复杂微通道内气泡在浮力作用下上升行为的格子Boltzmann方法模拟

本文采用气-液两相流格子Boltzmann方法模拟了复杂微通道内气泡在浮力作用下的上升过程，主要研究障碍物表面润湿性、浮力大小、障碍物尺寸和气泡初始位置对气泡变形、分裂、合并的动力学行为以及对气泡上升速度、终端速度和气泡剩余质量的运动特性的影响.研究发现，障碍物表面接触角较小时气泡能够完整地通过障碍物通道，随着障碍物表面接触角增加，气泡通过障碍物通道时严重变形，并会发生分裂行为，使得部分气泡黏附在障碍物表面，从而导致气泡到达终端时质量减少.相应地，气泡上升速度以及终端速度也随着微通道表面接触角的增加而减小.另一方面，随着浮力的增加，气泡在上升过程中更容易发生分裂和合并现象，且气泡剩余质量和终端速度随着浮力的增加呈对数形式增加.此外，随着微通道障碍物半径增加，气泡剩余质量首先缓慢减小然后快速减小，而气泡终端速度近似呈线性减小.最后，数值结果还表明当气泡初始位置偏离管道中间时，其上升速度、气泡剩余质量以及气泡终端速度都与初始位置在管道中间时的变化趋势一致，然而对应的数值均减小，且气泡在上升过程中变形更严重.

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

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.