纳米尺度Poiseuille流动中气体黏度的分子动力学研究

Molecular dynamics simulation of gas viscosity in nano-scale Poiseuille flow

受壁面作用和稀薄效应等的影响,微纳尺度通道内的气体流动有别于宏观流动现象.采用分子动力学方法,研究纳米通道中气体的Poiseuille流动,主要对通道内气体黏度特性进行了分析.利用牛顿粘性定律,定义了气体的当地等效黏度.根据模拟结果,可将纳米通道内气体划分为中心区和近壁区两个部分,中心区气体当地黏度与宏观黏度一致,但是在近壁面区,气体受到壁面原子的作用,气体的当地黏度小于宏观黏度值.研究发现:1)不同的气体密度、流固作用势能以及温度下,通道中心区域的气体当地等效黏度均符合对应温度和压强条件下的气体宏观实测黏度值;2)在纳米尺度气体流动中,气体密度越小,稀薄程度越高,气体偏离热力学平衡态越远,所以壁面对气体等效黏度的影响随密度的减少而增大,壁面影响厚度也随之增大;3)气体黏度的壁面影响厚度在10 nm量级,该厚度不随温度和流固作用势能的变化而变化,但是密度越小,壁面影响厚度越大.

Due to the effect of wall and rarefaction, gas flow characteristics in nano-scale channels are different from those in macro-scale flow. Gas Poiseuille flows in nano-scale channels were investigated by using the molecular dynamics simulation. The equivalent local viscosity was defined based on the Newton law of viscosity. According to simulation results, the gaseous flow in a nano-scale channel can be divided into two regions, the bulk region and the near-wall region. The equivalent viscosity in the bulk region agrees well with macro value. However, the equivalent viscosity decreases rapidly in the near-wall region. The gas equivalent viscosities always agrees well with macro values at different gas densities, temperatures and gas-wall interactions. In nano-scale channel flows, the wall-influence-depth increases with the decrease of gas density because of the rarefaction effect. The wall-influence-depth is at the order of 10 nm. This depth doesn＇t vary with gas temperature and fluid-wall interaction.