Cavitation Breakdown in an Axial Waterjet Pump: An Experimental Characterization of Flow Phenomena
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Cavitation breakdown results in the performance failure of a turbomachine, for which head rise and flow rate through the machine each drop significantly, as a result of an inlet pressure below a certain critical value. Extensive cavitation in the machine, and the resulting breakdown, limits the operating range of turbomachines and constrains their design. While cavitation in turbomachinery has been studied extensively, a definitive explanation for the mechanism of cavitation breakdown has not been adequately given and supported. Prior models attribute cavitation breakdown to various mechanisms including choking, blockage, acoustic shocks and the formation of perpendicular cavitating vortices (PCV’s). Previous research on cavitation in the Johns Hopkins turbomachinery lab demonstrates that the use of circumferential grooves to manipulate tip leakage flow provides minimal performance enhancement during breakdown conditions. The present thesis provides experimental evidence to elucidate the mechanism of cavitation breakdown due to cavitation-induced blockage. Evidence to explain the mechanism of breakdown is presented using high-speed flow visualization, performance testing, and stereoscopic particle image velocimetry (SPIV) at planes along the blade pressure side tip region to characterize passage flow in a Navy axial waterjet pump (AxWJ-2). Each method is evaluated at a range of cavitation indices, from conditions of minimal cavitation through severe performance breakdown. The results indicate that rapid performance breakdown occurs when a sufficient inlet pressure decrease causes the suction side attached cavitation to extend into the blade overlap region. The flow along the pressure side of the blade is accelerated due to cavitation blockage in the tip region. At this point, a shift in blade loading temporarily improves machine performance. Yet any further decrease in inlet pressure causes cavitation to rapidly expand, both axially and radially, to significant blade coverage. During this rapid expansion, flow blockage in the tip region increases and performance breakdown occurs. Performance worsens significantly with any further decrease in pressure. The results provide evidence that cavitation breakdown in the axial waterjet pump occurs due to this process, involving flow blockage in the tip region and resulting cavitation structures which further block the passage flow.