Evaporation fronts in porous media

Experimental and computational studies have been conducted to model the propagation of evaporating fronts through porous media. The results from the experiments are compared with a numerical model and the results agree qualitatively with the temperature distribution in the vapor and liquid regions obtained from the numerical solution. The condition for which a two-phase zone does not exist due to high heat flux is also examined. Results also confirm earlier analysis of the front stability.;In this thesis an implicit finite difference scheme is utilized to simulate the propagation of an evaporating front in a porous medium saturated with water and undergoing the phase change process. The following three numerical models are developed: (1) a one-equation model that assumes local thermal equilibrium; (2) a two-equation model that utilizes the lumped capacitance assumption to predict the heat transfer to the solid phase; and (3) a two-equation model that utilizes a more precise quasi-analytical approach to more accurately characterize the conduction in the solid phase. Results illustrate that the one-equation model does not yield accurate results when the thermophysical properties characterized by the volume weighted ratio of thermal diffusivities, C, is greater than 10 (within 5% error). Hence a two-equation model is necessary depending on the level of accuracy desired. In addition, consistent with the established "rule of thumb", for Biot number, Biv, is less than 0.1, the traditional two-equation model which makes the lumped capacitance assumption for the solid phase compares well with a two-equation model that more accurately predicts the time dependent diffusion in the solid phase using Duhamel's theorem.;High intensity drying is used to characterize those situations for which the drying medium is sufficiently above the saturation temperature of water to preclude the existence of a two-phase zone. High intensity drying is modeled numerically and the relationship between pressure, the drying conditions and material properties is examined since elevated pressure that can occur during high intensity drying is potentially destructive. A quasi two-dimensional numerical model of high intensity drying with specific application to underground coal gasification is presented. The anisotropy due to permeability of coal is considered and the results illustrate that a decrease in permeability, K (10-14 to 10-12 m2), results in faster front propagation. Front propagation for the same thick coal seam at two different depths indicated that it is faster when the depth increases. It was also found that as the thickness of coal seam decreases the front propagates faster. Decreasing the pressure or increasing the temperature in the cavity results in a faster front propagation. Groundwater contamination can be a potential problem when the pressure and temperature in the cavity are lowered.