[Kimia]

Hi all,
I am working on a micro scale simulation of evaporation inside a porous structure, using ANSYS FLUENT. It is a transient two phase model (VOF) considering water and vapor inside a micro scale pore. I am using UDF codes to apply the source terms relative to the mass transfer according to Schrage's model formulation.
The results that I get seem to be logical, but my problem is that the time steps that I have to use are very small ( in the order of ~1e-9 s), which make it impossible to reach convergence.
Every time I increase the time step, I face the error that the "Global Courant number is greater than 250.00. The velocity field is diverging. check the solution and reduce the time step".
I really appreciate it if you have any suggestions or solutions for this problem or you have any recommendations for me to increase the time steps to more realistic values.

[Ahmed Alkaisi]

Decrease URF to (0.2-0.5)
Use implicit

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[blue-sky]

Originally Posted by Kimia

Hi all,
I am working on a micro scale simulation of evaporation inside a porous structure, using ANSYS FLUENT. It is a transient two phase model (VOF) considering water and vapor inside a micro scale pore. I am using UDF codes to apply the source terms relative to the mass transfer according to Schrage's model formulation.
The results that I get seem to be logical, but my problem is that the time steps that I have to use are very small ( in the order of ~1e-9 s), which make it impossible to reach convergence.
Every time I increase the time step, I face the error that the "Global Courant number is greater than 250.00. The velocity field is diverging. check the solution and reduce the time step".
I really appreciate it if you have any suggestions or solutions for this problem or you have any recommendations for me to increase the time steps to more realistic values.

hi,I'm working on boiling flow inside a tube, and I meet the same problem with you. Can you have some new ways to solve this problem? And i meet other problems that the bobble couldn't separate from the wall. can you give me some advice. thank you very much

[hospital0968]

Courant number = v*dt/dx
if C is huge enough to lead to divergence, decrease dt or increase dx
hope this will help

[blue-sky]

Originally Posted by hospital0968

Courant number = v*dt/dx
if C is huge enough to lead to divergence, decrease dt or increase dx
hope this will help

thanks for your reply. the problem is that i drcrease the time step size ,but the problem also appear. i think the problem may be caused by the surface tension

[Ahmed Alkaisi]

Originally Posted by Kimia

Hi all,
I am working on a micro scale simulation of evaporation inside a porous structure, using ANSYS FLUENT. It is a transient two phase model (VOF) considering water and vapor inside a micro scale pore. I am using UDF codes to apply the source terms relative to the mass transfer according to Schrage's model formulation.
The results that I get seem to be logical, but my problem is that the time steps that I have to use are very small ( in the order of ~1e-9 s), which make it impossible to reach convergence.
Every time I increase the time step, I face the error that the "Global Courant number is greater than 250.00. The velocity field is diverging. check the solution and reduce the time step".
I really appreciate it if you have any suggestions or solutions for this problem or you have any recommendations for me to increase the time steps to more realistic values.

Do you have any update?

Sent from my SM-G900I using CFD Online Forum mobile app

[vidyadhar]

Originally Posted by Kimia

Hi all,
I am working on a micro scale simulation of evaporation inside a porous structure, using ANSYS FLUENT. It is a transient two phase model (VOF) considering water and vapor inside a micro scale pore. I am using UDF codes to apply the source terms relative to the mass transfer according to Schrage's model formulation.
The results that I get seem to be logical, but my problem is that the time steps that I have to use are very small ( in the order of ~1e-9 s), which make it impossible to reach convergence.
Every time I increase the time step, I face the error that the "Global Courant number is greater than 250.00. The velocity field is diverging. check the solution and reduce the time step".
I really appreciate it if you have any suggestions or solutions for this problem or you have any recommendations for me to increase the time steps to more realistic values.

Hello Kimia,


I have also written an UDF to simulate evaporation. The UDF can be found in the link below:
UDF with VOF method to predict evaporation mass/heat flux at vapor-liquid interface


But, I am also facing similar problems:

1) The interface is moving but faster than predicted by analytical solution of Stefan problem.
2) For large time steps, I am also facing divergence problems.


I request you to go through the above link and kindly let me know if there is any mistake in the UDF.
Also, how to specify the Standard state enthalpy(J/kgmol) values in Fluent so that required Latent heat(J/kg) is accomplished. Can I specify any values of standard state enthalpy such that the difference (in values between the two phases) divided by molecular weight of liquid or vapor gives the desired latent heat?


Thanks in advance!
vidyadhar

[Liz1219]

Can you upload your file and study it together？