On the suppression of oscillatory circulation inside an evaporating bi-component drop through acoustic streaming

Abstract

The present study investigates the effect of acoustics on the oscillatory internal circulation of evaporating methanol – water drops. The circulation is termed oscillatory when a periodic boosting and deceleration of flow is observed. The acoustic signal generated, is a sine wave of different frequencies at constant amplitude, with and without added white noise. Minimum acoustic boundary layer thickness and maximum evaporation rate are observed at lowest acoustic frequency of 30 Hz, with and without, added white noise. The evaporation rate is enhanced for all volume fractions (vf) of methanol when compared to the absence of acoustics. It is observed that the oscillation frequency for a given drop decreases when subjected to an acoustic field, and is lowest at 30 Hz for all drops tested. The presence of intense acoustic streaming around the evaporating drop creates large velocity gradients on the drop surface due to higher air entrainment from the surroundings at low acoustic frequencies, sweeping out more amount of evaporated mass from the surrounding of drop and augmenting the heat and mass transfer from the drop surface. Further, the higher rate of air entrainment into the drop, (i) suppresses the oscillatory behavior of internal circulation and (ii) breakup of the circulation ligaments. The breakup of circulation ligaments accelerates the circulation velocity which results in enhancement of the evaporation rate. The enhanced evaporation raises solutal Marangoni and Rayleigh convection, and again forces the circulation to be steadier.