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-   -   Nusselt numbers for fully developed laminar flow (http://www.cfd-online.com/Forums/cd-adapco/56851-nusselt-numbers-fully-developed-laminar-flow.html)

 Danro October 29, 2008 16:03

Nusselt numbers for fully developed laminar flow

Hi, I am using StarCCM+ to calculate the Nusselt numbers and friction factors for fully developed laminar flow. I chose the steady state, constant density, stationary, laminar, segregated fluid temperature physical model for a 3D (round) pipe, which is subjected to a constant temperature boundary. The Nusselt number is suppose to be 3.66 once the flow is developed, but the results shows that the Nusselt number starts at the inlet with a value of 132 and then continuously decreases up to a value of 2.1 at the outlet. I get the same result even if I make the pipe longer. I would be very thankful if anyone can help me! Thank you!

 anon October 31, 2008 18:32

Re: Nusselt numbers for fully developed laminar fl

Go to Tools>Field Functions>Nusselt Number and change the reference properties.

 Danro November 3, 2008 02:01

Re: Nusselt numbers for fully developed laminar fl

I already did, it doesn't help.

 rH November 7, 2008 18:24

Re: Nusselt numbers for fully developed laminar fl

I was running a similar problem a few months back and was able to achieve the analytical 3.66 Nusselt number. Several things to check, make sure that the length of your pipe is long enough for your flow to be thermally fully developed. Also, in my case I had to experiment with several different meshes (prism layers and refinement) before I was able to achieve the correct value.

For reference, how are you doing your postprocessing? Are you doing all the calculations in STAR-CCM for the Nusselt number? A detailed description of how you're doing the calcs and postprocessing may help in troubleshooting the issue.

 Danro November 10, 2008 10:56

Re: Nusselt numbers for fully developed laminar fl

I only used the Nusselt number field function in STAR-CCM to calculate the Nusselt number

 Amod November 10, 2008 13:42

Re: Nusselt numbers for fully developed laminar fl

The flow has to be developed Hydraulically as well as Thermally. You can ensure the hydraulically fully developed flow by assigning the parabolic profile U = Um(1-r/R)^2 available in any Fluid Mechanics book. You can put similar profile for temperature. This will expedite the process and you may not need very very long pipe!

To predict H.T.C. correctly, one should continuously do mesh convergence study near the walls. Within 2 ~ 3 iteration, you will get the theoretically correct result.

Hope this explains!

|A| Always Positive!

 moh1367 February 14, 2010 04:51

Hi guys!

My case is very simple , its laminar flow in a pipe with constant wall temperature. I take the length as long as the flow can reach to fully thermally and Hydrodunamically developed. When I get the Nusselt Number from the xyplot/Surface Nusselt Number it can seemed that the Nu begins from a large number(as predicted by theory) and go toward zero at the end of pipe (the theoric value is 3.66). Can anybody help me?

Thanks alot
.

 triple_r July 17, 2010 12:22

Hi,

I know this is an old thread, but just wanted to add this for later reference :-)

I don't know how Star-CCM calculates the Nusselt number, but I just wanted to mention that when you calculate the Nusselt number, you need the value of heat transfer coefficient. In order to get the heat transfer coefficient, you need a temperature difference. In a pipe flow, the temperature difference is defined to be the difference between the wall temperature and the bulk temperature.
In order to get bulk temperature, you need to calculate the mass-flow average of the temperature in the cross section that you want to calculate h (and in turn Nu).
I know in CFX you can define a reference temp, but can't define the bulk temperature to be reference value (or at least I don't know how). So maybe that is the problem that you all getting a value that doesn't match the theory.

 Goutam July 19, 2012 07:22

[QUOTE=Amod
;176668]The flow has to be developed Hydraulically as well as Thermally. You can ensure the hydraulically fully developed flow by assigning the parabolic profile U = Um(1-r/R)^2 available in any Fluid Mechanics book. You can put similar profile for temperature.

Hi, whats the thermally fully developed profile for temperature !!!

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