The Fundamental Investigation On The Boundary Layer Oscillating Aspiration Method Controlling Separation Flows In Highly-loaded Axial Compressor

The increasing demand for aero-engine thrust-to-weight ratio poses a huge challenge to the aerodynamic design of axial compressors.At the same time as the compressor load is increased,the accompanying violent separation flow not only seriously affects the efficiency,but even hinders the stability of the whole machine.Therefore,while ensuring high-load work,maintaining internal flow efficiency is the ideal state for turbomachinery practitioners.Today,the potential of passive flow control methods is gradually being fully explored.If the compressor stage load is to be further improved,the intervention of active flow control technology will inevitably be the future development focus of the compressor field.Based on an insightful understanding of the internal flow mechanism of the compressor,Engine Aerodynamic Research Centre of Harbin Institute of Technology propose the design concept of the low reaction compressor.Given the different situations in rotating and stationary components,the low reaction concept highlights releasing the expansion demand and concentrating on total pressure rise in the rotor whereas achieving static pressure rise mainly in the stator where the active flow control is employed.In this context,the control effect of the boundary layer aspiration technology becomes a critical factor determining whether the low reaction concept can be successfully realized.In order to further improve the effect of suction control,reduce the cost,and improve the sensitivity to variable working conditions,this paper introduces the oscillation excitation into the suction flow,and performed systematic investigations on this approach.Firstly,the two-dimensional uRANS and LES simulation calculations for the separation of the diffuser airfoil boundary layer were carried out.Under the same external conditions,the control effects of steady suction and oscillating suction in the large-scale separation flow field are compared.The modal analysis method was used to analyze the flow field,and the nonlinear physical process behind the oscillation suction control was explored.The results show that the oscillating suction control effect is affected by the unsteady control parameters.The oscillating suction can further reduce the cascade loss factor when the excitation frequency is appropriate.The optimal frequency is affected by the magnitude of the excitation,but is generally equal to the characteristic frequency of the trailing edge of the blade and its frequency multiplication.When the excitation frequency is constant,the control effect does not monotonously improve as the excitation amplitude increases.The oscillating suction inherits the advantage of the constant suction to weaken the separation degree of the boundary layer.On this basis,the reconstruction process of the flow field separation structure is realized by introducing the periodic excitation signal.The exchange of momentum between the low energy fluid in the recirculation zone and the mainstream is facilitated by inducing an additional vortex transport process.The flow field structure after the excitation is simpler.The large eddy simulation results demonstrate the reliability of the uRANS calculations for the qualitative description of the separation flow.The stability analysis of the uncontrolled flow field shows that the separated flow field is in a neutral stable state and is subject to change when subjected to external disturbances.When the suction control is introduced,the global stability of the separated flow field is improved,and the global stability characteristics of the flow field of the oscillation suction control are even better.After clarifying the action mechanism of the oscillation suction on the separation phenomenon of the two-dimensional boundary layer,the research was carried out in the three-dimensional plane diffuser cascade.Firstly,the concentrated vortex structure inside the plane diffuser cascade is explored.The primary and secondary contradictions of flow deterioration in the diffuser cascade at different load levels are clarified: in the conventional load,the main contradiction is the channel vortex structure;when the load is too high,the angular separation occurs,and the suction surface separation becomes the main cascade performance.On this basis,the effect of the separation and flow of the angular control zone based on the suction and suction of the suction surface is studied in depth.The results show that under the action of the oscillating excitation,the layered separation vortex in the original corner region is dispersed into independent vortex tubes and transported downstream.Flowing vortex structures(such as horseshoe vortices and channel vortices)are not greatly affected.In terms of performance,the newly formed discrete separation vortex structure exacerbates the local loss,but significantly reduces the time-average cascade loss and improves the cascade flow.The formation mechanism of the discrete separation vortex is basically consistent with the conclusion obtained in the two-dimensional analysis.The unsteady disturbance generated by the suction surface suction groove induces the separation shear layer to roll up the concentrated vortex,and the similar processes of different leaf heights jointly form the vortex tube.The discrete separation vortex promotes the momentum exchange between the mainstream and the low-energy fluid inside the recirculation zone,and improves the momentum level of the low-energy fluid,thereby reducing the loss and improving the flow.Subsequent calculations at the medium high Mach number show that the advantages of the oscillatory aspiration at higher Mach number can still be maintained.Finally,the control effect of the oscillating suction technique in the most complex environment is investigated.Initially verify the effectiveness of the technology.Detailed flow analysis of the stage flow was carried out to clarify the primary and secondary status of each flow phenomenon,in order to provide direction for improving the oscillation suction configuration.The results show that under different typical conditions,the main contradiction to determine the performance of the compressor class is different: under the near stall condition(NS),the separation flow in the stationary blade is the main source of loss;as the flow increases,The flow of the vane tends to be orderly.At this time,the leakage flow at the tip of the vane becomes the main factor of the left and right performance,especially in the case of approaching the plugging point(NC).The oscillating suction technology can exert its full advantages under the condition that the suction surface separation phenomenon is the main contradiction;however,under the condition that the static blade flow is better,the level performance is degraded due to the additional viscous dissipation.Although in the stage environment,the periodic sweep of the trail of the moving leaves induces the separation of the boundary layer of the vane separation,the characteristic frequency is consistent with the rotational frequency of the rotor,but the intensity is weak;the discrete suction surface induced vortex intensity induced by the oscillation suction Higher,the frequency is consistent with the excitation frequency,and the mechanism is generally consistent with the rules summarized in the planar cascade.