coupling two phase laminar flow and heat transfer in fluids

Hi guys,

I have serious convergence problem in my case of two phase laminar flow and heat transfer in fluids. It is not a big deal. Just a box with inlet and oulet - for the flow - and one temperature boundary source and time dependent analysis.. . At the beginning there is nitrogen inside and another gas is going through the inlet and is mixing with the nitrogen. At the same time, there is a temperature at one boundary. I just want to know what is happenig with the density of the mixture in time.

Is there some trick how to couple these two analyses? It works fine separately, of course...

Thank you in advance!

Katerina