(T4 À T4 2 ) T À TR ! (23-43) Here TR is a reference temperature, and T1 and T2 are the respective sur- face temperatures. In effect, equation (23-43) represents a straight-line approximation to the radiant heat transfer as illustrated in Figure 23.28. The factor, F, accounts for geometry and surface condition of the radiating and absorbing surface. By constructing a tangent to the relation curve at T ¼ T1, the following relations are obtained for hr and TR: hr ¼ 4sT3 1 F1À2 (23-44) and TR ¼ T1 À T4 1 À T4 2 4T3 1 (23-45) 23.13 CLOSURE Radiation heat transfer has been considered in this chapter. Radiant energy transfer is associated with the portion of the electromagnetic spectrum between 0.1 and 100 mm, which is generally referred to as the thermal band. The fundamental rate equation for thermal radiation, introduced in Chapter 15, is designated the Stefan–Boltzmann equation; it is expressed as Eb ¼ sT4 (23-12) where Eb is the black body emissive power, T is the absolute temperature, and s is the Stefan–Boltzmann constant, having units of W/m2 Á K4 in the SI system. Modifications to this relationship were made for nonblack surfaces and for geometric relationships between multiple surfaces in view of each other. The presence of absorbing and emitting gases between surfaces was also examined. The gases of principle interest in this regard are water vapor and carbon dioxide. T – T2 hr (T – TR ) 12 s(T 4 – T 2 4) T1 – T2 qr A1 Ᏺ Figure 23.28 Tangent approximation for hr . PROBLEMS 23.1 The sun is approximately 93 million miles distant from Earth, and its diameter is 860,000 miles. On a clear day solar irradiation at Earth’s surface has been measured at 360 Btu/h ft2 and an additional 90 Btu/h ft2 are absorbed by Earth’s atmo- sphere. With this information, estimate the sun’s effective sur- face temperature. 23.2 A greenhouse is constructed of silica glass that is known to transmit 92% of incident radiant energy between wavelengths of 0.35 and 2.7 mm. The glass may be considered opaque for wavelengths above and below these limits. Considering the sun to emit as a black body at 5800 K, deter- mine the percent of solar radiation that will pass through the glass. Problems 393
36. If the plants on the inside of the greenhouse have an average temperature of 300 K, and emit as a black body, what fraction of their emitted energy will be transmitted through the glass? 23.3 A tungsten filament, radiating as a gray body, is heated to a temperature of 40008R. At what wavelength is the emissive power maximum? What portion of the total emission lies within the visible-light range, 0.3 to 0.75 mm? 23.4 A radiation detector, oriented as shown in the sketch, is used to estimate heat loss through an opening in a furnace wall. The opening in this case is circular with a diameter of 2.5 cm. The detector has a surface area of 0.10 cm2 and is located 1 m from the furnace opening. Determine the amount of radiant energy reaching the detector under two conditions: a. the detector has a clear view of the opening; b. the opening is covered by a semitransparent material with spectral transmissivity given by tl ¼ 0:8 for 0 l 2 mm tl ¼ 0 for 2 mm < l < 1 Furnace 30° Detector Opening diameter = 25 cm T = 1500 K 23.5 The distribution of solar energy, incident on Earth, can be approximated as being from a black body at 5800 K. Two kinds of glass, plain and tinted, are being considered for use in windows. The spectral transmissivity for these two glasses is approximated as plain glass: tl ¼ 0 for 0 < l < 0:3 mm 0:9 for 0:3 < l < 2:5 m 0 for 2:5 mm < l tinted glass: tl ¼ 0 for 0 < l < 0:5 mm 0:9 for 0:5 < l < 1:5 m 0 for 1:5 mm < l Compare the fraction of incident solar energy transmitted through each material. Compare the fraction of visible radiant energy transmitted through each. 23.6 Determine the fraction of total energy emitted by a black body, which lies in the wavelength band between 0.8 and 5.0 mm for surface temperatures of 500, 2000, 3000, and 4500 K. 23.7 The sun’s temperature is approximately 5800 K and the visible light range is taken to be between 0.4 and 0.7 mm. What fraction of solar emission is visible? What fraction of solar emission lies in the ultraviolet range? The infrared range? At what wavelength is solar emissive power a max- imum? 23.8 A satellite may be considered spherical with its surface properties roughly those of aluminum. Its orbit may be con- sidered circular at a height of 500 miles above Earth. Taking the satellite diameter as 50 in., estimate the temperature of the satellite skin. Earth may be considered to be at a uniform temperature of 508F, and the emissivity of Earth may be taken as 0.95. Solar irradiation may be taken as 450 Btu/h ft2 of satellite disc area. 23.9 An opaque gray surface with e ¼ 0:3 is irradiated with 1000 W/cm. For an effective convective heat-transfer coefficient of 12 W/m2 Á K applying, and air at 208C adjacent to the plate, what will be the net heat flux to or from a 308C surface? 23.10 A black solar collector, with a surface area of 60 m2, is placed on the roof of a house. Incident solar energy reaches the collector with a flux of 800 W/m2. The surroundings are con- sidered black with an effective temperature of 308C. The con- vective heat-transfer coefficient between the collector and the surrounding air, at 308C, is 35 W/m2 Á K. Neglecting any con- ductive loss from the collector, determine: a. the net radiant exchange between the collector and its surroundings; b. the equilibrium temperature of the collector. 23.11 A 7.5-cm-diameter hole is drilled in a 10-cm-thick iron plate. If the plate temperature is 700 K and the surroundings are at 310 K, determine the energy loss through the hole. The hole sides may be considered to be black. 23.12 If the 7.5-cm-diameter hole in Problem 23.11 were drilled to a depth of 5 cm, what heat loss would result? 23.13 A sheet-metal box in the shape of a 0.70-m cube has a surface emissivity of 0.7. The box encloses electronic equipment that dissipates 1200 W of energy. If the surroundings are taken to beblackat280K,andthetopandsidesoftheboxareconsideredto radiateuniformly, what will be the temperatureof the box surface? 23.14 Two very large black plane surfaces are maintained at 900 and 580 K, respectively. A third large plane surface, having e ¼ 0:8, is placed between these two. Determine the fractional change in radiant exchange between the two plane surfaces due to the intervening plane and evaluate the temperature of this intervening plane. 23.15 The filament of an ordinary 100 W light bulb is at 2910 K and it is presumed to be a black body. Determine (a) the wavelength of maximum emission and (b) the fraction of emission in the visible region of the spectrum. 23.16 A small circular hole is to be drilled in the surface of a large, hollow, spherical enclosure maintained at 2000 K. If 100 W of radiant energy exits through the hole, determine (a) the hole diameter, (b) the number of watts emitted in the visible range from 0.4 and 0.7 mm, (c) the ultraviolet range between 0 and 0.4 mm, and (d) the infrared range from 0.7 to 100 mm. 23.17 A large cavity with a small opening, 0.0025 m2 in area, emits 8 W. Determine the wall temperature of the cavity. 394 Chapter 23 Radiation Heat Transfer
37. 23.18 Determine thewavelength of maximum emission for (a) the sun with an assumed temperature of 5790 K, (b) a light bulk filament at 2910 K, (c) a surface at 1550 K, and (d) human skin at 308 K. 23.19 A furnace that has black interior walls maintained at 1500 K contains a peephole with a diameter of 10 cm. The glass in the peephole has a transmissivity of 0.78 between 0 and 3.2 mm and 0.08 between 3.2 mm and 1. Determine the heat lost through the peephole. 23.20 A cryogenic fluid flows in a 20-mm-diameter tube with an outer surface temperature of 75 K and an emissivity of 0.2. A larger tube, having a diameter of 50 mm, is concentric with the smaller one. This larger tube is gray, with e ¼ 0:05 and its surface temperature is 300 K. The intervening space between the two tubes is evacuated. Determine the heat gain by the cryogenic fluid, in watts per meter of tube length. Evaluate the heat gain per meter of length if there is a thin walled radiation shield placed midway between the two tubes. The shield surfaces may be considered gray and diffuse with an emissivity of 0.04 on both sides. 23.21 A circular duct 2 ft long with a diameter of 3 in. has a thermocouple in its center with a surface area of 0.3 in.2. The duct walls are at 2008F, and the thermocouple indicates 3108F. Assuming the convective heat-transfer coefficient between the thermocouple and gas in the duct to be 30 Btu/h ft2 8F, estimate the actual temperature of the gas. The emissivity of the duct walls may be taken as 0.8 and that of the thermocouple as 0.6. 23.22 A heating element in the shape of a cylinder is main- tained at 20008F and placed at the center of a half-cylindrical reflector as shown. The rod diameter is 2 in. and that of the reflector is 18 in. The emissivity of the heater surface is 0.8, and the entire assembly is placed in a room maintained at 708F. What is the radiant energy loss from the heater per foot of length? How does this compare to the loss from the heater without the reflector present? 23.23 A 12-ft-long, 3-in.-OD iron pipe e ¼ 0:7, passes hori- zontally through a 12 Â 14 Â 9 ft room whose walls are main- tained at 708F and have an emissivity of 0.8. The pipe surface is at a temperature of 2058F. Compare the radiant energy loss from the pipe with that due to convection to the surrounding air at 708F. 23.24 The circular base of the cylindrical enclosure shown may be considered a reradiating surface. The cylindrical walls have an effective emissivity of 0.80 and are maintained at 5408F. The top of the enclosure is open to the surroundings, which are maintained at 408F. What is the net rate of radiant transfer to the surroundings? 6 ft 12 ft 23.25 The hemispherical cavity shown in the figure has an in- side surface temperature of 700 K. A plate of refractory material is placed over the cavity with a circular hole of 5 cm diameter in the center. How much energy will be lost through the hole if the cavity is a. black? b. gray with an emissivity of 0.7? What will be the temperature of the refractory under each condition? 15-cm radius 23.26 A room measuring 12 ft by 20 ft by 8 ft high has its floor and ceiling temperatures maintained at 85 and 658F, respec- tively. Assuming the walls to be reradiating and all surfaces to have an emissivity of 0.8, determine the net energy exchange between the floor and ceiling. 23.27 A dewar flask, used to contain liquid nitrogen, is made of two concentric spheres separated by an evacuated space. The inner sphere has an outside diameter of 1 m and the outer sphere has an inside diameter of 1.3 m. These surfaces are both gray- diffuse with e ¼ 0:2. Nitrogen, at 1 atmosphere, has a saturation temperature of 78 K and a latent heat of vaporization of 200 kJ/kg. Under conditions when the inner sphere is full of liquid nitrogen and the outer sphere is at a temperature of 300 K, estimate the boil-off rate of nitrogen. 23.28 A cylindrical cavity is closed at the bottom and has an opening centered in the top surface. A cross section of this configuration is shown in the sketch. For the conditions stated below, determine the rate of radiant energy passing through the
