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Old   February 12, 2012, 06:44
Question calculate Nu number
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Diana
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Hi, all,

I am using FLUENT to simulate heat transfer in pipe with twist tape inserts. my problem is how to calculate Nu number. I dont know how to calculate it by fluent.

any one have idea to help hoe=w to calculate Nu number for pipe under constant heat flux and inserted by twist tape?

Many thanks

Diana
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Old   February 13, 2012, 06:29
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Volker Pawlik
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Hi Diana,
that's quite a difficult problem, since for a correct calculation you needed the static temperature at the top of the boundary layer. Do you know it? That is due to the definition of Nu = (alpha*L)/lambda (alpha= convective heat transfer coefficient; chrateristic Length (i.e. diameter) and lambda = thermal conductivity. To get an image of the Nu-number it is possible to to the following: Define a custom-field-function for alpha by:alpha = q/(T_BL- T_w) with q=heat transfer rate, T_w= outer-surface-temperature of the wall following which both are Fluent-field-functions already. For T_BL you can take, as an approximation, the local static temperature at the wall-surface pointing to the fluid unless you have no shadow wall. The static temperature is the temperature of the fluid in the wall next cell. This approximation only works well for grids which do not resolve the boundary layer, hence for y+>30.

Do you only consider a periodic and short part of the pipe? Then temperature difference between inlet ans outlet temperature is quite small and you can use the already defined field function Nu with a reference temperature instead of the definition above. The reference teperature is then your average of the inlet and outlet temperture or even your volume averaged temperature which must be provided in the reference panel found under reports. But remember that your Nu numbers are at first local ones unless locality can be ignored due to the length of your flow object.
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Old   February 13, 2012, 07:53
Question Calculate Nu number
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Diana
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Hi Volker,

I do these steps but the results are not reasonable

1. Create points on the surface of cylinder
But it doesn’t work for all points.

2 calculate the Nusselt number using FLUENT parameters as follows:

Nu = qp D/ ((Ts - Tb)Kf

where,
qp = Heat flux at point p.
D = Location of point p, on the plate.
Ts = Temperature of surface
Tb = Bulk temperature of fluid .
K = Thermal conductivity of fluid.

3. Try to obtain the values of heat flux (qp) and bulk temperature to calculate Nu.
(a) Obtain the value of heat flux at cylinder.

Report -------Surface Integrals...
i. Select Vertex Average from the Report Type drop-down list.
ii. Select Wall Fluxes... and Total Surface Heat Flux from the Field Variables
drop-down lists.
iii. Try to select point-1 from the Surfaces selection list ( not work).
iv. Click Compute.
The value of heat flux will be updated at Area weighted average (w/m2) at cylinder
(b) Obtain the bulk temperature at outlet
i. Select Mass-Weighted Average from the Report Type drop-down list.
ii. Select Temperature... and Static Temperature from the Field Variable drop down lists.
iv. Click Compute and close the Surface Integrals panel.
(c) The value of temperature out will be updated for Mass-Weighted Average as To K. The value of thermal conductivity for fluid is calculated at Tb.



If there is any mistake in my procedure can you guide me please.


With regards






Last edited by Diana.N; February 13, 2012 at 07:56. Reason: change title
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Old   February 27, 2012, 14:40
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Volker Pawlik
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Hi Diana,
excuse for the delay.
If you evaluate the custom field function at a point-surface you cannot use vertex average. Use faceted average instead.
Also it is not possible to get a mass averaged value on a wall or a point on a wall.
Total surface heat flux (W/m2) should work for points.

Hence try faceted average and tell me, what you get.
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