A methodology for centrifugal compressor stability prediction

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2009.Cataloged from PDF version of thesis.Includes bibliographical references (p. 133-135).The stable operation of centrifugal compressors is limited by well-known phenomena, rotating stall and surge. Although the manifestation of the full scale instabilities is similar to the ones observed in axial machines, the stall inception in centrifugal compressors is less well understood. This thesis focuses on developing an integrated methodology to predict the stability limit and the stall inception pattern in highly-loaded centrifugal compressors with vaned diffusers. The approach, based on a body force representation of the blade rows, is different from previous research in that the prediction is independent of compressor stability correlations and a-priori knowledge of the compressor characteristics. The methodology consists of a control volume analysis to define the body force fields based on steady, single-passage 3-D RANS simulations and a look-up table approach to capture the dependence of the body forces on the local flow parameters. The body force model was implemented in a finite volume scheme of an existing unsteady Euler solver for a full-wheel domain consisting of only 68,000 cells. The body force based compressor model was validated against high-fidelity 3-D RANS calculations and was applied to investigate the stall inception in a pre-production, 5.0 pressure ratio, high-speed centrifugal compressor stage of advanced design. The body force based simulation at 75% corrected design speed agreed with experimental measurements available at design speed in that the diffuser becomes unstable at operating points with a positively sloped diffuser static pressure rise characteristic.(cont.) Modal stall precursors were observed and the backward-traveling character of the long-wavelength disturbance was found to be in agreement with predictions by a previously developed analytical centrifugal compressor stability model and experimental measurements in the same compressor. Additional investigations in a compressor with altered dynamic behavior demonstrated the capability of the body force-based compressor model to capture the formation of shortwavelength spike-like stall precursors. It is the first time that both backward-traveling modal waves and short-wavelength spike-like stall precursors are simulated in a centrifugal compressor with vaned diffuser, in agreement with experimental measurements. Further work is required to investigate the flow mechanisms responsible for the formation of the short-wavelength disturbances and to establish a criterion for their occurrence.by Björn Benneke.S.M