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A centrifugal compressor is a typical compressed air energy storage device.In order to ensure the safety of the compressed energy storage process in the compressor,the intal unsteady flow phenomena need to be closely monitored,especially some serious ones like stall and surge.It is necessary to explore the mechanism of flow instabilities under different conditions.A centrifugal air compressor was tested with a vaneless diffuser and a variable vaned diffuser with five different vane setting angles,respectively.Various diffuser types resulted in various modes of flow instabilities prior to surge.The vaneless region between the impeller and the diffuser was focused on.Multiple high-speed sensors were arranged along the circumferential direction.The pressure signals at all these positions were being measured and collected in real time as the compressor was slowly throttled into surge.This paper emphasizes on the influence of matching between the impeller and the diffuser on the flow instability.The experimental results showed that the diffuser vane setting angle affected the stall characteristics.Due to the asymmetry of the volute,the circumferential pressure distribution was always severely distorted prior to surge.A high-pressure region appeared near the volute tongue,and a low-pressure region was formed away from the volute tongue.In the case of the vaned diffuser with non-design installation angle and the vaneless diffuser,the rotating stall signal was originated in the low-pressure region and propagated circumferentially.However,in the case of the vaned diffuser with the design installation angle,the circumferential high-pressure region became the most sensitive region for the generation of stall,and another form of instability occurred there.Both the inducement and development of these flow instabilities have been studied.The dynamic experimental research on the compressor matching different types of diffusers could be a good case supplement.