Hi all!

I am attempting to model a simple air flow channel. In my application, oxygen diffuses out through the roof of the channel. I want to observe the change in mass fraction of oxygen down the length of the channel.

I have set this up in the following way:

Inlet face: mass flow rate inlet, Oxygen and Nitrogen with substance concentration set to 0.22 Oxygen and 0.78 Nitrogen volume fractions.

Roof: mass flow rate outlet, Oxygen only (using substance concentrations of 1 Oxygen and 0 Nitrogen)

Outlet face: static pressure opening, set to atmospheric pressure.

When I run the model, using a cut plot along the length of the channel there is no change in the mass fraction of Oxygen or Nitrogen, regardless of the inlet/outlet mass flow rates.

Does anyone have any suggestions as to why I am not observing a gradient in the mass fraction of Oxygen along the length of the channel?