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wallHeatFlux--how does it work?

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Old   July 17, 2012, 16:17
Default wallHeatFlux--how does it work?
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Mike
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Dear Foamers,

I am new to OpenFOAM and am currently using buoyantSimpleFoam to simulate heat loss through the walls of a channel. This is a 2D simulation right now with the geometry having a width of 5cm and a length of 16m. The airspeed is 16.32m/s and starts at 292K, and the walls are at 278K with U-values of 6.246 W/m^2/K.

First, I attempted to solve the problem analytically, assuming that the air was well-mixed across the width of the channel. I also assumed that the heat transfer coefficient was 8x higher than the U-value due to convective heat transfer. I was able to obtain a value for the heat flux through the walls from this.

Second, I set up the attached simulation for the the buoyantSimpleFoams solver. Then I used the wallHeatFlux utility to find the total heat flux through the walls.

Problem: the two different values I found for the heat flux through the walls (analytical and simulated) are off by an order of magnitude. At first I thought this was merely due to my assumption of a high convective heat transfer coefficient, but when I adjusted my analytical solution for this, my answers were still not close enough to say that the error was only due to the well-mixed cross-section assumption.

Does someone know how the wallHeatFlux utility works? I think I may be using it wrong or wrongly comparing it to the analytical answer I've gotten. The other possibilities are that I set up the simulation wrong (boundaries or thermophysical properties may be incorrect) or that I completely failed at obtaining an analytical solution. Even if it is the latter, I'd like to find out how the wallHeatFlux utility works so I might fix my heat flux calculation accordingly.

Thank you for your help!
-Mike
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Old   July 24, 2012, 09:38
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If someone could point me to where the source files should be, that would be great. I'm a windows person and am having trouble finding them on my virtual ubuntu machine.
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Old   July 24, 2012, 14:57
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$FOAM_UTILITIES/postProcessing/wall/wallHeatFlux is where you can find the source code. In general:

$FOAM_UTILITIES, $FOAM_SRC, $FOAM_SOLVERS are the starting directories, and everything is organized hierachically. It is also beneficial to use the doxygen documentation. If you don't want to compile your own, use the online version: http://www.openfoam.org/docs/cpp/
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Old   August 21, 2012, 04:07
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The alphaWall you are using as U, is it the thermal transmission coefficient for wall-fluid?
You have to be careful, if alphaWall is used for fluid-wall-fluid. Because that is how I would interpret it. Tinf would be the temperature on the other side of the wall (t infinite).

http://de.wikipedia.org/wiki/W%C3%A4rmedurchgang
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Old   December 19, 2013, 09:47
Default wallHeatFlux & wallHeatFluxIncompr: example?
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hi

Is there a example/tutorial on how to use the wallHeatFlux utility?
What should be included in the wallHeatFluxDict?
Maybe somebody who already used it on a simple case?

Thx!
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