Downsizing and high power density of turbopumps are achieved by increasing their rotational speed. Cavitation often becomes a problem while the influence of cavitation will be generally relieved by employing an inducer before the impeller. For general-use turbopumps with inducer, instability-free operation as well as high suction performance are required in wide flow rate range including extremely low flow rate. However, the low frequency and large amplitude of cavitation surge is often be a serious problem even with inducer when operated at very low flow rates. In this study, a reduced-diameter suction pipe (RSP) equipped with swirl brake (SB) was proposed for a suppression device of the inlet backflow as well as of the cavitation surge through removing swirling velocity component. The effectiveness of this device was investigated by CFD and experiments. First, several geometries of RSP with SB were examined by CFD, and it was found that the extension of inlet backflow was stopped at this device provided that the swirl brake had a sufficient radial or axial length. Then, one of the proposed RSP with SB was manufactured, and the experimental evaluation of the effectiveness of this device was conducted. It seemed that RSP with SB could well prevent the extension of inlet backflow. The cavitation surge was completely suppressed even at extremely low flow rates. As a result, the suction performance was also improved at low flow rates.

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In the present study, the gas-liquid two-phase flow performance of a turbopump with an inducer has been experimentally investigated. In the performance evaluation test, the air bubbles is released from dissolved air at the upstream valve to realize two-phase flow. By this method, a homogeneous bubbly flow is expected just after the valve. The amount of dissolved oxygen (DO) is an important parameter, and the experiment is carried out under the conditions of DO≅50%, 100%, 200%, where DO[%] indicates the relative amount of DO against the saturated value at the atmospheric condition. Under each DO condition, the experiment is performed at various water flow rates. As a result, a clear trend that the higher DO is, the worse the performance becomes, is observed. It is found that the mechanism for the head drop of the pump differs between at high and low flow rates. At the high flow rate, the performance does not deteriorate up to relatively high volumetric flow rate ratio of air and water. The cause of performance deterioration seems to be the increased number and/or size of bubbles released at the upstream valve. On the other hand, at the low flow rate, the performance deteriorates at relatively low air-water flow rate ratio. Although the air water volume flow ratio is low, the bubbles can precipitate in inlet flow recirculation and back flow vortices, which seems to cause the performance deterioration.