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Convergence with a Natural Convection Problem

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Old   June 23, 2008, 20:54
Default Convergence with a Natural Convection Problem
  #1
peznex
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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, density-based 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 F-Cycle 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/kg-k constant 1884

Thermal Conductivity w/m-k constant 0.80000001

Viscosity kg/m-s constant 0.00208

Molecular Weight kg/kgmol constant 28.966

L-J Characteristic Length angstrom constant 3.711

L-J Energy Parameter k constant 78.6

Thermal Expansion Coefficient 1/k constant 0.000363
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Old   June 24, 2008, 09:22
Default Re: Convergence with a Natural Convection Problem
  #2
prashant
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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 k-epsilon model works decently well. You can even try k-omega 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.
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Old   June 24, 2008, 11:45
Default Re: Convergence with a Natural Convection Problem
  #3
peznex
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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 k-epsilon 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.
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Old   June 25, 2008, 03:55
Default Re: Convergence with a Natural Convection Problem
  #4
prashant
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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 k-epsilon 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.
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Old   June 25, 2008, 08:29
Default Re: Convergence with a Natural Convection Problem
  #5
peznex
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I sent you an email with the Case. Thanks.
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