38. 5-mm-diameter cavity opening. What will be effective emissiv- ity of the opening? a. All interior surfaces are black at 600 K. b. The bottom surface is diffuse-gray with e ¼ 0:6, and has a temperature of 600 K. All other surfaces are reradiating. c. All interior surfaces are diffuse-gray with e ¼ 0:6 and are at a uniform temperature of 600 K. 40 mm 30 mm 2 3 1 23.29 A circular heater, measuring 20 cm in diameter, has its surface temperature maintained at 10008C. The bottom of a tank, having the same diameter, is oriented parallel to the heater with a separation distance of 10 cm. The heater surface is gray (e ¼ 0:6) and the tank surface is also gray (e ¼ 0:7). Determine the radiant energy reaching the bottom of the tank if a. the surroundings are black at 278C; b. the space between the two cylindrical surfaces is enclosed by an adiabatic surface. 23.30 Two parallel black rectangular surfaces, whose back sides are insulated, are oriented parallel to each other with a spacing of 5 m. They measure 5 m by 10 m. The surroundings are black at 0 K. The two surfaces are maintained at 200 and 100 K, respectively. Determine the following: a. the net radiant heat transfer between the two surfaces; b. the net heat supplied to each surface; c. the net heat transfer between each surface and the surroundings. 23.31 Two parallel rectangles have emissivities of 0.6 and 0.9, respectively. These rectangles are 1.2 m wide and 2.4 m high and are 0.6 mapart. The plate havinge ¼ 0:6 is maintained at 1000 K and the other is at 420 K. The surroundings may be considered to absorb all energy that escapes the two-plate system. Determine a. the total energy lost from the hot plate; b. the radiant energy interchange between the two plates. 23.32 If a third rectangular plate, with both surfaces having an emissivity of 0.8 is placed between the two plates described in Problem 23.31, how will the answer to part (a) of Problem 23.31 be affected? Draw the thermal circuit for this case. 23.33 Two disks are oriented on parallel planes separated by a distance of 10 in., as shown in the accompanying figure. The disk to the right is 4 in. in diameter and is at a temperature of 5008F. The disk to the left has an inner ring cut out such that it is annular in shape with inner and outer diameters of 2.5 and 4 in., respectively. The disk surface temperature is 2108F. Find the heat exchange between these disks if a. they are black; b. they are gray e1 ¼ 0:6, e2 ¼ 0:3. 10 in 2.5 in 4 in 1 2 23.34 Evaluate the net heat transfer between the disks described in Problem 23.33 if they are bases of a cylinder with the side wall considered a nonconducting, reradiating surface. How much energy will be lost through the hole? 23.35 Evaluate the heat transfer leaving disk 1 for the geometry shown in Problem 23.33. In this case the two disks comprise the bases of a cylinder with side wall at constant temperature of 3508F. Evaluate for the case where: a. the side wall is black; b. the side wall is gray with e ¼ 0:2: Determine the rate of heat loss through the hole in each case. 23.36 A heavily oxidized aluminum surface at 755 K is the source of energy in an enclosure, which radiantly heats the side walls of a circular cylinder surface as shown, to 395 K. The side wall is made of polished stainless steel. The top of the enclosure ismade of fire clay brick and isadiabatic. Forpurposes of calculation, assume that all three surfaces have uniform temperatures and that they are diffuse and gray. Evaluate the heat transfer to the stainless steel surface. Aluminum Stainless steel Fire clay brick 2 m 2 m 23.37 A gray, diffuse circular heater with a diameter of 15 cm is placed parallel to asecond gray,diffuse receiverwith aspacing of 7.5 between them. The backs of both surfaces are insulated and convective effects are to be neglected. This heater–receiver assembly isplaced in a large roomat a temperature of 275 K. The surroundings (the room) can be considered black and the heater surface emissivity is 0.8. When the power input to the heater is 300 W, determine a. the heater surface temperature; b. the receiver surface temperature; 396 Chapter 23 Radiation Heat Transfer
39. c. the net radiant exchange to the surroundings; d. the net radiant exchange between the heater and receiver. 23.38 A small (1/4 in. diameter  1 in. long) metal test specimen is suspended by very fine wires in a large evacuated tube. The metal is maintained at a temperature of 25008F, at which temperature it has an emissivity of approximately 0.2. The water-cooled walls and ends of the tube are maintained at 508F. In the upper end is a small (1/4-in.-diameter) silica glass viewing port. The inside surfaces of the steel tube are newly galvanized. Room temperature is 708F. Estimate a. the view factor from the specimen to the window; b. the total net heat-transfer rate by radiation from the test specimen; c. the energy radiated through the viewing port. 12 in 4 in Test specimen Viewing port 23.39 A duct with square cross section measuring 20 cm by 20 cm has water vapor at 1 atmosphere and 600 K flowing through it. One wall of the duct is held at 420 K and has an emissivity of 0.8. The other three walls may be considered refractory surfaces. Determine the rate of radiant energy transfer to the cold wall from the water vapor. 23.40 A gas of mixture at 1000 K and a pressure of 5 atm is introduced into an evacuated spherical cavity with a diameter of 3 m. The cavity walls are black and initially at a temperature of 600 K. What initial rate of heat transfer will occur between the gas and spherical walls if the gas contains 15% CO2 with the remainder of the gas being nonradiating? 23.41 A gas consisting of 20% CO2 and 80% oxygen and nitrogen leaves a lime kiln at 20008F and enters a square duct measuring 6 in. by 6 in. in cross section. The specific heat of the gas is 0:28 Btu/lbm F, and it is to be cooled to 10008F in the duct, whose inside surface is maintained at 8008F, and whose walls have an emissivity of 0.9. The mass velocity of the kiln gas is 0:4 lbm/ft2 :s and the convective heat-transfer coefficient between the gas and duct walls is 1:5 Btu/h ft2 F. a. Determine the required length of duct to cool the gas to 10008F. b. Determine the ratio of radiant energy transfer to that by convection. c. At what temperature would the gas leave the duct if the length of the duct were twice the value determined in part (a)? (Courtesy of the American Institute of Chemical Engineers.) Hint. As the response of the gas to emission and absorp- tion of radiant energy differs, an approximation for the radiant energy exchange between the enclosure and gas contained within an arbitrary control volume is given by AwFwÀgsew (egT4 g À agT4 w ): Problems 397
40. Chapter 24 Fundamentals of Mass Transfer The previous chapters dealing with the transport phenomena of momentum and heat transfer have dealt with one-component phases that possessed a natural tendency to reach equilibrium conditions. When a system contains two or more components whose concentrations vary from point to point, there is a natural tendency for mass to be transferred, minimizing the concentration differences within the system. The transport of one constituent from a region of higher concentration to that of a lower concentration is called mass transfer. Many of our day-to-day experiences involve mass transfer. A lump of sugar added to a cup of black coffee eventually dissolves and then diffuses uniformly throughout the coffee. Water evaporates from ponds to increase the humidity of the passing air stream. Perfume presents a pleasant fragrance that is imparted throughout the surrounding atmosphere. Mass transfer is the basis for many biological and chemical processes. Biological processes include the oxygenation of blood and the transport of ions across membranes within the kidney. Chemical processes include the chemical vapor deposition (CVD) of silane (SiH4 ) onto a silicon wafer, the doping of a silicon wafer to form a semiconducting thin film, the aeration of wastewater, and the purification of ores and isotopes. Mass transfer underlies the various chemical separation processes where one or more components migrate from one phase to the interface between the two phases in contact. For example, in adsorption or crystallization processes, the components remain at the interface, whereas in gas absorption and liquid–liquid extraction processes, the components penetrate the interface and then transfer into the bulk of the second phase. If we consider the lump of sugar added to the cup of black coffee, experience teaches us that the length of time required to distribute the sugar will depend upon whether the liquid is quiescent or whether it is mechanically agitated by a spoon. The mechanism of mass transfer, as we have also observed in heat transfer, depends upon the dynamics of the system in which it occurs. Mass can be transferred by random molecular motion in quiescent fluids, or it can be transferred from a surface into a moving fluid, aided by the dynamic characteristics of the flow. These two distinct modes of transport, molecular mass transfer and convective mass transfer, are analogous to conduction heat transfer and convective heat transfer. Each of these modes of mass transfer will be described and analyzed. As in the case of heat transfer, we should immediately realize that the two mechanisms often act simultaneously. However, in the confluence of the two modes of mass transfer, one mechanism can dominate quantitatively so that approximate solutions involving only the dominant mode need be used. 398
41. 24.1 MOLECULAR MASS TRANSFER As early as 1815, Parrot observed qualitatively that whenever a gas mixture contains two or more molecular species, whose relative concentrations vary from point to point, an apparently natural process results, which tends to diminish any inequalities of composition. This macroscopic transport of mass, independent of any convection within the system, is defined as molecular diffusion. In the specific case of gaseous mixtures, a logical explanation of this transport phenomenon can be deduced from the kinetic theory of gases. At temperatures above absolute zero, individual molecules are in a state of continual yet random motion. Within dilute gas mixtures, each solute molecule behaves independently of the other solute molecules, as it seldom encounters them. Collisions between the solute and the solvent molecules are continually occurring. As a result of the collisions, the solute molecules move along a zigzag path, sometimes toward a regionof higher concentration, sometimes toward a lower concentration. Let us consider a hypothetical section passing normal to the concentration gradient within an isothermal, isobaric gaseous mixture containing solute and solvent molecules. The two thin, equal elements of volume above and below the section will contain the same number of molecules, as stipulated by Avogadro’s law. Although it is not possible to state which way any particular molecule will travel in a given interval of time, a definite number of the molecules in the lower element of the volume will cross the hypothetical section from below, and the same number of molecules will leave the upper element and cross the section from above. With the existence of a concentration gradient, there are more solute molecules in one of the elements of volume than in the other; accordingly, an overall net transfer from a region of higher concentration to one of lower concentration will result. The net flow of each molecular species occurs in the direction of a negative concentration gradient. As pointed out in Chapters 7 and 15, the molecular transport of momentum and the transport of energy by conduction are also due to random molecular motion. Accordingly, one should expect that the three transport phenomena will depend upon many of the same characteristic properties, such as mean free path, and that the theoretical analyses of all three phenomena will have much in common. The Fick Rate Equation The laws of mass transfer show the relation between the flux of the diffusing substance and the concentration gradient responsible for this mass transfer. Unfortunately, the quantitative description of molecular diffusion is considerably more complex than the analogous descriptions for the molecular transfer of momentum and energy that occur in a one- component phase. As mass transfer, or diffusion, as it is also called, occurs only in mixtures, its evaluation must involve an examination of the effect of each component. For example,we will often desire to know the diffusion rate of a specific component relative to the velocity of the mixture in which it is moving. As each component may possess a different mobility, the mixture velocity must be evaluated by averaging the velocities of all of the components present. In order to establish a common basis for future discussions, let us first consider definitions and relations that are often used to explain the role of components within a mixture. Concentrations. In a multicomponent mixture, the concentration of a molecular species can be expressed in many ways. Figure 24.1 shows an elemental volume dV that contains a Molecule of species A Figure 24.1 Elemental volume containing a multicomponent mixture. 24.1 Molecular Mass Transfer 399
