Generation mechanism of micro-bubbles in a pressurized dissolution method

Micro-bubbles are in use in many industrial fields such as water treatment, purification of lake water, chemical engineering, washing processes and housing equipment by virtues of their large interfacial area concentration and long residence time in liquid. A pressurized dissolution method based on decompression of liquid with dissolved gas is one of promising methods for generating fine micro-bubbles at high number density. Since the mechanism of micro-bubble generation is not clarified yet, design and improvement of micro-bubble generators are based on trial and error. In this study, effects of liquid volume flux at a decompression nozzle and dissolved gas concentration in the upstream region of the nozzle on diameter and number density of generated micro-bubbles are examined to understand generation mechanism of micro-bubbles in a pressurized dissolution method. The diameter and the number density of micro-bubbles are measured by using phase Doppler anemometry (PDA) in the downstream region of the nozzle, and the flow patterns in the nozzle are visualized by using a high-speed camera. The experimental results show that diameter and number density of generated micro-bubbles depend on cavitation pattern at the nozzle, and that cavitation bubbles containing not only vapor but also non-condensable gas become micro-bubbles due to their shrink caused by condensation of the vapor in the downstream region of the nozzle. (C) 2014 Elsevier Inc. All rights reserved.

微气泡由于其较大的界面面积浓度以及在液体中较长的停留时间，被应用于许多工业领域，如水处理、湖水净化、化学工程、洗涤过程以及家居设备等。基于溶解有气体的液体减压的加压溶解法是一种在高数量密度下产生微小气泡的有前景的方法之一。由于微气泡产生的机理尚未明确，微气泡发生器的设计和改进是基于反复试验的。在这项研究中，考察了减压喷嘴处的液体体积流量以及喷嘴上游区域的溶解气体浓度对所产生微气泡的直径和数量密度的影响，以了解加压溶解法中微气泡的产生机理。通过使用相位多普勒测速仪（PDA）在喷嘴下游区域测量微气泡的直径和数量密度，并使用高速摄像机对喷嘴内的流型进行可视化。实验结果表明，所产生微气泡的直径和数量密度取决于喷嘴处的空化模式，并且由于喷嘴下游区域蒸汽的凝结导致收缩，不仅包含蒸汽而且包含不凝性气体的空化气泡会变成微气泡。（C）2014爱思唯尔公司。保留所有权利。