How to deal with source term when using RKschemes?
There are a lot of numerical schemes to deal with the derivatives in the NS equation. My question is how to deal with the chemical raction source term? For example: in RoungeKutta schemes there are several intermediate 'times steps' between n to n+1 i.e. v(n+1)=a*v1(n+1/3)+b*v2(n+1/2)+c*v(n) my question is how to deal with the energy and species source terms between those small "fraction time steps" such as from "n to n+1/3" or form "n+1/3 to n+1/2", let them to be constant or let them advance with the dependent variabes such as temperatures and so on?

Re: How to deal with source term when using RKsche
If you are using the RK scheme to march in "pseudotime" to a steadystate solution, then you should try updating source terms (and possibly viscous fluxes) on only a "few" stages (maybe first and last, for example) to save on CPU time for your simulation. You can experiment yourself with updating source terms every RK stage, versus only on some of the RK stages. This is a very typical procedure and since you are just using the RK procedure (in this case) as an iteration technique to find the steadystate solution, you can play around with what gets updated on each stage  all you care about is that the simulation converges and converges in some "reasonable" number of iterations.
If you are using the RK scheme to march explicity in physical time (ie, a timeaccurate simulation), then you should update your source terms every RK stage. Of course, you could evaluate the loss of accuracy in not doing so and make your own choice in accuracy loss versus CPU time reduction for a timeaccurate simulation using RK to march in physical time. 
Re: How to deal with source term when using RKsche
Though the intermediate t* are not clearly known evaluating the source term at all the intermediate steps using the same t seems to work for me.
H 
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