I'm performing a simple simulation on the mixture of watre vapor and air diffusing through a 2D domain. the water vapor takes more mass fraction that that of air (almost 2:1), I 've tried input them in a different order in the materieal window. the calculation show that there is almost no difference between the two cases (the species fraction and the temperature distribution).

although this example may be too simple, but could any one tell me why the the input order is very important for the species transport, FLUENT's user's manul did not give any further explaination.

Thanks

The last species in the list is computed by difference; a transport equation is not solved. Therefore, it should usually be the species with the highest concentration---not a species present only in trace amounts. The order of the rest of the list does not matter.