Rotating Stall Acoustic Signature in a Low Speed Centrifugal Compressor: Part 2 — Vaned Diffuser

Abstract

An experimental study is performed to identify spatially coherent pressure waves which would serve as precursors to the development of an instability in the Purdue Low Speed Centrifugal Research Compressor when configured with a vaned diffuser. To achieve this, sensitive electret microphones were uniformly distributed around the circumference in the inlet and diffuser sections of the compressor. Fourier analysis of simultaneously sampled data from these microphone arrays was employed to identify the development of dominant spatial modes in the pressure field in the compressor. Three different diffuser geometries were investigated, resulting in three different instability pathologies. The rotating stall patterns observed in the compressor demonstrated propagation rates near impeller speed, and from one to four cells. These instabilities would classically be described as impeller stall, although the conditions appeared to arise simultaneously in the diffuser and impeller and were typically of similar magnitude in both locations. The excitation of the pressure waves, as indicated by spatial Fourier analysis, occurred five to fifteen impeller revolutions before small changes were evident in the raw microphone signals, and fifteen to twenty-five revolutions before the stall condition could be considered fully developed.