[HumanistEngineer]

Hello Again,
I try to model the temperature change (of water) through an insulated pipe in time. So I solve:



The thermal resistance for the pre-insulated pipe is based on x:


The problem is that whenever I change the mesh size (&number) in space x while solving the convection diffusion equation (with heat sink) the overall thermal resistance Rt is changing so the water temperature profile through the pipe. For Example (According to steady-state calculations the plateau should be 68.07°C!):



QUESTION: How to avoid the mesh size effect on the heat sink? Are my formulations true in principle?