Research On Dynamic Characteristics Of Rotor System Considering Fluid-structure Interaction Of Impeller Front Shroud

Centrifugal impellers are important components of centrifugal pumps. Due to the complexity of structure and fluid environment with high pressure and speed, dynamic fluid forces are always produced on an impeller while it is working. These forces can make rotor system with centrifugal impeller develop complicated nonlinear dynamic behaviors, and consequently exert substantial impacts on the stable, reliable and safe operation of the system. With the consideration of fluid-structure interaction of impeller front shroud, some research work on dynamic characteristics of a rotor system with centrifugal impeller is done and substantial conclusions are obtained in the present dissertation. This work will provide certain theoretical guidance for the vibration monitoring and rotordynamic designs of high-speed centrifugal pump rotor systems.Based on Hirs’bulk-flow theory and Blasius’s friction factor equation, a set of nonlinear governing equations is derived for the leakage flow between impeller front shroud and volute. The dimensionless zeroth and first order perturbation equations are obtained by nondimensionalization and perturbation. Staggered grid and finite difference method are used to discretize the equations. The SIMPLE algorithm is used to develop the computational procedure for solving the equations. A MATLAB computer program in accordance with the computational procedure is written to conduct the numerical calculations. The steady-state pressure and velocity distributions are determined by solving the zeroth order equations. The transient pressure distribution is obtained by solving the first order equations. The fluid forces and moments on the impeller front shroud are defined according to the transient pressure distribution. The dynamic model of fluid-structure interaction of impeller front shroud is formed in sequence. The steady-state analysis shows that the pressure changes with the gap variation along the circumference, being large at small gap and small at large gap, and decreases gradually along the flow path from the inlet to the outlet of leakage flow, being maximal at the inlet and minimal at the outlet. The path velocity increases with the gap along the circumference and goes up gradually along the flow path from the inlet to the outlet of leakage flow. The circumferential velocity distributions along the circumference and the flow path are similar to the pressure distribution, except that the former is more drastic than the latter.Based on a six-degree-of-freedom method that represents lateral, axial and torsional vibrations of a rotor, a bending-torsion coupling mechanics model of a rotor under mass imbalances and a dynamic model of a rotor system with centrifugal impeller are developed in sequence with the consideration of fluid-structure interaction of impeller front shroud.Some nonlinear dynamic characteristics of the rotor system under internal and external bending-torsion couplings are studied with the fourth order Runge-Kutta method and modern numerical analysis tools for nonlinear rotordynamics. It is concluded that the lateral vibrations are characterized by synchronous periodic motion. With the increasing of speed lateral vibrations intensify, but keep stable on a whole. As the speed increases, effect of rotor’s bending-torsion coupling strengthen and trosional vibration grows up, resulting in low frequency components which include1/2synchronous frequency and1/4synchronous frequency. On a whole, the system keeps stable under internal and external bending-torsion doubling and maintains periodic motion.Some nonlinear dynamic characteristics of the rotor system with the consideration of nonlinear fluid-structure interaction forces of impeller front shroud are investigated with the fourth order Runge-Kutta method and modern numerical analysis tools for nonlinear rotordynamics. Conclusions are reached that varying degrees of vibrations occur on the six degrees of freedom of the rotor and intensify as rotating speed increases. In particular axial and torsional vibrations are more drastic than others. At high speed, the waveforms and spectrums of axial and torsional vibrations become complicated, with super harmonic components appearing in the spectrum of axial vibration, low frequency components appearing in the spectrum of torsional vibration. From orbits and Poincare maps of different rotating speeds, it can be seen that the rotor system at high speed can produce quasi-periodic and chaotic motions. It is concluded that nonlinear fluid-structure interaction forces of impeller front shroud have significant impacts on the stability of rotor system.