Experiment on Instability of Horizontal Rotor System Partially Filled with Small Amounts of Fluid

The rotor disk cavities of Marine gas turbine often cause self-excited vibrations due to fluid accumulation. The accumulated fluid forms by condensation of vaporized lubricating oil or water vapor. The rotor drives the liquid in the cavity to rotate, causing vibration instabilities. In this study, a horizontal flexible rotor system with two disks and two supports was examined experimentally. The rotor instability and stability restoration were observed, and the effects of the liquid volume and viscosity were studied to reveal the dynamic behaviors of small amounts of liquid in horizontal rotors. There was a threshold amount of liquid for the unstable vibrations, and instability occurred when the amount of fluid exceeded the threshold. A threshold speed of instability and recovery speed were found, both of which were higher than the critical speed. When the speed was higher than the threshold speed of instability, instability occurred. When the speed increased further, for speeds faster than the recovery speed, the instability disappeared. The amplitude of the total vibration increased suddenly, as did the sub-harmonic frequency. The fundamental and sub-harmonic frequencies were modulated and showed beat vibration characteristics. The threshold speed of instability decreased first and subsequently increased with the increase in fluid volume. With the increase in the viscosity coefficient of the fluid, the threshold speed of the fluid-filled rotor and amount of fluid increased. For unstable vibrations, the rate of amplitude growth increased rapidly.