Lattice Boltzmann simulations of gas-liquid bubbly flows

Process units employing gas-liquid bubbly flow are ubiquitous in the chemical industry. It is well known that the hydrodynamics play an important role in the performance of these bubble columns. Over the last three decades, the two-fluid model has emerged as a viable tool for simulating the hydrodynamics in such systems. The two-fluid model is obtained by averaging over a volume of bubbly suspension. This averaging erases details of flow behavior at the level of the bubbles, but their consequences appear through terms for which closures are required. Finding closure relations, therefore, is a major research frontier in the field of multiphase flow and is of both fundamental and practical interest.; We present closures for the drag, virtual mass and lift force terms appearing in this two-fluid model for flow of a mixture consisting of uniformly sized gas bubbles dispersed in a liquid. These closures were deduced through computational experiments performed using a novel implicit formulation of the lattice Boltzmann method with a BGK collision model. The closure relations obtained in our study are limited to a regular array of uniformly sized bubbles and were obtained by simulating the rise behavior of a single bubble in a periodic box. The effect of volume fraction on the rise characteristics was probed by changing the size of the box relative to that of the bubble. While spherical bubbles exhibited the expected hindered rise behavior, highly distorted bubbles tended to rise cooperatively. The closure for the drag force, obtained in our study through computational experiments, captures both hindered and cooperative rise.; Simple models for the virtual mass and lift force coefficients, applicable to both spherical and distorted bubbles, was also obtained by fitting simulation results. The virtual mass coefficient for isolated bubbles could be correlated to the aspect ratio of the bubbles. The single-bubble lift coefficient, determined by low-shear computational experiments, varies in a systematic manner with the aspect ratio of the bubbles. It was found that at high shear rates the lift force manifested a noticeable shear rate-dependence and it could even become negative.; Through a linear stability analysis of the uniformly bubbling state, it is demonstrated that the lift force can destabilize a uniformly rising array of highly distorted bubbles and give way to columnar structures.; In summary, the major accomplishments in this dissertation are (i) the development of an implicit formulation of the lattice Boltzmann method; (ii) the development of closure relations for drag, virtual mass and lift forces; and (iii) the discovery of a columnar instability in bubble columns.