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How To Calculate Average Heat Transfer Coefficient in CFD-Post?

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Old   April 29, 2016, 07:11
Default How To Calculate Average Heat Transfer Coefficient in CFD-Post?
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Shomaz ul Haq
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Dear all,

Hi. Hope all are well. I am doing heat transfer analysis in a circular pipe. The fluid is water. I wanted to ask how can I get average heat transfer coefficient and local heat transfer coefficient in CFD-Post along the length of the pipe? I searched for these variables in the variables list but all I found was wall heat transfer coefficient. When I used that in a line along the center of the pipe even though I got values I didn't get any values in the chart. Why is that? I couldn't change the boundary data from Hybrid to Conservative. Also is wall heat transfer coefficient equal to average heat transfer coefficient? Really urgently need to make graphs of average htc and local htc along the length of the pipe. Would be grateful for help. Thanks.
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Old   April 29, 2016, 12:18
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I'm sure this has been discussed many times. You should do a search for quicker answers.
Heat transfer coefficient is only a variable at the wall, not through the center of the pipe!?!
It is equal to:
Wall Heat Transfer Coefficient = Wall Heat Flux / (Wall Temperature - Wall Adjacent Temperature)
Which is not equivalent to the standard engineering equation which uses "bulk temperature" in place of the wall adjacent temperature. When using wall adjacent temperature, the returned value is going to be completely mesh dependent, and a coarser mesh will give you answers approaching the standard definition using bulk temperature, and a fine mesh would give you very large values.
You can plot Wall Heat Transfer Coefficient on a contour at the fluid side of the interface or boundary, or make a line along the pipe wall, and make a graph.

If you want the HTC to use a different value for reference temperature instead of Wall adjacent temperature, can do a couple things:

1.) use the expert parameter "Tbulk for HTC"
2.) make your own expression for bulk temperature along the pipe. I'll let you figure this out. Then make a new expression for:
MyHTC = Wall Heat Flux / (Wall Temperature - MyBulkTemperature).

The tricky part is how do you get "Wall Temperature"
Well from the original HTC equation:
Wall Heat Transfer Coefficient)= Wall Heat Flux / (Wall Temperature - Wall Adjacent Temperature)
Rearrange it to:
Wall Temperature = (Wall Heat Flux / Wall Heat Transfer Coefficient) + Wall Adjacent Temperature

Then make a variable from your "MyHTC" expression.
You can then make a contour or plot it on a graph with a line running along the wall.
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Old   April 30, 2016, 10:41
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Glenn Horrocks
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Hi Erik - Can you stick it in the FAQ page? Then just refer people to the FAQ page in future. It also means it can be reviewed by many people so we can make it the best response possible.

http://www.cfd-online.com/Wiki/Ansys_FAQ
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Old   May 2, 2016, 17:43
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Sure thing Glenn,
I just applied for the "user group" account so I can edit/expand the FAQ.
I will clean up my response and add it there.

EDIT: I added this to the FAQs, I feel special now :-)

Thanks,
Erik

Last edited by evcelica; May 3, 2016 at 16:59.
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Old   May 3, 2016, 22:14
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Excellent, thanks Erik. Feel free to do updates to any of the other comments on that page as well. Then you can feel even more special.
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Old   November 21, 2019, 09:05
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Hello, everybody,

I just came across this thread and have a question about it: I activated the expert parameter t bulk and added the fluid temperature of the environment. If I now get the htc output, I get a realistic number (<20 for free convection of air). However, the value differs from the value I get when I divide the heat flux by delta T. What could be the reason for this?

Many thanks in advance

Last edited by bemomb; November 21, 2019 at 10:30.
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Old   November 21, 2019, 10:28
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What delta T are you referring to? What expression and location exactly?
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Old   November 21, 2019, 10:39
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Hi evcelica,


I have a free convection of air on the outside of a cylinder. With tbulk I get a htc (wall heat transfer coefficient) around 4. My air has a temperature of 298 K. When I want to verify the htc by dividing the heat flux by (twall-tfluid) I get a value that is around 11. But it has to be in the area of 4 hasn't it?


