On the sensitivity of cavitation inception & dynamics for water quality

Abstract

The microbubble content is considered one of the most important fluid characteristics influencing cavitation inception and dynamics. However, the effects of microbubbles on cavitation are not yet precisely understood. The current work experimentally assesses the influence of microbubbles on cavitation around the runner of a hydraulic turbine model in a hydraulic test facility. Microbubbles are measured upstream of the modelusingIPI, while cavitation is observed visually. IPI measurements are performed under different DO concentrations, thereby changing the microbubble content in the facility, at different runner speeds. The results show an increase in microbubble sizes and concentrations for higher DO
concentrations. The sizes and concentrations of microbubbles increase accordingly with decreasing σ. This increase is not only the result of a pressure reduction at the measurement position but also the appearance of a cavitating vortex at the outlet of the runner, causing additional bubbles measured at the measurement position. These effects are highlighted once again after the onset of gap cavitation, showing the influence of cavitation on the measured microbubble content as no resorber is located in the test facility. A decrease in the microbubble concentration is observed after the onset of gap cavitation for increasing DO concentrations, as oxygen diffuses into cavitation bubble. Moreover, at low values of σ, a vacuum is applied to the system, resulting in a faster decrease in the DO concentration
at a higher initial DO concentration, resulting in a decrease of the microbubble content. A delay in the onset of gap cavitation is observed at lower DO concentrations due to the availability of fewer and smaller microbubbles in the facility. After the onset of cavitation, the development of gap cavitation is significantly altered by a change in the DO concentration. It is shown that gap cavitation covers a larger area of the runner at high DO concentrations, most likely caused by an increase in the vapour volume fraction at the outlet of the labyrinth seal, thereby increasing the number of vapour bubbles around the gap of the runner