Hi,

I am wondering, if absorptivity and emissivity coefficients depend on the temperature of a body? Example

I have two plates, plate 1 (T1=300 Kelvin) and plate 2 (T2=301K)

Both plates are not in thermal equilibrium, so plate 2 gives thermal heat in form of radiation to plate 1. So for plate 2: Q=Boltzmann*Area*Emissivity*ViewFactor21*(T2^4-T1^4) - Boltzmann*Area*Absorptivity*ViewFactor12*(T1^4-T2^4)

So at that the beginning, the emissivity of plate two higher than its absorptivity. But what about their absorption coefficients and emissivity coefficients? Do they change with temperature?

But when both bodies have the same temperature, then the emissivity = absorptivity coefficient according to Kirchhoff. So in some way they depend on temperature?

Can you follow me?

So in the end I am wondering, why in some CFD codes it is stated, that for technical surfaces, we can assume emissivity = absorptivity coefficient. If this is true, then the surfaces can not have same different temperatures because they would always loose the same mount of energy as the absorp?

I think I have a big misunderstanding in the Kirchhoff Law?