Effects of the divergent angle on bubble transportation in a rectangular Venturi channel and its performance in producing fine bubbles

Venturi-type bubble generator has a broad application in various fields and its performance depends on its geometrical size to a great extent. In present work, bubble movement, and breakup process in Venturi channels with different divergent angles were recorded by a high speed camera to further investigate the role of the divergent angle in bubble breakup. Three rectangular Venturi channels with the only difference in the divergent angles (7.5 degrees, 10.0 degrees and 12.5 degrees) were tested at the liquid velocity of 0.67-10.0 mis in the throat. Parameters relevant to bubble transportation, including bubble velocity, deceleration, breakup position and size distribution, etc., were obtained by Digital Image Analysis method from the video data. Results showed that bubbles experienced dramatic deceleration in the entrance of the diverging section, where an obvious stagnation region was observed. Strong interaction between the gas and liquid was resulted and gave rise to bubble breakup.The divergent angle had a significant influence on the deceleration process. For a larger divergent angle: (1) it took a shorter distance and time for bubbles to be decelerated to their minimum velocities; (2) more intensified interaction between the gas and liquid was resulted, generating smaller bubbles; (3) a lower liquid flow rate was required to induce bubble breakup. Finally, a modified correlation for predicting average bubble size in Venturi channels was proposed with the consideration of the effect of the divergent angle. (C) 2019 Elsevier Ltd. All rights reserved.