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Heat transfer report

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Old   February 26, 2018, 09:04
Question Heat transfer report
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FrancoisP
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Hello everyone,


I am new to this forum and have issues with a simulation which are rather puzzling. Hope you can help me out!

Here it is:
I have been simulating forced-convection of air in a 10cm-long, 3.5mm-high channel (heat exchanger). The width of the channel is 2.5mm, but the mesh and computation are done on a 2-dimensional cut.
The air enters the channel at 1 m/s, temperature is 20C. The walls have a constant temperature of 13C.
I want to extract the heat exchange coefficient on the top and bottom walls in this situation.

When I run a "Heat transfer report" on the top and bottom boundaries of my 2D design, I get 1.41 W.
When I run an "average" report of the "heat transfer" over the same surface, I get 8.7e-3 W.

I am really surprised the results are different. Based on simple calculations, the second value is in the correct order of magnitude.
Does someone know what the "Heat transfer report" actually reports?
I suspect there is a trick because I am working in 2D...


Thanks in advance !
FrancoisP
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Old   February 27, 2018, 08:04
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Marco Riedel
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Hi Francois,
the "heat transfer report" calculates the heat transfer in every face on the boundary and calculates the sum of that values. The "surface average report" of the previous done "heat transfer report" calculates the average of that value. And that is what you want.

Best regards
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Old   February 27, 2018, 10:22
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Hello MRI, thanks for your answer !


I tried to report the "sum" of the "boundary heat transfer" over the same surface. As you predicted, I got the same result as the "heat transfer report" : 1.41 W.
So, I understand now that the correct value was the average.

Out of curiosity, I still don't get what the other value is.
The heat transfer report needs a surface. I feed in these pink parts :


https://ibb.co/fUDEVx

What does the "sum" of the "boundary heat transfer" mean, exactly ?
Does it sum the heat transfer over the adjacent surfaces ? What is the application of this feature?


Thanks again for your quick answer !
FrancoisP
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Old   February 28, 2018, 01:30
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Quote:
Originally Posted by FrancoisP View Post
Hello MRI, thanks for your answer !


I tried to report the "sum" of the "boundary heat transfer" over the same surface. As you predicted, I got the same result as the "heat transfer report" : 1.41 W.
So, I understand now that the correct value was the average.

Out of curiosity, I still don't get what the other value is.
The heat transfer report needs a surface. I feed in these pink parts :


https://ibb.co/fUDEVx

What does the "sum" of the "boundary heat transfer" mean, exactly ?
Does it sum the heat transfer over the adjacent surfaces ? What is the application of this feature?


Thanks again for your quick answer !
FrancoisP
Buddy, In CFD sum means total heat going out/in from surface. For which CFD software calculates heat transfer from each element on surface and then takes it sum. So the another report which is average heat transfer would be like [(Sigma{heat transfer from element * element area})/total area]. But second report doesn't make much sense because I would rather have heat flux report for analysis.
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Old   February 28, 2018, 10:05
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Hello Ashokac,


We have the two following reports over the previous 50 mm surface :
  • "Sum" of "Boundary heat transfer" = "Heat transfer report" = \displaystyle\sum_{elem \in S}Q_{elem}, which gives us 1.41 W.
  • "Average" of "Boundary heat flux" = \frac{\displaystyle\sum_{elem \in S}\frac{Q_{elem}}{S_{elem}}S_{elem}}{S_{tot}}=\frac{\displaystyle\sum_{elem \in S}Q_{elem}}{S_{tot}}, which gives us 176 W/m.
But 1.41W / 50e-6m = 2.82e4 W/m... Why doesn't it work out ?
Could you show me where I am wrong ?



Thank you !
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Old   February 28, 2018, 10:15
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On the other hand, I have computed the "Sum" of "Boundary heat flux", which I think would be \displaystyle\sum_{elem \in S}\frac{Q_{elem}}{S_{elem}}. The output is 2.88e4 W/m : the value I expected previously.
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Old   February 28, 2018, 10:38
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First of all there is something wrong with your area of the boundary. If you calculate in 2D, Star-CCM+ refers to a standard width of 1m, so your area has to be in the range of *.e-3 m^2. And you should keep in mind that your Region consists of top and bottom surface (thats what I understand when I look on your pic), maybe you don't consider it in your simple calculations. For deeper knowledge its always good to look in the user guide under "statistic reports" or on the steve-portal. Special cases are described there very good.

Best regards
Marco
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Old   February 28, 2018, 11:59
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Hi again MRI,


Many thanks ! I suspected there was a trick due to the 2-dimensional calculation ! With this standard 1m-width, I get the correct order of magnitude.
The top and bottom surfaces were selected on purpose. No problem here.

Thanks again! (and I will look up steve portal)
FrancoisP
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