Investigation On Control Strategies And Mechanisms Of Aspiration In Controlling The Flow Separation And Tip Leakage Flow For Axial Flow Compressors

In the modern aeroengine with high load,high efficiency and wide stall margin,both the flow separations and tip leakage flow in axial compressors have a remarkable impact on the aeroengine performance.Therefore,carrying out the research on flow control techniques to remove or decrease the influence of flow separations and tip leakage flow in axial compressors has a crucial meaning for improving the performance of aeroengine compression system in our country.Aiming at the control strategies and relavent mechanisms in controlling the phenomena of bounder layer separation,corner separation and tip leakage flow in axial flow compressors,the following studies were conducted mainly by aspiration techanique in the current paper:1.As the basis of the whole research,firstly,the effectiveness of aspiration technique in reducing the loss of boundery layer separation and improving the compressor cascade performance was verified by experiments.Then,the reliability of numerical method was checked according to the experimental data of normal and aspirated cascades.Finally,the accuracy of numeircal approach in predicting the overall performance and capturing the flow details of compressor stage was validated.The results show that the numerical method used in this paper has reliable calculation accuracy.2.Research on the corner separation control and combined aspiration technique in compressor cascade.In the studies of full circumferential groove aspiration applied on the endwall,the results show that the control effectiveness and mechanisms are different for the implement of aspiration in different development stage of corner separation.The range of corner separation can be effectively reduced when the the implement of aspiration is in and before the initial stage of the corner separation.Further researches on the aspiration schemes by partial circumferntial grooves on the endwall show that the control laws are different for the implement of aspiration in different development stage of corner separation.At last,based on the optimal aspiration scheme applied on the blade suction surface by spanwise slot to control the boundary larer separation,the control laws and mechanisms corresponding to three different combined aspiration schemes were studied in detail.It is found that corner separation can be controlled most effectively by the two combined aspiration schemes applied in the development stage of corner separation in which partial spanwise slots and partial streamwise slots are set up on the blade suction surface near the endwall respectively.However,for the combined aspiration scheme in which streamwise slots are set up on the endwall near blade suction surface,the range of corner separation can be reduced more effectively by the aspiration applied in the initial stage of corner separation.3.Research on the control strategies and mechanisms of tip leakage flow effect in compressor cascade.Firstly,the influence law on the tip leakge flow is investigated in detail with the change of incidence angle and tip clearance size.The results show that the impact range of tip leakage vortex is enlarged with the increase of incidence angle and tip clearance size,while the starting position of the tip leakage vortex shows an opposite trend.The changes of spanwise location of the tip leakage vortex core are different with the increase of incidence angle and tip clearance size.The total pressure loss of compressor cascade at different incidence angles shows different trend with the increase of tip clearance size.Then,three different control strategies were discussed systematically in view of the tip leakage flow effect and the corresponding control laws and mechanisms were obtained.For the aspiration schemes by the cicumferential grooves set up on the casing endwall,the improvement of the cascade performance is because the mixing loss caused by the interaction of tip leakage flow,endwall boundary layer and main flow is reduced due to the attenuation of the endwall boundary layer by aspiration.The control effectiveness decreases gradually with the aspiration groove moving towards the blade trailing edge.Through the studies of two new types of aspiration forms,i.e.,implement streamwise slots on the casing endwall and the blade tip along the streamwise direction respectively,it was found that the control laws are different for the two aspiration schemes.Different from the control mechanism of the aspiration scheme by circumferential groove on the casing endwall,the cascade performance can be improved by the direct impact of aspiration on the structure and strength of tip leakage flow by the the two aspiration schemes.4.Research on the control of tip leakage flow by circumferential groove casing treatment in stage environment.A two stage counter-rotating axial compressor was shosen to investigate the effectiveness of circumferential groove casing treatment set up on the casing endwall over the rear rotor and the corresponding control laws and action mechanisms are obtained from the researches.The results show that the compressor stall margin can be effectively improved when the single groove is positioned from the blade leading edge to 40% axial tip chord,and the best position of the single groove should be located near about 20% axial tip chord in terms of the stall margin improvement.Additionally,the improvement of the compressor performance is more and more significant with the groove moving towards the blade leading edge.By comparing the the tip flow field before and after the casing treatment,it can be found that the flow field near the tip is improved remarkably with casing treatment and the intensity of tip leakage flow is weakened obviously.Additionally,the influence laws on the unsteady flow patterns are distinct for different casing treatment schemes.Frequency analysis for the static pressure signals near the blade tip shows that the disappearance of low frequency components is benefical to the stability improvement,and the compressor stall margin can be more effectively improved by controlling the tip leakage flow released from the mid-chord.5.Research on the control of tip leakage flow by circumferential groove casing aspiration in stage environment.A two stage counter-rotating axial compressor was shosen to investigate the effectiveness of aspiration applied on the shroud over the rear rotor.The control laws and influence mechanisms of different aspiration schemes were studied systematically.The results show that the improvement of compressor stall margin decreases gradually with the aspiration groove moving towards the blade trailing edge.After the aspiration,the compressor performance near the stall condition is increased,while it decreases near the peak efficiency point and within high flow condition due to the impact of casing groove on the main flow.The leading edge spillage and double leakage phenomena near the tip are suppressed significantly after the casing aspiration,and the intensity of tip leakage flow is also decreased.Different aspiration schemes have different effects on the structure of the tip leakage flow and compressor performance can be more effectively enhanced by controlling the tip leakage flow released from the mid-chord.Frequency analysis of mass-flow signals at oulet plane shows that both the disappearance of the low frequency components and the reduction of fluctuating strength are benefical to the improvement of compressor preformance and working stability.