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heat exchanger, 2d axial symmetry

Posted Jun 21, 2010, 8:11 a.m. EDT 5 Replies

fabrizio alberti
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hi,
i'm trying to model a pipe (stationary, 2d axial symmetry, coupled fluid dynamic and heat transfer) where oil flow and is heated at the wall by an imposed heat flux condition.
when I check the solution in postprocess I should have a sum for "ntflux_htgh" (surface integral for axissymmetric model) equal to 0 on the only three boundary that aren't insulated:
1) oil entrance
2) oil exit
3) the heated wall.

In my case this is not appending! and thus the FIRST PRINCIPE of thermodynamic is not respected!

is that a numeric problem? how can I have the correct integral solution?


5 Replies Last Post Jun 21, 2010, 10:25 a.m. EDT
fabrizio alberti
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Posted: 1 decade ago Jun 21, 2010, 8:17 a.m. EDT
here you can fin my model:
docs.google.com/leaf?id=0BwjKLImxn8FNYjM5NzdjMmYtMTQ3Ny00YWZmLWJhNWQtMTY1YjdkY2IyYThh&hl=en
here you can fin my model: https://docs.google.com/leaf?id=0BwjKLImxn8FNYjM5NzdjMmYtMTQ3Ny00YWZmLWJhNWQtMTY1YjdkY2IyYThh&hl=en
Edgar Kaiser
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Posted: 1 decade ago Jun 21, 2010, 8:24 a.m. EDT
Hi,

you will hardly get a true zero in a numerical system. Did you check how big the discrepancy is (in comparison to the total heat flux from the heated wall?
You can try to refine the mesh and see if the discrepancy gets smaller. Checking such discrepancies is actually one way to verify for proper meshing.

Best regards
Edgar
Hi, you will hardly get a true zero in a numerical system. Did you check how big the discrepancy is (in comparison to the total heat flux from the heated wall? You can try to refine the mesh and see if the discrepancy gets smaller. Checking such discrepancies is actually one way to verify for proper meshing. Best regards Edgar
fabrizio alberti
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Posted: 1 decade ago Jun 21, 2010, 8:35 a.m. EDT
hi,
the discrepancy is quite big
the difference between inlet and outlet for the fluid is 64 W, and is not matching with the 35 W I'm imposing at the wall.
It seems like there is an internal generation of heat, that doesn't make any sense.

I refined the mesh but the difference is still the same.


How can i impose the respect of such condition?

thank you very much for your help

fabrizio
hi, the discrepancy is quite big the difference between inlet and outlet for the fluid is 64 W, and is not matching with the 35 W I'm imposing at the wall. It seems like there is an internal generation of heat, that doesn't make any sense. I refined the mesh but the difference is still the same. How can i impose the respect of such condition? thank you very much for your help fabrizio
Edgar Kaiser
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Posted: 1 decade ago Jun 21, 2010, 9:38 a.m. EDT
Fabrizio,

I had a quick look into your model. one comment:

You are defining constants with short names. One or more of them might be internal names of COMSOL. COMSOL likes to use short names like x, y, z. This will get you into troubles. I try to avoid that by adding a certain prefix to all my variables.

Regards
Edgar

Fabrizio, I had a quick look into your model. one comment: You are defining constants with short names. One or more of them might be internal names of COMSOL. COMSOL likes to use short names like x, y, z. This will get you into troubles. I try to avoid that by adding a certain prefix to all my variables. Regards Edgar
fabrizio alberti
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Posted: 1 decade ago Jun 21, 2010, 10:25 a.m. EDT
Thank you very much Edgar,
but I don't think that's the problem. I still have same results.
Thank you very much Edgar, but I don't think that's the problem. I still have same results.

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