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Old   July 17, 2013, 06:18
Default Average Nusselt Number
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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...
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Old   July 17, 2013, 12:06
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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).
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Old   July 17, 2013, 12:18
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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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