Rice powder is of interest in Japan, because it can be processed into various foods. However, conventional methods of manufacturing rice-powder generate heat when crushing the rice. National Institute of Technology, Okinawa College (ONCT) has been developing a pressure vessel for producing rice powder using underwater shock waves. The merits of this process are the dry condition and the lack of heating, and a decrease in the manufacturing cost can be expected, using this method. A power supply for generating the shock waves, a pressure vessel for crushing and, a device for the transportation of the rice were developed. The amount of flour that could be milled by the first prototype device was approximately 450g per hour. Then, the pressure vessel was improved to increase the rate of milling. Toward achieving this target, the characteristics of rice processing using shock waves were evaluated. First, a shock wave crushed the rice, which was trapped in transparent acrylic blocks. At the same time, the transmitted shock wave was observed using a high speed camera. From the result of the observation, the speeds of the wave passing through the acrylic blocks and rice was calculated. A linear relationship between the shock wave velocity (Us) and particle velocity (Up) in a material has been empirically found. The propagation of a shock wave in the pressure vessel was calculated by computer simulation. Moreover, a cylindrical pressure vessel with an internal diameter of 150mm was developed. Silicone hoses are installed in the pressure vessel, and the rice passes through in the hoses. The shock wave is generated by the electrical collapse induced by supplying a high voltage to the gap between electrodes in the center of the vessel. The rice is milled into flour by these phenomena, and rice is continuously supplied to the pressure vessel by a classification device and the transport device. The amount of flour milled per hour was verified experimentally.

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