How to define boundary condition
 

Hi dear all Can somebody.I am applying fluent5.4 to simulate the burning of solid fuel .Vaporizing of the fuel is primarily sustained by the rate of heat transfering from gas to the surface by conduction and radiation.The surface regression rate depends on the fuel surface temperature.Now I don't how to define the boundary condition of wall.How can I get the fuel surface temperature. I've tried everything I can imagine!!! Thanks a lot, in advance......