# Converge problem for unsteady gas solid flow

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 February 28, 2012, 14:27 Converge problem for unsteady gas solid flow #1 New Member   kong Join Date: Sep 2011 Posts: 21 Rep Power: 7 I am simulating gas solid flow for euler euler model in 10m circulating fluidized bed. When I using a 18000cells mesh ,it converged well. and i can get the final solution about 12s flow time. After refined it to 36000 and 54000 cells mesh, there is a converge problem. the refined mesh only got converge before 4m or 2m from the inlet of bed, after that, the values monitored in rest of bed are keeping fluctuating when the flow time is more than 30s, although each time step shows that it converged. The time step i am using is 0.0001s, the absolute criteria is 0.0001 can any one help me, thank you

 February 28, 2012, 21:54 #2 Member   Musango Lungu Join Date: Dec 2011 Location: China Posts: 73 Rep Power: 7 Rule of thumb for setting time step is del t< delx/v where del t= time step,del x = smallest mesh size, v= velocity. So make sure your time step is small enough for your refined mesh size. Next you can try and lower the under relaxation factor of pressure, momentum, volume fraction and K-epsilon if you are you using a turbulent viscous model.

 February 29, 2012, 11:59 #3 New Member   kong Join Date: Sep 2011 Posts: 21 Rep Power: 7 for the 36000 cells mesh .the delx=0.00056m,and V=2.3m/s, so i think the time step is small enough, and i am using implicit scheme for volume fraction, and I check the manual of fluent, implicit scheme dont require the courant number

 February 29, 2012, 23:40 #4 Member   Musango Lungu Join Date: Dec 2011 Location: China Posts: 73 Rep Power: 7 Okay, have you tried lowering the under relaxation factors?

 March 1, 2012, 17:28 #5 New Member   kong Join Date: Sep 2011 Posts: 21 Rep Power: 7 now i am using default underrelaxtion factors, cuz i found in each time step iteration, it got converge, if i change it, which one should i change?any clue to do this? thank you for your reply

 March 1, 2012, 17:31 #6 New Member   kong Join Date: Sep 2011 Posts: 21 Rep Power: 7 now ,i tried to use lower time step to see whether it works

 Tags converge, fluidized bed, gas solid

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