1. More Radiation
EGR 4345
Heat Transfer

2.  =  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.

3. 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

4. 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 )

5. 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 )

6. Gray Surface

7. 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

8. Gray Surface

9. 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

10. Spectral Distribution of Solar Radiation

11. Scattering of Solar Radiation

12. Directional Distribution of Solar Radiation

13. Solar Absorptivity and Emissivity