buoyancy driven flows

brian

My model is a vertical downflow model. I have a mass flow inlet at the top. In it the heat flux from the walls is great enough so natural convection occurs. I suspect the overall flow through the domain is constant, but there will be areas of recirculation near the hot walls.

My question is what outlet do I use? Fluent says not to use outflow for varying density flows. So I would suspect pressure outlet, but what does this impose?

I understand if I were to use pressure inlet and pressure outlet, by varying the DP in the domain, I'd vary the flowrate, but when i've already set the mass inlet flowrate, how does varying the pressure outlet affect the model? Thanks