Pressure specified boundary w/t natural convection

Atit Koonsrisuk

I model the flow in a equipment with high chimney. This equipment allow the working fluid to flow in through the inlet portion and flow out through a chimney. The flow arise due to natural convection within the equipment. Actually I am not sure about specify pressure as the boundary conditions in CFX. I always set the Inlet as 'Openning' with zero total pressure, and Outlet as 'Outlet' with zero average static pressure. When the working fluid is air, the result seems reasonable. (I add the hydrostatic pressure to the pressure obtained when I inspect the result.) However, when I chose water as the working fluid, the water flow in from the chimney and out at the inlet portion. I try to set the temperature in the chimney portion to be higher than the temperature in other portions of the equipment as the initial guess. The result is that the temperature in the chimney is higher that others, but the water still flow down. So I suspect if I have to specify the hydrostatic pressure at the boundary. Please suggest me. Thank you very much.