Abstract:

The application of ultrasound to liquid freezing got much attention over the last few years and its potential seems very promising. In order to make clear droplet freezing assisted by ultrasound, the heat and mass transfer characteristic was studied based on ultrasound theory, penetration theory of mass transfer and energy conservation. The ultrasound frequency, ultrasound intensity and operation time were studied in the process of the liquid drop freezing. The results showed that ultrasound could accelerate mass transfer and make droplet rapidly cooling. In the effect of ultrasound, the bubble size in the droplet was decreased with ultrasound frequency, and the bubble number in the droplet was increased with ultrasound frequency. Mass transfer coefficient of droplet was increased with ultrasound intensity and reduced with ultrasound frequency. For the mass transfer and heat transfer, the direction were same in the droplet freezing process, the heat transfer was strengthened by mass transfer in the droplet freezing process. Comparing with no ultrasound, droplet temperature with ultrasound (ultrasound frequency 20000 Hz and ultrasound intensity 400 W·m^-2) was lower 2.0-2.5℃ after the same time (60 s). Hence the ultrasound helps to cool and freeze droplet. This study is favor to understanding the freezing by ultrasound and its application.

Key words: ultrasound, freezing, mass transfer, phase transformation, liquid

摘要：

超声波液体辅助冻结在食品、海水淡化及冰蓄冷等方面的应用得到了广泛关注。结合声场理论，在对液滴界面处热质传递分析的基础上，建立了超声波作用下的液滴冷却冻结数学模型，讨论了不同超声波频率、强度及作用时间对液滴界面处热质传递的影响，得到了超声波作用下液滴的冷却冻结规律。结果表明：超声波作用下界面处的传质系数随着超声波强度的增加而增大，但随超声波频率的增加而减小；在液滴冷却冻结过程中，质量传递与热量传递的方向相同，液滴的冷却冻结在超声波作用下得到了强化。在超声波频率为20000 Hz（强度为400 W·m^-2），经过相同的时间（60 s），超声波作用下的液滴温度比无超声作用的温度低2.0~2.5℃。研究将有助于深入理解超声波辅助冷却冻结机理，并为其工程应用提供了指导和参考。

关键词: 超声波, 冻结, 传质, 相变, 溶液