
[Sponsors] 
November 2, 2010, 10:40 
Petukhov, Gnielinski HTC

#1 
New Member
Join Date: Nov 2010
Posts: 4
Rep Power: 8 
I have a "simple" simulation that's driving me nuts. I'm trying to prove the accuracy of the PetukhovGnielinski heat transfer coefficient correlation for thermalentry length in a pipe (see http://www.cheresources.com/convection.shtml under section 13) using the following model: A 2.2m long pipe with inner diameter of 9.7mm. There's an unheated section 1.6m long (to achieve a fullydeveloped velocity profile), followed by a 0.6m long section with constant wall heat flux. Water is the fluid, and I'm using SST turbulence model. The Reynolds number is around 52000. The pipe wall temperature, massflow averaged mean temperature, and the resulting Nusselt number all have the correct shape, but the Nusselt number magnitude is off by a few thousand. I've tried all sorts of meshes. Any advice?


November 2, 2010, 11:50 

#2 
Senior Member
Join Date: Apr 2009
Posts: 532
Rep Power: 14 
Are you using the default wall heat transfer coefficient, which would be based on the nearwall temperature? If so, you probably want to set a reference temperature for the HTC calculation using the expert parameter 'tbulk for htc'.


November 2, 2010, 12:50 

#3 
New Member
Join Date: Nov 2010
Posts: 4
Rep Power: 8 
Unfortunately, I can't use the nearwall temp, nor a constant value for 'tbulk'. It needs to be a velocityaveraged temperature (since water is incompressible, I can use massaveraged temp.). Basically I have a bunch planes marching down the pipe and use the equation massFlowAve(Temperature)@Plane1 to find tbulk throughout the axial length of pipe.


November 4, 2010, 13:20 

#4 
Senior Member
Michael P. Owen
Join Date: Mar 2009
Posts: 196
Rep Power: 10 
How are you calculating the Nusselt number?


November 4, 2010, 13:34 

#5 
New Member
Join Date: Nov 2010
Posts: 4
Rep Power: 8 
Nu = h*dia./k
q" = h(Twall  Tmean) q" and the pipe diameter are constant input values. k is a constant material property. Twall I get from a line generated at the wall, and Tmean comes from "massFlowAve(Temperature)@Plane x" where I have multiple planes normal to the flow at various axial locations in the pipe. Then I calculate the heat transfer coefficient from q", Twalls (at the same axial locations as my planes), and Tmeans, then get the Nusselt Numbers along the pipe length. 

November 4, 2010, 14:44 

#6 
Senior Member
Michael P. Owen
Join Date: Mar 2009
Posts: 196
Rep Power: 10 
Twall should be the solid surface temperature. This is NOT the fluid temperature at the wall; there is a discontinuity in temperature at the fluid solid interface. Using the fluid temperature will give you an artificially small delta T in the calculation, meaning your heat transfer coefficient and hence your Nusselt number will be artificially high.


November 4, 2010, 15:53 

#7 
New Member
Join Date: Nov 2010
Posts: 4
Rep Power: 8 
I originally had the model as three domains: two solids (one heated pipe and one unheated pipe), and one fluid (water). The heated pipe had a constant heat flux boundary condition at its OD. I saw backeffects of the heated portion of the pipe into the water prior to the water entering the heated portion even though I had an adiabatic wall separating the two pipes. Also, the resultant heat flux on the inner pipe wall was not constant. Therefore, I switched to two domains: one unheated water and one heated water which has the heat flux boundary at the OD (which would be the ID of the pipe, if it were there). I adjusted my heat flux value to account for the difference in area of the original pipe OD and the new water OD, although that shouldn't change the Nusselt Number. Should I put the pipe back in? If so, how could I ensure no energy gets transferred to the water prior to my x=0 point (the point where the heated wall starts)?


November 4, 2010, 16:23 

#8 
Senior Member
Michael P. Owen
Join Date: Mar 2009
Posts: 196
Rep Power: 10 
Yeah. The problem is that you don't know the temperature of the pipe, which is the temperature that goes into the heat transfer coefficient calculation.


Tags 
htc, nusselt number, petukhov, pipe flow 
Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
HTC problem with multiphase  shogologo  CFX  0  April 21, 2010 09:26 
define htc BC as function of Qwall in transients  richard  CFX  0  October 10, 2008 10:41 
How to find Heat Transfer Coefficient (HTC)?  Raja  CFX  5  March 4, 2008 14:20 
hi how to calculate the HTC  kathir  Siemens  0  March 26, 2006 09:31 
Why so different 'htc' results...?  azt  CFX  0  November 9, 2005 17:15 