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

The flow rate will be constant as defined in inlet boundary condition. The outlet pressure will be used as a reference pressure to calculate the pressure in the remaining part of domain, using the dp obtained by solving the governing eqns.

if you vary the output pressure, you can see that input pressure also varies such that delta P will be constant.

For buoyancy driven flows use presto or bfw pressure scheme with incomp-ideal-gas or ideal gas with gravity on. For v.low temp ranges you can use Boussinesq for density.