RESEARCH ON SUPPRESSION OF CAVITATION INSTABILITIES IN INDUCERS

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A guideline for cascade design for the suppression of cavitation instabilities was provided based on a stability analysis of cavitating flow through cascades. Several new methods for the suppression are proposed and their validity was shown by experiments. The mechanism of the suppression was clarified through numerical simulations.It was shown by a stability analysis of the flow through circular arc and piano-convex hydrofoil cascades that the occurrence of various cavitation instabilities can be delayed by decreasing the flow angle measured from the shockless flow angle through the adjustments of blade camber and thickness distribution. The effects of casing geometry on the instabilities were examined in detail. It was found that the cavitation instabilities are suppressed by enlarging the tip clearance or the casing diameter at the inlet of an inducer. It was found that the inlet backflow is increased and the incidence angle at the inlet is decreased by the modifications of the casi … More ng.Next, the following three methods for the suppression were examined: (1) rounding of blade tip, (2) equalizing the circumferential pressure distribution by using an accumulator at the inlet, (3) injecting circumferential jets at the inlet. It was shown by experiments that the range of the cavitation number with rotating cavitation can be decreased by all of the three methods. The effectiveness of the method (2) was also shown by a stability analysis. In the methods (1) and (3), the suppression was closely related with the enhancement of the inlet backflow.The steady and unsteady characteristics of the inlet backflow were investigated by using numerical simulations. It was first clarified by calculations using the k-E turbulence model that the angular momentum of the backflow is supplied by a flow from the pressure surface of the blades turning around the swept leading edge. The backflow vortex system surrounding the backflow region was successfully simulated by a LES calculation. It was found that the backflow vortex responds to the flow rate fluctuation as a first order delay element. This shows that the backflow vortex cavitation have a destabilizing effects on low frequency disturbances but have a stabilizing effects for disturbances with the frequency higher than about 10% of the rotational frequency. This shows that the unsteady response of the backflow is more important than the existence of the backflow, for the suppression of the cavitation instabilities. This suggests that an active control of backflow by circumferential jet would be a powerful tool for the suppression of cavitation instabilities. Less