毛细管内气液Taylor流动的气泡及阻力特性

采用相对坐标系方法,研究毛细管(d 2mm)内充分发展垂直上升气液Taylor流动,分析两种工作介质下Taylor气泡的形状、上升速度、液膜厚度以及压降特性。结果表明:随着两相表观速度(V_tp)增大,Taylor气泡长度增大,气泡尾部曲率半径增大。气泡长度及内部回流区随着气泡体积分数(ξ_g)增大而增大,量纲1液膜厚度与气泡上升速度与毛细数(Ca)正相关,模拟结果与经验公式吻合较好。摩擦阻力因子(f_c)随V_tp与ξ_g的增大而降低,N₂/乙二醇为工质的Taylor流动f_c低于单相情况,而N₂/水为工质的Taylor流动f_c高于单相情况。Kreutzer等的流型依赖公式以及Lockhart等的分离模型可较好预测本文的两相压降,模拟结果与预测值的误差在±10%以内,常规通道所推荐C 5仍然适用于本文毛细管情况。     

方肋微通道内流动沸腾的气泡动态与传热特性分析

为揭示方肋微通道热沉内流动沸腾的传热传质机理,本文基于耦合VOF方法与“饱和界面”相变模型对微通道内单个气泡绕流加热方肋的传热传质过程进行了数值研究。通过分析该过程中气泡增长速率与方肋壁面传热系数的变化,重点讨论了初始气泡体积和入口雷诺数Re对相变传热效率和流动结构的影响。结果表明：在气泡流经加热方肋过程中,气泡与方肋表面之间形成一层薄液膜,该薄液膜的相变蒸发极大强化方肋表面的换热效果,换热系数较相同条件下的单相流动提升6倍以上。此外液膜厚度随Re增大而变厚,液膜热阻相应增大,液膜蒸发对换热的促进作用随Re增大而降低。最后考察了气泡体积对方肋壁面换热的影响,结果表明：初始体积大的气泡具有更薄的液膜厚度及更大的蒸发面积,表现出更高的相变传热效率;而小气泡对壁面温度影响较小。     

多排平直翅片管换热器表面气液降膜流动特性的三维数值模拟

基于VOF模型建立了考虑重力、表面张力及界面摩擦力源项的多排平直翅片管换热器表面气液两相降膜流动三维瞬态CFD模型。不同气流速度下液膜厚度模拟结果与文献中试验值吻合较好,最大偏差小于5%,表明所建立CFD模型是可靠的。通过研究壁面接触角为30°时不同气液Reynolds数下液膜流动特性,结果表明：翅片管表面满膜流的临界Reynolds数Re_l为239,临界喷淋密度为0.06 kg/(m·s);在239 ≤ Re_l ≤ 995内,其平均液膜厚度较Nusselt理论解高16.8%～35.1%;气液逆流和顺流时气相Reynolds数Re_g应分别小于2190.7和3286.0,其主要原因在于过高的Re_g会导致气液界面摩擦力快速增大,从而引发液膜破裂和液滴脱落等现象恶化设备性能。总之,气液顺流更有利于在较高气相Reynolds数下实现翅片管表面的较薄满膜流动。     

0.5mm毛细管内气-液Taylor流动换热数值研究

采用移动计算域方法研究0.5mm毛细管内充分发展的气液Taylor流动换热特性,分析了Taylor气泡的形状、压降与换热特性。结果表明,随着入口Reynolds数Re的增大,气泡尾部的不稳定区域增大,液膜厚度逐渐增大,气泡长度变长;随着气泡体积分数ξ_g的增大,气泡形状基本不变而长度逐渐增大。阻力因子f随Re、ξg增大而降低,两相阻力系数高于单相的情况。平均Nusselt数Nu_(tp)随Re增大而增大,增大趋势逐渐降低;随ξg增大而线性降低。Taylor流的Nu_(tp)为单相的1.2~3倍,强化换热效果。     

0.5 mm毛细管内气-液Taylor流动换热数值研究

采用移动计算域方法研究0.5 mm毛细管内充分发展的气液Taylor流动换热特性,分析了Taylor气泡的形状、压降与换热特性。结果表明,随着入口Reynolds数Re的增大,气泡尾部的不稳定区域增大,液膜厚度逐渐增大,气泡长度变长;随着气泡体积分数ξ_g的增大,气泡形状基本不变而长度逐渐增大。阻力因子f随Re、ξ_g增大而降低,两相阻力系数高于单相的情况。平均Nusselt数Nu_tp随Re增大而增大,增大趋势逐渐降低;随ξ_g增大而线性降低。Taylor流的Nu_tp为单相的1.2～3倍,强化换热效果。     

