# Centrifugal compressor analysis at Choke and Surge condition

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 July 12, 2018, 05:26 Centrifugal compressor analysis at Choke and Surge condition #1 New Member   seunghyeon, yun Join Date: Apr 2018 Posts: 18 Rep Power: 8 Hello. I'm working on centrifugal compressor of the turbocharger for car. Geometry has 4 domains : 1. Inblock(Inlet domain) 2. Passage(Blade) 3. Outblock(Diffuser) 4. Volute Condition for analysis is Inlet total pressure and Outlet mass flow rate. 0. steady state 1. Inlet total pressure: 1atm(reference pressure: 0atm) 2. Outlet mass flow rate: 26.7 g/s(@Surge), 69.9 g/s(@Choke) 3. Rotating Speed: 230,000 rpm 4. Frozen Rotor condition at all interfaces. 5. Passage domain and Outblock(Wall Configuration: Yes) domain are rotating condition. Design point of this compressor is 46.0 g/s and 230,000 rpm. In this case, I have a very good converged result and analysis result at surge condition is so so. But, Problem is choke condition. I tried to input for initial condition of converged result at design point. But.... In this case isn't convergence... So, I tried to condition that has 57.95 g/s condition then It is convergence. Now, I tried to 63.925 g/s... How to set analysis condition if i should get converged result? Thanks for read.

 July 14, 2018, 07:01 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,767 Rep Power: 143 Choked flow is often tricky to converge. The standard tips for improving numerical stability apply: * Improve mesh quality * Smaller time steps * Double precision numerics * Better initial condition __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

September 20, 2018, 22:07
#3
New Member

seunghyeon, yun
Join Date: Apr 2018
Posts: 18
Rep Power: 8
Quote:
 Originally Posted by ghorrocks Choked flow is often tricky to converge. The standard tips for improving numerical stability apply: * Improve mesh quality * Smaller time steps * Double precision numerics * Better initial condition

I have a one more question.
To my knowledge, it recommend that setting the time step is 1/RPM(rad/s) from cfx guide.
But, I used to very large time step(about 20) than recommend value.
This time step give good convergence.

I am confuse always

Have a nice day!!

 September 21, 2018, 06:34 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,767 Rep Power: 143 If it is a steady state simulation and it converges with a large time step then that is OK. It means your simulation is quite stable and should converge quickly. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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