Simulation of bubble dynamics in a microchannel using a front-tracking method

By using a front-tracking approach for moving boundaries, whose surface properties are solved in terms of an immersed-boundary method, the dynamics of bubble transport in a microchannel is computationally studied. This methodology allows the simulation of a liquid-gas system with a realistically large density ratio. In accordance with the pressure-driven inlet/outlet condition generally encountered in experiments, a projection method enforcing incompressibility is implemented as the solution scheme. The approach is then applied to two unique problems of bubble dynamics in microchannels. The first is concerned with bubble transport in a channel with sudden contraction. A bubble slug is placed in the microchannel embedded with a pair of side blocks. In the flow driven by pressure difference, the bubble slug would pass through or be stuck by the blocks, depending on the variations of Reynolds number and Weber number. With such variations of key parameters, furthermore, the bubble slug may deform in different manners. In the second part, the ascending dynamics of multiple bubbles is investigated, specifically regarding their interactions. It is found that in the confined space of small scale, the behaviors of bubbles are constrained by the walls, and not much interaction between bubbles is observable particularly when the flow is dominated by an imposed pressure difference. If the channel is sufficiently wide, for a pair of rising bubbles which are lined in a row, substantial interactions between the bubbles and the walls are observed. By changing the dimensionless parameters of rising bubbles and the channel width, variations in bubble shape, rising trajectory, and the wakes behind bubbles are discussed based on such essential mechanisms as the interplay of vortices and nonuniform pressure field. Moreover, a third bubble is inserted at the center and the flow structure is analyzed.