# How to find Mean Nusselt Number?

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 December 21, 2012, 04:00 How to find Mean Nusselt Number? #1 New Member   Abhishek Join Date: Dec 2012 Posts: 1 Rep Power: 0 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.

 December 22, 2012, 03:24 #2 Senior Member   Lucky Tran Join Date: Apr 2011 Location: Orlando, FL USA Posts: 1,976 Rep Power: 26 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 area-weighted 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.

December 12, 2016, 08:21
hi
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 Originally Posted by LuckyTran 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 area-weighted 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.
What you mean by length scale, when I use length in a rectangular cavity do you mean depth ( hydrolics diameter )

 December 12, 2016, 10:50 #4 New Member   a0407 Join Date: Aug 2016 Posts: 6 Rep Power: 3 Hi, u need to precribed the bulk temperature and characteristic length in the reference value in order to calculate the Nusselt number. djamila77 likes this.

December 12, 2016, 15:15
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 Originally Posted by azizi0407 Hi, u need to precribed the bulk temperature and characteristic length in the reference value in order to calculate the Nusselt number.
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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