流体力学与 及传热课件RADIATION HEAT TRANSFER.pptVIP

4.6. RADIATION HEAT TRANSFER ;All substances at temperatures above absolute zero emit radiation that is independent of external agencies.;Fundamental facts concerning radiation;Radiation as such is not heat, and when transformed into heat on absorption, it is no longer radiation.;The fraction that is absorbed is called absorptivity α.; When bodies at different temperatures are placed in sight of one another inside an enclosure, the hotter bodies loss energy by emission of radiation faster than they receive energy by absorption of radiation from the cooler bodies. Temperatures of hotter bodies decrease.;Wavelength of radiation ; Although radiation of any wavelength is, in principle, convertible into heat on absorption by matter, the portion of the electromagnetic spectrum that is of importance in heat flow lies in the wavelength range between 0.5 and 50 μm. ;Visible light covers a wavelength range of about 0.38 to 0.78 μm;Emissive power; At constant surface temperature, a curve can be plotted showing the rate of energy emission as a function of the wavelength. ; For the entire spectrum of the radiation from a surface, the total radiating power W is the sum of all the monochromatic radiations from the surface, or , mathematically,;Blackbody radiation;Emissivities of solids;Practical source of blackbody radiation No actual substance is a blackbody, although some materials, such as certain grades of carbon black, do approach blackness.;The distribution of energy in the spectrum of a blackbody is known accurately. It is given by Planck’s law;Planck’s law can be shown to be consistent with the Stefan-Boltzmann law by substituting Wb,λ from Eq 4.6-6 into Eq 4.6-2 and integrating.; A basic relationship for blackbody radiation is the Stefan-Boltzmann law, which states that the total emissive power of a blackbody is proportional to the fourth power of the absolute temperature, or ;Absorption of radiation by opaque solids;Thu