The Application Of Discrete Unified Gas Kinetic Scheme In Micro Flows

Owing to a high volume ratio of micro-fluidic devices,the micro flows can cover continuous flow regime,slip flow regime and even transition flow regime,capturing the multi-scale flows characteristics of micro flows is a big problem.At present,most numerical methods are only valuable in specific flow regime,which can not meet the requirements of accuracy and efficiency.In order to solve the problem,based on the original discrete unified gas kinetic scheme(DUGKS),an isothermal DUGKS model with a force term and a new DUGKS model with total energy distribution function are developed and verified.Then,isothermal micro-channel flow and micro-nozzle flow were studied.For the low-speed isothermal flow,the isothermal DUGKS model with a force term is developed,and the isothermal micro-channel flow is analyzed.Based on the model,the long micro-channels gas flow driven by pressure is simulated.The solutions of DUGKS are consistent with those of DSMC and experiment,which validate the codes.Besides,the micro-channel gas flow driven by a constant external force is also simulated.With increase of Knudsen number,gas rarefied effect is enhance,and the slip velocity is increased,and a break point is emerged for mass flow rate.The simulation results are in good agreement with the reference solution,which verifies the model can be qualified for the whole flow regime.For the high temperature and high speed flow,a new DUGKS model with total energy distribution function is developed,and the flow characteristics and propulsion performance of the micro-nozzle are studied.The effects of back pressure and wall viscosity on the flow characteristics are analyzed.When the back pressure is applied,the gas flow will be separated near the wall and returned from the outlet,and the expansion-compression wave will be formed.The inviscid wall will reduce the wave effect.Besides,the propulsion performance of micro-nozzle is also analyzed.Increasing of the inlet pressure and micro-nozzle size can obtain larger thrust and higher specific impulse.Increasing the inlet temperature can get a higher specific impulse,but at the expense of the propulsion efficiency.The results show that the propulsion performance of the micro-nozzle can be characterized by the Knudsen number.A better propulsion performance can be obtained by adjusting the inlet pressure,nozzle size and inlet temperature to obtain smaller Knudsen number.