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Download A Fully-Coupled Finite Element Code for Modelling Thermo
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FEM Modelling of Coupled Thermo-Hydro-Mechanical Processes in Porous Geological Media Lanru Jing Royal Institute of Technology, Stockholm, Sweden The presentation describes a new FEM method and code for simulating fully-coupled thermo-hydro-mechanical processes in porous geological media, which may be applicable for design, operation, performance and safety assessments of rock/soil engineering in general and geological radioactive waste repositories in particular. The governing equations are based on the theory of mixtures applied to the multiphysics of porous media. Among the phenomena accounted for are solid-phase deformation, liquid-phase flow, gas flow, heat transport, thermally-induced water flow, phase change of water, and swelling deformation. For three-dimensional problems, eight governing equations are presented to describe the coupled THM processes. Three displacement components, temperature, water pressure, gas pressure, vapor pressure and porosity of teh media are chosen as the eight primary variables. Parameters such as the density, viscosity, thermal expansion coefficient and bulk modulus of the water and gas are expressed in terms of the basic variables. The elastic moduli, permeability and thermal expansion coefficient of the solid are taken to be constant, or to depend on stress, pressure and temperature without requiring fundamental modifications to the code. The relative permeability of both the liquid and vapor phase are functions of the liquid saturation. The eight governing continuum equations are discretized using the Galerkin finite element formulation. This leads to a non-symmetric matrix equation that has many small entries along its diagonal, and is therefore ill-conditioned. An interlaced solution approach was developed to solve the global matrix equation. The code was validated against several classical analytical solutions to problems in poroelasticity and thermoelasticity, including the Mandel-Cryer problem for a porous cylinder. It was applied for simulating a laboratory experiment on compacted MX-80 bentonite (as a buffer material between radioactive waste canisters and the host rock in a geological repository), which was performed by the French Commission of Atomic Energy from 2003 to 2004. Good agreement was found between the numerical predictions and experimental data, and continued development is in progress for more comprehensive constitutive models of rock and fluids, and equation solution efficiencies.