Pore scale analyses of evaporation from porous media

Evaporation from porous media is a key process in the hydrologic cycles, waste isolation, biological and food processes and other engineering application. Drying rate and patterns in porous media are strongly influenced by transport properties of the medium and the external evaporative demand. Drying rate from porous media often exhibit a transition from a nearly constant and high evaporation rate (stage 1) supplied by capillary liquid flow to a lower rate (stage 2) supplied by vapor diffusion. During stage 1, continuous liquid pathways connect the receding drying front with evaporating surface and sustain high evaporation rates. At a certain drying front depth, gravity overcomes capillary driving forces and a transition from liquid flow-supported stage-1 to diffusion-supported stage-2 evaporation occurs. The front depth at this transition marks a characteristic length defined by the width of pore size distribution. The extent and existence of hydraulic connections through the unsaturated zone above the drying front has been predicted theoretically and demonstrated experimentally using high resolution x-ray tomography and neutron radiography. In addition to provide detailed information regarding to the dynamics of drying front and water content distribution during evaporation, the concept of capillary characteristic length was extended to partially wettable porous media by systematically considering capillarity of clusters of hydrophilic and hydrophobic grains. Simplified geometrical and statistical assumptions have been applied to describe the porous media with mixed wettability. The theoretical predictions were successfully validated using a set of experiments with sands with different fraction of hydrophobic grains. Additionally, neutron radiography was used to investigate the drying of uniform hydrophilic and hydrophobic sand at high spatial and temporal resolutions to investigate the impact of wettability on drying front and water content dynamics. To explore the effect of liquid continuity and clarify the mass transport mechanisms supplying different stages of evaporation, the evaporation from porous media including hydrophobic sand layers was investigated. Results confirm interruption of capillary flow by hydrophobic layers and overall reduction in evaporative mass loss. Persistence of capillary flow to the interface between hydrophilic and hydrophobic layers was evident by accumulation of dye tracer indicating formation of vaporization plane. Evaporation flux across the hydrophobic layer was purely diffusive and proportional to the diffusion length to the surface (thickness of hydrophobic layer). In a further, theoretical and experimental studies were conducted to analyze evaporation behavior of layered porous media affected by the extent and sequence of layering and capillary characteristics of each layer (wettability, pore size distribution). The proposed model shows that the combination of intrinsic capillary characteristic length and the position of a layer below evaporation surface define the ultimate depth of drying front at the end of stage-1 evaporation. The model was tested in laboratory experiments using Hele-Shaw cells filled with layers of coarse and fine sand. Transition to stage-2 evaporation occurring at a depth defined by exceeding of the weakest "capillary link" in the sequence was confirmed. In addition to analyze the drying behavior of porous media under different conditions, a critical review was performed questioning the concept of enhanced water vapor fluxes from partially-saturated porous media relative to fluxes predicted based on Fick's law. This concept led to various mechanistic and phenomenological enhancement factors to reconcile apparent discrepancies between experiments and predictions made based on macroscopic diffusion theory. These concepts were reevaluated considering the role of capillary-induced liquid flow extending from saturated zone to vaporization plane. It was proved that the continuous capillary liquid flow is the main reason to measure experimentally fluxes more than the prediction by Fick's law of diffusion.


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