纳米通道内气体剪切流动的分子动力学模拟

Molecular dynamics simulations of surface effects on Couette gas flows in nanochannels

采用分子动力学模拟方法研究了表面力场对纳米通道内气体剪切流动的影响规律.结果显示通道内的气体流动分为两个区域:受壁面力场影响的近壁区域和不受壁面力场影响的主流区域.近壁区域内,气体流动特性和气体动力学理论预测差别很大,密度和速度急剧增大并出现峰值,正应力变化剧烈且各向异性,剪切应力在距壁面一个分子直径处出现突变.主流区域的气体流动特性与气体动力学理论预测相符合,该区域内的密度、正应力与剪切应力均为恒定值,速度分布亦符合应力-应变的线性响应关系.不同通道高度及密度下,近壁区域的归一化密度、速度及应力分布一致,表明近壁区域的气体流动特性仅由壁面力场所决定.随着壁面对气体分子势能作用的增强,气体分子在近壁区域的密度和速度随之增大,直至形成吸附层,导致速度滑移消失.通过剪切应力与切向动量适应系数(TMAC)的关系,得到不同壁面势能作用下的TMAC值,结果表明壁面对气体分子的势能作用越强,气体分子越容易在壁面发生漫反射.

Molecular dynamics method is used to investigate gas flows in nanoscale channels. A set of Couette gas flows with the same Knudsen number but different channel heights and densities is simulated to study the dimensional effects on dynamically similar flow conditions. Results show that the gas flow in the channels is divided into two regions： near wall region affected by a wall force field and bulk flow region affected by no wall force field. The flow characteristics in the bulk flow region are in good accordance with the kinetic theory predictions, which are characterized by constant density,normal stress, shear stress and linear velocity distribution while within the near wall region, the velocity, density, normal stress and shear stress distributions exhibit deviations from the kinetic theory predictions. The density and velocity sharply increase, accompanied with a single peak appearing. The normal stress which is dominated by the surface virial is anisotropic and changes drastically. Shear stress value is constant in bulk flow region and part of the near wall region,while the surface virial induces variation at a place about one atom diameter far from the wall. In the near wall region, the normalized density, velocity and stress tensor are constant under different channel heights and densities, which indicates that the gas flow characteristics in this area are determined by the wall force field. Besides, the tangential momentum accommodation coefficient（TMAC） values for different cases can be obtained through the relationship between TAMC and shear stress. It is found that under the same Knudsen number, the TMAC remains constant no matter what the height and density are. Furthermore, another set of Couette gas flows with different gas-surface potential strength ratios but the same channel height and density is simulated to study the gas-surface interaction effects on nanoscale gas flow.The results show that the gas density and velocity in the near wall region increase with increasing potential strength ratio between wall atoms and gas molecules. Large potential strength ratio cases（C≥3.0） result in velocity sticking on the surface, which is induced by the gas molecule accumulation and surface adsorption. Using the same approach, the TMAC values for various potential strength ratios are calculated, varying from 0.63 to 0.96 for different cases（C = 0.5-4.0）,which indicates that the stronger the potential energy acting on the gas molecules, the more easily the gas molecules generate the diffuse reflection on the walls