Thx
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Old   November 21, 2019, 20:54
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Where does twall and tfluid come from?
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Old   November 22, 2019, 01:29
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Tfluid is my ambient temperature, that i chose to be 298 K (that's why tbulk has that value). And twall is the temperature on the cylinder surface, that I get from post. So what I want to do is basically compare the htc I get from post (which is around 4) and the htc I get by taking the heat flux on the cylinder surface and dividing it by delta t (around 11) and my problem is that they are different.
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Old   November 22, 2019, 01:54
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Quote:
Originally Posted by bemomb View Post
Tfluid is my ambient temperature, that i chose to be 298 K (that's why tbulk has that value). And twall is the temperature on the cylinder surface, that I get from post. So what I want to do is basically compare the htc I get from post (which is around 4) and the htc I get by taking the heat flux on the cylinder surface and dividing it by delta t (around 11) and my problem is that they are different.
If you calculate twall using areaAve(T) function then it takes conservative values of T i.e. average over control volume near the wall but not wall values (hybrid). Take a look this post and below it.
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Old   November 22, 2019, 04:32
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I just read the post and it sounds quite complicated. So basically the main problem is that Twall is computed conservative? When I change it to hybrid in post, the difference is .03 K. I don't get it.



In addition: I want to simplify my simulation by replacing the big atmosphere domain with a htc bc on the outside of my cylinder. Therefore I tried to find the htc in that first simulation.
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Old   November 22, 2019, 04:53
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As Antanas states, details are important. Even the way you average it over the surface is important. So please tell us exactly how you have calculated the numbers you are comparing. Include the integration/averaging scheme you used, which exact variable and if you select hybrid or conservative values.
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Old   November 22, 2019, 05:15
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So I have calculated the WHTC with areaAve(T) on the fluid side of the interface, where the same value comes out for conservative and hybrid, namely 4.8 W/m^2K.
For comparison, I also calculated the wall heat flux with areaAve(T) on the fluid side of the interface, 210 W/m^2. Again, it doesn't matter if conservative or hybrid. Then I calculated the wall temperature also on the fluid side of the interface, also with areaAve(T). Here conservative and hybrid differ by only 0.03 K. In the appendix you can see a picture. What might be important is that the convection takes place only on the outer surface, the side surfaces are adiabatic. Maybe this causes problems, on the picture you can see on closer inspection that the WHTC rises towards the edges.

Many thanks in advance and a compliment from me that all of you really try to help!


Edit: The temperature scale is for the plane, the colours on the surface of the cylinder represent the WHTC!
Attached Images
File Type: png Unbenannt.PNG (194.1 KB, 39 views)

Last edited by bemomb; November 22, 2019 at 06:29.
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Old   November 23, 2019, 05:32
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No, your problem is more fundamental than that. If A and B are field variables, then, in general; average(A)*average(B) does not equal average(A*B).

This means you cannot use an average of T in this calculation. You should define a new variable as Wall Heat Flux/(T-Tamb). This is the number you should be comparing to the HTC from CFX.
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Old   November 23, 2019, 06:05
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Okay, but how do I define T? What's also interesting is that if I enter the two different htc into the simulation as Bc (without atmosphere), the temperatures in the simulation will match for the calculated htc 11 (wall heat flux/delta t), not for the htc from cfx 4. This means that the problem is not the value calculated by myself (wall heat flux/delta t), but the value calculated by the system
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Old   November 24, 2019, 19:57
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This means that when you are comparing heat transfer coefficients they need to have the same averaging process. So if you are comparing the areaAve(HTC) then you need to define your value as a variable field defined as Wall Heat Flux/(T-Tamb), and then you do an areaAve on that variable.
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