Problems with a compressor simulation
Hello CFX Users,
I have been calculating an centrifugal compressor about for one year. I have developed the CFD model from the point of view of some important things. I got a measuring to this compressor two months ago. I tried to validate the CFD model, but the critical pressure ratio did not agree with the measuring. The main problem is the surge limit. According to the measuring the surge shapes at about 3,5 pressure ration, while in the case of my simulation this value is about 2,5.
At first I approached the full process with more steady state simulations. (I increased the pressure at the outflow in every cases until the appearance of surge.) In the last case I approached the phenomenon with one transient simulation but this was unsuccesfull too, the surge limit about 2,5.
The applied turbulence model is SST. The y+ is between 5-100 because the velocity is very different in the total flow field.
Is there anybody who has experience in this problem?
Thanks your help;
how many stages do you have in your compressor simulation?
Also, can you have a match for pressure and mass flow rate (say, if you put pressure conditions on both inlet and outlet, the simulated mass flow rate should match measurement, or you use pressure condition at outlet and mass flow at inlet, the simulated inlet pressure should match measurement).
I use pressure boundary conditions at the inflow and the outflow too. The transient simulation starts from 1.1 pressure ratio with 120000 RPM. The total time is 0.2 s, the time step is 0.0001 s. The OUTLET pressure begins to increase from 0.05 s. Finally the pressure ratio will be 3.5 at 0.2 s.
In the case of my last simulation the model was a rotational periodic geometry, I calculated only 60 degrees from the full model. The mesh was very fine. The surge limit increases, it was at ~3.3 pressure ratio, but the mass flow is too little still. The measured mass flow is ~0.5kg/s at start of the process while the mass flow is in the simulation ~ 0.3 kg/s...
I want to do a compressor simulation with periodic geometry. To do that, I need a periodicity in geometry and in physics too. In cf. compressor, there is two way to create a periodic geometry:
1. cutting along two blades, so I get a pure blade passage, bounded by one-one side of a blade as walls or
2. cutting exactly between two blades, so I get two half blade passages (as periodic boundary condition) with a whole blade in the middle (as wall of course)
The first one sounds easier to create, but in references, we found the second type at most.
What do You recommend?
Thanks a lot,
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