A tube heat transfer simulation
 

Hi, I want to simulate a pipe in the air, with heated fluid flowing into the pipe, and try to get the temperature of the outlet flow.

My first model is just a pipe, with boundary conditions: velocity inlet and temperature, pressure outlet and temperature, coupled interface between the fluid and the tube inner wall, and tube outside wall with convection boundary(as I just focus on the change along the pipe not the area around it, I don't include the area around the pipe in the model, thinking of the convection boundary could transfer the heat out). the geometry and mesh is shown in Fig.1

The second try add the area around the pipe, with boundary conditions: velocity inlet and temperature, pressure outlet and temperature, coupled interface between the fluid and the tube inner wall, coupled interface between the outer tube wall and the area around the pipe (air), and wall with constant temperature boundary (assuming the temperature at some distance away from the tube is free constant air temperature). The geometry and mesh is shown in Fig.2

The results of the two models are different, I have no idea which is right, or both wrong? Is there any mistake in my understanding of this problem model?
Regards,

Jing