Numerical Simulation of Gas-Liquid Bubbly Flows

Gas-liquid bubbly flows exist in many engineering processes. However, limitations in understanding prevent the optimal design and operation of multiphase equipment. The bubble size distribution is a key parameter in such flows as it governs the interfacial area and the rate of exchange of mass, momentum and energy between the phases. Evolution of the bubble population is to a large extent driven by the coalescence and breakup of bubbles. Due to the lack of experimental studies of these phenomena, accurate predictions from numerical models are of value in improving understanding, and for use in developing engineering models. The work described furthers our insight of and ability to predict bubbly flows by combining large eddy simulation and Lagrangian bubble tracking. Horizontal and vertical channel flows of water over a range of shear Reynolds numbers and air bubble diameters are considered. Coalescence and breakup are favoured in upflows, with high turbulence levels impacting bubble interaction. Coalescence is dominant at low turbulence levels, and increases with decreasing bubble size, whereas breakup is favoured at high turbulence levels. The breakup of air bubbles, under the flow conditions studied, is almost negligible. The simulations are therefore extended to bubbles of refrigerant R134a, with a considerably lower surface tension than air bubbles, with significant levels of breakup detected at high Reynolds numbers. The investigation is a novel contribution to the literature and provides a comprehensive study of next generation predictive techniques. The model developed can predict microbubble behaviour in turbulent flows up to the level of four-way coupling, where inter-bubble collisions, coalescence and breakup are accounted for. Its application extends existing knowledge of these flows, including the effect of bubbles on the carrier fluid. Overall, the tool developed and the understanding generated are of value to industry in allowing the design of more efficient flow processes.