Study On Solute Transport And Heat Transfer Process In Porous Media By Pore-scale SPH Simulation

As an important topic for environmental geotechnical engineering,study on solute transport and heat transfer in porous media is of significance.Traditionally,corresponding researches are carried out on the representative elementary volum(REV)scale.For REV scale,pore structure,the most essential characteristics for porous media,is ignored,so there are limitations.According to the viewpoint of pore scale,the macroscopic properties of the medium are related to the pore structure,composition of the medium and the physical processes occurring in pore space.Thus,the physical essence of solute transport and heat conduction in porous medium can be understood more clearly by pore scale.In this work,a pore-scale simulation experiment method is developed to reveal the relationship between the pore structure characteristics of porous media and the macroscopic solute transport and heat transfer characteristics,based on smoothed particle hydrodynamics(SPH)method.The process of solute transport and heat transfer in porous media mainly involves mechanical dispersion,solute diffusion and heat conduction.The controlling factors are dispersivity,tortuosity and effective thermal conductivity,respectively.This work focuses on the influence of changes in pore structure and composition of porous media on the above three parameters,and studies the variations of dispersivity,tortuosity and effective thermal conductivity with other characteristic parameters of pore structure,and further establishes some macro mathematical models for engineering application based on the laws of parameters obtained by pore-scale simulation experiments.The main contents include:(1)To determine the dispersivity,the tortuosity and the effective thermal conductivity value,three kinds of simulation experiments are designed,i.e.,soil column penetration test,unsteady solute diffusion experiment and steady-state heat conduction experiment.Navier-Stokes equations are used to describe pore water movement,and the linear homogeneous second-order differential equation is used to describe solute diffusion and heat conduction process.SPH method is employed to solve these equations.The accuracy of the simulation experiment is verified by comparing the simulation results with the analytical solutions and other numerical solutions.Simulation experiments are based on digital model of porous media.3D modeling methods are proposed to reconstruct the digital models,and algorithms considering the wetting and freezing processes of porous media are proposed.(2)Dispersivity,tortuosity and effective thermal conductivity are closely related to the characteristics parameter of pore structure for porous media.Through numerical experiments,the experimental conditions and the characteristics of the medium can be precisely controlled.This paper gives full play to the advantage.Based on a large number of simulation results,the correlativity between dispersivity,tortuosity,effective thermal conductivity and other characteristic parameters of pore structure are discussed.Specifically,the correlation between dispersivity and the velocity difference,the tortuous path length difference,porosity tortuosity,the correlation between diffusive tortuosity and particle shape,dimension,porosity and specific surface area,and the correlation between effective thermal conductivity and porosity,saturation and freezing rate,are investigated.(3)Based on the rules concluded by the simulation experiments,corresponding algorithms and models are proposed or improved.To describe the convective dispersion process in fractured porous media,a simplified analytical solution for the two-region model is proposed.In order to establish the relationship among the tortuosity,the inclination and particle size for lamellar media,a geometric tortuosity model based on the probability distribution of the tortuous path is proposed.In addition,in order to describe the change of the thermal conductivity for unsaturated porous media during the humidification and freezing process,a thermal conductivity model suitable for frozen soil is established,by introducing the relationship between the thermal conductivity and the freezing rate to the heat conductivity model of normal temperature.It is applied to the water-heat coupling transfer problem for frozen soil.