Adrian Bejan(Editor), Allan D. Kraus (Editor). Heat transfer Handbok (776115), страница 91
Текст из файла (страница 91)
ASME, 75, 489–499.Rotem, Z., and Claassen, L. (1969). Natural Convection above Unconfined Horizontal Surfaces, J. Fluid Mech., 39, 173–192.Schuh, H. (1948). Boundary Layers of Temperature, in Boundary Layers, W. Tollmien, ed.,British Ministry of Supply, German Document Center, Ref. 3220T, Sec. B.6.Sparrow, E. M., and Gregg, J. L.
(1956). Laminar Free Convection Heat Transfer from theOuter Surface of a Vertical Circular Cylinder, Trans. ASME, 78, 1823–1829.Sparrow, E. M., and Gregg, J. L. (1958). Similar Solutions for Free Convection from a Nonisothermal Vertical Plate, J. Heat Transfer, 80, 379–386.BOOKCOMP, Inc. — John Wiley & Sons / Page 570 / 2nd Proofs / Heat Transfer Handbook / Bejan[570], (46)Lines: 1345 to 1397———0.0pt PgVar———Custom Page (2.0pt)PgEnds: TEX[570], (46)REFERENCES123456789101112131415161718192021222324252627282930313233343536373839404142434445571Tennekes, H., and Lumley, J.
L. (1972). Introduction to Turbulence, MIT Press, Cambridge,MA.Torrance, K. E. (1979). Open-Loop Thermosyphons with Geological Applications, J. HeatTransfer, 101, 677–683.Turner, J. S. (1973). Buoyancy Effects in Fluids, Cambridge University Press, Cambridge.Vliet, G. C. (1969). Natural Convection Local Heat Transfer on Constant Heat Flux InclinedSurfaces, J. Heat Transfer, 9, 511–516.Vliet, G. C., and Liu, C. K. (1969). An Experimental Study of Turbulent Natural ConvectionBoundary Layers, J.
Heat Transfer, 91, 517–531.Vliet, G. C., and Ross, D. C. (1975). Turbulent, Natural Convection on Upward and DownwardFacing Inclined Heat Flux Surfaces, J. Heat Transfer, 97, 549–555.Warner, C. Y., and Arpaci, V. S. (1968). An Experimental Investigation of Turbulent NaturalConvection in Air at Low Pressure along a Vertical Heated Flat Plate, Int. J.
Heat MassTransfer, 11, 397–406.Yang, K. T. (1987). Natural Convection in Enclosures, in Handbook of Single-Phase Convective Heat Transfer, S. Kakaç, R. K. Shah, and W. Aung, eds., Wiley-Interscience, New York,Chap. 13.Yang, K. T., and Lloyd, J.
R. (1985). Turbulent Buoyant Flow in Vented Simple and ComplexEnclosures, in Natural Convection: Fundamentals and Applications, S. Kakaç, W. Aung,and R. Viskanta, eds., Hemisphere Publishing, New York, pp. 303–329.Yuge, T. (1960). Experiments on Heat Transfer from Spheres Including Combined Natural andForced Convection, J. Heat Transfer, 82, 214–220.BOOKCOMP, Inc. — John Wiley & Sons / Page 571 / 2nd Proofs / Heat Transfer Handbook / Bejan[571], (47)Lines: 1397 to 1419———*279.119pt PgVar———Custom Page (2.0pt)* PgEnds: PageBreak[571], (47)123456789101112131415161718192021222324252627282930313233343536373839404142434445CHAPTER 8Thermal RadiationMICHAEL F.
MODESTCollege of EngineeringPennsylvania State UniversityUniversity Park, Pennsylvania[First Page]8.1Fundamentals8.1.1 Emissive power8.1.2 Solid angles8.1.3 Radiative intensity8.1.4 Radiative heat flux8.2 Radiative properties of solids and liquids8.2.1 Radiative properties of metalsWavelength dependenceDirectional dependenceHemispherical propertiesTotal propertiesSurface temperature effects8.2.2 Radiative properties of nonconductorsWavelength dependenceDirectional dependenceTemperature dependence8.2.3 Effects of surface conditionsSurface roughnessSurface layers and oxide films8.2.4 Semitransparent sheets8.2.5 Summary8.3 Radiative exchange between surfaces8.3.1 View factorsDirect integrationSpecial methodsView factor algebraCrossed-strings method8.3.2 Radiative exchange between black surfaces8.3.3 Radiative exchange between diffuse gray surfacesConvex surface exposed to large isothermal enclosure8.3.4 Radiation shields8.3.5 Radiative exchange between diffuse nongray surfacesSemigray approximation methodBand approximation method[573], (1)Lines: 0 to 90———11.3931pt PgVar———Normal Page* PgEnds: PageBreak[573], (1)573BOOKCOMP, Inc.