42. mixture of components, including species A. As each molecule of each species has a mass, a mass concentration for each species, as well as for the mixture, can be defined. For species A, mass concentration, rA , is defined as the mass of A per unit volume of the mixture. The total mass concentration or density, r, is the total mass of the mixture contained in the unit volume; that is, r ¼ å n i¼1 ri (24-1) where n is the number of species in the mixture. The mass fraction, vA, is the mass concentration of species A divided by the total mass density vA ¼ rA å n i ri ¼ rA r (24-2) The sum of the mass fractions, by definition, must be 1: å n i¼1 vi ¼ 1 (24-3) The molecular concentration of species A, cA , is defined as the number of moles of A present per unit volume of the mixture. By definition, one mole of any species contains a mass equivalent to its molecular weight; the mass concentration and molar concentration terms are related by the following relation: cA ¼ rA MA (24-4) where MA is the molecular weight of species A. When dealing with a gas phase, concentrations are often expressed in terms of partial pressures. Under conditions in which the ideal gas law, pAV ¼ nART, applies, the molar concentration is cA ¼ nA V ¼ pA RT (24-5) where pA is the partial pressure of the species A in the mixture, nA is the number of moles of species A, V is the gas volume, T is the absolute temperature, and R is the gas constant. The total molar concentration, c, is the total moles of the mixture contained in the unit volume; that is, c ¼ å n i¼1 ci (24-6) or for a gaseous mixture that obeys the ideal gas law, c ¼ ntotal/V ¼ P/RT, where P is the total pressure. The mole fraction for liquid or solid mixtures, xA , and for gaseous mixtures, yA , are the molar concentrations of species A divided by the total molar density xA ¼ cA c (liquids and solids) yA ¼ cA c (gases) (24-7) For a gaseous mixture that obeys the ideal gas law, the mole fraction, yA , can be written in terms of pressures yA ¼ cA c ¼ pA/RT P/RT ¼ pA P (24-8) 400 Chapter 24 Fundamentals of Mass Transfer
43. Equation (24-8) is an algebraic representation of Dalton’s law for gas mixtures. The sum of the mole fractions, by definition, must be 1: å n i¼1 xi ¼ 1 å n i¼1 yi ¼ 1 (24-9) A summary of the various concentration terms and of the interrelations for a binary system containing species A and B is given in Table 24.1. EXAMPLE 1 The composition of air is often given in terms of only the two principal species in the gas mixture oxygen, O2, yo2 ¼ 0:21 nitrogen, N2, yN2 ¼ 0:79 Determine the mass fraction of both oxygen and nitrogen and the mean molecular weight of the air when it is maintained at 258C (298 K) and 1 atm (1.013 Â 105 Pa). The molecular weight of oxygen is 0.032 kg/mol and of nitrogen is 0.028 kg/mol. Table 24.1 Concentrations in a binary mixture of A and B Mass concentrations r ¼ total mass density of the mixture rA ¼ mass density of species A rB ¼ mass density of species B vA ¼ mass fraction of species A ¼ rA /r vB ¼ mass fraction of species B ¼ rB /r r ¼ rA þ rB 1 ¼ vA þ vB Molar concentrations Liquid or solid mixture Gas mixture c ¼ molar density of mixture ¼ n=V c ¼ n/V ¼ P/RT cA ¼ molar density of species A ¼ nA/V cA ¼ nA/V ¼ pA/RT cB ¼ molar density of species B ¼ nB/V cB ¼ nB/V ¼ pB/RT xA ¼ mole fraction of species A ¼ cA/c ¼ nA/n yA ¼ cA/c ¼ nA/n ¼ pA/p xB ¼ mole fraction of species B ¼ cB/c ¼ nB/n yB ¼ cB/c ¼ nB/n ¼ pB/p c ¼ cA þ cB c ¼ cA þ cB ¼ pA RT þ pB RT ¼ P RT 1 ¼ xA þ xB 1 ¼ yA þ yB Interrelations rA ¼ cAMA xA or yA ¼ vA/MA vA/MA þ vB/MB (24-10) vA ¼ xAMA xAMA þ xAMA or yAMA yAMA þ yBMB (24-11) 24.1 Molecular Mass Transfer 401
44. As a basis for our calculations, consider 1 mol of the gas mixture oxygen present ¼ (1 mol)(0:21) ¼ 0:21 mol ¼ (0:21 mol) (0:032 kg) mol ¼ 0:00672 kg nitrogen present ¼ (1 mol)(0:79) ¼ 0:79 mol ¼ (0:79 mol) (0:028 kg) mol ¼ 0:0221 kg total mass present ¼ 0:00672 þ 0:0221 ¼ 0:0288 kg vO2 ¼ 0:00672 kg 0:0288 kg ¼ 0:23 vN2 ¼ 0:0221 kg 0:0288 kg ¼ 0:77 As 1 mol of the gas mixture has a mass of 0.0288 kg, the mean molecular weight of the air must be 0.0288. When one takes into account the other constituents that are present in air, the mean molecular weight of air is often rounded off to 0.029 kg/mol. This problem could also be solved using the ideal gas law, PV ¼ nRT. At ideal conditions, 0C or 273 K and 1 atm of 1:013 Â 105 Pa pressure, the gas constant is evaluated to be R ¼ PV nT ¼ (1:013 Â 105 Pa)(22:4 m3) (1 kg mol)(273 K) ¼ 8:314 Pa Á m3 mol Á K (24-12) The volume of the gas mixture, at 298 K, is V ¼ nRT P ¼ (1 mol) 8:314 Pa Á m3 mol Á K   (298 K) 1:013 Â 105 Pa ¼ 0:0245 m3 The concentrations are cO2 ¼ 0:21 mol 0:0245 m3 ¼ 8:57 mol O2 m3 cN2 ¼ 0:79 mol 0:0245 m3 ¼ 32:3 mol N2 m3 c ¼ å n i¼1 ci ¼ 8:57 þ 32:3 ¼ 40:9 mol/m3 The total density, r, is r ¼ 0:0288 kg 0:0245 m3 ¼ 1:180 kg/m3 and the mean molecular weight of the mixture is M ¼ r c ¼ 1:180 kg/m3 40:9 mol/m3 ¼ 0:0288 kg/mol Velocities. In a multicomponent system the various species will normally move at different velocities; accordingly, an evaluation of a velocity for the gas mixture requires the averaging of the velocities of each species present. 402 Chapter 24 Fundamentals of Mass Transfer
45. The mass-average velocity for a multicomponent mixture is defined in terms of the mass densities and velocities of all components by v ¼ å n i¼1 ri vi å n i¼1 ri ¼ å n i¼1 ri vi r (24-13) where vi denotes the absolute velocity of species i relative to stationary coordinate axes. This is the velocity that would be measured by a pitot tube and is the velocity that was previously encountered in the equations of momentum transfer. The molar-average velocity for a multicomponent mixture is defined in terms of the molar concentrations of all components by V ¼ å n i¼1 civi å n i¼1 ci ¼ å n i¼1 civi c (24-14) The velocity of a particular species relative to the mass-average or molar-average velocity is termed a diffusion velocity. We can define two different diffusion velocities vi À v; the diffusion velocity of species i relative to the mass-average velocity and vi À V, the diffusion velocity of species i relative to the molar-velocity average According to Fick’s law, a species can have a velocity relative to the mass- or molar-average velocity only if gradients in the concentration exist. Fluxes. The mass (or molar) flux of a given species is a vector quantity denoting the amount of the particular species, in either mass or molar units, that passes per given increment of time through a unit area normal to the vector. The flux may be defined with reference to coordinates that are fixed in space, coordinates that are moving with the mass- average velocity, or coordinates that are moving with the molar-average velocity. The basic relation for molecular diffusion defines the molar flux relative to the molar- average velocity, JA . An empirical relation for this molar flux, first postulated by Fick1 and, accordingly, often referred to as Fick’s first law, defines the diffusion of component A in an isothermal, isobaric system: JA ¼ ÀDAB rcA For diffusion in only the z direction, the Fick rate equation is JA,z ¼ ÀDAB dcA dz (24-15) where JA,z is the molar flux in the z direction relative to the molar-average velocity, dcA /dz is the concentration gradient in the z direction, and DAB , the proportionality factor, is the mass diffusivity or diffusion coefficient for component A diffusing through component B. A more general flux relation that is not restricted to isothermal, isobaric systems was proposed by de Groot2 who chose to write flux ¼ À overall density   diffusion coefficient   concentration gradient   1 A. Fick, Ann. Physik., 94, 59 (1855). 2 S. R. de Groot, Thermodynamics of Irreversible Processes, North-Holland, Amsterdam, 1951. 24.1 Molecular Mass Transfer 403
46. or JA,z ¼ ÀcDAB dyA dz (24-16) As the total concentration c is constant under isothermal, isobaric conditions, equation (24-15) is a special form of the more general relation (24-16). An equivalent expression for jA,z , the mass flux in the z direction relative to the mass-average velocity, is jA,z ¼ ÀrDAB dvA dz (24-17) where dvA/dz is the concentration gradient in terms of the mass fraction. When the density is constant, this relation simplifies to jA,z ¼ ÀDAB drA dz Initial experimental investigations of molecular diffusion were unable to verify Fick’s law of diffusion. This was apparently due to the fact that mass is often transferred simultaneously by two possible means: (1) as a result of the concentration differences as postulated by Fick and (2) by convection differences induced by the density differences that resulted from the concentrationvariation. Steffan (1872) and Maxwell (1877), using the kinetic theory of gases, proved that the mass flux relative to a fixed coordinatewas a result of two contributions: the concentration gradient contribution and the bulk motion contribution. For a binary system with a constant average velocity in the z direction, the molar flux in the z direction relative to the molar-average velocity may also be expressed by JA,z ¼ cA(vA,z À Vz) (24-18) Equating expressions (24-16) and (24-18), we obtain JA,z ¼ cA(vA,z À Vz) ¼ ÀcDAB dyA dz which, upon rearrangement, yields cAvA,z ¼ ÀcDAB dyA dz þ cAVz For this binary system, Vz can be evaluated by equation (24-14) as Vz ¼ 1 c (cAvA,z þ cBvA,z) or cAVz ¼ yA(cAvA,z þ cBvB,z) Substituting this expression into our relation, we obtain cAvA,z ¼ ÀcDAB dyB dz þ yA(cAvA,z þ cBvB,z) (24-19) As the component velocities, vA,z and vB,z , are velocities relative to the fixed z axis, the quantities cA vA,z and cB vB,z are fluxes of components A and B relative to a fixed z coordinate; accordingly, we symbolize this new type of flux that is relative to a set of stationary axes by NA ¼ cAvA 404 Chapter 24 Fundamentals of Mass Transfer
47. and NB ¼ cBvB Substituting these symbols into equation (24-19), we obtain a relation for the flux of component A relative to the z axis NA,z ¼ ÀcDAB dyA dz þ yA(NA,z þ NB,z) (24-20) This relation may be generalized and written in vector form as NA ¼ ÀcDAB = yA þ yA(NA þ NB) (24-21) It is important to note that the molar flux, NA , is a resultant of the two vector quantities: ÀcDAB =yA the molar flux, JA, resulting from the concentration gradient: This term is referred to as the concentration gradient contribution; and yA(NA þ NB) ¼ cAV the molar flux resulting as component A is carried in the bulk flow of the fluid: This flux term is designated the bulk motion contribution: Either or both quantities can be a significant part of the total molar flux, NA . Whenever equation (24-21) is applied to describe molar diffusion, the vector nature of the individual fluxes, NA and NB , must be considered and then, in turn, the direction of each of two vector quantities must be evaluated. If species A were diffusing in a multicomponent mixture, the expression equivalent to equation (24-21) would be NA ¼ ÀcDAM =yA þ yA å n i¼1 Ni where DAM is the diffusion coefficient of A in the mixture. The mass flux, nA , relative to a fixed spatial coordinate system, is defined for a binary system in terms of mass density and mass fraction by nA ¼ ÀrDAB =vA þ vA(nA þ nB) (24-22) where nA ¼ rA vA and nB ¼ rB vB Under isothermal, isobaric conditions, this relation simplifies to nA ¼ ÀDAB =rA þ vA(nA þ nB) As previously noted, the flux is a resultant of two vector quantities: ÀDAB =rA , the mass flux, jA , resulting from a concentration gradient; the concentration gradient contribution: vA(nA þ nB) ¼ rA v; the mass flux resulting as component A is carried in the bulk flow of the fluid; the bulk motion contribution: 24.1 Molecular Mass Transfer 405
48. If a balloon, filled with a color dye, is dropped into a large lake, the dye will diffuse radially as a concentration gradient contribution. When a stick is dropped into a moving stream, it will float downstream by the bulk motion contribution. If the dye-filled balloon were dropped into the moving stream, the dye would diffuse radially while being carried downstream; thus both contributions participate simultaneously in the mass transfer. The four equations defining the fluxes, JA , jA , NA , and nA are equivalent statements of the Fick rate equation. The diffusion coefficient, DAB , is identical in all four equations. Any one of these equations is adequate to describe molecular diffusion; however, certain fluxes are easier to use for specific cases. The mass fluxes, nA and jA , are used when the Navier– Stokes equations are also required to describe the process. Since chemical reactions are described in terms of moles of the participating reactants, the molar fluxes, JA and NA , are used to describe mass-transfer operations in which chemical reactions are involved. The fluxes relative to coordinates fixes in space, nA and NA , are often used to describe engineering operations within process equipment. The fluxes JA and jA are used to describe the mass transfer in diffusion cells used for measuring the diffusion coefficient. Table 24.2 summarizes the equivalent forms of the Fick rate equation. Related Types of Molecular Mass Transfer According to the second law of thermodynamics, systems not in equilibrium will tend to move toward equilibrium with time. A generalized driving force in chemical thermo- dynamic terms is Àdmc/dz where mc is the chemical potential. The molar diffusion velocity of component A is defined in terms of the chemical potential by vA,z À Vz ¼ uA dmc dz ¼ À DAB RT dmc dz (24-23) where uA is the ‘‘mobility’’ of component A, or the resultant velocity of the molecule while under the influence of a unit driving force. Equation (24-23) is known as the Nernst–Einstein relation. The molar flux of A becomes JA,z ¼ cA(vA,z À Vz) ¼ ÀcA DAB RT dmc dz (24-24) Equation (24-24) may be used to define all molecular mass-transfer phenomena. As an example, consider the conditions specified for equation (24-15); the chemical potential of a componentinahomogeneousidealsolutionatconstanttemperature andpressureisdefinedby mc ¼ m0 þ RT ln cA (24-25) Table 24.2 Equivalent forms of the mass flux equation for binary system A and B Flux Gradient Fick rate equation Restrictions nA =vA nA ¼ ÀrDAB =vA þ vA(nA þ nB) =rA nA ¼ ÀDAB =rA þ vA(nA þ nB) Constant r NA =yA NA ¼ ÀcDAB =yA þ yA(NA þ NB) =cA NA ¼ ÀDAB =cA þ yA(NA þ NB) Constant c jA =vA jA ¼ ÀrDAB =vA =rA jA ¼ ÀDAB =rA Constant r jA =yA JA ¼ ÀcDAB =yA =cA JA ¼ ÀDAB =cA Constant c 406 Chapter 24 Fundamentals of Mass Transfer
