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Energy equation blows up in simple channel

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Old   October 31, 2017, 15:18
Default Energy equation blows up in simple channel
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Shamoon Jamshed
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Hello experts,

I am solving a channel flow with conjugate heat transfer. So it has solid regions apart from fluid domains. This problem never occurred to me before. And I tried my best to solve it but no clue.
I am doing simulation of water through this channel which is convex in the middle of its axial length. The walls are made of copper. I simulate the flow very well with all equations converged below to 1e-06. But when I turn on energy , the residual goes to 1e14 (in simple terms, it diverges). I tried coupled, simple, coupled with pseudo transient etc. but did not find a solution. MEsh is ok as well. I am totally stuck. Why the solution is diverging?

The temperature is room temperature 300K and no heat source. But I need to turn energy on because later I have to solve CHT problem by giving flux at the bottom of channel.
Please help
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Old   November 1, 2017, 12:26
Default Better shell concuction instead of solid mesh?
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Moritz Kuhn
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Hello Shamoon,
It is possible that your solid mesh is not properly connected with the fluid mesh? Try to solve your problem without the solid mesh and use for your channel wall a defined wall thickness in conjunction with "shell conduction" instead.

Moritz
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Old   November 1, 2017, 13:03
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You could try to make a sketch with all the boundary conditions, material properties and initial values you used and upload it here. That might give us a hint if there is a problem with the setup.
When you say "turn on energy equation": do did you remember to initialize the temperature with a reasonable value before proceeding with the calculation?
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Old   November 1, 2017, 13:31
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Quote:
Originally Posted by MKuhn View Post
Hello Shamoon,
It is possible that your solid mesh is not properly connected with the fluid mesh? Try to solve your problem without the solid mesh and use for your channel wall a defined wall thickness in conjunction with "shell conduction" instead.

Moritz
Actually, I believe that shell conduction does not work. I tried it many times for simple pipe flows, but it does not give correct results.
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Old   November 1, 2017, 13:45
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Quote:
Originally Posted by flotus1 View Post
You could try to make a sketch with all the boundary conditions, material properties and initial values you used and upload it here. That might give us a hint if there is a problem with the setup.
When you say "turn on energy equation": do did you remember to initialize the temperature with a reasonable value before proceeding with the calculation?
I would defnitely put initial value for temperature if it did have to change. But when even I am giving constant temperature, why it is chaging at all in the convex region of the domain. ITs basically a 3d geometry but I am attaching a 2d sketch. I had to later put flux on lowaer wall.
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File Type: png channel.png (8.9 KB, 7 views)
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Old   November 2, 2017, 09:44
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Quote:
Originally Posted by Shamoon Jamshed View Post
Actually, I believe that shell conduction does not work. I tried it many times for simple pipe flows, but it does not give correct results.
I've some good experience with shell conduction, but thanks for the hint, that the solution could be not correct.

Try to locate the error after some iterations, maybe you have some local very high (respectively low) temperatures, this could indicate poor mesh at this location.
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