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In cases where multiple species enter the domain through multiple inlets with one of the species having a flow rate orders of magnitude less than the rest, it is almost impossible to get good species mass balance for the trace specie. This is because at the inlet (mass flow or velocity) for the trace specie, the mass fraction of that specie is set at 1 and 1 cell next to the inlet, the mass fraction is very small. This huge gradient makes it almost impossible to converge that particular species conservation equation.
To overcome the problem described above, the work-around is to make that specie inlet as wall and separate one cell region next to this wall from the rest of the fluid. Then one can specify mass and species source terms for this one cell region next to wall such that the total mass of species flowing into the domain is same as that specified by the mass flow (or the velocity) inlet earlier. With application of source terms, the gradients are not severe and the flow and species equation converge nicely and overall species mass balance is obtained quickly.
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