PCM boundary condition - Density variation with temperature
 

I am trying to simulate Phase change energy storage model, which include melting and solidification of phase change material (PCM). Heat is transferred from heat transfer fluid to PCM (See picture attached). Since PCM is heated, density of pcm varies with temperature.

My question is, since my pcm mesh volume is constant, as pcm heats up density changes and pcm expands. I am currently using "wall" boundary condition which I think will not allow any expansion of pcm.

What is the appropriate PCM boundary condition to accommodate the expansion of PCM. I appreciate any help regarding this issue.

Thanks,
Abe