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June 1, 2011, 23:53 
why my result is 100% decided by initialization?

#1 
Member
Ji Guozhao
Join Date: Jan 2011
Posts: 37
Rep Power: 7 
Dear all:
i was trying to simulate a gas mixture(H2/Argon) going through a gas separation module. there is a change in H2 fraction(or Argon fraction) due to the selectivity of the membrane(more H2 is going through the membrane). but the H2 fraction distribution result is 100% decided by my initial guess in initialization. if i set 0.1 as H2 fraction in initialization, i get 0.9925 at retentate outlet; if i set 0.3 as H2 fraction in initialiazation, i get 0.293072 at retentate; when i set 0.5 initially, i get 0.491635 at retentate; for setting 0.7 initially, i get 0.692509 finally. in all the cases above, the solving can converge to 10e5. so which one is the really result? (in experiment i think at the beginning it is transient state, but after 5 minutes or a little more, it reaches steady state. in the simulation i did steady state, one reason is that i just want to have a look at what is going on after 5 minutes, i do not care the first 5 minutes; another reason is my limited capability in cfx only allows me do steady state at the moment.the boundary at inlet is 1E6kg/s and 0.7 H2 molar fraction, 0.3 molar fraction of Argon. retentate outlet is 500kpa pressure. the module is 1 meter long and about 0.3 meter in diameter.) is there anyone can help me? kind regards Ji Guozhao g.ji@uq.edu.au Last edited by jiguozhao; June 3, 2011 at 02:45. 

June 2, 2011, 13:23 

#2 
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A.R. Baserinia
Join Date: Jan 2010
Location: Canada
Posts: 24
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You have not provided enough info; is the simulation steadystate or transient? What are the inlet boundary conditions?


June 2, 2011, 17:57 

#3  
Member
Ji Guozhao
Join Date: Jan 2011
Posts: 37
Rep Power: 7 
Quote:
to answer your first question, in experiment i think at the beginning it is transient state, but after 5 minutes or a little more, it reaches steady state. in the simulation i did steady state, one reason is that i just want to have a look at what is going on after 5 minutes, i do not care the first 5 minutes; another reason is my limited capability in cfx only allows me do steady state at the moment. for the second question, the boundary at inlet is 1E6kg/s and 0.7 H2 molar fraction, 0.3 molar fraction of Argon. retentate outlet is 500kpa pressure. the module is 1 meter long and about 0.3 meter in diameter. thanks again! 

June 3, 2011, 09:30 

#4 
Senior Member
Join Date: Apr 2009
Posts: 530
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Your timestep might be way too small.


June 3, 2011, 12:06 

#5 
New Member
A.R. Baserinia
Join Date: Jan 2010
Location: Canada
Posts: 24
Rep Power: 8 
This is indeed interesting! You said it takes five minutes or longer for the actual device to reach steadystate conditions. That means the diffusion time scale is much larger than the momentum time scale. So if you want to get a reasonable steadystate solution, you should use a larger time scale for the mass fraction equation. If you use the same time scale for momentum and mass fraction, the changes would be so small that CFX thinks the solution has converged.
Last edited by baserinia; June 3, 2011 at 17:04. 

June 4, 2011, 07:18 

#6  
Member
Ji Guozhao
Join Date: Jan 2011
Posts: 37
Rep Power: 7 
Quote:
thank you for your answer! i just set time scale in solver control, and i can just set one value. can i set different time scale for different equations? if so, how can i set them separately? thanks again! Ji Guozhao 

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