Free stream temperature in internal flows
 

Hi,
I'm trying to simulate flow of hot gas in a rectangular duct with some thickness, which gets heated up by the flow. As the gas flows inside the duct, there is a temperature drop as it loses some heat to the inner walls. So, while giving convection boundary condition for inner walls, what should be the free stream temperature? ( constant (avg) or variable?). Inlet temperature is around 400 degree C