Droplet Coalescence and Departure on a Vibrating Film During Humid Air Condensation

Abstract

This paper investigates an approach to collect evaporated water from a moist air stream, a scenario found in many power plant cooling towers which utilize evaporation to cool, thereby resulting in evaporative water losses. For example, a 500 MW power plant may lose about 27m3/h (7133 gal/h) of water to evaporation during operation. When a cooled surface is placed in a warm humid environment, water condenses on the surface. The condensed liquid forms a thermal resistance, thereby reducing the condensation rate and water collected. The concept presented in this paper is to vibrate the cooled surface, thereby rapidly removing more droplets than gravity alone. With forced movement and through droplet coalescence, new droplets can nucleate in the space created by departing water droplets. Droplet nucleation, coalescence, and departure were investigated on vibrating and stationary Teflon films (contact angle 105°) in an environmental chamber at 30°C and 50% RH. Film vibrations of approximately 100 Hz were investigated. Droplet departure diameters were approximately 2–3 mm diameter on the vibrating surface and 6 mm on the stationary surface.