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In order to extend the working range of high-speed submerged water jets and to improve its accuracy confidence, the jet working characteristics are studied under the condition of widely varying ambient pressures P2. The erosion tests were systematically carried out by using aluminum-alloy specimens, and the aspects of the cavitation clouds around the jets were simultaneously observed. Clearly, the pressure P2 drastically affects the aspects of cavitation and its occurrence region. With increasing P2, the length of the cavitating jet decreases, so that the standoff distance, where the most severe erosion appears, shifs upstream. In short, the intensity and the distribution of the impulsive pressures induced by the cavitating jets also drastically change with P2. The impinging erosion characteristics are also sensitive to the standoff distance and the injection time.

In order to clarify structures of high-speed submerged water jets and to predict the jet working capacity, we systematically measured the cavitation noise around the jets using a hydrophone, for several nozzles. The power spectra and the rms values of the cavitation noise are carefully analyzed. Clearly, cavitation noise created by the jets is related to the behavior of cavitation bubble clouds, and the cavitation noise characteristics are markedly changed with the injection pressure and nozzle geometry.

High-Speed submerged water jets have two characteristic peaks with respect to the stand-off distance, in the same manner as high-speed water jets in air. In order to clarify the behavior of the impulsive pressures at these two peaks, we systematically carried out erosion tests using an aluminum-alloy specimen in a wide range of the injection pressure P1 from 10 to 70 MPa, and microscopically observed the eroded surfaces by SEM. We find that the mass loss rate Δm^^· at the 1st peak is decidedly due to the water-column impact, so that it is very sensitive to the injection pressure P1, while Δm^^· at the 2nd peak is due to the shock pressures of cavitation bubble collapse, so that it is not necessarily sensitive to P1. We also indicate that the form and the speed of erosion change markedly with the differences in the jet structure with the stand-off distance x/D.

In this paper, we attempt to clarify the jet structure and the behavior of severely erosive cavitation clouds around a high-speed submerged water jet, using a high-speed movie camera with a framing rate of ten thousand frames. The effects of the injection pressure and the nozzle geometry on cavitation are also investigated. The experiments are performed with both a free jet and an impinging jet. It is clearly found that the cavitation clouds are periodically discharged. The cavitation clouds are also closely related to downstream instability and to the impinging erosion.

In order to establish techniques of cutting, drilling, peening and flushing by high-speed submerged water-jets, we systematically observe the eroded surface on aluminum-alloy test-pieces in relation to the aspects of cavitating jets, especially for the impinging jet through lucite-test-pieces, for several types of nozzles. The impulsive pressures induced by the impinging jets are also measured by means of pressure-sensitive film. It is concluded that the erosion characteristics are quite different in two regions differentiated by the standoff distance. Cylindrical, small and deep pits occur in one of the regions, while small lots of plastic deformations occur in the other region. The former is suitable for cutting, while the latter is for peening.

This paper aims to clarify peculiar phenomena arising around a ultra high-speed submerged water jet accompanied by very severe cavitation. By means of a flow visualization technique with a xenon flash light, three kinds of water jets are carefully observed, and the spatial distributions of cavitation-pressure pulses around the jets are also measured with pressure-sensitive film. The effects of the working pressure and the shape of nozzles are clarified systematically. Thus, the characteristics and the structures of ultra high-speed cavitating submerged water jets are clearly shown.

In order to elucidate the atomizing mechanism of Coal Water Mixture (CWM), several experiments on the atomization of CWM will be carried out with twin fluid atomizers. The effect of coal sizes on the formation of CWM spray droplets is investigated in detail. It is also found that large droplets of CWM spray consist of numerous fine coal particles and that large coal particles do not form "Nucli" of large spray droplets. From these experimental results, the breakup mechanism of CWM uneer high shear stress by gas-blasting at the exitport of the atomizer is proposed.