Average Nusselt Number

vipingawande · July 17, 2013, 06:18

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...

triple_r · July 17, 2013, 12:06

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).

vipingawande · July 17, 2013, 12:18

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?

July 17, 2013, 06:18	Average Nusselt Number	#1
vipingawande New Member Vipin Join Date: Jul 2013 Posts: 4 Rep Power: 12	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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July 17, 2013, 12:06		#2
triple_r Senior Member Reza Join Date: Mar 2009 Location: Appleton, WI Posts: 116 Rep Power: 17	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
vipingawande New Member Vipin Join Date: Jul 2013 Posts: 4 Rep Power: 12	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?