Investigation of Slip Models for High-Speed Centrifugal Compressors

Abstract

Slip in centrifugal compressors arises from imperfect guidance of the flow by the impeller blades and reduces the work input delivered by the impeller. Slip models are used to predict slip in preliminary design. However, slip models are typically calibrated with data that are not representative of modern aerospace compressors (i.e., pumps, turbochargers, or industrial compressors) and do not account for the variation of slip factor with operating condition. This Paper investigates the efficacy of slip models to predict the slip factor and work input for modern, high-speed impellers at design and off-design conditions. Three slip models are used to predict the performance of four well-known impellers in the open literature. All three slip models generally overpredict the slip factor, and the largest error in the prediction of slip factor typically occurs at maximum operating speed (design speed). In addition, the analysis shows a close correlation between slip factor and two key design parameters of machine Mach number and loading coefficient over the entire compressor operating range. Finally, error propagation analysis shows that the error in impeller work input is proportional to the error in slip factor scaled by the square of machine Mach number and reveals the inherent challenge in accurate prediction of work input for high-speed machines.