A Study on the Improvement of Submerged Water Jet Performation by the Efficient Use of Cavitation

本研究旨在开发一种替代的水下水射流系统，该系统在中等较低的排放压力下运行，并能够提高通常使用的系统的运行效率和安全性。本文的主要研究成果如下：(1)发展了一种新的实验方法来产生液体中的微气泡并测量其性质。通过旋转多孔圆盘将压缩空气注入水中，可以产生直径为10~40[μm]、数密度为(1~4)×10^5[数/cm~3]的微气泡。(2)研究了水下水射流出口速度为15~20[m/cm2]时，水中空化气泡的产生和发展规律。发现在射流发展的早期阶段产生的气泡的一个显著特征是形成类似于涡环的形状。利用一种新型的浸没水射流系统进行了水中有机物的分解实验，空化水射流通过特殊设计的喷嘴排放。结果表明，在比现有系统低得多的放电压力下，可以实现有机物的分解。(4)提出了一种基于双流体模型的水下水射流计算方法。通过考虑气液界面上液体的表面平均压力和界面上的相变，可以描述空化气泡破裂时局部高压的发生。

The present study has been done to develop an alternative submerged water jet system which is operated at a moderately low level of discharge pressure and is capable to improve the efficiency and safety in operation of customarily used systems. The main results of the study are as follows.(1)Novel experimental methods were developed to generate micro-bubbles in liquid and to measure their properties. The injection of compressed air into water through a rotating multi-hole disk enabled to produce micro-bubbles with their diameter of 10〜40[μm] and number density of (1〜4) x10^5[number/cm^3] which were measured by a newly-developed image processing technique.(2)The generation and development of cavitation bubbles in a starting submerged water jet were investigated at jet exit speeds of 15〜20[m/s]. A prominent feature of bubbles generated at the early stage of jet development was found to form a shape similar to a vortex ring. The average trajectory, size and geometry of the ring-like bubbles were evaluated from photographic data.(3)Experiments on the decomposition of organic compounds in water were made using a new submerged water jet system where the cavitating water jet was discharged through a specially designed nozzle. The result indicated that the decomposition of organic compounds could be achieved under the discharge pressure much lower than that of currently available systems.(4)A new computational method based on the two-fluid model was developed to calculate submerged water jet flows. By taking into account of both a surface average pressure of liquid at the gas-liquid interface and a phase change across the interface, it became possible to describe the occurrence of locally high pressure at the collapse of cavitation bubbles.