Vaneless diffuser rotating stall is a serious problem for centrifugal compressors since it limits their working range. In particular the last stage seems to be the most critical from this point of view. In the literature some good correlations can be found for predicting stall inception but they hardly cover the case of last stage configuration, especially for very low blade-outlet-width-to-impeller-radius-ratio impellers typically used in high-pressure applications. Extensive research has been planned to define diffuser stall limits for this family of stages: three impellers characterized by different blade-outlet-width-to-impeller-radius-ratios are tested. The basic configuration comprises a 1:1 geometrical scale stage with a return channel upstream, a 2D impeller with a vaneless diffuser and a volute with a constant cross sectional area downstream. Several diffuser types with different widths, pinch shapes and diffusion ratios were tested. The results for the first impeller have already been reported in part I and II. In this part the effects of the above mentioned geometric parameter changes on stage stability and performance are presented with respect to the second impeller.

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