Bubbly Flow Physics for Applications in Aerated Hydroturbines and Underwater Transport

Abstract

Bubbly flows occur in a wide variety of situations and are frequently employed in industry and other engineering applications. The current work deals with two such examples of bubbly flows: speed enhancement and controllability of next-generation high–speed underwater vehicles for naval defense applications and environment-friendly power generation through aerating hydroturbines. A complete understanding of the bubbly flow physics is imperative to leverage it for these different applications. These investigations into bubbly flow physics are accomplished by studying the fundamental fluid dynamics of bubbles at different size scales. A large bubble is used to envelop an underwater vehicle causing tremendous reduction in flow resistance while the small bubbles are employed for aeration applications in a hydroturbine. Our experiments have provided critical insights into the design and development of operational strategies and models for these novel technologies. For the dispersed bubbly flow regime, bubble size characteristics in the wake of a ventilated turbine blade is measured using shadow imaging and a newly developed image processing approach. Simultaneous mass transfer measurements in the wake have shown an interrelationship with the bubble size and high speed imaging of the bubbles reveal the physical mechanisms of bubble breakup and coalescence and its effect upon the bubble sizes in the wake. In the supercavity bubble regime, systematic studies are carried out to investigate the supercavity closure mechanisms in detail, and a unified theory is proposed to predict different closure modes. Further insights are provided into the interrelationship between supercavity closure, ventilation demand and gas entrainment behaviors of supercavity. The effect of ocean waves on the stability of supercavity bubbles and its closure are also investigated by replicating the ocean waves in the high speed water tunnel. Specifically, a novel aspect of the current research pertains to the visualization and quantification of the internal flows inside supercavity bubbles and drops.