49. where m0 is a constant, the chemical potential of the standard state. When we substitute this relation into equation (24-24), the Fick rate equation for a homogeneous phase is obtained JA,z ¼ ÀDAB dcA dz (24-15) There are a number of other physical conditions, in addition to differences in con- centration, which will produce a chemical potential gradient: temperature differences, pressure differences, and differences in the forces created by external fields, such as gravity, magnetic, and electrical fields. We can, for example, obtain mass transfer by applying a temperature gradient to a multicomponent system. This transport phenomenon, the Soret effect or thermal diffusion, although normally small relative to other diffusion effects, is used successfully in the separation of isotopes. Components in a liquid mixture can be separated with a centrifuge by pressure diffusion. There are many well-known examples of mass fluxes being induced in a mixture subjected to an external force field: separation by sedimentation under theinfluence of gravity, electrolytic precipitation due to an electrostatic force field, and magnetic separation of mineral mixtures through the action of a magnetic force field. Although these mass-transfer phenomena are important, they are very specific processes. The molecular mass transfer, resulting from concentration differences and described by Fick’s law, results from the random molecular motion over small mean free paths, independent of any containment walls. The diffusion of fast neutrons and molecules in extremely small pores or at very low gas density cannot be described by this relationship. Neutrons, produced in a nuclear fission process, initially possess high kinetic energies and are termed fast neutrons because of their high velocities; that is, up to 15 million meters per second. At these high velocities, neutrons pass through the electronic shells of other atomsormoleculeswithlittlehindrance.Tobedeflected,thefastneutronsmustcollidewitha nucleus, which is a very small target compared to the volume of most atoms and molecules. The mean free path of fast neutrons is approximately one million times greater than the free paths of gases at ordinary pressures. After the fast neutrons are slowed down through elastic- scattering collisions between the neutrons and the nuclei of the reactor’s moderator, these slower moving neutrons, thermal neutrons, migrate from positions of higher concentration to positions of lower concentration, and their migration is described by Fick’s law of diffusion. 24.2 THE DIFFUSION COEFFICIENT Fick’s law of proportionality, DAB , is known as the diffusion coefficient. Its fundamental dimensions, which may be obtained from equation (24-15) DAB ¼ ÀJA,z dcA/dz ¼ M L2t   1 M/L3 Á 1/L   ¼ L2 t are identical to the fundamental dimensions of the other transport properties: kinematic viscosity, n, and thermal diffusivity, a, or its equivalent ratio, k/rcp. The mass diffusivity has been reported in cm2/s; the SI units are m2/s, which is a factor 10À4 smaller. In the English system ft2/h is commonly used. Conversion between these systems involves the simple relations DAB(cm2/s) DAB(m2/s) ¼ 104 DAB(ft2/h) DAB(cm2/s) ¼ 3:87 (24-26) 24.2 The Diffusion Coefficient 407
50. The diffusion coefficient depends upon the pressure, temperature, and composition of the system. Experimental values for the diffusivities of gases, liquids, and solids are tabulated in Appendix Tables J.1, J.2, and J.3, respectively. As one might expect from the consideration of the mobility of the molecules, the diffusion coefficients are generally higher for gases (in the range of 5 Â 10À6 to 1 Â 10À5 m2/s), than for liquids (in the range of 10À10 to 10À9 m2/s), which are higher than the values reported for solids (in the range of 10À14 to 10À10 m2/s). In the absence of experimental data, semitheoretical expressions have been developed which giveapproximations, sometimes as valid as experimental values due to the difficulties encountered in their measurement. Gas Mass Diffusivity Theoretical expressions for the diffusion coefficient in low-density gaseous mixtures as a function of the system’s molecular properties were derived by Sutherland,3 Jeans,4 and Chapman and Cowling,5 based upon the kinetic theory of gases. In the simplest model of gas dynamics, the molecules are regarded as rigid spheres that exert no intermolecular forces. Collisions between these rigid molecules are considered to be completely elastic. With these assumptions, a simplified model for an ideal gas mixture of species A diffusing through its isotope AÃ yields an equation for the self-diffusion coefficient, defined as DAAÃ ¼ 1 3 lu (24-27) and l is the mean free path of length of species A, given by l ¼ kT ffiffiffi 2 p ps2 A P (24-28) where u is the mean speed of species A with respect to the molar-average velocity u ¼ ffiffiffiffiffiffiffiffiffiffiffi 8kNT pMA r (24-29) Insertion of equations (24-28) and (24-29) into equation (24-27) results in DAAÃ ¼ 2T3/2 3p3/2s2 A P k3N MA  1=2 (24-30) where MA is the molecular weight of the diffusing species A, (g/mol), N is Avogadro’s number (6:022 Â 1023 molecules/mol), P is the system pressure, T is the absolute temperature (K), K is the Boltzmann constant (1:38 Â 10À16 ergs/K), and sAB is the Lennard–Jones diameter of the spherical molecules. Using a similar kinetic theory of gases approach for a binary mixture of species A and B composed of rigid spheres of unequal diameters, the gas-phase diffusion coefficient is shown to be DAB ¼ 2 3 K p   3/2 N1/2T3/2 1 2MA þ 1 2MB   1/2 P sA þ sB 2  2 (24-31) 3 W. Sutherland, Phil. Mag., 36, 507; 38, 1 (1894). 4 J. Jeans, Dynamical Theory of Gases, Cambridge University Press, London, 1921. 5 S. Chapman and T. G. Cowling, Mathematical Theory of Non-Uniform Gases, Cambridge University Press, London, 1959. 408 Chapter 24 Fundamentals of Mass Transfer
51. Unlike the other two molecular transport coefficients for gases, the viscosity and thermal conductivity, the gas-phase diffusion coefficient is dependent on the pressure and the temperature. Specifically, the gas-phase diffusion coefficient is  an inverse function of total system pressure DAB / 1 P  a 3/2 power-law function of the absolute temperature DAB / T3/2 As equation (24-31) reveals, and as one of the problems at the end of this chapter points out, the diffusion coefficients for gases DAB ¼ DBA. This is not the case for liquid diffusion coefficients. Modern versions of the kinetic theory have been attempted to account for forces of attraction and repulsion between the molecules. Hirschfelder et al. (1949),6 using the Lennard–Jones potential to evaluate the influence of the molecular forces, presented an equation for the diffusion coefficient for gas pairs of nonpolar, nonreacting molecules: DAB ¼ 0:001858T3=2 1 MA þ 1 MB ! 1/2 Ps2 AB VD (24-33) where DAB is the mass diffusivity of A through B, in cm2/s; T is the absolute temperature, in K; MA , MB are the molecular weights of A and B, respectively; P is the absolute pressure, in atmospheres; sAB is the ‘‘collision diameter,’’ a Lennard–Jones parameter, in A ˚ ; and VD is the ‘‘collision integral’’ for molecular diffusion, a dimensionless function of the temperature and of the intermolecular potential field for one molecule of A and one molecule of B. Appendix Table K.1 lists VD as a function of kT/eAB, k is the Boltzmann constant, which is 1:38 Â 10À16 ergs/K, and eAB is the energy of molecular interaction for the binary system A and B, a Lennard–Jones parameter, in ergs, see equation (24-31). Unlike the other two molecular transport coefficients, viscosity and thermal conductivity, the diffusion coefficient is dependent on pressure as well as on a higher order of the absolute temperature. When the transport process in a single component phase was examined, we did not find any composition dependency in equation (24-30) or in the similar equations for viscosity and thermal conductivity. Figure 24.2 presents the graphical dependency of the ‘‘collision integral,’’ VD, on the dimensionless temperature, kT/eAB. The Lennard–Jones parameters, r and eD, are usually obtained from viscosity data. Unfortunately, this information is available for only a very few pure gases. Appendix Table K.2 tabulates these values. In the absence of experimental data, the values for pure components may be estimated from the following empirical relations: s ¼ 1:18 V1/3 b (24-34) s ¼ 0:841 V1/3 c (24-35) s ¼ 2:44 Tc Pc   1/3 (24-36) eA/k ¼ 0:77 Tc (24-37) 6 J. O. Hirschfelder, R. B. Bird, and E. L. Spotz, Chem. Rev.,44, 205 (1949). 24.2 The Diffusion Coefficient 409
52. and eA/k ¼ 1:15 Tb (24-38) where Vb is the molecular volume at the normal boiling point, in (cm)3/g mol (this is evaluated by using Table 24.3); Vc is the critical molecular volume, in (cm)3/g mol; Tc is the critical temperature, in K; Tb is the normal boiling temperature, in K; and Pc is the critical pressure, in atmospheres. For a binary system composed of nonpolar molecular pairs, the Lennard–Jones parameters of the pure component may be combined empirically by the following relations: sAB ¼ sA þ sB 2 (24-39) and eAB ¼ ffiffiffiffiffiffiffiffiffi eA eB p (24-40) 0.00 0.00 0.50 1.00 1.50 2.00 2.00 4.00 6.00 8.00 10.00 2.50 3.00 Dimensionless temperature, kT/e ΑΒ Collision integral for diffusion, W D Figure 24.2 Binary gas- phase Lennard–Jones ‘‘collision integral.’’ Table 24.3 Atomic diffusion volumes for use in estimating DAB by method of Fuller, Schettler, and Giddings Atomic and structure diffusion-volume increments, v C 16.5 Cl 19.5 H 1.98 S 17.0 O 5.48 Aromatic ring À20.2 N 5.69 Heterocyclic ring À20.2 Diffusion volumes for simple molecules, v H2 7.07 Ar 16.1 H2 O 12.7 D2 6.70 Kr 22.8 CCIF2 114.8 He 2.88 CO 18.9 SF6 69.7 N2 17.9 CO2 26.9 Cl2 37.7 O2 16.6 N2 O 35.9 Br2 67.2 Air 20.1 NH3 14.9 SO2 41.1 410 Chapter 24 Fundamentals of Mass Transfer
53. These relations must be modified for polar–polar and polar–nonpolar molecular pairs; the proposed modifications are discussed by Hirschfelder, Curtiss, and Bird.7 The Hirschfelder equation (24-33) is often used to extrapolate experimental data. For moderate ranges of pressure, up to 25 atm, the diffusion coefficient varies inversely with the pressure. Higher pressures apparently require dense gas corrections; unfortunately, no satisfactory correlation is available for high pressures. Equation (24-33) also states that the diffusion coefficient varies with the temperature as T3/2/VD varies. Simplifying equation (24-33), we can predict the diffusion coefficient at any temperature and at any pressure below 25 atm from a known experimental value by DABT2 ,P1 ¼ DABT1 ,P1 P1 P2   T2 T1   3/2VD j T1 VD j T2 (24-41) In Appendix Table J.1, experimental values of the product DAB P are listed for several gas pairs at a particular temperature. Using equation (24-41), we may extend these values to other temperatures. EXAMPLE 2 Evaluate the diffusion coefficient of carbon dioxide in air at 20C and atmospheric pressure. Compare this value with the experimental value reported in appendix table J.1. From Appendix Table K.2, the values of s and e/k are obtained s; in A 8 eA/k; in K Carbon dioxide 3:996 190 Air 3:617 97 The various parameters for equation (24-33) may be evaluated as follows: sAB ¼ sA þ sB 2 ¼ 3:996 þ 3:617 2 ¼ 3:806 A 8 eAB/k ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi (eA/k)(eB/k) p ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi (190)(97) p ¼ 136 T ¼ 20 þ 273 ¼ 293 K P ¼ 1 atm eAB kT ¼ 136 293 ¼ 0:463 kT eAB ¼ 2:16 VD (Table K:1) ¼ 1:047 MCO2 ¼ 44 and MAir ¼ 29 Substituting these values into equation (24-33), we obtain DAB ¼ 0:001858 T3/2(1/MA þ 1/MB)1/2 Ps2 AB VD ¼ (0:001858)(293)3/2(1/44 þ 1/29)1/2 (1)(3:806)2(1:047) ¼ 0:147 cm2/s 7 J. O. Hirschfelder, C. F. Curtiss, and R. B. Bird, Molecular Theory of Gases and Liquids, John Wiley & Sons, Inc., New York, 1954. 24.2 The Diffusion Coefficient 411
