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Second Thermal and Fluids Engineering  Conference

ISSN: 2379-1748
ISBN: 978-1-56700-430-4

Droplet coalescence and departure mechanisms on flexible flapping films


Power plants account for 41% of fresh water withdrawals in the United States, 90% of which is utilized for condenser cooling. Evaporative losses in the cooling tower can be significant, and reclaimed water could be used for industrial or agricultural uses. This work investigates droplet motion on a flapping polymer, and these findings will be employed to design flapping condensers for water collection in cooling towers. In order to systematically investigate physical mechanisms of flapping on droplets, consistent droplet spays were generated. Droplets impinged on stationary and flapping thin (i.e., 0.127mm) vertical sheets of 0.61m tall and 0.36m wide. Clear perfluoroalkoxy (PFA) sheets were coated with Rain-X for increased hydrophobicity and observed average water contact angles was 106 degrees. The vibration system provided the film with flapping motion of ±1 cm amplitude and 11 Hz frequency. Droplet impingement, coalescence, and sweeping were captured with a high-speed camera, and droplet departure diameter, sweeping frequency and coverage area over time were analyzed. On the stationary surface, droplets were shed through gravity once the droplet exceeded the departure size (i.e., 3−4mm), and droplets elongated in the vertical direction. Droplets shed from the flapping surface moved vertically down the film or were ejected from the surface, termed "shooting droplets". On the flapping film, droplets remained round and were on average 44% smaller than on the stationary surface and departed more rapidly. The removal of smaller droplets benefits condensation by increasing available surface area thereby increasing water collection.

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