声场作用下含盐液滴冻结过程及组分迁移特性的研究

Study on freezing process and component migration characteristics of salty droplet under acoustic field

为了揭示超声波辅助冻结的内部作用机理,明确声场作用下相变冻结过程中的热质传递规律及组分迁移特性,根据声场理论分析了超声波的空化作用和热效应,并在冻结过程能质守恒的基础上建立了超声波作用下液滴相变冻结及盐分迁移数学模型,研究了超声波对液滴冻结过程中气泡状态和液滴温度的影响,分析了不同盐浓度下液滴冻结过程中固液界面、溶液比例、盐度及盐水残余率的变化规律。结果表明,超声波空化作用强化了界面处的热质传递,液滴温度下降较快,有利于液滴的冻结;液滴盐浓度越高,凝固界面的移动越慢,液滴直径为2 mm时,盐浓度5wt%时达到冻结点的时间为15 s,盐浓度8wt%时达到冻结点的时间为20 s;超声波作用下盐浓度越低,冻结过程中盐分迁移变化越剧烈。

In order to reveal the internal mechanism of ultrasonic-assisted freezing and clarify heat and mass transfer law and component migration characteristics during the freezing under the action of sound, the cavitation and thermal effects of the ultrasonic wave were analyzed according to the sound field theory. Based on the conservation of energy and mass, a mathematical model of droplet freezing and salt migration in the effect of the ultrasonic wave was established, which presented the heat and mass transfer and the salt migration law. The model can reflect the effect of the so und field on the freezing and was used to analyze the phase change freezing process, which involved the ultrasonic cavitation effect and the state change of bubbles. In different conditions, some important parameters, s uch as temperature variation, liquid ratio,concentration ratio, and salinity were studied, which can provide the path to improve the freezing efficiency and the brine separation effect. The results showed that ultrasound was helpful for heat and mass transfer. The dropping temperature drops rapidly, ultrasound was conducive to the freezing of drops. It was that the low-frequency ultrasound will increase the growth cycle of bubbles in droplets a nd the maximum diameter of bubbles, to strengthen heat and mass transfer. The higher the salt concentration was, the slower the freezing process was. It was that the smaller the diameter, the greater the mass transfer and heat dissipation caused by the cavitation effect. When the droplet diameter was 2 mm, the time to reach the freezing point was 15 s when the salt concentration was 5wt%, and20 s when the salt concentration was 8wt%. When the concentration of salt in the droplet was low, the change range of salt migration became more intense. It was that the lower the concentration of droplets, the stronger the driving force of salt migration potential difference, and the range of salt migration was becoming more and more intense.