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Defining an additional variable in CFX

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Old   December 10, 2023, 06:53
Default Defining an additional variable in CFX
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Xavier
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Hello everyone,
My problem originates from calculating the Nusselt number for heat transfer of the flow in a circular pipe with uniform wall heat flux. The theoretical value is different from that calculated with Nu=\frac{q D}{(Tw-Tbulk)k}.
I'm following a solution suggested in:
Convective heat transfer issues in CFX
by defining an additional variable for Tw. For some reason, which I don't know, I can't just calculate the area-average for temperature at the wall. He provide a code for Tw as
Code:
LIBRARY: 
  ADDITIONAL VARIABLE: Tw
    Boundary Only Field = On
    Option = Definition
    Tensor Type = SCALAR
    Units = [K]
    Variable Type = Unspecified
  END
END

FLOW: Flow Analysis 1
  DOMAIN: HeXe
    FLUID MODELS: 
      ADDITIONAL VARIABLE: Tw
        Additional Variable Value = T
        Option = Algebraic Equation
      END
    END
  END
END
I'm not sure how to use this code in CFX. Will appreciate any help.
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Old   December 10, 2023, 16:21
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Glenn Horrocks
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The starting point for a calculation like this is to use areaAve(T)@Wall for the wall temperature, whatever your defined bulk temperature is for Tbulk, and the other properties as evaluated at the wall.

The thread you link to goes into a lot more detail on this and discusses things to consider to get much better accuracy. But only look at this more complex stuff if the basic stuff I just described is inadequate.

So before you look at the complex solution discussed in that thread, have you looked at a basic Nusselt number calculation? What does that say?
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Old   December 12, 2023, 06:25
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Thanks ghorrocks for your response and sorry for the late reply. In fact, yes I'm looking for better accuracy if possible because the error is about 10% for this simple case. If the mentioned solution wouldn't reduce my error, I will accept what I am getting. But for the sake of knowledge, how can I implement this code? As a mathematician, I don't have much experience with CFD software but I had to work with it because implementing a numerical technique for a turbulent model from scratch is a waste of time.
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Old   December 12, 2023, 17:03
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The text snippet is CFX command language, CCL. In CFX-Pre you can edit the CCL directly and add the snippet into the CCL of your run.

Note that as you refine the mesh the error of the simple areaAve(T)@Wall should also reduce. So if you have refined your mesh enough to give an accurate simulation then the simple wall temeprature approach might be good enough.
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