倾斜通道内泡状流空泡份额分布特性

对窄矩形通道(3.25 mm×43 mm)和常规圆管(i.d.50 mm)内泡状流空泡份额分布特性进行了实验研究,探究倾斜对气泡和气液界面分布的影响。实验以空气和纯净水为工质,倾斜角度为5°、10°和15°。结果表明窄矩形通道内空泡份额主要呈壁峰型分布,未观察到核峰型分布;常规圆管内存在核峰型和壁峰型两种分布类型。倾斜对窄矩形通道和常规圆管内壁峰型分布影响相似,即随着倾斜角度的增加,靠近上壁面的峰值被加强,靠近下壁面的峰值被削弱直至消失。对于圆管内核峰型分布,倾斜导致中间宽峰向通道上部倾斜,且峰值随倾斜角度增加而增大。此外,竖直和倾斜条件下壁峰型分布的峰值较常规圆管更加接近通道中心位置。     

倾斜通道内泡状流空泡份额分布特性

对窄矩形通道（3.25 mm×43 mm）和常规圆管（i.d.50 mm）内泡状流空泡份额分布特性进行了实验研究,探究倾斜对气泡和气液界面分布的影响。实验以空气和纯净水为工质,倾斜角度为5°、10°和15°。结果表明窄矩形通道内空泡份额主要呈壁峰型分布,未观察到核峰型分布;常规圆管内存在核峰型和壁峰型两种分布类型。倾斜对窄矩形通道和常规圆管内壁峰型分布影响相似,即随着倾斜角度的增加,靠近上壁面的峰值被加强,靠近下壁面的峰值被削弱直至消失。对于圆管内核峰型分布,倾斜导致中间宽峰向通道上部倾斜,且峰值随倾斜角度增加而增大。此外,竖直和倾斜条件下壁峰型分布的峰值较常规圆管更加接近通道中心位置。     

矩形截面螺旋通道内弹状流的流动特性

对水平放置矩形截面螺旋通道内弹状流的流动特性进行了实验研究.通过实验获得了不同周角下的气弹演变过程和局部流动特征,结果表明,其流动特性会随着螺旋周角位置的变化而变化.根据实验数据分析发现,同一工况下,不同转角气弹的运动速度、频率和长度分布不尽相同.重力和离心力的相对大小决定着内外壁面液膜的厚度,给出了同一条件下,不同时刻的液膜厚度的演变过程.最后对下降液膜的运动速度展开了分析研究,在螺旋上升过程中,液膜下降速度逐渐减小,在螺旋下降段,液膜速度明显增大.     

捕捉微通道内Taylor流特性的一种渐变网格划分方法

提出了一种适用于捕捉通道内Taylor流液膜厚度的渐变网格划分方法。为了平衡模拟精度和计算效率,提出了针对此方法的具体规则来获得一种高效的网格划分。利用CFD软件Fluent进行了一系列的二维数值模拟,采用VOF模型来捕捉微通道内气液两相界面,所用微通道横截面宽度分别为0.1,0.2,0.3,0.4,0.5,0.8和1.0mm。采用提出的网格划分方法获得的模拟结果与关联式计算值和实验数据很接近。基于模拟结果,提出了新的气液弹长度的关联式,并且利用实验数据对此关联式进行了验证。     

壁面润湿性对蛇形微通道内两相流流动特性影响的格子Boltzmann模拟

在90°Y形汇流的矩形截面蛇形微通道内,采用格子Boltzmann方法对不同接触角的蛇形微通道内气液两相流动进行了数值计算。首先以空气和水为工作流体对气液两相流动进行模拟研究并通过实验进行验证。验证模型合理性后,根据模拟计算结果,以气液相流速为坐标绘制了不同接触角下的流型图并分析其差异性及原因;同时深入研究了液相黏度和接触角对于弹状流流体力学性质的综合影响;比较了具有不同接触角壁面的蛇形微通道内两相流压降、摩擦因子、壁面摩擦系数和剪切应力的分布规律,并讨论了蛇形微通道内气液两相流动的影响因素。研究表明疏水壁面即接触角大于90°时,微通道内两相流压降、摩擦因子、壁面摩擦系数和剪切应力均低于亲水壁面微通道内相关参数,更利于流体流动。     

