Characteristic Lengths Affecting Evaporation From Porous Media With Sharp Textural Contrasts

Displacement of evaporation drying front into initially saturated porous medium results from capillary gradients between large pores at the front supplying the evaporation process from smaller pores at the surface. Like other displacement processes, the drying front may exhibit irregular spatial patterns due to pore size variations. In heterogeneous media containing sharp textural contrasts, drying front displacement patterns follow preferential liquid flow from coarse textured regions in support of evaporation from saturated fine textured regions connected to the surface. Hele-Shaw cells with vertical and horizontal sharp textural interfaces between coarse and fine sand domains were used to study water distribution during evaporation using neutron transmission technique and imagery with dyed water. For vertical textural interfaces, evaporation from saturated fine sand was sustained by liquid flow from adjacent coarse sand resulting in preferential advance of the drying front exclusively into coarse sand region. Direct evidence of water flow pathways from coarse to fine sand w obtained with neutron radiography using heavy water as a tracer. A characteristic length defining maximum drying front depth (in the coarse medium) is determined by the difference in air-entry values of the two media. In our experiments, viscous resistance exerted no effect on maximum front depth even when flow cross section (fine sand relative area) was reduced from 75% to 5% for similar external evaporative conditions (viscous limitations would be important for clayey media). For horizontal layers of fine over coarse sand, the drying front initially propagates in fine sand until air first enters the coarse sand resulting in an abrupt and disproportionally large displacement water from coarse to overlaying fine layer driven by capillary pressure difference between air entry values of the sands. Subsequent to rapid pressure relaxation, drying front invades preferentially the coarse layer with no changes in liquid distribution in the overlaying fine layer. Experiments with layers of different thicknesses and positions (depths) relative to evaporative surface revealed the importance of another characteristic length spanned by pore size distribution of the medium. The combination of intrinsic capillary characteristic length and the position of a textural interface below the surface defines the ultimate depth of drying front in layered media (hence magnitude of evaporative losses). Preferential evaporation patterns from texturally-heterogeneous media during capillary driven liquid flows result in an increase in overall evaporative losses relative to porous media represented by homogenous effective properties.