复杂微通道内非混相驱替过程的格子Boltzmann方法

Lattice Boltzmann simulation of immiscible displacement in the complex micro-channel

本文采用改进的基于伪势模型的格子Boltzmann方法研究复杂微通道内的非混相驱替问题.这种方法克服了原始伪势模型中计算结果对网格步长的依赖.首先用Laplace定律验证模型的正确性,然后用该方法研究壁面润湿性、粗糙结构、黏性比以及距离对非混相驱替过程的影响.模拟结果表明:与壁面粗糙结构和黏性比相比,壁面润湿性的影响是决定性的因素.随着接触角的增加,驱替效率增加,当接触角大于某一值后,驱替效率不再变化;随着黏性比的增加,驱替效率增加;而壁面粗糙性对驱替过程的影响较复杂,只有凸起半圆的半径在一定范围内增加时,驱替效率增加;距离较小时将促进驱替过程.

The immiscible displacement process in micro-channel, which widely existes in daily life and industrial production, is an important research subject. This subject is a typical contact line problem involving complicated fluid-fluid interactions and fluid-solid interactions which have attracted the interest of many scholars. Although the immiscible displacement in micro-channels has been studied by some researches, the problem is still not fully understood because the mechanism of the immiscible displacement is very complex. In order to further explain the physical mechanism of immiscible displacement process in micro-channels, detailed numerical simulations are carried out in a complex micro-channel containing a semicircular cavity and a semicircular by bulge using an improved pseudo-potential lattice Boltzmann method（LBM）. This model overcomes the drawback of the dependence of the fluid properties on the grid size, which exists in the original pseudo-potential LBM. Initially, the cavity is filled with the liquid and the rest of the area is filled with its vapour. The semicircular bulge represents the roughness of the micro-channel. The approach is first validated by the Laplace law. The results show that the numerical results are in good agreement with the theoretical predictions. Then the model is employed to study the immiscible displacement process in the micro-channel. The effects of the surface wettability, the surface roughness, the viscosity ratio between the liquid phase and the gas phase, and the distance between the semicircular cavity and the semicircular bulge are studied. The simulation results show that the influence of the surface wettability on the displacement process is a decisive factor compared with other factors.With the increase of the contact angle, the displacement efficiency increases and the displacement time decreases. When the contact angle is larger than a certain value, all of the liquid can be displaced from the cavity. At that time, the displacement efficiency is equal to 1. The above results are consistent with the theoretical prediction that with the increase of the contact angle, the liquid is easily driven out of the cavity because the adhesion force of the liquid in the cavity decreases. On the other hand, the influence of the surface roughness on the displacement process is more complex.The displacement efficiency increases with the radius of the semicircle bulge increasing in a certain range. When the radius is larger than a certain value, the liquid cannot be ejected from the cavity due to the velocity around the cavity is too small. Furthermore, the liquid cannot be displaced from the cavity at a small viscosity ratio. As the viscosity ratio increases, the displacement efficiency increases and the displacement time decreases. As for the distance between the semicircular bulge and the semicircular cavity, it promotes the displacement process at an early stage. When the distance exceeds a certain value, it has little effect on the displacement process.