微纳尺度气体流动的速度滑移及流动特性研究

Micro-scale gaseous flow behaviors differ greatly from those at macro-scale. With the decrease of characteristic length scale, the surface to volume ratio increases rapidly and the wall plays a more and more important role. The velocity slip models, which are usually developed from rarefied gas dynamics, work well in rarefied slip or early transition regimes. But these models are not appropriate in micro/nano-scale rarefied gaseous flows. With the increase of Knudsen number, the flow deviates from equilibrium. The Navier-Stokes equations, which are derived basing on linear constitute relationship between stress and strain, are no longer valid. In present project, the molecular dynamics method is adopted to investigate the gas flow characteristics near the boundary and the high-order Burnett equations are used to study the flow characteristics of micro/nano-scale gas flows. Numerical simulation and experimental study will be carried out and we aim to achieve the following targets. First, the gaseous flow characteristics near the wall will be investigated. The effects of interaction strength between gas-wall molecules, the wall roughness, the rarefied effect and the system temperature will be investigated and the tangential moment accommodation coefficient will be obtained. Second, the gaseous velocity slip characteristics on the wall will be investigated by taking into consideration the effects of normal velocity gradient, the tangential temperature gradient and the external force field. The general velocity slip boundary conditions will then be obtained. Third, the stability of the Burnett equations will be studied and the Burnett equations together with the new derived velocity slip boundary conditions will then be applied to study the flow characteristics of micro/nano-scale gas flows. The flow mechanism of rarefied micro/nano-scale gas flows can be revealed.

微纳尺度下的气体流动常常出现明显不同于常规尺度下的流动现象。随着特征尺度的减小，与壁面相关的物理量作用增强，现有根据稀薄气体运动推导出的速度滑移模型，不能体现出微纳尺度流动中壁面效应的影响；随着努森数的增大，基于应力和应变线性本构关系的Navier-Stokes方程不再适用。本项目采用分子动力学方法模拟近壁面非平衡区的气体运动特性，采用高阶的Burnett方程研究微纳尺度下的稀薄气体流动，通过数值模拟和实验研究两种手段，达到以下目标：给出近壁面非平衡区气体运动特性，获得固壁上气体的切向动量协调系数；构建法向速度梯度场、切向温度梯度场和外加力场对壁面气体速度滑移影响的模型，给出速度滑移普适表达式；建立Burnett方程结合滑移边界条件模拟微纳尺度气体流动特性的研究方法，揭示各种微纳通道中的气体流动规律。通过本项目研究，完善速度滑移模型，加深对微纳尺度气体流动的认识，为实际应用提供理论依据。

微纳尺度下的气体流动与宏观流动有明显不同。随着特征尺度的减小，壁面对流动的影响逐渐增大，现有根据稀薄气体运动推导得到的速度滑移模型，不能描述微纳尺度流动中壁面效应的影响；随着努森数的增大，基于应力和应变线性本构关系的Navier-Stokes方程不再适用。本项目主要采用分子动力学方法研究了近壁面非平衡区的气体运动特性。提出了粒子反射膜方法，实现了压力驱动的分子动力学模拟，研究了纳米尺度泊肃叶流动和后向台阶流动，分析了微纳米尺度下的流动特性。提出了虚拟壁面方法，简化了模拟时壁面原子与流体分子之间作用力的计算，提高了计算效率。提出了等效粘度的概念，获得了纳米尺度通道中平板库塔和泊肃叶流动的粘度分布规律。分析了法向速度梯度场、切向温度梯度场、外加力场以及这些场的耦合作用对壁面附近气体流动特性的影响，构建了这些场耦合作用下壁面上气体的速度滑移模型。分析了壁面和气体分子间作用强度、固壁和流体温度、壁面粗糙元的形状和尺寸、通道尺寸引起的稀薄效应等因素对近壁面分子运动特性的影响，揭示了近壁面非平衡区稀薄气体运动特性。结合速度滑移模型，完善了二维和三维Burnett方程的通用求解程序，模拟分析了各种微/纳机电系统典型流场中的气体流动，获得微纳尺度下的气体流动特性，揭示了微纳尺度气体流动规律。