垂直及倾斜上升管内气液两相弹状流壁面切应力的模拟

用VOF模型对垂直及倾斜上升管内弹状流壁面切应力进行数值模拟。计算结果表明,垂直上升流动时,液膜厚度始终小于对应位置倾斜上升弹状流的液膜厚度,壁面切应力从气弹头部至尾部逐渐增大至恒定不变,在尾流区呈杂乱无章状态。倾斜上升流动时,气泡头部顶点偏向管中心线上方,倾角越小,相同轴向位置处测得的液膜厚度越大。当FrTB较小时,倾斜管内弹状流上管壁面的切应力曲线在液膜区有明显波动,而下管壁面在对应区域的切应力分布则比较光滑。随着FrTB的增大,上下壁面切应力分布曲线越来越靠近。     

垂直上升气液两相弹状流模型

基于等效弹单元思想,改进了预测垂直上升管中充分发展气液弹状流流动特性的模型。 模型中考虑了界面切应力对液膜运动的影响;并在液弹空隙度预测中引入临界气体夹带速度的概念,以此来描述弹状流中大气泡尾部的混合特性。本文提出的模型还考虑了管径对液弹空隙度的影响。弹状流模型的计算结果得到本文及其他作者实验数据的验证。     

垂直管内弹状气泡上升时的壁面传质

Wall-liquid mass transfer for Taylor bubbles rising through liquid column in vertical tubes is an important and fundamental topic in industrial processes.In this work,the characteristics of wall-liquid mass transfer for this special case of slug flow were studied experimentally by limiting diffusion current technique (LDCT). Based on the experimental results and the analysis of hydrodynamic mechanisms,it was proposed that four different zones exist,i.e.the laminar falling film zone,the turbulent falling film zone,the wake region and the remaining liquid slug zone.The corresponding correlations for all these zones were developed.     

垂直管内弹状气泡上升中壁面传递的实验研究

垂直管内弹状流壁面传递是诸多工业应用中需研究的重要问题之一。今用极限扩散电流技术,对弹状气泡上升时瞬时壁面剪应力和传质系数进行了测定,结果显示:当基于表观气速的Froude数FrG < 0.74时,壁面剪应力随弹状气泡和液塞的到来呈现方向相反的交替变化,壁面传质系数亦相应变化;而当FrG > 0.74时,剪应力方向一直向下,说明液膜向下流动,且弹状气泡和液塞的到来对壁面传质系数的影响很小。这说明下落液膜射流穿透了液塞段,控制了整个壁面传递过程。研究还对下落液膜区、尾迹区及液塞段的不同传递特征及机理进行了分析, 并结合气泡塔熔融结晶器中弹状气泡上升时的传热,对结晶操作条件的合理选择进行了讨论。     

Void fraction measurement and calculation model of vertical upward co-current air–water slug flow

The focus of this paper is on the measurement and calculation model of void fraction for the vertical upward co-current air–water slug flow in a circular tube of 15 mm inner diameter. High-speed photography and optical probes were utilized, with water superficial velocity ranging from 0.089 to 0.65 m·s^-1 and gas superficial velocity ranging from 0.049 to 0.65 m·s^-1. A new void fraction model based on the local parameters was proposed, disposing the slug flow as a combination of Taylor bubbles and liquid slugs. In the Taylor bubble region, correction factors of liquid film thickness C_δ and nose shape C_Z^* were proposed to calculate α_TB. In the liquid slug region, the radial void fraction distribution profiles were obtained to calculate α_LS, by employing the image processing technique based on supervised machine learning. Results showed that the void fraction proportion in Taylor bubbles occupied crucial contribution to the overall void fraction. Multiple types of void fraction predictive correlations were assessed using the present data. The performance of the Schmidt model was optimal, while some models for slug flow performed not outstanding. Additionally, a predictive correlation was correlated between the central local void fraction and the cross-sectional averaged void fraction, as a straightforward form of the void fraction calculation model. The predictive correlation showed a good agreement with the present experimental data, as well as the data of Olerni et al., indicating that the new model was effective and applicable under the slug flow conditions.     

