Energy equation convergence problem

Reza

I have asked this before but have not found a good answer. I have to solve the energy equation along with the momentum and continuity equation for a solidification problem. The problem is that at certain points the energy equation does not converge. Also I am running a multi-component mixture so I have mushy zones and it is impossible to use the expressions used in the literatue for updating the enthalpy. What I am doing at this moment is updating the enthalpies in the source term of energy equation based on the availble temperature. Also I have to under-relax the temperature at each iteration. But for some strange reason the convergence fails after some time. I have run the code for single phase and it was ok. Can anybody give me a hint or help me with some ideas?

Thank you for your considerations.