The leidenfrost phenomenon: film boiling of liquid droplets on a flat plate

Abstract

The film boiling of small droplets of liquid on a hot flat surface in the atmosphere is commonly termed the Leidenfrost Phenomenon after J. G. Leidenfrost who first studied the process in 1756. In the present study, the total evaporation times were determined for small droplets (< 0.1 ml) of water, carbon tetrachloride, ethanol, benzene, and n-octane on a stainless steel plate at surface temperatures ranging from 150°C to 500°C. Most of the data were taken in the film boiling regime though data were also taken in the nucleate and transition boiling regimes. The Leidenfrost point, defined as the plate temperature at which the droplet evaporation time is greatest, was determined. The Leidenfrost point was found to be 100–105 degC above the saturation temperature for all liquids except water; for water, the exact value of the Leidenfrost point appears to depend upon the surface and the method of depositing the droplet and varies from 150 degC to 210 degC above saturation. The Leidenfrost point is independent of droplet size over the range studied. An analytical model of the Leidenfrost phenomenon is postulated: Heat is transferred to the droplet by conduction through the vapor film on the bottom half and by radiation to the entire droplet. Mass is removed from the droplet by evaporation on the lower surface to supply the vapor film and by diffusion from the upper surface. The droplet is supported by the excess pressure in the vapor film. The droplet is presumed spherical and isothermal at the saturation temperature. The instantaneous evaporation rate is found by satisfying the heat, mass, and momentum balances for this model; total evaporation times are calculated by integrating the evaporation rate. Calculated and experimental evaporation times agree within 20 per cent except for n-octane at high temperatures, where some thermal cracking may have occurred.