Hydrodynamics and local mass transfer characterization under gas–liquid–liquid slug flow in a rectangular microchannel

Gas–liquid–liquid three-phase slug flow was generated in a glass microreactor with rectangular microchannel, where aqueous slugs were distinguished by relative positions to air bubbles and organic droplets. Oxygen from bubbles reacted with resazurin in slugs, leading to prominent color changes, which was used to quantify mass transfer performance. The development of slug length indicated a film flow through the corner between bubbles and the channel wall, where the aqueous phase was saturated with oxygen transferred from bubble body. This film flow results in the highest equivalent oxygen concentration within the slug led by a bubble and followed by a droplet. The three-phase slug flow subregime with alternate bubble and droplet was found to benefit the overall mass transfer performance most. These results provide insights into a precise manipulation of gas–liquid–liquid slug flow in microreactors and the relevant mass transfer behavior thereof.     

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。     

Film and slug behaviour in intermittent slug–annular microchannel flows

Whilst there are numerous experimental, theoretical and computational studies of Taylor flow in microchannels, the intermittent slug–annular regime has largely been neglected. In this paper time-resolved micro-PIV data are collected and used to study the flow characteristics of a gas–liquid system for flow regimes spanning Taylor to annular flow. The experimental work used a 1.73 mm diameter channel with water and nitrogen as the working fluids, for gas and liquid superficial velocity ranges of 0.35–8.65 m s^?1 (40<Re_G<1000) and 0.071–0.18 m s^?1 (120<Re_L<300), respectively. Time-averaged velocity profiles were obtained in the liquid film surrounding the gas bubbles (or the gas core in the pseudo-annular flow regime) and in the liquid slugs (which changed from regular slugs to annular rings as the gas superficial velocity was increased). These data showed that the velocity in the liquid film relaxed back to an equilibrium value following the passage of each liquid slug or annular ring. In contrast rather flat velocity profiles were observed in the liquid slug. Based on a simple representation of the flow structure, average gas holdups were estimated using independent experimental data obtained by the micro-PIV technique and by direct observation of the flow structure. A phenomenological model of intermittent slug flow, based on the representation of the flow structure as a train of slugs and bubbles moving over a liquid film, is used to interpret the experimental data. The modelling work highlights the different behaviour of the limiting cases of slug and annular flow, in terms of the gas–liquid interfacial shear and its influence on the pressure field.     

Influence of operating conditions on the observed reaction rate in the single channel monolith reactor

The catalytic hydrogenation of nitrobenzoic acid (NBA) to the aminobenzoic acid was used as a model reaction for a quantitative study of influences of the operating conditions on the observed reaction rate in a single channel monolith reactor operated in Taylor flow regime. A simple mathematical model was derived and used for the analysis of hydrogenation experiments carried out in batch mode. Results showed that in the investigated concentration range of NBA, i.e. 0.0005–0.02 mol/l and under the hydrogen pressure of 1 bar, the observed reaction rate is considerably limited by mass transport. At higher concentrations of NBA, the reaction is controlled by the hydrogen mass transport while at lower concentrations the mass transport of NBA is dominant. The analysis of experimental results, which were obtained when the length of gas bubbles and liquid slugs were varied, showed that the reaction took place in the thin liquid film surrounding the gas bubble. The liquid slug serves as exchanger of reactants and reaction products between bulk liquid slug and liquid film surrounding the catalyst surface.     

Hydrogenation of 2‐ethylanthraquinone under Taylor flow in single square channel monolith reactors

The hydrogenation of 2‐ethylanthraquinone (EAQ) to 2‐ethylanthrahydroquinone (EAHQ) was carried out under Taylor flow in single square channel monolith reactors. The two opening ends of opaque reaction channel were connected with two circular transparent quartz‐glass capillaries, where Taylor flow hydrodynamics parameters were measured and further used to obtain practical flow state of reactants in square reaction channels. A carefully designed gas‐liquid inlet mixer was used to supply steady gas bubbles and liquid slugs with desired length. The effects of various operating parameters, involving superficial gas velocity, superficial liquid velocity, gas bubble length, liquid slug length, two‐phase velocity and temperature, on EAQ conversion were systematically researched. Based on EAQ conversion, experimental overall volumetric mass transfer coefficients were calculated, and also studied as functions of various parameters as mentioned earlier. The film model, penetration model, and existing semi‐empirical formula were used to predict gas‐solid, gas‐liquid, and liquid‐solid volumetric mass transfer coefficients in Taylor flow, respectively. The predicted overall volumetric mass transfer coefficients agreed well with the experimental ones. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

The pressure drop experiment to determine slug lengths in multiphase monoliths

The length of the liquid slugs, that separate the elongated bubbles in Taylor flow, is an important parameter for mass transfer, flow stability and pressure drop in capillary microchannels. In this work, pressure drop measurements are used to determine the length of slug in Taylor flow in downflow monoliths. The method is sensitive if the slugs are relatively short, less than 10 times the channel diameter. The pressure drop measurements are a cheap and fast alternative to tomographic or electric methods. Experiments using different distributors indicate that the slug length varies significantly with changes in the hydrodynamics in the feed section of the monoliths. Slug length correlations that are based on parameters inside the channels can therefore not safely be used for a different setup. As a result, the slug length should be measured in each experimental setup, which makes a inexpensive and robust method to do so very welcome.     

