在不同操作条件下对水包油系统中微液滴的形成过程及粒径大小进行了系统实验观察和测试,同时利用VOF/CSF模型对液滴形成过程进行了数值模拟,模拟结果与实验非常吻合;在此基础上,考虑两相流速、黏度、界面张力及壁面效应等因素,对液滴形成过程进行了进一步的模拟研究.结果表明,与传统油包水系统相比,水包油系统液滴的形成也有滴流(dripping)和射流(jetting)两种流型,但水包油系统能在更高的两相流速下保持滴流操作,而射流主要出现在离散相速度极高的条件下且很不稳定;滴流条件下生成的液滴具有很高的单分散性,且可改变流速提高产生频率,而射流条件下产生的液滴单分散性差,且易于出现卫星液滴;滴流条件下影响液滴粒径的显著因素是连续相流速、黏度、界面张力和壁面效应,其中前三者的影响可用毛细管数表征;研究对各影响因素的作用机理进行了阐释,并综合实验与模拟结果,给出了不同操作条件和装置尺寸下液滴生成粒径的预测关联式.
Droplet formation and droplet size of an oil-in-water system was investigated under different conditions. A numerical method based on VOF/CSF models was successfully applied to simulate droplet formation, and the results are in good agreement with that of experiments. Furthermore, the effects of two phase velocity, viscosity, interfacial tension and wall effects on droplet formation were numerically investigated.Similar as typical water-in-oil systems, the oil-in-water system also have two flow modes (dripping and jetting)in droplet generation, but the dripping mode in the oil-in-water system is happened at higher two phase velocities with good monodispersity. Meanwhile, the jetting mode appears only when disperse phase velocity becomes extremely high, and the generated droplets are usually poly-dispersed because of high instability of jetting flow in the oil-in-water system. The velocity and viscosity of the continuous phase, interfacial tension and wall effects are the most important factors that affect droplet diameters, and the first three factors can be combined using a capillary number to reflect their correlative effects. The mechanism of how these parameters affect droplet size is discussed. Finally, an empirical equation is presented to predict droplet diameter under different conditions and channel sizes.