Study On The Discriminant Criterion Of Wide And Narrow Vanless Diffuser In Centrifugal Compressor

Centrifugal compressor is a kind of mechanical equipment which can effectively improve static pressure,which is widely used in petrochemical,refrigeration,air-conditioning and other industrial fields.However,it will produce rotating stall under small mass flow,accompanied by large pressure fluctuation,flow blockage and other characteristics,resulting in the centrifugal compressor running in unstable conditions,which has an adverse impact on the whole system.Centrifugal compressor with vaneless diffuser is more widely used because of its simple structure and wide operating range.Therefore,the study of the induction mechanism and criterion of the stall of the vaneless diffuser with different width is of great significance to the stable operation and stall control of the compressor.Study on the relationship between the width ratio of diffuser and the mechanism of stall induction.The effect of width ratio on stall induction of vaneless diffuser is investigated,and the width ratio thresholds of different stall induction mechanisms are determined.the differences in the radial induced position of the stall precursor,the manifestation of the stall precursor and the evolution details of the key flow field in the diffuser near the width ratio threshold are compared,and the similarities and differences of the stall induction mechanism between the wide diffuser and the narrow diffuser are described.in order to reveal the hydrodynamics mechanism induced by rotating stall in wide diffuser more clearly.Firstly,this paper studies the relationship between the width ratio of diffuser and the mechanism of stall induction.By investigating the effect of width ratio on stall induction of vaneless diffuser,this paper expounds the similarities and differences of stall induction mechanism between wide diffuser and narrow diffuser,in order to reveal more clearly the hydrodynamic mechanism induced by rotating stall of wide diffuser.The study found that:Reducing the width ratio of the diffuser can increase the stability of the compressor.The stall critical flow of the wide vaneless diffuser is larger than that of the narrow vaneless diffuser,and the energy loss in the diffuser increases with the decrease of the flow rate.After the rotational stall occurs,the energy loss increases rapidly,and the energy loss reaches the maximum when the stall is fully developed.On the whole,the energy loss of the wide vaneless diffuser is larger than that of the narrow vaneless diffuser.The stall of the narrow vaneless diffuser is related to the boundary layer separation.In the non-stall stage,the reflux occurs first on the walls on both sides of the cover,but there is no reflux in the core flow area.In the stall-induced stage,there is an obvious local reflux in the boundary layer on the side of the cover and extends inward in the direction of leaf height.This leads to a complete stall.Therefore,under the condition of near stall point,strong flow separation and a large amount of reflux are necessary conditions for stall in a narrow diffuser.The stall of the wide vaneless diffuser is related to the core flow distortion.In the non-stall stage,there is a high-speed flow on the inlet side of the diffuser due to the jet structure,and a slight backflow on the cover side due to little work done at the outlet of the impeller.In the stall-induced stage,the reflux occurs at the entrance of the cover side,the exit of the cover side and the boundary layer in the middle of the disk side,which leads to the distortion of the core flow.The mixture of twisted core flow and backflow causes most of the diffuser to become a uniform low-speed zone except for the inlet.Then the reflux appears in the whole core area from the outlet to the inside,and the radial velocity of the reflux becomes larger and larger.When a stall mass occupying the whole blade height direction is formed in the diffuser,the complete stall occurs.The discriminant criterion for the wide and narrow vaneless diffuser is determined from two aspects: the width ratio and the stall induction mechanism.according to the width ratio,the boundary of the wide and narrow vaneless diffuser is between 0.083 and 0.124.According to the different stall mechanism,the stall of the wide vaneless diffuser is caused by the core flow distortion,while the stall of the narrow vaneless diffuser is caused by the separation of the boundary layer.The stall induction mechanism of wide and narrow vaneless diffuser is different,but the stall mass characteristics caused by stall are the same.The stall mass rotates at the speed of part of the impeller.