微小通道内低Reynolds数液-液两相流动与换热特性实验研究

实验研究了圆形微小通道内液-液两相流的流动和换热特性。选用去离子水为分散相,高黏度二甲基硅油为连续相。通过处理高速摄像所拍摄的可视化图像,总结了液-液两相流流型和液滴的长度/形状特征。并在此基础上考察了低Reynolds数下液-液弹状流对微小通道的换热作用。结果表明,平均Nusselt数随着Reynolds数的增加而增加,且油水比越大传热系数增加幅度越明显。Nu随着含水率的增加而降低。虽然含水率增加会使两相平均热容量提高,但在低Reynolds数下,这种提高被其长液滴内较弱的循环强度所抵消。选用三种不同形式接头在相同混合速度和含水率的情况下生成不同长度的液滴,发现短液滴更有利于换热。相同工况下,液滴长度的优化可以使整体传热系数提高近26%。     

管内垂直下降液膜速度与厚度分布特性

对洗涤冷却管内垂直降膜的流动特性进行研究,采用超声波多普勒测速仪对管内不同周向以及轴向位置的液膜厚度和速度进行了无接触式的测量,液膜Reynolds数范围为1.0×10⁴~3.1×10⁴。结果表明:在0°周向位置上液膜厚度与速度均达到最大值,导致该位置局部液膜厚度过大而不能保持稳定,部分液体脱离液膜表面,此外还造成了8°和16°位置的液膜厚度激增。在轴向上,当Reynolds数小于2.0×10⁴时,液膜速度在重力作用下随流动距离增加而增加,反之,液膜速度因为流动阻力会随距离增加而减小。随着Reynolds数的增大,液膜平均厚度和速度呈增大趋势。此外,Reynolds数的增大还会使得液膜更加不稳定。     

管内垂直下降液膜速度与厚度分布特性

对洗涤冷却管内垂直降膜的流动特性进行研究,采用超声波多普勒测速仪对管内不同周向以及轴向位置的液膜厚度和速度进行了无接触式的测量,液膜Reynolds数范围为1.0×10~4~3.1×10~4。结果表明:在0°周向位置上液膜厚度与速度均达到最大值,导致该位置局部液膜厚度过大而不能保持稳定,部分液体脱离液膜表面,此外还造成了8°和16°位置的液膜厚度激增。在轴向上,当Reynolds数小于2.0×10~4时,液膜速度在重力作用下随流动距离增加而增加,反之,液膜速度因为流动阻力会随距离增加而减小。随着Reynolds数的增大,液膜平均厚度和速度呈增大趋势。此外,Reynolds数的增大还会使得液膜更加不稳定。     

低Reynolds数下水平管降膜滴状流动的实验研究

为了充分了解水平管降膜滴状流动现象,以水为实验流体,采用高速高清摄像仪对不同管距下水平管降膜滴状流动进行了可视化的实验研究。实验观察了低Reynolds数Re≤200、管间距s≤40 mm 范围内的管间滴状流动过程。根据流动形态的特征,将滴状流动划分为堆积滴状流、不完全滴状流、吊坠滴状流、完全滴状流和不完全回缩滴状流。依据实验结果,绘制了滴状流动形态分区图。提出了分离长度的概念,并讨论了流量对分离长度的影响。研究了低Reynolds数下液滴流动的滴落点间距与流量的关系。结果表明,滴状流的管间流动形态不仅受流量的影响,同时也受管间距的影响。分离长度随着Re的增大呈两段式线性增大。液滴流动的滴落点间距在Re≤100范围内随Re增大而减小,在100<Re≤200范围内趋于平稳。此外,分别建立了分离长度和滴落点间距与Re的关系式。     

