Research On Nonlinear Dynamic Response Of Blade And Rotor Of Aero-Engine Under Airflow Excitation

Rotor and blade are key components of aeroengine. The blades drived by high-speed rotor work under the airflow to transform thermal energy into available mechanical energy. Simultaneously, it causes intense vibration of rotor and blade that is the main factor inducing their fatigue failure. Due to the combined action of airflow excitation and mechanical excitation, and because of the nonlinear effects caused by assembled clearance, crack, rub-impact, etc, the rotor and blade always exhibit complex nonlinear behaviors such as nonlinear flutter, main resonance, combined resonance, bifurcation, chaos and so on. These complex dynamic phenomena put forward higher requirements on the optimization design of aeroengine parameters. In the practical region of physical parameters, the flutter mechanism and evolving rule of rotor and blade are investigated under airflow excitation, and the principle-resonance and combined-resonance transition characteristics of blade with clearance or crack are discussed under the rotor displacement excitation as well. The details are as follows:The compressor blade and rotating hub are usually joined through mortise connection that may change the flexural rigidity of blade. Then the restoring force in flexural vibration will exhibit piecewise linear features. Considering the piecewise linear and cubic nonlinearity in torsional vibration, the bending-torsional coupled flutter of blade is investigated under unsteady, incompressible and inseparable airflow in this dissertation. The response stability of limit cycle is studied and the threshold speed of the linear flutter is obtained by Routh’s criterion. The response equations of limit cycle oscillations are deduced by using the averaging method. The results reveal that the “double-critical” Hopf-bifurcation exists in blade flutter because of the clearance nonlinearity. With the decrease of clearance, the jump of amplitude in limit cycle oscillation get reduced or disappeared and the flutter characteristic changes from “double-critical” Hopf-bifurcation to supercritical Hopf-bifurcation, which enhance the stationarity of blade flutter. The flutter amplitude is largely affected by the natural frequencies ratio of bending and torsion and the nonlinear rigidity coefficients in torsional vibration which can be used to inhibit the flutter amplitude to prevent fracture damage of blade.Under different operation conditions, the blade with clearance works in different states of airflow turbulence which have different effects on the bending-torsional coupled flutter of blade. The principle-resonance of bending-torsional coupled vibration of blade is further investigated in the steady airflow under lateral displacement excitation of the rotor. The amplitude-frequency response equation of principle-resonance is obtained by the averaging method, and the transition sets for topological structure and bifurcation diagrams in different persistent areas are presented in the practical region of physical parameters based on the singularity theory. The results show that the assembled clearance between blade and hub is the source of constrained bifurcation, and the excitation amplitude of rotor displacement and nonlinear stiffness ratio in torsional vibration are important parameters to change the topological structure of resonance curve.Long-term action of dynamic load will induce fatigue crack in compressor blade. However, the opening and closing of crack along with vibration response will make the blade exhibit complex nonlinear dynamic behaviors. The combined-resonance research on blades with breathing crack is of practical significance to nondestructive testing of blade crack. A single degree of freedom blade model is established based on the Galerkin method. Under the combined action of varying compliance excitation by breathing crack and displacement excitation from rotor, the first order approximate equation of amplitude-frequency response for combined resonance is obtained by the multiple-scale method. The effects of depth of crack, location of crack and amplitude difference between the horizontal and vertical displacements on amplitude-frequency response are analyzed. The results indicate that the variation of depth and location of crack changes the parametric excitation amplitude and bifurcation modes in resonance zone. The variety of displacement excitation amplitude changes the value, scope and hysteresis trend of the resonance peak of amplitude-frequency curve.The labyrinth seal is one of main non-contact seal structures in aeroengine. When high-pressure airflow is flowing through labyrinth seals in aeroengine, the aerodynamic loads in seals will induce instability of rotor system. Taking into consideration the airflow loads in labyrinth seals of high-pressure drum, the flutter response of dual-rotor coupled system is studied by numerical simulation. The flutter mechanism and evolving rule of dual-rotor coupled system are presented with the variety of aerodynamic parameters in seal structure and the rotor supporting stiffness. The swirl speed ratio of circumferential flow at seal inlet is regarded as the main aerodynamic parameter to cause flutter of the system.