Surface tension coefficient

alirezame · March 11, 2016, 03:40

Hello,

I am modeling condensation of CO2 through a nozzle with RK EOS model.
I do every thing correct according to the user guide and tutorials, but I can not get convergence.
inlet : total pressure 9.1 MPa and total temperature 37 C
outlet static pressure 3MPa
turbulence k-w SST
I have kept the nucleation check box off for getting the initial solution first, but even without nucleation I can not get convergence.
In my opinion problem maybe because of tension surface coefficient.
I have used a CEL to define the coefficient which I found in a paper. but even by unchecking the tension surface coefficient check box I cannot get convergence.

I would appreciate if you could guide me regarding this issue.

ghorrocks · March 11, 2016, 04:52

Your question is very general, so I can only give a general answer. See this FAQ on a closely related convergence problem: http://www.cfd-online.com/Wiki/Ansys...do_about_it.3F

You are modelling phase change, have a complex EOS, is it high Mach flow as well? That sounds like a really challenging simulation to me and I would expect convergence difficulties with it. The best way to approach complex models like this is to get the models working one at a time - phase change first in a simple geometry, then a model with compressible flow using ideal gas, then add the EOS with no phase change.

alirezame · March 11, 2016, 05:44

thanks for your answer.
shape is simple now,a 2D converging diverging nozzle.
Much number is below 1 along the whole nozzle, according to the experimental data.
I could get results in single phase and ideal, but even by using those as an initial solution I couldn't get a result in non-equilibrium mode.

ghorrocks · March 11, 2016, 06:20

I really can't add much beyond what the FAQ says: Use a better mesh, smaller time step, double precision, a better initial condition, ramp the complexity in.

alirezame · March 11, 2016, 06:41

thanks for your response.
actually I am not an amateur CFD worker. But it seems that there are not many expert cfd worker in this forum to answer or even know much about this problem. anyway, thanks for your comments.

ghorrocks · March 12, 2016, 05:29

If you had the basics under control then why didn't you say so? You will get a better answer quicker if you describe important information like this.

But you are right - your application is quite specialised so your chances of finding somebody on the forum with experience in your exact area is low.

March 11, 2016, 03:40	Surface tension coefficient	#1
alirezame Member Join Date: Nov 2013 Posts: 57 Rep Power: 12	Hello, I am modeling condensation of CO2 through a nozzle with RK EOS model. I do every thing correct according to the user guide and tutorials, but I can not get convergence. inlet : total pressure 9.1 MPa and total temperature 37 C outlet static pressure 3MPa turbulence k-w SST I have kept the nucleation check box off for getting the initial solution first, but even without nucleation I can not get convergence. In my opinion problem maybe because of tension surface coefficient. I have used a CEL to define the coefficient which I found in a paper. but even by unchecking the tension surface coefficient check box I cannot get convergence. I would appreciate if you could guide me regarding this issue.

March 11, 2016, 04:52		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Your question is very general, so I can only give a general answer. See this FAQ on a closely related convergence problem: http://www.cfd-online.com/Wiki/Ansys...do_about_it.3F You are modelling phase change, have a complex EOS, is it high Mach flow as well? That sounds like a really challenging simulation to me and I would expect convergence difficulties with it. The best way to approach complex models like this is to get the models working one at a time - phase change first in a simple geometry, then a model with compressible flow using ideal gas, then add the EOS with no phase change. alirezame likes this.

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March 11, 2016, 05:44		#3
alirezame Member Join Date: Nov 2013 Posts: 57 Rep Power: 12	thanks for your answer. shape is simple now,a 2D converging diverging nozzle. Much number is below 1 along the whole nozzle, according to the experimental data. I could get results in single phase and ideal, but even by using those as an initial solution I couldn't get a result in non-equilibrium mode.

March 11, 2016, 06:20		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	I really can't add much beyond what the FAQ says: Use a better mesh, smaller time step, double precision, a better initial condition, ramp the complexity in.

March 11, 2016, 06:41		#5
alirezame Member Join Date: Nov 2013 Posts: 57 Rep Power: 12	thanks for your response. actually I am not an amateur CFD worker. But it seems that there are not many expert cfd worker in this forum to answer or even know much about this problem. anyway, thanks for your comments.

March 12, 2016, 05:29		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	If you had the basics under control then why didn't you say so? You will get a better answer quicker if you describe important information like this. But you are right - your application is quite specialised so your chances of finding somebody on the forum with experience in your exact area is low.