[mehdiyeganeh]

hi everybody
How absorption & reflection coefficient can be applied to walls, because only emissivity can be entered for walls.
thanks.
mehdi.

[LuckyTran]

In general you cannot enter an absorption coefficient for a wall boundary condition because it is not a boundary condition, it is a physical/material property. Reflection is even more complicated because it is bi-directional.

But are you trying to use the surface to surface radiation model? Even if not explicitly, you might be thinking about the prob using the s2s model and applying the gray-diffuse model for thick bodies.

In thick body radiation, one can have: emissivity, reflectivity, absorptivity, & transmissitivity.

By energy balance the sum of reflectivity, absorptivity, & transmissivity must be 1. If the surface is opaque and thick enough, you can eliminate transmissivity because all light is absorbed. In thermal equilibrium, Kirchoff's Law gives that absorptivity equals emissivity. Hence, all of these properties are uniquely defined once you know the surface emissivity.