Numerical Study On The Mechanism Of Rotating Stall In Centrifugal Compressor With Vaned Diffuser

The air flow near the outlet of the centrifugal compressor impeller shows obvious spatial heterogeneity under the effect of centrifugal force and Coriolis force,which is concretely manifested in the "jet-wake" structure along circumference and the flow angle difference along spanwise.In order to meet the application requirements of the high pressure ratio and high efficiency,more and more centrifugal compressors adopt the design scheme of installing guide vanes in diffusers.However,the existence of vanes will produce additional negative effects,among which flow separation is the most common phenomenon.Considering that the centrifugal compressor will be affected by the disturbance of pipe networks,and its internal flow is inherently unsteady due to rotor-stator interaction during operation,the flow separation will further develop into stall or even surge,thus reducing the steady operating range of the centrifugal compressor.Therefore,it is of great significance to study the generation and development mechanisms of the stall phenomenon in the centrifugal compressors with vaned diffusersIn this paper,a centrifugal compressor stage(model number:LB561 50 K Y 108)fabricated by Shenyang Blower Group Co.Ltd.is taken as the investigated subject.A low-frequency rotating stall phenomenon propagating along the opposite direction of impeller rotation is detected near the shroud side of vaned diffuser passages during the unsteady numerical simulation for the full-annulus flow passages of the stage near design condition.The cause of this phenomenon is analyzed from the perspective of the matching between the installation angle of diffuser vanes and the air flow angle by retrofitting diffuser vanes;a new theoretical explanation for the development mechanism of this phenomenon is proposed from the perspective of the accumulation and release of the air flow energy in impeller passages using the Energy Gradient Theory for reference.The main work in this paper can be summarized under four parts as follow:1.Experimental data of external characteristic parameters and a series of independent verification methods are used to analyze the reliability of the computation model.The results show that it is necessary to adopt the unsteady computation method for the whole flow passages of all stage components in the study of rotating stall phenomenon.Compared with URANS SST and DES methods,the efficiency of computation processes and the reliability of computation results can be simultaneously taken into account by using the URANS k-? method.2.The URANS k-? method is used to compute the whole flow field of the centrifugal compressor stage under design condition.The circumferential propagation characteristics and the generation mechanism of rotating stall cells are analyzed.The results show that the generation of stall cells is caused by the mismatching between the flow angle near the diffuser inlet and the installation angle of diffuser vanes due to the dramatic variation of the flow angle near the impeller outlet along spanwise,which has nothing to do with the circumferential asymmetry of the blade number between the impeller and the vaned diffuser.The stall cells inside the centrifugal compressor are restrained after changing the installation angle of diffuser vanes,and the stage performance is improved.3.Numerical computations of the centrifugal compressor stage are carried out under a series of small mass flow conditions.The internal flow characteristics of the centrifugal compressor stage under each mass flow condition are analyzed and compared.The circumferential development mechanism of the rotating stall cells is studied using the Energy Gradient Theory for reference The results show that the circumferential propagating speed of stall cells increases with the reduction of mass flow.The driving force for the reverse propagation of stall cells comes from the air flow mechanical energy gradient along the normal direction of the streamline in diffuser passages,and the mechanical energy gradient originates from the gradual accumulation and rapid release of the air flow energy in upstream impeller passages due to the blockage of downstream stall cells4.The aerodynamic stability of the centrifugal compressor during a transient process with sudden-dropped mass flow is computed and analyzed by establishing a variable mass flow boundary condition.The results show that with the sudden decrease of mass flow,the flow in the centrifugal compressor gradually becomes unstable.Before the radial resultant force applying on the rotor suddenly increases nearly by an order of magnitude,which will further increase the risk of mechanical failure,the radial force components of the rotor show an obvious early warning characteristic,which has an important guiding significance for the safe and reliable operation of the centrifugal compressor in engineering practices.The rationality of the explanation that the propagation of stall cells is driven by the energy accumulation and release is further confirmed by combining the results of the above-mentioned four parts.In addition,there is an inherent design problem for the investigated stage,where the installation angle of diffuser vanes does not match the air flow angle near the diffuser inlet,which indicates that the installation angle of diffuser vanes should be taken as a primary modification factor in the retrofit and optimization design.