Thin wall
 

I have to find increase in temperature of water as it flows through a triangular pipe with heat flux incident on it.thickness of pipe is 3 mm which i am taking into account by modeling it in designmodeler separately(2 bodies 1 solid wall 2 inner fluid region) and joining them together using form new part.i am having trouble generating a good quality mesh in wall part as it is very thin as compared to lenghth(3mm:3m).i tried using proximity advance size function.what else can i do?

You could control the size of the mesh locally by imposing sizing for the edges and the pipe surfaces. Also try matching for the surfaces shared by the interface. If I were you I would mesh one of the end surfaces and use sweep mesh method.