Research On The Tip Vortex Structure And Unsteady Flow Control Of Compressor Cascade

In this paper,NACA64-A905 diffuser cascade with tip clearance is adopted to study the tip vortex flow characteristics and the flow control methods mechanisms with the way of numerical simulation.Based on the sufficient study on the three-dimensional flow field and characteristics of the diffuser cascade,a means of method that put the suction hole on the casing is proposed to achieve the unsteady control to the tip region vortex structure.The control mechanism of this form of unsteady excitation for the flow field is explored and the influence of the excitation position on the control effect is studied.The main contents and conclusions are as follows:First of all,end-wall moving speed,flow angle of attack,tip clearance height these three factors' influence law to the tip flow field is studied using the method of steady numerical simulation combined with the Q criterion.The results show that the movement of the end-wall will dissipate the vortex and change the shape of the leakage vortex while increasing the axial angle.Through the entrance attack angle from the rated attack angle to the stalling attack angle,it is found that the radial separation vortex at the suction side of the blade transforms to the axial separation vortex at the tip of the trailing edge when the attack angle is large.On the one hand,the separation vortex continuously sucks the leakage flow so as to reduce the leakage speed of the leakage flow in the circumferential direction and makes it no longer leaks into the leakage vortex.On the other hand,the separation vortex also reduces the effective flow area in the tip clearance and has a certain inhibitory effect on the leakage flow.When the angle of attack is constant,with the change of tip clearance,there is a gap that maximizes the inhibition effect.In this case,the tip clearance is the optimum clearance,and the total pressure loss at this tip clearance height is the minimum.By the means of numerical simulation with large eddy simulation method,the vortex structure under near-stall condition is studied and the existence of secondary leakage vortex in tip region is found.The main leakage vortex downstream of the development process will be broken,resulting in tip plugging and entropy losses,while the secondary leakage vortex in the downstream channel into the main leakage vortex will also be broken under the near-stall condition.The main frequency of the tip leakage vortex is obtained by the means of FFT.As the tip leakage vortex develops to the downstream,the amplitude of tip leakage vortex frequency decreases and the amplitude of miscellaneous frequency increases.Through the analysis of the flow field at different time points,the dynamic coupling process between the separation vortex and the leakage vortex is obtained.It is found that the periodic oscillation of the leakage vortex and the periodic vortex shedding of the separation vortex are the main reason of the unsteady flow in the tip area.In this paper,the control effect and mechanism to the flow field at different excitation locations are analyzed.The results show that when the excitation position is at the leading edge of the cascade,the total pressure loss coefficient decreases by 4.5%.When the excitation is at the trailing edge of the cascade,the total pressure loss of the cascade can not be reduced.Both excitation positions can reduce the load on the leading edge of the tip region and help to improve the stability of the compressor.Through the study on the flow field of the tip area in the controlled state,it is found that the dominant frequency of the leakage vortex is larger than the dominant frequency in the uncontrolled state,and the dominant frequency is close to the excitation frequency,and the dominant frequency is obvious.Uncontrolled state of miscellaneous frequency amplitude larger than the state of control and the tip leakage vortex frequency is not obvious.Controlled state of the magnitude of miscellaneous frequency amplitude is significantly inhibited.At this time,the flow field at the tip area is in a more orderly resonance state through the non-linear effect of the unsteady excitation signal and the flow field.