# Average Nusselt Number

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 July 17, 2013, 06:18 Average Nusselt Number #1 New Member   Vipin Join Date: Jul 2013 Posts: 4 Rep Power: 4 i am simulating a 2D problem with a rectangular duct and heat flux at the middle. i am calculating the nusselt number using Nu = hDh/K Dh i have calculated from 3D duct dimensions... from fluent when i am calculating report - surface integral-wall fluxes-surface heat transfer coefficient---it is not matching with Dittus bolter relation. even when i am calculating the h by h = q/ ( Ts-Tf) ..i am using q= constant value still it is not matching with Dittus Bolter relation.. Please tell me the suitable procedure...

 July 17, 2013, 12:06 #2 Member   Reza Join Date: Mar 2009 Location: Appleton, WI Posts: 99 Rep Power: 8 What do you mean by "heat flux at the middle"? Dittus Boelter is only applicable to pipe-like fully-developed and fully-turbulent internal flows. And even with that, the correlation might have more than 5% error. Also, if you simulate a rectangular duct as a 2D problem (parallel plate) then you need to use the 2D hydraulic diameter with your simulation results (two times the distance between the plates). And finally, make sure Tf in your equation is the bulk temperature of the fluid (mass flow averaged temperature in a cross section).

 July 17, 2013, 12:18 #3 New Member   Vipin Join Date: Jul 2013 Posts: 4 Rep Power: 4 sir, Heat flux is given at the middle of the duct... flow is fully developed how to set reference parameters in fluent for nusselt number?

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