T型微通道内浆料体系中气泡的生成动力学

实验研究了T型微通道内浆料体系中弹状气泡的生成动力学,重点考察了颗粒粒径的影响。气泡的生成过程可划分为四个阶段：填充阶段、挤压阶段、过渡阶段和快速夹断阶段。在填充阶段、挤压阶段和快速夹断阶段,气泡最小颈部宽度与时间呈幂律关系。在过渡阶段,气泡最小颈部宽度与时间呈线性关系。浆料体系中气泡的挤压阶段和过渡阶段随颗粒粒径的减小而缩短。连续相流量及颗粒粒径对填充阶段幂律指数无显著影响;挤压阶段和快速夹断阶段的幂律指数以及过渡阶段的线性斜率均随颗粒粒径的增大而减小,随连续相流量的增大而增大。     

台阶式单微通道内气泡生成动力学

采用高速摄像系统研究了台阶式微通道中气泡生成机理和尺寸变化规律。探究了气液相流量、液相黏度以及台阶宽度对气泡生成过程的影响机制。实验中分别采用不同质量浓度的甘油水溶液和氮气作为液相和气相。发现台阶式微通道中气泡的生成过程分为扩展、夹断和蓄能阶段。增大气相、液相流量,减小液相黏度使各阶段所消耗的时间减少。随液相黏度的增大夹断阶段消耗的时间逐渐变长,成为气泡生成过程中起主导作用的阶段。最后,基于台阶式微通道中气泡的生成机理,构建了气泡体积的预测式。     

聚焦十字型微通道内高黏流体中气泡生成动力学

利用高速摄像仪观察了聚焦十字型微通道内高黏度（630 mPa·s）的甘油-水溶液中氮气气泡的生成过程。研究了气泡生成过程以及气泡体积和最小颈部半径的变化规律。结果表明,高黏流体内气泡生成过程可分为回缩、膨胀、挤压和最终破裂4个阶段。气泡体积在膨胀和挤压阶段均随时间线性增长,但挤压阶段的斜率大于膨胀阶段的斜率。气泡最小颈部半径随时间变化分为两个不同的阶段：在挤压阶段,颈部半径随剩余时间呈幂率关系;而在最终破裂阶段,颈部半径随时间呈线性关系。     

聚焦十字型微通道内高黏流体中气泡生成动力学

利用高速摄像仪观察了聚焦十字型微通道内高黏度(630 m Pa·s)的甘油-水溶液中氮气气泡的生成过程。研究了气泡生成过程以及气泡体积和最小颈部半径的变化规律。结果表明,高黏流体内气泡生成过程可分为回缩、膨胀、挤压和最终破裂4个阶段。气泡体积在膨胀和挤压阶段均随时间线性增长,但挤压阶段的斜率大于膨胀阶段的斜率。气泡最小颈部半径随时间变化分为两个不同的阶段:在挤压阶段,颈部半径随剩余时间呈幂率关系;而在最终破裂阶段,颈部半径随时间呈线性关系。     

Y型微通道内纳米颗粒稳定气泡的完全阻塞破裂动力学

研究了Y型微通道吸附型纳米颗粒稳定气泡的完全阻塞破裂的动力学,破裂过程可划分为挤压阶段和快速夹断阶段,两阶段内无量纲气泡最小颈部宽度与时间均呈幂率关系。气泡破裂过程的颈部动力学表明颗粒的存在并不影响两阶段转变的临界颈部宽度,但吸附在气泡表面的颗粒层会减弱挤压阶段中连续相对气泡颈部的挤压作用,以及快速夹断阶段角区中连续相液体回流对气泡的挤压作用,进而阻碍气泡颈部的形变,延长了气泡的破裂过程。纳米颗粒稳定的气泡的指前因子m及幂率指数α均小于常规气泡,但其差值随着毛细管数Ca和气泡长度l₀的增大而减小,颗粒对气泡破裂过程的影响逐渐减弱。此外,纳米颗粒稳定的气泡的头部曲率略小于常规气泡,颗粒对完全阻塞破裂过程气泡头部动力学的影响可以忽略。     

