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Centrifugal compressor analysis at Choke and Surge condition

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Old   July 12, 2018, 05:26
Default Centrifugal compressor analysis at Choke and Surge condition
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seunghyeon, yun
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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.
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Old   July 14, 2018, 07:01
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Glenn Horrocks
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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
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Old   September 20, 2018, 22:07
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Quote:
Originally Posted by ghorrocks View Post
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
Thanks your reply

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.

How about your opinion this situation?
I am confuse always

Thanks for read
Have a nice day!!
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Old   September 21, 2018, 06:34
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Glenn Horrocks
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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.
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