Nusselt number in ribbed channel.
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Hi,
I am simulating a 2d ribbed channel heat transfer problem at (Re=10000 and wall heat flux q=1200 w/m2), and to simplify the solution i simulate half of the channel, but when computing Nusselt number from surface integrals i get a Nusselt number that is half of the right value of Nusslet number:confused:. (I am using the air temperature as reference temperature). This is a picture of the geometry and mesh. Thank you. 
Did you check the reference values in fluent. To report nusselt iot makes use of reference length which is set as default 1 in the refernce values.

Thank you for your reply,
I used the hydraulic diameter of the channel as length reference value. 
Any solutions? :confused:

Is this problem brcause of using standard kw model, or because of using 2d study instead of using 3d study.
I hope that someone could advice me. Thank you in advance. 
Hi,
the 2D simulation is appropriate for ribbed channels with high aspect ratio (AR=depth x height). Anyway, RANS model hardly give accurate heat transfer results for flows with adverse pressure gradients (and thus recirculation). You can check this paper: A. Ooi, G. Iaccarino, P.A. Durbin, M. Behnia, Reynolds averaged simulation of flow and heat transfer in ribbed ducts, Int. Jou. Heat and Fluid Flow, 23, 2002 
Thank you for your reply, it was helpful.

Hi
Is your problem with the nusselt number solved? The problem i am working on also needs the variations of nusselt number across the area. But i am getting less values as you have mentioned. If your problem got fixed, could you please tell me the changes i need to make in the numerical model? I have used the reference values just as you have mentioned in the previous posts. 
Hi,
The main problem in my study was using 2d study (2d study can't predict the secondary flows that occur in the channel). But if your study is 3d you could check the paper that marluc suggested, where you will find that also 3d study won't give you the same experimental results. Quote:

A problem with calculating nusselt number
I am new to cfd, And i am sure this question will seem easy to experts,
I am simulating a 3d ribbed channel, can anyone tell me exactly how should I calculate average nusselt number on ribbs in fluent ?? I use reports>surface integrals>areaweighted average>wall flux > surface nusselt number then I chose my ribbed wall and calculate nusselt Is this ok for calculating average nusselt number ?? I use symmetry bc to simulate half of a channel should i divide result on 2 to get the proper result ?? Your great help will be really appreciated 
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