[pnguyen]

Hi all,

I'm relatively new to CFD and OpenFoam, and am having problems with a case, so please bare with me. I am an undergrad student doing a project for my supervisor comparing openfoam with fluent.

I am setting up a conjugate heat transfer case inside an oven, with 3 solid regions inside the oven. 2 nozzles direct flow into the oven. The mesh is a fluent mesh that my supervisor gave me.

I have set up the case using the chtMultiRegionSimpleFoam tutorial (multiRegionHeater), changing the 0/ files to suit my problem. I have an inlet velocity of 19m/s with a temp of 478K and the outlet is at zeroGradient for both velocity and temperature, and held at a constant pressure of 1 bar. You can find attached my 0/ files for my fluid and solid regions.

When I run the simulation, it diverges after 130 iterations. The temperature field and density seems to start diverging once the solid heats up. The region where the lowest temperature is on the last iteration via paraview is attached.

Could someone please give me any ideas on what is going wrong with the simulation? Could it be a boundary condition set up, solver set up or mesh? Any replies will be greatly appreciated.

Sincerely,

Peter Nguyen

[psosnows]

Hello Peter,

here are some ideas, maybe you will find something useful:
*) The time step; how did you set it up? Is it small enough with respect to the minimum mesh size, diffusivity, maximum velocity? Remember that the fvc:: schemes are in fact explicit, disregarding the time scheme you have chosen!
*) Boundary conditions; check them again. Yes, I know you have done it twice already- but do it one more time. Well, maybe twice
*) Corrections; I am not very familiar with the new chtMRSFoam code, but I suppose there are some corrections for non-orthogonality or so. Increase the number of those corrections.
*) Recirculation; does it appear? If so- you need to prevent it.
*) Outlet condition; What comes in, must come out. Calculate mass flux going through the inlets and set the outlet with this value as a fixed one.

I hope that one of those ideas will work. Good luck!

Best,
Pawel

[pnguyen]

Hi Pawel,

Thanks a lot for your reply and the suggestions. I just have a few questions regarding some of them.

1) Time step: My fvSchemes is attached if you want to take a look, but I am not sure I understand what you mean when you say to change the time step. Since this is steady state solver, wouldn't the delta t in controlDict just be the iteration number?

2) Outlet condition: Just wondering if you know the BC i could use to do mass flux?

I'll post any improvements I obtain.

Sincerely,

Peter

[psosnows]

Hi Peter,

I really have to find some time and dig into new OF cht solvers... Of course the time step should not matter in a steady state solver.

Regarding outlet- I usually simply use fixedValue. With constant density and know diameters of the outflow the calculation is quite straightforward

Best,
Pawel

[Daniele111]

Hi

I would use chtMRSF to simulate cooling of solid by water forced convection, and calculate solid temperature distribution. Can I use chtMRST for my application?
Thanks
Daniele

[Daniele111]

Hi
No helps?
Daniele

[psosnows]

Well Daniele,

this solver is supposed to perform conjugated heat transfer simulations.
As soon as you manage to set-up your case (make mesh, set properties etc.), then you are ready to go. If you do everything "right", you will get some nice plots and simulation. Then there is the question of interpretation and analysis of the results and used methods, but that is another story...

At the same time I strongly recommend to study the solver itself (I still have some doubts about its performance).

Good luck.