— John Wiley & Sons / Page 573 / 2nd Proofs / Heat Transfer Handbook / Bejan574123456789101112131415161718192021222324252627282930313233343536373839404142434445THERMAL RADIATION8.4Radiative properties of participating media8.4.1 Molecular gases8.4.2 Particle cloudsSootPulverized coal and fly ash dispersionsMixtures of molecular gases and particulates8.5 Radiative exchange within participating media8.5.1 Mean beam length method8.5.2 Diffusion approximation8.5.3 P-1 approximation8.5.4 Other RTE solution methods8.5.5 Weighted sum of gray gases8.5.6 Other spectral modelsNomenclatureReferences[574], (2)Lines: 90 to 1428.1———FUNDAMENTALS-1.32596pt PgVar———Radiative heat transfer or thermal radiation is the science of transferring energy inNormal Pagethe form of electromagnetic waves.
Unlike heat conduction, electromagnetic wavesdo not require a medium for their propagation. Therefore, because of their ability to * PgEnds: Ejecttravel across vacuum, thermal radiation becomes the dominant mode of heat transfer in low pressure (vacuum) and outer-space applications. Another distinguishing[574], (2)characteristic between conduction (and convection, if aided by flow) and thermal radiation is their temperature dependence. While conductive and convective fluxes aremore or less linearly dependent on temperature differences, radiative heat fluxes tendto be proportional to differences in the fourth power of temperature (or even higher).For this reason, radiation tends to become the dominant mode of heat transfer inhigh-temperature applications, such as combustion (fires, furnaces, rocket nozzles),nuclear reactions (solar emission, nuclear weapons), and others.All materials continuously emit and absorb electromagnetic waves, or photons, bychanging their internal energy on a molecular level.
Strength of emission and absorption of radiative energy depend on the temperature of the material, as well as on thewavelength λ, frequency ν, or wavenumber η, that characterizes the electromagneticwaves,λ=c1=νη(8.1)where wavelength is usually measured in µm(= 10−6 m), while frequency is measured in hertz = cycles/s), and wavenumbers are given in cm−1 . Electromagneticwaves or photons (which include what is perceived as “light”) travel at the speed oflight, c. The speed of light depends on the medium through which the wave travelsand is related to that in vacuum, c0 , through the relationBOOKCOMP, Inc.
— John Wiley & Sons / Page 574 / 2nd Proofs / Heat Transfer Handbook / BejanFUNDAMENTALS123456789101112131415161718192021222324252627282930313233343536373839404142434445c0nc=c0 = 2.998 × 108 m/s575(8.2)where n is known as the refractive index of the medium. By definition, the refractiveindex of vacuum is n ≡ 1.
For most gases the refractive index is very close to unity,and the c in eq. (8.1) can be replaced by c0 . Each wave or photon carries with it anamount of energy determined from quantum mechanics as = hνh = 6.626 × 10−34 J · s(8.3)where h is known as Planck’s constant. The frequency of light does not change whenlight penetrates from one medium to another because the energy of the photon must beconserved. On the other hand, the wavelength does change, depending on the valuesof the refractive index for the two media.When an electromagnetic wave strikes an interface between two media, the waveis either reflected or transmitted. Most solid and liquid media absorb all incomingradiation over a very thin surface layer. Such materials are called opaque or opaquesurfaces (even though absorption takes place over a thin layer).
An opaque materialthat does not reflect any radiation at its surface is called a perfect absorber, blacksurface, or blackbody, because such a surface appears black to the human eye, whichrecognizes objects by visible radiation reflected off their surfaces.8.1.1Emissive PowerEvery medium continuously emits electromagnetic radiation randomly into all directions at a rate depending on the local temperature and the properties of the material.The radiative heat flux emitted from a surface is called the emissive power E, andthere is a distinction between total and spectral emissive power (heat flux emittedover the entire spectrum or at a given frequency per unit frequency interval), so thatthe spectral emissive power Eν is the emitted energy/time/surface area/frequency,while the total emissive power E is emitted energy/time/surface area.
Spectral andtotal emissive powers are related by ∞ ∞E(T ) =Eλ (T,λ) dλ =Eν (T,ν) dν(8.4)00It is easy to show that a black surface is not only a perfect absorber, but it is also aperfect emitter, that is, the emission from such a surface exceeds that of any othersurface at the same temperature (known as Kirchhoff’s law). The emissive powerleaving an opaque black surface, commonly called blackbody emissive power, can bedetermined from quantum statistics asEbλ (T,λ) =2πhc02n2 λ5 (ehc0 /nλkT − 1)(n = const)(8.5)where it is assumed that the black surface is adjacent to a nonabsorbing mediumof constant refractive index n. The constant k = 1.3806 × 10−23 J/K is known asBOOKCOMP, Inc. — John Wiley & Sons / Page 575 / 2nd Proofs / Heat Transfer Handbook / Bejan[575], (3)Lines: 142 to 184———-1.13882pt PgVar———Normal PagePgEnds: TEX[575], (3)576Visible part of spectrum7105000K1021000KK20003000K103/)C3104T=5105762 K106=(TBlackbody emissive power Eb (W/m2 .
µm)108E b123456789101112131415161718192021222324252627282930313233343536373839404142434445THERMAL RADIATION05010 1K100Wavelength λ , µm[576], (4)101Lines: 184 to 211Figure 8.1 Blackbody emissive power spectrum.———-2.8009pt PgVarBoltzmann’s constant. The spectral dependence of the blackbody emissive power intovacuum (n = 1) is shown for a number of emitter temperatures in Fig.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