54. From Appendix Table J.1 for CO2 in air at 273 K, 1 atm, we have DAB ¼ 0:136 cm2/s Equation (24-41) will be used to correct for the differences in temperature DAB,T1 DAB,T2 ¼ T1 T2   3/2 VD jT2 VD jT1   Values for VD may be evaluated as follows: at T2 ¼ 273 eAB/kT ¼ 136 273 ¼ 0:498 VD j T2 ¼ 1:074 at T1 ¼ 293 VD j T1 ¼ 1:074 (previous calculations) The corrected value for the diffusion coefficient at 208C is DAB,T1 ¼ 293 273   3/2 1:074 1:047   (0:136) ¼ 0:155 cm2/s (1:55 Â 10À5 m2/s) We readily see that the temperature dependency of the ‘‘collision integral’’ is very small. Accordingly, most scaling of diffusivities relative to temperature include only the ratio ðT1/T2 Þ3/2. Equation (24-33) was developed for dilute gases consisting of nonpolar, spherical monatomic molecules. However, this equation gives good results for most nonpolar, binary gas systems over a wide range of temperatures.8 Other empirical equations have been proposed9 for estimating the diffusion coefficient for nonpolar, binary gas systems at low pressures. The empirical correlation recommended by Fuller, Schettler, and Giddings permits the evaluation of the diffusivity when reliable Lennard–Jones para- meters, si and ei, are unavailable. The Fuller correlation is DAB ¼ 10À3T1:75 1 MA þ 1 MB   1/2 P Â (Sv)1/3 A þ (Sv)1/3 B Ã 2 (24-42) where DAB is in cm2/s, T is in K, and P is in atmospheres. To determine the v terms, the authors recommend the addition of the atomic and structural diffusion-volume increments v reported in Table 24.3. Danner and Daubert10 have recommended the atomic and structure diffusion-volume increments for C to be corrected to 15.9 and for H to 2.31 and the diffusionvolumes for H2 to be corrected to 6.12 and for air to 19.7. 8 R. C. Reid, J. M. Prausnitz, and T. K. Sherwood, The Properties of Gases and Liquids, Third Edition, McGraw-Hill Book Company, New York, 1977, Chapter 11. 9 J. H. Arnold, J. Am. Chem. Soc., 52, 3937 (1930). E. R. Gilliland, Ind. Eng. Chem., 26, 681 (1934). J. C. Slattery and R. B. Bird, A.I.Ch.E. J., 4, 137 (1958). D. F. Othmer and H. T. Chen, Ind. Eng. Chem. Process Des. Dev., 1, 249 (1962). R. G. Bailey, Chem. Engr., 82(6), 86, (1975). E. N. Fuller, P. D. Schettler, and J. C. Giddings, Ind. Eng. Chem., 58(5), 18 (1966). 10 R. P. Danner, and T. E. Daubert, Manual for Predicting Chemical Process Design Data, A.I.Ch.E. (1983). 412 Chapter 24 Fundamentals of Mass Transfer
55. EXAMPLE 3 Reevaluate the diffusion coefficient of carbon dioxide in air at 208C and atmospheric pressure using the Fuller, Schettler, and Giddings equation and compare the new value with the one reported in example 2. DAB ¼ 10À3T1:75 1 MA þ 1 MB   1/2 P (åv)1/3 A þ (åv)1=3 B h i 2 ¼ 10À3(293)1:75 1 44 þ 1 29   1/2 (1)[(26:9)1/3 þ (20:1)1/3]2 ¼ 0:152 cm2/s This value compares very favorably to the value evaluated with Hirschfelder equation, 0.155 cm2/s, and its determination was easily accomplished. Brokaw11 has suggested a method for estimating diffusion coefficient for binary gas mixtures containing polar compounds. The Hirschfelder equation () is still used; however, the collision integral is evaluated by VD ¼ VD0 þ 0:196d2 AB TÃ (24-43) where dAB ¼ (dAdB)1/2 d ¼ 1:94 Â 103m2 p VDTD (24-44) mp ¼ dipole moment, Debye Vb ¼ liquid molar volume of the specific compound at its boiling point, cm3/g mol Tb ¼ normal boiling point, K and TÃ ¼ kT/eAB where eAB k ¼ eA k eB k   1/2 e/k ¼ 1:18(1 þ 1:3 d2)Tb (24-45) d is evaluated with (24–44). And VD0 ¼ A (TÃ)B þ C exp(DTÃ) þ E exp(FTÃ) þ G exp(HTÃ) (24-46) 11 R. S. Brokaw, Ind. Engr. Chem. Process Des. Dev., 8, 240 (1969). 24.2 The Diffusion Coefficient 413
56. with A ¼ 1:060,36 E ¼ 1:035,87 B ¼ 0:156,10 F ¼ 1:529,96 C ¼ 0:193,00 G ¼ 1:764,74 D ¼ 0:476,35 H ¼ 3:894,11 The collision diameter, sAB, is evaluated with sAB ¼ (sAsB)1/2 (24-47) with each component’s characteristic length evaluated by s ¼ 1:585 VD 1 þ 1:3 d2   1/3 (24-48) Reid, Prausnitz, and Sherwood12 noted that the Brokaw equation is fairly reliable, permitting the evaluation of the diffusion coefficients for gases involving polar compounds with errors less than 15%. Mass transfer in gas mixtures of several components can be described by theoretical equations involving the diffusion coefficients for the various binary pairs involved in the mixture. Hirschfelder, Curtiss, and Bird13 present an expression in its most general form. Wilke14 has simplified the theory and has shown that a close approximation to the correct form is given by the relation D1Àmixture ¼ 1 y0 2 /D1À2 þ y0 3 /D1À3 þ Á Á Á þ y0 n /D1Àn (24-49) where D1Àmixture is the mass diffusivity for component 1 in the gas mixture; D1Àn is the mass diffusivity for the binary pair, component 1 diffusing through component n; and y0 n is the mole fraction of component n in the gas mixture evaluated on a component-1-free basis, that is y0 2 ¼ y2 y2 þ y3 þ Á Á Á yn ¼ y2 1 À y1 In Problem 24.7 at the end of this chapter, equation (24-49) is developed by using Wilke’s approach for extending the Stefan and Maxwell theory in order to explain the diffusion of species A through a gas mixture of several components. EXAMPLE 4 In the chemical vapor deposition of silane (SiH4 ) on a silicon wafer, a process gas stream rich in an inert nitrogen (N2 ) carrier gas has the following composition: ySIH4 ¼ 0:0075, yH2 ¼ 0:015, yN2 ¼ 0:9775 The gas mixture is maintained at 900 K and 100 Pa total system pressure. Determine the diffusivity of silane through the gas mixture. The Lennard–Jones constants for silane are eA/k ¼ 207:6 K and sA ¼ 4:08 A ˚. 12 R. C. Reid, J. M. Prausnitz, and T. K. Sherwood, The Properties of Gases and Liquids, Third Edition, McGraw-Hill Book Company, New York, 1977, Chapter 11. 13 J. O. Hirschfelder, C. F. Curtiss, and R. B. Bird, Molecular Theory of Gases and Liquids, Wiley, New York, p. 718. 14 C. R. Wilke, Chem. Engr. Prog., 46, 95–104 (1950). 414 Chapter 24 Fundamentals of Mass Transfer
57. The binary diffusion coefficients at 900 K and 100 Pa total system pressure estimated by the Hirschfelder equation (24-33) are DSiH4 ÀN2 ¼ 1:09 Â 103cm2/s and DSiH4 ÀH2 ¼ 4:06 Â 103cm2/s The binary diffusion coefficients are relatively high because the temperature is high and the total system pressure is low. The composition of nitrogen and hydrogen on a silane-free basis are y0 N2 ¼ 0:9775 1 À 0:0075 ¼ 0:9849 and y0 H2 ¼ 0:015 1 À 0:0075 ¼ 0:0151 Upon substituting these values into the Wilke equation (24-49), we obtain DSiH4 Àmixture ¼ 1 y0 N2 DSiH4 ÀN2 þ y0 H2 DSiH4 ÀH2 ¼ 1 0:9849 1:09 Â 103 þ 0:0151 4:06 Â 103 ¼ 1:10 Â 103 cm2 s This example verifies that for a dilute multicomponent gas mixture, the diffusion coefficient of the diffusing species in the gas mixture is approximated by the binary diffusion coefficient of the diffusing species in the carrier gas. Liquid-Mass Diffusivity In contrast to the case for gases, where we have available an advanced kinetic theory for explaining molecular motion, theories of the structure of liquids and their transport characteristics are still inadequate to permit a rigorous treatment. Inspection of published experimental values for liquid diffusion coefficients in Appendix J.2 reveals that they are several orders of magnitude smaller than gas diffusion coefficients and that they depend on concentration due to the changes in viscosity with concentration and changes in the degree of ideality of the solution. Certain molecules diffuse as molecules, while others that are designated as electrolytes ionize in solutions and diffuse as ions. For example, sodium chloride, NaCl, diffuses in water as the ions Naþ and ClÀ. Though each ion has a different mobility, the electrical neutrality of the solution indicates that the ions must diffuse at the same rate; accordingly, it is possible to speak of a diffusion coefficient for molecular electrolytes such as NaCl. However, if several ions are present, the diffusion rates of the individual cations and anions must be considered, and molecular diffusion coefficients have no meaning. Needless to say, separate correlations for predicting the relation between the liquid mass diffusivities and the properties of the liquid solution will be required for electrolytes and nonelectrolytes. Two theories, the Eyring ‘‘hole’’ theory and the hydrodynamical theory, have been postulated as possible explanations for diffusion of nonelectrolyte solutes in low- concentration solutions. In the Eyring concept, the ideal liquid is treated as a quasi- crystalline lattice model interspersed with holes. The transport phenomenon is then described by a unimolecular rate process involving the jumping of solute molecules into the holes within the lattice model. These jumps are empirically related to Eyring’s theory of reaction rate.15 The hydrodynamical theory states that the liquid diffusion coefficient is related to the solute molecule’s mobility; that is, to the net velocity of the molecule while under the influence of a unit driving force. The laws of hydrodynamics provide 15 S. Glasstone, K. J. Laidler, and H. Eyring, Theory of Rate Processes, McGraw-Hill Book Company, New York, 1941, Chap. IX. 24.2 The Diffusion Coefficient 415
58. relations between the force and the velocity. An equation that has been developed from the hydrodynamical theory is the Stokes–Einstein equation DAB ¼ kT 6prmB (24-50) where DAB is the diffusivity of A in dilute solution in D, k is the Boltzmann constant, T is the absolute temperature, r is the solute particle radius, and mB is the solvent viscosity. This equation has been fairly successful in describing the diffusion of colloidal particles or large round molecules through a solvent that behaves as a continuum relative to the diffusing species. The results of the two theories can be rearranged into the general form DABmB kT ¼ f(V) (24-51) in which f(V) is a function of the molecular volume of the diffusing solute. Empirical correlations, using the general form of equation (24-51), have been developed, which attempt to predict the liquid diffusion coefficient in terms of the solute and solvent properties. Wilke and Chang16 have proposed the following correlation for none- lectrolytes in an infinitely dilute solution: DABmB T ¼ 7:4 Â 10À8(FBMB)1/2 V0:6 A (24-52) where DAB is the mass diffusivity of A diffusing through liquid solvent B, in cm2/s; mB is the viscosity of the solution, in centipoises; T is absolute temperature, in K; MB is the molecular weight of the solvent; VA is the molal volume of solute at normal boiling point, in cm3/g mol; and FB is the ‘‘association’’ parameter for solvent B. Molecular volumes at normal boiling points, VA , for some commonly encountered compounds, are tabulated in Table 24.4. For other compounds, the atomic volumes of each element present are added together as per the molecular formulas. Table 24.5 lists the contributions for each of the constituent atoms. When certain ring structures are involved, corrections must be made to account for the specific ring configuration; the following Table 24.4 Molecular volumes at normal boiling point for some commonly encountered compounds Compound Molecular volume, in cm3/g mol Compound Molecular volume, in cm3/g mol Hydrogen, H2 14.3 Nitric oxide, NO 23.6 Oxygen, O2 25.6 Nitrous oxide, N2 O 36.4 Nitrogen, N2 31.2 Ammonia, NH3 25.8 Air 29.9 Water, H2 O 18.9 Carbon monoxide, CO 30.7 Hydrogen sulfide, H2 S 32.9 Carbon dioxide, CO2 34.0 Bromine, Br2 53.2 Carbonyl sulfide, COS 51.5 Chlorine, Cl2 48.4 Sulfur dioxide, SO2 44.8 Iodine, I2 71.5 16 C. R. Wilke and P. Chang, A.I.Ch.E.J., 1, 264 (1955). 416 Chapter 24 Fundamentals of Mass Transfer
59. corrections are recommended: for three-membered ring, as ethylene oxide deduct 6 for four-membered ring, as cyclobutane deduct 8.5 for five-membered ring, as furan deduct 11.5 for pyridine deduct 15 for benzene ring deduct 15 for naphthalene ring deduct 30 for anthracene ring deduct 47.5 Recommended values of the association parameter, FB, are given below for a few common solvents. Solvent FB Water 2.2617 Methanol 1.9 Ethanol 1.5 Benzene, ether, heptane, and other unassociated solvents 1.0 If data for computing the molar volume of solute at its normal boiling point, VA , are not available, Tyn and Calus18 recommend the correlation VA ¼ 0:285V1:048 c where Vc is the critical volume of species A in cm3/g. mol. Values of Vc are tabulated in Reid, Prausnitz, and Sherwood.19 Table 24.5 Atomic volumes for complex molecular volumes for simple substancesy Element Atomic volume, in cm3/g mol Element Atomic volume, in cm3/g mol Bromine 27.0 Oxygen, except as noted below 7.4 Carbon 14.8 Oxygen, in methyl esters 9.1 Chlorine 21.6 Oxygen, in methyl ethers 9.9 Hydrogen 3.7 Oxygen, in higher ethers Iodine 37.0 and other esters 11.0 Nitrogen, double bond 15.6 Oxygen, in acids 12.0 Nitrogen, in primary amines 10.5 Sulfur 25.6 Nitrogen, in secondary amines 12.0 yG. Le Bas, The Molecular Volumes of Liquid Chemical Compounds, Longmans, Green & Company, Ltd., London, 1915. 17 The correction of FB is recommended by R. C. Reid, J. M. Prausnitz, and T. K. Sherwood, The Properties of Gases and Liquids, Third Edition, McGraw-Hill Book Company, New York, 1977, p. 578. 18 Tyn, M.T. and W.F. Calus, Processing, 21, (4): 16 (1975). 19 R.C. Reid, J.M. Prausnitz and, T.K. Sherwood, The Properties of Gases and Liquids, Third Edition, McGraw-Hill Book Company, New York, 1977, Appendix A. 24.2 The Diffusion Coefficient 417
