Boundary condition of species transport for Pressure outlet Boundary condition
 

Hello! Everyone! I have a question in Fluent, as you can see the following figure, for the Pressure outlet, if we set the species mass fraction=0, it means that we set the mass fraction equal zero in the case of back flow, but if there is no backflow velocity, so the mass fraction is non-zero, and all the species will come from the interior and can pass through the outlet to the outside, am I right? Thank you!
The following link is my figure:
https://previews.dropbox.com/p/thumb...ue&size_mode=5

I am struggling to understand this BC too... could anyone shed some light on the matter?:)

EDIT. I found it. Check out this thread:
https://www.cfd-online.com/Forums/fl...onditions.html

Finally, what I said above is correct. When I set mass fraction at the outlet equal zero, it means that mass fraction will be zero in the case of there is backflow in to the domain (there is reversed flow into the domain at the outlet). I already tried this case and I got the results like what I think.

That is correct. In cases, you don't have any backflow, you can set it any values.