INVESTIGATION INTO FLOW MECHANISM LEADING TO THE STEP CHANGE IN AERODYNAMIC MODES OF ROTATING INSTABILITIES IN A SUBSONIC AXIAL COMPRESSOR ROTOR

Rotating instability (RI) as a prestall wave is closely related to tip clearance noise, blade vibration and rotating stall/surge of axial compressors, so lots of researchers have focused on it. In this paper, the experimental and numerical investigations for a subsonic compressor are carried out to explore the flow mechanism leading to the step change in aerodynamic modes of rotating instabilities at different rotor speeds. The measurement results show that RIs only appear at operating points in a narrow mass flow range near the stability limit of the test rotor both at a medium and at a high rotor speeds. The dominant mode order frequency of RI decreases from increased rotor speed. The simulations show that circumferential traveling waves also appear near the stability limit of the test rotor at different rotor speeds. The dominant frequency/mode of the circumferential traveling waves decrease with the raise of rotor speed. Details of the simulated flow fields reveal that the periodical behavior of TLF will form the flow unsteadiness in one passage. There is a time shift of pressure oscillations in between rotor passages. RI originates when the flow unsteadiness propagates circumferentially no matter at MRS or HRS. In the rotating frame, the time shift of pressure oscillations reduces and the period of the static pressure oscillation increases from the increased rotor speed. The variation of the time shift of pressure oscillations and the period of the static pressure oscillation lead to the step change in the dominant frequency/mode of RI.