Deformation and Breakup of Finite-sized Bubbles in Intense Turbulence

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Johns Hopkins University
From rain droplets in clouds to entrained gas bubbles in oceans, the majority of fluid mechanics problems in nature and industry are turbulent and consist of multiple phases. In such flows, bubbles and droplets experience complex deformation. Though this deformation occurs at small-scale interfaces, it plays important roles in many large-scale processes e.g. the overall heat and mass transfer in two-phase energy systems. To understand the fundamental physics behind the interaction between turbulence and deformable bubbles, simultaneous 3D measurements of both phases are essential. However, obtaining such measurements is a very challenging task. To address this problem, a unique vertical water tunnel (V-ONSET) capable of generating energetic turbulence is designed. V-ONSET is equipped with six high-speed cameras uniformly distributed around its test section to obtain high-resolution images of both bubbles and the turbulent carrier phase simultaneously. To reconstruct the 3D shapes of bubbles, a new algorithm addressing the limited-angle reconstruction problem by using the physical constraint of minimum surface energy is developed. Moreover, to quantify turbulence, tracer particles in the surrounding flow are tracked with an in-house OpenLPT code. Leveraging such unique simultaneous measurements of bubbles and their surrounding turbulent flow, we investigate the mechanisms in turbulence responsible for the deformation and breakup of bubbles. We identify and evaluate two key mechanisms namely, the coarse-grained turbulent strains and the slip velocity between the two phases. Interestingly, two Weber numbers based on these two mechanisms show that in strong turbulence, the rather ignored mechanism of the slip velocity has a comparable magnitude to the other mechanism of turbulent strains. The distributions of these two Weber numbers are modeled based on turbulent flow characteristics. This also helps to estimate bubble breakup probability in turbulence. Furthermore, we investigate the orientation dynamics of bubbles with respect to the aforementioned deformation mechanisms. It elucidates that bubbles exhibit the strongest alignment with the slip velocity direction indicating the dominant role played by the compression induced by the slip velocity. Finally, a Lagrangian model including both deformation mechanisms is proposed to predict bubble deformation and orientation in turbulence.
Turbulence, Multiphase Flow, Bubble Dynamics