Sorry if I did not clarify my question properly. What I want to say: I have (air+Co2) mixture and I want to define the value of concentration of Co2 instead of the mass fraction on the boundaries and I want to set it as a constant value (as 350 ppm)? I just want Fluent to consider different concentrations between the walls? knowing that concentration is a dimensional quantity not the same as the mass fraction which it non-dimensional? so is that possible?

Many thanks