Droplet Evaporation and Leidenfrost Phenomena on Metallic Surfaces

Abstract

This experiment presents a detailed and thorough parametric study of the Leidenfrost point which serves as the boundary of the transition and film boiling regimes. The evaporation time in film boiling region of a sessile drop of liquid on a hot metallic surface has been analyzed in the present study. The time of evaporation for the droplet on the hot metallic surface was measured. With the time-temperature plot of these experimental data, the Leidenfrost phenomena has been clarified and explained. Sessile drop of three different liquids namely Distilled Water, Methanol and Ethanol having diameter of 1.3mm was used These liquid were dropped as sessile droplets from heights of 20mm and 50mm to conduct the experiment for a wide range of solid surface temperatures of 50-320⁰C. Three solid surfaces of Brass, Aluminum and Mild Steel were used to conduct the experiment. Graphs were plotted by placing evaporation time of liquids against surface temperature of metal blocks. The temperature at which time required for evaporation is maximum is called the Leidenfrost point. These variations in liquid types, heights of droplets and metals have been done to present a clear statement that the Leidenfrost temperature range does not change for a certain liquid. The only change obtained from varying liquid, height and metal is the time of evaporation. Among these three liquids, the liquid which has higher boiling point will take more time to evaporate. Water has highest boiling point (100⁰C) compared to methanol (64.7⁰C) and ethanol (78.3⁰C). So water takes the highest time to evaporate. Among the three metal surfaces, aluminium has the maximum thermal conductivity of 205W/mK & mild steel has the minimum thermal conductivity of 43W/mK. Higher thermal conductivity results in higher Leidenfrost time because high thermal conductivity allows fast heat transfer from the metal to the vapor film, keeping the vapor film stable and thus preventing direct contact of the drop with the metal surface. But due to some unknown error, the leidenfrost time we got for water on aluminium surface is 112.03s and for water on mild steel, it is 114.72s. Here too, water takes higher time to evaporate and the temperature is 180⁰C. So, for all the metal blocks and test liquids dropped from different heights used in this experiment, Leidenfrost temperature is within the range of 150⁰C -180⁰C and a specific temperature for each liquid. This concludes to the fact that Leidenfrost point for a certain liquid does not change for any parameter.