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Old   October 2, 2013, 23:16
Default Phase change
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I am working on phase change problem where part of the liquid changes its phase to vapor. This happens when the liquid jet expands and impinges on a surface.The volume fraction of the resultant mixture impinging the surface is around 50%. CFX post outputs Liquid.Temperature and Vapor.Temperature. Both the temperatures are different values. Each point should have one temperature value. Any suggestions on evaluating surface temperature?
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Old   October 3, 2013, 08:28
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Why should each point have one temperature value? The way you have set it up means the vapour and liquid can have different temperatures - and this is physically correct for some multiphase flows.

If you want the liquid and gaseous phase to share a temperature field then turn the energy equation option to "homogenous".

Alternately you can do a weighted average of the gaseous and liquid phases to get a single representative temperature. I will leave what form of weighting up to you - probably mass weighted, but maybe of enthalpy, internal energy or m*Cp*T.
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Old   October 4, 2013, 11:47
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Thank you for your response. It is very helpful. Yes, weighted average would be the best option. Volumes of liquid and vapor are close to being 50-50%. If we were to do a mass weighted average the liquid would have a dominating effect on the temperature. This is a cryogenic process and the temperature needs to drop as it expands through the exit port and phase changes.The liquid temperatures however are not in cryogenic range. If you were to monitor the gas temperature they are however in the cryogenic range, but the weighted averaging reduces that effect.

If we choose the homogeous "energy equation" option, I cannot specific the saturation temperature in the FLuid Pairs tab. I created a homogenous binary material mixture to accomodate for the phase change. The resulting temperature(one value for Liquid and Vapor) I see are higher than cryogenic temperatures.
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Old   October 4, 2013, 17:24
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Quote:
The liquid temperatures however are not in cryogenic range. If you were to monitor the gas temperature they are however in the cryogenic range
Then you have the different phases at different temperatures so the homogeneous assumption is not valid.

So doesn't this answer your question?
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Old   October 4, 2013, 17:44
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Thank you very much for your reply. Yes, if I were to plot a contour of temperature the plot for Liquid.temperature (range around 263-250K)and Vapor.temperature (range around 200-210k) shows different impingment patterns and temperature ranges resulting from jet impingements . For the model that does not have the homogeneous energy equations, the liquid mass dominates but the liquid is not displaying the cryogenic temperature. That confusing me.
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Old   October 5, 2013, 06:25
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Have you activated an interphase heat transfer model?
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Old   October 14, 2013, 10:48
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yes, I have
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Old   October 14, 2013, 17:27
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There is no way we can help you with the sketchy details you have provided so far. http://www.cfd-online.com/Wiki/Ansys...ible_answer.3F
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Old   November 15, 2013, 20:10
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Attaching excerpts from .out file below. I have used the following before-time scale -physical timescale(10e-6, 10e-7 and 10e-8)
The fluid enters in as vapor and then there is a possibility of phase change. Also, run into density errors. Any help is highly appreciated.


.

Parallel run: Received message from slave
-----------------------------------------
Slave partition : 4
Slave routine : EX_TABLE
Master location : End of Continuity Loop
Message label : 009100008
Message follows below - :
+--------------------------------------------------------------------+
| ****** Notice ****** |
| While evaluating |
| Fluid 2.Specific Heat Capacity at Constant Volume |
| on domain "Default Domain", |
| the variable |
| Fluid 2.Static Temperature |
| went outside of its upper limit. Its maximum value was |
| 3.5142E+04. The bounds error was handled by clipping. |
| If this situation persists, consider increasing the table range. |
+--------------------------------------------------------------------+

Parallel run: Received message from slave
-----------------------------------------
Slave partition : 4
Slave routine : EX_TABLE
Master location : End of Continuity Loop
Message label : 009100008
Message follows below - :
+--------------------------------------------------------------------+
| ****** Notice ****** |
| While evaluating |
| Fluid 2.Local Speed of Sound |
| on domain "Default Domain", |
| the variable |
| Fluid 2.Static Temperature |
| went outside of its upper limit. Its maximum value was |
| 3.5142E+04. The bounds error was handled by clipping. |
| If this situation persists, consider increasing the table range. |
+--------------------------------------------------------------------+
+--------------------------------------------------------------------+
| Notice: The maximum Mach number is 2.804E+07. |
+--------------------------------------------------------------------+
================================================== =======
OUTER LOOP ITERATION = 474 CPU SECONDS = 1.268E+05
----------------------------------------------------------------------
| Equation | Rate | RMS Res | Max Res | Linear Solution |
+----------------------+------+---------+---------+------------------+
+--------------------------------------------------------------------+
| ERROR #004100018 has occurred in subroutine FINMES. |
| Message: |
| Fatal overflow in linear solver. |

Last edited by PYJG; November 21, 2013 at 13:46.
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Old   November 16, 2013, 06:08
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Use double precision numerics, define a better initial condition, use local time stepping to start the simulation off, improve mesh quality. And this assumes your physics is correctly set up - nothing will work if it is not set up correctly.
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Old   November 19, 2013, 08:59
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Thanks Glenn. I had already tried all these recommendations. The problem was solved by a simple "reset" solution Thank you for all your help.
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