Molecular Dynamics Studies On Flow Characteristics Of Gas In Micro/Nano-scale Channels

With the development of nanotechnology, micro/nano-electro-mechanical systems have been widely used. In these systems, micro/nano scale fluid flows are usually encountered. With the decrease of characteristic length scale, the flow behaviors deviate from those at macro scale. But up to now, the study in this field is far from enough and further researches are needed. Molecular dynamics method is used in the present study to investigate the gas flow characteristics in nano-scale channels.Firstly, a reflecting particle membrane method is developed to produce a pressure-driven gas flow in nano-scale channel. The pressure ratio, which is defined as the ratio of pressure difference between inlet and outlet to the average pressure along the channel, is found to increase with membrane’s reflecting probability and channel aspect ratio, but be independent of Knudsen number and temperature. An empirical equation is then concluded based on the simulation, which can be used to determine the membrane’s reflecting probability when channel aspect ratio and pressure ratio are given. The pressure-driven and gravitation-driven flows are compared and application scope of reflecting particle membrane method is discussed.The computation requirements in time and memory are usually very large in molecular dynamics simulation because of the large number of solid wall atoms. Based on the periodicity of surface atoms, a virtual wall model is proposed to reduce the computation. By saving the interactions between gas and wall molecules in memory, this method can significantly reduce the computing time, especially for the low density fluid. The results of virtual wall model are found to agree very well with those of real wall model, for Poiseuille flow, Couette flow and rough wall flows.Finally, the molecular dynamics results are compared with those of the Burnett equations. The same flow geometry, pressure ratio and initial gas density are used. The same mass flows in two methods are obtained by adjusting the tangential momentum accommodation coefficient in the Burnett simulation. When the accommodation coefficient is set between0.4 and 0.5, the flow properties, such as velocity, temperature and pressure distributions agree with each other in the bulk region of the channel. But in the near wall region the molecular dynamics simulation shows its advantage, where the wall force field dominates the flow properties.