Abstract:

Evaporation crystallization of brine micro-droplets plays a very important role in seawater desalination, crystal preparation, drug premixing and particle screening. Herein, a computational hydrodynamic model is developed to simulate the evaporation process of NaCl aqueous droplets on a hydrophobic platform with different heat transfer coefficients and diameters at the hydrophobic interface, mainly to predict the circulation evolution mechanism of the fluid inside the droplet under the effect of temperature and concentration differences. The simulative results were validated by optical visualization and infrared thermal imager. The results indicated that the circulation inside the NaCl aqueous droplet is dominated by Rayleigh convection and the solute Marangoni effect at room temperature evaporation. Different heat transfer coefficients at the solid-liquid interface affect the heating rate and temperature distribution of the droplet, and the thermal Marangoni effect can dominate the internal circulation of the droplet when the temperature difference is greater than a certain value. A phase diagram of the evolution of the flow field based on the modulation intervals of the Rayleigh and Marangoni numbers was produced to effectively predict the circulation state within the droplet. The direction and duration of circulation within the brine droplet can be controlled by tuning the thermal Marangoni effect, which affects the crystal morphology and deposition distribution of the final evaporative crystallization. This work can provide a theoretical basis for evaporation interface design and process control of evaporation crystallization.

Key words: evaporation of saline droplets, Rayleigh convection, Marangoni convection, interfacial tension, computational fluid dynamics, numerical analysis

摘要：

盐水微液滴蒸发结晶在海水淡化、晶体制备、药物预混合和颗粒筛选中具有非常重要的作用。通过计算流体动力学模型，对不同疏水界面传热系数和不同直径疏水平台上的NaCl盐水液滴蒸发过程开展模拟研究，主要通过模拟预测了液滴内流体在温度和浓度差作用下的环流演变机制。模拟结果通过光学可视化和红外热成像仪进行了验证。结果表明，常温蒸发下，NaCl液滴内部的环流主要由Rayleigh对流和溶质Marangoni效应主导。不同的固液界面传热系数会影响液滴的升温速率和温度分布，当温差大于一定值时，热Marangoni效应可以主导液滴内部环流。基于Rayleigh数和Marangoni数的调控区间制得了流场演变相图，有效地预测液滴内部的环流状态。盐水液滴内环流方向和持续时间可以通过调整热Marangoni效应来控制，从而影响最终蒸发结晶的晶体形貌和沉积分布。这项工作可以为蒸发界面设计、蒸发结晶的过程控制等提供理论依据。

关键词: 盐水液滴蒸发, Rayleigh对流, Marangoni对流, 界面张力, 计算流体力学, 数值分析