Advanced Vadose Zone Simulations Using TOUGH

The vadose zone can be characterized as a complex subsurface system in which intricate physical and biogeochemical processes occur in response to a variety of natural forcings and human activities. This makes it difficult to describe, understand, and predict the behavior of this specific subsurface system. The TOUGH nonisothermal multiphase flow simulators are well-suited to perform advanced vadose zone studies. The conceptual models underlying the TOUGH simulators are capable of representing features specific to the vadose zone, and of addressing a variety of coupled phenomena. Moreover, the simulators are integrated into software tools that enable advanced data analysis, optimization, and system-level modeling. We discuss fundamental and computational challenges in simulating vadose zone processes, review recent advances in modeling such systems, and demonstrate some capabilities of the TOUGH suite of codes using illustrative examples

Numerical modeling of compositional two-phase reactive transport in porous media with phase change phenomena including an application in nuclear waste disposal

Non-isothermal compositional two-phase flow is considered to be one of the fundamental physical processes in the field of water resources research. The strong non-linearity and discontinuity emerging from phase transition phenomena pose a serious challenge for numerical modeling. Recently, Lauser et al.[1] has proposed a numerical scheme, namely the Nonlinear Complementary Problem (NCP), to handle this strong non-linearity. In this work, the NCP is implemented at both local and global levels of a Finite element algorithm. In the former case, the NCP is integrated into the local thermodynamic equilibrium calculation. While in the latter one, it is formulated as one of the governing equations. The two different formulations have been investigated through several well established benchmarks and analyzed for their efficiency and robustness. In the second part of the thesis, the presented numerical formulations are applied for application and process studies in the context of nuclear waste disposal in Switzerland. Application studies comprehend the coupling between multiphase transport model and complex bio-geo-chemical process to investigate the degradation of concrete material due to two major reactions: carbonation and Aggregate Silica Reaction(ASR). The chemical processes are simplified into a lookup table and cast into the transport model via source and sink term. The efficiency and robustness of the look-up table are further compared with a fully reactive transport model

Advective-diffusive gaseous transport in porous media: the molecular diffusion regime

1993 Spring.Includes bibliographical references.Traditional mathematical models for advective-diffusive transport in porous media fail to represent important physical processes when fluid density depends on composition. Such is the case for gas mixtures comprised of species with differing molecular masses, such as found in the vadose zone near chlorinated hydrocarbon sources. To address problems of this nature, a more general advection-diffusion (A-D) model is presented, which is valid for porous media with permeabilities exceeding 10-10 cm2 (where Klinkenberg and Knudsen effects are negligible). The new mathematical model is derived by thermodynamic means, based on identifying the meaning of Darcy's advective reference velocity in terms of a weighted average of species drift velocities~ The resulting model has no additional parameters, and introduces no additional complexity or nonlinearity when compared to the traditional A-D model most commonly used in hydrology and environmental science. Because the form of traditional A-D models is retained, the new formulations fit readily into existing numerical simulators for the solution of subsurface transport problems. The new model is equivalent to the Dusty-Gas Model of Mason et al. (1967) for cases where the molecular diffusion regime prevails and pressure, temperature, and forced diffusion are negligible. Further support of the model is provided by hydrodynamic analysis, accounting for the diffusive-slip flux identified by Kramers and Kistemaker (1943). The new model is analytically compared to two existing A-D models, one from the hydrology literature, where Darcy's law is assumed to yield a mass-average velocity, and one from the chemical engineering literature, where Darcy's law is assumed to yield a mole-average velocity. Significant differences are shown to exist between the three transport models. The new model is shown to match closely with the experimental data of Evans et al. (1961a), while the existing A-D models are shown to fail in this regard

Pore-scale study of the multiphase methane hydrate dissociation dynamics and mechanisms in the sediment

Methane hydrate is a promising energy resource, but the hydrate development still faces technical difficulties due to the complicated multiple physicochemical and thermal processes during the multiphase hydrate dissociation in the sediment. In this study, a pore-scale numerical model based on the lattice Boltzmann method was proposed to simulate methane hydrate dissociation considering multiphase flow, heat and species transport, heterogeneous reaction and hydrate structure evolution. The single-phase hydrate dissociation was firstly simulated to identify the convection and diffusion transport-limited regimes according to the Péclet number. Effects of the connate water saturation and the Péclet number on the multiphase hydrate dissociation were then investigated to understand the varying dissociation dynamics and dissociation mechanisms. The competitive mass-transfer-limitation and heat-transfer-limitation were quantified to elucidate the interplay between multiphase mass transport and heat transport on the hydrate recovery efficiency. The regime diagram of the methane hydrate dissociation was mapped to exhibit five dissociation regimes according to the connate water saturation and the Péclet number. Empirical correction of the permeability and the specific surface area was obtained to improve the REV (Representative Element Volume)-scaled modeling accuracy of the volume-averaged transport and geometric properties with three typical dissociation patterns. The insights from the pore-scale multiphase dissociation studies can enlighten the accurate REV-scaled simulation with the addressed non-negligible physics

