Microbubble coalescence and breakup in turbulent vertical channel flows

Large eddy simulation coupled to Lagrangian bubble tracking is used to study four-way coupled turbulent bubbly flow in channels, including bubble collision, coalescence and breakup. Upward and downward vertical channel flows of water at shear Reynolds numbers of 150 and 2000 are examined, with air bubbles of diameter db= 220 µm dispersed within the flows. Additional simulations are performed for the case of refrigerant R134a at a shear Reynolds number of 1154. The ability of the model to predict coalescence and breakup is evaluated, as well as the impact of the flow condition on the two phenomena. Coalescence and breakup are favoured in upflow conditions, with turbulence found to significantly impact the level of bubble interaction. Coalescence is dominant at low turbulence levels and breakup, which was only detected in the R134a flow, is favoured at high turbulence. The results demonstrate the capabilities of the overall model to predict bubble coalescence and breakup, and its usefulness for predicting flows that are of industrial relevance where interfacial area and bubble size distribution govern interfacial mass, momentum and heat transfer processes.