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August 26, 2011, 20:01 |
Steam pipline
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#1 |
New Member
Milan Banic
Join Date: Mar 2011
Posts: 2
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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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August 26, 2011, 22:15 |
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#2 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,729
Rep Power: 143 |
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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August 27, 2011, 09:24 |
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#3 |
New Member
Milan Banic
Join Date: Mar 2011
Posts: 2
Rep Power: 0 |
I have already read the convergence tips. I can't change the advection scheme because I have demands for high accuracy. I didn’t 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 don’t know what is the model parameter which is not properly defined. |
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August 28, 2011, 06:33 |
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#4 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,729
Rep Power: 143 |
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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