CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > FLUENT

Confuse about Nu Number in Laminar Flow

Register Blogs Members List Search Today's Posts Mark Forums Read

Like Tree1Likes
  • 1 Post By Alek

Reply
 
LinkBack Thread Tools Display Modes
Old   January 30, 2014, 08:40
Default Confuse about Nu Number in Laminar Flow
  #1
Senior Member
 
Join Date: Jan 2011
Posts: 163
Rep Power: 6
mariam.sara is on a distinguished road
Hi I did simple heat transfer problem of flow throw a circular pipe with constant heat flux at wall. Usually for laminar flow with constant heat flux the Nusselt Number should be 4.36 ? when I view surface nusselt number from Report>Wall Fluxes>Surface Nusselt number it give me Nusslet number as 11.61 ? Why this deffer from the theortical value as (4.36)?

Mariam
mariam.sara is offline   Reply With Quote

Old   January 30, 2014, 08:54
Default
  #2
New Member
 
Smith
Join Date: Jan 2014
Posts: 9
Rep Power: 3
Alek is on a distinguished road
dear mariam and sara,
In fact FLUENT compute h (heat transfer coefficient) and NU number wrong and you should compute them manually as follows:
h=mdat*cp*(Twall-Tbulk)
Nu=hD/k

Sincerely
Alek is offline   Reply With Quote

Old   January 30, 2014, 08:59
Default
  #3
Senior Member
 
Join Date: Jan 2011
Posts: 163
Rep Power: 6
mariam.sara is on a distinguished road
Hi Alek thanks a lot for the quick respond. You mentioned in your relation Twall & Tbulk? the problem I not have both of these temperatures because I used fixed heat flux at the wall not fixed wall temperature? So how I can predict Twall in this case?
mariam.sara is offline   Reply With Quote

Old   January 30, 2014, 09:23
Default
  #4
New Member
 
Smith
Join Date: Jan 2014
Posts: 9
Rep Power: 3
Alek is on a distinguished road
The Twall which is computed with FLUENT is right and you can use it.
Alek is offline   Reply With Quote

Old   January 30, 2014, 09:25
Default
  #5
Senior Member
 
Join Date: Jan 2011
Posts: 163
Rep Power: 6
mariam.sara is on a distinguished road
Do you mean I need to plot Twall along the wall? what about Tbulk how i can compute it?
mariam.sara is offline   Reply With Quote

Old   January 30, 2014, 09:27
Default
  #6
New Member
 
Smith
Join Date: Jan 2014
Posts: 9
Rep Power: 3
Alek is on a distinguished road
If you send me an email I can send you a good document.
Alek is offline   Reply With Quote

Old   January 30, 2014, 10:33
Default
  #7
Senior Member
 
François Grégoire
Join Date: Jan 2010
Location: Laval University, Canada
Posts: 366
Rep Power: 8
macfly is on a distinguished road
Quote:
Originally Posted by Alek View Post
dear mariam and sara,
In fact FLUENT compute h (heat transfer coefficient) and NU number wrong and you should compute them manually as follows:
h=mdat*cp*(Twall-Tbulk)
Nu=hD/k
Sincerely
Careful maria, the equation Alek gave you for h is wrong: It's not the equation for h, it's the equation for the heat transfer.

Fluent calculates the heat transfert coefficient (h) from various correlations depending on the physics. It's your homework to verify in the Theory Guide which correlation Fluent uses according to the physics of your model, then decide if you should modify it or not.

Last edited by macfly; January 30, 2014 at 12:00.
macfly is offline   Reply With Quote

Old   January 30, 2014, 11:44
Default
  #8
Senior Member
 
Join Date: Jan 2011
Posts: 163
Rep Power: 6
mariam.sara is on a distinguished road
Hi Macfly I not have problem with the theory. I tried solve problem with constant heat flux at a cylinder wall with laminar flow case it must give me Nu=4.36 but it give different value? I am not one of fluent designer to know how FLUENT predict Nu values you answer is vague to me?
mariam.sara is offline   Reply With Quote

Old   January 30, 2014, 12:08
Default
  #9
Senior Member
 
François Grégoire
Join Date: Jan 2010
Location: Laval University, Canada
Posts: 366
Rep Power: 8
macfly is on a distinguished road
Quote:
Originally Posted by mariam.sara View Post
Hi Macfly I not have problem with the theory. I tried solve problem with constant heat flux at a cylinder wall with laminar flow case it must give me Nu=4.36 but it give different value? I am not one of fluent designer to know how FLUENT predict Nu values you answer is vague to me?
You don't have problem with the theory but it seemed like you bought what Alek said.

- maybe your mesh is not fine enough?
- maybe the flow is not fully developped?

What I'm saying wouldn't sound vague if you took a look at the theory guide in order to understand what Fluent is doing. I guess you're doing some engineering homework and you have to put some thinking into it. You don't become an engineer just clicking on buttons expecting to get the right number on the output.
macfly is offline   Reply With Quote

Old   January 30, 2014, 12:19
Default
  #10
New Member
 
Smith
Join Date: Jan 2014
Posts: 9
Rep Power: 3
Alek is on a distinguished road
Dear maryam and Sara
you can use the following relations to compute h and NU manually:
Q=q''Px=mdat*C*(Tbulk,x-Tbulk,in)->Tbulk,x=(q''Px/mdat*C)+Tbulk,in
h(x)=q''/(Twall-Tbulk,x)

Twall is calculated by Fluent
macfly likes this.
Alek is offline   Reply With Quote

Old   February 9, 2014, 02:12
Default How to make inlet velocity fixed along a pipe
  #11
Senior Member
 
Join Date: Jan 2011
Posts: 163
Rep Power: 6
mariam.sara is on a distinguished road
Hello
I have a query to how i can make the inlet velocity value fixed from pipe inlet until pipe outlet? is that possible in Fluent? knowing that the flow is laminar?
mariam.sara is offline   Reply With Quote

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
decomposePar pointfield flying OpenFOAM Running, Solving & CFD 28 December 30, 2013 16:05
AMI interDyMFoam for mixer danny123 OpenFOAM Running, Solving & CFD 4 June 19, 2013 04:49
parallel code samiam1000 SU2 3 March 25, 2013 05:55
Stable boundaries marcoymarc CFX 33 March 13, 2013 07:39
AMI speed performance danny123 OpenFOAM 19 October 24, 2012 07:44


All times are GMT -4. The time now is 04:45.