Research On The Stall Mechanism Of Centrifugal Compressor And Optimization Design Of The Self-recirculating Casing Treatment

With the widespread application of micro gas turbines in aviation,aerospace,industry,energy and other fields,the centrifugal compressor,as an important component,has continuously increasing requirements for aerodynamic performance,including high pressure ratio,high efficiency,and broad flow range.Therefore,in recent decades,how to identify the unstable flow phenomenon of the centrifugal compressor under small flow rate and how to improve the working flow range of the centrifugal compressor have been hot and difficult research issues.Although there has been a lot of research on the unstable flow of compressors,for centrifugal compressors,the high reverse pressure gradient,large curvature of the meridian flow path,supersonic tangential velocity at the blade tip,and highly distorted two-dimensional blade shape all further increase the complexity and difficulty of researching unstable flow.In addition,in order to broaden the working flow range of the centrifugal compressor,the self-recirculating casing treatment has been widely used as a passive flow control method.However,there is no clear design method and criteria for self-recirculating casing treatment design,and there is a lack of specific analysis and discussion on the flow control mechanism of self-recirculating casing treatment in the unstable process.Therefore,this paper uses numerical simulation methods to investigate the unstable flow phenomenon of the centrifugal compressor,establish a self-recirculating casing treatment design method and optimization approach,and explore the unsteady flow structure and flow control method of the self-recirculating casing treatment.First,the paper studied the aerodynamic characteristics and flow loss mechanism of the centrifugal compressor using a single-channel model simulation of the centrifugal compressor.By analyzing the evolution process of the aerodynamic parameters of the compressor at different speeds,it was found that when the flow rate decreases continuously,the flow direction at the exit of the impeller and the inlet of the diffuser develops into an unstable flow state,and there is a large circumferential distortion at the inlet of the diffuser.Based on one-dimensional stability parameter analysis,it was determined that the stability of the impeller and diffuser gradually decreases as the flow rate decreases,and the stability is worst at the leading edge of the diffuser,and the stability of the diffuser decreases as the speed increases.Secondly,the paper studied the unstable flow structure of the centrifugal compressor based on a full-circumference numerical simulation method.In the steady-state calculation,the circumferential non-uniformity of the aerodynamic parameters of the full-circumference model was found to continuously increase as the flow rate decreases and develops downstream by comparing the differences between the full-circumference and single-channel model flow fields.In the full-circumference model,the circumferential non-uniformity of the Mach number near the choke point increased from 8.13% to 25.52% from the impeller leading edge to the diffuser inlet,and from 15.18% to 24.12% at the design point.In the unsteady simulation method,the throttle valve outlet model was used to approximate the unstable flow state of the compressor.The pressure signals at key positions of the centrifugal compressor were analyzed by timefrequency analysis,amplitude-frequency characteristics,and low-pass filtering to identify the flow characteristics.The results showed that the centrifugal compressor exhibited rotational instability at the impeller leading edge,and the 1815.75 Hz rotational disturbance signal propagated circumferentially at 40% impeller speed.At the diffuser inlet,various lowfrequency disturbances were superimposed to form an 87.17 Hz rotational stall characteristic frequency.Furthermore,based on the flow field analysis under stall conditions,the formation mechanism of the stall mass in the diffuser was revealed,and the principle of the stall mass propagating backward in the diffuser channel at 4.17% impeller speed was explained.Then,based on the research on the flow loss mechanism of the compressor single-channel and the unstable flow phenomenon of the centrifugal compressor,a preliminary design of the self-circulating structure of the compressor was carried out.Four key geometric structural parameters of the self-recirculating casing treatment were studied,including the upstream groove width,downstream groove width,downstream groove position,and airflow reflux angle,and three self-recirculating casing treatment structures were designed based on the orthogonal experiment method.By comparing the compressor characteristics and flow fields,the superiority of the self-recirculating casing treatment design method with the optimal comprehensive stability enhancement ability as the goal was determined,and the working flow range at the design speed was increased by 10.95%.In addition,a diffuser casing structure was designed for the case of diffuser throat blockage,which effectively improved the flow state of the compressor near the choke point.An active control method coupled with impeller/diffuser casing was proposed to further improve the performance of the compressor.Finally,the flow control method of the optimal comprehensive stability enhancement casing was further studied.Based on steady-state and unsteady simulation methods,the influence law and flow control mechanism of the self-recirculating casing treatment on the internal flow field of the centrifugal compressor under different flow rate conditions were investigated.Under high flow rate conditions,the self-recirculating casing treatment can reduce the negative incidence angle of the impeller inflow,weaken the channel shock wave caused by the pressure surface,and significantly improve the circumferential non-uniformity of the impeller inflow.Under low flow rate conditions,the self-recirculating casing treatment can effectively reduce the vortex structure developed on the suction surface of the impeller leading edge through the suction effect of the downstream groove,alleviate the accumulation of lowenergy fluid in the blade tip region,and reduce the low-frequency disturbance phenomenon mainly caused by rotational disturbance,significantly improving the stability of the compressor.Under deep stall conditions,the self-recirculating casing treatment induces an entire backflow in the tip of impeller,causing the unstable flow structures to alternate both in the impeller and the diffuser,and through damping,it can slow down the oscillation of the airflow in the compressor and achieve stability enhancement.