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-   -   A Simple question about Nusselt No.?! (http://www.cfd-online.com/Forums/fluent/48871-simple-question-about-nusselt-no.html)

 Yalu July 26, 2008 06:39

A Simple question about Nusselt No.?!

Dear All,

I am a new user of Fluent. I need to calculate Average value of Nusselt No. for an internal flow in a pipe with constant heat flux. I have checked the calculation with Report > Surface Integrals> area-weighted average> surface heat transfer coefficient : & Nu=(h*d)/k; using wall average Static Temperature and pipe area and diameter in reference values. But unfortunately the results are so different with literature, especially when changing the Re. No. The length of the pipe is 0.8 m & the diameter is : 0.01m.

Any urgent help would be appreciated.

All the best:Yalu

 AAA July 26, 2008 08:50

Re: A Simple question about Nusselt No.?!

Hi

Check the fluid properties (k). The mesh at the wall has to be fine enough.

regards

AAA

 Yalu July 28, 2008 07:22

Re: A Simple question about Nusselt No.?!

Dear AAA, Thanks alot for your prompt answer. I checked the properties.They are O.K. Also about the mesh I have used 100*150 grid: 100 in r-direction and 150 in x-direction.? I think it suits with my dimension. Anyway I would like to know is there any reliable procedure for calculating Average Nusselt number, which consists with literature? I look forward to hear from anybody who can help me....

Best Wishes

 deepthi saini July 28, 2008 16:24

Re: A Simple question about Nusselt No.?!

hi,

for calculation of nusselt number .Follow this procedure.

STEP 1 as you are modeling a constant heat flux problem use newtons law of cooling to calculate nusselt number

STEP2

i.e; q=h (TS-TF)

ONCE h IS OBTAINED

then use h to calualte Nu =hD/K

your solution doesnt match the analytical solution as fluent uses length to calculate Nusselt number so foolow this steps These will help IF YOU NEED ANY OTHER HELP YOU ARE WELCOME BYE GOOD LUCK

 Yalu August 19, 2008 05:15

Re: A Simple question about Nusselt No.?!

Thanks a lot for your answers, but can you help me how to find TS and TF (wall temperature & fluid temperature) from FLUENT?

All the Best

 deepthi saini August 21, 2008 22:26

Re: A Simple question about Nusselt No.?!

hi

you can plot wall temperature using the icon plot-xy plot-

and select your boundary wall to get the surface temperatures at each point

step 2

click on write option in the same panel /store in excel file

for fluid temperature

go to mass weighted average in surface integral option and select outlet if you need outlet fluid temperature

but if in case you need the average fluid temperature or all the temperature along the tube

you need to create vertical lines/rakes in the tube using surface -point option /line option in fluent

after creating lines calculate the same mass weighted average along that lines using surface integral option

this gives the fluid temperatures along the flow.

good luck seems lengthy but this wil work

bye

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