随机粗糙表面上剪切变稀流体液滴的沉积过程模拟

采用计算流体力学相场方法模拟了单个剪切变稀非牛顿流体液滴在随机粗糙表面的沉积过程,并分析揭示了随机粗糙表面形貌对液滴运动状态及平衡状态的影响。结果表明,在指定的相同操作条件下,即使在光滑表面,剪切变稀流体液滴比牛顿流体液滴铺展更大且回缩至平衡所需时间更少,不存在二次铺展;剪切变稀流体液滴最大铺展直径随均方根粗糙度Rr与Wenzel粗糙度Wr的增加而略有增加。Wr相同时,随着Rr增大,液滴最终铺展系数减小,高度系数增大,平衡接触面积及接触角有所减小。在Rr相同情况下,随着Wr增大,液滴达到平衡所需时间缩短,平衡接触面积线性增大。     

随机粗糙表面上剪切变稀流体液滴的沉积过程模拟

采用计算流体力学相场方法模拟了单个剪切变稀非牛顿流体液滴在随机粗糙表面的沉积过程,并分析揭示了随机粗糙表面形貌对液滴运动状态及平衡状态的影响。结果表明,在指定的相同操作条件下,即使在光滑表面,剪切变稀流体液滴比牛顿流体液滴铺展更大且回缩至平衡所需时间更少,不存在二次铺展;剪切变稀流体液滴最大铺展直径随均方根粗糙度Rr与Wenzel粗糙度Wr的增加而略有增加。Wr相同时,随着Rr增大,液滴最终铺展系数减小,高度系数增大,平衡接触面积及接触角有所减小。在Rr相同情况下,随着Wr增大,液滴达到平衡所需时间缩短,平衡接触面积线性增大。     

剪切变稀流体中液滴运动的研究

采用VOF方法,基于非牛顿流体的流变模型对剪切变稀流体中液滴的运动进行数值模拟。结果发现剪切变稀流体中液滴的上升较稳定,上升速度前期增长较快,后期趋于平缓,轨迹呈直线型;单液滴初始半径r越大,上升过程中产生的形变越明显;溶液的稠度指数k越大,液滴上升的速度v越小;幂律指数n越大,液滴上升的速度v越大,k的作用效果比n效果明显。     

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。     

聚丙烯剪切流变测试与分析

在不同温度和剪切速率下,利用高压毛细管流变仪测试聚丙烯流变性能。结果表明,聚丙烯属于典型的剪切变稀型非牛顿流体,其粘流活化能较低,熔体粘度对温度敏感度低。     

变截面通道内流动聚焦微液滴生成数值研究

化学分析研究的不断深入,对液滴微流控精度提出了更高的要求。设计了不同孔径尺寸的流动聚焦通道模型,模拟油-水两相流动剪切微液滴生成过程。采用Level Set方法处理两相流动界面,研究了流动聚焦模型下通道尺寸和油-水两相流量比对生成液滴尺寸的影响。研究结果对实现微液滴生成过程的精准操控具有一定的参考意义。     

非牛顿流体中气泡群传质的研究

CO2气泡群在液相中的传质在化工过程中广泛存在.对CO2气泡群在3种不同流体(牛顿流体、非弹性剪切变稀流体和黏弹性剪切变稀流体)中的气液传质过程进行了研究.利用CO2探针测定了不同操作条件下CO2的体积传质系数,考察了液相浓度,气体流量以及流变性质对体积传质系数的影响.结果表明:在3种流体中,体积传质系数均随气体流量的...     

微通道液-液两相流动特性实验研究

利用高速摄像机与Canny算法,以硅油为离散相,含0. 5%SDS的蒸馏水为连续相,研究了凹穴型微通道内液-液两相流动特性。结果表明,直通道内观察弹状流、过渡流、滴状流3种流型。随着毛细数的增大,液滴形成机理由挤压机制向剪切机制转变,液滴速度逐渐增大,液滴长度逐渐减小。随着连续相流量的增加,液滴形成时间逐渐减小,且挤压机制生成液滴的时间大于剪切机制。凹穴结构减弱了壁面对液滴的限制,液滴速度降低,T型交汇处压差降低,相同工况下的液滴尺寸大于对冲T型微通道的液滴尺寸。     

