Hi @schaiburum, thanks for posting
Let me try and answer your questions as best as I can.
→ Since the system is closed, there is no need to define any boundary condition to spcify an opening, you can leave them undefined
→ Since you said there aren’t openings and the air is just recirculating within the domain, you shouldn’t use the fan boundary condition. If you were to use it, you’d be inserting mass into the system which wouldn’t be able to leave it
→ You could use either a momentum source or a rotating zone. The difference is that a momentum source will accelerate the fluid but won’t model the actual fan behavior (fluid-blades interaction) whereas the rotating zone will. If you’re not concerned with that, just go with the momentum source approach for simplicity
→ As momentum sources or rotating zones. Continuity errors will appear if mass is not conserved (e.g. you have an inlet but no outlets)
Cheers
Igor