| With the improvement of aerodynamic performance of axial turbomachines,the stage load of fan/compressor continually increasing,while the weight is going to be lighter and lighter.This causes blade flutter problem being increasingly prominent,which gradually becomes a bottleneck restricting the development of modern aero-engine.Therefore,the improvement of unsteady analysis method of cascades,the establishment of efficient and reliable prediction platform of blade flutter and the exploration of aeroelastic instability mechanism of blades manifest very important theoretical significance and engineering application value.This article developed the implicit method to solve harmonic balance governing equations based on source term linearization technique and extended the capability of unsteady simulation in the multi-perturbation frequency domain compared with the original solver.Then,combining the energy method,based on frequency domain nonlinear analysis model,realized the efficient prediction of aeroelastic stability of blades.Later taking a transonic compressor blade and a linear turbine as research objects,the accuracy and reliability of steady and unsteady numerical simulation were verified.On this basis,the following research work was done in the paper:Firstly,based on structural dynamics analysis,unsteady flow around oscillating blades was numerical simulated choosing the transonic compressor blades by frequency domain nonlinear technique in the isolated blade row.The aerodynamic damping coefficients of blades with different clearances were investigated systematically for the first three vibration mode.Through comparing and analysing of aerodynamic work on the blade surfaces for cases with different clearances,the influencing mechanism of leakage flow on blade flutter characteristics was discussed.Results showed that the leakage flow of larger tip gaps caused the static pressure unload near the leading edge of blade tip and formed a strong pressure fluctuation in the corresponding region which induced local positive damping effect for any vibration mode.Secondly,the first rotor of two-stage transonic fan was numerical predictioned about the blade flutter characteristics in the multi-stage enviroment using frequency domain/steady hybrid calculation model.Based on analysis of time averaged flow field and static pressure fluctuation,the physical factors inducing aerodynamic work on the blade surfaces were explained.The study revealed that there was not instability risk for the first rotor at the design rotation speed and part-speed.However the supersonic stall flutter maybe occur for the blades at 105% and 110% design rotation speed because the aerodynamic damping coefficients were negative.Moreover,the positive work consentration area originated from different unsteady perturbations on the blade surfaces.The oscillation of shock wave on the pressure surface maybe produce positive work consentration for the most situations.Additionally,supersonic zone and separation flow after shock wave on the suction surface were also the source of positive work.Thirdly,taking a forward swept fan as the research case,the influence of sine wave distortion and BLI distortion(Boundary Layer Ingestion)on aeroelastic stability of blades was studied using multi-harmonic frequency domain calculation method.The results showed that variation of time averaged flow field and unsteady characteristics of shock wave induced by inlet total pressure distortion,leading to the differences of aerodynamic work on the blade surfaces,which is the main reason for blade flutter characteristics.Lastly,a hybrid computational model and a multi-disturbance frequency domain analysis method were used to study the flutter characteristics of the downstream rotor of a high load two-stage counter-rotating fan,and the differences of aerodynamic work and unsteady pressure response on the blade surfaces were investigated.The effects of row-to-row interaction on aeroelastic stability of the downstream blades were discussed.The results revealed that considering periodic disturbance from upstream blade row resulted in reduction of aerodynamic damping of the downstream rotors.The main reason for the differences of flutter characteristics included the changes of shock wave form in the time averaged flowfield,the fluctuation of attack angle induced by upstream wakes and the time-variation of the circumferential static pressure caused by row-to-row potential disturbance. |
PDF Full Text Request