Dear friends,

I am using steady-state, converged case and data files for getting residence time distribution. I am using water as tracer in a tank containing water( main fluid) (one inlet and outlet). Properties of both tracer and main fluid is same. I am injecting tracer at the inlet (0.0005 Mass fraction) and trying to note down the mass fraction of tracer at the outlet.

My doubt is regarding the equations to be solved. I am not changing any flow condition at all. only change is the mass fraction at the inlet. hence as per my thinking there should not be any change in the flow field. Therefore, what are the equation i should solve? can it be only species equation? OR flow, turbulence (K-epsilon), species, energy? waitig for CFD fundoo's comments.

thanking you.

I only solve the species equation and it works fine. In a few exceptional cases I had to solve the flow as well, but you should be able to see whether it is necessary or not. If your result does not seem illogical with only the species solved it should be fine.

Dear piran Thanks for reply. I tried to use only speceis eqn but mass balance of tracer is not maintained. actually i have inputed 4.4gm of water but only 3.68gm is comming out. Remaining (i.e. 4.4-3.68) is lost some where. that is why i tried with flow, turbulence,speceis equation then i got mass balance of tracer. The reason is not clear to me. Why i have to solve all equations for tracer study( when i am disturbing flow filed at all)? Pl. reply at the earliest.

Dear piran Thanks for reply. I tried to use only speceis eqn but mass balance of tracer is not maintained. actually i have inputed 4.4gm of water but only 3.68gm is comming out. Remaining (i.e. 4.4-3.68) is lost some where. that is why i tried with flow, turbulence,speceis equation then i got mass balance of tracer. The reason is not clear to me. Why i have to solve all equations for tracer study (I am not disturbing flow filed at all. Tracer properties are same as that of main fluid, Total inflow is also same only mass fraction is changed)? Pl. reply at the earliest.

Well it is a mystery to me as well. Normally you should only have to solve the species equation. But since the result is faulty and by solving the other equations the result is correct I am afraid you will have to solve all the equations. I can not explain it, I have had the same problem only a couple of times and in those cases there was also no apparent explication or reason for this behavior. Maybe it is a bug in the software. I do not know. Sorry I can't help you more!

Dear Piran, Thanks for your help and quick reply. L.G.Patil