# Turbulent thermal conductivity

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 May 9, 2006, 07:16 Hello, Im creating a case w #1 newbee Guest   Posts: n/a Hello, Im creating a case whera a fluid of incompressible fluid is beeing heated from a constant heat flux from one of the walls. As temperature B.C for that wall I have put upp the condition fixed gradien (uniform) which works fine. Th problem is which value to use for the konstant temperature gradient. Im using the formula deltaT=q''/kEff. The problem is to find a value for the constant kEff=kTurb+k (where kTurb is the problem) Deos anyone know how I can find this value out? Thanks /Erik

 November 25, 2011, 04:47 #2 Senior Member   Anne Gerdes Join Date: Aug 2010 Location: Hamburg Posts: 152 Rep Power: 6 As far as I know k_eff = v/Pr + nu_t/Prt where nu is the kinematic viscosity, nu_t the turbulent viscosity and Pr and Pr_t Prandtl-number, laminar and turbulent, respectively. Goutam likes this.

 August 29, 2012, 18:47 #3 New Member   Anonymous Join Date: Aug 2012 Posts: 8 Rep Power: 4 Hi Anne and Erik, I'm new to OpenFOAM and CFD. I'm trying to determine the heat transfer from a problem I setup. It is a heated concentric cylinder problem that I solved with the k-epsilon model with buoyantBoussinesqPimpleFoam. It appears to solve for kappat (kinematic turbulent thermal conductivity), k (turbulent kinetic energy), nut (turbulent viscosity), alphat (thermal diffusivity), epsilon (kinetic energy dissipation rate), and a few others. I'm a little lost as to what equation to use to get the effective thermal conductivity though. If I can get k_eff, I have a formula for my particular setup to get the heat transfer. But I'm stuck on how to get k_eff. Any suggestions?

 September 10, 2012, 06:05 #4 Senior Member   Anne Gerdes Join Date: Aug 2010 Location: Hamburg Posts: 152 Rep Power: 6 Hey Tiffany, kappa_eff is the sum of kappa_t and kappa. The ladder is computed from nu/Pr which are defined in constant/transportProperties. Kind Regards Anne

 December 29, 2013, 09:25 #5 Member   Peter Join Date: Nov 2011 Posts: 45 Rep Power: 5 Hi, Tiffany! Have you solve your problem yet? I have a similar question. As mentioned by Anne, k_eff = v/Pr + nu_t/Prt, then I guess k_eff is not a constant because nu_t is not. Am I right? If it is so, then how to set up the temperature boundary condition for a give heat flux? Regards Peter

January 8, 2014, 09:08
#6
Senior Member

Anne Gerdes
Join Date: Aug 2010
Location: Hamburg
Posts: 152
Rep Power: 6
Hey Peter,

there are wall functions for kappat, namely

Quote:
 kappatJayatillekeWallFunction
which you can use at the wall.

On the other boundaries you can set kappat to "calculated" as it is computed from nut, which is computed from the turbulent properties (omega, k, epsilon,...). You can also use "symmetryPlane" or "empty", depending on the case that you would like to simulate.

Kind Regards
Anne

January 8, 2014, 21:10
#7
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Peter
Join Date: Nov 2011
Posts: 45
Rep Power: 5
Quote:
 Originally Posted by palmerlee Hi, Tiffany! Have you solve your problem yet? I have a similar question. As mentioned by Anne, k_eff = v/Pr + nu_t/Prt, then I guess k_eff is not a constant because nu_t is not. Am I right? If it is so, then how to set up the temperature boundary condition for a give heat flux? Regards Peter
The following code is for those who need to set a fixed heat flux boundary:

Quote:
 wall { type groovyBC; value uniform 300; valueExpression "300"; gradientExpression "gradT"; fractionExpression "0"; variables ( "heatFlux=100;" "rho=1.1767;" "cp=1006.6;" "nv=1.58e-05;" "pr=0.714;" "kappa=nv/pr;" "gradT=heatFlux/((kappa+kappat)*rho*cp);" ); }

January 12, 2014, 21:07
#8
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Peter
Join Date: Nov 2011
Posts: 45
Rep Power: 5
Hi, Anne!

Thank you for reply. Here is my kappt boundary:
Quote:
 wall { type fixedValue; value uniform 0; } top { type calculated; value \$internalField; } inlet { type calculated; value \$internalField; } outlet { type calculated; value \$internalField; }
The yPlus in my case is small (y+ < 3) so that I do not use wall funtion. Instead, I set it to zero at the wall. Please correct me if I am wrong. I use LES turbulent model.
By the way, my simulation seems to blow up after thousands of time steps and I have no idea what causes that. Could it be the fvSchemes?

Regards
Peter

 January 13, 2014, 04:32 #9 Senior Member   Anne Gerdes Join Date: Aug 2010 Location: Hamburg Posts: 152 Rep Power: 6 Dear Peter, I think if your mesh is fine enough you can set kappat like that. What kind of boundary is boundary "Top" in your case? In which sense does the simulation blow up? Residuals? Continuity error?

January 13, 2014, 06:46
#10
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Peter
Join Date: Nov 2011
Posts: 45
Rep Power: 5
Dear Anne,

"Top" is an open boundary which I treated the same way as the "outlet" boundary. This is U at "top" boundary:
Quote:
 { type inletOutlet; inletValue uniform (0 0 0); value uniform (0 0 0); }
I set "adjustTimeStep" on to keep Co<0.5. For the first thousands of time steps, delta t is at the order of e-6 second, as it supposed to be. After that, delta t become extremely small. The latest time step is 2.46198e-18 second, for instance. Then I checked the velocity field and found it a total mess. The field became nonphysical.

Perhaps I should use the QUICK scheme for the discretization of the energy equation, the same way as a simulation work in literature.

Best,
Peter

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