Evaporation of pendant water droplets in fractures

Abstract

This paper investigates the evaporation of pendant water droplets in fractured tuffaceous rock. The droplets are assumed to exist near asperities and cavities as a result of gravity‐dominated flow through fractures. Because of the curvature of the droplet, the vapor pressure near the liquid‐vapor interface is greater than the saturated vapor pressure of water held in the matrix at the system temperature. Expressions for the vapor pressure and evaporation rate of water droplets are developed as a function of droplet radius and relative humidity of the surroundings. The relative humidity is calculated using Kelvin's equation to account for vapor pressure lowering in the surrounding tuffaceous rock. Results indicate that the vapor pressure of water droplets is not significantly increased above the saturated vapor pressure of freestanding water when the droplet size is greater than 100μm. However, if the relative humidity of the surroundings is exactly equal to 1 or if the droplet radius is very small (≪1μm), the effects of interface curvature will enhance the evaporation of water droplets. Finally, expressions are developed to determine the minimum liquid flux required to propagate flow through either a single fracture or a network of fractures without being evaporated.