Impact of Leidenfrost Drops on Spherical Targets.

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2018-08
Authors
McLean, Mitchel L.
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Mechanical Engineering
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Abstract
Johann Leidenfrost first proposed a physical explanation for the levitation of a water drop on a metal surface which was at a temperature well above the drop’s boiling point. The drop floats above the surface and may be propelled vertically by the lateral flow of the vapor layer below. This phenomenon is known as the Leidenfrost effect. The Leidenfrost effect has been studied extensively for use in applications as well as its possible detrimental effects on heat transfer processes. Research advances have been made on impinging drops on superheated planar surfaces over the past fifteen years. The impingement of drops on spherical targets is less understood and this has important implications for petroleum processing such as in fluid catalytic cracking. In this work, an experimental system was designed to heat metallic targets beyond the Leidenfrost temperature for drop impingement studies. Water drops with Weber numbers ranging from 10 to 45 were impinged on planar and spherical targets with temperatures from 160 °C to 220 °C. Impinging drops covered a larger surface area on a spherical compared to a planar surface. In addition, it was observed that the Leidenfrost temperature depended on the ratio of the drop diameter to the spherical target diameter. For Weber numbers from 10 to 15, drops were observed to rebound off the target. At Weber numbers from 15 to 30, hole formation was observed and analyzed. For greater Weber numbers and temperatures, liquid toroids were observed. The toroidal drop dynamics were measured with high speed photography and these results were compared with existing models for planar surfaces. The toroids broke up into a discrete number of smaller drops as a result of a Plateau-Rayleigh instability.
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Leidenfrost drops, impinging drops, spherical target, experimental fluid dynamics, heat transfer
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