Classification of Cavitation in Hydraulic Machines using Vibration Analysis

This paper describes vibration analysis techniques that have been adapted and used to classify cavitation in a model Francis turbine. This analysis can be used to compliment standard cavitation tests. Cavitation can be classified in various ways using vibration analysis. Cavitation attached to the blades of the runner is modulated by instabilities caused by the interaction with the blades and the guide vanes, as well as by the outlet swirl. The shape of this modulation is not sinusoidal. Cavitation on the runner is non-uniform; i.e. some blades are, on average, experiencing more aggressive cavitation than others are. Analysis techniques employed include a demodulation technique that considers the envelope auto-power spectrum, joint time-frequency analysis and phase averaging.