Study On Dynamic Characteristics Of Centrifugal Impeller-rotor System

This thesis is a part of 'Research on Mechanism of Fluid-structure Interaction in a Whirling Centrifugal Impeller and Rotordynamics of Turbo Machine'(ZS031-B25-009-G) supported by the Nature Science Fund of GanSu Province.As an important device of power transformation, turbo machine is widely used in power, petrochemical, communication & transport, refrigeration & hypothermia, machine manufacture, air condition of construction, food & medicine industry, and so on. Therefore the dynamic characteristics of turbo machine are studied widely by more and more specialists.Although there are many sophisticated methods to analyze the dynamic characteristics of a known rotor system and the inner flow field in impeller at present, but there are not perfect for the researches on the dynamic exciting force on a whirling impeller in flow field and rotordynamics of turbo machine. Based on the existing materials, the deep research on dynamic characteristics of centrifugal impeller-rotor system is given. The main research contents and conclusions are as follow:Based on the unsteady incompressible potential flow theory, the flow exciting force on whirling impeller caused by fluid-structure interaction in inner flow field is derived and calculated. It is concluded that the tangential force f_θis negative when rotor is in backward whirling, and positive when in forward whirling. The radial force f_r decreases when backward whirling and increases when forward whirling in vaneless diffuser. This means that the fluid exciting force exerts a damping effect on whirling impeller; and for the whirling impeller in vaned diffuser, the components undergo significant fluctuation due to impeller-guidvane interactions while the time-averaged values appear to be unaffected.The centrifugal impeller system is simplified as single disk rotor model, which is a symmetry single mass flexible rotor supported by two dynamic pressure hydrodynamic bearings. The critical speed, the instable speed, amplitude, phase and the shaft journal center trajectory are given by analyzing dynamic characteristics of system vibration while considering the exciting force. The significant differences of impeller system imposed by fluid exciting force is easily got by comparison, they are indicated in ways of significant interference in phase, amplitude and disturbance about shaft journal center orbit. The motion differential equations are derived by simplifying the impeller rotor system as Jeffcott rotor, taking into account outside exciting force and using nonlinear oil film forces model of short hydrodynamic bearing. The system response in different rotating speed of rotor is calculated by using Runge-kutta method. The bifurcation characteristics of rotor system are studied by bifurcation diagram, trajectory, phase diagram and Poincare maps. The bifurcation characteristics of rotor system are compared by changing the eccentric mass and the clearance of bearing, and the results show that the eccentric mass and the bearing clearance will influence the stability of system.The main causes for dynamic faults of turbo machine are discussed in the last part of this thesis, these include the crack of rotor, wear and tear of impeller, critical speed, out-of-balance, surging, disengaging of rotation, exciting vibration of fluid, poor alignment, friction of exciting vibration and nonlinear fractional harmonic resonance.