A Study on the Evaporation of Water–Ethanol Mixture Using Rainbow Refractometry

Evaporation of droplets of liquid mixture is a subject of interest in combustion studies, e.g., combustion of bioethanol blends. In this paper, experimental investigation, using rainbow refractometry, on the variations of droplet diameter and composition during the evaporation of water–ethanol droplet in quiescent atmosphere is studied. The droplet is suspended on the tip of 125 μm-diameter fiberglass rod. The initial diameter is around 1000–1100 μm, and the initial composition is varied from 0% to 100% of ethanol by volume. The scattered rainbow signal from the evaporating droplet is fitted to the Airy theory to extract information on the diameter and refractive index of the liquid droplet against evolution time. To determine the accuracy of droplet diameter measurements using this technique, the diameter is also measured from the shadow image of droplet simultaneously. At 0–60% of ethanol by volume, the diameter and volume fraction accuracies are within ±30 μm and 10%, respectively, even though the temperature and composition gradients inside a droplet are neglected. The results show that the water–ethanol mixture evaporates faster at the beginning due to the higher amount of the volatile component, i.e., ethanol. The D2–t curve appears as a series of two straight lines of different slopes: a steep one initially and a moderate one at later stage. The slope at the initial or the transition stage increases with the ethanol composition, while the slope at later stage (steady stage) is equivalent to that of pure water. Likewise, the refractive index decreases rapidly at the beginning and becomes steady reaching a final value of 1.333, which is close to the refractive index of pure water.