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Heat Transfer. Cooling instead of heating.
Posted Dec 15, 2023, 5:06 a.m. EST Heat Transfer Version 6.1 5 Replies
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Dear community,
I am trying to model the self-heating of a PCB. My model has a CPU as heat source and I want to know the temperature at 3 different spots on the board.
Even though I have the initial temperature values and the ambient temperature at 37°C, and am adding energy to the system through the heat source, the temperature drops below 37°C at one of my points of interest. How is that possible? What am I doing wrong?
Best, Matthias
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I think you have a time-stepping problem. I re-ran your problem, manually forcing it to take 20s time steps and it gave far more believable results. Go to Study --> Solver Configurations --> Solution 1 --> Time-Dependent Solver 1. In the Settings panel, click on Time Settings and change the value of "Steps taken by solver:" from "free" to "Manual." Then change "Time step:" from its default (.01) to some number like 20 (in your case, this is 20 seconds). Then re-run your model and you'll notice a very different result! I encourage you to also experiment with other different time steps and different meshings. Also, your present mesh is too coarse (in my humble opinion), in some places.
-------------------Scientific Applications & Research Associates (SARA) Inc.
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I second Robert's view on the coarseness of your mesh. You may want to try a swept mesh, your geometry lends itself to it.
Jeff
-------------------Jeff Hiller
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Thank you very much for both of your replies!
Unfortunately both suggestions don't work out for me. I tried really hard to create a swept mesh but am unable to. I always get errors. I read through the tutorial and tried to gain knowledge from the bipolar transistor example but I simply can't produce a swept mesh.
As for the time-stepping: Once I change it to 20 seconds (I also tried 10s), it produces very weird jumps in temperature, that are also not explainable. I attached an image of the table.
I would be very grateful for additional help.
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I rebuilt the whole model step by step and found out that the weird results came by adding the vias. Because when adding the vias, I could not create a swept mesh anymore as the vias throw a "Unsupported topology of linking face" error.
That's why I exchanged the vias with cylinders with the same volume as the ring of the via. I can now create a swept mesh and get results that seem plausible.
This is not a solution however, just a workaround.
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Hello Matthias,
Page 850 of the Reference Manual, version 6.1, explains the requirements that your geometry needs to meet for a swept mesh to be possible. This will guide you on how to create your vias correspondingly.
Best,
Jeff
-------------------Jeff Hiller