超声空化气泡动力学仿真及其影响因素分析

Dynamic simulation of ultrasonic cavitation bubble and analysis of its influencing factors

为获得最佳的超声空化效果,构建了空化气泡运动的动力学模型,并对模型方程进行数值仿真,探讨了超声频率、声压、空化泡初始半径、反应体系主体温度和绝热指数对空化气泡运动的影响。模拟结果表明,随着超声频率的增加,空化效应减弱;随着声压幅值的增大,空化泡最大振幅增加,崩溃时的最高温度和最大压力先增大后减小;气泡的初始半径较小,并且反应体系温度较低时,空化效果较好;绝热指数取值的不同会导致空化模拟计算结果有所差异,该研究为超声空化技术的广泛应用提供参考。

Ultrasonic waves can be found in many different areas such as chemistry, biology, cleaning, medicine, etc. The mechanical interaction between ultrasonic waves and bubbles in liquids leads to a phenomenon described as ultrasonic acoustic cavitation. A cavitation bubble in a liquid undergoes cycles of growth, rapid collapse, and damped rebounds in response to ultrasonic sound waves. Due to the very short lifetime of an ultrasonic cavitation bubble, the high temperature and pressure from its collapse haven’t hitherto been measurable, but the cavitation process can be simulated by constructing a dynamic model of a cavitation bubble. This paper explores physical conditions under which the best ultrasonic cavitation effect can be obtained and provides theoretical guidance for extensive applications of ultrasonic cavitation. Based on the Rayleigh-Plesset equation, we perfected bubble dynamic motion in an ultrasonic cavitation model by considering viscosity, surface tension, vapour pressure, adiabatic exponent, and acoustic radiation damping as dynamic factors. Since temperature variations influence physical properties of water, physical models of water saturation vapor pressure, surface tension, sound velocity and viscosity with temperature changing were also built. Thus, influences of ultrasonic frequency, acoustic pressure amplitude, initial bubble radius, bulk solution temperature, and adiabatic index on the evolution process of an ultrasonic cavitation bubble are discussed accordingly. The simulation results indicate that the cavitation effect decreases as ultrasonic frequency increases. With an increase of ultrasonic sound pressure, the radius of cavitation bubble amplitude increases, and both the highest temperature and maximum pressure first increase and then decrease when a bubble collapses. In addition, the cavitation effect’s best condition occurs when the initial radius of a bubble is smaller and the reaction system temperature is relatively low. Moreover, different adiabatic indexes cause variations in our cavitation simulation results. Therefore, in order to obtain a good cavitation effect, the following conditions must be satisfied： 1） The frequency of an ultrasonic generator should be lower than 40 kHz, and the lower the frequency, the better the results; 2） The ultrasonic power should be moderate with a suggested sound pressure amplitude within the range 0.2-0.35 MPa; 3） The temperature of the reaction system should not be higher than 320 K, and the lower the temperature, the better the results.