Quantifying Air Convection through Surface‐Exposed Fractures: A Laboratory Study

Abstract

Fractures in arid regions, especially within low‐permeability rocks, have a significant effect on the hydrological cycle. Recent studies have revealed large amounts of salt precipitates along fracture walls in the upper few meters of surface‐exposed fractures (SEFs). However, measured salt‐precipitation rates are far too high to be accounted for by diffusive venting of moist fracture air. An alternative mechanism that could explain enhanced evaporation from fractures is thermal convection, which also vents moist air from the fractures. This paper describes a series of experiments conducted in Hele‐Shaw chambers aimed at quantifying the potential venting capacity of air convection through natural fractures as a function of thermal gradient and fracture aperture. Using smoke to visualize air flow, we quantified air velocities, mass flux, and convection‐cell morphology as a function of Rayleigh number (Ra). Thermal differences of 0, 5, 10, and 13°C along the 50‐cm‐high Hele‐Shaw chambers and apertures of 1 and 2 cm were explored. Velocity was found to increase linearly with Ra while mass transfer increased exponentially with Ra, likely due to the concomitant increase in the number of convection cells per chamber width. For example, a thermal difference of 13°C between ambient air and chamber air for a 2‐cm aperture resulted in an increase of approximately two orders of magnitude in the mass‐transfer rate relative to pure diffusive flux (i.e., when no thermal gradient is imposed). The air‐mass‐transfer rate in the Hele‐Shaw chamber was found to be linearly proportional to the Sherwood number.

Soil–aquifer phenomena affecting groundwater under vertisols: a review

“…vious work on subsurface evaporation and salinization in rock fractures (e.g., Weisbrod and Dragila, 2006;Nachshon et al, 2008;Kamai et al, 2009;Weisbrod et al, 2009). In DCIS, vertical convective flow of air in the cracks is driven by instability due to cold (and dense) air in the crack near the surface and warmer air down in deeper parts of the crack at night or other surface-cooling periods.…”

Falling through the cracks: The role of fractures in Earth‐atmosphere gas exchange

“…This new hypothesis is able to successfully account for the amount of enhanced soil drying and salt accumulation observed. Nachshon et al [2008] and Kamai et al [2009] quantified this mechanism under controlled laboratory conditions for different thermal gradients and showed that fractures with apertures greater than 1 cm convect freely, while thinner apertures are affected by thermal and viscous dissipation. If, in fact, this mechanism exists under natural field conditions, its importance to atmospheric gas exchange could be quite significant, because convective venting would affect not only gas flux but also speciation.…”

Impact of thermal convection on air circulation in a mammalian burrow under arid conditions

“…10). Based on the results of the air replacement experiment, that the venting rate by TCV is two orders of magnitude higher than the mammal's _ V CO2 , and considering that the venting rate increases with the increase of Ra (Nachshon et al, 2008), it can be deduced that higher venting rates will occur in larger burrows. White (2005) showed a linear relationship for mammals between a burrow's cross-sectional area and its inhabitant's body mass ðR 2 fMÞ.…”

“…These gradients are expected to be especially prevalent in arid environments where dayenight temperature differences are high (Weisbrod and Dragila, 2006). This mechanism was quantified in the laboratory (Kamai et al, 2009;Nachshon et al, 2008) and the field ) for a fractured rock.…”

“…The morphology of free convection cells (e.g., Nachshon et al, 2008) could also complicate direct measurements in the field as it requires high spatial density measurements. Even if one could overcome the extreme technical difficulties and measure these velocities in situ, it would still be unfeasible to distinguish between the various mechanisms that can control air motion, such as wind velocities and barometric pressure variations and, of course, TCV.…”