Hi @Rob. Thank you so much for your quick response.
Yes, it is the species equation. All species mix in the gas phase and enter from a single inlet. Actually, I am running several cases using different density functions such as density as a constant (volume-weighted mixing law) and density as an ideal EOS. In ideal gas conditions, it started with good convergence and then the solution stopped converging in 40 iterations. It is a transient solver with PISO and PRESTO. The same model I used without species transport (just mixing rule for density calculation) is converging very well.
Thanks & regards