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The generation mechanism of a diffuser stall in a centrifugal compressor with a vaneless diffuser was experimentally and computationally investigated, and the main findings are presented in this paper. Diffuser stall is generated when the mass flow through the compressor decreases and approaches the compressor surge limit. It was observed that diffuser stall cells rotate at 25-30 % of the impeller rotational speed, with diffuser stall fluctuations observed at 180° from the cutoff (volute tongue). It was found that the magnitude of the diffuser stall fluctuation gradually increased near the cutoff. According to the CFD analysis, the mass flow fluctuations at the diffuser exit showed a low mass flow region, rotating at approximately 25% of the impeller rotational speed. The diffuser stall fluctuations began at 180° from the cutoff and developed as this region approached the cutoff. Therefore, the diffuser stall could be simulated by CFD analysis. First, the diffuser stall cell originated at 180° from the cutoff by the hub side boundary layer separation. Then, the diffuser stall cell further developed by the boundary layer separation accumulation and the induced low velocity area. The low velocity region formed a blockage across the diffuser passage span. The diffuser stall cell expanded due to the boundary layer separations occurring on the shroud and hub wall by turns. Finally, the diffuser stall cell vanished when it passed the cutoff, because the mass flow recovery occurred. The current paper presented the main results of this phenomena of great importance in centrifugal compressors. INTRODUCTION A centrifugal compressor is mostly used in a wide range of applications due to a high pressure ratio and small size. Centrifugal compressors with a vaneless diffuser are often employed due to their wider stability range, compared to those with a vaned diffuser. However, the operating range is limited by the onset of rotating stall and surge. Prior to the onset of the surge, the rotating stall was generally occurred. Therefore, to enhance the operating range and suppress the surge onset, one should must focus on understanding the rotating stall generation mechanism. Numerous researchers have reported rotating stall behavior in centrifugal compressors with vaneless diffuser. The tornado-type separation vortex caused by full blade leading-edge separation was observed under the flow field of a developed rotating condition (Iwakiri et al. 2009). Zhang et al. investigated the volute influence on stall inception onset and development. Under the volute influence, the spillage vortices at a leading edge of impeller blades were developed by a static pressure peak near the tongue (Zhang et al. 2019). Therefore, the centrifugal compressor impeller stall was strongly related to a suction side separation and tip-leakage vortex. Additionally, many reports discuss a diffuser stall behavior in a vaneless diffuser. Senoo et al. were the first to investigate the relationship between reverse flow and the initiation of a vaneless diffuser rotating stall in a centrifugal blower. They proposed initiation criteria for a vaneless diffuser rotating stall (Senoo et al. 1977, 1978). Ohuchida et al. suggested both a boundary layer separation pattern and a stall propagation mechanism in a vaneless diffuser by PIV experiments (Ohuchida et al. 2013). They indicated that a

Generation Mechanism of Diffuser Stall in a Centrifugal Compressor with Vaneless Diffuser