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

Dani Or, Nima Shokri
2008
Article

Résumé

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.

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https://infoscience.epfl.ch/record/160818?ln=fr

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Dani Or, Nima Shokri
2008
Article

Résumé

Official source

https://infoscience.epfl.ch/record/160818?ln=fr

À propos de ce résultat

Concepts associés (12)

Concepts associés

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Publications associées (3)

Publications associées

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Quantifying evaporation drying front dynamics using neutron radiography

Dani Or, Nima Shokri

Soil evaporation plays an important role in the hydrologic cycle and affects near surface energy balance. Changes in evaporative drying rates reflect complex interplay between soil pore space and transport properties and atmospheric conditions. Typically, initial drying rate from the wet soil surface is high and is limited by atmospheric conditions. Subsequently, when hydraulic connection between the receding drying front and atmosphere can no longer be maintained, the drying rate becomes dominated by vapor diffusion through the air-filled pores. We show that the depth of drying front where a transition from atmospheric (high) to diffusion controlled (slow) evaporation takes place, is related to air entry value of the medium (or to a critical pore size). Fine textured soils with small pores sustain higher drying rates than coarser ones. We delineated drying front position and morphology of sand-filled Hele-Shaw cells using neutron transmission technique. Insights on water content distribution and dynamics of drying front in sands of different textures will be presented and compared with theoretical predictions.

2006

Chargement

Evaporation from porous media involves mass and energy transport including phase change, vapor diffusion, and liquid flow, resulting in complex displacement patterns affecting drying rates. Force balance considering media properties yields characteristic lengths affecting the transition in the evaporation rate from a liquid-flow-based first stage limited only by vapor exchange with air to a second stage controlled by vapor diffusion through the medium. The characteristic lengths determine the extent of the hydraulically connected region between the receding drying front and evaporating surface (film region) and the onset of flow rate limitations through this film region. Water is displaced from large pores at the receding drying front to supply evaporation from hydraulically connected finer pore at the surface. Liquid flow is driven by a capillary pressure gradient spanned by the width of the pore size distribution and is sustained as long as the capillary gradient remains larger than gravitational forces and viscous dissipation. The maximum extent of the film region sustaining liquid flow is determined by a characteristic length L-C combining the gravity characteristic length L-G and viscous dissipation characteristic length L-V. We used two sands with particle sizes 0.1-0.5 mm ("fine") and 0.3-0.9 mm ("coarse") to measure the evaporation from columns of different lengths under various atmospheric evaporative demands. The value of LG determined from capillary pressure-saturation relationships was 90 mm for the coarse sand and 140 mm. for the fine sand. A significant decrease in drying rate occurred when the drying front reached the predicted LG value (viscous dissipation was negligibly small in sand and LC LG). The approach enables a prediction of the duration of first-stage evaporation with the highest water losses from soil to the atmosphere.

2008

Chargement

On the Diurnal Soil Water Content Dynamics during Evaporation using Dielectric Methods

Ivan Lunati, Marc Parlange, Scott W. Tyler

The water content dynamics in the upper soil surface during evaporation is a key element in land–atmosphere exchanges. Previous experimental studies have suggested that the soil water content increases at the depth of 5 to 15 cm below the soil surface during evaporation, while the layer in the immediate vicinity of the soil surface is drying. In this study, the dynamics of water content profiles exposed to solar radiative forcing was monitored at a high temporal resolution using dielectric methods both in the presence and absence of evaporation. A 4-d comparison of reported moisture content in coarse sand in covered and uncovered buckets using a commercial dielectric-based probe (70 MHz ECH2O-5TE, Decagon Devices, Pullman, WA) and the standard 1-GHz time domain reflectometry method. Both sensors reported a positive correlation between temperature and water content in the 5- to 10-cm depth, most pronounced in the morning during heating and in the afternoon during cooling. Such positive correlation might have a physical origin induced by evaporation at the surface and redistribution due to liquid water fluxes resulting from the temperature-gradient dynamics within the sand profile at those depths. Our experimental data suggest that the combined effect of surface evaporation and temperature-gradient dynamics should be considered to analyze experimental soil water profiles. Additional effects related to the frequency of operation and to protocols for temperature compensation of the dielectric sensors may also affect the probes response during large temperature changes.

2010

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