微通道内气-液两相传质过程行为及其应用

微通道内气-液两相体系中Taylor流和泡状流具有气泡尺寸均一、停留时间分布窄、可调控性强和比表面积高等优点,具有广泛的应用前景。从Taylor气泡和泡状气泡的传质过程出发,系统综述了微尺度下气泡的溶解规律、传质过程机理和传质/溶解模型等方面的研究进展,并介绍上述流型在反应或过程强化、基础物性及动力学数据测量和微纳材料合成方面的应用。最后总结并展望了技术领域的研究难点与研究方向。     

超声微反应器内气液传质过程的介尺度强化机制

采用CFD方法对超声微反应器内的Taylor气液两相流的传质过程进行了模拟。针对传质过程中主要的介尺度结构,包括气泡表面波、空化声流、液相内的局部浓度,分析了其空间分布和时间演化规律。模拟结果有效捕捉了实验难以观测的液膜区域,并将液膜厚度与气泡表面波振动进行了关联,阐释了气液界面附近的空化声流对传质过程的强化作用。根据超声微反应器内Taylor流的传质特点,分别研究了不同流动和超声条件对液弹内和液膜处传质过程的影响,比较了各局部传质对整体传质效率的贡献。通过分析整体/局部Sherwood数与Peclet数间的关系,研究了超声效应对气液传质速率的影响。分析结果从介尺度角度验证了文献关于超声微反应器传质系数的计算,完善了超声微反应器内气液传质过程的强化理论。     

微通道内气-液弹状流动及传质特性研究进展

气-液弹状流,又称Taylor流,是一种以长气泡和液弹交替形式流动的流动形态。微通道内气-液弹状流因其气泡与液弹尺寸分布均一、停留时间分布窄、径向混合强等优点,是一种适于强化气-液反应的理想流型。本文首先介绍了微通道内气泡的生成机理、气泡和液弹长度,以及气泡生成阶段的传质特征。其次系统综述了主通道中弹状流动及传质过程的研究进展,包括气泡形状与液膜厚度、液弹内循环和泄漏流特征、气-液传质系数的测量与预测,以及物理与化学吸收过程中的传质特性等方面内容。最后阐述了当前研究的不足并展望了气-液弹状流的研究方向。     

微通道气液两相流研究进展

介绍了国内外微化工发展背景及发展现状,微通道内气液两相流流动气泡特性和传质特性。微通道中(横向及竖向微通道)气液两相流流型划分主要有泡状流、弹状流、环状流、搅拌流等。气泡形成过程中流体挤压力对气泡表面进行破坏致使分离,表面张力则在整个过程中维持着气泡形状及长度。着重介绍了微通道内气泡形成过程及经验长度计算,比较了不同研究者提出的经验公式中气液相表观速度比和气泡长度的关系,得出气泡长度均随气液相表观速度比的增加而增加,但依据研究者实验条件不同增加趋势也不尽相同。传质方面,研究基本集中在气液相比表面积较大的泡状流、环状流上,而气液表观速度、当量直径、压强等都会影响传质系数。微通道气液两相流虽然在传热、传质方面有很大的应用前景,但仍存在研究手段单一、理论数据不完善等问题,指出在未来的研究中研究者们要扩大领域范围,为传质传热的实际应用提供更可靠的理论依据。     

微通道内气液两相流行为研究进展

综述了微通道内的气液两相流行为及传质特性。在微通道内流型一般分为泡状流、弹状流、环状流和弹状-环状流,没有分层流。气液传质效率比常规尺度中的提高了2～3个数量级。讨论了气泡对气液两相流的影响及其生成、生长和聚并规律。介绍了微通道内气液两相流的计算机模拟结果。从实验、理论和数值模拟3个方面对微通道内气液两相流的研究和应用前景进行了展望。     

微通道内Taylor流的计算流体力学数值模拟研究进展

简要阐述了计算流体力学(CFD)中流体体积函数法(VOF)的概念和原理。重点综述了采用CFD方法研究微通道中两相Taylor流的最新进展,包括两相流速度场、气泡或微液滴的生成、发展等流动特性、壁面处液膜厚度的影响因素、微通道内的传质传热特性等。最后展望了微通道内多相流CFD数值模拟的研究重点和应用方向。     

