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How to calculate average Nu ???

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Old   April 4, 2012, 06:12
Question How to calculate average Nu ???
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Sairamu Madala
Join Date: Jan 2011
Location: IIT Kanpur
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Sir/Madam,
I am solving a problem "Natural convection from a circular cylinder in an enclosure" using Fluent. I took radius of cylinder 'r' as 0.2 and side of the square enclosure 'L' as 1 such that the ratio r/L =0.2. But later, while calculating the local/average Nusselt number, I found that irrespective of the characteristic dimension (r) i'm using, Fluent is taking it as 1. But I want the local/avg. Nusselt number based on the diameter. For this to happen, I created a 'custom field function' as "Nusselt number * d(=0.4)" and I used this to extract the data. I found that the results of local Nusselt number varied with the factor mentioned i.e., 0.4 but with average Nusselt number were not varying with this factor exactly. They are coming lesser than what it should actually come.
But at the same time, when I took only the local heat transfer coefficient data from the fluent and did all other calculations manually in determining the average Nusselt number (similar to the way fluent do), my average Nusselt number is varying with exactly the same factor (=0.4) as it should come.
Now my question is why I am not able to get the same expected value when I used custom field function. How is it calculating the area weighted average of any variable??
Someone please help me in this regard.

Thanks in advance...

regards
Sairamu.

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M.Sai Ramu,
M.Tech., Dept. of Chemical Engineering,
Indian Institute of Technology Kanpur,
Kanpur, U.P. -208016.
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area weighted average, average nu, custom field function, heat transfer coefficient, natural convection

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