CFD Simulation of hydrodynamics of air sparging in a vertical tubular membrane

Computational fluid dynamics (CFD) simulation of the hydrodynamics of slug flow which is generated by air sparging in a vertical tubular membrane has been investigated. The results of simulation have been reported in the form of parameters such as shape, velocity profile, surface shear stresses and gas slug (Taylor bubble) rising velocities, and evaluated with experimental data which were presented in previous articles. This study showed that CFD modeling is able to accurately simulate the shape and velocity field around the gas slugs. Also the shear stress induced by slug flow passage and rising velocity of gas slugs for high-velocity liquid and low-velocity gas fit appropriately to values in reference data. Simulation results for gas slug rising velocity showed about 0.35–9% error in the different conditions investigated in respect to experimental data.     

The effect of mass transfer on flow characteristics in vertical two-phase unstable flow

Studies carried out on the absorption of hexane from inert carrier gases into paraffin oil have shown that mass transfer can affect flow characteristics in unstable vertical two-phase flow. In the absence of mass transfer chain-flow patterns prevailed. These patterns were broken when mass transfer took place from the gas to the liquid phase. Bullet shaped slugs were then formed, the slugs pairing and coalescing as they rose up the column. The direction of mass transfer was critical no effect on flow pattern being observed when mass transfer took place from the liquid to the gas phase. Slug frequencies and slug sizes were found to be influenced by mass transfer and an effect of overall absorber length was observed. Individual slug sizes and separation distances were measured and varied widely even under constant flow conditions. Rise velocities were correlated using an equation of the form:where C₂ was a function of the fluid flow-rates and system physical-properties.     

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。     

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。     

Numerical study on gas and liquid slugs for Taylor flow in a T-junction microchannel

The rapid development of microfabrication techniques creates new opportunities for applications of microchannel reactor technology in chemical reaction engineering. The extremely large surface-to-volume ratio and the short transport path in microchannels enhance heat and mass transfer dramatically, and hence provide many potential opportunities in chemical process development and intensification. Multiphase reactions involving gas/liquid reactants with a solid as a catalyst are ubiquitous in chemical and pharmaceutical industries. The hydrodynamics of the flow affects the reactor performance significantly; therefore it plays a prominent role in reactor design. For gas/liquid two-phase flow in a microchannel, the Taylor slug flow regime is the most commonly encountered flow pattern. The present study deals with the numerical simulation of the Taylor flow in a microchannel, particularly on gas and liquid slugs. A T-junction empty microchannel with varying cross-sectional width (0.25, 0.5, 0.75, 1, 2 and 3 mm) served as the model micro-reactor, and a finite volume based commercial computational fluid dynamics (CFD) package, FLUENT, was adopted for the numerical simulation. The gas and liquid slug lengths at various operating and fluid conditions were obtained and found to be in good agreement with the literature data. Several correlations in the T-junction microchannel were developed based on the simulation results. The slug flows for other geometries and inlet conditions were also studied.     

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.     

IMPROVED HYDRODYNAMIC MODEL OF TWO-PHASE SLUG FLOW IN VERTICAL TUBES

Merits of the Fernandes model(Fernandes et al.1983)for two-phase slug flow in verticaltubes are reviewed in this paper.While predicting many macroscopic parameters of slug flow in verti-cal tubes,it fails to present correctly the trend that the average voidage in liquid slugs increases asthe rising velocity of Taylor bubbles is increased.It is also desirable to extend its application toelectrolyte systems, and to churn flow conditions.Based on the diagnostic analysis,the model equa-tion for gas entrainment by falling liquid film is reformulated and the influence of surface tension isalso accounted for.Development of the falling liquid film is recognized in the revised model in or-der to suit the case of short Taylor bubbles as well.The modified model predicts the variation of av-erage voidage in liquid slugs in good agreement with available experimental data.     

