Effect of Droplet-Laden Fibers on Aerodynamics of Fog Collection on Vertical Fiber Arrays.

Abstract

Growing population, along with rapid urbanization, has led to severe water scarcity, necessitating development of novel techniques to mitigate this looming problem. Fog contains water in the form of liquid droplets suspended in air, which can be collected on a porous structure placed in the path of the fog flow. We first develop an artificial fog-generating system using the thermodynamic principle of mixing of air streams followed by condensation, which closely mimics the liquid water content and droplet size distribution of natural fog. We then investigate how collected fog droplets growing on fiber surfaces alter the aerodynamics of fog flow across vertical fiber arrays, called harps, thus affecting their fog collection efficiency. As deposited droplets grow on the fiber surface, they increase the area occluded by droplet-laden fibers, thus increasing the effective shade coefficient (SCact), which increases with time from its initial geometric value (SCgeo), eventually reaching a quasi-steady state, as droplet shedding due to gravity and droplet growth due to fog collection balance each other. We find that this difference in the SCgeo and SCact is governed by local fiber geometry and its physico-chemical morphology; the process dynamics is captured by a nondimensional number, SC*, which increases with the length scale corresponding to the critical volume of droplet shedding relative to the fiber diameter, V*. Thus, there is a significantly greater increase in the effective shade coefficient for thin fibers having larger values of V* as compared to fibers with larger diameters which have lower V* values. On hydrophobic fibers, the quasi-steady state is achieved faster, and the time-averaged SCact is lower as compared to hydrophilic fibers due to the lower critical volume of droplet shedding. The shape of droplets growing on harp fibers affects the aerodynamics of fog flow, its inertial capture mechanism, and efficiency, which can guide design considerations for fog harps toward achieving optimal fog collection performance.