水平管段塞流持液率的波动特性

气液两相段塞流是液塞和长气泡在空间和时间上的交替,在流动过程中表现出间歇性和不稳定性.今对水平管中段塞流持液率的波动特性进行了分析.结果表明:在同一折算液速下,随着折算气速的增加,段塞单元的平均持液率和液膜持液率先快速下降再缓慢下降,而液塞持液率先缓慢下降再快速下降.段塞流持液率的概率密度分布为双峰分布,高持液率峰对应于液塞区,低持液率峰对应于液膜区;概率密度函数中较完好的峰所对应的持液率与光滑分层液膜区和液塞区的平均持液率相一致.     

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。     

Hydrodynamics of an external-loop gas-lift system with restrictions located in the downcomer

Experiments were conducted to study the fluid dynamics in the case that slug flow occurs in the riser of an external-loop gas-lift system with a restriction section located in the downcomer. Complex fluctuation behaviors of the liquid circulation velocity and the wall shear stress in the riser were observed and discussed. Based on the slug flow hydrodynamic behaviors and the balance of momentum and pressure drop over the circulating loop, a model was developed to predict the main parameters of the system: the liquid circulation velocity, the void fraction, the length and velocities of Taylor bubbles and liquid slugs. The predicted results of these parameters were compared with the experimental data and a good agreement was obtained.     

水平管气液两相段塞流的波动特性

气液两相段塞流是液塞和长气泡在空间和时间上的交替,在流动过程中表现出间歇性和不稳定性。系统地研究了水平管中段塞流持液率、压力和压差的波动特性。结果表明,段塞流持液率的概率密度分布为双峰分布,高持液率峰对应于液塞区,低持液率峰对应于液膜区;在压力的概率密度分布中,当压力测试点到管道出口之间的段塞单元数目少时,压力分布出现双峰分布;当压力测试点到管道出口之间的段塞单元数目多时,压力分布出现单峰分布;压差信号分布呈单峰分布。这些特征为流型识别提供了可靠的段塞流标识。     

CFD approaches for the simulation of hydrodynamics and heat transfer in Taylor flow

Previous studies on heat and mass transfer in the Taylor flow regime in microchannels have shown the transport (heat/mass) rates to be dependent on the length of the liquid slug. In order to understand the effect of slug length on transport rates and to have a one-to-one comparison with experimental data, a computational approach is required to simulate flows with liquid slugs and bubbles of controlled lengths.Here we describe and benchmark two approaches. The first, and conceptually simplest, is to generate bubbles and slugs in a long tube using a time-dependent boundary condition. In the second method, the flow and heat transfer in a single unit cell, consisting of a bubble surrounded by liquid slugs, is solved in a frame of reference moving with the bubble velocity. Both methods were implemented in ANSYS-Fluent.Simulations for a two-phase (liquid-only) Reynolds number of 713, Capillary number of 0.004 and void fraction of 0.366 for nitrogen-water flow were performed to compare the two techniques. There was a very large difference between the required computational mesh sizes and times for the two methods, with a wall clock time of 38 h on a single processor for the moving domain compared with 1460 h using four processors for the stationary domain approach. In addition, for a constant wall heat flux boundary condition, even with 14 bubbles present in a long tube thermal development was not achieved. The hydrodynamic and heat transfer results obtained from the two approaches were found to be very similar to each other and with results from our earlier verification and validation studies, giving a high degree of confidence in the implementation of both methods.     

