Hello,

I was wondering if anyone could help me understand the proper way to setup a conjugate heat transfer problem. I have run a few Navier-Stokes (NS) problems with Elmer, and feel pretty confident about getting consistent results, even with thermally driven flow. The issue arises when I try to excite the system by applying a heat load to a solid, which is then connected to a fluid. The thermal system converges almost immediately, but the fluid system doesn't seem to advance. I have two systems of equations, one for the fluid (air), which includes the Heat Equation and NS, with calculated convection, and the second which is just the heat equation applied to the solid. I have tried running it both steady and transient without success. The attached sif file is indicative of what I have tried, playing with the solvers, preconditioning and solver order. I can try to share the mesh, but it has 4M elements (it is a very simple geometry, so I am not sure why there is an issue), and is around 500MB as a unv file. (Images attached)

I have also tried this problem with slightly different boundary conditions (which are more reflective of what I am trying to do) in which I have both solid and fluid thermal excited boundary conditions, and those seem to crash the most. When I apply the thermal boundary as a heatflux, there doesn't seem to be any excitation, but when I apply it as a temperature, I can get very small time steps to converge, but it ultimately diverges as the time progresses.

My main questions is if I have setup the solvers correctly to reflect the solid-fluid thermal relationship, and what could be the cause of my divergence/lack of convergence problem?