60. EXAMPLE 5 Estimate the liquid diffusion coefficient of ethanol, C2H5OH, in a dilute solution of water at 108C. The molecular volume of ethanol may be evaluated by using values from Table 24.5 as follows: VC2 H5 OH ¼ 2VC þ 6VH þ VO VC2 H5 OH ¼ 2(14:8) þ 6(3:7) þ 7:4 ¼ 59:2 cm3/mol At 10C, the viscosity of a solution containing 0.05 mol of alcohol/liter of water is 1.45 centipoises; the remaining parameters to be used are T ¼ 283 K FB for water ¼ 2:26 and MB for water ¼ 18 Substituting these values into equation (24-52), we obtain DC2 H5 OHÀH2 O ¼ 7:4 Â 10À8(2:26 Â 18)1/2 (59:2)0:6 ! 283 1:45   ¼ 7:96 Â 10À6 cm2/s (7:96 Â 10À10 m2/s) This value is in good agreement with the experimental value of 8:3 Â 10À10m2/s reported in Appendix J. Let us compare this value of the liquid diffusivity of ethanol in a dilute solution of water at 10C, 7:96 Â 10À6cm2/s, with the value of the gas diffusivity of ethanol in air at 10C and 1 atm pressure, 0:118 cm2/s. This emphasizes the order of magnitude difference between the values of the liquid and gas diffusivities. Performing a similar calculation, the liquid diffusion coefficient of water in an infinite dilute solution of ethanol at the same 10C temperature predicts that the diffusion coefficient DBA is equal to 1:18 Â 10À5 cm2/s. It is important to note that liquid diffusivities DABL and DBAL are not equal as were the gas diffusivities at the same temperature and pressure. Hayduk and Laudie20 have proposed a much simpler equation for evaluating infinite dilution diffusion coefficients of nonelectrolytes in water DAB ¼ 13:26 Â 10À5mÀ1:14 B VÀ0:589 A (24-53) where DAB is the mass diffusivity of A through liquid B, in cm2/s; mB is theviscosity of water, in centipoises; and VA is the molal volume of the solute at normal boiling point, in cm3/g Á mol. This relation is much simpler to use and gives similar results to the Wilke–Chang equation. If we substitute the values used in example 4 into the Hayduk and Laudie relationship, we would obtain a diffusion coefficient for ethanol in a dilute water solution of 7:85 Â 10À6cm2/s; this value is essentially the same value obtained using the Wilke–Chang equation. Scheibel21 has proposed that the Wilke–Chang relation be modified to eliminate the association factor, FB, yielding DABmB T ¼ K V1/3 A (24-54) 20 W. Hayduk and H. Laudie, A.I.Ch.E. J., 20, 611 (1974). 21 E. G. Scheibel, Ind. Eng. Chem., 46, 2007 (1954). 418 Chapter 24 Fundamentals of Mass Transfer
61. where K is determined by K ¼ (8:2 Â 10À8) 1 þ 3VB VA   2/3 " # except 1. For benzene as a solvent, if VA < 2VB, use K ¼ 18:9 Â 10À8. 2. For other organic solvents, if VA < 2:5VB, use K ¼ 17:5 Â 10À8. Reid, Prausnitz, and Sherwood22 recommend this equation for solutes diffusing into organic solvents; however, they noted that this equation might evaluate values that had errors up to 20%. Most methods for predicting the liquid diffusion coefficients in concentration solutions have combined the infinite dilution coefficients, DAB and DBA, in a simple function of composition. Vignes23 recommended the following relationship: DAB ¼ (DAB)xB (DBA)xA where DAB is the infinitely dilute diffusion coefficient of A in solvent B, DBA is the infinitely dilute diffusion coefficient of B in solvent A, and xA and xB are the molar fraction composition of A and B. This Vignes equation has been less successful for mixtures containing an associating compound, such as an alcohol. A modification for this type of concentrated solution has been proposed by Leffler and Cullinan24 DABm ¼ (DABmB )xB (DBAmA )xA As the values of liquid diffusion coefficients reported in the literature were obtained in the neighborhood of the ambient temperature, Tyne25 recommended the following equation for extrapolating to higher temperatures (DAB T1 ) (DAB T2 ) ¼ Tc À T2 Tc À T1   n (24-55) where T1 and T2 are in K, Tc is the critical temperature of solvent B in K, and n is the exponent related to the latent heat of vaporization of solvent, DHv, at its normal boiling point temperature. This exponent may be evaluated from the following table: The properties of electrically conducting solutions have been studied intensively for more than 75 years. Even so, the known relations between electrical conductance and the liquid diffusion coefficient are valid only for dilute solutions of salts in water. DHv, (kJ/kmol) 7,900–30,000 30,000–39,000 39,000–46,000 46,000–50,000 >50,000 n 3 4 6 8 10 25 M. J. Tyne, Trans. I. Chem. E., 9, 112 (1981). 22 R. C. Reid, J. M. Prausnitz, and T. K. Sherwood, The Properties of Gases and Liquids, Third Edition, McGraw-Hill Book Company, New York, 1977, Chapter 11. 23 A. Vignes, Ind. Eng. Chem. Fundam., 5, 189 (1966). 24 J. Leffler and H. T. Cullinan, Ind. Eng. Chem., 9, 84 (1970). 25 M. J. Tyne, Trans. I. Chem. E., 9, 112 (1981). 24.2 The Diffusion Coefficient 419
62. The diffusion coefficient of a univalent salt in dilute solution is given by the Nernst equation DAB ¼ 2RT (1/l0 þ þ 1/l0 À)F (24-56) where DAB is the diffusion coefficient based on the molecular concentration of A, in cm2/s; R is the gas constant, 8.316 joules/(K)(g mol); T is absolute temperature, in K, l0 þ, l0 À are the limiting (zero concentration) ionic conductances in (amp/cm2) (volt/cm) (g equivalent/cm3), and F is Faraday’s constant, 96,500 coulombs/g equivalent. This equation has been extended to polyvalent ions by replacing the numerical constant 2 by (1/nþ þ 1/nÀ), where nþ and nÀ are the valences of the cation and anion, respectively. Pore Diffusivity There are many instances where molecular diffusion occurs inside the pores of porous solids. For example, many catalysts are porous solid pellets containing catalytically active sites on the pore walls. The porous catalyst possesses a high internal surface area to promote chemical reactions at the catalytic surface. The separation of solutes from dilute solution by the process of adsorption is another example. In an adsorption process, the solute sticks to a feature on the solid surface that is attractive to the solute. Many adsorbent materials are porous to provide a high internal surface area for solute adsorption. In both examples, the molecules must diffuse through a gas or liquid phase residing inside the pores. As the pore diameter approaches the diameter of the diffusing molecule, the diffusing molecule can interact with the wall of the pore. Below, we describe two types of pore diffusion: the Knudsen diffusion of gases in cylindrical pores and the hindered diffusion of solutes in solvent-filled cylindrical pores. Knudsen diffusion. Consider the diffusion of gas molecules through very small capillary pores. If the pore diameter is smaller than the mean free path of the diffusing gas molecules and the density of the gas is low, the gas molecules will collide with the pore walls more frequently than with each other. This process is known as Knudsen flow or Knudsen diffusion. The gas flux is reduced by the wall collisions. The Knudsen number, Kn, given by Kn ¼ l dpore ¼ mean free path length of the diffusing species pore diameter is a good measure of the relative importance of Knudsen diffusion. If the Kn number is much greater than one, then Knudsen diffusion can be important. At a given pore diameter, the Kn number goes up as the total system pressure P decreases and absolute temperature T increases. In practice, Knudsen diffusion applies only to gases because the mean free path for molecules in the liquid state is very small, typically near the molecular diameter of the molecule itself. Consequently, Kn for liquids is very small. The diffusivity for Knudsen diffusion is obtained from the self-diffusion coefficient derived from the kinetic theory of gases DAAÃ ¼ lu 3 ¼ l 3 ffiffiffiffiffiffiffiffiffiffiffi 8kNT pMA r (24-57) For Knudsen diffusion, we replace path length l with pore diameter dpore, as species A is now more likely to collide with the pore wall as opposed to another molecule. In this 420 Chapter 24 Fundamentals of Mass Transfer
63. instance, the Knudsen diffusivity for diffusing species A, DKA, is DKA ¼ dpore 3 u ¼ dpore 3 ffiffiffiffiffiffiffiffiffiffiffi 8kNT pMA r (24-58) DKA ¼ dpore 3 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 8 p 1:38 Á 10À16 g Á cm s2K   6:023 Á 1023 molecules mol   s ffiffiffiffiffiffiffi T MA r ¼ 4850dpore ffiffiffiffiffiffiffi T MA r This simplified equation requires that dpore has units of cm, MA has units of g/mol, and temperature T has units of K. The Knudsen diffusivity, DKA, is dependent on the pore diameter, species A molecular weight, and temperature. We can make two comparisons of DKA to the binary gas phase diffusivity, DAB. First, it is not a function of absolute pressure P, or the presence of species B in the binary gas mixture. Second, the temperature dependence for the Knudsen diffusivity is DKA / T1=2, vs. DAB / T3=2 for the binary gas phase diffusivity. Generally, the Knudsen process is significant only at low pressure and small pore diameter. However, there are instances where both Knudsen diffusion and molecular diffusion (DAB) can be important. If we consider that Knudsen diffusion and molecular diffusion compete with one another by a ‘‘resistances in series’’ approach, then the effective diffusivity of species A in a binary mixture of A and B, DAe, is determined by 1 DAe ¼ 1 À ayA DAB þ 1 DKA (24-59) with a ¼ 1 þ NB NA For cases where a ¼ 0 (NA ¼ ÀNB), or where yA is close to zero, equation (24-59) reduces to 1 DAe ¼ 1 DAB þ 1 DKA (24-60) The above relationships for the effective diffusion coefficient are based on diffusion within straight, cylindrical pores aligned in a parallel array. However, in most porous materials, pores of various diameters are twisted and interconnected with one another, and the path for diffusion of the gas molecule within the pores is ‘‘tortuous.’’ For these materials, if an average pore diameter is assumed, a reasonable approximation for the effective diffusion coefficient in random pores is D0 Ae ¼ e2DAe (24-61) where e ¼ the volume occupied by pores within the porous solid total volume of porous solid ðsolid þ poresÞ e is the volume void fraction of the porous volume within the porous material. This ‘‘void fraction’’ is usually experimentally determined for a specific material. The four possible types of pore diffusion are illustrated in Figure 24.3, each with their respective diffusivity correlation. The first three, pure molecular diffusion, pure Knudsen 24.2 The Diffusion Coefficient 421
64. diffusion, and Knudsen and molecular combined diffusion, are based on diffusion within straight, cylindrical pores that are aligned in parallel array. The fourth involves diffusion via ‘‘tortuous paths’’ that exist within the compacted solid. EXAMPLE 6 One step in the manufacture of optical fibers is the chemical vapor deposition of silane (SiH4) on the inside surface of a hollow glass fiber to form a very thin cladding of solid silicon by the reaction SiH4(g) ! Si(s) þ 2H2(g) as shown in Figure 24.4. Typically, the pro- cess is carried out at high temperature and very lowtotal system pressure. Optical fibers for high bandwidth data transmission have very small inner pore diameters, typically less than 20 mm (1 mm ¼ 1 Â 10À6 m). If the inner diameter of the Si-coated hollow glass fiber is10 mm, assess the importance of Knudsen diffusion for SiH4 inside the fiber lumen at 900 K and 100 Pa (0.1 kPa) total system pressure. Silane is diluted to 1.0 mol % in the inert carrier gas helium (He). The binary gas phase diffusivity of silane in helium at 25C (298 K) and 1.0 atm (101.3 kPa) total system pressure is 0:571 cm2/s, with sSiH4 ¼ 4:08 A ˚ and eSiH4 /k ¼ 207:6 K. The molecular weight of silane is 32 g/mol. The gas-phase molecular diffusivity of SiH4–He, Knudsen diffusivity for SiH4, and effective diffusivity for SiH4 at 900 K and 100 Pa total system pressure must be calculated. The gas-phase Pure molecular diffusion Pore wall A B D AB P s2 AB W D 0.001858T3/2 1 M A 1 M B + = 1/2 Pure knudsen diffusion Pore wall dpore d pore A D KA = 8 k N T p M A Random porous material D' Ae = e 2 D Ae Pore wall A B Knudsen + molecular diffusion 1 D Ae 1 D AB 1 D KA + @ 3 Figure 24.3 Types of porous diffusion. Shaded areas represent nonporous solids. SiH4 gas H2 gas dpore = 10 µm 900 K, 100 Pa Optical fiber Si thin film Hollow glass fiber Figure 24.4 Optical fiber. 422 Chapter 24 Fundamentals of Mass Transfer
