Natural convection + Species in a room

Fran

Dear All, I am modelling a room with air. This room has got a heat source and a species source (hidrogen). The room has got an inlet from which air could enter due to the buoyancie effect and an outlet from which air+species scapes to the exterior (another control volume). I would like to model the natural convection within the room, and how the air goes out from the room. I have got some problems with the mass balance and convergence. Can anyone recommend me wich is the best set up in order to get a good convergence? Cheers