流体力学与传热 ：4-6- RADIATION HEAT TRANSFER

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 ddiation
Known electromagnetic radiations cover an enormous range of wavelengths, from the short cosmic rays to long wave broadcasting wave.
Visible light covers a wavelength range of about 0.38 to 0.78 µm
At temperature above about 5000ºC ，heat radiation in the visible spectrum become significant.
The sum of these fractions must be unity, or
ρ+ α + τ =1
4.6-1
The maximum possible absorptivity is unity. A body which absorbs all incident radiation is called a black body.
The higher the temperature of the radiating body, the shorter the predominant wavelength of the thermal radiation emitted by it.
Emissive power
The monochromatic energy emitted by a radiating surface depends on the temperature of the surface and on the wavelength of the radiation.
Some types of material will emit radiation when they are treated by external agencies.
All substances at temperatures above absolute zero emit radiation that is independent of external agencies.
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.
Emission of radiation
The radiation emitted by any given mass of substance is independent of other material in sight of , or in contact with, the mass.
The fraction that is transmitted is called transmissivity τ.
The fraction of the radiation falling on a body that is reflected is called reflectivity ρ.
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.
4.6. RADIATION HEAT TRANSFER
Radiation, which may be considered to be energy streaming through space at the speed of light, may originate in various ways.
Radiation that is the result of temperature only is called thermal radiation.
Fundamental facts concerning radiation
Radiation moves through space in straight lines, or beams, and only substances in sight of a radiating body can intercept radiation from that body.
The net energy gained or lost by a body is the difference between the energy emitted by the body and that absorbed by it from the radiation reaching it from other bodies.