[Julian121]

Does choking occur in low pressure compressors?
I have experimental data for a compressor and I am using CFX to obtain performance curves.
In order to calculate choking mass flow, is it necessary to lower static pressure at outlet while inlet pressure should be kept constant?
I have done this but the mass flow keep increasing, while I lower the static pressure.
In experimental data choking mass flow is 2.5 kg/s.
In simulations, I have reached 3 kg/s and it keeps increasing!
The Mach number does not reach unity at the throat of compressor. At most it reaches 0.29 at the tip.
Can choking mass flow be predicted in low speed compressor?
If choking does not occur in low speed compressors, how was this mass flow calculated in experimental tests?

[ghorrocks]

If the simulation is set up correctly then, yes, CFX can predict choking mass flow on low pressure compressors (or just about any system for that matter, as long as the compressible Navier Stokes equations are a good model for the fluid).

Read the FAQ: https://www.cfd-online.com/Wiki/Ansy..._inaccurate.3F

And read the best practices guide int he CFX documentation on rotating machinery.

[Julian121]

In experimental tests, mass flow was limited by a throttling valve which was not included in numerical simulations.
In numerical study, a bellmouth, IGV, Rotor, Stator and Outlet have been considered.
Is it not the reason why the choking mass flow cannot be predicted correctly since the throttling vale was not considered?
As I said, the calculated experimental mass flow is 2.5 kg/s, but in simulations as far as the static pressure is lowered in outlet, the mass flow keeps increasing!
The maximum pressure ratio the compressor can provide is 1.1.

[ghorrocks]

Of course the throttling valve can affect the results.

Comparing the experiment with a throttling valve and the simulation without one appears to be meaningless. Your experiment and simulation need to look at the same thing - isn't that obvious?