Vaneless diffuser rotating stall is a serious problem for centrifugal compressors, since it limits their working range. In the literature some good correlations can be found for predicting stall inception but they have limited coverage of last stage configuration case, especially for very low blade-outlet-width-to-impeller-radius-ratio impellers typically used in high-pressure applications. In addition, stall inception is strictly bounded to diffuser geometry (for example, diffuser width and diffusion ratio). As a part of a wide range activity on rotating stall, a stage with a blade-outlet-width-to-impeller-radius-ratio of 0.1 has been tested. The stage configuration is made up by 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. Diffusers with three different widths and two diffusion ratios were tested in order to find their influence on stage stability and performance. Results obtained for impellers with lower blade-outlet-width-to-impeller-radius-ratios have been published in previous papers. The purpose of this paper is to comment the obtained results and increase the amount of experimental data available on vaneless diffuser rotating stall behaviour.

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