Unsteady flow mechanism of a rotating instability in a 1.5-stage axial compressor

Abstract

The unsteady flow mechanism of rotating instability (RI) in a 1.5-stage axial compressor is investigated through experimental and numerical analyses. The total pressure increase characteristics of the rotor blades show that a kink point occurred in a case involving a wide clearance, whereas the RI occurred below the kink point. The RI appears in the power spectrum as a gentle hump whose frequency is approximately 20%–40% of the blade-passing frequency at the blade tips. The mode of the RI disturbance increases as the flow rate decreases, whereas the propagation velocity remains unchanged. This causes the RI frequency to increase. The double-phase-locked averaging method is used to visualize the flow field inside the rotor blade when the RI occurred. The results show that as the flow rate decreases, the spacing of the pressure patterns inside the rotor blade decreases and the number of RI modes increases. Results of numerical analysis show that the RI is formed by the repeated collision and separation of the tip leakage vortex onto the pressure surface of the adjacent blade. The fluctuation in the inflow angle of the vortex entering the adjacent blade changes the blade-tip loading. The vortex fluctuations propagate in the circumferential direction.