Study On Dynamic Characteristics Of Planetary Gear Transmission System Of Geared Turbofan Engine With Sudden Imbalance Under Windmilling

Compared with the conventional double rotor engine,the geared turbofan engine has the advantages of lower maintenance cost,lower noise level and lower emissions,and the market demand is huge.The fan driven gearbox is one of the keys to the development of this configuration engine.In the geared turbofan engine,the fan driven gearbox works at high speed and high load? Not only it is complicated in structure,but also the gear and bearings work under bad conditions.Therefore,it is a system with frequent failures and poor reliability.Fan blade off is a must for high bypass ratio turbofan engine.Fan blade off will cause imbalance excitation.And for the geared turbofan engine,the influence of planetary gearbox characteristics has to be considered.The geared Turbofan engine is facing new problems of imbalance caused by blade off.It is necessary to explore new technologies to adapt to the new structure.Dynamic analysis is one of the most important technologies.In this paper,the world's smallest geared turbofan engine-a small civil high bypass ratio turbofan engine,is chosen as the research object.Aiming at the special structure of the geared Turbofan engine gear transmission system,its dynamic model is established.Considering the influence of time-varying meshing stiffness,time-varying meshing error and unbalance excitation caused by blade off,the vibration response of gear transmission system is studied.The influence of system parameters on dynamic characteristics is revealed.The main contents are as follows:(1)According to the structural characteristics of the gear transmission system in the geared turbofan engine,the Euler-Bernoulli beam element considering shear deformation is adopted according to the virtual shaft equivalent theory.The finite element basic element models of shaft segment unit,gear meshing unit and bearing unit are established respectively.The sudden imbalance excitation caused by fan blade off,the time-varying meshing stiffness and meshing error are considered.The dynamics model of fan-gear-shaft-bearing-casing system is coupled by basic elements.(2)According to the system eigenvalue equation,the natural frequency of the gear transmission system in the geared turbofan engine is solved.The distribution of modal energy of the system is studied.And the effects of mesh stiffness and bearing stiffness on natural frequencies are studied.It lays the foundation for the vibration response analysis of the gear transmission system in the geared turbofan engine.(3)Under windmilling condition with a single blade completely lost,the vibration characteristics of the fan shaft of the turbofan engine with and without a gearbox system are compared.The effect of the gear system on the vibration of the fan shaft under different rotating speeds is examined.The results show that the orbit of the fan shaft center in the turbofan engine with the gearbox system exhibits a multi-frequency whirling motion and has a stable limit cycle.Under windmilling condition,the meshing frequency and modal frequency have multiple intersection points.The critical speed is dense and the peak value of the transient vibration of the geared turbofan engine gearbox shows a wave rise with an increase in speed.The results of this study could provide a reference for the parameter design and optimization of the geared turbofan engine.