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Wrong result Heat Transfer in a Cavity

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Old   February 19, 2014, 22:34
Default Wrong result Heat Transfer in a Cavity
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I am trying to simulate one of the cases given in following paper by
Metzger Cavity Heat Transfer on a Transverse Grooved Wall in a Narrow Flow Channel. in Fluent

Re= 15000. The geometry is a 2D section
I am using Standard K-Omega Model

I am trying to get the Nusselt number. I first specify Total temperature at Inlet i.e. 320K and T_Wall= 300. I got wrong results.

Then I tried opposite way: T_Wall=320K and Total Temp_Inlet=300k. I got some results and tried to plot Nusselt. Attached are the Results.

I defined the Nusselt number through Custom Defined function as Nu= (Surface Heat Transfer coeff * Cavity Depth)/ Thermal Conductivity.

I used the Thermal conductivity of Fluid i.e. k=0.02 w/m-k.

I am new to Heat transfer area so I am not sure what I am doing wrong. Cases converged there was no problem with the case setup. I checked the dimensions of the geometry that's right. Please help.
Thanks
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Old   February 20, 2014, 04:55
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Read the manual how Fluent computes heat transfer coefficients. It differs from the usual engineering definition of a heat transfer coefficient.
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Old   February 20, 2014, 18:28
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I read the manual. But first of all I wanna know how the temperatures should be specified. Is the Wall Temperature higher or the Inlet Temperature. First of all I wanna make sure that my case setup is right.
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Old   February 21, 2014, 04:49
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It doesnt matter. Both cases should yield the same result when set up correctly, provided you dont account for any temperature dependent material properties.
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Old   February 21, 2014, 11:52
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Sorry, did'nt look at the experimental data properly. It was given in the form of string distance. I was getting the right results all the way. But the position of the inlet does affect the Nu distribution and the exit conditions.

I also a question about the stability of the discretization schemes.

Is it OK to use Third Order MUSCL scheme or Second Order Upwind. I read that second order upwind is stable and reduces numerical dissipation.

Thanks for the help
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Old   February 21, 2014, 12:31
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The general rule of thumb for the interpolation schemes for the convective terms is: with higher order schemes you trade in stability for accuracy.

To answer your question: The second order upwind scheme is more stable compared to a third order MUSCL scheme and reduces numerical diffusion compared to the first order upwind scheme.
Always use the scheme with the highest order that gives stable results.
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