循环射流混合槽内液液两相混合特性数值模拟

循环射流混合槽作为一种高效的混合装置在化工过程强化处理技术中具有潜在的工业应用前景。由于缺乏对其内多相体系流动和混合行为的研究,制约了循环射流混合反应器的优化设计与工业化应用。本文选取水和二甲基硅油两相体系,采用计算流体力学软件ANSYS Fluent V16.1中Eulerian-Eulerian多相流模型和SST k-?湍流模型,对两种不同加料方式下循环射流混合槽内液液两相射流中心线速度、离析强度、拉伸率等参数进行研究。研究结果表明：分散相浓度（α_d）增大射流卷吸能耗增大,在l/s<0.4内α_d=1.80%和2.86%量纲为1的射流中心线速度衰减趋势与α_d=6.00%相比减弱51%和21%;在低分散相浓度时,量纲为1的射流中心线速度随Re的增大衰减趋势变化小,在l/s<0.24内Re=6346、9519和12692量纲为1的射流中心线速度衰减趋势与Re=3173相比分别减弱2.60%、2.87%和12.69%。离析强度随混合时间的增大而减小,随周向角度增大呈W形变化趋势。在相含率和雷诺数相同时,对称球状较圆柱状加料达到混合时间减少65.5%;不同喷嘴之间的拉伸率随迹线长度的增大而增大,jet1和jet9位置处的拉伸率与其余喷嘴相比较大;相同喷嘴之间拉伸率随Re的增大而增大,Re=6346、9519和12692的拉伸率与Re=3173相比分别提高289%~320%、418%~454%和607%~667%。     

MAXIMIZING SELECTIVITY OF LIQUID-LIQUID REACTION SYSTEMS. CONTROL OF THE DISPERSION PROCESS

Currently available information on droplet coalescence and break-up rates in turbulent flows in mixing vessels can be used to control drop sizes in dispersed phase equipment. The effect of drop size distributions on the selectivity and productivity in multi-reaction systems is examined in this paper.

The reaction system features the primary desired product (C) as resulting from reaction (in the bulk phase) between a reactant (A) in the drop phase and a second reactant (B) in the bulk phase. An adverse reaction is also envisaged which consumes (C) by further reaction with (B) to form a waste product. While small drops promote conversion because of large interfacial area, larger drops promote selectivity because of the facility of the product to re-enter the drop phase avoiding further reaction (to form waste) in the bulk phase. The effect of the bivariate distribution of drop size and reactant (A) concentration in the feed to a continuous stirred tank reactor on the selectivity and productivity of (C) is investigated within the framework of film theory while neglecting drop dynamics such as coalescence and break-up.

The results show the selectivity can be substantially improved by controlling drop size and distribution of the reactants among the differently sized droplets. Contrary to conventional wisdom which emphasizes creation of interfacial area by promoting very small droplets, it emerges that optimal distributions of drop size and reactant concentration which maximize productivity of the desired product exist. The practical implications are discussed.     

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。     

The Experimental Observation and Modelling of Film Thinning and Film Retraction during the Interfacial Coalescence of Biodiesel and Glycerol Droplets

This paper is concerned with the way interfacial coalescence of a single drop of biodiesel or glycerol occurs at a glycerol/biodiesel interface. Two stages of interfacial coalescence were studied: the thinning of the trapped liquid film between the rising or falling droplet and bulk fluid interface, and the retraction of the film after the film had ruptured. Unexpectedly, the thinning time for the high viscosity glycerol film around a rising biodiesel droplet was found to be much shorter than that for a low viscosity biodiesel film around a sedimenting glycerol droplet. Squeeze flow modelling showed that the film of glycerol around a biodiesel droplet was bounded by relatively inviscid biodiesel and therefore flowed with high slip at its interfaces, resulting in rapid film thinning. The biodiesel film around a glycerol droplet was bounded by highly viscous glycerol and flowed with little slip at the interfaces, resulting in slower film thinning. After rupture, film retraction was found to be much faster for biodiesel droplets than for glycerol droplets. The drag exerted by the fluid surrounding the film was found to control the retraction kinetics. The results are of particular relevance to the separation of glycerol from biodiesel and of general relevance to coalescence kinetics for immiscible drops at an interface.     

界面物性对液滴聚结的影响

推导了分散相液滴运动轨迹模型和分层二相流基本流模型,并优化了计算方法。直接数值模拟结果表明：当层膜厚度和板宽变化时,流场的密度、黏度以及界面张力对界面处的液滴的作用方式不相同;黏度始终对液滴聚结产生明显的影响,而密度和界面张力对聚结的影响却因流场结构的变化而有所不同;密度、黏度及界面张力的变化使得液滴的顺时针或逆时针旋转运动更加具有倾向性。因此,密度和黏度在一定程度上可用于改善流场的聚结条件。数值模拟的结果与已知的实验结果吻合较好,而且,基本流的介入使得该模型能较好地反映液滴在分层二相流流场中的运动特性。     

