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Getting (Nusselt vs Re) & (friction vs Re) in a ribbed duct

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Old   November 13, 2012, 00:56
Unhappy Getting (Nusselt vs Re) & (friction vs Re) in a ribbed duct
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shayan
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Hello

I have a ribbed duct as blow :
http://imageshack.us/photo/my-images/109/51677694.jpg/

With the following boundary conditions:

Duct height (H)=20 mm
Rib height (e)=3 mm (square rib)
p/e=13.3
Inlet length=245 mm
Uniform heat at bottom surface=1100 W/m2
(the surface below a rib is considered insulated)
Aspect ratio (AR)=5
Pitch p=40 mm
Length of test section=280 mm
Outlet length=115 mm
Width of duct=100 mm
Reynolds number=8900.

How can I get the following curves in?

http://imageshack.us/photo/my-images/209/17778613.jpg/

http://imageshack.us/photo/my-images/23/35025581.jpg/

*I use (Fluent 6.3.26)*

Thank you in advance

Shayan
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Old   November 13, 2012, 19:29
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Lucky Tran
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To get Nu vs Re, you need to run multiple simulations (multiple Re).

For each Re, calculate Nu and then save it into your favorite data manipulator (excel, etc) and make the plot from there.
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Old   November 13, 2012, 23:27
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Quote:
Originally Posted by LuckyTran View Post
To get Nu vs Re, you need to run multiple simulations (multiple Re).

For each Re, calculate Nu and then save it into your favorite data manipulator (excel, etc) and make the plot from there.
Thanks Lucky
In Fluent , I can get the "Surface Nusselt Number vs Position". You mean that I should write the reports of that for my specific Re number and calculate the average of those Nusselt number?
In order to change the Re number, is it needed to change the "inlet velocity" only or any other terms should be considered?
Lastly,the above curve is called "Ave(Nu)-Re" or "Nu-Re"?

Regards
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Old   November 13, 2012, 23:50
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You need to define Re. And then change the appropriate boundary condition to achieve the desired change in Re.

The curve is labeled Nu but it is undecipherable from the plot what Nu is. It could mean a lot of things. How Nu is defined will determine what you need to do in order to plot it.

Fluent can calculate the overall average Nu (area weighted average) so you do not need to plot Nu vs x and then average yourself. Fluent can calculate those directly.
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Old   November 14, 2012, 00:09
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Quote:
Originally Posted by LuckyTran View Post
You need to define Re. And then change the appropriate boundary condition to achieve the desired change in Re.

The curve is labeled Nu but it is undecipherable from the plot what Nu is. It could mean a lot of things. How Nu is defined will determine what you need to do in order to plot it.

Fluent can calculate the overall average Nu (area weighted average) so you do not need to plot Nu vs x and then average yourself. Fluent can calculate those directly.
If you check the following link, you can see that for example for Re=10000 ,there is a specific Nu number.But, when I get Nu vs Position, I'll have many different Nu numbers for different positions. How I can relate "one" Nu number for "one" Re number?

http://imageshack.us/photo/my-images/209/17778613.jpg/

Thanks
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Old   November 14, 2012, 00:24
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It's up to you.

The overall average Nusselt number is one way to get one Nusselt number. But there are many many many non-trivial ways to define average Nusselt number or just Nusselt number.
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Old   November 14, 2012, 04:42
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Hi,

It seems the best you can do is changing velocity to have different Re and calculate the Nu for each model. Even you can do it changing viscosity or density, whatever you want if you keep consistency with the Re.

The only part I see a bit more difficult is how deal with this low Re number around 2000-10000. You will have to try different turbulence methods and find out which is able to handle this low Re region.

Rgds.
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