Scaling of heat transfer coefficient with Thickness in 2D heat transfer/fluid flow model

Hi,

I'm modeling heat transfer of a simple cylinder in convective flow in 2d, with a defined volumetric heat source (General Source in W/m^3).

If I add a constant heat flux to the outside of the cylinder, and change the heat transfer thickness dz, the temperature of the cylinder is unchanged, as I would expect (since the heat source is volumetric).

However, if I add CFD turbulent flow and do a multiphysics solution, the temperature changes dramatically when dz is changed (ie, the effective heat transfer coefficient has changed).

Anyone have a clue what may be going on here?