Abstract
In this study, droplet characteristics including droplet length and formation time, and mixing efficiency in droplets were investigated via the volume of fluid (VOF) method coupled with a user defined scalar (UDS) model. A cross-shaped junction with a square cross-section was designed and used for droplet formation. An initial arrangement which differed from that of a conventional operation was adopted. Results show that when the droplet superficial velocity is constant, the exchange between the dispersed phase velocity and the continuous phase velocity has a marginal effect on the droplet formation time. However, the exchange has a great effect on droplet length. These findings provide a valuable guide for future operation of droplet formation. In addition, the results show that the mixing efficiency in the droplet forming stage can be classified into time-dominated and length-dominated regimes according to the droplet superficial velocity. When a droplet flows in a microchannel, a higher droplet superficial velocity increases mixing efficiency due to the faster inner circulation and shorter droplet length.
概要
目 的
液滴表面流速是决定液滴混合性能的重要参数, 可影响微流控系统中不同界面的更新频率和总传输时间. 本文旨在建立具有正方形截面的十字型微通道, 并基于 VOF 方法耦合标量方程, 探究液滴表面流速对液滴长度、 液滴形成时间以及液滴混合效率的影响机制, 以期为微流控系统的进一步优化设计提供依据.
创新点
1. 设计了具有正方形截面的十字型微通道用于液滴形成, 改变了分散相和连续相的注入方式; 2. 建立数值模型, 自定义标量方程, 实现了混合过程的可视化, 并利用无量纲数定量分析了不同液滴表面流速下液滴长度、 形成时间以及混合效率等液滴特性.
方 法
1. 建立具有正方形截面的十字型微通道数值计算模型, 验证数值方法的准确性 (图3); 2. 自定义标量方程, 调加标量至分散相中, 实现混合过程的可视化分析, 并以分散相液滴内标量的浓度变化作为混合效率的评价指标; 3. 改变液滴表面流速, 并采用无量纲数定量分析液滴表面流速对液滴长度 (图5)、 液滴形成时间(图6)以及液滴混合效率(图10)的影响机制.
结 论
1. 液滴表面流速的增加可导致液滴长度和液滴形成时间减少; 恒定液滴表面流速下, 液滴长度随分散相分率的增加而增加, 分散相和连续相的速度相互交换, 液滴生成频率几乎不变; 分散相体积分率越小, 液滴混合效率越好. 2. 液滴在微通道内流动时, 液滴内部产生内循环; 内循环可分为主循环和次循环: 主循环由液滴和壁面相互作用产生, 而次循环由液滴和连续相流体相互作用产生. 3. 液滴混合效率可分为时间主导和长度主导两个阶段, 且这两个阶段中液滴表面流速对混合效率的影响规律相反.
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Acknowledgments
This work is supported by the National Natural Science Foundation of China (No. 52175067), the Zhejiang Provincial Natural Science Foundation of China (No. LY20E050016), the Key R&D Plan of Zhejiang Province (No. 2021C01021), and the Youth Funds of the State Key Laboratory of Fluid Power and Mechatronic Systems (Zhejiang University) (No. SKLoFP-QN-1801), China.
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Jin-yuan QIAN designed the research. Jin-yuan QIAN and Xiao-juan LI processed the data. Jin-yuan QIAN and Lei ZHAO wrote the first draft of the manuscript. Zhi-jiang JIN and Wen-qiang LI helped to organize the manuscript. Lei ZHAO revised and edited the final version.
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Jin-yuan QIAN, Lei ZHAO, Xiao-juan LI, Wen-qing LI, and Zhi-jiang JIN declare that they have no conflict of interest.
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Qian, Jy., Zhao, L., Li, Xj. et al. Effect of droplet superficial velocity on mixing efficiency in a microchannel. J. Zhejiang Univ. Sci. A 23, 783–794 (2022). https://doi.org/10.1631/jzus.A2200159
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DOI: https://doi.org/10.1631/jzus.A2200159