Thermal simulations – when is FEM sufficient and when is CFD needed

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Thermische Simulation

FEM calculations are more common than CFD calculations in the calculation departments of our customers. Now there are especially thermal tasks that can be calculated both structurally with FEM and fluidically with CFD.

Examples are the temperature distribution in printed circuit boards, computers, electronic boxes, but also in measuring instruments and optical devices.

We often encounter the argument that the seemingly simpler and faster way via FEM, where heat transfer values and fluid temperatures are prescribed, is perfectly sufficient.
This is not entirely true, as there can be great differences, especially in high-precision optics where temperature has a significant influence on precision.
But this also plays a major role in the cooling of power supply units PSUs.

Just imagine a simple shoe box open at the bottom. If there is now heat on the top side, it cannot simply disappear on the bottom side. An insulating cushion of warm air is formed.
The real temperatures are then higher than the temperatures calculated by FEM.

The averaged heat transfer values of the heat atlas are already here no longer suitable for an exact representation of the conditions – even if the inclination of the vertical wall is taken into account.

The enclosed images of an investigation of the same model with FEM as well as with CFD show considerable differences in the course of the same scaling. Also the maximum temperatures differ by about 10%.

Image 1 Temperature distribution with calculated, free convection, CFD calculation
Image 2 Temperature distribution at given heat transfer values, FEM calculation

This difference can be decisive for the correct conclusions.

In the present case, the component temperatures of 87°C exceed the value of 85°C, which is often assumed to be permissible for printed circuit boards, while the FEM calculation signals that everything is fine at a maximum temperature of 79°C.

Image 3 shows how the warm air remains trapped in the shoe box and heats up. Here, the temperature of the air no longer corresponds to the ambient temperature of 25°C, but is far higher at about 40-60°C.
This means that less heat is dissipated and the component temperatures of the electronic elements increase.

Image 3 Temperature of the flow field with surrounding air in section from two views

Whenever a housing comes into play, this effect leads to up to 20% higher temperatures in the more precise CFD calculation.

We know from a multitude of projects when a simplified approach using FEM is sufficient and when we have to resort to the more precise CFD calculations. Of course, the required accuracy also plays an important role here.

However, it is no longer the case that the CFD calculation must always be much more complex than the FEM calculation.

Among other things, we also have to evaluate the temperatures on complete vehicles and are under enormous time pressure here as well.
Accordingly, we have suitable systems and methodologies to investigate even the most complex models quickly and efficiently.
And if an FE model is sufficient for the accuracy requirement and can be calculated much faster and cheaper, we will of course calculate your thermal tasks with FEM.

This is the great advantage of a calculation service provider who is at home in both simulation worlds, flow and structure.

My proposal to you:

You send me the result of a thermal calculation via FEM together with your requirements for accuracy.
We will evaluate for you free of charge whether we consider a calculation with FEM to be sufficient in this case.

If we believe that the accuracy required by you (maximum value, spatial distribution) is exceeded, you will receive a cost estimate for the thermal analysis via CFD.
If you then agree, we will carry out the CFD calculation for you.
You pay for this CFD examination only if the CFD calculation also shows that the difference is greater than the tolerance you require.

Does that sound like a fair deal to you?
Then we are looking forward to your application cases.
Just send me the data with the note TestFEMCFD.

Your Stefan Merkle

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Stefan Merkle

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