Research On Dynamic Characteristics Of Counter- Rotating Dual-rotor System And Intermediate Bearing In Aeroengines

To improve the overall performance, counter-rotating dual-rotor system with intermediate bearing is adopted by many advanced aeroengines. As the main source of vibration in aeroengines, dynamic characteristics of the counter-rotating dual-rotor system will be one of the most important topics in the development of Chinese aeroengines. In this paper, systematic and thorough research on dynamic modeling, solution method of the nonlinear system, dynamic characteristics analysis and intermediate bearing analysis are conducted with five-support dual-rotor system as the engineering background. Results of this paper provide basis and methodology for dynamic characteristic design of the counter-rotating dual-rotor system for aeroengines. The main contents and innovations of this paper are listed as follows:(1) A dynamic modeling method for the dual-rotor system is introduced with the finite element method and fixed interface modal synthesis method applied. Dynamic equation of a dual-rotor system is established with mass and stiffness matrixes as well as modal parameters obtained from ANSYS in which the finite element model is established. Dimension reduction is then achieved by fixed interface modal synthesis method to improve the computational efficiency. Dynamic characteristics of the counter-rotating dual-rotor system, including critical speeds and transient response characteristics of the uniformly variable process, are analyzed systematically with this method. Experimental verification on transient response is conducted. Simulated and experimental results show that: the differences of critical speeds between co-rotating and counter-rotating dual-rotor system are mainly caused by gyroscopic torque. Besides, due to dynamic coupling of the inner and outer rotor, counter-rotating dual-rotor system has unique transient response patterns and characteristics which are different from those of the single rotor system. This methodology is of great engineering value because advantages of the finite element method, accuracy and applicability, are retained without complex programming while more efficient.(2) Mechanical analysis model of the intermediate bearing is developed considered with the roller, ring and cage. The influence of cage riding method to dynamic characteristics of the bearing is studied with the inner ring connected to inner rotor and the outer ring connected to outer rotor. The experimental results, which agree quite well with those of the calculation, indicate that the cage riding method has a significant influence to intermediate bearing on cage speed, rotational speed of the rollers and cage eccentricity, while it has little effect on the contact fatigue life of bearing. The results provide basis and references for intermediate bearing’s designs.(3) Based on the equation of motion established with finite element method, two approaches to acquire dynamic characteristics of the high-dimensional nonlinear dual-rotor system are developed considering the locality of nonlinear force. The first approach, in which the modal synthesis method and Newmark’s Method of Direct Integration are utilized, is applied to dual-rotor system with nonlinear force acting on the interfaces of supports. The second one, which is a semi-analytical method for dual-rotor system with nonlinear force acting on any nodes, utilizes the free interface modal synthesis method and Duhamel’s integral. With these two methods, research on nonlinear dynamic response of the counter-rotating dual-rotor system is conducted with two unbalance forces and rub-impact considered separately. The results show that: due to the unbalance forces, nonlinear forces of supports and rub-impact forces, nonlinear characteristics and a lot of frequency components emerge in responses of the counter-rotating dual-rotor system. Both methods make up the finite element method for inefficiency in predicting dynamic characteristics of the high-dimensional nonlinear rotor system.(4) A modeling method for the bearing-squeeze film damper-high dimensional dual-rotor system is proposed by introducing the concept of connection substructure for practical supports. Taking a dual rotor test rig as research object, the dual-rotor system is divided into modal substructures and connection substructures. The finite element method and fixed interface modal synthesis method are used respectively to model the modal substructure and reduce the dimensions. The supports are retained in physical space as independent connection substructures. Then the substructures are assembled according to linking conditions between interfaces before the equations of motion are numerically solved with an improved Newmark’s Method. Based on this method, which is verified by experiments, unbalance response of the dual-rotor system is studied. The study results show that: in the range of rotational speed in which the analysis is carried out, frequency of variable stiffness of the rolling bearing, unbalance excitation frequency and their combinations exist in response of the rotor system. Besides, response of the rotor system is closely related to node positions, unbalance and vibration shape. This is an effective method to introduce nonlinear characteristics of the supports to analysis of the dynamic response of dual-rotor system.As stated above, systematic research on modeling method and nolinear characteristics analysis of the counter-rotating dual-rotor system are conducted in this paper. The results are of significant theoretical and engineering value to improve the overall performances of the counter-rotating dual-rotor system adopted in modern aeroengines.