微通道内浆料体系中的气泡生成特性及尺寸预测

利用高速摄像仪对T型微通道内浆料体系中的气泡生成频率和气泡尺寸进行了研究。以氮气作为分散相,含0.35%（质量分数）表面活性剂（SDS）不同浓度玻璃珠的甘油-水溶液为连续相。实验考察了弹状流下气液两相流量、颗粒浓度以及浆料表观黏度对气泡生成频率及气泡尺寸的影响。结果表明：在弹状流下,当分散相流量一定时,随着连续相流量的增大,气泡的生成频率增大而气泡尺寸减小。当连续相流量一定时,随着分散相流量的增大,气泡生成频率和气泡尺寸均增大。随着颗粒浓度的增大,浆料的表面张力减小,表观黏度增大,气泡生成频率增大而气泡尺寸减小。提出了T型微通道内浆料体系中生成气泡尺寸的预测模型,模型具有良好的预测精度。     

完全阻塞气泡在T型不等宽微通道分岔口的破裂动力学

采用高速摄像仪对T型不等宽微通道分岔口的气泡完全阻塞破裂动力学行为进行了研究。实验以N₂作为分散相,含0.5%十二烷基硫酸钠(SDS)的蒸馏水与甘油混合液作为连续相。结果表明:气泡完全阻塞破裂过程可以分为两个阶段:挤压阶段和快速夹断阶段。挤压阶段又可分为快速挤压阶段和慢速挤压阶段,快速挤压阶段尚未发现普遍的变化规律,慢速挤压阶段量纲为1气泡颈部最小宽度与时间存在指数关系:(1-w_m/w₀)∝t^0.62。表观流速和液相黏度的增加可使颈部变细速率加快,而气泡初始长度对气泡颈部变化的影响可以忽略。在快速夹断阶段,量纲为1气泡颈部最小宽度与剩余时间存在指数关系:w_m/w₀∝(T-t)^0.32。     

微通道内催化表面气泡生长及脱离的可视化实验

采用聚二甲基硅氧烷材料(PDMS)制备矩形截面的微通道,并在微通道壁面上沉积MnO₂作为催化剂,采用高速摄影仪对通流过程中过氧化氢催化分解生成氧气气泡的过程进行了可视化实验研究,分析了反应物的浓度和流量对气泡生长速度及脱离直径的影响.结果表明:气泡在微通道内催化表面的生长及脱离过程呈周期性变化的趋势;气泡生长可以分为快速生长和缓慢生长两个阶段,当t<3 s时气泡处于快速生长阶段,催化反应主要受动力学控制,当t≥3 s时扩散控制占主要地位,气泡生长速度随反应物浓度的升高而增大;气泡脱离直径受反应物浓度影响较小,受反应物流量影响较大,而且随液相反应物Reynolds数的增大线性降低.     

流动聚焦微通道内牛顿微液滴在幂律剪切致稀流体中的生成研究

采用开源CFD软件OpenFOAM中的interFoam求解器对流动聚焦微通道内微液滴的形成过程进行了数值模拟。通过与文献中的实验数据进行对比,验证了VOF模型和幂律非牛顿流体模型的准确性。并以此为基础模拟了幂律剪切致稀流体中牛顿微液滴的形成过程,研究了幂律流体的幂律指数n和稠度系数K对微液滴生成的影响。研究表明,在滴状和挤压状流型中,离散线颈部宽度与周期内剩余时间呈幂律关系;离散线长度在坍塌阶段呈现线性缓慢增长,在夹断阶段呈现近似指数迅速增长的趋势。随着n和K的增大,液滴的尺寸逐渐减小,而生成频率则逐渐增大,且n的变化比K的变化对其产生的影响更明显。     

微通道壁面浸润性对气-液两相流的影响规律研究

通道壁面浸润性对微通道内的气-液两相流具有重要影响。利用等离子体辅助接枝改性,将甲基丙烯酰乙基磺基甜菜碱（SBMA）及1H, 1H, 2H, 2H-全氟癸基三乙氧基硅烷接枝在聚甲基丙烯酸甲酯（PMMA）材料表面,得到了10°、40°、70°和110°四种接触角的微通道,并考察了浸润性对流型、气泡长度和压降的影响。结果表明,随接触角增大,气泡截断位置下移,膨胀阶段缩短,挤压阶段变长;低流量时,气泡长度随接触角增加而增大,高流量时则减小;建立了与材料表面水接触角相关的气泡尺寸预测关联式,与Garstecki经典预测关联式相比,预测精度更高;θ<90°时,接触角增加,压降减小;θ>90°时,三相接触线使流动阻力和压降增加。     

Formation dynamics and size prediction of bubbles for slurry system in T-shape microchannel

