Abstract:

Based on the computational fluid dynamics (CFD) and experimental method, the cavitation characteristics of liquid nitrogen induced by ultrasonic waves with a frequency of 20 kHz are studied. The simulations were based on the mixture model with the Singhal cavitation model for phase change calculations, and the Realizable k-ε turbulence model was adopted for turbulence consideration. The sinusoidal oscillation of the vibrator was realized with the dynamic mesh method to simulate the ultrasonic generator. The periodic variation characteristics of the ultrasonic cavitation structure were obtained, which are in good agreement with the experimental observations. Based on the CFD models, the changes of pressure and temperature in the ultrasonic affected zone were obtained to make up for the lack of experimental conditions. It was found that the ultrasonic cavitation of liquid nitrogen had different characteristics due to the thermal effect compared with ultrasonic cavitation of water. The effects of different amplitudes, ultrasonic frequencies and pressures of cavitation were studied. The expression of the ultrasonic induced cavitation number was proposed, and the result proved that the smaller the cavitation number, the more likely to occur cavitation and the larger the cavity. The results reveal the mechanism of liquid nitrogen ultrasonic cavitation.

Key words: CFD, liquid nitrogen, ultrasonic cavitation, gas-liquid flow, dynamic modeling, cavitation number

摘要：

基于计算流体力学（CFD）和实验方法，研究频率为20 kHz的超声波诱导的液氮空化特性。数值建模采用Mixture多相流模型，Singhal空化模型和Realizable k-ε湍流模型，并通过动网格方法实现边界正弦振荡来模拟超声波发生器界面。模拟计算获得超声空化结构的周期变化特性，与实验观察相比发现现象上一致性较好。基于数值结果，分析了超声影响区压力和温度等参数的变化特性；发现由于热效应，使得液氮超声空化相比水超声空化具有不同特性。模拟了振子振幅、超声频率及系统压力等参数对空化的影响特性，并提出超声波诱导的空化数表达式，结果证明空化数越小越容易发生空化。计算结果深入揭示了液氮超声空化机理。

关键词: 计算流体力学, 液氮, 超声空化, 气液两相流, 动态建模, 空化数