Dynamical Modeling And Nonlinear Vibration Study Of Aero-engine Rotor System

The aircraft engines with complex structure operate in extremely serious environment and occur vibration faults frequently,which restricts the development of aviation of our country.Most of the aircraft engines are dual-rotor structure supported by rolling bearings,and contain multilevel disks a nd blades,as well as squeeze film dampers(SFDs).Those engines are imposed on by complex dynamic loads such as double-frequency excitation and flow perturbation.Study on the dynamic characteristics of complex rotor systems is especially important to the design of aircraft engines.Also,clarifying their nonlinear vibration behaviors and fault mechanism has important engineering significance and theoretical value.In this dissertation,the complex dual-rotor system of the actual aircraft engine is considered.Its discrete dynamic model is firstly established,and the reversible structure order reduction of the rotor system is conducted.Based on the reduced model containing the nonlinear features of inter-shaft bearing,the effects of inter-shaft roller bearing clearance and external load on the nonlinear dynamic responses of dual-rotor system supported by inter-shaft bearing are studied,as well as the effects of radial clearance of ball bearing on the time-varying stiffness of the rotor system.Then,the crack model of hollow shaft is established,and the dynamic characteristics of dual-rotor system with hollow shaft crack fault are studied with the combination of analytical and numerical methods.Finally,the effects of blade parameters on the dynamic characteristics and bifurcation behaviors of rotor-bearing system are studied.The detailed contents and results are discussed as follows:For a dual-rotor system with six-point support,the dynamic modeling and model reduction are studied,and the application condition of different simplified models is given.A relatively complex discrete dynamic model is firstly established based on the finite element method,and the basic dynamic behaviors of the system are analyzed.Then,based on the actual structure behaviors,the order reduction of discrete dynamic model of dual-rotor is studied,and the reduced models with four or three disks are obtained,as well as their application conditions.The comparison between critical speeds of different reduced models reveals that the one with four disks can reserve the dynamic characteristics of the original system in any supporting conditions.Considering the high pressure rotor system,the reversible issues of the reduced model are discussed,and the results indicate that t he error of the first three critical speeds of the new reduced model is less than 3%,which demonstrates that the order reduction method is reversible.The effects of inter-shaft roller bearing on the nonlinear vibration characteristics of dual-rotor system are studied.Considering the structural and operating features of actual dual-rotor and the interaction between rolling bearing and dual-rotor,a four-disk model supported by inter-shaft bearing is established with the previous modeling method.The effects of radial clearance of inter-shaft bearing,external load and speed ratio on the nonlinear dynamic responses are analyzed with numerical method.The results indicate that the radial clearance can significantly affects the system's nonlinear vibration characteristics.As the clearance increases,the rotating speed of forced resonance decreases,and the linear resonance at the two peaks of forced vibration changes to nonlinear behavior with hard rigidity.The larger the clearance is,the more obvious the h ard rigidity is.Moreover,with the increasing of radial external load,the hard rigidity changes to soft rigidity near the amplitude-frequency curve's forced resonance peak.The effects of the ball bearing parameters on the nonlinear characteristics of th e dual-rotor-supporting system,such as time-varying stiffness and primary resonance,are discussed.The coupled dynamic model of ball bearing,dual-rotor and supporting are established by considering the nonlinear factors of the 4# ball bearing,and the effects of bearing clearance on the time-varying stiffness and primary resonance are analyzed in detail by the numerical methods.The results reveal that the system's equivalent stiffness and the rotating speed of forced resonance decrease as the clearance increases.In the low-speed zone(?1<800rad/s),the zero-stiffness can be seen,and the number of zero-stiffness in one motion period grows with the clearance increasing.In the case of small even zero clearance,the amplitude-frequency curves still behave hard rigidity in forced resonance domain,and have hysteresis and jump phenomena.Based on the proposed modeling method for the reduced model,a four-disk discrete dynamic model is established for the rotor system with hollow shaft crack.Considering the variation of the sectional moment of inertia of the element with crack and the crack breathing effect,the hollow shaft crack model is established by extending the solid shaft crack model.With the finite element method,the dynamic equations of rotor system with hollow shaft crack is established,which contain double frequency unbalanced excitation,gravity and time varying stiffness.The system steady dynamic responses are calculated by using two-dimensional harmonic balance method and Newmark-? method,then the effects of crack position and depth on the responses are analyzed.The results indicate that the presence of crack can arouse resonance of reverse critical speed and lead to super-harmonic resonance,which will be more obvious as the crack depth increases.The crack signals(2?1 and 3?1 frequency components)of high pressure rotor can transmit to low pressure rotor via inter-shaft bearing.Therefore,an idea can be proposed that the high pressure crack information can be detected via low pressure rotor.The hollow shaft crack model given by this dissertation is appropriate for the study on nonlinear dynamics of rotor system with hollow shaft crack.The influences of blade parameters on the dynamic characteristics of rotor-rolling bearing system is analyzed in depth.Taking the coupling effect of the blade-disk and the rotor system into account,the effects of blade parameters on the dynamic characteristics of dual-rotor system with six-point support and the bifurcation behaviors of low-pressure compressor rotor system supported by rolling bearings are studied.The blade is simplified as cantilever structure,and the dynamic equations are obtained with the finite element method.The effects of blade parameters on the dual-rotor system's dynamic characteristics are studied with the amplitude-frequency curves and orbit diagrams.The results indicate that blades can decrease the critical speed of the system,and affect its dynamic characteristics obviously.The high pressure rotor and low pressure turbine r otor can excite the first order resonance of dual-rotor system,and the low pressure compressor rotor can excite the second order resonance.The dual-rotor system with six-point support can further simplified as single span single rotor model or dual-span dual-rotor model with four-point support.Moreover,combining bifurcation diagrams with time history curves,orbits of shaft center,Poincaré map diagrams and frequency spectrum diagrams,the effects of blade length and elastic modulus on the bifurcation behaviors of low pressure compressor rotor-rolling bearing system are analyzed emphatically.The conclusions are made that the blades can cause motion forms more complex and lead to the chaotic motion of low pressure compressor rotor-rolling bearing system in the case of high speed.