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PCM

Posted 23 sept. 2015, 09:09 UTC−4 Heat Transfer & Phase Change, Mesh, Results & Visualization Version 4.4 3 Replies

Astrid van Sprang
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I want to model the heat transfer in a PCM. But the problem is that every time I run the model (with the exact same settings) I get different results..
How is this possible and how do I know what the right solution is?

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3 Replies Last Post 25 sept. 2015, 10:47 UTC−4
Frank van Gool COMSOL Employee

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Posted: 9 years ago 25 sept. 2015, 09:21 UTC−4
Dear Astrid,

In your model, you can expect a large change in gradient of the solution in the x direction, and none in the y direction. If there is a gradient anyway, that is typically caused by a suboptimal mesh.

Attached your model where the mesh is changed to be more accurate, and also the relative tolerance is decreased in the time dependent step. In your case, make sure that the mesh is fine enough to capture the phase transition.

To check whether your model is correct, always do a mesh convergence study:
www.comsol.com/blogs/how-to-implement-mesh-refinement-study/
www.comsol.com/blogs/meshing-considerations-linear-static-problems/
And ideally also a tolerance convergence, where you decrease the tolerances in your model, and check the effect.

Best regards,
Frank

ps. I notice that the phase change interval is only 0.1K, try out setting this to a larger value. You might also want to limit the timesteps manually, and see if this has a large effect.
Dear Astrid, In your model, you can expect a large change in gradient of the solution in the x direction, and none in the y direction. If there is a gradient anyway, that is typically caused by a suboptimal mesh. Attached your model where the mesh is changed to be more accurate, and also the relative tolerance is decreased in the time dependent step. In your case, make sure that the mesh is fine enough to capture the phase transition. To check whether your model is correct, always do a mesh convergence study: https://www.comsol.com/blogs/how-to-implement-mesh-refinement-study/ https://www.comsol.com/blogs/meshing-considerations-linear-static-problems/ And ideally also a tolerance convergence, where you decrease the tolerances in your model, and check the effect. Best regards, Frank ps. I notice that the phase change interval is only 0.1K, try out setting this to a larger value. You might also want to limit the timesteps manually, and see if this has a large effect.

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Astrid van Sprang
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Posted: 9 years ago 25 sept. 2015, 09:53 UTC−4
Thanks a lot for your response.

I already tried to change the relative tolerance and to make the mesh smaller but still I didn't have the same results for different runs...

Unfortunately I can not open the model you send my because I only have the 5.0 version and PCM_fix.mph is made in a newer version.

Astrid
Thanks a lot for your response. I already tried to change the relative tolerance and to make the mesh smaller but still I didn't have the same results for different runs... Unfortunately I can not open the model you send my because I only have the 5.0 version and PCM_fix.mph is made in a newer version. Astrid
Frank van Gool COMSOL Employee

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Posted: 9 years ago 25 sept. 2015, 10:47 UTC−4
Dear Astrid,

Here the version in 5.0, where the following is changed:
- transition interval 1K
- fine mesh in x direction
- intermediate stepsize, which limits the timestep to 10s

Best regards,
Frank
Dear Astrid, Here the version in 5.0, where the following is changed: - transition interval 1K - fine mesh in x direction - intermediate stepsize, which limits the timestep to 10s Best regards, Frank

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