Backward whirling behaviors of a twin-spool rotor system with coupling inter-shaft and rotor-stator rubbings

The aim of this paper is to investigate the backward whirling behaviors of an aero-engine twin-spool rotor system with coupling rubbings. Firstly, considering the effects of the inter-shaft and rotor-stator rubbings, the dynamic model for a twin-spool rotor system without inter-shaft bearing is developed by means of finite element method. Subsequently, using three-dimensional full spectrum and rotor orbits, the backward whirling behaviors of twin-spool rotor system are illustrated, and the effects of rotational direction and speed ratio on the whirling direction of the rotor are discussed as well. The results reveal that backward inter-shaft and rotor-stator frictions are the physical reasons of backward whirl in the rubbing rotor. For the twin-spool rotor system with inter-shaft rubbing, when the speed ratio is positive, the high-pressure rotor can strengthen the rotation of the low-pressure rotor through the inter-shaft rubbing point, and the low-pressure rotor maintains in forward whirl. In contrast, when the speed ratio is negative, the larger inter-shaft friction may induce backward whirl for the low-pressure rotor. Regardless of whether the speed ratio is positive or negative, the rotor-stator rubbing always contributes to promote the backward whirl of the rotor. As the speed ratio goes up, the initial speed of the backward whirl drops and the opposite is also true. These findings contribute to understanding the mechanism of backward whirling behaviors for the rubbing twin-spool rotor system.