65. molecular diffusivity of silane in helium is scaled to process temperature and pressure using the Hirschfelder extrapolation, equation (24-41) DSiH4 ÀHe 900 K 0:1 kPa ¼ 0:571 cm2 s 900 K 298 K   1:5 101:3 kPa 0:1 kPa   0:802 0:668   ¼ 3:32 Â 103 cm2 s      It is left to the reader to show that the collision integral VD is equal to 0.802 at 298 K and 0.668 at 900 K for gaseous SiH4–He mixtures. Note that the gas phase molecular diffusivity is high due to high temperature and very low system pressure. The Knudsen diffusivity of SiH4 inside the optical fiber is calculated using equation (24-58), with dpore ¼ 1 Â 10À3 cm (10 mm) DK,SiH4 ¼ 4850 dpore ffiffiffiffiffiffiffiffiffiffiffiffi T MSiH4 s ¼ 4850(1 Â 10À3) ffiffiffiffiffiffiffiffi 900 32 r ¼ 25:7 cm2 s As the SiH4 is significantly diluted in He, the process is dilute with respect to SiH4 and so equation (24-60) can be used to estimate the effective diffusivity DSiH4 ,e ¼ 1 1 DSiH4 ÀHe þ 1 DK;SiH4 ¼ 1 1 3:32 Â 103 þ 1 25:7 ¼ 25:5 cm2 s The effective diffusivity for SiH4 is smaller than its Knudsen diffusivity, reflecting the resistance in series approach. Finally, we calculate the Knudsen number for SiH4 l ¼ kT ffiffiffi 2 p ps2 A P ¼ 1:38 Â 10À16 erg K 1 N m 107 erg 900 K ffiffiffi 2 p p 0:408 nm 1 m 109 nm   2 100 N m2 ¼ 1:68 Â 10À4 m ¼ 168 mm Kn ¼ l dpore ¼ 168 mm 10 mm ¼ 16:8 As Kn ) 1 and the effective diffusivity is close to the Knudsen diffusivity, then Knudsen diffusion controls the silane transport inside the optical fiber if no external bulk transport is supplied. Hindered solute diffusion in solvent-filled pores. Consider the diffusion of a solute molecule through a tiny capillary pore filled with liquid solvent. As the molecular diameter of the solute approaches the diameter of the pore, the diffusive transport of the solute through the solvent is hindered by the presence of the pore and the porewall. General models for diffusion coefficients describing the ‘‘hindered diffusion’’ of solutes in solvent-filled pores assume the form of DAe ¼ D AB F1(w)F2(w) (24-62) The molecular diffusion coefficient of solute A in the solvent B at infinite dilution, D A  e , is reduced by two correction factors, F1(w), and F2(w), both of which are theoretically bounded by 0 and 1. Furthermore, both correction factors are functions of the reduced pore diameter w w ¼ ds dpore ¼ solute molecular diameter pore diameter (24-63) If w > 1, then the solute is too large to enter the pore. This phenomena is known as solute exclusion, and is used to separate large biomolecules such as proteins from dilute aqueous 24.2 The Diffusion Coefficient 423
66. mixtures containing solutes of much smaller diameter. As w approaches 1, both F1(w) and F2(w) decrease asymptotically toward zero so at w ¼ 1, the effective diffusion coefficient is zero. The correction factor F1(w), the stearic partition coefficient, is based on simple geometric arguments for stearic exclusion, that is, F1(w) ¼ flux area available to solute total flux area ¼ p(dpore À ds)2 pd 2 pore ¼ (1 À w)2 (24-64) and holds for 0 F1(w) 1:0 The correction factor F2 ðwÞ, the hydrodynamic hindrance factor, is based on the compli- cated hydrodynamic calculations involving the hindered Brownian motion of the solute within the solvent-filled pore. Equations for F2(w), assuming diffusion of a rigid spherical solute in a straight cylindrical pore, have been developed. The analytical models are generally asymptotic solutions over a limited range of w, and ignore electrostatic or other energetic solute–solvent-pore wall intereactions, polydisperity of solute diameters, and noncircular pore cross sections. The most common equation, developed by Renkin,26 is reasonable for 0 w 0:6 F2(w) ¼ 1 À 2:104w þ 2:09w3 À 0:95w5 (24-65) EXAMPLE 7 It is desired to separate a mixture of two industrial enzymes, lysozyme and catalase, in a dilute, aqueous solution by a gel filtration membrane. A mesoporous membrane with cylindrical pores of 30 nm diameter is available (Figure 24.5). The following separation factor (a) for the process is proposed a ¼ DAe DBe Determine the separation factor for this process. The properties of each enzyme as reported by Tanford27 are given below. ds,B = 10.44 nm ds,A = 4.12 nm dpore = 30 nm Bulk solvent Figure 24.5 Hindered diffusion of solutes in solvent-filled pores. 26 E. M. Renkin, J. Gen. Physiol., 38, 225 (1954). 27 C. Tanford, Physical Chemistry of Macromolecules, John Wiley & Sons, New York, 1961. 424 Chapter 24 Fundamentals of Mass Transfer
67. The transport of large enzyme molecules through pores filled with liquid water represents a hindered diffusion process. The reduced pore diameters for lysozyme and catalase are wA ¼ ds,A dpore ¼ 4:12 nm 30:0 nm ¼ 0:137 and wB ¼ ds;B dpore ¼ 10:44 nm 30:0 nm ¼ 0:348 For lysozyme, F1 ðwA Þ by equation (24-64) and F2 ðwA Þ by the Renkin equation (24-65) are F1(wA ) ¼ (1 À wA )2 ¼ (1 À 0:137)2 ¼ 0:744 F2(wA ) ¼ 1 À 2:104wA þ 2:09w3 A À 0:95w5 A ¼ 1 À 2:104(0:137) þ 2:09(0:137)3 À 0:95(0:137)5 ¼ 0:716 The effective diffusivity of lysozyme in the pore, DAe is estimated by equation (24-62) DAe ¼ D AÀH2 O F1(wA )F2(wA ) ¼ 1:04 Â 10À6 cm2 s (0:744)(0:716) ¼ 5:54 Â 10À7 cm2 s Likewise, for catalase F1(wB ) ¼ 0:425; F2(wB ) ¼ 0:351, and DBe ¼ 6:12 Â 10À8 cm2/s: Finally,the separation factor is a ¼ DAe DBe ¼ 5:54 Â 10À7 cm2/s 6:12 Â 10À8 cm2/s ¼ 9:06 It is interesting to compare the value above with a0, the ratio of molecular diffusivities at infinite dilution a0 ¼ D AÀH2 O D BÀH2 O ¼ 1:04 Â 10À6 cm2/s 4:1 Â 10À7 cm2/s ¼ 1:75 The small pore diameter enhances thevalue for a because the diffusion of the large catalase molecule is significantly hindered inside the pore relative to the smaller lysozyme molecule. Solid Mass Diffusivity The diffusion of atoms within solids underlies the synthesis of many engineering materials. In semiconductor manufacturing processes, ‘‘impurity atoms,’’ commonly called dopants, are introduced into solid silicon to control the conductivity in a semiconductor device. The hardening of steel results from the diffusion of carbon and other elements through iron. Vacancy diffusion and interstitial diffusion are the two most frequently encountered solid diffusion mechanisms. In vacancy diffusion, the transported atom ‘‘jumps’’ from a lattice position of the solid into a neighboring unoccupied lattice site or vacancy, as illustrated in Figure 24.6. The atom continues to diffuse through the solid by a series of jumps into other neighboring vacancies that appear to it from time to time. This normally requires a distortion of the lattice. This mechanism has been mathematically described by assuming a unimolecular rate process and applying Eyring’s ‘‘activated state’’ concept, as discussed in the ‘‘hole’’ theory for liquid diffusion. The resulting equation is a complex equation relating the diffusivity in terms of the geometric relations between the lattice positions, the length of the jump path, and the energy of activation associated with the jump. Lysozyme (species A) Catalase (species B) MA ¼ 14 100 g/g mol MB ¼ 250 000 g/g mol ds;A ¼ 4:12 nm ds;B ¼ 10:44 nm Do AÀH2O ¼ 1:04 Â 10À6 cm2/s Do BÀH2O ¼ 4:10 Â 10À7 cm2/s 24.2 The Diffusion Coefficient 425
68. An atom moves in interstitial diffusion by jumping from one interstitial site to a neighboring one, as illustrated in Figure 24.7. This normally involves a dilation or distortion of the lattice. This mechanism is also mathematically described by Eyring’s unimolecular rate theory. Excellent references are available for a more detailed discussion on the diffusion characteristics of atoms in solids (Barrer; Shewmon; Middleman and Hochberg; Kou.28 Appendix Table J.3 lists a few values of binary diffusivities in solids. Figure 24.8 illustrates the dependence of solid-phase diffusion coefficients on temperature, specifically for the diffusion of common dopants in solid silicon. The solid-phase diffusion coefficient has been observed to increase with increasing temperature according to an Arrhenius equation of the form DAB ¼ DoeÀQ/RT (24-66) or ln(DAB) ¼ À Q R 1 T þ ln(Do) (24-67) where DAB is solid diffusion coefficient for the diffusing species A within solid B, Do is a proportionality constant of units consistent with DAB, Q is the activation energy (J/mol), R is the thermodynamic constant (8.314 J/mol Á K), and T is the absolute temperature (K). Energy State 2 State 1 3 1 Activation energy State 2 State 3 Figure 24.7 Solid-state interstitial diffusion. Energy State 2 State 1 3 1 Activation energy State 3 State 2 Figure 24.6 Solid-state vacancy diffusion. 28 R. M. Barrer, Diffusion In and Through Solids, Cambridge University Press, London, 1941; P. G. Shewmon, Diffusion of Solids, McGraw-Hill Inc., New York, 1963; S. Middleman and A. K. Hochberg, Process Engineering Analysis in Semiconductor Device Fabrication, McGraw-Hill Inc., New York, 1993; S. Kou, Transport Phenomena and Materials Processing, John Wiley & Sons Inc., New York, 1996. 426 Chapter 24 Fundamentals of Mass Transfer
69. Data from Figure 24.8 can be used to estimate Q for a given dopant in silicon using equation (24-67). Tables 24.6 and 24.7 provide the diffusion data needed to evaluate DAB by equation (24-66) for self-diffusion in pure metals and interstitial solutes in iron. These tables point out the significant energy barrier that must be surpassed when an atom jumps between two lattice sites by vacancy diffusion (Table 24.6) and a significantly smaller energy barrier encountered in interstitial diffusion (Table 24.7). Diffusion coefficients and solubilities of solutes in polymers are reported by Rogers,29 and by Crank and Park.30 Diffusivities of solutes in dilute biological gels are reported by Friedman and Kramer31 and by Spalding.32 DAB, B = Si (cm2/s) 1000/T(K–1) 0.6 0.7 0.8 In 0.9 10–14 10–13 10–12 10–11 10–10 10–9 Al Sb B, P Ga Figure 24.8 Diffusion coefficients of substitutional dopants in crystalline silicon. Table 24.6 Data for self-diffusion in pure metals Do Q Structure Metal (mm2/s) (kJ/mole) fcc Au 10.7 176.9 fcc Cu 31 200.3 fcc Ni 190 279.7 fcc FeðgÞ 49 284.1 bcc FeðaÞ 200 239.7 bcc FeðdÞ 1980 238.5 29 C. E. Rogers, Engineering Design for Plastics, Reinhold Press, New York, 1964. 30 J. Crank and G. S. Park, Diffusion in Polymers, Academic Press, New York, 1968. 31 L. Friedman and E. O. Kramer, J. Am. Chem. Soc., 52, 1311 (1930). 32 G. E. Spalding, J. Phys. Chem., 3380 (1969). 24.2 The Diffusion Coefficient 427
70. 24.3 CONVECTIVE MASS TRANSFER Mass transfer between a moving fluid and a surface or between immiscible moving fluids separated by a mobile interface (as in a gas/liquid or liquid/liquid contactor) is often aided by thedynamic characteristics of themovingfluid. Thismode of transfer is called convective mass transfer, with the transfer always going from a higher to a lower concentration of the species being transferred. Convective transfer depends on both the transport properties and the dynamic characteristics of the flowing fluid. As in the case of convective heat transfer, a distinction must be made between two types of flow. When an external pump or similar device causes the fluid motion, the process is called forced convection. If the fluid motion is due to a density difference, the process is called free or natural convection. The rate equation for convective mass transfer, generalized in a manner analogous to Newton’s ‘‘law’’ of cooling, equation 15.11 is NA ¼ kc DcA (24-68) where NA is the molar mass transfer of species A measured relative to fixed spatial coordinates, DcA is the concentration difference between the boundary surface concentration and the average concentration of the fluid stream of the diffusing species A, and kc is the convective mass-transfer coefficient. As in the case of molecular mass transfer, convective mass transfer occurs in the direction of a decreasing concentration. Equation (24-68) defines the coefficient kc in terms of the mass flux and the concentration difference from the beginning to the end of the mass- transfer path. The reciprocal of the coefficient, 1/kc , represents the resistance to the transfer through the moving fluid. Chapters 28 and 30 consider the methods of determining this coefficient. It is, in general, a function of system geometry, fluid and flow properties, and the concentration difference DcA : From our experiences in dealing with a fluid flowing past a surface, we can recall that there is always a layer, sometimes extremely thin, close to the surface where the fluid is laminar, and that fluid particles next to the solid boundary are at rest. As this is always true, the mechanism of mass transfer between a surface and a fluid must involve molecular mass transfer through the stagnant and laminar flowing fluid layers. The controlling resistance to convective mass transfer is often the result of this ‘‘film’’ of fluid and the coefficient, kc , is accordingly referred to as the film mass-transfer coefficient. It is important for the student to recognize the close similarity between the convective mass-transfer coefficient and the convective heat-transfer coefficient. This immediately suggests that the techniques developed for evaluating the convective heat-transfer coeffi- cient may be repeated for convective mass transfer. A complete discussion of convective mass-transfer coefficients and their evaluation is given in Chapters 28 and 30. Table 24.7 Diffusion parameters for interstitial solutes in iron Do Q Structure Solute (mm2/s) (kJ/mole) bcc C 2.0 84.1 bcc N 0.3 76.1 bcc H 0.1 13.4 fcc C 2.5 144.2 428 Chapter 24 Fundamentals of Mass Transfer
