Convection in a liquid: density change in a closed volume
 

Hi all,
I am trying to simulate a melting problem: hot oil is flowing in a heat pipe, warming up a mass of PCM (phase change material). When melting occurs I need to compute the convection movements in the liquid phase, which are very important for heat-exchange. Obviously, in order to see convection movements, I have to set a temperature changing density for my PCM. My problem is: how can I assume a density changing flow like that, in a closed volume? I tried opening the top section of my volume (setting it like "outflow") but i was not able to achieve convergence in steady initial solution. Can anyone help me?
I hope I've been clear
Regards and thank you all.