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Valued Contributor
JohnTomasik6493
Posts: 74
Registered: ‎11-21-2011
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No thermal results.

344 Views, 24 Replies
12-19-2013 04:03 PM

I've been trying to run a simple static thermal study.  Even with 25X the heat generation applied as boundary conditions, I'm getting no thermal change.  I've gone back through tutorials and previous studies, checked everything I can think of, but I can't identify where the issue is.   Attached is a screen capture showing as much as I could, as well as the flatline temp.

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wildej
Posts: 778
Registered: ‎08-25-2011

Re: No thermal results.

12-20-2013 02:15 AM in reply to: JohnTomasik6493

Hi John,

 

I cannot see any Temperature BC's, so CFD does not have a reference point to start from. If you have heat lost from this box and do not wish to model the external air, the quickest way is through a film coeff. Try 5W/m2/K at your ambient temp on the external faces. (5W/m2/K is an approximation for natural convection, this value would be higher for forced convection externally).

 

Kind regards,

Jon



Jon Wilde
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Valued Contributor
JohnTomasik6493
Posts: 74
Registered: ‎11-21-2011

Re: No thermal results.

12-20-2013 06:51 AM in reply to: wildej

Jon,

 

What temp boundary conditions do I need?  I referred to the Sim CFD "Electronics" tutorial when setting this up, and the only boundary conditions they had that were temp generating were three "Total Heat Generation" conditions.  They did not have me apply any surface temps in that tutorial, if I remember correctly.  In that tutorial, the "Total Heat Generation" BC's created a change in temperature in that study.  I have two of the same BC's in my study, but, it is not creating a substantial change in temp (it's on the order of E^-13).  I did try to apply a temperature boundary condition to my outside air, but then the whole study simply remained that temp throughout the analysis.

 

At the wattage I applied, this thing should be on fire.

 

 

John

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wildej
Posts: 778
Registered: ‎08-25-2011

Re: No thermal results.

12-20-2013 07:57 AM in reply to: JohnTomasik6493

Hi John,

 

It should have had a temp on the inlet, along with a flowrate.

 

For this, use what I said. Film coefficients - if you have no actual inlets or outlets.

 

Kind regards,

Jon



Jon Wilde
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Valued Contributor
JohnTomasik6493
Posts: 74
Registered: ‎11-21-2011

Re: No thermal results.

01-07-2014 10:34 AM in reply to: wildej

Jon,

 

Applying a temp on either inlet or outlet produced results.  If I set gravity with the temperature BC at the "inlet" side, the temps would skyrocket.  A 20C temp with about 1.5W of heat generation on the board would send temps beyond 500C.   If I placed the temp on the outlet side, I got more reasonable results.  I went back into some of the tutorials, and I noticed temps were on all surfaces of the air external volume, yet those studies (with heat generation on the order of 5W+) didn't do that, even though the temp BC was also on the inlet side.  I'm trying to compare studies thoroughly to understand this. 

 

Let me describe the situation.  The client has 3 boards they've prototyped and ran in the lab (outside of an enclosure, exposed to ambient air in the lab).  They've taken thermal image photographs.  They're able to give me input power and also Jc and Jb for some components.  I'm trying to replicate their results, because we'll eventually take those three boards, put them all in one enclosure, and put them in outdoor environments in a variety of environmental temps.  I'd like to be able to quickly predict hot spots in those conditions on the boards, and create thermal conductor solutions to relieve those hot areas and minimize component temps.

 

I thought I'd use an external air volume and get an idea what my heat generation loads on the hot spot chips (identified in the thermal photographs) needed to be to reproduce the thermal image.  

 

If I use the film coefficient, I can't use a two-resistor (compact thermal model).  This might produce questions by the client.   I'm just guessing.

 

I'm setting up that analysis now and am going to run it.  I'll post updates.  But, in the interim, I'd like to know what I need to show/upload so that someone can tell me why I see over 500C from the board if that temperature BC is put on the "inlet" side of the air space, where it doesn't do the same in the tutorials.

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Valued Contributor
JohnTomasik6493
Posts: 74
Registered: ‎11-21-2011

Re: No thermal results.

01-08-2014 01:32 PM in reply to: wildej

Jon, to use the film coefficient, and since this board is presently being tested on a bench in open air, are there any rules of thumb that will help me estimate air volume/size/aspect ratio around the board?  In fact, are there any guidelines that would help me size an air space around any object so that I'll get the closest results to an open-air environment?

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wildej
Posts: 778
Registered: ‎08-25-2011

Re: No thermal results.

01-09-2014 01:45 AM in reply to: JohnTomasik6493

Hi John,

 

I suggest you refer to the guide here for domain sizing.

 

If this is a natural convection study, is the unit placed on a bench or will it have airflow beneath it also? It looks as though it will have air all around it so you should use the 'chimney approach'.

 

This means Ambient Temp and a P=0 BC on the bottom surface and just a P=0 on the top surface of the air domain.

 

Feel free to share a CFZ (support/share) file once you have it setup.

 

Kind regards,

Jon



Jon Wilde
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Valued Contributor
JohnTomasik6493
Posts: 74
Registered: ‎11-21-2011

Re: No thermal results.

01-13-2014 01:24 PM in reply to: wildej

Attached is the CFZ file.  Note the lack of temperature differential on the board.  The real-life unit has a delta T on the board assembly of at least 30C.  

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wildej
Posts: 778
Registered: ‎08-25-2011

Re: No thermal results.

01-13-2014 02:58 PM in reply to: JohnTomasik6493

Hi John,

 

You have air set to fixed, so it was only a conduction analysis. Select the air and switch to variable so that the density can change.

 

 

Air Variable.jpg

 

With that change the model looks like this after 100 iterations. You can also set the temperature scale to the board by right clicking on the legend and choosing 'set to part' like I have below. You do have a rather conductive PCB so that may explain the uniform temperatures. It is mainly copper.

 

PCB Temps.png

 

I hope that helps.

 

Kind regards,

Jon



Jon Wilde
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Valued Contributor
JohnTomasik6493
Posts: 74
Registered: ‎11-21-2011

Re: No thermal results.

01-13-2014 03:28 PM in reply to: wildej

Ahhhh....basic gas laws stuff.  I'm feeling real smart about now....lol.  Thanks much, Jon.

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