To study the stall mechanism in a wide vaneless diffuser,a three-dimensional centrifugal compressor model was established.The numerical simulation method was used to study the fluid characteristics in a centrifugal compressor with a wide vaneless diffuser under stall conditions.The simulated results showed that the wide vaneless diffuser stall was related to the instability of the core flow.Due to the influence of the reflow,the core flows moved between the hub and shroud side and the directions were not consistent.This was called core flow distortion,which triggered the rotating stall.The reflow first appeared at the outlet boundary of the diffuser,and the reflux gradually expanded to the inside of the diffuser as the impeller rotated.Eventually,the reflux zones grew and merged,forming a stable low-speed fluid regiment,which produced a stall cell.The stall cells rotated around themselves with an opposite direction with impeller.