壁面润湿性对微通道内Taylor流动特性的影响

壁面润湿性不仅影响着Taylor气泡的形状,同时对通道内流体流动、相变换热等有着关键的作用。采用VOF模型对T型微通道内气液两相Taylor流动进行三维数值模拟,重点研究了接触角改变对Taylor气泡流体动力学特性的影响。模拟结果与他人实验数据对比基本吻合,验证了模型的有效性。结果表明：随着接触角增大,气泡周围液含量逐渐降低,相界面也由外凸形变为内凹形。壁面越接近润湿（或疏水）状态,气液接触面的曲率就越大;当120°≤θ≤150°时Taylor气泡稳定性变差。当θ≥150°时“拖曳流态”出现,分析指出在大接触角下气体更易贴附壁面导致接触区内流场发生变化,形成的涡流减弱了水对气相的水平剪切作用,进而引起流型转变。接触角对通道内压力有着重要影响,通道中心轴向压力曲线以θ=90°为过渡,润湿状态下呈凸函数递减且p _G>p _L,疏水状态下气液进口处的压力分配改变,曲线趋势相反。     

气体入口角度和截面宽高比对微通道内泰勒气泡行为的影响

微通道的几何参数对于气泡或者液滴在其中的形成过程有着显著的影响。采用流体体积法(VOF法)研究了不同气体入口角度以及不同通道截面宽高比对微通道内气液两相流流动状况的影响。在所研究的操作范围内,各个微通道内的两相流流型均为泰勒流,并且在大多数情况下气泡长度分布均匀。在通道截面宽高比为0.5～2条件下,60°的气体入口角度有利于产生较短的气泡;如果通道截面宽高比达到4或8时,45°的气体入口角度更有利于形成较短的气泡。此外,随着通道截面宽高比的增大,通道内气泡的量纲1长度也随之增大,气泡长度分布的均匀性也逐渐变差。当通道截面宽高比增大到8时气泡长度分布变得很不均匀。     

蛇形微通道气液两相流动特性

以空气、去离子水为实验工质,在θ=50°的Y型混合器内充分混合,利用高速摄像仪对当量直径177.8 μm的小曲率蛇形微通道进行可视化实验,对比了两种气液混合方式(上气下液型、上液下气型)下的流动特性。实验发现的主要流型有弹状流、波状分层流、弥散流等,对此分别研究了气弹的形状和长度、液膜厚度以及气流携带液滴的份额,并提出新的预测液滴含量的关联式。此外,针对这两种不同的混合方式,在弯道处发现圆弧可以诱导产生弹状流,二者均经历拉伸和断裂过程,区别在于后者在拉伸之前,先要进行膨胀。不同的气液混合方式对各相的流动会产生一定的影响,两相流体在通道壁面附近以及弯道处的分布也有所区别。     

并行微通道内气液相分配规律

微反应器的集成放大对于微化工技术的工业应用具有重要意义。利用高速摄像仪对4个并行微通道内气液两相流动状况及相分配规律进行了研究,考察了气液两相流量及液相黏度对两相分布均匀性的影响。实验所用液相为含0.3%表面活性剂十二烷基硫酸钠（SDS）的蒸馏水-甘油溶液,气相为氮气（N₂）。实验观察到了6种典型的两相流型。对各支通道均为弹状流情况下气泡长度和气泡速度的分布规律进行了研究。在一定气相流率下,各支通道气泡长度的相对标准偏差随液相流率的增大而增大,气泡速度的相对标准偏差值随液相流率的增大先升高到一定值然后逐渐减小。气相分配不均匀性随液相流率和黏度的增大而增大,液相分配不均匀性随液相黏度的增大而减小,气相流率的变化对于两相分布影响不明显。研究结果有助于并行微通道的结构设计与优化,以实现更为均匀的气液两相流动分配。     

