Unsteady Flow Structure of Rotating Instability in a 1.5-Stage Axial Compressor

Abstract

The unsteady flow structure of the rotating instability (RI) in a 1.5-stage axial compressor is investigated through experimental and numerical analyses. In the tested compressor, the total pressure rise of the rotor stagnates at a certain flow coefficient before it increases again toward a lower flowrate in a case involving a wide tip clearance. The RI appears beyond this stagnant point as the compressor is throttled. The RI indicates a gentle hump in the frequency spectra of wall pressure or the flow velocity near the tip in a range of approximately 20–40% of the blade-passing frequency. As the flowrate decreases, the mode order of the RI increases, in contrast to the more commonly reported tendency, while the propagation velocity remains constant. These features are well captured by detached eddy simulation (DES) of the half-annulus model, with which the unsteady flow structure is further investigated. At its onset, RI is formed by the collision of the tip leakage vortex onto the pressure surface of the adjacent blade and subsequent vortex segmentation. This vortex structure spans two blade passages and propagates in the direction opposite to the rotor rotation. As the compressor is throttled, RI becomes more dominated by the circumferential propagation of vortex breakdown of tip leakage vortices, which occur simultaneously among neighboring passages with slight phase differences. The mechanism is discussed in relation to the temporal change in the blade tip loading. The frequency of vortex shedding increases with the reduction in flowrate and thus the increase in the number of RI disturbances.