Study On The Effect Of Pre-Swirl On The Performance Of Centrifugal Compressor

Centrifugal compressors are widely used in various fields of industry as a machine for increasing gas pressure.With the continuous development of modern industry,the requirement for single-stage centrifugal compressors is increasing,together withthe impeller rotating speed.Therefore,the stability of the compressor deteriorates and the operating range become narrow.Therefore,in order to pursue high efficiency,wide stable range of operation under high pressure ratio conditions,many compressor stability expansion technologies have been developed,in which pre-swirl and casing treatment are two important methods.In this paper,the influence of pre-swirl on compressor performance is studied,and the influence of pre-swirl and self-recirculation casing treatment on compressor stability is analyzed.Firstly,the Eckardt impeller was studied through the combination of theory and numerical simulation.The influence of pre-swirl on the performance curve of the impeller was discussed.The influence of pre-swirl on internal flow is analyzed mainly according to the inlet angle of attack,blade tip relative Mach number and total pressure along streamwise.The influence of pre-swirl on the internal flow and aerodynamic performance is also described.It is clear that the pre-swirl has a significant effect on the inlet angle of attack and the relative Mach number of the blade tip.It is concluded that positive pre-swirl will reduce the angle of attack and the relative Mach number of the inlet flow.meanwhile,it reduces the load at the leading edge of the blade,and improves the efficiency and stability of the impeller.Negative pre-swirl enhances the impeller’s working force,and the total pressure ratio rises,while positive pre-swirl makes the front blade’s working force worse,so that the low-pressure fluid will accumulate in the middle and rear blades,leading to the decrease of the total pressure ratio.Secondly,the influence of large pre-swirl and linear pre-swirl on the performance of the compressor are analyzed for the Eckardt impeller.The results show that the large pre-swirl deteriorates the compressor inlet conditions,resulting in a narrower operating range and a lower total pressure ratio and efficiency.Linear pre-swirl can alleviate the negative effects of constant pre-swirl on compressor performance,making pre-swirl applications more flexible and extensive.On this basis,this paper used pre-swirl to enhance the stability extension ability of a transonic high-pressure ratio centrifugal impeller with self-circumferential casing treatment.The flow field analysis is used to clarify the stability extension mechanism of the impeller with casing treatment in the near stall condition and the near choke condition,and constant pre-swirl and variable pre-swirl are applied to the impeller.The influence mechanism of pre-swirl on the stability extension of the impeller with casing treatment was explored.The results show that the pre-swirl angle within 10° of the impeller with casing treatment will have a significant impact on the stable range of operation.The negative pre-swirl to counteract the positive pre-swirl caused by the backflow of the casing treatment can delay the occurrence of stall,but the effect is not significant.When the shroud side +5°,hub side-5°pre-swirl is used,the stable operating range of the impeller can be increased by 7.5%.