EVAPORATION OF A LIQUID DROPLET ON A HOT PLATE

The evaporation of a small liquid droplet impinging on a hot stainless steel plate is investigated. The liquids include water, various pure and mixed hydrocarbon fuels with drop sizes ranging from 0.07 to 1.8 mm. The wall temperatures varying from 63 to 605-degrees-C cover the entire spectrum of heat transfer characteristics from film evaporation (below boiling temperature) to spheroidal vaporization (above Leidenfrost temperature). A strobe-video visual system is used to record the transient process and to measure the droplet lifetime. Qualitatively, the droplet lifetime curve as a function of wall temperature is similar for all liquids and all drop sizes. The maximum heat transfer rate occurs at about 50-60-degrees-C above the boiling temperature for all pure liquids and the Leidenfrost heat transfer rate occurs at about 120-degrees-C for pure fuels and 180-degrees-C for water above the boiling temperature. The maximum evaporation rate can significantly exceed the burning rate of fuel droplets. Visual observations show that beyond the maximum heat transfer point, larger droplets levitate above the surface whereas smaller droplets bounce up and down to many diameters above the surface. Thus for the smallest droplets beyond the Leidenfrost temperature, the droplet lifetime actually increases with wall temperature which is in contrast to the larger droplets. The maximum heat transfer rate is independent of drop sizes for all fuels and shows a small decrease with increasing drop sizes in the case of water. At the Leidenfrost temperature, the data show that the heat transfer rate is maximum at a droplet diameter of approximately 0.5 mm.