Advanced drying theory of capillary porous media from high-performance-computing pore network simulations
Final Report Abstract
The main goal of this project was to address and overcome the shortcomings in the prediction capacity of existing macroscopic continuum models for drying capillary porous media. In this respect, we developed successfully a two-equation (one equation for the liquid phase and one equation for the vapor phase) continuum model of drying that describes realistically the physics of drying porous media at the macro scale. This achievement has been accomplished in two successive steps. In the first step we focused on a simplified situation, upscaling the process of evaporation and vapor diffusion in a partially saturated porous medium, in which the liquid phase is immobile. We successfully formulated and validated a two-equation continuum model that captures important physical effects (such as non-local equilibrium effect) for the limiting case of immobile liquid phase. Interestingly, our formulation of the macroscopic diffusivity suggested that (contrary to previous considerations) the liquid phase acts just as an obstruction to the macroscopic vapor diffusion, similarly to the obstruction by the solid phase. In the next step, we considered the more complex situation of a fully saturated porous medium where the macroscopic capillary transport in the liquid phase is also taken into account. Here, we also addressed the modeling of mass transport at the porous medium surface (which is still an unresolved issue) by presenting correlations for the boundary conditions of the liquid and vapor transport equations. We validated the solution of two-equation continuum model through independent reproduction of phase distributions, the drying kinetics and the non-local equilibrium effect. As planned, we also evaluated the commonly used continuum model with dissolved species transport (DST) based on the classical macroscopic advective-diffusive transport equation using pore network simulations. Our analysis was limited to the first drying period (till the porous medium surface stays wet), since the most likely place of crystallization of the dissolved solute is the surface. We observed that the classical continuum model with DST could not reciprocate the solute concentration profiles in the region near the medium surface. To investigate the underlying reasons for this discrepancy, we performed a detailed analysis based on pore network simulations. Our analysis clearly revealed that the consideration of liquid fragmentation process is imperative to resolve the dynamics of solute transport. This is not possible just by incorporating the sharp saturation variation in the region near the surface known as the edge effect (which appears to be intrinsic to drying capillary porous media). We also worked on the development of continuum models with secondary capillary structures (SCS), i.e. liquid films. Here, we studied the limiting case of hyperdeveloped continuous liquid films by means of pore network simulations. The theoretical continuum model formulations developed in this project can be applied in similar way to study a full drying situation, where the vapor phase is also present inside the porous medium. Overall, we worked on various fundamental aspects of drying capillary porous media and have made a sound progress in bridging the gap between microscopic discrete models of drying and macroscopic continuum models. The newly developed continuum models can be used to simulate mass transport in most diverse scales and at various boundary conditions. They are expected to empower our ability of improving drying processes and the respective products, with a plethora of potential applications in practice.
Publications
-
Advanced modeling of the drying process in porous media. Oral presentation in Jahrestreffen der ProcessNet-Fachgruppe Wärme- und Stoffübertragung, Trocknungstechnik und Mischvorgänge, 18 – 20 March, 2019, Essen, Germany
Ahmad, F., Kharaghani, A., Prat, M., Tsotsas, E.
-
Two-equation continuum model of drying: a limiting case of immobile phase. Interpore, 6 – 10 May, 2019, Valencia, Spain
Ahmad, F., Kharaghani, A., Tsotsas, E., Prat, M.
-
Insights on solute transport in drying porous media gained from discrete and continuum model simulations, Jahrestreffen der ProcessNet-Fachgruppe Wärme- und Stoffübertragung, 12 – 13 March, 2020, Erfurt, Germany
Ahmad, F., Rahimi, A., Kharaghani, A., Tsotsas, E.
-
Non-local equilibrium continuum modeling of partially saturated drying porous media: Comparison with pore network simulations, Chemical Engineering Science, 228, 115957, 2020
Ahmad, F., Talbi, M., Prat, M., Tsotsas, E., Kharaghani, A.
