Heat transfer and evaporation rates of small liquid droplets on heated horizontal surfaces

The evaporation rates of small liquid droplets were observed and quantified in order to measure heat transfer rates when individual droplets evaporate on a horizontal heated surface. Single water droplets with a post-impact diameter of approximately 1 mm and n-heptane droplets approximately 5 mm in diameter were gently placed on aluminum and copper surfaces that were maintained at temperatures below the saturation temperatures of the liquids, and the evaporation was observed at atmospheric pressure. The droplets were videotaped throughout the entire evaporation process and transient variations of the diameter, height were measured. From the measured quantities, the contact angle and evaporation rates were calculated and used to predict the heat transfer rate on the surface. The results show that the trends in the wetted diameter, height and contact angle for water droplets are fundamentally different than the geometric changes for heptane droplets. Even though the evaporation rates of the n-heptane droplets were an order of magnitude greater than the water droplets, the heat fluxes for the n-heptane droplets are approximately an order of magnitude less than the water drops. The experimental results are compared with recent analytical models which predict the evaporation rates of small liquid droplets.