[Olds88]

Hi all,

I am trying to model the cooling of a heat sink with a heat source in Fluent. I'm generating the mesh in Pointwise and then exporting to Fluent. I've attached a picture to help clarify what I'm trying to do. The small box below the sink is the heat source.

I've created the mesh and then the boundary conditions (inlet, outlet, etc.). There is a shared domain between the heat source and the heat sink. I also specified volume conditions for the sink and source (both solid) and for the fluid. Once I export to Fluent and open the case in Fluent I get this message:

Warning: materials in neighbor cell threads (2 and 7) of
interior zone 11 are of different types (air and aluminum).
This problem MUST be fixed before solving!
Warning: materials in neighbor cell threads (2 and 7) of
interior zone 11 are of different types (air and aluminum).
This problem MUST be fixed before solving!
Warning: materials in neighbor cell threads (2 and 9) of
interior zone 13 are of different types (air and aluminum).
This problem MUST be fixed before solving!
Warning: materials in neighbor cell threads (2 and 9) of
interior zone 13 are of different types (air and aluminum).

I changed these zones from interior zones to wall zones so I could get shadow walls to allow for heat transfer. Now I have coupled walls between the sink an the fluid, the sink and the source, and the source and the fluid.

I've set up the source the have a source of 10 W. I have an inlet velocity of 1 m/s. However, when I run the simulation, the following message appears:

temperature limited to 5.000000e+03 in 55766 cells on zone 2 in domain 1
temperature limited to 5.000000e+03 in 140077 cells on zone 7 in domain 1
temperature limited to 5.000000e+03 in 1195 cells on zone 9 in domain 1

What am I doing wrong? I don't think 10 W is too much so there must be something wrong with my mesh or setup.

Eric

[AlphaKapla]

Originally Posted by Olds88

I've set up the source the have a source of 10 W
Eric

Dear Eric,

have you paid attention that in Fluent, energy source term is in W/(m^3)?

[Olds88]

Yes,

I have accounted for that. The source is 14.5 x 14.5 x 2 (mm). So I specified a source of 2.38e7 W/m3. Which should come out to a 10 W source.

[AlphaKapla]

Well this is a very little source, the power density is quite high but it should be ok...

I don't know how is your convergence, but this warning message is OK if you are at the beginning of your run, especially if your mesh in the source part is not very refined. You can decrease the limit of the max temperature if you want to improve your convergence (in case that your setup is good and that you have a good guess about the max temp., which I don't think so), or you can increase the limit to see how far you overpass the 5000K even after many iterations.

but first of all I would check the report flux and see how your energy is dissipating, it should help to see if it comes from your setup or your mesh.

Good luck

[Olds88]

So I think I figured out my problem. I created the mesh in mm but I didn't scale it upon bring it into Fluent. So I think Fluent thought everything was in meters. I scaled it and now everything seems to be working better. The flux reports look good. Thanks for your help!

[Olds88]

I have another question for you.

If I have two adjacent cell zones, Fluent defines an "interior" boundary condition between them. For example, between the heat source and the heat sink, where they are connected there is an "interior" boundary condition. I have a conformal mesh.

In order to get the correct heat transfer between the two zones, do I need to change the interior into a wall/shadow wall? I have tried both and they yield very similar results.