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June 23, 2008, 20:54 
Convergence with a Natural Convection Problem

#1 
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Hello, I have setup and ran my fluent problem. It seems to not want to converge. There is a circular heater rod in the middle of a cylinder of fluid. Version: axi, dp, dbns imp, lam (axi, double precision, densitybased implicit, laminar) Release: 6.3.26
This is Axisymmetric. The axis is the short piece under the heater. Everything else is a wall. Is this correct? Thank you. I have included pictures and some information. Let me know if you need any more information. Velocity http://i30.tinypic.com/2i1jvoo.png Residuals http://i28.tinypic.com/e14wev.png Solver Controls  Equations Equation Solved  Flow yes Numerics Numeric Enabled  Absolute Velocity Formulation yes Relaxation Variable Relaxation Factor  Solid 1 Linear Solver Solver Termination Residual Reduction Variable Type Criterion Tolerance  Flow FCycle 0.1 Discretization Scheme Variable Scheme  Flow Second Order Upwind Time Marching Parameter Value  Solver Implicit Courant Number 5 ...........Material: moltensalt (fluid) Property Units Method Value(s)  Density kg/m3 boussinesq 2005 Cp (Specific Heat) j/kgk constant 1884 Thermal Conductivity w/mk constant 0.80000001 Viscosity kg/ms constant 0.00208 Molecular Weight kg/kgmol constant 28.966 LJ Characteristic Length angstrom constant 3.711 LJ Energy Parameter k constant 78.6 Thermal Expansion Coefficient 1/k constant 0.000363 

June 24, 2008, 09:22 
Re: Convergence with a Natural Convection Problem

#2 
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look for few check and try various combinations:
1) What are your URF values? 2) I have worked on few turbulence models concerning natural convection and have found v2f model to be most suitable. This was using the openFOAM solver. maybe I overheard, RNG formulation of kepsilon model works decently well. You can even try komega SST model but I do not feel it is something which is affecting your convergence. 3)Use PRESTO! pressure formulation 4) If the changing the URM values does not bring in much change, I will ask you to consider mesh refinement. 

June 24, 2008, 11:45 
Re: Convergence with a Natural Convection Problem

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Thank you very much for the response.
1. I am using the desity based solver so I only have a solid variable with a Relaxation Factor of 1. I did this because I am using boussineq for the fluid density with a Thermal Expansion Coefficient of 0.000363 2. The velocity is less than 4cm/s and it has a small Prandtl number. That is why it is laminar, but I could try the kepsilon and mess with the RNG. 3.Do I have to use a pressure based solver with this? 4.What should I be looking for in the mesh? If you want, I could email you my Case and Mesh if it is easier for you to see. 

June 25, 2008, 03:55 
Re: Convergence with a Natural Convection Problem

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[[[[1. I am using the desity based solver so I only have a solid variable with a Relaxation Factor of 1. I did this because I am using boussineq for the fluid density with a Thermal Expansion Coefficient of 0.000363]]]]]
No problems here. [[[[[2. The velocity is less than 4cm/s and it has a small Prandtl number. That is why it is laminar, but I could try the kepsilon and mess with the RNG.]]]]] sounds okay to me. Turbulence in optional based on accuracy in results that you require. Though you cannot neglect the turbulence just on low values of reynolds number and prandtl number. Even for a low prandtl number flows the effect of buoyancy on turbulence can be enormous based of effects like heat transfer through conduction if the fluid conductivity is high. If your fluid has high thermal conductivity, it will cause turbulent effect on heat transfer. [[[[3.Do I have to use a pressure based solver with this?]]]] NO [[[[[4.What should I be looking for in the mesh? ]]]]]] Boundary refinement at the walls. If its possible, send me the case and i will have a look at it. 

June 25, 2008, 08:29 
Re: Convergence with a Natural Convection Problem

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I sent you an email with the Case. Thanks.


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