Modal characteristics and unbalance responses of fan rotor system with flexible support structures in aero-engine

This paper investigates the vibration patterns, i.e. rigid motions of shaft and elastic deformation of support structures, of fan rotor system in aero-engine, which differs from traditional flexible rotor systems, and together with its shaft transverse motions due to unbalanced mass. The fan rotor system commonly is composed of one rigid shaft and two flexible support structures (such as squirrel cages), which is effective to decrease the critical speeds avoiding serious shaft vibration due to unbalance. Scaled test rig for realistic fan rotor system is set up according to similarity principles, governing differential equations of which are deduced by means of Lagrangian approach with four degrees of freedom. In contrast to modeling a traditional flexible rotor system, the system stiffness is not determined by the shaft but the two flexible support structures. The rigid shaft only contributes to the inertial items of the governing equations. Parameter values of dynamic model are identified from measurements on the scaled test rig, the modal shapes and the modal energy distributions are calculated. These modal characteristics of the fan rotor system are quite different from those of a traditional flexible rotor system whose stiffness mainly contributed by its elastic shaft even the system values are consistent. The obtained modal characteristics are compared and confirmed by using the simulation results of a corresponding finite element model, in which shaft is built by rotating beam elements and its flexible structures are built by equivalent spring elements. Campbell diagrams of the fan rotor system are used to illustrate the gyroscopic effect with the increasing speeds. And then the unbalance responses are calculated through the deduced analytical formula rapidly and comparisons, including the response spectrum and orbits, the amplitude and phase frequency response curves, and operating deflection shapes, are carried out in the sub- and super-critical range.

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