Multi Mechanism Gas Flow Transmission Model And LBM Numerical Simulation Analysis Of Gas-bearing Coal

Gas-bearing coal pore and fissure structure is complex,and coal seam pore and fissure has a span of small to nanoscale to centimeter scale.Gas is mainly stored in the nanoscale pore.The gas flow in the coal is more complex,with the both linear and nonlinear flow.Therefore,the flow of gas in the coal contains a variety of flow modes.Gas mainly exists in linear viscous flow,slider continuous flow,concentration diffusion and surface diffusion.Accurately revealing the Multi mechanism of gas flow is an indispensable part of the gas flow theory system.It is of great significance to the development and utilization of gas and the occurrence of coal mine safety accidents.Based on the microscale gas flow characteristic experimental platform,the paper studies the microscale pore gas flow under different pore sizes and different gas pressure conditions,and establishes the multi mechanism transmission model by combining seepage mechanics,Langmuir adsorption theory and fluid mechanics.Considering the surface diffusion and gas slip flow transmission,it can accurately describe the transport mechanism of coalbed methane in the micro-nano pore.The visual permeability model is effectively validated by experimental data and the gas flow characteristics is studied at the microscale based on the LBGK-D2Q9 model.The lattice Boltzmann method considers surface diffusion due to the density gradient in the surface boundary conditions of the adsorption layer.Gas adsorption was incorporated into the lattice Boltzmann method by using the BSR boundary conditions on the adsorbed layer surface.The effect of the adsorption layer surface diffusion on the velocity distribution and apparent permeability is discussed using the lattice Boltzmann method.The results show that the multi mechanism model of gas considers the influence of slip viscous flow,concentration diffusion and surface diffusion and applies to gas flow in different stages.Gas pressure and pore size are the main factors affecting gas flow.The gas flow driven by pressure gradient is mainly controlled by slip flow and concentration diffusion.With the increase of the Knudsen number(Kn),the proportion of viscosity flow gradually decreases and concentration diffusion and surface diffusion increase,indicating that the larger the Knudsen number,the more obvious the multi mechanism gas flow characteristics,the adsorption properties are studied by lattice Boltzmann numerical simulation at the pore-scale level.The results directly reflect the pore gas transmission characteristics under different Knudsen numbers and reproduce the microscopic gas flow process.The gas adsorption effect can reduce the permeability of micro nano-scale coalbed methane.Compared with the experimental results,the error of the lattice Boltzmann numerical simulation results is between 7.65% and 27.25%,and the overall error is 13.21%,which can effectively verify the reliability of the Boltzmann numerical method.