Numerical Research Of Tip Clearance Flow And Its Control In A Low-reaction Transonic Compressor Rotor

Further development of modern aircraft engines has promoted increasingly high standards for compressor loading and efficiency.As a new technology that can ensure efficient flow and high pressure ratio,low reaction aspirated compressor(LRAC)is of great significance for improving the performance of aero-engine compression systems.Tip clearance flow(TCF)has a significant impact on the overall performance and internal flow stability of the compressor.However,the current understanding of the TCF in the LRAC is still incomplete,and the internal flow mechanism still remains to be explored.In this paper,the first-stage rotor of a three-stage highly-loaded LRAC was taken as the research object,and a detailed numerical research on the mechanism and control of the TCF was presented.The main research contents and conclusions are as follows:Firstly,the effect of TCF on the macroscopic characteristics of LRAC transonic rotor is studied.Since the gap size is the most direct geometric parameter that determines the TCF,the flow field with different gap sizes is numerically simulated for every working conditions.It is founded that the TCF is the main reason for the deterioration of the rotor aerodynamic performance.As the gap size increases,the massflow rate at the congestion point decreases.In the meantime,the total pressure ratio and efficiency decrease throughout the working range with the increment of the gap,but the stall margin shows a trend of first increasing and then decreasing.Then,in order to improve the understanding of the flow mechanism in the tip region of the LRAC transonic rotor,multi-passage unsteady simulation was applied to study the microscopic characteristics of the rotor with different gap sizes.Shock wave/tip leakage vortex interference,tip leakage vortex fluctuation and pre-stall state are discussed in detail.The results show that the stall source of the LRAC transonic rotor is located in the tip region,and the stall mechanism of the rotor varies when the tip gap size changes.With a small tip clearance size,shockwave-induced boundary layer separation formed a large-scale blockage in the suction surface-casing corner.And the flow blocage was the main cause of the compressor stall.However,with a relatively large tip clearance size,there appears a cross-channel rotation disturbance that propagates along the circumferential direction in the blade tip region.The vortex breakdown causes the rotation disturbance to occur,which eventually results in the compressor stall.In order to effectively suppress the formation of tip clearance flow or tip leakage vortex,thereby improving the aerodynamic performance of the compressor,curved blade technology was applied to reduce the effect of TCF based on the above research.A series of parametric studies were carried out for the two bending forms,namely leaned blade and dihedral blade.By comparing and analyzing the flow field of the prototype and curved blades,the influence of geometric design parameters on the aerodynamic performance and flow feature of the rotor was obtained.It is founded that a larger negative bending angle and a lower bending height help to weaken the main leakage intensity,thereby reducing shock wave/ tip leakage vortex interaction.The weakened interaction is beneficial to reduce shock wave loss and tip leakage loss in the blade tip region.The role of curved blade in the LRAC transonic rotor is mainly reflected in the control of shock wave strength and tip leakage vortex intensity.The speed line and flow structure of the rotor under two bending forms show different characteristics.With respect to the dihedral blade,the leaned blade can improve the total pressure ratio and efficiency without decreasing the flow capacity of the blade passage.In addtion,the leaned blade has a more obvious control effect on the main leakage vortex strength and shock wave position,and it also has a stronger effect on narrowing the range of lowmomentum fluid accumulated in the casing-suction surface corner.Therefore,it is recommended to adopt negatively leaned blade in the aerodynamic design of LRAC transonic rotors.Large bending angle and low bending height are beneficial under the premise of sufficient strength.Considering that highly-complex three-dimensional blades are prone to strength problem and blade flutter,a study on the control of TCF with circumferential groove casing treatment was carried out in the LRAC transonic rotor.A series of configurations were first generated by changing the axial position,depth and coverage area of the circumferential grooves within a certain range.Then,the performance of these casing treatment configurations were quickly evaluated by means of numerical simulation.At last,the influence of geometric design parameters on the flow field was analyzed in detail,so that the design criteria of circumferential groove casing treatment under the background of LRAC could be obtained.The results show that when the position of the circumferential groove changes within the range of the first 40% of the axial chord,the single circumferential groove can expand the stable working range of the rotor without losing efficiency.The change in groove depth only significantly changes the stall margin at 0% and 10% axial position,and the maximum increase in stall margin is up to 6%.Among the first 30% of the axial chord,the efficiency gain gradually decreases with the increase of the groove depth;but the change of the groove depth has no effect on the peak efficiency in the rear 70% of the axial chord.Analysis of the flow filed shows that the multi-circumferential groove layout not only effectively controls the generation and development of the tip leakage vortex,but also alleviates the flow blockage in the tip region.The circumferential groove destroys the formation mechanism of the tip leakage flow,significantly improves the overall performance of the rotor,and delays the compressor stall caused by the leakage flow.