| The design of high-load compressor is one of the effective ways to increase compressor ratio,reduce compressor weight,and improve operation economics of military/civil engine.However,the increase in blade load will inevitably lead to greater adverse pressure gradient and blade tip load,and result in enhancing tip leakage flow,intensifying flow separation and increase losses,which greatly limits high efficiency and wide stall margin of a compressor.Casing treatment is one of the effective strategies to extend compressor operating range.In fact,during compressor accelerating,stable operating,decelerating and varying operating conditions,the rotor tip clearance size will increase or decrease more or less.The increase or decrease of tip clearance affects the secondary flow and interacting mechanisms between casing treatment and flow field in tip region for a compressor with spike-type stall.This paper focuses on the fundamental research about effects of tip clearance on internal flow mechanisms in a high-load and poor stability compressor with and without casing treatment.Based on the main research objective of mixed-flow compressor and secondary objective of lowspeed axial compressor,the flow instability,interaction mechanisms between casing treatment and tip region flow,and internal flow loss in compressor are studied in this paper.Adopting combination method of experimental and numerical simulation,the following four aspects of research work are carefully carried out:(1)The influence of tip clearance on stall mechanisms in spike-type compressors: The micro mixed-flow compressor rotor and low-speed compressor rotor are taken as the research objectives.Using unsteady simulation method with throttle valve model,the spatial and temporal evolution of the flow structures casing compressor stall,and circumferential propagation characteristics of stall cells are discussed in detail.The influence mechanisms of tip clearance on compressor stall are revealed.The flow structures resulting in low pressure disturbance are analyzed in detail.It is found that with throttling compressor,under zero tip clearance,the corner separation becomes serious and spills out of the blade passage,which induces compressor stall;under large tip clearance,the interface formed by incoming flow and tip leakage flow moves towards to rotor leading edge,and finally spills out at blade leading edge and causes compressor stall;the low pressure disturbances are caused by leading edge separation or radial vortex formed by strong shear interaction between incoming flow and backflow regardless of which flow structure initiates compressor spike-type stall.(2)The influence of geometric parameters of slot casing treatment on the performance of mixed-flow compressor and the effect of tip clearance on mechanisms of extending stall margin: Based on understanding mechanisms of compressor stall,taking single-stage micro mixed-flow compressor as the objective,the slot type casing treatment is parameterized.Full factorial design method is applied and the main effect of design factors are analyzed in detail.The results show that the length and open area ratio are the two parameters obviously affecting the performance of the mixed-flow compressor.Choosing one designed case with wide stall margin and high efficiency,based on unsteady numerical simulation results,the effect of tip clearance on mechanisms of extending stall margin is discussed in detail.The mechanisms of improving blade tip load of the compressor with slot-type casing treatment are revealed.It is found that the slot sucks the flow with high relative total pressure and reinjects it into incoming flow.The high energy fluid flows with incoming flow together,then is captured by rotor blade and it attaches on rotor blade,which leads to local high pressure.This further deepen the understanding of the interaction between the casing treatment and compressor rotor tip flow field.(3)The influence of slot casing treatment on compressor stall mechanisms and stall characteristics: Under the condition of casing treatment,dynamic/static measurements on the single-stage micro mixed-flow compressor are carried out.The compressor stall type and the characteristics of spatiotemporal stall disturbance are studied.The experimental results show that the compressor stalls with spike-type stall characteristics regardless of casing treatment and propogation speed of stall cells are about 80%?90% rotor speed.The experimental results measured at rotating speed of 30000 rpm are analyzed deeply.It is clarified that the initiated disturbance is caused by secondary flow near rotor trailing edge.Further,taking the single compressor rotor with casing treatment as the objective,unsteady numerical simulations with throttle valve model are used to study its stall mechanisms.It is found that the compressor stall is caused by interface spillage.The criteria of determining compressor stall are re-checked.The results show that there is only interface spillage without tip leakage backflow from rotor trailing edge,which expands the application criteria for determining compressor stall.(4)The mechanisms of internal flow loss in compressor with casing treatment: On the basis of the results that casing treatment improves compressor efficiency,the singlestage micro mixed-flow compressor is taken as the objective.Based on the assumption that thermodynamic parameters varying periodically during compressor operating stably,steady numerical simulations and local entropy generation model are applied.Exploratory analysis is made on the distributions of entropy generation rate in compressor with and without casing treatment.The positive and negative effects on internal flow loss are discussed.The results show that slot casing treatment can weaken tip leakage flow and its mixing effect with main flow,and can increase loss upstream of rotor leading edge for injection effect of slot.According to the entropy gradient calculated at rotor trailing edge plane,several zones are divided.The integrated loss in each zone are quantified.The results show that loss in zone near casing reduces about 20%,which significantly reduces the internal flow loss in the compressor. |
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