耦合双泡声空化特性的理论研究

当双泡中心间距足够小时,由于气泡间辐射压力波的存在,作用在气泡上的压力不等于外部驱动压力.通过考虑双泡之间的辐射压力波,利用改进的Keller-Miksis方程,分别计算了不同大小、不同间距、含不同惰性气体的双泡在声空化过程中半径的变化、次Bjerknes力的变化和双泡内温度的变化.计算结果表明,当双泡大小不同时,小气泡受到的抑制作用较强,温度变化也比较大.随着双泡间距离从100μm增大到1 cm时,气泡间的次Bjerknes力的数量级从10~(-4)N减小到10~(-8)N.含不同惰性气体的耦合双泡在回弹阶段表现出明显不同的振荡规律.

Theoretical study of coupling double-bubbles ultrasonic cavitation characteristics

When the distances between bubbles are small enough, the pressure acting on the bubble is not the same as the external driving pressure, because of the radiation pressure wave of the neighboring bubbles. The force between two bubbles due to the bubble-radiated pressure waves by the neighboring bubbles is called the secondary Bjerknes force. Considering the bubble-radiated pressure waves and using the modified Keller-Miksis equation and van der Waals equation, the changes of the radius, the secondary Bjerknes force and the temperature of the double bubbles, which have different sizes, interspaces in between, and noble gases, in the process of ultrasonic cavitation are calculated. The calculations are based on the assumption that the locations of double bubbles stay unchanged in the oscillation process and their shapes always keep spherical. The double bubbles can also oscillate synchronously under the influence of the driving ultrasonic field. Because the sound propagation speed in water extremely fast, the time-delay effect on the secondary Bjerknes force is neglected. From the calculated results, the following conclusions can be drawn: when the sizes of double bubbles are different, the smaller bubble is more restrained and the temperature change is larger. When the sizes of double bubbles are the same, the Bjerknes force is negative, indicating that the coupled double bubbles are attracted to each other during the oscillation and the Bjerknes force has two radial oscillations in one driving period. As the interspace between double bubbles increases from 100 μm to 1 cm, the secondary Bjerknes force decreases from 10^-4 N to 10^-8 N, indicating that the interaction between double bubbles increases with the decreasing of the distance between the bubbles. The coupling double bubbles with different noble gases have only a small difference in maximum radius in the stage of expansion, but have different oscillation patterns clearly in the stage of rebound. This is because the bubble expansion process can be seen as an isothermal process, the effective polytropic exponent γ is approximately equal to 1. The collapse process can be regarded as an adiabatic process, so the effective polytropic exponent γ of noble gas with large molecules changes rapidly, and the influence of the oscillation of the bubbles becomes large. Our work provides a theoretical basis for establishing the acoustic cavitation model of different-number bubbles, and calculating the interaction force between different-number bubbles.