Convection values on a thermal simulation

[mario.brizida]

Hi all;

I'm a beguinner on using Fusion thermal simulation.

I saw your presentations about the theme, and made some tests and respective simulations, in order to understand how it works. I use thermal simulation mainly to calculate and dimension aluminium heatsinks for electronic heat management.

I still don't understand which should be correct value of field "Convection Value" when defining a thermal load of a convection type load. Changing this value has dramatic influence on simulation, and in your instructions I didn't find any reference or table to get it. In your isntructions, sometimes you use "1000", other times any other value.

Could somebody explain how to get it?

Thank you.

MB

[John_Holtz]

Hi @mario.brizida 

You need to calculate the convection coefficient (the "Convection Value"). Engineering textbooks on Heat Transfer will devote a chapter or two to how to calculate the value. It depends on the shape (flat, circular), orientation (facing up, facing down),  air speed (natural convection, forced by a fan), and so on.

The only exception is when you want the face to be at the specified temperature. Then you enter an arbitrarily large convection coefficient (1000 W/m^2 C is somewhat large). The large convection coefficient indicates that a large quantity of heat can be added or removed, so the calculated temperature on the surface approaches the entered temperature. The larger the value, the closer the calculated temperature becomes. But too large of a value can cause numerical problems.

Let us know if you have any questions.

[mario.brizida · Accepted solution]

Hi John;

Thanks for your answer.

As far as I understood, it's very hard to calculate convection coefficient, because is dependent of many variables, and some of them tepend each other. Given that, and given that is mandatory to introduce that value to get simulation, I will never have an accurate simulation. Is that correct? I tested some values, and simulation is competely different if I introduce a value of 15 or 50! So, for a given heatsink I need to simulate, which value must I introduce to get an error of 5%? As I understood by reading about theme, the best way to calculate convection coefficient is to use a simulation program, because that number is also dependent of heatsink temperature. That value I want to get from simulation, so I must calculate a number that needs an input that is something I want to get, so I think is an impossible equation. Could you please clarify?

Thank you;

MB

[John_Holtz · Accepted solution]

Hi @mario.brizida 

Let me start with your second question: "which [convection] value must I introduce to get an error of 5%?"

Chances are the theory of natural convection (or even forced convection) is not accurate to 5%, so I think that you will never get a thermal analysis to be accurate to within 5% without performing physical tests and changing the input to match the test. (I'm not even sure what "5%" means in your thermal analysis.)

And your first question in reference to what convection coefficient should you use is this: "I will never have an accurate simulation. Is that correct?"

That is not entirely correct. If you are going to use a simplified simulation program such as Fusion, then you need to do more work to get an accurate simulation. This does not mean that it cannot be done. (Of course, we are assuming that there is a theory that is "accurate" as I indicated in my first answer.) The procedure to perform the simulation in Fusion 360 would be like the following:

1. Guess what the heat sink temperature will be. (You can do that from performing one analysis.)
2. Based on the calculated temperature, re-calculate the convection coefficient.
3. Enter the new convection coefficient.
4. Run the analysis.
5. Repeat steps 2 through 4 until the calculated temperature no longer changes.

P.S. One of these two programs would provide a more sophisticated simulation:

• Inventor Nastran, where you can enter a table of convection coefficient versus calculated temperature, and the software would iterate until the solution converges. This analysis would be moderately complex.
• Autodesk CFD, where you would model the fluid (air?) and the simulation would calculate the air movement and heat transfer. This analysis would be considerably complex.

[mario.brizida]

Hi John.

Thanks for your clarification.

Best regards;

MB.