The bubble formation dynamics and size manipulation in the slurry of polystyrene microspheres in the microfluidic T-junction were visually investigated by a high-speed camera. Based on the evolution of the bubble neck with time, the formation process of bubbles is divided into three stages: filling, squeezing and pinch-off. The particle concentration has an obvious effect on the squeezing stage, while less impact on the filling and pinch-off stages. In the squeezing stage, the evolution of the dimensionless minimum neck width of bubbles with time could be described by a power-law relationship. The increase of the particle concentration or continuous phase flow rate could lead to the increase of body flow of the continuous phase and the enhancement of the squeezing force acted on the bubble neck, correspondingly, the power-law index α in the squeezing stage enlarges. Moreover, the bubble size increases with the increase of the gas phase flow rate and the decrease of the particle concentration and continuous phase flow rate. However, the effect of the particle concentration on the bubble size weakens with the increase of the continuous phase flow rate. In addition, a new prediction correlation of the bubble size for the slurry system in a T-shape microchannel was proposed with good prediction accuracy.     

Numerical Study on Bubble Formation of a Gas‐Liquid Flow in a T‐Junction Microchannel

Bubble emergence in a gas‐liquid flow in a T‐junction microchannel of 100 μm diameter, operated under a squeezing regime, was simulated with the computational fluid dynamics method. In general, bubble formation in channels includes three stages: expansion, collapse and pinching off. After analyzing and comparing quantitatively the three forces of pressure, surface tension and shear stress acting on the gas thread in the whole process, their effects in the different stages were identified. The collapse stage was the most important for bubble formation and was investigated in detail. It was found that the collapse process was mostly influenced by the liquid superficial velocity, and the rate and time of collapse can be correlated with empirical equations including the liquid superficial velocity, the capillary number and the Reynolds number.     

Formation characteristics of Taylor bubbles in a T-junction microchannel with chemical absorption

This study focuses on the effect of chemical absorption on the formation dynamic characteristics and initial length of Taylor bubbles. The temporal evolutions of neck width and length of gaseous thread and initial length with and without chemical absorption were investigated with the Capillary number and Hatta number between 0.0010–0.0073 and 1.8–5.8 respectively. The squeezing regime with typical three stages, expansion, squeezing and pinch off is observed for both two processes. Compared with the non-absorption process, the increase of formation time in the chemical absorption process arises mainly from the expansion stage, and the decrease of initial length is from the necking stage. In addition, the temporal length evolution satisfies the power-law scale with the same exponent but a smaller pre-exponential factor. The correlations of neck width for stage transition and initial length with Hatta number demonstrate the enhancement effect of chemical absorption on bubble formation dynamics and initial length at relatively high chemical reaction rates and long formation time. This study provides insight into the bubble formation mechanism and helps to regulate the bubble initial size with chemical absorption.     

The effect of system pressure on gas‐liquid slug flow in a microchannel

Characteristics of gas‐liquid two‐phase flow under elevated pressures up to 3.0 MPa in a microchannel are investigated to provide the guidance for microreactor designs relevant to industrial application. The results indicate that a strong leakage flow through the channel corners occurs although the gas bubbles block the channel. With a simplified estimation, the leakage flow is shown to increase with an increase in pressure, leading to a bubble formation shifting from transition regime to squeezing regime. During the formation process, the two‐phase dynamic interaction at the T‐junction entrance would have a significant influence on the flow in the main channel as the moving velocity of generated bubbles varies periodically with the formation cycle. Other characteristics such as bubble formation frequency, bubble and slug lengths, bubble velocities, gas hold‐up, and the specific surface area are also discussed under different system pressures. © 2013 American Institute of Chemical Engineers AIChE J, 60: 1132–1142, 2014     

Formation of viscoelastic droplets in a step-emulsification microdevice

In this article, polyethylene oxide glycerin aqueous solution is used as the dispersed phase, and cyclohexane is used as the continuous phase to study the formation mechanism of viscoelastic fluid droplets in a step-emulsification microdevice. The formation process of viscoelastic droplets mainly includes three stages: two-dimensional expansion stage, necking stage, and pinch-off stage. The elasticity has basically no effect on the two-dimensional expansion stage, but it can delay the process of the necking stage and cause the beading phenomenon of droplets in the pinch-off stage. The results show that the effect of elasticity on the droplet size is weak, but it cannot be ignored. Moreover, the elasticity will be coupled with the viscous force to reduce the operating range of the dripping regime. Finally, the influence of the elasticity on the formation of satellite droplets is analyzed, and the method to prevent the influence of satellite droplets is proposed.