小曲率蛇形微通道弯头处弹状流流动及传质特性的数值研究

采用CLSVOF（coupled level set and volume of fluid）方法,以空气和水为工作流体对小曲率矩形截面蛇形微通道内气液两相流动进行模拟研究。验证模型的合理性后,研究了曲率对弯通道内压降的影响,曲率及气相速度对弹状流气泡及液塞长度的综合影响;同时深入分析了弯管内气液两相流动的传质特性,包括不同曲率下气泡长度的变化,弯管内液侧体积传质系数与液膜体积传质系数的比较,曲率及气相速度对液相体积传质系数的影响。同时,对比了回转弯道与直微通道传质系数的差异,发现弯微通道可以强化传质。     

蛇形管内汽液泡状流与传热的数值模拟

建立了含有相变过程的汽液泡状流VOF数值模型,通过同实验进行流型对比,验证了模型的有效性。模拟研究了蛇形管内汽液泡状流传热过程,结果表明：二次流导致冷热流体混合;在单汽泡流动时,弯管壁面上温度场在汽泡经过后具有良好的恢复能力;在汽泡串流动时,二次流将弯管下半部温度较高的流体带入上半部,有效提升上半部壁温,从而造成上壁面温度在汽泡经过前后出现较大的变化。     

Gas-liquid two-phase flow in serpentine microchannel with different wall wettability

Gas-liquid flow in serpentine microchannel with different surface properties exhibits drastically different flow behavior.With water and air as working fluids,the method of numerical simulation was adopted in this paper based on CLSVOF (coupled level set and volume of fluid method) multiphase model.After verifing the reasonability of the model through experiment,by changing wall properties and Re number (Re ＜ 1500),the influences of contact angle and surface roughness on flow regime and Po number were discussed.Moreover,the difference of pressure drop between curve and straight microchannel was also calculated.Beyond that,the combined effect of curve channel and wall properties on flow resistance was analyzed.This paper finds that wall properties have great influence on gasliquid flow in microchannels not only on flow regime but also flow characteristics.Meanwhile,the pressure drop in curve microchannels is larger than straight.It is more beneficial for fluid flowing when the straight part of microchannel is hydrophilic smooth wall and curve part is hydrophobic with large roughness.     

On the leakage flow around gas bubbles in slug flow in a microchannel

The leakage flow is that liquid does not push gas bubbles and leaks through the channel corners. This leakage flow was confirmed by tracking particles moving in the liquid film with a double light path method and was quantified by tracking the gas–liquid interface movement. The results show that leakage flow varies during bubble formation process. The average net leakage flow Q_net‐leak in a bubble formation cycle at T‐junction can be as large as 62.4% of the feeding liquid flow rate, depending on the liquid properties. Q_net‐leak for regular flow at main channel is much smaller, ranging from about 0 to 30% of the feeding liquid flow rate. The difference between the two leakage flows would lead to an increase in liquid slug length after generation. Finally, the effects of parameters such as phase flow rates, surface tension, and viscosity were investigated. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3964–3972, 2015     

滑移壁面蛇形微通道两相流数值模拟

基于Fluent平台,采用CLSVOF方法对滑移壁面蛇形微通道气液两相流动进行了数值计算。计算选用的方法与理论结果具有较好的一致性,同时可以表明疏水壁面会产生滑移现象,且在高度较小的微通道内滑移效果更显著,从而减小通道内流体流动阻力,实现减阻;不同壁面性质通道内流体流动情况的计算结果表明,滑移壁面对截面速度分布趋势几乎没有影响,但上下壁面疏水性不同会影响通道截面最大速度分布。此外接触角及相对粗糙度对滑移特性影响较大,合理设计壁面润湿性及微粗糙元结构可以最大限度发挥滑移现象引起的减阻效果;与无滑移壁面相比,滑移壁面微通道内传热效果更好,且随滑移速度的增大,通道换热增强。     