十字交叉微通道内微液滴生成过程的数值模拟

采用VOF模型对十字交叉微通道内微液滴的生成进行三维数值模拟,获得了拉伸挤压、滴状剪切、单分散射流等单分散微液滴的生成机制以及紊乱射流、节状形变流、管状流和滑移流等两相流型,模拟与实验结果相吻合验证了模拟的有效性。液液两相流型主要受两相流速、两相界面张力以及连续相黏度的影响,发现随着连续相的流量增大,微液滴的生成尺寸减小,生成频率增大;而离散相流量的影响则相反。两相表面张力与连续相黏度分别在低连续相Ca数和高连续相Ca数条件下分别起主导作用。在低连续相Ca数(U_d<0.03 m·s^-1)的拉伸挤压和滴状剪切流流型下,微液滴生成尺寸随着表面张力系数的减小而减小,在射流条件下反而增大,微液滴的生成频率变化则相反。在高连续相Ca数(U_d>0.03 m·s^-1)下,微液滴的生成尺寸随着连续相黏度的增大而减小,微液滴的生成频率变化则相反。另外,壁面接触角在拉伸挤压流型下对微液滴生成无太大影响,但在滴状剪切和单分散射流流型下,接触角减小会导致微液滴无法稳定生成。     

剪切变稀流体中的单气泡生成行为

为了探究剪切变稀流体的流变性对气泡生成的影响,采用高速摄像仪对单气泡在剪切变稀流体[羧甲基纤维素钠(CMC)水溶液]及牛顿流体(甘油水溶液)中的气泡生成过程进行研究,考察了剪切变稀流体的流变性对气泡在生成过程中的体积及形状的影响。结果表明:气泡生成体积及生成时间随流体黏度的增大而增大,在气泡颈形成之前的阶段,气泡体积增长速度随流体黏度的增大而减小,在气泡颈形成之后,气泡体积增长速度则随流体黏度的增大而增大,不同流体中气泡的长径比随着生成过程的进行均呈增大趋势,但在生成初期,长径比的增长速度随剪切变稀性的增大而减小,在生成后期,气泡长径比的增长速度则随剪切变稀性的增大而增大。     

Concentration by Membrane Ultrafiltration of a Shear-Thinning Fluid

Abstract

Many commercial membrane processes involve fluids whose rheological properties are non-Newtonian. However, very little has been published on ultrafiltration of non-Newtonian fluids. The aim of this work is to show some experimental results concerning the concentration by membrane ultrafiltration of fluids whose viscosity is high and shear-thinning. Experiments were performed with xanthan solutions as model shear-thinning fluids. The variations of permeate flux with respect to the operating parameters show the unusual effects of some of these parameters. It is shown that when the feed solution in an ultrafiltration process has shear-thinning properties, those properties have an enormous influence in determining the operation efficiency.     

Velocity Evolution for the Coalescence of Two In-Line Bubbles Rising in Non-Newtonian Fluids

This study experimentally investigates velocity evolution for the coalescence of two in-line bubbles rising in non-Newtonian fluids by using a high speed camera. Due to the wake of the leading bubble and the shear-thinning effect of non-Newtonian fluids, the following bubble is accelerated to approach the leading bubble, leading to the coalescence of the two bubbles. Based on the Newton’s second law and Schlichting’s wake theory, a theoretical model was developed to estimate the instantaneous rising velocity of the following bubble in the coalescence process of the two bubbles. The predicted values by the present model showed a good agreement with the experimental data.     

液滴流微反应器的基础研究及其应用

综述了近些年来快速发展的液滴微流控技术, 回顾了微流控系统中液滴的基本行为, 如液滴的生成、运动、聚并和分裂等研究进展, 重点探讨微液滴作为反应器其内部的流动、传质和反应过程, 以及液滴流微反应器已有的和潜在的重要应用价值。通过精确调控液滴在微尺度上的行为(产生、聚并与分裂、内部的混合与反应等), 使单个液滴成为新型受限空间内的微型间歇反应器, 而微通道内的液滴流进而形成了若干间歇反应器构成的连续流反应器新型式。除了微流控技术普遍具有的微小尺寸效应带来传质传热强化、易于放大等优势外, 液滴流微反应器还具有诸如避免试剂交叉污染、液滴内部可控混合、易于独立调控、便于高通量筛选或者制备等独特特点, 使得其在功能材料制备、化学合成以及生物化工方面有着广泛的应用。     

Recent developments in scale-up of microfluidic emulsion generation via parallelization

Microfluidics affords precise control over the flow of multiphasic fluids in micron-scale channels. By manipulating the viscous and surface tension forces present in multiphasic flows in microfluidic channels, it is possible to produce highly uniform emulsion droplets one at a time. Monodisperse droplets generated based on microfluidics are useful templates for producing uniform microcapsules and microparticles for encapsulation and delivery of active ingredients as well as living cells. Also, droplet microfluidics have been extensively exploited as a means to enable highthroughput biological screening and assays. Despite the promise droplet-based microfluidics hold for a wide range of applications, low production rate (<<10mL/hour) of emulsion droplets has been a major hindrance to widespread utilization at the industrial and commercial scale. Several reports have recently shown that one way to overcome this challenge and enable mass production of microfluidic droplets is to parallelize droplet generation, by incorporating a large number of droplet generation units (N>>100) and networks of fluid channels that distribute fluid to each of these generators onto a single chip. To parallelize droplet generation and, at the same time, maintain high uniformity of emulsion droplets, several considerations have to be made including the design of channel geometries to ensure even distribution of fluids to each droplet generator, methods for large-scale and uniform fabrication of microchannels, device materials for mechanically robust operation to withstand high-pressure injection, and development of commercially feasible fabrication techniques for three-dimensional microfluidic devices. We highlight some of the recent advances in the mass production of highly uniform microfluidics droplets via parallelization and discuss outstanding issues.     

