Conjugate Heat Transfer Analysis Problem
 

I am doing a transient conjugate heat transfer analysis between a solid zone and a fluid zone in 2 steps in Ansys Fluent. In step 1, the fluid zone has fluid 1 with high velocity (forced convection). However, in step 2, I have stationary fluid 2 (natural convection) in the fluid cell zone.

I changed the material of the fluid cell zone in step 2 and also changed the BC as required. The simulation runs okay, but the results are not right. For some reason the heat transfer in step 2 is slowed down significantly. Even the temperature change in the solid zone is very low which is not acceptable. All on a sudden, the system is heading towards steady state, but that is not the actual case.

Any suggestion?

The results of the first step is okay. The heat transfer as well as flow velocity looks okay. The problem is with the second step only.