[katie13]

Hi All,

I am working on a UDF to facilitate evaporation between two layers. One is a liquid (water) and the other is a vapor(water vapor). My goal is to track the interface as the water evaporates.

I have a model built in ANSYS fluent, real simple, just a two zone 2D rectangle with half patched to liquid and the primary phase being water vapor.

Now, I've tried using ANSYS' built in evaporation-condensation Lee model to evaporate the water layer into the gas vapor. This results in the liquid layer dispersing upward which makes no sense. My understanding is that their model applies the evaporation mass flux to all cells that satisfy T>Tsat and thus it doesn't care where the interface is.

So, I need a UDF instead. I want the UDF to 1. find the interface and 2. apply an evaporative mass flux to the cells there so they will 3. show the interface "receding" from the vapor.

My UDF uses DEFINE_MASS_TRANSFER (and is very similar to the sample UDF ANSYS provides for this macro). I have an if-statement that checks the VOF and then if that indicates that I'm at the interface it calculates the mass flux.

The Problem when I run the below code I never enter the if-statement ie, it doesn't think there is an interface. However, as I said before, this is a two-zone model with vapor the primary (default) and then I patch half the domain to liquid so there is definitely an interface at the start of the analysis.

My questions are: 1. Does my approach make sense and 2. Any idea why the if-statement would not be executing?

Thanks in advance for any help on this!

My UDF: (I've removed the energy equations for ease of reading)
------------------------------------------------------------------------------

// UDF to define a simple mass transfer based on VOF.The "TO" phase is the gas and the "FROM" phase is the liquid phase

#include "udf.h"
#include "sg_mphase.h"

DEFINE_MASS_TRANSFER(liq_gas_source, cell, thread, from_index, from_species_index, to_index, to_species_index)
{
real m_lg;
real T_SAT = 373.15;
Thread* liq = THREAD_SUB_THREAD(thread, from_index);
Thread* gas = THREAD_SUB_THREAD(thread, to_index);
Message("\n \n UDF IS DOING SOMETHING");
m_lg = 0.;
face_t f;

if ((C_VOF(cell, liq) > 0.05) && (C_VOF(cell, liq) <= 0.95))
{
Message("\n \n I found the interface!");

if (C_T(cell, liq) > T_SAT)
{ /* Evaporating */
}
else if (C_T(cell, gas) < T_SAT)
{ /* Condensing */
}


}
return (m_lg);
}

[Cooper24]

Writing a UDF for interface tracking is really challenging. The method(CVOF range between 0 and 1) you are using is more of a Layman's approach and does not yield accurate results. To track the interface at every iteration or time-step, you need to use DEFINE_ADJUST or DEFINE_EXECUTE_AT_END and write a code which will track the cells with volume fraction between 0 and 1. You can store the cell data in a UDMI and use it somewhere else in your UDF.

[bidax]

hi! have you been solve the problem?

[bidax]

Originally Posted by Cooper24

Writing a UDF for interface tracking is really challenging. The method(CVOF range between 0 and 1) you are using is more of a Layman's approach and does not yield accurate results. To track the interface at every iteration or time-step, you need to use DEFINE_ADJUST or DEFINE_EXECUTE_AT_END and write a code which will track the cells with volume fraction between 0 and 1. You can store the cell data in a UDMI and use it somewhere else in your UDF.

hi would you ming see my udf ,in my blogs. im write a udf for eva