[akidess]

I cannot help you with Elmer, but I'm 100% confident that given the correct settings you should be able to obtain a stable solution. You might want to describe your problems on the appropriate forum if you haven't so already. Depending on your case it may be impossible to expect to solve the case with a single core in reasonable time, but everyone has access to multicore processors these days. Elmer runs parallel out of the box, as does OpenFOAM.

To specify a volume heat source in OpenFOAM, you need a scalarSemiImplicitSource (you define the regions via cellSets). At surfaces, you specify temperature gradients, that is the heat source in [W/m2] divided by the thermal conductivity. So it's possible, but I'm not sure if your experience will be any better than Elmer.