How can I set Initial composition of air in the cellar?
 

Hi guys,

I would like to ask someone for one problem.

Description of my idea:
I´m trying to solve flow of methane through porous media. I attachedd a picture of my idea. There is a cellar in this porous media and there is a small gap (crack) in this cellar. There is a space for stairs, I suppose that it could be a velocity or mass-flow inlet. The air should flow from this inlet to the cellar. All rest sides of cellar are walls.

My idea is that there will be an air in the cellar and methane flow from underground will flow through small crack and it will be mixed with air. I would like to know concentration of methane in the cellar after finishing of solving.

For Inlet of methane I suppose mass-flow inlet.
For Surface I would like to have pressure outlet or outlflow.
Crack is Interior.
Spece for stairs: it could be velocity inlet, pressure-inlet or outlet (I don´t know)

How can I set that there will be an air (21% O2, 1% CO2 and 78% N2) during solution initiatization?