How can I prevent reversed flow from outlet Pressure Boundaries in Fluent 6.3 ?
 

I am modeling a 3D solar air heater using FLUENT 6.3. The air heater is tilted about 40 deg form the horizon and its inlet is open to the atmosphere. The collector outlet is connected to a vertical chimney which is also opened to the atmosphere.
The updraft force and the natural convection shall drive the air up in the chimney.

1- I have chosen the absorber to be the source of energy by applying heat flux.
2- The collector cover (glass) is selected to participate in both convection and radiation
3- Other walls (including the chimney walls) are selected as adiabatic walls.
4- The inlet is selected to be as “pressure inlet”
5- The outlet is selected to be an “Pressure outlet”
6- The radiation DO model is selected.
7- Heat flux is taken about 350 W/m^2
8- Gravity acceleration is also considered.

Everything till now is ok. Right?

When I start the simulation, first the program show a message after each iteration that reversed flow is occurred in cell No. …
Then when I checked the results I found that the flow is coming as a downdraft instead of an updraft.

I tried to close the inlet and outlet and consider them as adiabatic walls and then everything works fine and the natural updraft and circulations were reasonable in the closed system

Please advise how can get the correct run and solve this problem?