Actually, I am simulating a problem having cuboid shape geometry. It contains two inlets and one outlet.
No. 1 inlet = hydrogen is flowing inside the geometry with a known value of mass flow rate.( So species mass fraction of hydrogen= 1)
No. 2 inlet = air is coming inside the geometry with known velocity value.( Species mass fraction of oxygen= 1).
No. 3 outlet = as mixture of hydrogen and oxygen is going out so pressure outlet is selected ( species mass fraction values= ? )
What can be the solution?
Thank you.
If it is a true outlet, i.e. flow can only leave your fluid domain through that surface then you do not need to set mass fractions at that boundary condition. If the outlet is actually an opening with a pressure boundary condition then it is possible that flow can re-enter your model domain and in that case you must define what the likely mass fractions are in that re-entrant flow. As you know what the flow rate of oxygen and hydrogen at your two inlets is, then if those two flows become well mixed while passing through your domain it would seem reasonable to use the mass fractions of the mixture to define your opening boundary condition. If the gases do not become well mixed by the time they reach your opening and the local mass fraction is important I would suggest you need to model more of your system, perhaps including the downstream geometry.
@Tristan Davenne
Thank you for your support.
If I add exhaust fan as an outlet boundary condition for the mixture (hydrogen and oxygen) to escape out from the domain then what value of species mass fraction I should put?
I think a fan at the outlet would probably mean no re-entrant flow through that boundary so perhaps you could model it as a true outlet with no possibility of re-entrant flow and hence you won't need to define mass fractions at that boundary. If you are using an opening as the boundary one test you could do is see if your result is sensitive to the values of mass fraction that you choose at the opening. In this case I would start with a value based on the ratio of the two inlet flows and then try a few different values either side and see if it effects your result.
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