Effects of initial bubble size on geometric and motion characteristics of bubble released in water

Abstract

To seek and describe the influence of bubble size on geometric and motion characteristics of the bubble, six nozzles with different outlet diameters were selected to inject air into water and to produce different bubble sizes. High-speed photography in conjunction with an in-house bubble image processing code was used. During the evolution of the bubble, bubble shape, traveling trajectory and the variation of bubble velocity were obtained. Bubble sizes acquired varied from 0.25 to 8.69 mm. The results show that after the bubble is separated from the nozzle, bubble shape sequentially experiences ellipsoidal shape, hat shape, mushroom shape and eventually the stable ellipsoidal shape. As the bubble size increases, the oscillation of the bubble surface is intensified. At the stabilization stage of bubble motion, bubble trajectories conform approximately to the sinusoidal function. Meanwhile, with the increase in bubble size, the bubble trajectory tends to be straightened and the influence of the horizontal bubble velocity component on the bubble trajectory attenuates. The present results explain the phenomena related to relatively large bubble size, which extends the existing relationship between the bubble terminal velocity and the equivalent bubble diameter.

摘要

为了探究和描述气泡尺寸对气泡的几何形态及运动学特征的影响,使用6 种不同直径的喷嘴采 用向水中通气的方法产生了不同尺寸的气泡,采用高速摄像系统对气泡图像进行采集,并结合专门开 发的软件分析了气泡发展过程中的形状、运动轨迹和速度的变化。实验过程中获得的气泡尺寸范围为 0.25~8.69 mm。结果表明,气泡脱离喷嘴后,气泡形态依次经由椭球形、球帽形、蘑菇形,最终转变 为稳定的椭球形。随着气泡尺寸的增大,气泡表面的波动加剧。在气泡运动的稳定阶段,气泡的轨迹 可近似用正弦曲线来描述。同时,随着气泡尺寸的增大,气泡轨迹有被拉直的趋势,横向分速度对气 泡轨迹的影响减弱。研究结果解释了与大尺寸气泡相关的现象,拓展了已发表的气泡终速度与气泡当 量直径之间的关系。