| Modern aero-engines are faced with the challenges of high stability,high thrustto-weight ratio,and high stall margin.One approach to tackling these challenges is reducing the stage numbers of the compressor to enhance the load capacity.However,each stage must then undertake a higher pressure ratio.By reasonably organizing the shock waves in the cascade passage to pressurize,the supersonic diffuser cascade can obtain a high-pressure ratio.The supersonic diffuser cascade,which is situate at the head of the flowpath of the air-breathing propulsion system,connects the airframe and the propulsion system aerodynamically.Due to the supersonic incoming flow,shock and vortex structures in the supersonic diffuser cascade passage play a leading role in supplying sufficient,steady,and compressed airflow to the engine.A great deal of research work has been carried out on the evolution of shock and vortex in the rectangular supersonic/hypersonic inlet passage.The flowfield structure is affected by many factors such as background waves,incoming turbulence,incoming Mach number and backpressure.Due to the curved profile of the supersonic diffuser cascade,a series of compression and expansion waves form from the suction surface and pressure surface,leading to the generation of a complex background wave system.The curved profile also leads to complex vortex structures and instability flow.Therefore,this topic focuses on the evolution process of shock and vortex,the mechasnism of instability flow and the monitoring and control of flowfield in the curved supersonic casade.Firstly,to clarify the evolution process of vortex in different operation modes,three typical operation modes of curved supersonic diffuser cascade,namely,the flow-through mode,the high-backpressure mode,and the low-speed mode are analyzed numerically.The impinging and glancing of shock waves on the suction surface boundary layer and the corner boundary layer drive the low-kinetic energy fluid to move spanwise and interact with the flow of the other portions,which results in the generation of the vortex.By using the velocity gradient,the cross-flow topology of the subsonic vortex can be accurately described.The evolution process of the supersonic vortex is found to obey Zhang’s theory which excludes the incorrect solutions generated by the description of the velocity gradient along the vortex axis.The pressure gradient prove to be the main factors dominating the evolution of the supersonic vortex.The Hopf bifurcation process begins and limit cycle appears when the vortex cores of the subsonic vortex and supersonic vortex experience the zerovelocity-gradient and zero-pressure-gradient region,respectively.The appearance of limit cycle does not immediately change the rotating direction of the vortex and the bifurcation process is gradual.Secondly,wind tunnel experiments with linear increase back pressure are carried out in the supersonic isolator of M∞1.85/M∞2.70 under the condition of uniform incoming.High-frequency wall static pressure measurements are performed along the primary and corner regions.The fine structures of the shock train are recorded using schlieren visualization with multiple knife edges.The pressure results show that the shock train leading shock at M∞2.70 is more three-dimensional.The flow field exhibits the following features near the corner: the pressure fluctuation amplitude is smaller,the shock train leading shock is closer to the upstream regions.Schlieren snapshots obtained using horizontal and vertical knife edges show shock train structures with alternating distributions of the vertical and horizontal density gradients.Further application of color knife edges clearly distinguishes these regions.The experimental study on the throttling process of curved supersonic diffuser cascade is further carried out.The impingement of the shock train leading edge on the suction surface leads to the generation of a large subsonic separation region.After passing through the shock train leading edge,the static pressure increases and the total pressure decreases significantly.Compared with rectangular channel,the motion of shock train in the curved channel is significantly affected by the background waves.The forward movement and oscillation of the shock train are suppressed in the expansion wave region and strengthened in the compression wave region.During the instability flow,the shock train leading shock oscillates between the detached state and attached state to the suction surface tip,which results in a large fluctuation of the pressure in the whole cascade channel at a frequency of 52.49 Hz.The upstream shock oscillation near the blade tip is highly coupled with the parameter fluctuation in the downstream cascade channel.The phase results indicate that the upstream oscillations of the shock occur first,leading to the subsequent fluctuation in the downstream cascade channel.Thirdly,the characteristics of flow instability of supersonic diffuser cascade are studied.During the stage II of the instability flow,the detachment distance of the shock wave is large,and the large shock oscillation leads to the violent fluctuation of pressure.In order to accurately monitor the occurrence of stage II of the flow instability,the magnitude,standard deviation,derivative and power spectral density of pressure data are extracted.The analysis shows that these characteristics can distinguish the stage II of instability flow from other stages.The instability flow can be monitored with an advance time.The method of instability flow monitoring for supersonic diffuser cascade is studied and the monitoring method flowchart of instability flow is defined.Instability flow monitoring is carried out based on schlieren image and wall pressure.Based on the intensity-time series on the schlieren images,the occurrence of instability flow can be effectively detected.The trigger mechanism of alarm is that the shock train leading edge reaches the monitoring point and leads to the sharp drop of light intensity value.However,instability flow monitoring based on schlieren image requires precise optical instruments and accurate optical path adjustment.Instability flow monitoring based on schlieren image is limited in practical application.Therefore,the instability flow monitoring based on the pressure-time series is proposed.The study shows that the monitoring method based on derivative algorithm can effectively detect the occurrence of instability flow with a certain margin by calculating the pressure-time series at S5 near the suction surface tip.Compared with other monitoring algorithms,the derivative monitoring algorithm has high accuracy and less calculation consumption.Finally,in order to effectively improve the flow field performance and the maximum backpressure ratio of the cascade,the boundary layer suction control of the curved supersonic diffuser cascade is studied.The results show that the suction pressure ratio which can give full play to the maximum suction efficiently of the suction slot should be less than 0.2.The interaction between the shock train leading edge and the suction surface boundary layer is the key position to exert the boundary layer suction control.The angle of suction slot ranges of 60° to 90° and-90° to-60°can effectively improve the maximum backpressure ratio of the flow field.In the steady flow,the boundary layer suction matches the downstream backpressure condition by enhancing the strength of shock train leading shock.When the backpressure is greater than the critical backpressure,the corner instability first occurs,and then the separation region spreads to the midspan and further causes the instability of the whole cascade channel.In the instability flow,the boundary layer suction matches the downstream backpressure condition by adjusting the pressure distribution downstream of the shock train leading shock.By using the improved combined suction scheme with one spanwise slot on the suction surface and one streamwise slot on the endwall,the maximum backpressure can be improved by 20%under a suction mass flow of 8% of the capture mass. |
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