Defining an additional variable in CFX

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 December 10, 2023, 06:53 Defining an additional variable in CFX #1 New Member   Xavier Join Date: Dec 2023 Posts: 2 Rep Power: 0 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.

 December 10, 2023, 16:21 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,702 Rep Power: 143 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? shearThickening likes this. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

 December 12, 2023, 06:25 #3 New Member   Xavier Join Date: Dec 2023 Posts: 2 Rep Power: 0 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.

 December 12, 2023, 17:03 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,702 Rep Power: 143 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. shearThickening likes this. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

 Tags cfx, nusselt number, temperature bc