Selectiong the right boundary type
 

Hello
I am Panos and i am working as researcher at the NTU Athens. I am new on cfd and im trying to simulate heat transfer phenomena and air flow in a heater. The geometry is quite simple. A cylinder that is heated by resistances. on the upper side of the cylinder i have the following boundaries. A velocity inlet, a mass inlet and wall. i have air flow from the top to the bottom and ore at the same time is imported from a hole near the air inlet hole. My question is about the right outlet boundary type i have to choose. I am between pressure outlet and outflow. can anybody advice me or explain me the difference between those two types?
in addition, regardless of talking about the furnace exit it is possible to have some air coming in the furnace from the outlet of the system
Thank you in advance:)