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Nu No for fully developed region

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Old   March 5, 2014, 02:30
Default Nu No for fully developed region
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mina
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Hello
I have a problem, I am doing simulation of a laminar water flow in a 2D channel but I cannot reach the desired Nu=7.54. I don't know where the problem exists. could anybody help me

channel: 75mm (H) x 1500mm (L)
thickness=0.6 mm
Re=80
Dh (hydraulic diameter)= 75mm
T inlet=298K
T wall=400K
inlet b.c= normal speed according re=80
outle b.c=mass flow=p*u*A , A=0.6*75
but the Nu number i obtained is around 4.98
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Old   March 5, 2014, 05:23
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Matthias Voß
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Please post a picture of the setup. Do you reach a converged solution?Did you do a mesh convergence study?
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Old   March 5, 2014, 05:25
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FAQ: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F
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Old   March 5, 2014, 07:07
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The Nusselt number is the non-dimensional heat transfer coefficient, based on local thermal conductivity and length scale.

What reference temperature do you think CFX bases the heat transfer coefficient on?
And what reference temperature do you think literature bases the heat transfer coefficient on?

Don't use CFX's definition of the Nusselt number / heat transfer coefficient. I.e. if the literature base these values on some bulk temperature then use the bulk temperature in CFX.
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Old   March 6, 2014, 03:08
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would any body please help me???
i want to reach incropera Nu no 7.54 for constant wall tempof channel.
and i use this expression for Nu no:
((ave(Wall Heat Flux)@Line 128)/(400 [K]-(ave(T)@Line 128)))*(.15[m]/.6069[W m^-1 K^-1])
Dh=2h=2*.075=.15

http://www.imageupload.co.uk/TOs
http://www.imageupload.co.uk/TOx
http://www.imageupload.co.uk/TOz
http://www.imageupload.co.uk/TO8
http://www.imageupload.co.uk/TXK
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Old   March 6, 2014, 04:56
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There are three posts on this thread responding to your request which all suggest important things to consider. If you are going to ignore the advice you get on this forum then why bother posting on it?

You have a velocity inlet and a mass flow outlet. This is not good. You need to make one of these boundaries a pressure boundary. Read the section in the documentation about selection of boundary conditions.
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Old   March 7, 2014, 03:39
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dear Glenn
I change the mesh. b.cs(static pressure,opening for outlet), channel lenght and write an expression according Tbulk for Nu.
but always the same result
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Old   March 7, 2014, 04:40
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You should definitely check your boundary layer - i remember that to get the temperature gradient/wallheatflux right, you need a pretty super fine resolution at the wall. Like twice the resolution for the drag/lift-forces to remain constant when refining the mesh resolution.
please post some pictures.
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