Analysis of Unsteady Flow Features in a Multi-Stage Compressor Operating Near the Stability Boundary

Abstract When multi-stage compressors are operating close to the stall boundary, local flow separations and shear layer fluctuations create an inherently unsteady flow. Often, this unsteadiness can be described as a circumferentially traveling disturbance with characteristic frequencies and wave numbers. The nature of the unsteadiness is of interest as it can serve as a precursor to stall and also cause vibrations of the rotor blades. The paper uses full-annulus URANS computational fluid dynamic (CFD) simulations to investigate the unsteady flow features using a 1.5 stage compressor domain representative of the front stages found in modern high-speed core compressors. In experiments, the first rotor row experienced vibration in the second torsional vibration mode when operating at part-speed conditions close to the stall boundary, which was caused by unsteadiness local to the tip of the rotor. The present study concentrates on this part-speed operating condition and, in particular, investigates the evolution of stall features by comparing three different operating points on the compressor characteristic approaching the stability boundary. Several sources of unsteadiness are identified: Before the rotor reaches peak pressure rise, short length scale disturbances are present near the rotor tip. Transiently, larger length scale disturbances of lower amplitude can also be detected. The features near the rotor tip are accompanied by larger regions of reversed flow at the stator hub. As the rotor is throttled, the large length scale disturbances grow in strength and reach further upstream.