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Laminar-turbulent slope change in heat transfer

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Old   May 7, 2012, 12:56
Default Laminar-turbulent slope change in heat transfer
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Hello everyone, I hope you guys can give me a hand about the next question:

I have calculated the heat transfer rate to the walls from hot fluid along a pipe typical of a heat exchanger (the walls have some turbulence promoters). The CFD code I am using is Ansys CFX.

I did this for several Reynolds numbers in the laminar range (Re=100-3000), I used no turbulence model here. Then I went into the turbulent range (Re=2000-30000) using the SST model.

Finally I plotted an averaged Nusselt number over the walls surface vs inlet Reynolds number. Contrarily to what I was expecting, I did not observe a change in slope in the Nusselt number for the two ranges considered. I was expecting to get a steeper slope in the turbulent region, as turbulence is supposed to enhance heat transfer.

Do you guys have any idea as to why I am not getting this slope change? Just to clarify, I am not trying to predict laminar-turbulent transition, since I guess this is far beyond this code capabilities.

Alright, thanks a lot for your help.
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