Development and application of a three-dimensional finite element model of subsurface flow, heat transfer, and reactive chemical transport.

摘要

This thesis presents a three-dimensional finite-element numerical model designed to simulate chemical transport in subsurface systems with temperature effect taken into account. The hydrological environment to which the model can be applied to is a heterogeneous, anisotropic, saturated-unsaturated subsurface medium under either transient or steady flow conditions. The temperature within the system of interest can be both time- and location-dependent. For steady-state simulations, strong coupling among subsurface flow, chemical transport, and heat transfer is used in the model. For transient simulations, weak coupling is used, but a density effect is still considered in computations. The model employs chemical equilibrium to describe the relationship among chemicals. The chemical reactions included in the model are aqueous complexation, multi-site adsorption/desorption, multi-site ion-exchange, precipitation/dissolution, redox, and acid-base reactions. The element used in the model can be a hexahedral, a triangular prism, or a tetrahedral element. Two approaches are presented for the chemical transport module in this thesis. The first approach uses the pore velocity and dispersion coefficient to handle advection and dispersion, respectively, for aqueous components, whereas the second approach employs the retarded pore velocity and the retarded dispersion coefficient. Both approaches are designed to solve the problems with dominating precipitated species involved. The governing equations of subsurface flow, chemical transport, chemical equilibrium, and heat transfer are stated and/or derived. The numerical approaches with the finite element method to solve the governing equations are described. Twenty five examples have been used to verify the model. Four applications, including a three-dimensional subsurface flow example, a three-dimensional reactive chemical transport example, a three-dimensional heat transfer example, and a three-dimensional coupled flow-transport-transfer example, are presented to demonstrate the capability of the model. A calibration exercise is also included. The modeling experience from this study is summarized.