Free convection in a pipe
 

Hi there I am trying to model the heat transfer inside of a rectangular cavity with a pipe running through the cavity which is responsible for cooling the cavity. The first test will be to model the pipe with air inside which cools the cavity by means of natural convection (drawing in cool air from the bottom, while the hot air is removed at the top). The problem I am experiencing is reverseed flow inside of the pipe. I currently have the inlet and outlet of the pipe BC's set as a pressure outlet. Can anyone please advise me on the BC's, if they are correct or any help would be much apreciated.

Thanks