Design of mixing in microfluidic liquid slugs based on a new dimensionless number for precise reaction and mixing operations

This paper reports mixing characteristics inside a microfluidic liquid slug using the computational fluid dynamics (CFD) simulations. Each slug is modeled as a single-phase flow domain. Slug-based microfluidics offers rapid mixing by internal circulation and transport with narrow residence time distribution, making it suitable for precise reaction and mixing operations. Miniaturizing the slug size to microscale allows high interactions between the slug internal fluid and the channel wall, leading to a highly effective internal circulation. However, quantitative understanding of mixing characteristics and the influences of operating parameters on mixing rate is crucial for the design of a liquid slug that ensures desired mixing rates. The simulation results provide insights into the influences of operating parameters on slug-based mixing rates. Based on the simulation results, the modified Peclet number, , is proposed for designing mixing in liquid slugs. A novel method using Pe^* to estimate mixing rates and design liquid slugs to obtain desired mixing rates is discussed. Using this method, both short (ms) and long (min) mixing timescales can be accessed in the same microfluidic device by simply varying the slug velocity.     

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。     

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.     

Slug flow in curved microreactors: Hydrodynamic study

The need for eco-friendly and energy saving processes which are substantially compact and give higher efficiency has led to the concept of process intensification (PI). Curved microchannel is such innovative device, which has potential for the intensification of processes currently carried out in conventional straight or T-type microchannels. Curved microchannels utilize the benefits of centrifugal force to its advantage. The present study deals with the numerical simulation of the Taylor flow in curved microchannels, particularly on gas and liquid slugs with varying curvature ratios (i.e., coil to tube diameter=5, 10, 20 and 30). The three-dimensional, unsteady slug flow development in the curved microchannel was carried out using control volume finite difference method (CVFDM). The gas and liquid slug lengths at various operating and fluid conditions were obtained. The slug flow development for different inlet conditions and geometries (premixed feed, T-type and Y-type inlets) was also studied in the curved microchannels. It was found that for low curvature ratio (D/d=3), the phenomenon of flow reversal and slug freezing takes place due to centrifugal and buoyancy forces. For the similar process conditions, with an increase in curvature ratio to 5 and 10, the phenomenon of flow reversal and slug freezing observed was very minor. The non-uniformity in the slug formation was observed for low curvature ratio as compared to the higher curvature ratios. Further the influence of surface tension, viscosity and wall adhesion was studied on slug flow development in the curved microchannels. From the results it was observed that the surface tension, viscosity and wall adhesion have significant influence on slug flow development in curved microchannels.     

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.     

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.     

气液界面摩擦因子对分层流向段塞流过渡的影响

根据气液两相流段塞稳定性理论,结合分层流理想化模型,对直径2.54 cm水平管内空气-水两相流出现段塞流时的各相临界表观速度和临界液层高度进行了理论预测,并且分析了气液两相界面的摩擦因子对于预测流型转变的影响.由于在不同的气相流速范围内,段塞流由不同形式的界面波形成,当气相表观速度小于 4 m(s(1时,由大振幅波形成段塞流,而当气相表观速度大于 4 m(s(1,段塞流由小波长的滚动波合并而成,因而根据实测数据对界面摩擦因子的计算进行了补充修正,并且应用到对应的气相流速范围.基于所给出的计算方法解得发生分层流向段塞流转变的临界表观速度和临界液层高度,并和实验数据进行了比较. 结果表明所做的修正可以避免以往理论预测中出现的低估情况,求得的各临界特征参数值和实验结果吻合得较好.     

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.     

Characteristics of liquid slugs in gas–liquid Taylor flow in microchannels

The hydrodynamics of liquid slugs in gas–liquid Taylor flow in straight and meandering microchannels have been studied using micro Particle Image Velocimetry. The results confirm a recirculation motion in the liquid slug, which is symmetrical about the center line of the channel for the straight geometry and more complex and three-dimensional in the meandering channel. An attempt has also been made to quantify and characterize this recirculation motion in these short liquid slugs (L_s/w<1.5) by evaluating the recirculation rate, velocity and time. The recirculation velocity was found to increase linearly with the two-phase superficial velocity U_TP. The product of the liquid slug residence time and the recirculation rate is independent of U_TP under the studied flow conditions. These results suggest that the amount of heat or mass transferred between a given liquid slug and its surroundings is independent of the total flow rate and determined principally by the characteristics of the liquid slug.     