多种类型液滴蒸发综述

液滴蒸发过程是伴随着复杂变化但又没有统一且充分认知的的过程,是目前一个重要的研究热点,在许多科学应用中起到关键作用。本文介绍了液滴蒸发的历史研究过程,综述了3种不同类型液滴,即纯液滴、二元混合溶液液滴和聚合物溶液液滴蒸发过程的研究成果,分析了液滴蒸发过程中和蒸发结束后沉积物的影响因素,简述了液滴的研究成果在实际生活中的应用。现有研究表明,不同类型液滴的蒸发过程受到多种因素的影响,比如溶液中的纳米粒子、环境温度和压力等。这些因素还会影响到沉积物的图案和大小。目前,研究人员已经研究出典型的液滴蒸发过程（接触线固定和接触角固定模式）,讨论出液滴蒸发基本理论。对一些常见的二元及多元溶液,研究人员已经发现它们与纯溶液蒸发过程的不同之处,并且已经在科学界进行了大量的研究及讨论,建立出数学模型。最后重点介绍了液滴蒸发在医学领域的研究成果、应用和未来发展的方向,比如通过生物液滴蒸发后的沉积物的纳米层,跟正常沉积物对比结果来检测疾病等。最后对液滴蒸发理论的现状、潜力和未来发展需求进行了总结和展望。     

Turbulent flow of shear-thinning liquids in stirred tanks—The effects of Reynolds number and flow index

LDA measurements are reported on the turbulent velocity fields in vessels agitated by a Rushton turbine and containing Newtonian as well as non-Newtonian, shear-thinning fluids. Ten different liquids were investigated, with flow indices varying from 1.00 down to 0.56. Experiments were performed in three vessel sizes, viz. 28.6, 44.1, and 62.7 cm in diameter, at various impeller speeds. The main issue of the paper is the question whether or not, and if so to what extent, turbulent flow of shear-thinning fluids differs from that of Newtonian liquids, or – in other words – whether and when turbulent flow of shear-thinning liquids exhibits Reynolds number similarity.     

Characterization of helical impellers by circulation times

Measurements of the circulation time during batch mixing of Newtonian and non-Newtonian fluids were used to analyse and compare the performance of six different helical-ribbon impellers and a screw impeller inside a draft coil. The distribution of circulation times obtained from two hundred repeated measurements is shown to depend markedly on the impeller geometry. Mixing time data with the helical-ribbon impellers are correlated with the mean value and the reduced standard deviation of the circulation times. Effective (rapid) mixing corresponds to a large circulation capacity and a wide distribution of the circulation times. The mixing mechanism of Newtonian fluids with the helical-ribbon impellers is qualitatively described by the Voncken model. The helical impellers' circulation capacities are not affected by the shear-thinning properties of non-Newtonian fluids. However, in highly shear-thinning fluids, the presence of important stagnant zones is responsible for much longer mixing times which consequently do not correlate with the circulation parameters. The relative efficiencies of the different impellers in Newtonian fluids are compared using a criterion based on the the total energy required to achieve a certain degree of mixing. The wide blade impeller is the most efficient of the helical-ribbon impellers but is considerably less efficient than the screw impeller in a draft coil.     

Experimental investigation of the flow dynamics of rheologically complex fluids in a Maxblend impeller system using PIV

The Maxblend^® mixer is used in processes involving Newtonian, shear-thinning or viscoelastic fluids and, as for many impellers, little is known regarding the relationship between the rheological behavior and the flow and mixing properties. An experimental study of the hydrodynamics in the Maxblend was carried out using Newtonian and non-Newtonian fluids in laminar and early transitional regimes. Flow fields were measured by two-dimensional particle image velocimetry (PIV) to evaluate the effect of highly shear-thinning and viscoelastic behavior on the performance of the mixer. The experimental setup consisted of a 35-L tank equipped with two baffles. A total of 100 measurements were proven to be sufficient in order to attain convergence of the velocity components for all studied regimes. Isolated zones in the highly shear-thinning case were observed by means of a decolorization technique. Elasticity in the laminar regime produced a reversal of the flow and a solid body rotation in the bottom region of the tank. Fluid velocity magnitude and spread were observed to be reduced by the presence of elastic forces.