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.

中文翻译：

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通过声流抑制蒸发双组分液滴内的振荡循环

摘要 本研究调查了声学对蒸发甲醇-水滴振荡内循环的影响。当观察到流动的周期性加速和减速时，循环被称为振荡。产生的声学信号是具有恒定幅度的不同频率的正弦波，带有或不带有添加的白噪声。在 30 Hz 的最低声频下观察到最小声学边界层厚度和最大蒸发率，有和没有添加白噪声。与没有声学的情况相比，甲醇的所有体积分数 (vf) 的蒸发速率都提高了。观察到，当受到声场时，给定液滴的振荡频率会降低，并且对于所有测试的液滴，在 30 Hz 时最低。由于在低声频下从周围环境夹带的空气较多，蒸发液滴周围存在强烈的声流在液滴表面产生较大的速度梯度，从而从液滴周围清除更多的蒸发质量并增强传热和传质从液滴表面。此外，较高的空气夹带率（i）抑制内部循环的振荡行为和（ii）循环韧带的断裂。循环韧带的断裂加速了循环速度，从而导致蒸发率增加。增强的蒸发提高了溶质 Marangoni 和 Rayleigh 对流，并再次迫使循环更加稳定。