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 Diana.N February 18, 2012 00:56

Problem in Nu number

Hello everyone.
I have one problem related to heat transfer forced convection.
Problem statement: pipe ( 1.5m length & 1inch diameter) is heated under constant heat flux.

I simulated it in fluent and I got the unsatisfactory results .

My Calculation sample in fluent is;
1. Create iso- surfaces on the surface of cylinder (0,0.3,0.6……etc.).

2 calculate the Nusselt number for each surface using FLUENT parameters as follows:

Nu = qp Di/ ((Ts - Tb)Kf
where
qp = Heat flux at specified surface (Report--- Surface integral------area weighted average).
Di = inside diameter of pipe
Ts = Temperature of surface
Tb = Bulk temperature of fluid at the out let of the surface (Report- Surface integral--mass weighted average).
K = Thermal conductivity of fluid.

3- The result as compared with correlation are not satisfied.

I don’t know if there is a mistake in my procedure can anyone help me please.

With regards

Diana.N

 Zigainer February 20, 2012 06:08

Hi,

in principle your approach is all right.
When I have to handle with Nu, I always create Nu as a "custom field function". But that should not be the problem. Are you sure, that your correlation uses the same Di? Which correlation do you use (fully-developed flow vs starting flow)? Is this correlation valid for an area-averaged Nu number? What is your inlet condition (in terms of boundary profile)? Probably you have the effect of boundary-layer deployment, which results in massive change of heat transfer compared to a fully developed boundary layer.

Cheers

 Onur10 March 2, 2012 07:18

Hey there;

I have the same question about the Nusselt Number.

I am a bit new to Fluent, and I am currently working on a cylindrical pipe with turbulent flow.

Geometry: My pipe is 1 meter long and has a 8 mm diameter.

Working Fluid: I am using standart water as the working fluid, and compute turbulent flow with Re ranges of 6000 to 10000. Tried some random other fluids too after recieving bad results with water.

Boundary Conditions: I work for both constant wall temperature and constant heat flux boundary conditions.

Mesh: For 2D I use a standart mesh with bias on both ends to the wall to approach the Nu correctly, for 3D geometry I use Multizone mesh and inflation with a good mesh statistics.

Nu calculation:

Nu = h*D/k

h = q" / (Tw - Tb)

I get bulk Temperature by creating planes (or lines for 2D) at crossections of the pipe and getting surface integral / mass weighted Temperature average of them.

For constant heat flux, q" is already given, for constant wall Temperature, I read from post process - wall heat flux at the given position.

For constant Twall i use the given Twall, for constant q", I draw a line at the wall surface parallel to flow direction at post process, and read the inner wall temperature on that.

Other variables are given already at the start of the analysis.

First I calculate h from q", Tb and Tw. Then I calculate Nu from h, D and k.

Problem: For both 2D and 3D approach my Nu to a developed region Nu and stays constant there. However the approached Nu is very big compared to theoretical values.

As an example, theoretical (and experimental as well) values suggest Nu = 43 for a case, but I get both with 2D and 3D models I get Nu of 160 or something like that. Similarly for different cases the Nu in simulations is 3 to 4 times the actuel Nu.

What I have tried:
- Changed the mesh parameters, mesh type etc. Eliminated the possibility of mesh dependent solution

- Changed the problem geometry geometry

- Changed the fluid in consideration

- Changed boundary conditions (by having the same Re)

- Changed Re (flow velocity etc.)

- Changed simulation dimensions (both 2d and 3d)

* * *

I dont know what else I should do. I always get the same "much higher" Nu and there is a developed region there for sure. Do I use false calculations? I always approach a Nu about 3 times greater than the realistic Nu.

Anyone has any solution? It is a bit urgent, so I would really appreciate the help.

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