Transient evaluations of the temperature and diameter of an evaporating droplet stream by using rainbow technique and simulation

The transient evaporation characteristics of free-falling ethanol droplets under ambient temperatures of 373–473 K are investigated with the rainbow technique. The size and equivalent temperature of droplets have been simultaneously extracted by analyzing the rainbow patterns. It indicates that the rainbow signal shifts to lower scattering angles by enhancing ambient temperature. While larger droplets tend to have more peaks within the same scattering angle range. Besides, the variation trend of diameter and equivalent temperature obtained by the experiment confirms well with the simulation results. However, the temperatures retrieved from rainbow measurements and predicted by the model are not quantitatively identical with each other due to the temperature gradient. Furthermore, to account for the inhomogeneous temperature in the liquid phase, a parabolic profile of the refractive index was adopted and thus the temperature values inside the droplet were proposed from the experimental results. The temperature values from the experiment and model are agreeable with each other for smaller droplets. By contrast, the deviation of the temperatures between experimental measurements and the predictions is visible for larger droplets. This might be attributed to the shape change of larger droplet from spherical to spheroidal one.