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Divergence detected in AMG solver

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Old   January 2, 2023, 12:37
Post Divergence detected in AMG solver
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Rahmani Zakaria
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Hi, I am simulating a 3D transient multiphase flow using the mixture model with 3 phases and the evaporation-condensation model (Lee model). After few iterations the calculation diverges and show this message:
temperature limited to 1.000000e+00 in 80 cells on zone 4 in domain 1
temperature limited to 1.000000e+00 in 313 cells on zone 4 in domain 1
Stabilizing temperature to enhance linear solver robustness.
temperature limited to 5.000000e+03 in 377818 cells on zone 4 in domain 1
Stabilizing y-momentum to enhance linear solver robustness.
Stabilizing z-momentum to enhance linear solver robustness.
Stabilizing temperature to enhance linear solver robustness.
temperature limited to 1.000000e+00 in 756903 cells on zone 4 in domain 1
temperature limited to 1.000000e+00 on 338 faces
temperature limited to 5.000000e+03 on 6 faces
Stabilizing x-momentum to enhance linear solver robustness.
Stabilizing y-momentum to enhance linear solver robustness.
Stabilizing z-momentum to enhance linear solver robustness.
Stabilizing pressure correction to enhance linear solver robustness.
Stabilizing temperature to enhance linear solver robustness.
temperature limited to 1.000000e+00 in 1151916 cells on zone 4 in domain 1
temperature limited to 5.000000e+03 in 18 cells on zone 4 in domain 1
temperature limited to 1.000000e+00 on 133 faces

reversed flow in 346 faces on outflow 7.
Stabilizing x-momentum to enhance linear solver robustness.
Stabilizing y-momentum to enhance linear solver robustness.
Stabilizing z-momentum to enhance linear solver robustness.
Stabilizing temperature to enhance linear solver robustness.
temperature limited to 5.000000e+03 in 18 cells on zone 4 in domain 1
temperature limited to 1.000000e+00 on 39 faces

reversed flow in 262 faces on outflow 7.
Stabilizing x-momentum to enhance linear solver robustness.
Stabilizing y-momentum to enhance linear solver robustness.
Stabilizing z-momentum to enhance linear solver robustness.
Stabilizing temperature to enhance linear solver robustness.
Stabilizing temperature using GMRES to enhance linear solver robustness.

Divergence detected in AMG solver: temperature
Error at host: floating point exception
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Old   January 11, 2023, 11:19
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Andrew
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There can be a lot of different reasons for divergence, and it's very dependent on your model. If you search "temperature divergence" on this forum, you'll find a lot of threads with good replies that should help you troubleshoot. I can't summarize them all here, but some common things to to try would be:

- Evaluate the mesh quality (orthogonality, skewness). Try to keep max. skewness below 0.95, and average skewness around 0.33 or less. Best practice is also to keep minimum orthogonal quality above 0.1, and average values as close to 1 as possible. Improve the mesh if necessary.

- Refine the mesh to smaller elements. If there's specific regions where divergence begins, try refining at these locations first.

- Lower your timestep

- Try adding more iterations per timestep to better converge each timestep, and/or lower URFs a bit. For temperature divergence, i've found that lowering the energy URF (down to maybe 0.5 or 0.1) is a good place to start. I don't recommend lowering it too much more than this though.

- Check that your standard state enthalpy values for your phases are correct for each material
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Old   January 11, 2023, 13:44
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Thanks sir,
I have another question; the domain is filled with Hydrogen and Ammonia liquid enters the domain with T=26C and P=25bar. When Ammonia becomes in contact with hydrogen its partial pressure and temperature drops to P=1bar and T=-33C (according to Dalton's law) and the evaporation of ammonia begins. The question how to do that in Ansys Fluent supposed this drop in pressure and temperature results automatically.
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Old   January 11, 2023, 14:28
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What is the mechanism that causes the ammonia's partial pressure and temperatures to drop? Is it a reaction with the hydrogen, or a result of saturation conditions? I'm not sure about this without more information
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Old   January 11, 2023, 15:09
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We have a mixture of Hydrogen and Ammonia; the total pressure is 25bar. The presence of hydrogen plays its role to reduce the partial pressure of ammonia in the evaporator according to Dalton’s law and to allow ammonia to evaporate.
T_saturation = -33C (P=1bar)
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