TAILIEUCHUNG - Heat Transfer Handbook part 60

Heat Transfer Handbook part 60. The Heat Transfer Handbook provides succinct hard data, formulas, and specifications for the critical aspects of heat transfer, offering a reliable, hands-on resource for solving day-to-day issues across a variety of applications. | RADIATIVE PROPERTIES OF SOLIDS AND LIQUIDS 583 the enclosure. Thus in the case of opaque walls it is customary to speak of absorption by and emission from a surface although a thin surface layer is implied. If radiation impinging on a solid or liquid layer is considered a fraction of the energy will be reflected reflectance p often also referred to as reflectivity another fraction will be absorbed absorptance a often also referred to as absorptivity and if the layer is thin enough a fraction may be transmitted transmittance t often also referred to as transmissivity . Because all radiation must be either reflected absorbed ortransmitted p a t 1 If the medium is sufficiently thick to be opaque then t 0 and p a 1 All surfaces also emit thermal radiation or rather radiative energy is emitted within the medium some of which escapes from the surface . The emittance e is defined as the ratio of energy emitted by a surface as compared to that of a black surface at the same temperature the theoretical maximum . All of these four properties may vary in magnitude between the values 0 and 1 for a black surface which absorbs all incoming radiation and emits the maximum possible a e 1 and p t 0. They may also be functions of temperature as well as wavelength and direction incoming and or outgoing . One distinguishes between spectral and total properties an average value over the spectrum and also between directional and hemispherical properties an average value over all directions . It may be shown through another variation of Kirchhoff s law that at least on a spectral directional basis a TfK s e TfK s This is also true for hemispherical values if either the directional emittance or the incoming radiation are diffuse they do not depend on direction . It is also true for total values if eitherthe spectral emittance does not depend on wavelength orif the spectral behavior of the incoming radiation is similar to blackbody radiation at the same temperature. Typical

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