[er_ijaz]

I am working on drafts/tutorialing in ANSYS Fluent to explore gas distribution in an HVAC duct system using the species transport model. The setup includes a vertical main duct supplying multiple floors, with branch ducts at each level that open into rooms. The top of the main duct is also open to the atmosphere. Air is the primary medium, and the gas is introduced from the first floor, mixing with air as it moves upward.

I tested two different boundary conditions for the branch outlets (room inlets):

Pressure Outlet (0 Pa, atmospheric):

Reverse flow occurs at the branch outlets, allowing room air to enter and mix with the gas.
A significant portion of the gas escapes through the top opening rather than being evenly distributed through the branches.
Outflow Boundary Condition:

Gas exits more uniformly through the branch ducts.
There is no reverse flow into the system.
Since each branch opens into a room at atmospheric pressure, I am unsure whether Pressure Outlet or Outflow is the most appropriate boundary condition to accurately model gas transport and mixing behavior.

Which one should I use to best represent real flow behavior in an HVAC system while ensuring correct gas distribution across different levels?

[MKuhn]

I would recommend the pressure outlet. A reversflow is not an error, if you define the back flow conditions properly (temperature, ...)