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声场中液滴稳定性理论的完善对超声雾化技术和超声悬浮技术的发展具有重要价值。本文通过实验、理论、数值模拟相结合的方式，研究了驻波声场（19.8 kHz）中的液滴失稳现象及其动力学机制。结果表明，随着声场强度的增加，液滴失稳模式由圆盘失稳转变为边缘锐化失稳，更重要的是，液滴在失稳过程中其赤道面扩张加速度存在自发增大的现象。经分析，本文揭示了液滴变形过程中其赤道处声辐射负压与长径比之间的正反馈机制，前者与后者的二次方成正比，阐明了液滴自加速失稳的形成原因。之后，建立了包含表面张力和考虑正反馈机制的声辐射压力的液滴界面平衡方程，最终得到了声致液滴失稳的无量纲判据，即当声韦伯数Wea≤1时，液滴界面保持平衡；Wea>1时，赤道声吸力大于表面张力，液滴发生失稳，该理论判据与实验结果吻合良好。The advancement of the theory of droplet stability in the acoustic field is of significant value to the advancement of ultrasonic atomization and ultrasonic levitation technologies. In order to reveal the detailed mechanism of acoustic droplet instability and give the instability criterion for easy application. In this paper, a kinetic study of droplet Instability in standing wave acoustic field (19.8 kHz) is carried out through a combination of experiment, theoretical derivation and numerical calculation. The acoustic Instability of the droplet occurring near the wave node is mainly manifested in two typical modes: disk Instability and edge-sharpening Instability. The appearance of these two Instability modes depends on the relative magnitude of the standing wave field strength. Specifically, with the gradual enhancement of the intensity of the standing wave field, the Instability mode of the droplet will gradually change from disc instability to edge-sharpened instability.The droplets show obvious self-accelerating expansion in the equatorial plane during the Instability process. The positive feedback between the droplet aspect ratio and the negative pressure of acoustic radiation at the equator of the droplet, is the reason for the above self-accelerating behavior. The theoretical results obtained by derivation show that the amplitude of the negative acoustic radiation pressure at the droplet equator is proportional to the quadratic of the droplet aspect ratio. The surface tension of the droplet is the main factor hindering the deformation of the droplet, and the acoustic radiation suction at the equator is the main factor driving the deformation of the droplet. Based on this, the force equilibrium equation of the droplet interface is established, and the dimensionless criterion of acoustic droplet instability, i.e., the acoustic Weber number Wea, is derived. when Wea≤1, the droplet interface stays in equilibrium, and when Wea>1, the equatorial acoustic suction is larger than the surface tension, and the droplet instability occurs, and the average error between the experimental results and the theoretical results is only 9%.
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Keywords:
- Ultrasonic standing wave /
- Droplet /
- Instability /
- Dynamics
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