Experimental on spatial-temporal evolution characteristics of the blade tip cavitation in a mixed flow waterjet pump

Tip cavitation remains a complex and critical concern for waterjet pumps, affecting the safety and efficiency of pump operation. This paper aims to explore the cavitation scale's evolution law, the cavitation structure's evolution characteristics and mechanisms, and how these factors affect pump performance. A synchronous experimental system consisting of a test pump, a high-speed camera, and pressure transducers has been established. Results show that the tip cavitation mainly consists of primary leakage vortex cavitation (PLTVC), secondary leakage vortex cavitation (STLVC), and suction surface attached cavitation near the blade tip (SSAC). The gray level-based image processing method is used to reveal the law of cavitation scale growth rate and vapor fraction distribution. The tip cavitation scale's growth rate increased as the cavitation number decreased. The rapid expansion of the attached cavitation promotes the formation of a re-entrant jet, cavity shedding, and the formation of perpendicular cavitating vortices (PCVs). It was then found that for every 0.1 decrease in cavitation number, the PCV scale in the blade overlap region increased by about 14.1 % during σ =0.256–0.206, which severely blocked the tip passage flow and accelerated the degradation of pump performance. The spatial-temporal evolution of the PCVs is classified into three stages: the shedding stage of the attached cavity, the stage that the shedding cavities change the migrating direction and form the PCVs, and the collage stage of the PCVs. With cavitation numbers of 0.256 and 0.223, the PCV evolves at roughly 194 Hz and 144 Hz, which results in pressure fluctuations of approximate frequency.