The evaporation rate of a droplet was explained in relation to the thickness of the boundary layer and the condition near the droplet’s surface. However, the number of results obtained from experiments is very limited. This study aims to investigate the thickness of the boundary layer of an ethanol–water mixture droplet and its effect on the evaporation rate by Z-type Schlieren visualization. Single and double droplets are tested and compared to identify the effect of the second droplet on the average and instantaneous evaporation rate. The double droplet’s lifetime is found to be longer than the single droplet’s lifetime. The formation of a larger vapor region on the top of the droplet indicates a higher instantaneous evaporation rate. The thickness of the boundary layer is found to increase with increase in ethanol concentration. Furthermore, a larger vapor distribution area is found in the case of higher ethanol concentration, which explains the faster evaporation rate at higher ethanol concentration.
In this study, effect of the ambient relative humidity on the vapor concentration of ethanol-water mixture droplet on PTFE substrate was observed. Ethanol-water mixtures were prepared at 100 vol% (pure ethanol), 75 vol%, 50 vol%, 25 vol%, and 0 vol% (Pure water) by ethanol volume fraction. Relative humidity inside the test chamber was controlled at 33%RH, 52%RH, and 75%RH using saturated salt solution technique. Vapor concentration field was acquired by both applying Abel inverse transform to the Schlieren image and simulating with OpenFOAM source code. Pure ethanol had smaller value of mole fraction at the center than ethanol water mixture which was similar to the refractive index field. Two peaks of the mole fraction value were observed at the side of droplet in the case of pure ethanol from the concentration field, which was due to the presence of the droplet that blocked the downward movement of the ethanol vapor which was heavier than air and forced it to horizontally move to the edge of the droplet. On the other hand, one peak at the middle was found in the case of ethanol-water mixtures. Comparison of the concentration field between experimental result and simulation showed consistency in pure ethanol case and deviation became larger as ethanol portion decreased. Results of pure water and low ethanol concentration case (25 vol%) were obtained from simulation alone due to the limitation of Z-type Schlieren.
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