Radiation in Fluent
 

Hello Everyone;
I want to make an radiation heat transfer analysis in fluent and I am confused about the internal emissivity and external emissivity term,what they are exactly mean and what they are used for. I have 2 parallel duct and one is radiating to the environment and I want to analyse the effect of the radiating duct to the another duct. When I am defining boundry conditions; if I use a solid cell zone ;I cannot assign it an internal emissivity.(Fluent allows only external radiation to the solid cell zones).Could you help me? Thank you very much.