| Popis: |
The subject of the paper is a high speed unshrouded centrifugal compressor in which rotating instabilities have been measured near the stage stall point. The impeller is studied numerically by means of unsteady RANS simulations, and the results are compared to experimental measurements. Instead of rotating instabilities, the numerical results directly capture a rotating stall pattern in which a tornado-like separation vortex is shed due to a separation at the impeller leading edge, and propagates around the circumference. The vortex has one end attached to the casing and the other end attached to the pressure side of the blade. Its propagation speed is within 10% from the measured speed of the rotating instabilities. Because of the high pressure gradient the tip leakage flow crosses the impeller front and thus convects the vortex in front of the adjacent blade. The spillage of the vortex in the adjacent channel convects radial and azimuthal vorticity onto the next blade. This triggers the inception of a new vortex, and induces the propagation of the rotating stall cells. |
