An experimental study of the Leidenfrost evaporation characteristics of emulsified liquid droplets

Experimental observations of the evaporation characteristics of water-in-fuel emulsion droplets in film boiling on a hot horizontal surface are reported. Detailed measurements of the temporal variation of equivalent droplet diameter for water-in-heptane and water-in-decane emulsions are made at surface temperatures of 565,620 and 680 K. Experiments are performed at atmospheric pressure and initial water volume concentrations of 10, 20, 35 and 50%. More qualitative characteristics are observed for emulsions of water in octane, nonane and dodecane. It is found that water/heptane and water/decane emulsions exhibit preferential vaporization such that either the water or hydrocarbon evaporate first (for decane or heptane emulsions, respectively). This result is revealed by an apparent discontinuity in the evolution of equivalent droplet diameter and is explained on the basis of a simple distillation limit model for evaporation. Results also show that the droplet evaporation rate decreases, and the total droplet evaporation time increases, with increasing water content. The initially opaque colored droplets appear to clear up during vaporization for several of the emulsions tested. Coalescence of the internal phase for water/octane and water/dodecane emulsions is also observed. Of the emulsions tested, only the water/heptane emulsions exhibit a disruptive characteristic at the end of the evaporation process.