HYBRID VOLUME OF FLUID (VOF) AND LAGRANGIAN APPROACH FOR SIMULATING INTERACTIONS BETWEEN DISPERSED BUBBLES AND LARGE INTERFACES IN TWO-PHASE FLOW

In many two-phase flow and heat transfer processes, gas and liquid structures of widely varying physical scales interact. For example, in flow-boiling, mm-scale dispersed bubbles may depart from nucleation sites on walls, interact with bulk liquid flow, and ultimately coalesce with cm-scale gas slugs. It can be prohibitively computationally expensive to directly resolve this range of scales with interface capturing simulation methods, such as the Volume of Fluid (VOF) approach. For such flows, we propose that the dynamics of small bubbles can be modeled with Lagrangian approaches, which treat them as point particles tracked on a coarse mesh that resolves large scale transport. In this study, a hybrid VOF-Lagrangian solver is developed to characterize the coupling between micro-scale bubble transport and macro-scale hydrodynamics in such multiscale two-phase flows. The Lagrangian model tracks the trajectory of individual injected discrete small bubbles, accounting for effects such as buoyancy, pressure, virtual mass, drag, and turbulent dispersion. Once bubbles exceed a threshold packing density or overlap with VOF structures, they are converted to the grid-scale vapor phase. An empirical bubble-lifetime model is implemented to account for the finite coalescence times of bubbles at free surfaces. Contributions of this effort include programmable closure for bubble lifetime at the free surface (before popping/coalescence), a pinning force method for bubbles at the free surface, and Lagrangian-to-VOF transition of bubbles based on packing density. The accuracy of this approach is being assessed using experimental high-speed video data for bubble trajectories at free surface of a bubble column.