Numerical Simulation On Miscible Displacement Of Gas Flowing Through Microchannels By Lattice Boltzmann Method

The miscible displacing process of gas flowing through microchannels is widely used in oil extraction,underground storage of carbon dioxide,and aerospace.More and more focus is put on this topic because of the wide range of applications of miscible displacement.The lattice Boltzmann method is a typical method of the mesoscopic methods and is used to study the miscible displacement process in the slip region of fluid flow.There are two main problems in the microchannel flow in the slip region.One is how to connect the relaxation time with the Knudsen number together,the other is how to implement the boundary conditions where the slip velocity can be generated.The single-component gas and two-component gas in the slip region were studied based on the solution of these two problems.In this paper,the model based on the two-fluid model and the combination of the no-slip bounce-back and the free-slip specular reflection boundary scheme are used to study the displacement processes in slip regime.The variety of the air concentration at the outlet,the velocity profiles of the intermediate interface,the displacement time and the effects of different roughness elements on the displacement process are analyzed.Before the miscible displacement simulation,the following work should be solved:(1)The Poiseuille flow is used to verify the selected the combination of the no-slip bounce-back and the free-slip specular reflection boundary scheme.The Knudsen number is introduced into the model through the relaxation time,and the slip velocity,the relative slip length,friction coefficient and the gas flow rate for different Knudsen numbers are showed.(2)The Peclet numbers and the different density ratios between the two fluids during the displacement process were studied separately using the control variable method.The result shows,the larger the Peclet number is,the less the displacement time is,the greater the density ratio is,the less the displacement time is.The displacement of air by xenon and helium is studied based on above work.The initial slip velocity at the boundary related to the process of helium displacing air is greater than that of xenon displacing air because of the higher.No matter in the smooth microchannel or in the rough microchannel the time of xenon displacing air is shorter than that by helium.This isdue to the higher Pelican number and greater density ratio between helium and air.Regardless of whether the displacement fluid is helium or xenon,the presence of roughness elements in microchannel leads to an increase in resistance during the displacement process,so that the displacement time in the rough microchannel is longer than that in the smooth microchannel.With the increase of the high of the roughness elements and the decrease of the distance between roughness elements,the resistance becomes larger and the displacing time becomes longer.