Numerical Investigation on Rotating Instability from the Oscillation of Tip Leakage Flow in Transonic Axial Compressor Rotor

摘要

A numerical simulation of the full annulus was performed on a transonic axial compressor rotor to investigate the unsteady features of the tip leakage flow. The unsteady results showed that no oscillation occurred on the tip leakage vortex and the tip leakage flow was almost steady under the high mass flow condition(peak efficiency condition); while the tip leakage vortex intensively fluctuated under the low flow condition (near stall condition). Furthermore, tip leakage vortex impinged on the adjacent blade pressure surface, and periodical interaction of the tip leakage flow between adjacent passages emerged in the full annulus computations. As a result, a phenomenon called "rotating instability" (RI), caused by the fluctuation of the tip leakage vortices and its circumferential propagation, was found. The dominating cell number of RI is nearly 40% of the rotor blade number, e.g. the cell size (respectively wavelength) of the disturbance is about 2～3 rotor blade pitches. A simple method was set up to identify the frequency marked as "rotating instability" obtained in a stationary frame. The relationship between the rotating instability and rotating stall inception is also analysed. During the stall process, the 2nd harmonics increase quickly around one revolution, whereas the 7th harmonics (RI harmonics) decrease. Two long length-scale stall cells emerge within only one revolution.