Rotating Stall Measurements in the Vaneless Diffuser of a Radial Flow Compressor

Results from an experimental study of flow behavior at the inlet of a vaneless diffuser of a centrifugal compressor are presented. Hot-film measurements and measurement results from a crossed hot-wire probe are discussed for operating points having inlet flow coefficients ranging from 0.006 to 0.019 at different Reynolds numbers. Instantaneous, time-averaged, and phase-averaged absolute velocity and flow angle at the diffuser inlet are deduced from the hot-wire signals after correction for mean density variations. These results show how flow behavior varies in stable, rotating stall and surge regimes of compressor operation. The critical flow angle at the onset of rotating stall shows agreement with other measurements confirming the importance of diffuser inlet width and diffuser inlet flow angle on the onset of rotating stall in vaneless diffusers.