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Member
mjasper33
Posts: 4
Registered: ‎04-10-2012

Thermal Analysis Heat Flux Through Surfaces

443 Views, 3 Replies
04-10-2012 01:27 PM

I have a question regarding the heat flux through a surface using the “Wall Results” in Sim.CFD-2012.

 

My analysis consists of basically a big box with a heat generation source inside (2000 BTU/hr). There is an inlet flow and outlet flow (using CFM in and P=0 out) through small pipes in the side wall and ceiling.

 

All external surfaces have been given a Film Coefficient of 0.35 BTU/hr-ft2-F.

 

The analysis converges with the flow traces and thermal profile looking very good. However, when I check the heat flux through the exterior surfaces, it is telling me that each surface has a heat flux of ~5000-9000 BTU/hr, or in other words, WELL over the total heat generation in the model (2000 BTU/hr) for each surface!

 

I was wondering if I was using this measurement tool wrong, reading the data incorrectly, or does a bug exist?

 

Thank you for your help.

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Valued Contributor
nhahn
Posts: 92
Registered: ‎01-18-2006

Re: Thermal Analysis Heat Flux Through Surfaces

04-11-2012 11:48 AM in reply to: mjasper33

Perhaps you have already checked this and just didn't mention it.

 

What is the reference temperature for your film coefficients?   If it is much different than your fluid inflow temperature you will get a large amount of heat flowing through the walls, regardless of your heat generation.

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Member
mjasper33
Posts: 4
Registered: ‎04-10-2012

Re: Thermal Analysis Heat Flux Through Surfaces

04-16-2012 08:59 AM in reply to: mjasper33

My Film Coefficient ref temp = 80°F on all exterior surfaces. My fluid Inlet temperature is 70°F. Are you saying that maybe I am seeing a heat flux into the box from the "reference temp to fluid inlet temp" deltaT?

 

I have attached a heat flux surface measurment image, showing a hot face (due to heat generation within box) and the corresponding heat flux.

 

 

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Valued Contributor
nhahn
Posts: 92
Registered: ‎01-18-2006

Re: Thermal Analysis Heat Flux Through Surfaces

04-16-2012 09:26 AM in reply to: mjasper33

I would expect that, yes... if the fluid is 70 and the ref temp is 80, there will be net heat flow into your fluid (I think it's as straightforward as 0.35 * (80-70) = 3.5 BTU/sq ft). 

 

That said it does seem like the numbers are larger than you'd expect, though I'm not sure of the physical size of your model.  I've had mixed results using the heat flux summary.

 

If it were my problem I'd try running it with fluid inflow temp=ref temp to take that out of the equation for debugging purposes, and if you're still getting odd numbers then maybe something else is wrong.  But at least then you'll know the only energy input is from your heat generation.

 

Maybe someone else will chime in if this is a known issue.

 

EDIT: you could also try enabling the wall heat flux result quantity.  This should show you where on your walls you have positive or negative heat flux.

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