duct fin system with conjugate heat transfer
 

My subject is related with conjugate heat transfer through a duct attached with external fin.With the boundary conditions of velocity inlet at the duct entrance and outflow at outlet(high aspect ratio) the conjugateness is attempted to solve by imposinng an extended domain where the fins are present. As the fins are given wall boundary condition with solid facing,drop in temperature as expected is not found.I want to know the reason for this whether my imposed boundary conditions are not appropriate or the drop in fin temperature will be less as obtained.For turbulent flow inside the duct I have used spalart allmaras one equation model. For extended domain I have used pressure outlet for all the three sides exposed to the atmosphere with wall and wall shadow condition on the side which is having the duct and fin system.