| In the past few decades,China’s rapid economic and technological development has produced serious soil and groundwater pollution problems in the process of rapid industrialization.For example,chlorinated hydrocarbons derived from the chemical industry for dry cleaning,and petroleum-like substances leaked during mining,transportation,storage and use,are important sources of soil and water environmental pollution.Such contaminants are called non-aqueous phase liquids(NAPLs)contaminants due to their minimal solubility in water.The transport of non-aqueous fluids in soil media belongs to the seepage problem of multiphase fluids in porous media.Mainly due to the interfacial tension dominated by the capillary pressure and gravity drive,its flow has a strong nonlinear characteristics.The corresponding numerical solution method is at the forefront of international scholarship and is a problem worthy of breakthroughs and in-depth study.In this paper,the law of multiphase fluid migration under the action of capillary force and gravity in porous media is revealed.Starting from the problems of groundwater seepage and oil-water two-phase seepage,a system that can describe NAPLs pollution in soil saturated and unsaturated zones is finally established.The three-phase fluid mathematical model of water,NAPLs,and air transported by the material is presented,and the corresponding finite element numerical discrete solution method is given,and the three-dimensional finite element program Mig-3PF3 D is written in Fortran language.The following contents in the three-phase system of water,NAPLs and air are comprehensively analyzed: interfacial tension and capillary pressure;the relationship between the difference in fluid wettability and its distribution in pores;the complex relationship between relative permeability coefficient-saturation-capillary pressure.Linear relationships and interaction mechanisms between fluids.The Fortran program called Mig-3PF3 D established according to the numerical model given in this paper can complete the three-dimensional simulation of the complete migration process of non-aqueous pollutants infiltrating into the ground through the soil vadose zone and saturated zone.It has quantitative guiding significance for the evaluation of soil water pollution level and the design of purification treatment.This paper mainly studies the following contents:(1)Combined with the distribution law of different wettability fluids in porous media,the distribution of water,NAPL and air in pores in porous media is given.The role of capillary pressure dominated by interfacial tension in NAPL transport in soil vadose zone is introduced in detail.Starting from the mathematical model of groundwater seepage,a system of partial differential equations governing the three-phase flow of water,NAPL and air in porous media is deduced.The numerical solution difficulties encountered in the calculation process of the single-type equation due to the strong nonlinearity of the S-p curve are discussed,and the mixed-type governing equation is used to solve the problem well.The governing equations are discretized in space using the Galerkin finite element method,and a backward discretization scheme is used in time.Finally,a numerical model that can completely describe the infiltration of NAPL into the ground and flow through the soil vadose zone and saturation zone is established.(2)According to the finite element discrete format deduced in this paper,the Fortran high-level language is used to write a three-dimensional numerical simulation program Mig-3PF3D(Migration-three phase flow 3D),which can calculate the three-phase flow in porous media.The establishment process of the numerical simulation program is given in detail,and the correctness of the self-developed program is verified using the finite element calculation program NAPL simulator developed by the US Environmental Protection Agency.Finally,the typical LNAPL seepage problem and DNAPL seepage problem were simulated using the self-developed program. |
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