Generation mechanism of micro-bubbles in a pressurized dissolution method

• Published in: Experimental thermal and fluid science Vol. 60; pp. 201 - 207
• Main Authors: Maeda, Yasunari, Hosokawa, Shigeo, Baba, Yuji, Tomiyama, Akio, Ito, Yoshihiro
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01.01.2015
• Subjects: Anemometry; Bubble diameter; Bubble number density; Cavitation; Density; Dissolution; Doppler effect; Liquids; Micro-bubble; Multiphase flow; Nozzles; PDA; Pressurized dissolution method; Bubble number density; Multiphase flow; Micro-bubble; Bubble diameter; Cavitation; Pressurized dissolution method
• ISSN: 0894-1777; 1879-2286
• DOI: 10.1016/j.expthermflusci.2014.09.010

•Micro-bubbles are generated when cavitation occurs in the decompression nozzle.•Mean diameter of micro-bubbles increases with dissolved gas concentration xGD∗.•Bubble number density depends on xGD∗ only when bubble cavitation takes place.•Micro-bubbles are mainly generated by shrink of vapor bubbles formed by cavitation. 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.