1、1CHAPTER 14Radiation Heat Transfer2Content n14.1 INTRODUCTIONn14.2 EMISSION OF RADIATIONn14.3 ABSORPTION OF RADIATION BY OPAQUE SOLIDS n14.4 RADIATION BETWEEN SURFACES n 14.5 RADIATION TO SEMITRANSPARENT MATERIALS n14.6 COMBINED HEAT TRANSFER BY CONDUCTION-CONVECTION AND RADIATION 314.2 EMISSION OF

2、RADIATIONn1. Wavelength of radiationn2. Emissive powern3. Blackbody radiation; emissivityn4. Emissivities of solidsn5. Practical source of blackbody radiationn6. Laws of blackbody radiation43. Blackbody radiation; emissivity Emissivity (黑度/發(fā)射率)， The ratio of the total emissive power W of a body to t

3、hat of a blackbody Wb (at constant surface temperature ).bWW(14.3) 5 Monochromatic Emissivity the ratio of the monochromatic emissive power of a body to that of a blackbody at the same wavelength(at constant surface temperature ). (14.4) ,bWW Graybody (灰體) If the monochromatic emissivity of a body i

4、s the same for all wavelengths, the body is called a gray body. 64. Emissivities of solidsn Emissivities:npolished metals: 0.03 to 0.08;nmost oxidized metals: 0.6 to 0.85; nnon-metals such as refractories(耐火材料), paper, boards, and building materials: 0.65 to 0.95;npaints, other than aluminum paint(鋁

5、粉漿): 0.80 to 0.96. nEmissivity usually increases with temperature.75. Practical source of blackbody radiationnNo actual substance is a blackbody.nAn experimental equivalent of a blackbody is an isothermal enclosure containing a small peephole. 86. Laws of blackbody radiation Plancks law Varying laws

6、 of monochromatic emissive p o w e r o f b l a c k b o d y w i t h t h e wavelengths.(Fig14.1)(14.7) 1/51,2TCbeCW216110742. 3mWCKcmC439. 129FIGURE 14.1 Energy distribution in spectra of blackbodies and gray bodies. 10nBecause 0,dWWbbStefan-Boltzmann law Total emissive power of a blackbody is proport

7、ional to the fourth power of the absolute temperature4TWb(14.5) Stefan-Boltzmann constant:428/10672. 5KmW11Wiens displacement law the maximum monochromatic radiating power max is inversely proportional to the absolute temperature. (14.8) CTmaxThe constant C is 2,890 when max is in micrometers and T

8、is in kelvins.121314.3 ABSORPTION OF RADIATION BY OPAQUE SOLIDSn1. Reflectivity and absorptivity of opaque solids n2. Kirchhoffs law 141. Reflectivity and absorptivity of opaque solidsnopaque solids (不透熱體不透熱體)nTransmissivity = 0，nAbsorptivity + reflectivity =1152. Kirchhoffs lawnAt temperature equil

9、ibrium, the ratio of the total radiating power of any body to its absorptivity depends only upon the temperature of the body. 2211WW(14.9) This law applies to both monochromatic and total radiation.16 If the first body is a blackbody, 1=1, (14.10) 221WWWbbWW22(14.11) 222bWW(14.12) But, by definition

10、, the emissivity of the second body 2211WW(14.9) 17FIGURE 14.1 Energy distribution in spectra of blackbodies and gray bodies. Solid lines for blackbody Dotted line for graybody1814.4 RADIATION BETWEEN SURFACESn1. Black surfacesn2. Nonblack surfaces19nThe total radiation for a unit area of an opaque

11、body of area A1, emissivity 1, and absolute temperature T1 is4111TAq(14.14) 201. Black surfacesn(1) Two large parallel black planes41T42TT2T1 FIGURE 14.3 (a)The net loss of energy per unit area by the first plane and the net gain by the second are q12/A)(/4241424112TTTTAqW/m221(2) General black surf

12、acesFIGURE 14.322)(424112TTAFqq12 = net radiation between two surfacesA = area of either of two surfaces, chosen arbitrarilyF = dimensionless geometric factor F :view factor or angle factor(角系數(shù)), depends upon the geometry of the two surfaces, their spatial relationship with each other, and the surfa

13、ce chosen for A.(14.25)23(14.26)If surface A1 is chosen for A )(424112112TTFAqIf surface A2 is chosen)(424121212TTFAq(14.27)212121FAFA(14.28)Thus 242. Nonblack surfacesn(1) a small body that is not black surrounded by a black surface A1,T1A2,T2The net energy loss by surface A1 is 421212411112TFATAq(

14、14.34)25But by A2F21=A1 Eq. (14.34) becomes)(421411112TTAq(14.35) If surface A1 is gray, 1 =1 and )(42411112TTAq(14.36) In general, for gray surfaces, )()(4241212424112112TTFATTFAq(14.37) where F12 and F21 are the overall interchange factors and are functions of 1 and 2. 421212411112TFATAq(14.34)26(

15、2) Two large parallel planesthe total amount of radiation originating at surface 1 that is absorbed by surface 2 per unit area: )1 ()1 ()1)(1 (1 /2221212141221TAq27Some of the energy originating at surface 2, is reflected by surface 1 and returns to surface 2, where part of it is absorbed. The amoun

16、t of this energy, per unit area, is )1 ()1 ()1 (/22122122242222TAq28nThe total amount of energy absorbed by a unit area of surface 2 is Finally, getor)()(4241212424112112TTFATTFAq1/1/112121F222221212/AqAqAq1/1/1)(214241212TTAq2914.6 COMBINED HEAT TRANSFER BY CONDUCTION-CONVECTION AND RADIATIONnassuming black surroundings, the total loss is, )()(44TTTThAqAqAqwwwcrcT(14.44)Equation (14.44) is sometimes writt