Monte Carlo Simulation Of Rarefied Gas Flow Process In Microchannels

In recent years,due to the development requirements of modern high technology,the improvement of processing and control technology and the needs of actual engineering,miniaturization,integration and intelligence have become the trends of modern industry.With the rapid development of miniaturization technology,vacuum technology is increasingly involved in gas flow problems in microchannels.In micro-electromechanical systems(MEMS)and biochip micro-nano flow control,in helium mass spectrometry leak detection and high pressure dynamic sampling technique for mass spectrometry analysis,in the study of the gas diffusion process between the power capacitor and the compression film of the degassing process of the power battery,research on microchannel gas flow process is becoming increasingly important and prominent.In this paper,we use OpenFOAM software and the direct simulation Monte Carlo method to simulate and study the rarefied gas flow process in microchannels.First of all,we verified the applicability of the direct simulation Monte Carlo method in transition flow region and molecular flow region.The results showed that the simulation results are in good agreement with the classical theoretical calculations.It laid a solid foundation for the simulation of subsequent content.For gas flow process in vacuum leaks,pressure distribution,velocity distribution,temperature distribution,flow region distribution of the helium and air in the vacuum leak and the effect of the structure on the reaction time is analyzed.Research shows that the pressure distribution along the vacuum leak changes nonlinearly,gas velocity increase continuously and the temperature has a significant drop in the vacuum exit.Compare with the air,the change of the helium is more significant.As the simulation time increases,the gas flow state in the vacuum leak is converted from contain slip flow and transition flow simultaneously to contain slip flow,transition flow and molecular flow simultaneously.As the time goes,the proportion of transition flow is declining,the proportion of molecular flow is rising,and the main gas flow state in the vacuum leak is the transition flow.Moreover,with the same theoretical leak rate,the reaction time increases remarkably with the increase of the leak diameter.For gas flow process in high vacuum section in mass spectrometry dynamic throttling sampling device,the pressure distribution,velocity distribution,temperature distribution and mass segregation of nitrogen and hydrogen in microchannels with different aspect ratio and inlet pressure ratio are discussed.The results show that the pressure distribution along the channel changes linearly,axial pressure radial distribution remains constant,the aspect ratio affects pressure change rate and the pressure variation,gas pressure ratio at the inlet has almost no effect on pressure distribution.Velocity along the channel increase continuously,the axial velocity radial distribution is parabolically distributed and the aspect ratio affects velocity change rate.The greater the proportion of hydrogen,the higher the corresponding velocity.The temperature along the channel has a significant drop at the exit,the smaller the aspect ratio,the larger the proportion of hydrogen,the more significant the temperature decrease.The larger the aspect ratio and the proportion of hydrogen,the mass segregation is more notable.For the gas migration process in the slit channel,a vacuum glazing was taken as an example for simulation.The gas flow characteristics of vacuum glazing pumping through two methods in molecular flow region was studied.One is pumping through the exhaust port and another is place unsealed vacuum glazing in a vacuum chamber for pumping.Research shows that when pumping through the exhaust port,the time to achieve stable is short,the steady pumping speed is low and the pressure variation is small,pressure distribution along the glazing is approximately linear,axial pressure radial distribution remains constant,velocity and temperature along the glazing are basically stable,the larger the vacuum glazing size,the smaller the pressure change,and the vacuum glazing size has no effect on the speed and temperature along the vacuum glazing.When pumping through the vacuum chamber,the time to achieve stable is long and steady pumping speed is large,a high degree of vacuum can be achieved.Pressure distribution along the glazing changes nonlinearly,axial pressure radial distribution remains constant,the larger the vacuum glazing size,the smaller the pressure change.Velocity increase continuously along the glazing,the larger the vacuum glazing size,the higher the stabilization velocity and the smaller the velocity increment.Temperature decrease at the end of the vacuum glazing,the larger the size,the more pronounced the temperature drop at the end of the vacuum glazing.Through the Monte Carlo simulation of the rarefied gas flow in microchannels,we can study the gas flow characteristics in microchannels at the molecular level,analyze the physical mechanism and gas motion law in microchannels from a microscopic point of view.It can help people to understand complex flow phenomena,provide an evidence for people to discover new flow phenomenon and establish new concepts and physical models.Moreover,it is of great significance for the development of gas flow theory and practical engineering applications.