Lattice Boltzmann simulations of rising bubble driven by buoyancy in a complex microchannel

Abstract

The movement of bubbles in the viscous fluid is a typical process in many industrial applications, such as in evaporators of refrigeration cycles, petroleum refining, boiling process, steam bubble rising in boiler tubes and heat exchangers, etc. It is an important research problem in engineering and physics. Although this kind of problem has been extensively studied, their flow details are largely unknown due to the complexity of the interface dynamics, which hinders the understanding of the physical mechanism. In order to further study underlying physics of the issue, a gas bubble rising under buoyancy in a complex micro-channel is investigated by using a gas-liquid two-phase flow lattice Boltzmann method. Initially, the model as well as a classical problem of bubble rising in a smooth vertical microchannel is tested by Laplace law. Then it is then applied to the study of a bubble rising in a complex micro-channel. Specially, the dynamic behaviors of the bubble deformation, breaking up, coalescence, and the following movement in the micro-channel are presented. The rising velocity, terminal velocity and residual mass of the bubble under the influence of micro-channel surface wettability, buoyancy force, obstacle size and the initial position of bubble are examined. The simulation results show that the surface wettability of the obstacle has a significant influence on the bubble motion. For smaller values of the contact angle, the whole bubble passes through the channel with obstacles successfully. For higher values of contact angle, the bubble is attracted to the obstacle surface of the micro-channel in the movement process. In this case, an appreciable deformation of the bubble is observed. After detachment, part of the bubble is attached by the obstacle surface, so only the rest of the bubble can go through the micro-channel, which leads the the bubble residual mass to decrease. Correspondingly, the rising velocity and terminal velocity of the bubble decrease with the wettability of the micro-channel obstacle increasing. On the other hand, with the increase of buoyancy force the detachment and coalescence phenomenon happen easily, and the bubble residual mass and terminal velocity increase logarithmically. Furthermore, as the radius of the obstacle structure increases, the bubble clings more tightly to the obstacle surface when it rises in the micro-channel. And the bubble residual mass decreases first slowly and then rapidly, while the bubble terminal velocity approximately decreases linearly. Finally, the numerical results also show that when the bubble is located at the sidewall initially, the variation trend of bubble rising velocity, terminal velocity and residual mass are consistent with that of initial position placed in the middle of the micro-channel, however all of the corresponding values decrease and the bubble deformation is more significant in the rising process.