71. EXAMPLE 8 A pure nitrogen carriergas flows parallel to the 0:6 m2 surface of a liquid acetone in an open tank. The acetone temperature is maintained at 290 K. If the average mass-transfer coefficient, kc , for the mass transfer of acetone into the nitrogen stream is 0.0324 m/s, determine the total rate of acetone release in units of kg.mol/s. The total molar rate of acetone transfer from the liquid to the gas phase can be evaluated by WA ¼ NAA ¼ kcA(cAs À cA1) The mass transfer area is specified as 0:6 m2: At 290 K, acetone exerts a vapor pressure of 161 mmHg or 2:148  104 Pa: Therefore, the concentration of acetone in the gas phase at the acetone surface is cAs ¼ PA RT ¼ 2:148  104 Pa 8:314 Pa  m3 kg mol  K   (290 K) ¼ 8:91 kg mol m3 and the concentration of acetone in the nitrogen carrier gas is near zero because the molar flowrate of the carrier gas is in a large excess relative to the rate of acetone transfer. Thus WA ¼ kcA(cAs À cA1) ¼ 0:0324 m s   (0:6 m2) 8:91 kg:mol m3 À 0   ¼ 0:1732 kg:mol s 24.4 CLOSURE In this chapter, the two modes of mass transport, molecular and convective mass transfer, have been introduced. As diffusion of mass involves a multicomponent mixture, funda- mental relations were presented for concentrations and velocities of the individual species as well as for the mixture. The molecular transport property, DAB, the diffusion coefficient or mass diffusivity in gas, liquid, and solid systems, has been discussed and correlating equations presented. The rate equations for the mass transfer of species A in a binary mixture are as follows: molecular mass transfer: JA ¼ ÀcDAB =yA molar flux relative to the molar-average velocity jA ¼ ÀrDAB =vA mass flux relative to the mass-average velocity NA ¼ ÀcDAB =yA þ yA(NA þ NB) molar flux relative to fixed spatial coordinates nA ¼ ÀrDAB =vA þ vA(nA þ nB) mass flux relative to fixed spatial coordinates convective mass transfer: NA ¼ kc DcA PROBLEMS 24.1 Liquified natural gas, LNG, is to be shipped from the Alaskan Kenai Peninsula by an ocean carrier to processing plant on Yaquina Bay, Oregon. The molar composition of the com- mercial LNG is determine a. the weight fraction of ethane; b. the average molecular weight of the LNG mixture; c. the density of the gas mixture when heated to 207 K and at 1.4 Â105 Pa; d. the partial pressure of methane when the total pressure is 1:4  105 Pa; e. the mass fraction of carbon dioxide in parts per million by weight. methane, CH4 93.5 mol % ethane, C2 H6 4.6% Propane, C5 H8 1.2% Carbon dioxide, CO2 0.7% Problems 429
72. 24.2 In the manufacture of microelectronic devices, a thin film of solid silicon (Si) is uniformly deposited on a wafer surface by the chemical decomposition of silane (SiH4 ) in the presence of H2 gas. If the gas composition is maintained at 40 mol % SiH4 and 60 mol % H2 , determine a. the weight fraction of these species; b. the average molecular weight of the gas mixture; c. the molar concentration, cA , of SiH4 if the feed gas is maintained at 900 K and a system pressure of 60 torr. 24.3 Air is contained in a 30 m3 container at 400 K and 1:013 Â 105 Pa: Determine the following properties of the gas mixture: a. mole fraction of O2 ; b. volume fraction of O2 ; c. weight of the mixture; d. mass density of O2 ; e. mass density of N2 ; f. mass density of the air; g. mass density of the air; h. average molecular weight of the gas mixture. 24.4 Starting with Fick’s equation for the diffusion of A through a binary mixture of species A and B as given by NAz ¼ ÀcDAB dyA dz þyA ðNAz þ NBz Þ and Fick’s equation for the diffu- sion of B through the same binary mixture given by NBz ¼ ÀcDBA dyB dz þyB(NBz þ NAz), prove the two gas diffusivities, DAB and DBA, are equal. Does the Hirshfelder equation for gas evaluating gas diffusivities verify this same equality? 24.5 Starting with the Fick’s equation for the diffusion of A through a binary mixture of components A and B NA ¼ ÀcDAB =yA þ yA(NA þ NB) derive the following relations, stating the assumptions made in the derivations: a. nA ¼ ÀDAB =rA þ wA ðnA þ nB Þ b. JA ¼ ÀDAB =cA 24.6 Starting with Fick’s equation for the diffusion of A through a binary mixture of A and B, prove a. NA þ NB ¼ cV; b. nA þ nB ¼ rv c. jA þ jB ¼ 0: 24.7 Stefan and Maxwell explained the diffusion of A through B in terms of the driving force dcA, the resistances that must overcome the molecular mass transfer, and a proportionality constant, b. The following equation expresses mathematically the resistances for an isothermal, isobaric gaseous system: ÀdcA ¼ b rA MA rB MB ðvAz À vBz Þdz Wilke33 extended this theory to a multicomponent gas mixture. The appropriate form of the Maxwell-type equation was assumed to be À dcA dz ¼ bAB rA MA rB MB ðvAz À vBz Þ þ bAC rA MA rC MC ðvAz À vCz Þ þbAD rA MA rD MD ðvAz À v0z Þ þ Á Á Á Using this relation, verify equation (24-49). 24.8 Determine the value of the following gas diffusivites using the Hirschfelder equation: a. carbon dioxide/air at 310 K and 1:5 Â 105 Pa b. ethanol/air at 325 K and 2:0 Â 105 Pa c. carbon monoxide/air at 310 K and 1:5 Â 105 Pa d. carbon tetrachloride/air at 298 K and 1:913 Â 105 Pa 24.9 The isomerization of n-butane to iso-butane is carried out on a catalyst surface at 2.0 atm and 4008C. What is the gas-phase molecular diffusion coefficient of n-butane in iso-butane? Com- pare values obtained from both the Hirschfelder and Fuller– Schettler–Giddings equations. 24.10 Determine the diffusivity of methane in air using (a) the Hirschfelder equation and (b) the Wilke equation for a gas mixture. The air is at 373 K and 1:5 Â 105 Pa. 24.11 An absorption tower is proposed to remove selectively ammonia from an exhaust gas stream. Estimate the diffusivity of ammonia in air at 1:013 Â 105 Pa and 373 K using the Brokaw equation (24-43). The dipole moment for ammonia is 1.46 debye. Compare the evaluated valuewith the experimental value reported in Appendix Table J.1. 24.12 Highly purified tetrachlorosilane (SiCl4 ) gas is reacted with hydrogen gas (H2 ) to produce electronic-grade polycrys- talline silicon at 8008C and 1:5 Â 105 Pa according to the equction: SiCl4 ðgÞ þ 2H2 ðgÞ ! SiðsÞ þ 4HClðgÞ: There are concerns that the reaction experiences diffusional limitations at the growing Si solid surface. Estimate the mole- cular diffusion coefficient for (a) SiCl4 in H2 and (b) SiCl4 in a gas phase mixture containing 40 mol % SiCl4 , 40 mol % H2 , and 20 mol % HCl. The Lennard–Jones parameters for SiCl4 (species A) are eA/k ¼ 358 K; sA ¼ 5:08 A ˚ . 24.13 An absorption tower has been proposed to remove selectively two pollutants, hydrogen sulfide (H2 S) and sulfur dioxide (SO2 ), from an exhaust gas stream containing Estimate the diffusivity of hydrogen sulfide in the gas mixture at 350 K and 1:013 Â 105 Pa: The critical temperature (TC ) of H2 S is 373.2 K and the critical volume (VC ) of H2 S is 98.5 cm3/mol. H2 S 3 vol % SO2 5 vol % N2 92 vol % 33 C. Wilke, Chem. Eng. Prog., 46, 95 (1950). 430 Chapter 24 Fundamentals of Mass Transfer
73. 24.14 The Stokes–Einstein equation is often used to estimate the molecular diameter of large spherical molecules from the molecular diffusion coefficient. The measured molecular diffu- sion coefficient of the serum albumin (an important blood protein) in water at infinite dilution is 5:94 Â 10À7 cm2=s at 293 K. Estimate the mean diameter of a serum albumin mole- cule. The known value is 7.22 nm. 24.15 Estimate the liquid diffusivity of the following solutes that are transferred through dilute solutions: a. oxygen in ethanol at 293 K; b. methanol in water at 283 K; c. water in methanol at 288 K; d. n-butanol in water at 288 K. Compare this value with experimental value reported in Appendix J.2. 24.16 Water supplies are often treated with chlorine as one of the processing steps in treating wastewater. Determine the liquid diffusion coefficient of chlorine in an infinitely dilute solution of water at 289 K using (a) the Wilke–Chang equation and (b) the Hayduk–Laudie equation. Compare the results with the experi- mental value reported in Appendix J.2. 24.17 Benzene (species A) is often added to ethanol to dena- ture the ethanol (species B). Estimate the liquid-phase diffusion of benzene in ethanol and ethanol in benzene at 288 K by (a) the Wilke–Chang equation and (b) the Scheibel equation. Does DAB ¼ DBA ? 24.18 The aeration of water is an important industrial opera- tion. Determine the liquid diffusion coefficient of oxygen in an infinitely dilute solution of water at 288 K using (a) the Wilke– Chang equation and (b) the Hayduk–Laudie equation. 24.19 A silicon wafer is doped with phosphorus. From Figure 24.8, the nominal value of the diffusion coefficient for phos- phorus in silicon at 1316 K is 1 Â 10À13 cm2/s and at 1408 K is 1 Â 10À12 cm2/s: Determine the value of the diffusion coeffi- cient at 1373 K. 24.20 The case-hardening of mild steel involves the diffu- sion of carbon into iron. Estimate the diffusion coefficient for carbon diffusing into fcc iron and bcc iron at 1000 K. Learn about the structures of fcc and bcc iron in a materials science textbook, and then explain why the diffusion coefficients are different. 24.21 Determine the effective diffusion coefficient for hydro- gen gas (H2 , species A) diffusing into nitrogen gas (N2 , species B) at 1008C and 1.0 atm within the following materials: a. Straight 100 A ˚ pores in parallel array; b. Random pores 100 A ˚ in diameter with void fraction of 0.4; c. Random pores 1000 A ˚ in diameter with void fraction of 0.4; d. Straight 20,000 A ˚ pores in parallel array. 24.22 Researchers are proposing the development of a ‘‘nano- channel reactor’’ for steam reforming of methane (CH4 ) to fuel- cell hydrogen gas to power microscale devices. Gas phase diffusion in nanochannel Nanochannel A = CH4, B= H2O 20 mol% CH 4 300°C, 0.5 atm 200 nm NA/NB = 0.25 A+B As each channel diameter is so small, the gas flow is likely to be very small within a given channel. Hence, gas diffusion pro- cesses mayplay a role inthe operation ofthis device, particularly during the mixing and heating steps. We are specifically inter- ested in evaluating the effective diffusion coefficient of methane gas (species A, MA ¼ 16g/g: mol) in water vapor (species B, MB ¼ 18 g/g: mol) at 3008C and 0.5 atm total system pressure. The diameter of the channel is 200 nm ð1 Â 109 nm ¼ 10 mÞ: A feed gas containing 20 mol % CH4 in water vapor is fed to the nanochannel with a flux ratio NA/NB ¼ 0:25. What is effective diffusion coefficient of CH4 in the nanochannel at the feed gas conditions? Is Knudsen diffusion important? 24.23 Diffusion experiments were conducted with a binary mixture of synthesistic gas containing H2 (species A) diluted in a large excess of CO (species B) at 2.0 atm total system pressure and 808C in a porous material of monodispersed pore size distribution and void volume fraction of 0.3. From the measure- ments, the measured effective diffusion coefficient of hydrogen was 0.036 cm2/s. What is the mean pore size ðdpore Þ of this material? 24.24 A mixture 1.0 mol % O2 (species A) in a helium carrler gas (species B) enters the microscale chamber. The chamber consists of a channel that is 5:0 mm (microns) in diameter. The total system pressurewithin the chamber is 300 Pa, which is very small. The chamber temperature is maintained at 1008C. The molecular weight of oxygen is 32 g/g. mol and helium is 4.0 g/g. mol. a. What is the molar concentration of oxygen gas at the entrance to the microchamber? b. What is the effective diffusion coefficent of O2 (DAez ) within the microchamber? 24.25 Consider a single, porous, spherical, inert mineral par- ticle. The pores inside the particle are filled with liquid water (species B). We are interested in analyzing the molecular diffusion of the contaminant benzene C6 H6 , species A within thewater-filled pores of the particle. The average diameter of the pores is 150 nm and the void fraction is 0.40. The benzene solute does not adsorb onto the intersurfaces of the pores. Benzene is very sparingly soluble in water and has a molecular diameter of 0.15 nm. The process is isothermal at 298 K. The concentration of dissolved benzene in the water surrounding the particle, CAc , Problems 431