Nima Shokri

Drying front and water content dynamics during evaporation from sand delineated by neutron radiography

Evaporative drying of porous media is jointly controlled by external ( atmospheric) conditions and by media internal transport properties. Effects of different atmospheric potential evaporative demand on observed drying rates were studied in a series of laboratory experiments using sand-filled Hele-Shaw cells. We examined two potential evaporation rates of about 8 and 40 mm per day. The evolution and geometry of the drying front (marking the interface between saturated and partially dry regions) and water content distribution above the drying front were measured every 5 min at 0.1 mm spatial resolution using neutron radiography. Water loss rates decreased with time for both rates, but the decrease was more pronounced for high evaporative demand. External evaporative demand had no effect on drying front geometry or spatial water content distribution for depths below 2 mm. Cycles of roughening and smoothing of drying fronts due to interfacial pinning and unpinning were observed. The water content above the front showed irregular patterns due to formation of isolated liquid clusters with the general profile showing a decrease in mean water content with deepening drying front. Measured water content profiles support the hypothesis that liquid flow supply surface evaporation during stage 1 and water content distribution were not affected by external drying rates. Additionally, observed saturation profiles indicate that the corresponding hydraulic conductivity supports fluxes larger than the highest drying rate measured for sand, suggesting that decreasing drying rate was limited by vapor exchange between progressively drying surface and the viscous boundary layer above.

Pore-scale dynamics of salt transport and distribution in drying porous media

Understanding the physics of water evaporation from saline porous media is important in many natural and engineering applications such as durability of building materials and preservation of monuments, water quality, and mineral-fluid interactions. We applied synchrotron x-ray micro-tomography to investigate the pore-scale dynamics of dissolved salt distribution in a three dimensional drying saline porous media using a cylindrical plastic column (15 mm in height and 8 mm in diameter) packed with sand particles saturated with CaI2 solution (5% concentration by mass) with a spatial and temporal resolution of 12 μm and 30 min, respectively. Every time the drying sand column was set to be imaged, two different images were recorded using distinct synchrotron x-rays energies immediately above and below the K-edge value of Iodine. Taking the difference between pixel gray values enabled us to delineate the spatial and temporal distribution of CaI2 concentration at pore scale. Results indicate that during early stages of evaporation, air preferentially invades large pores at the surface while finer pores remain saturated and connected to the wet zone at bottom via capillary-induced liquid flow acting as evaporating spots. Consequently, the salt concentration increases preferentially in finer pores where evaporation occurs. Higher salt concentration was observed close to the evaporating surface indicating a convection-driven process. The obtained salt profiles were used to evaluate the numerical solution of the convection-diffusion equation (CDE). Results show that the macro-scale CDE could capture the overall trend of the measured salt profiles but fail to produce the exact slope of the profiles. Our results shed new insight on the physics of salt transport and its complex dynamics in drying porous media and establish synchrotron x-ray tomography as an effective tool to investigate the dynamics of salt transport in porous media at high spatial and temporal resolution.

Fundamental investigation of foam flow in a liquid-filled Hele-Shaw cell

The relative immobility of foam in porous media suppresses the formation of fingers during oil displacement leading to a more stable displacement which is desired in various processes such as Enhanced Oil Recovery (EOR) or soil remediation practices. Various parameters may influence the efficiency of foam-assisted oil displacement such as properties of oil, the permeability and heterogeneity of the porous medium and physical and chemical characteristics of foam. In the present work, we have conducted a comprehensive series of experiments using customised Hele-Shaw cells filled with either water or oil to describe the effects of foam quality, permeability of the cell as well as the injection rate on the apparent viscosity of foam which is required to investigate foam displacement. Our results reveal the significant impact of foam texture and bubble size on the foam apparent viscosity. Foams with smaller bubble sizes have a higher apparent viscosity. This statement only applies (strictly speaking) when the foam quality is constant. However, wet foams with smaller bubbles may have lower apparent viscosity compared to dry foams with larger bubbles. Furthermore, our results show the occurrence of more stable foam-water fronts as foam quality decreases. Besides, the complexity of oil displacement by foam as well as its destabilizing effects on foam displacement has been discussed. Our results extend the physical understanding of foam-assisted liquid displacement in Hele-Shaw cell which is a step towards understanding the foam flow behaviour in more complex systems such as porous media.

Free-Flow–Porous-Media Coupling for Evaporation-Driven Transport and Precipitation of Salt in Soil

Evaporative salinization of soil is a common issue observed in arid and coastal regions. This process is driven by mass, momentum and energy exchange between the porous medium and the free-flow region. To analyze such coupled systems, we present a representative elementary volume-scale model concept for coupling non-isothermal multi-phase compositional porous-media flow and single-phase compositional laminar free flow. Our numerical results illustrate evaporation behavior from a porous medium initially saturated with NaCl solution, manifesting its influence on dissolved salt distribution, salt precipitation and porous-media properties. We show that the new model is capable to capture the evaporation physics for different stages of evaporative salinization and compare the numerical results to two different experimental datasets: (1) cumulative mass loss of water and dissolved salt during stage-1 of saline water evaporation and (2) evaporation rate for different stages of evaporative salinization. In addition, influence of the initial salt concentration on the saline water saturation vapor pressure and transition to stage-2 evaporation are analyzed and discussed.

Drying patterns of porous media containing wettability contrasts

Porous media containing sharp wettability discontinuities may occur in natural systems due to depositional processes, accumulation of organic layers or modification of soil wettability following intense forest fires all of which are known to significantly modify water flow and transport processes. We studied evaporation from sand columns containing hydraulically-interacting domains with sharp wettability contrasts. We employed neutron transmission technique to map liquid phase dynamics during evaporation, and conducted laboratory experiments to evaluate evaporative fluxes affected by interactions across wettability contrast. We explained the preferential drying front displacement in the hydrophobic domain and the spatial extent of capillary flow supporting the vaporization plane using a physically-based model. The model provides description of observed liquid phase patterns and dynamics observed in neutron radiography measurements and evaporative fluxes from laboratory experiments. Our results provide new insights into evaporation induced capillary exchange and preferential liquid phase distribution during evaporation from hydraulically interacting vertical porous domains with differing wettability properties and offer opportunities for design of selectively drying of porous media in natural and engineered systems.

Effects of Grain and Pore Size on Salt Precipitation During Evaporation from Porous Media

Salt precipitation in saline porous media during evaporation is important in many processes including

Effects of intermediate wettability on entry capillary pressure in angular pores

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Effects of substrate on cracking patterns and dynamics in desiccating clay layers

CO2 sequestration, soil salinity which is a global problem as well as the preservation of monuments and buildings. In this study, X-ray micro-tomography was used to investigate the evolution of salt precipitation during evaporation to study the effects of particle and pore sizes on salt precipitation patterns and dynamics. The packed beds were saturated with NaCl solution of 3 Molal, and the time-lapsed X-ray imaging was continued for one day to obtain pore- scale information associated with the evaporation and precipitation dynamics and patterns. The results show that the presence of preferential evaporation sites (associated with fine pores) on the surface of the sand columns influences significantly the patterns and dynamics of NaCl precipitation. They confirm the formation of an increasingly thick and discrete salt crust with increasing grain size in the sand column due to the presence of fewer fine pores (preferential precipitation sites) at the surface compared to the sand packs with finer grains. Fewer fine pores on the surface also result in shorter stage-1 precipitation for the columns with larger grain sizes. A simple model for the evolution of salt crust thickness based on this principle shows a good agreement with our experiments. The findings of this study offer new insights about the dynamics and patterns of salt precipitation in drying porous media.