How to find Mean Nusselt Number?
I am doing an computational analysis on natural convection heat transfer through rectangular fins attached to a wall. After the simulation, Surface Nusselt number for hot wall which I am getting in higher than (twice) the experimental results. The way I am following for finding the mean nusselt number is : Report> Surface Integrals>Facet Average>Wall Fluxes>Surface Nusselt number.
Please provide me the solution. Also let me know whether I am using the right way of finding the nusselt number or not. If not then please provide me an alternate way of finding the Average Nusselt Number. 
Two things:
1) Heat transfer coefficient needs a reference temperature 2) Nusselt number needs a reference length scale. Both of these reference quantities are specified on the reference values task pane. Did you remember to specify them? I have always used areaweighted average and not facet average, but I believe they yield similar results. You should be able to get the average Nusselt number the way you are doing. 
hi
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Hi, u need to precribed the bulk temperature and characteristic length in the reference value in order to calculate the Nusselt number.

i already did
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but my Question is what means length in CFD when you use 3dpp. My problem is in three dimensional length is it the hydraulics Diameter 
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