Experimental investigation on dynamic behaviour of bubbles emerging from micro-capillary orifice in a flow channel

The flow phenomenon of liquid with bubbles is widespread in various industrial fields, which determines the mass transfer characteristics of the equipment. In this work, the dynamic behaviour of bubbles emerging from micro-capillary orifice in a flow channel was studied by a visualization experiment, while the effects of gas flow rate and liquid flow rate on these processes of bubble growth, departure, and inrush were explored. The experimental results showed that one bubble formation cycle can be divided into three stages: Waiting, departure, and inrush, as well as the dynamic behaviour of bubble emerging from micro-capillary orifice in a flow channel, were significantly affected by gas flow rate and liquid flow rate. At a higher gas flow rate, the growth time and the departure time were shorter, as well as the departure volume of the leading bubble and the inrush volume of the trailing bubble were smaller, while the transverse longitudinal ratio fluctuated more violently, and the swing amplitude of the bubble centroid was greater. With an increasing liquid flow rate, the growth time, the departure time, and the inrush time shortened, while the departure volume of the leading bubble decreased and the fluctuation of the bubble centroid weakened. These findings are conducive to improving the performance of the equipment by optimizing the design of the aerator to regulate the dynamic behaviour of bubbles.     

The effect of liquid viscosity and modeling of mass transfer in gas–liquid slug flow in a rectangular microchannel

Both chemical (by adding 0.05 M NaOH) and physical absorption of CO₂ into aqueous glycerol solutions with viscosity up to 45.6 mPa·s in a microchannel are investigated. The concentration distribution pattern, absorption time, and mass transfer coefficient are analyzed and discussed. A new concentration distribution pattern is observed with the lowest concentration locating at the channel center. It is shown for the first time that presents a positive relationship with liquid viscosity, which is explained by the essential role of the mass exchange between the liquid film and bulk liquid slug. This mass exchange may lead to a rise in k _L when increasing the liquid viscosity under some cases in chemical absorption. A mass transfer model is successfully applied to predict the bubble size evolution in physical absorption. The model also shows about 10–46% of the mass transfer contribution from liquid films before saturation.     

Liquid‐liquid two‐phase flow patterns in a new helical microchannel device

Flow patterns of liquid‐liquid two‐phase fluids in a new helical microchannel device were presented in this paper. Three conventional systems were considered: kerosene‐water, n‐butyl acetate‐water, and butanol‐water. Six different flow patterns, slug flow, continuous parallel flow, discontinuous deformation parallel flow, discontinuous deformation parallel‐droplet flow, droplet‐slug flow, and filiform‐droplet flow, were observed. The influence of interfacial tension, microchannel structure, and rotation rate on two‐phase flow patterns were studied, and a universal flow pattern map was presented and discussed. The systems without mass transfer (0.1 g/g (10 %) tri‐n‐butyl phosphate (TBP)‐water, 0.2 g/g (20 %) TBP‐water, and 0.8 g/g (80 %) TBP‐water) and the system with mass transfer (0.8 g/g (80 %) TBP‐0.62 g/g (62 %) H₃PO₄) were used to verify the validity of the proposed universal flow pattern map in predicting flow patterns. The results showed that the former compared with the latter can be predicted more accurately by the universal flow pattern map.     

天然气管道泄漏速率的确定

天然气泄漏速率是模拟评价天然气管线泄漏事故过程中的重要因素之一,对其研究将有利于提高模拟评价结果的正确性。应用热力学和气体动力学理论,结合理想气体状态方程、绝热方程和能量守恒方程,研究分析了天然气管道的泄漏过程,分别给出了在临界泄漏阶段以及亚临界泄漏阶段的泄漏速率计算公式,为天然气管输安全工程的相关计算提供了可靠的理论依据。