微气泡耦合生物反应器的研究进展

微气泡具有气液接触面积大、气体溶解速率快、上升速度慢和水中停留时间长等理化特征，非常适合于高气液传质效率需求的生物发酵过程。本文介绍了能够耦合生物反应器的几种微气泡发生装置，分别为微气泡分散器、微孔膜、流体振荡器耦合微孔膜和微气泡曝气搅拌桨；并简述了微气泡发生装置耦合搅拌式生物反应器、气升式生物反应器和生物膜反应器在生物反应过程的应用进展；最后回顾了二氧化碳微气泡在生物反应器的应用研究进展。指出微气泡耦合生物反应器的研究仍处于起步阶段，在放大规律和能耗方面仍处于研究空白。微气泡耦合生物反应器的发展对工业生物技术、石油化工、污水处理和资源再利用等的发展具有重要的意义。     

泰勒流传质机制研究进展

微通道内气液两相泰勒流具有周期性、停留时间短以及比表面积大等优点,使其具有广泛的应用前景。全文阐述了气泡生成过程中的传质,介绍了泰勒流传质研究的模拟计算模型,以及相关实验得到的经验式。最后概括了目前泰勒流传质的研究进展并展望了泰勒流以后的研究方向。     

基于微吸收器的CO2吸收过程研究进展

微化工技术在流体流动、过程强化、传质与反应过程等领域备受关注,本文归纳整理了3种不同类型的微吸收器（微降膜吸收器、微通道吸收器和微网格吸收器）捕集CO₂过程中的水力学性质和传质过程及其机理研究进展,并对3种微吸收器吸收CO₂过程中存在的问题进行分析总结,同时对微吸收器能快速工业化提出展望。其中重点介绍了微通道泰勒流吸收器的水力学流动特性,包括泰勒流气泡的生成机制、气泡和液弹的长度、气泡的输运和运动速度、气泡截面形状及液膜厚度和气液两相流压降;归纳了微通道泰勒流吸收过程的传质过程机理和传质系数的模型以及不同影响因素（通道截面尺寸,通道长度,主通道结构及入口形状,气、液相组成及其流速,吸收剂和系统压力）作用下CO₂吸收效率和传质系数的研究进展。     

壁面润湿性对微通道内二氧化碳-水两相流流动及传质性能的影响

在1 mm×1 mm矩形截面下微通道内,以二氧化碳-水为工作流体,研究壁面润湿性和气液表观流速对气-液两相流型和气液传质的影响,并研究了气、液表观流速对弹状流流体力学性质的影响。在亲水微通道中观测到了泡状流、泡状-弹状流、弹状流;在疏水微通道中观测到了非对称弹状流、拉长的非对称弹状流、分层流。实验表明亲水微通道中弹状流区域下气泡长度大体上随气相表观流速的增大而增大,随液相表观流速的增大而减小;液弹长度大体上随气相表观流速的增大而减小,随液相表观流速的增大先增大后减小;液侧体积传质系数k_La均随气、液相表观流速的增大而增大,随通道壁面润湿性的增强而增大。     

Hydrodynamik in Blasensäulen – Messung von relativem Gasgehalt und Blasengröße

In bubble column reactors mass transfer kinetics are determined by local hydrodynamic processes (bubble formation, velocity, coalescence, break‐up, etc.). For a better understanding of the influence of pressure and particle load on these processes, a measurement system for detection of local bubble size distributions in opaque bubbly flows is needed. Therefore, in this work an optical needle probe was applied in a bubbly flow. In a first step, the needle probe was validated by means of single‐bubble measurements. The measurement technique shows high potential for application in more complex and opaque systems.     

Interfacial area density in bubbly flow

The interfacial area per unit volume is one of the key parameters in bubbly flow. Momentum, mass and energy transfer occur through the interface between the phases. The functionality of two phase reactors with bubbly flow depends mainly on these three transfer processes. Thus, the design process of a reactor requires the prediction of interfacial area density. In the present work a simple equation for the interfacial area density is derived from the population balance, taking into account the events of coalescence and bubble break-up for each bubble fraction. The system of partial integro-differential equations is simplified. Since the integrals in these equations complicate a numerical treatment. This reduces the balance to one single partial differential equation. An approximate analytical solution is given. If the resulting equation is applied to large gas fluxes, the instability of the coalescence process causes large bubbles and gas plugs to develop. From the instability the volume fraction of the large bubbles and gas plugs may be predicted. Additives may hinder the coalescence process. Experiments show that coalescence hindrance changes the coalescence kernel only by a factor. Calculations are done for bubble columns and vertical pipe flow.     

