Initial Results from the Experimental and Computational Study of Microbubble Generation

Abstract

A novel design of microbubble aeration is proposed to increase dissolved oxygen (DO) levels in water, by reducing the mean bubble diameter distribution down to the 20-50 µm range. Microbubble generators (MBGs) find application in aquaculture farms, where water oxygenation is crucial for sea lives and for disinfection through free radicals. A Venturi MBG is taken as baseline design for comparison with previous literature, to understand the pressure-recovery mechanism responsible for bubble breakup. Based on Venturi performance, a helicoidal body-at three different pitch angles-is added ahead of the throat to create sufficient swirling motion, with the aim of intensifying turbulent pressure fluctuations. Results from a validated Computational Fluid Dynamics (CFD) setup and flow visualisation are compared to assess the accuracy of present experimental results, in terms of MBG performance at different volumetric qualities. Backlight imaging and binary processing are implemented to estimate microbubble distributions. Probability distribution and mean diameter plots show an initial disagreement of experimental data from the literature and present predictions. This is presumably attributed to a possible improvement to be done, in terms of light illumination and the use of a microscopic lens, which can better spot small microbubbles. Furthermore, results from CFD show a reduction in the mean diameter when using a helicoid pitch angle of 20 degrees. Of particular interest is a mean diameter near 25μm at very low volumetric quality. By contrast, the configuration at pitch angle of 30 degrees gives a distribution with mean diameter near 70μm, for same volumetric quality. Although this paper only focuses on the bubble diameter aspect, DO measurements coupled with the analysis of microbubble distributions will reinforce authors assumption that small microbubbles represent the optimal condition to maximise water aeration and to enhance treatment of organic particles.