气液两相垂直管流中弹状流型数学模型

本文对气液两相垂直管流的弹状流型的流动机理进行了分析,建立了数学模型。模型的特点是采用了以弹状气泡速度运动的相对座标系,在弹状气泡周围下落液膜流动、液体段空隙率计算和压降计算等方面部提出了新的观点。在两种管径中用空气-水系统进行了实验以验证模型,与一些其他作者的实验数据和理论模型也进行了比较,结果都表明这个机理模型计算的弹状流型的各种流体力学参数值与实验值吻合良好,也优于其他作者提出的模型。     

Experimental study and CFD modelling of a two-phase slug flow for an airlift tubular membrane

The aim of the present study was to develop a computational fluid dynamics (CFD) model to study the effect of slug flow on the surface shear stress in a vertical tubular membrane. The model was validated using: (1) surface shear stresses, measured using an electrochemical shear probe and (2) gas slug (Taylor bubble) rising velocities, measured using a high speed camera. The length of the gas slugs and, therefore, the duration of a shear event, was observed to vary substantially due to the coalescing of gas slugs as they travelled up the tube. However, the magnitude of the peak surface shear stress during a shear event was not observed to vary significantly. The experimental conditions significantly affected the extent to which the gas slugs coalesced. More coalescing between gas slugs was typically observed for the experiments performed with higher gas flow rates and lower liquid flow rates. Therefore, the results imply that the frequency of shear events decreases at higher gas flow rates and lower liquid flow rates.Shear stress histograms (SSH) were used as a simple approach to compare the different experimental conditions investigated. All conditions resulted in bi-modal distributions: a positive surface shear peak, caused by the liquid slug, and a negative shear peak caused by the gas slugs. At high gas flow rates and at low liquid flow rates, the frequency of the shear stresses in both the negative and positive peaks were more evenly distributed. For all cases, increasing the liquid flow rate and decreasing the gas flow rate tends to result in a predominant positive peak. These results are of importance since conditions that promote evenly distributed positive and negative peaks, are likely to promote better fouling control in membrane system. At high liquid and low gas flow rates, the frequencies obtained numerically and experimentally were found to be similar, deviating by less than approximately 10%. However, at high gas and low liquid flow rates, the differences were slightly higher, exceeding 20%. Under these conditions, the CFD model simulations over predicted the shear stresses induced by gas slugs. Nonetheless, the results indicate that the CFD model was able to accurately simulate shear stresses induced by gas slugs for conditions of high liquid and low gas flow rates.     

低温气液两相弹状流动中Taylor气泡的统计特征?I. Taylor气泡长度

为了研究倾角(q)和管径对低温气液两相流中Taylor气泡长度分布规律的影响,在6种管路倾角下,以液氮为工质,使用高速相机对4种内径透明抽真空Pyrex玻璃管内的弹状流动进行了可视化实验. 结果表明,低温弹状流动中Taylor气泡长度分布的统计特征可由对数正态分布函数描述,Taylor气泡长度分布的标准偏差受到液膜射流直接影响,并随q减小先增大后减小. Taylor气泡无量纲平均长度随管路轴向位置增大而线性增加,随管路内径增加呈指数下降趋势,随q减小而增大,最小值出现在70o≤q≤90o.     

Hydrodynamic studies of liquid–liquid slug flows in circular microchannels

The aim of the work presented was to clarify the existence of a wall film and its influence on the hydrodynamics of liquid–liquid slug flow capillary microreactor.The methodology of the laser induced fluorescence (LIF) was adopted for visualisation purposes. The measurement of the light intensity profiles revealed a fully developed wall film for a variety of aqueous–organic two-phase systems in glass and PTFE capillaries of 1 mm internal diameter. In addition an acid as a quenching agent enabled the observation of the internal circulation patterns within the liquid slugs, as the fluorescent dye was deactivated by the acid diffusing in from the dye-free phase. A well-defined internal circulation pattern was always present in the wetting phase, i.e. that forming the wall film, leading to uniform mixing in the slugs of this phase. Stagnant zones and local circulation vortices, indicated by variations in the concentrations of the quenched dye, were observed in the non-wetting dispersed phase. These more complex flow structures varied little with the slug velocity, but were strongly dependent on the physical properties of the liquid–liquid system. To predict slug shape and hydrodynamics within the liquid slugs, CFD simulations were carried out using the volume-of-fluid method (VOF) based on the incompressible Navier–Stokes equation with appropriate boundary conditions between the two phases. The slug generation process was studied in a T-junction with 1 mm internal diameter inlets. The implementation of the wetting contact angle, measured in the visualisation experiments for the various systems, led to realistic slug lengths and shapes. The velocity vector plot indicated a fully developed internal circulation pattern within the simulated slugs. Calculations for a single slug with a non-wetting condition gave rise to a wall film in the simulated system.The results obtained demonstrate the significance of the wall film in the hydrodynamics and mass transfer liquid–liquid slug flow and reveal the presence of hitherto unsuspected complex patterns in place of simple single Taylor vortex flow assumed in the past.