Research On The Mechanism And Criteria For Discrimination Of Rotating Stall In Wide And Narrow Vaneless Diffusers Of Centrifugal Compressors

Centrifugal compressor is widely used in metallurgy,chemical industry and other industries,However,the operation of centrifugal compressor under small flow rate will produce unsteady flow phenomenon such as rotating stall and surge.This phenomenon will greatly affect the stability and safety of the compressor.Centrifugal compressor is usually composed of centrifugal impeller and diffuser.The vaneless diffuser is widely used because it has a more simple structure and wide running range compared than the vaned diffuser.Therefore,it has great significance to study the stall mechanism of the vaneless diffuser of centrifugal compressor to improve the stable operating range of centrifugal compressor and the active control of stall.In this paper,two centrifugal with wide vaneless diffuser and narrow vaneless diffuser respectively are disigned as the research subjects,the stall induction mechanism and the criterion of wide and narrow vaneless diffuser were studied by numerical simulation of wide and narrow vaneless diffuser,and the following conclusions are drawn:The wide and narrow leafless diffuser has different stall induction mechanism.For the wide vaneless diffuser,the reflux zone in the diffuser first appears on the hub.Over time,the reflux zone on the hub moves to the entrance of the diffuser and the reflux zone is also generated at the outlet of the diffuser shroud,with the joint influence of the reflux zone on the hub and shroud,the core flow in the wide vaneless diffuser appears radial movement from the shroud to the hub and the flow direction is not uniform,that is,the distortion of the core flow which leads to rotating stall in the wide vaneless diffuser.About the narrow vaneless diffuser,the reflux zone is also firstly appear in the diffuser hub at the stall process,over time,the reflux zone appears on the side of the coil on the outlet of the diffuser,and merges with each other with the lateral reflux area of the outlet of the impeller rotating diffuser,resulting in the distortion of the core flow in the narrow leafless diffuser Middle Ages and induced by the shrinkage of the narrow leafless diffuser.Over time,the reflux zone appears on the hub and shroud at the outlet of the diffuser,with the impeller rotary the reflux zone on the hub and shroud at the outlet of the diffuser merge with each other.which lead the distortion of the core flow in the narrow vaneless diffuser and cause the rotating stall appear on the narrow vaneless diffuser.The wide and narrow vaneless diffuser has different stall induction mechanism,But the wide and narrow vaneless diffuser have some similar flow characteristics in the stall process,and the reflux zone first appear at the hub of the diffuser both in the wide and narrow vaneless diffuser,and the stall of the wide and narrow leafless diffuser is related to the reflux at the outlet of the diffuser cover side.And the stall of the wide and narrow vaneless diffuser are both related to the reflux at the outlet of the diffuser shroud.The criterion of discriminating between the wide and narrow vaneless diffuser in this paper is determined from two aspects:width ratio and stall induced mechanism.According to the width ratio,the boundary between the wide and narrow leafless diffuser is between 0.093~0.13.According to the stall induced mechanism,the stall of the wide vaneless diffuser is caused by the distortion of the core flow,while the narrow vaneless diffuser is caused by the disorder of the boundary layer.The stall induced mechanism of the wide and narrow vaneless diffuser is different but the stall cell has the same characteristic.The speed of stall cell in the diffuser is 0.4~0.6 w_r,and the stall cell has rotates,the rotation direction is the opposite of the impeller rotation direction.