Research On Dynamic Characteristics Of A Dual Rotor System With Typical Faults

The dual rotor system has been widely used as the main structure of the aircraft engines, which has unique structural features:the inner and outer concentric rotors of different speeds with complex rotor supporting forms are coupled together through the intermediate bearing. In addition, the aircraft engines work repetitively at a poor circumstance of high temperature, high pressure and high rotating speed so that its structure will be affected by the mechanical excitations, aerodynamic excitations and thermal field influences, which would make the vibration problem of the dual rotor system very complex. Therefore, it is significant to research on dynamic characteristics of aircraft engines and the dynamic response characteristics with typical faults to improve the engine performance and promote the development of the aviation industry of our country.The paper mainly concerns the dynamic characteristics of dual rotor system, and its dynamic response characteristics in rub-impact fault are analyzed; in addition, the finite element model of blisk-rotor system are established to investigate its coupling characteristics and research on the influence of the mistuned blisk on the blisk-shaft system. The main research works are as follows:Firstly, numerical algorithm theories involed in this paper are introduced, such as the typical theory of solving the dynamic characteristics of rotor systems and the Lanczos method for solving large sparse matrix eigenvalues, then the Newmark-β method and Newton-Raphson method of solving kinetic equations are derived, moreover the nonlinear finite element theory of solving contact problems is described.Secondly, the finite element model of dual-rotor system is established, and the static frequency and the critical speed characteristic of this model are analyzed; the law of the natural frequency and the critical speed influenced by the supporting stiffness of each bearing are researched, and the comparative analysis of modal shapes of the dual rotor system with different supporting stiffness of the intermediate bearing is accomplished; what’s more, the correctness of the critical speed calculation has been validated by the amplitude-frequency response method. And numerical analysis shows that the natural frequencies and the critical speeds of the dual rotor system both tend to constant values with the increasing of the bearing surpporting stiffness, and the supporting stiffness of the intermediate bearing has a great influence on the modal shapes due to the coupling effect, which should be analyzed emphatically in the design phase.Thirdly, the dynamic model of the dual rotor system with unbalance is established, and then the steady state unbalance response is studied to compare with the dynamic response characteristics in rub-impact fault. The dynamic model in rub-impact fault is established, and the dynamics response characteristics are researched when the rubbing fault occurs on two disks respectively, and results show strong nonlinear characteristics:combined frequencies and continuous spectrum appear; in addition the lighter instability phenomenon has happened on the rotor without rubbing. Taking the left disk of the inner rotor in rub-inpcat fault for example, the influence law of dynamic response characteristics is researched when some rubbing parameters such as rub clearance change, according to what the severity of rubbing fault could be judged.Finally, the finite element model of blade, tuned blisk, tuned blisk-rotor system with different blade length (0.25m,0.225m,0.2m) and mistuned blisk and mistuned blisk-rotor system are established respectively, and then their natural frequency and modal shape characteristics are analyzed. Studies show that the coupling modal shapes appear in the blisk-rotor system, and the frequency spit phenomenon also appears; in addition, the mode localization appears on the blisk structure due to the mistuning, which cause certain influence on the blisk-rotor system. It should attract the attention of relevant designers in the design phase of the rotor system.