UDF of saturation temperature
Hi there,
I am trying to simulate condensation in the building environment, using the evaporation-condensation model in Fluent. Mixture multiphase model was used, with the primary phase as water liquid and a mixture of water vapour and air as a secondary phase. I understand I need a UDF to describe the relationship between saturation temperature and water vapour pressure. The UDF I used is listed below: #include "udf.h" DEFINE_PROPERTY(saturation_temp,c,t) { real sat_t; real vapor_p; real static_p = C_P(c,t); real operating_p = RP_Get_Real ("operating-pressure"); vapor_p = static_p + operating_p; // absolute pressure = static pressure + operating pressure sat_t = (1723.6425/(8.05573-((double)log10((double)vapor_p/133.322)))-233.08)+273.15; // Pressure in Pa to mmHg (Yaws) return sat_t; } A transient model is used and the results diverged after 5 iterations. The error is shown as below: Experiencing convergence difficulties - temporarily relaxing and trying again... Experiencing convergence difficulties - temporarily relaxing and trying again... Experiencing convergence difficulties - temporarily relaxing and trying again... Experiencing convergence difficulties - temporarily relaxing and trying again... Stabilizing pres sure coupled to enhance linear solver robustness. Stabilizing pressure coupled using GMRES to enhance linear solver robustness. Experiencing convergence difficulties - temporarily relaxing and trying again... Divergence detected in AMG solver: pressure coupled Stabilizing phase-2-species-0 to enhance linear solver robustness. Stabilizing phase-2-species-0 using GMRES to enhance linear solver robustness. Stabilizing temperature to enhance linear solver robustness. Stabilizing temperature using GMRES to enhance linear solver robustness. temperature limited to 1.000000e+00 in 751264 cells on zone 10 in domain 1 temperature limited to 5.000000e+03 in 399712 cells on zone 10 in domain 1 Divergence detected in AMG solver: pressure coupled Divergence detected in AMG solver: pressure coupled Divergence detected in AMG solver: pressure coupled Error at host: floating point exception Error at Node 0: floating point exception Error at Node 1: floating point exception Error at Node 2: floating point exception Error at Node 3: floating point exception ===============Message from the Cortex Process================================ Compute processes interrupted. Processing can be resumed. ================================================== ============================ Interrupting client... Done. Registering Udf, ("H:\rail humid_files\dp4\FLU\Fluent\sattemp1.txt") Calculation complete. It is really killing me, somebody please help me!!! Thank you so much. |
Does the same happen if you vaatwanden the relevant line to
Code:
sat_t = 300; |
Hi Pakk,
I am not sure if I have got you right, but I have tried the follows code: #include "udf.h" DEFINE_PROPERTY(saturation_temp,c,t) { return (real)373.1545154; } which works well. |
Hi Pakk,
yes, I just replaced Quote:
Quote:
|
OK, then the problem is in that equation.
Is the pressure in your simulation within the range where that equation is valid? |
Hi Pakk,
Yes, I guess so. May I ask what static_p = C_P(c,t) represent? Cuz I copy the code from somewhere else and not quite familiar with it. |
If C_P(c,t) is the static pressure of the mixture, then I think the above UDF is wrong.
Cuz the saturation temperature is a function of partial vapour pressure but not the static pressure of the mixture, am I right? |
The UDF implements a certain equation from physics. You have to say which equation should be implemented.
If you tell Fluent to use the wrong equation, then you will get the wrong results. I can not tell you if the equation is wrong: you are the one who is doing the simulation, you are the one who decided that a certain equation should be used. So you are the one who should know how the equation should look like. |
HI Pakk,
Thanks, I think I have already solved it! |
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