Experimental Investigation of the Evaporation of Droplets in Axial Acoustic Fields

The effect of axial acoustic fields on the evaporation of individual droplets is investigated both experimentally and theoretically. A setup was developed in which images of droplets moving through an acoustic field were acquired using a state-of-the-art, high-speed, intensified video system. The evaporation rates of droplets were determined from the droplet diameters measured from these images. Experimental investigations using methanol droplets showed that the presence of an acoustic field significantly enhances the evaporation of droplets. The evaporation rate is a strong function of the acoustic amplitude, but increases only slightly with frequency. The effect of acoustic oscillations on water and methanol droplet evaporation was theoretically modeled by numerically integrating the differential equations for droplet mass, momentum, and heat transfer. The model uses quasi-steady correlations for momentum heat and mass transfer. The model predicted the measured trends correctly. However, the quantitative agreement of these predictions with the experimental data depends strongly on the correlations for Nussult and Sherwood numbers used to model the energy and mass transfer, respectively. (Author)