Nonlinear rotordynamics in the presence of a hybrid bearing
Résumé
The paper investigates the nonlinear dynamic behavior of a rotor whose design is similar to spatial centrifugal turbopumps. Depending on the rotor speed of rotation, these pumps can be supported by ball bearings with loose fit, i.e. having a clearance between their outer race and their fixed housing, and by annular seals which develop radial fluid forces thanks to the Lomakin effect. Moreover, such turbopumps often operate in supercritical regime, which means that they must pass through critical speeds while experiencing the lowest vibration levels to ensure the pump safety. In this context, the paper aims to analyze the case of an academic rotor test bench designed so as to present a bending rotordynamics similar to that of spatial centrifugal turbopumps and built for experimental validation purposes. The investigation focuses only on the test bench equipped with hydrodynamic bearings and one ball bearing with loose fit. The two hydrodynamic bearings are located at the two shaft ends and the ball bearing is located at the middle of the shaft. The finite element model of this rotor is first detailed
and the test bench is described in details. Then, the nonlinear dynamic response of the rotor to mass unbalance and rotor bow when passing through its first critical speed is assessed both numerically and experimentally. Finally, base excitation with random and chirp sine profile are imposed to the rotor, which operates in a supercritical regime, so as to trigger the rotor-stator contacts in the ball bearing. The numerical results are computed using either a full transient step-by-step time integration technique or an arc-length continuation method. The main objective is to evaluate how the contact nonlinearity from the ball-bearing loose fit alters the original rotordynamics. Major nonlinear phenomena are highlighted such as the flattening of the resonance peak at the critical speed and the many harmonics of the shaft speed of rotation, owing to the hydrodynamic bearing and contact nonlinearities.