i'm interested in modeling the transport of chemical species dissolved in water. two questions: 1) if the species transport/reactions is entirely decoupled from the flow , can i solve the flow equations first, and then do separately the species transport? is it as simple as not selecting the flow solution in the Solver/Solution Controls box and only selecting the species solution? 2) if the above is possible, should the mass fraction of water which presumably is always 1 as the bulk fluid be of any relevance? it seems as if most of the fluent examples are directed at gaseous phase transport and reactions, where bulk mass fractions change.

Yes. Yes. Set the bulk species to be the last in the Materials panel list - then it is not solved and is 1 - sum of other species.