Hello everyone, Im studying an airflow inside a finned tubular heat exchanger at various inlet velocities in Fluent. Starting at 10 ms inlet velocity it runs smoothly and converges but with increasing velocity it starts to diverge (at 12,5ms) and i just cant explain why. Ive tried many different things: k-epsilon and omega SST models, different inlet turbulence intensities and settings (initial pressure), restricted backflow reversal, lower relaxation factors, etc.. Im using NIST real gas model of air. My initial guess was that it has to do with the mesh quality and y+ value, but after doubling the number of cells i still get the same results. Ideal gas model works fine at even larger velocities. Analyzing the results, my turbulent Reynolds number just skyrockets at the end of the domain. I would appreciate any help .
1. Did you do a grid independent study before starting your work? If no, please do.
2. Try reducing the relaxation factor for the energy equation to relax the solution while heat transfer is occurring.
3. You can also solve the flow first without enabling energy and afterwards, you enable the energy equation and solve it as well, depending on whether you are solving a coupled or decoupled problem.
For more information check out this link:
https://www.afs.enea.it/project/neptunius/docs/fluent/html/ug/node468.htm#:~:text=ANSYS%20FLUENTuses%20a%20default%20under-relaxation%20factor%20of%201.0,can%20usually%20retain%20the%20default%20value%20of%201.0.
Hello! To be able to help you with this issue, we need to know more about your case setup. Did you use the pressure-based or density-based solver? In case of DB did you use explicit or implicit formulation? What discretization schemes did you use?
In general, RG simulations converge slower than IG ones. Start with the discretization scheme 1st Order Upwind and try using very low UR factors or CFL number depending on your setup. Also, sometimes helps to start with FMG initialization or a converged solution for an IG case.
Make sure that the specified BCs lie within the range of database of RG. Properties should remain within this range in the entire domain.
Hope that helps, take care!
Jan Kracik So i did it always in pressure based solver, in density based it is actually converging, but the inlet pressure values are much lower than my previous values. i study pressure drop too, so im confused which values are right. Furthermore if I use pressure based but with a mass flow (not velocity as previous) inlet so the corresponding velocity value is around 15ms the simulation converges. I think these are some fluent limitations
Georgiy Tanasov Yes, it also could be a limitation. In my experience with Fluent, it sometimes behaves strangely and unpredictably, especially for larger meshes, parallel calculations, and UDFs.
Sometimes it is a trial-and-error process that need to be performed for a specific problem. So, let us know if you succeed in your simulation. Good luck!
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