毛细管中泰勒流液侧传质特性及其强化机理的CFD模拟

采用计算流体力学（CFD）的方法,研究了圆管中泰勒流的液侧传质特性,分析了泰勒气泡上局部传质特性,并研究了气泡上升速度、液膜长度和液栓长度对液膜处和气泡半球帽处平均传质系数的影响。结果表明,泰勒气泡表面局部传质系数存在3个峰值,液膜处的平均传质系数随气泡上升速度增大显著增大,随液膜长度增大而减小,而半球帽处的平均传质系数随气泡速度和液膜长度的增大变化较小,即膜接触时间增加时,液膜处的传质系数降低,而半球帽处传质系数变化较小。另外,引入场协同原则对单元胞内速度场和浓度场进行分析,解释了局部传质特性及强化机理。最后,给出了分别预测短和长膜接触时间下泰勒流液侧体积传质系数的关联式,该式在较宽的管径尺度范围（0.25～3 mm）内的预测误差在±20%以内。     

CFD simulations of wall mass transfer for Taylor flow in circular capillaries

Computational fluid dynamics is used to investigate the mass transfer from the liquid phase to the channel wall for Taylor flow of bubbles rising in circular capillaries. The separate influences of the Taylor bubble rise velocity, unit cell length, gas holdup, and liquid diffusivity on mass transfer were investigated for capillaries of 1.5, 2 and 3 mm diameter. A correlation is proposed for estimation of the wall mass transfer coefficient and this correlation has been tested against published experimental data.     

计及气泡诱导与剪切湍流的气泡破碎、湍流相间扩散及传质模型

在以鼓泡塔为代表的气液鼓泡流动中,存在着气泡诱导湍流（BIT）和剪切湍流两种湍流机制,并且二者在不同的时间、空间范围内既相互竞争又共同作用。受制于BIT动能能谱的形式和特性不够完整清晰,过去的研究中关于BIT如何对气泡破碎聚并、相间作用力、相间传热传质等相间相互作用过程产生影响的结论比较模糊。因此,本文在具有波数κ^-3特性的BIT能谱的基础上,提出了在不同工况下考虑BIT与剪切湍流共同作用的研究思路。研究结果表明,考虑两种湍流机制的气泡破碎模型和湍流相间扩散模型对BIT在整体或局部占据不同程度主导地位的情况,都能很好地捕捉气液鼓泡流动的动力学特性,为进一步准确揭示气液相间传质过程的内在机理提供了基础。     

Degassing of molten polymers

Degassing is a key-step in polymer processing. Low-molecular-weight components are removed from a polymeric system. The transport of these components takes place by diffusion to the polymer-vapour interface. This interface can be formed by free surfaces of single-phase polymer melts or by bubbles. In this study, the transport with and without bubble nucleation is investigated independently from each other in a special designed apparatus similar to a degassing extruder.The mass transport in thin films and in rotating pools with surface renewal is measured. High surface renewal rates and thick films enhance the mass transfer for single phase flow and bubbly flow. Dimensionless mass transfer coefficients are given as a function of the surface renewal rate, the area of the free surface and the total mass of the polymer. The conditions for bubble nucleation and foam formation are investigated. The bubble nucleation is observed in the rotating pool in the area of high shear velocity.     

Experiment and lattice Boltzmann simulation of two-phase gas-liquid flows in microchannels

The two-phase flows in microchannels have many advantages in heat and mass transfer compared to single-phase flows. In particular, segmented flows such as bubbly and slug flows are often used in microfluidic devices. In the present study, experiments and Lattice Boltzmann simulations were carried out to study the gas-liquid flow in microchannels under various conditions. Two types of mixer geometries were used, including the cross-shape and the converging shape channels. The bubble shape, bubble size, and formation mechanism were investigated for different flow rates and different mixer geometries. The simulation results and the experimental results were compared based on dimensionless numbers, and good agreement was found in general. Different flow regimes with different bubble shapes were found depending on the Capillary number of the flow. The simulation data confirmed that the breakup was induced by the pressure difference in the two phases for small Capillary numbers. The geometry of the mixing section was also observed to have an impact on the size of the gas and liquid slugs.     

绝热层流泡状流运动的双流体模型

绝热层流泡状流是泡状流研究中的一个基础范例 .目前描述绝热层流泡状流常采用的双流体模型由于相间作用考虑欠缺而适用性差 .本文结合理论和实验研究结果导出了描述壁面“排斥”作用的表达式 ,并建立了一个封闭的双流体模型 .模型预测值和实验值的比较表明 ,由于相间作用的合理考虑 ,扩大了该模型的适用范围