Abstract The evaporation of brine droplets has critical impacts on the zero-liquid-discharge thermal desalination process. A good understanding of the evaporation of brine droplets provides guidelines for the design of thermal desalination systems, and offers insights for applications when solvent droplet evaporation processes are involved. In this study, brine droplets with different salt mass concentrations were placed in a chamber with controlled temperature and relative humidity. The evaporation and crystallization processes were then visualized through a high speed camera by employing the pendant droplet method. The results showed that the evaporation rate increases with the decrease of relative humidity, the increase of temperature, and the decrease of salt mass concentration. After the evaporation was finished, we can observe that the crystal grew along the filament during the evaporation and remained on it. A salt shell was formed at the outside, while the droplet still contained some amount of brine inside, when the evaporation rate was low. Consequently, the evaporation mechanism was changed once the salt shell was formed, the water molecules needed to overcome the pressure difference inside and outside the salt shell, and diffused through the shell for further evaporation. A multi-variable fitted quadratic regression model was developed with R2 = 0.974 to describe the relationship among the evaporation rate, mass concentration, relative humidity and temperature. Because the regression model was based on experimental data with temperature varying from 30 °C to 60 °C, a predicted result of brine droplet's evaporation rate with various mass concentrations under 75 °C and 0% RH showed a good agreement with the experiment data. Therefore the developed regression model can be extended for high temperature (

中文翻译：

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不同温湿度条件下盐水滴蒸发结晶的实验研究

摘要 盐水液滴的蒸发对零排液热脱盐过程具有重要影响。对盐水液滴蒸发的充分理解为热脱盐系统的设计提供了指导，并为涉及溶剂液滴蒸发过程的应用提供了见解。在这项研究中，具有不同盐质量浓度的盐水滴被放置在温度和相对湿度受控的室内。然后使用悬滴法通过高速相机观察蒸发和结晶过程。结果表明，蒸发速率随着相对湿度的降低、温度的升高和盐质量浓度的降低而增加。蒸发完成后，我们可以观察到晶体在蒸发过程中沿着灯丝生长并留在灯丝上。当蒸发速率较低时，在外部形成盐壳，而液滴内部仍含有一定量的盐水。因此，一旦盐壳形成，蒸发机理就发生了变化，水分子需要克服盐壳内外的压力差，通过盐壳扩散进一步蒸发。建立了R2 = 0.974 的多变量拟合二次回归模型来描述蒸发率、质量浓度、相对湿度和温度之间的关系。由于回归模型是基于温度从 30 °C 到 60 °C 变化的实验数据，因此盐水滴的预测结果' 在 75 °C 和 0% RH 下，不同质量浓度的蒸发率与实验数据显示出良好的一致性。因此，开发的回归模型可以扩展到高温（