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UDF of saturation temperature

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Old   June 13, 2021, 02:36
Default UDF of saturation temperature
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Stanlee
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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.

Last edited by stanlee8; June 13, 2021 at 02:37. Reason: the color is annoying
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Old   June 13, 2021, 02:48
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Does the same happen if you vaatwanden the relevant line to

Code:
sat_t = 300;
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"The UDF library you are trying to load (libudf) is not compiled for parallel use on the current platform" is NOT the error after compiling. It is the error after loading. To see compiler errors, look at your screen after you click "build".
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Old   June 13, 2021, 06:41
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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.
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Old   June 13, 2021, 07:07
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Hi Pakk,

yes, I just replaced
Quote:
sat_t = (1723.6425/(8.05573-((double)log10((double)vapor_p/133.322)))-233.08)+273.15;
with
Quote:
sat_t = 300;
and it works fine.
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Old   June 13, 2021, 13:00
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OK, then the problem is in that equation.

Is the pressure in your simulation within the range where that equation is valid?
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"The UDF library you are trying to load (libudf) is not compiled for parallel use on the current platform" is NOT the error after compiling. It is the error after loading. To see compiler errors, look at your screen after you click "build".
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Old   June 14, 2021, 23:22
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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.
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Old   June 14, 2021, 23:30
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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?
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Old   June 16, 2021, 04:01
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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.
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Old   June 27, 2021, 07:12
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HI Pakk,

Thanks, I think I have already solved it!
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