Distributed and Lumped Parameter Models for Fuel Cells

The chapter presents a review of modeling techniques for three types of fuel cells that are gaining industrial importance, namely, polymer electrolyte membrane (PEMFC), direct methanol (DMFC), and solid oxide (SOFC) fuel cells (FCs). The models presented are both multidimensional, suitable for investigating distributions, gradients, and inhomogeneities inside the cells, and zero-dimensional, which allows for fast analyses of overall performance and can be easily interfaced with or embedded in other numerical tools, for example, for studying the interaction with static converters needed to control the electric power flow. Thermal dependence is considered in all models. Some special numerical approaches are presented, which allow facing specific problems. An example is the Proper Generalized Decomposition (PDG) that allows overcoming the challenges arising from the extreme aspect ratio of the thin electrolyte separating anode and cathode. The use of numerical modeling as part of identification techniques, particularly by means of stochastic optimization approaches, for extracting the material parameters from multiple in situ measurements is also discussed and examples are given. Merits and demerits of the different models are discussed

Water flow and transport of chloride in unsaturated concrete

Concrete structures deteriorate in their operating environment under the combined action of harsh environmental conditions and external loading. Although the applied load can lead to a certain degradation of the structure, the main long-term deterioration mechanism involves moisture movement and the transport of chlorides within concrete. In order to build durable and reliable structures, it is necessary to be able to accurately predict the movement of moisture and chlorides within concrete. In the case of unsaturated concrete, the transport of chloride ions is integrally associated with prediction of moisture fluxes in concrete. Even the diffusion of chloride ions depends on the degree of saturation of the concrete since concrete must have a continuous liquid phase for diffusion to occur. Therefore, simple diffusion theory, used in the current literature, is not sufficient to predict the diffusion of chloride ions in the case of unsaturated concrete. Most diffusion models described in the current published literature are applicable to concrete structures that are permanently wet and invariably underestimate the amount of chlorides penetrating the concrete of structures subjected to wetting and drying cycles. The research presented in this thesis reviews current knowledge, mathematical models and test methods pertinent to the movement of moisture and transport of chloride ions in unsaturated concrete. A laboratory testing program was established to characterize the material properties of concrete mixes with water-cement ratios 0.4, 0.5 and 0.6. Concrete was characterized by its saturated hydraulic conductivity, moisture retention function and dependence of diffusion coefficient on degree of saturation. A geotechnical centrifuge was used to determine the saturated hydraulic conductivity of the concrete samples. Values of the saturated hydraulic conductivity of the samples were in the range of 10-11-10-12 m/s. The moisture retention function of concrete samples was determined using a vapour equilibrium technique. The experimental moisture retention data was used to determine van Genuchten parameters for each of the concrete mixtures and subsequently used to determine the capillary pressure-degree of saturation relationship and relative permeability-degree of saturation relationship as a ``closed- form`` analytical expression. An electrical resistivity technique was used to determine the dependence of the chloride diffusion coefficient on the degree of saturation of the concrete. The result was compared with the Millington and Quirk model. Most of the experimental results should be useful to researchers in the field, as well as the engineering community at large, considering that they are rarely found in the concrete literature. Simulations were made to determine the influence of various parameters measured during experiment on movement of moisture and transport of chloride ions in unsaturated concrete using TOUGH2, a multiphase, multicomponent, model that simulates coupled heat, moisture and salt transport in saturated and unsaturated rocks

Towards predicting DNAPL source zone formation to improve plume assessment: Using robust laboratory and numerical experiments to evaluate the relevance of retention curve characteristics

© 2020 The Authors We conducted multiple laboratory trials in a robust and repeatable experimental layout to study dense non-aqueous phase liquid (DNAPL) source zone formation. We extended an image processing and analysis framework to derive DNAPL saturation distributions from reflective optical imaging data, with volume balance deviations \u3c 5.07%. We used a multiphase flow model to simulate source zone formation in a Monte Carlo approach, where the parameter space was defined by the variation of retention curve parameters. Integral and geometric measures were used to characterize the source zones and implemented into a multi-criteria objective function. The latter showed good agreement between observation data and simulation results for effective DNAPL saturation values \u3e 0.04, especially for early stages of DNAPL migration. The common hypothesis that parameters defining the DNAPL-water retention curves are constant over time was not confirmed. Once DNAPL pooling started, the optimal fit in the parameter space was significantly different compared to the earlier DNAPL migration stages. We suspect more complex processes (e.g., capillary hysteresis, adsorption) to become relevant during pool formation. Our results reveal deficits in the grayscale-DNAPL saturation relationship definition and laboratory estimation of DNAPL-water retention curve parameters to overcome current limitations to describe DNAPL source zone formation