More Radiation EGR 4345 Heat Transfer

 =  Kirchhoff’s Law For any surface in the enclosure No real surface can have an emissive power exceeding that of a black surface at the same temperature.

Kirchhoff’s Law For spectral conditions; less restrictive – good if irradition is diffuse or surface is diffuse  =  For no restrictions; inherent surface properties (independent of spectral and directional distributions of emitted and incident radiation) , =  , We will use this for two body interactions

Gray Surface If , =  , then  =  If either of the following is true The irradiation is diffuse (I,i is independent of  and ) The surface is diffuse (, and  , are independent of  and )

Gray Surface If =  then  =  If either of the following is true The irradiation corresponds to emission from a blackbody at the surface temperature T, in which case G() = E ,b( ,T) and G = Eb(T). The surface is gray ( and  are independent of )

Gray Surface

Gray Surface =  is not always true nor does  =  have to be true over the entire spectrum. GRAY SURFACE – one for which  and  are independent of  over the spectral regions of the irradiation and the surface emission

Gray Surface

Extraterrestrial Solar Irradiation Gs,o = Sc (f) cos Sc – solar constant Flux of solar energy incident on a surface normal to sun’s rays Value of 1353 W/m2 f – correction factor To account for the eccentricity of the earth’s orbit Value between 0.97 and 1.03

Spectral Distribution of Solar Radiation

Scattering of Solar Radiation

Directional Distribution of Solar Radiation

Solar Absorptivity and Emissivity