水平圆形与方形微小通道内R134a冷凝数值模拟

利用数值模拟研究了水平圆形与方形微小通道内R134a的冷凝换热阻力特性,制冷剂饱和温度为320 K。结果表明:传热系数与摩擦压降梯度随着质量流量、干度的升高而升高,而干度大于0.85时,摩擦压降梯度随着干度的升高而降低。方形通道的换热与阻力均高于圆形通道,数值结果与文献冷凝换热、阻力公式吻合较好。圆形通道内冷凝液膜集聚在通道下部,而方形通道内液膜集中在角落区域。薄液膜区域所占的比例随着干度的增大而增大,方形通道内的液膜厚度要小于圆形通道,换热效果优于圆形通道。     

两种烧结通道的沸腾传热和阻力特性对比

多孔材料对沸腾换热的强化是能源化工领域的重要主题。本文针对两种不同的烧结结构——并联微通道和扁平通道（仅有烧结底层）,以去离子水为工质,进行了过冷流动沸腾换热实验对比研究。研究发现：并联微通道的传热系数和临界热流密度远高于扁平通道,这和并联微通道优异的毛细供液性能相关。底厚粒径比对并联微通道的沸腾换热性能影响较大,过大的底厚粒径比会造成换热性能的下降。质量通量对小粒径样品的沸腾曲线和换热性能均影响较大,对大粒径（d=120μm）样品的沸腾曲线影响较小。烧结并联微通道的平均压降大于扁平通道。相同底厚下,平均压降随着微通道粒径的增大而增大。可视化观察表明：两种通道在中高热流密度流型不同,其主要相变机制均为薄液膜蒸发模式。     

An experimental study of air-water Taylor flow and mass transfer inside square microchannels

Flow and mass transfer properties under air-water Taylor flow have been investigated in two square microchannels with hydraulic diameters of 400 and 200 μm. Experimental data on Taylor bubble velocity, pressure drop and liquid side volumetric mass transfer coefficient (k_La) have been presented. It was shown that the measured Taylor bubble velocity in square microchannels could be well interpreted based upon an approximate measurement of the liquid film profile therein. Then, the obtained two-phase frictional pressure drop values in both microchannels were found to be significantly higher than the predictions of the correlation proposed by Kreutzer et al. [2005b. Inertial and interfacial effects on pressure drop of Taylor flow in capillaries. A.I.Ch.E. Journal 51, 2428-2440] when the liquid slug was very short, which can be explained by the inadequacy of their correlation to describe the excess pressure drop caused by the strong inner circulation in such short liquid slugs. An appropriate modification has been made to this correlation in order to improve its applicability in microchannels. Finally, the experimental (k_La) values in the microchannel with hydraulic diameter of 400 μm were found to be in poor agreement with those predicted by the existing correlations proposed for capillaries with diameters of several millimeters. The observed deviation was mainly due to the fact that mass transfer experiments in this microchannel actually corresponded to the case of short film contact time and rather poor mixing between the liquid film and the liquid slug, which was not in accordance with mass transfer assumptions associated with these correlations. A new empirical correlation has been proposed to describe mass transfer data in this microchannel.     

油水两相分散流液滴粒径预测模型

油水两相分散流的液滴粒径及其分布在很大程度上影响管路压降等流动参数,研究液滴粒径预测模型对揭示油水两相流的流动特性具有重要意义。通过研究湍流脉动动能与乳状液的界面能之间的平衡、管流径向速度脉动与摩擦速度之间的关系以及泵的剪切作用,建立了油水两相管流中分散相液滴粒径预测模型;在水平管道上对油水两相分散流的液滴特性进行了实验研究,采用高速摄像和显微镜拍摄获得液滴数据,探索含油率、流量和温度等因素对粒径的影响。预测模型计算结果与不同流量、温度和含油率条件下的实验数据吻合较好。根据预测模型计算了有泵和无泵情况下分散流液滴粒径,发现泵的剪切和扰动作用使得分散液滴具有更小的粒径,泵对液滴粒径及其分布起到了显著作用。     

ANALYTICAL SOLUTION FOR LAMINAR-LAMINAR TWO-PHASE STRATIFIED FLOW IN CIRCULAR CONDUITS

Many attempts made in modelling stratified two-phase flow for predicting the operational flow characteristics (pressure drop, holdup, etc.) assume mostly plane interface between the phases. Obviously, the interaction between the stratified layers and the resulting flow characteristics may be significantly affected by the configuration of the interface. Moreover, complete analytical solutions for stratified flows in circular conduits a re not yet available even for laminar flows with plane interface and most of previous studies resort to average two-fluid modelling.

The present study presents analytical solutions for laminar stratified two-phase flows in pipes with plane and curved interfaces.