This study will be described details on influence of movable diffuser such as local reverse flow patterns and pressure fluctuation in a vaneless diffuser during unstable operation of a centrifugal compressor. The experimental study is carried out in centrifugal compressor immersed into a water reservoir. Particle Image Velocity (PIV) is used to observe a secondary flow pattern and pressure transducer is used for investigating the onset and development of rotating stall inside vaneless diffuser. The reverse flow zone observes near hub wall of impeller exit at relatively low flow rate. This reverse flow on hub side wall brings about the rotating stall. This paper be simultaneously discussed on the effects of diffuser contraction to remove local reverse flow zone near hub wall, which are including variation of flow field in a vaneless diffuser and the influence on the pressure fluctuation. A contraction due to movable diffuser three cases (40%, 53% and 86% of diffuser width) was inserted into vaneless diffuser from a shroud wall. According to the results, the rotating stall involving single and two cells is enveloped by the outer lobe of the Rossby wave. Under the rotating stall onset, the stall propagation rate is 1.25Hz, 25% of the impeller speed in the same direction as the impeller in case of no contraction. In case of 40% contraction, onset of rotating stall is not delayed. In contrast of that, onset of rotating stall is delayed in case of 53%, 87% contraction. There exist several secondary vortices inside vaneless diffuser under onset of rotating stall. The size and location of them vary as flow rate decreases.

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