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Old   August 26, 2011, 20:01
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Milan Banic
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I have convergence and results problems in my simulation. I am trying to simulate a 150 m steam pipeline in order to determine energy losses. The transport fluid is dry steam (water ideal gas) with 180C and 10 m/s at inlet. Outlet pressure is set to 9.8 bar, while reference pressure is 0 bar. I use a rough wall, with Heat Transfer Coefficient = 10 [W m^-2 K^-1], outside temperature 20C and included viscous term. The overall mesh size is 7000000 elements. Boundary layer has 15 layers with total thickness of 1.5 mm and grow rate of 1.1. The mesh was refined on all the pipeline curves. I am using the k-e model, high resolution for turbulence numerics and advection control with double precission.
The simulation converges just fine with low values of outlet pressure (I used the outlet pressure of 0 bar) but the results are unrealistic. The steam was cooled down for 30C which is impossible.
When I use the real value of outlet pressure (9.8 bar) the solution fails to converge below RMS 10-4.
I have attached the convergence diagrams.
Note that for all monitors other than h energy domain imbalance is 0%. For h energy it is 0.001%.
The results are again unrealistic because temperature drop between inlet and outlet is now 18C which is huge for a given problem.

Does anybody knows what might be a problem with my setup?

Thank you in advance.
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Old   August 26, 2011, 22:15
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Glenn Horrocks
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There are some general tips here:
http://www.cfd-online.com/Wiki/Ansys...gence_criteria

But I suspect your problem lie with your specific setup. I am no expert on steam flow so will not be able to help you much.

What steam properties are you using?
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Old   August 27, 2011, 09:24
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Milan Banic
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I have already read the convergence tips. I can't change the advection scheme because I have demands for high accuracy. I didnít try the time step increase so I will try it now.
The steam properties were taken from the thermodynamics handbook. Below is the CEL definition of the material.

MATERIAL: Water Ideal Gas
Material Description = Water Vapour Ideal Gas (180 C and 10 bar)
Material Group = Dry Steam
Option = Pure Substance
Thermodynamic State = Gas
PROPERTIES:
Option = General Material
EQUATION OF STATE:
Molar Mass = 18.02 [kg kmol^-1]
Option = Ideal Gas
END
SPECIFIC HEAT CAPACITY:
Option = Value
Specific Heat Capacity = 2553 [J kg^-1 K^-1]
Specific Heat Type = Constant Pressure
END
REFERENCE STATE:
Option = Specified Point
Reference Pressure = 10 [bar]
Reference Specific Enthalpy = 2778000 [J/kg]
Reference Specific Entropy = 6586 [J/kg/K]
Reference Temperature = 180 [C]
END
DYNAMIC VISCOSITY:
Dynamic Viscosity = 9.4E-06 [kg m^-1 s^-1]
Option = Value
END
THERMAL CONDUCTIVITY:
Option = Value
Thermal Conductivity = 193E-04 [W m^-1 K^-1]
END
ABSORPTION COEFFICIENT:
Absorption Coefficient = 1.0 [m^-1]
Option = Value
END
SCATTERING COEFFICIENT:
Option = Value
Scattering Coefficient = 0.0 [m^-1]
END
REFRACTIVE INDEX:
Option = Value
Refractive Index = 1.0 [m m^-1]
END
END
END
END

I also think that the problem is in model setup, especially because of unrealistic results. But obviously I donít know what is the model parameter which is not properly defined.
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Old   August 28, 2011, 06:33
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Glenn Horrocks
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You could also try using the IAPWS (I think that is the name) water properties. That would tell you whether it is your material properties - and it will be more accurate than the material properties you are using anyway, providing you are modelling reasonably pure water.
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