Auto-Oscillation of Cavitating Inducers

Abstract

This paper presents details of measurements on the instability know as auto-oscillation which occurs in systems with cavitating pumps. Specific measurements are made of onset cavitation number and auto-osciallation frequency for a range of inducers. It has been shown that auto-oscillation is a system instability caused by the active dynamic characteristics of the cavitating pump. A system anslysis is presented which utilized previously measured dynamic transfer functions for the inducers; the resulting predictions of instability are consistent with the observations. Though the onset cavitation number is a function of the entire system it is also show that, given the onset cavitation number, the auto-oscillation frequency is only weakly dependent on the system and primarily a function of the pump dynamics. Detailed measurements of the amplitude and phase of fluctuating pressures and flow rates during auto-oscillation are also presented. These strongly suggest that the pump dynamics are primarily determined by the complicated flow at inlet to the inducer which involves pre-swirl generated by a strong backflow. Some data on the non-linear effect of auto-osciallation on overall mean performance are also presented.

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