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May 7, 2014, 16:49 
Natural convection convergence problem

#1 
Senior Member
Bruno Machado
Join Date: May 2014
Posts: 225
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Hi,
I'm trying to solve a square cavity (1x1m) with natural convection heat transfer. Both top and bottom are insulated, left surface is T=330K and right surface at T=300K. The Boussinesq approximation is use for density. Thermophysical properties: rho=1.127 Cp=1007 mu=1.9114e5 k=0.0271 Beta=0.006092 gravity= 9.81 I already tried to start the simulation with default solution methods and after a few iterations chance momentum and velocity values in order to achieve convergence of the result, but it doesn't worked. Any other helpful suggestion? Thanks in advanced 

May 20, 2014, 09:01 

#2 
Member
Hoo
Join Date: Feb 2012
Location: USA
Posts: 83
Rep Power: 6 
Hello!
Please share your case file to be checked. Regrads 

May 21, 2014, 02:21 

#3 
Senior Member
Flavio
Join Date: Sep 2011
Location: Brescia, Italy
Posts: 181
Rep Power: 6 
Hi Bruno,
I suggest setting up a transient simulation (best way for natural convection in closed domain). Otherwise you have to control under relaxation factors. Regards
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Bionico 

May 21, 2014, 06:49 

#4  
Member
Simas
Join Date: Mar 2012
Posts: 37
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Quote:
Could you expand your thought about why you need to do transient calculation for convective internal flow? It is because of iterations graph going up? I am doing calculation also with natural convection in closed domain and would like to know more 

May 21, 2014, 08:06 

#5 
Senior Member
Flavio
Join Date: Sep 2011
Location: Brescia, Italy
Posts: 181
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Hi zombiaska,
according to ANSYS Guidelines (and my experience with air flow in rooms) it's difficult to reach convergence, for continuity equation specially: this is due to a problem of mass balance. ANSYS suggests 2 ways to solve this problem: 1) Set up a transient simulation: the domain starts with a certain mass (calculated from initial conditions, of course) and mantains this value during the entire simulation. 2) Boussinesq approximation: it helps the convergence by using a constant density in some equations. Furthermore I suggest using RNGkepsilon as turbulence model in case of heat transfer in rooms. Anyway it's very difficult to find an ideal model for this kind of case: it takes a lot of time and effort! Best Regards
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Bionico 

May 21, 2014, 10:39 

#6 
New Member
ahmady.a
Join Date: Oct 2013
Posts: 19
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Hi Bruno
your problem my be is turbulent.check rayleigh number. 

May 21, 2014, 12:10 

#7 
New Member
sofia
Join Date: Apr 2010
Location: world
Posts: 19
Rep Power: 8 
Hi
For natural convection use body force weighted for the pressure and power law scheme for the momentum and the energy equations. 

May 21, 2014, 14:59 

#8 
New Member
Josue Fraire
Join Date: Mar 2013
Location: Monterrey, Mexico
Posts: 3
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try to set a initial velocity for the domain in the "Solution initialization", try this to be close to the average velocity you expect or less


May 30, 2014, 15:38 

#9  
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sudhir
Join Date: Mar 2009
Location: india
Posts: 56
Rep Power: 9 
Hi bruno,
Reduce the relaxation factors of momentum to 0.5 and reduce energy to 0.95 and check your solution..if its not converged, go till 0.1 for momentum and 0.9 for energy.. sudhir Quote:


June 2, 2014, 14:25 
Great advice for natural convection problems!

#10 
New Member
T
Join Date: Jun 2014
Posts: 2
Rep Power: 0 

Tags 
convergence, fluent 14.5, natural convection 
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