An experimental study of gas nuclei-assisted hydrodynamic cavitation for aquaculture water treatment

We present an experimental study on hydrodynamic cavitation generated by accelerating liquid through a series of constrictions in the presence of gas bubbles and explore its possible applications in water treatment with particular example in aquaculture industry. The formation of intense cavitation bubbles is visualized using a high-speed photography. The cavitation is initiated when a gas bubble moves towards a narrow cylindrical constriction where it accelerates, expands and then splits into smaller bubbles owing to the sharp pressure gradient of the liquid flow inside the constriction section. As the bubbles emerge downstream from the constrictions, they are exposed to a higher pressure region and collapse violently forming a cloud of bubbles. Smaller and more dispersed bubbles are produced by further passing the bubbles through a second series of constrictions. By introducing gas bubbles that serve as cavitation nuclei prior to the constriction, it is unnecessary to force the liquid flow below its vapor pressure to produce intense cavitation, thus enhancing the cavitation activities. We also present experimental evidences of a significant reduction of gram-negative Escherichia coli concentration after exposing them to the cavitation bubbles. Yet, the cavitation bubbles are found to be not sufficiently strong to lyse endospores Bacillus subtilis that are widely used in aquacultures.