Optical Flow Measurement of Cavitation in a Converging-Diverging Nozzle Using High-Speed Imagery

Abstract

Cavitation is a phenomenon where liquids will vaporize when subjected to low pressures. Essentially, the pressure is reduced sufficiently such that the liquid boils at the given temperature. The highest pressure at which cavitation could occur is called the vapor pressure. However, the pressure associated with the onset of cavitation could be lower than the vapor pressure. This indicates the liquid exists under a meta-stable condition. The current research is investigating different aspects of cavitation and cavitating flow characteristics. Particle tracking using high-speed photography provided further insight as to what the velocity profile of cavitating flow may resemble. The research has shown that the cavitation that occurred in the current nozzle appears to have a laminar velocity profile. In the experiments that were conducted, it was also observed that as the back pressure of the downstream decreased, the volumetric flow rate would increase. However, a maximum volumetric flow rate was measured once the flow had begun cavitation regardless of the back pressure. This indicated that choked flow conditions likely exist within the nozzle. Choked flow within the nozzle indicates that near the region of the throat the fluid velocity has reached the speed of sound. Using high-speed photography, visualization of flow separation and recirculation was recorded. The information obtained from the research provides a more detailed description of the velocity profile near the onset of cavitation. The main objectives of this research were to obtain measurements of the overall flow for support of on-going research and analysis of nozzle flow cavitation. This study will provide a foundation for further and more detailed research into cavitation phenomena.