Have you set the operating density in the system?
No, I didn't. I left the default operating conditions without specifying operating density. If I should, then the question is which density should be used the low air density or the high oil density or a mean density between them. There is only one field for one operating density.
Thank you very much Rob for your suggestion. Now I set the operating density to be that of air and I got results similar to the ones I get when each side is simulated separately. Of course I still need to judge the results more accurately but they seem OK from the first look.
To understand how this actually worked I found the following paragraph from Fluent user guide
Even though I've read this before but just now I can notice the highlighted part "averaging over all cells".
Does that mean that fluent has calculated the operating density by averaging densities of air and oil?! how about densities of the solids in the heat exchanger are they included as well?
Also, since now I've used air density as an operating density, can that make the results for the oil inaccurate some how?
Given it's in the buoyant flow section of the manual I assume it uses the fluid properties only. The same section also notes that setting the operating density helps with convergence and shouldn't influence the end result: so now it's got a fixed value to work with the solver should be happier. CFX has an independent (maybe detached, @DrAmine will know) volume option for their equivalent operating pressure and density settings, and we've asked for the same in Fluent given the confusion this can cause.
Yes, it averages over fluid properties only because when I simulate air side with the solid parts added I still got correct results.
I agree that calculating operating density by averaging causes confusion especially in applications having non-mixing 2 or more fluids with large density differences (like in many heat exchanges). I think for this case there should be separate operating conditions for each fluid in the system.
Finally I'm happy with the results and thank you all for your help :)
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