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微通道内两相流流型及液滴生成研究
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摘要
近年来,微流体技术发展迅速,多相流作为其研究和应用的基础,受到人们极大关注。本文研究了微通道内两相流流型及液滴的生成。
     采用激光影像放大技术,观测了截面尺寸为40×160μm的竖直矩形微通道内空气-水两相流的流型,主要有弹状流、液环流和平行流。以两相流速为坐标绘制了流型图,得出了流型转换线,并与文献中的结果进行了对比。
     利用高速摄像仪观察了对流T型、Y型微通道内液液两相流流型。油相为环己烷,水相为加入0.3%表面活性剂十二烷基硫酸钠(sodium dodecyl sulfate, SDS)的水溶液。观测到了滴状流、弹状流和平行流三种流型,实验发现流型转换线受微通道尺寸和进口方式的影响。以两相流的表观速度为坐标轴绘制了流型图,并与已有报道进行了对比。
     利用高速摄像仪研究了对流T型和垂直型微通道内液滴的生成过程及机理。考察了微通道结构和尺寸、两相流率、流体物理性质(黏度、界面张力)等对液滴生成的影响。对于弹状流流型中的液滴,其生成主要是由两端压力降(积压)控制,液滴生成尺寸与两相流量比呈正比关系;对于滴状流中的小液滴,其破裂主要受剪切力和界面张力控制,液滴的生成尺寸与毛细管数呈指数关系。基于两端压力降和剪切力的共同作用机理,用毛细管数Ca和两相流量比φ对不同进口方式的微通道内液滴生成尺寸进行了关联和预测,预测值与实验结果吻合良好。
In recent years, microfluidic technology has developed rapidly and been widely used. The multiphase flow, as the basis for research and application of microfluidics, has been attracted much attention. In this paper, two-phase flow patterns and droplet formation in microchannels were studied.
     The air-water two-phase flow patterns, including slug, annular and stratified flow, in vertical rectangle microchannels with the dimension of 40×160μm were investigated by means of laser image magnifying technique. A flow patterns map was constructed to obtain the transitions lines with the flow rates of both phases as the coordinates, and was compared with the results in the literature.
     A high-speed camera was used to observe the liquid-liquid two-phase flow patterns in the T and Y-junction microchannels. Cyclohexane and demonized water with 0.3% surfactant sodium dodecyl sulfate (SDS), were employed as the oil and water phase, respectively. Three main flow patterns were observed: droplet, slug and parallel flow. The transition lines of flow patterns were affected by microchannel size and the way of feeding. A flow patterns map was constructed with the apparent flow rates of both phases as the coordinates, and was compared with the existing reports.
     The processes and the mechanisms of droplet formation in the microchannels with head-on T-junctions and vertical-junction were studied using the high-speed camera. The effects of the geometry and size of microchannels, flow rates of both phases and physical properties (viscosity and interfacial tension) of fluids on droplet formation were investigated. In the slug flow regime, the droplet formation is mainly dominated by the accumulated pressure drop across the thread of the dispersed phase, and the size of formed droplet is proportional to the ratio of flow rates of dispersed/continuous phases. In the drop flow regime, the break-up of the droplet is mainly dominated by the shear force and interfacial tension, and the size of formed droplet has an exponential relation with capillary number. On the basis of interplay of the accumulated pressure drop and shear force, the correlations were proposed for predicting the size of formed droplet with different feeding ways, taking into account the capillary number Ca and the ratio of oil/water flow ratesφ, and the predicted values agreed well with the experimental data.
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