Study on Unstable Structure of Three-dimensional Vortices and its Oscillation induced by Cavitation Interaction

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Cavitation instabilities such as rotating cavitation and cavitation surge are one of the most important problems to be solved for realization of small-sized high-speed turbopumps. It is important to understand complex vortex structures in the internal flow and the unsteady cavity behavior, both of which play dominant roles in the cavitation instabilities. In the present study, we focused on the cavitation surge in helical inducers, in which the complex vortical flow structures including strong inlet back flow appear.It is known that the cavitation surge is strongly associated with the inlet back flow and the installation of the obstacle plate upstream of inducer is effective for the suppression of the cavitation surge. In the present study, in FY2005, we measured the inlet flow of inducers with the different blade tip angle, and we investigated the effect of the inlet ring plate, which blocks the back flow near the tip region, on the inlet back flow as well as on the onset region of the cavitation surge. As a result, it was found that the angular momentum in the back flow is increased by installing the inlet plate, resulting in the increase of the static pressure near the tip region and the suppression of the cavitation surge. In FY2006, we carried out the numerical flow analysis of the inducer, which supported the previous experimental results. Next, we measured the inlet flow distortion due to the inlet baffle plate which blocks the back flow partially in the circumferential direction. And also we developed the multi-cameras observation system which enables us to observe the simultaneous cavity pattern from blade to blade. In FY2007, we clarified the suppression mechanism of the cavitaion surge by installing the above two kinds of obstacle plate through the flow measurement, the cavity observation and the numerical simulation.