Assessment of mass transfer coefficients in coalescing slug flow in vertical pipes and applications to tubular airlift membrane bioreactors

One of the operational challenges associated with membrane bioreactors (MBRs) is the fouling of the membranes. In tubular side-stream MBRs, fouling reduction can be achieved through controlling the hydrodynamics of the two-phase slug flow near the membrane surface. The two-phase slug flow induces higher shear stresses near the membrane surface, which generate high mass transfer coefficients from the surface to the bulk region. However, measuring the mass transfer coefficient is difficult in complex heterogeneous mixtures like activated sludge and existing techniques (e.g. electrochemical methods) cannot be applied directly. As an alternative, in this work, a multidisciplinary approach was selected, by exploiting dimensionless analysis using the Sherwood number. Mass transfer coefficients were measured at various superficial velocities of gas and liquid flow in a tubular system. Due to the variability of the mass transfer coefficient obtained for each experimental condition, the results were compiled into, mass transfer coefficient histograms (MTH) for analysis. A bimodal MTH was observed, with one peak corresponding to the mass transfer induced by the liquid flow, and the other peak induced by the gas flow. It was noted that coalescence of bubbles affects the MTH. Coalescence increased the “width” of the peaks (i.e. the estimate of the variability of the mass transfer coefficient) and the height of the peak (i.e. amount of time that a mass transfer coefficient of a given value is maintained). A semi-empirical relationship based on the Lévêque relationship for the Sherwood number (mass transfer coefficient) was formulated for the laminar regime. A test case comparison between water and activated sludge was performed based on full-scale airlift MBR operational conditions. It was found that the Sherwood number in the non-Newtonian case is 8% higher than that in the Newtonian case.     

气液两相流段塞流持气率快关阀法优化设计

快关阀法(quick closing valve,QCV)是气液两相流流动实验中常用持气率标定手段。特别是由于段塞流中气塞与液塞表现为随机可变流动特性,不合理的快关阀间距及截取次数选择将会导致持气率测量误差增大。提出了一种持气率快关阀法优化设计方案。首先,采用环形电导传感器上下游阵列信号计算流体相关流速,根据相关测速结果提取上游传感器信号对应流动工况的气塞与液塞间隔长度序列,采用Maxwell方程提取液塞中含泡持气率;在此基础上,再依气塞在管道内占比模拟计算不同快关阀间距时捕获的持气率波动序列。通过分析持气率序列波动,从统计学角度指出了95%置信度及5%允许误差情况下所需最低截取次数。最后,在快关阀门间距为1.55 m的条件下对段塞流所需截取次数进行了实验验证。通过对快关阀法持气率测量误差进行统计分析,证明了设置两个快关阀门间距的充分条件。     

Influence of slug flow on flow fields in a gas–liquid cylindrical cyclone separator:A simulation study

A simulation method for slug flow based on the VOF multiphase flow model was implemented in ANSYS? Fluent via a user-defined function(UDF) and applied to the dissipation of liquid slugs in the inlet pipe of a gas–liquid cylindrical cyclone(GLCC) separator while varying the expanding diameter ratio and angle of inclination. The dissipation of liquid slug in inlet pipe is analyzed under different expanding diameter ratios and inclination angles.In the inlet pipe, it is found that increasing expanding diameter ratio and inclination angle can reduce the liquid slug stability and enhancing the effect of gravity, which is beneficial to slug flow dissipation. In the cylinder, increasing the expanding diameter ratio can significantly reduce the liquid carrying depth of the gas phase but result in a slightly increase of the gas content in the liquid phase space. Moreover, increasing the inclination angle results in a decrease in the carrying depth of liquid in the vapor phase, but enhances gas–liquid mixing and increases the gas-carrying depth in the liquid phase. Taking into consideration the dual effects of slug dissipation in the inlet pipe and carrying capacity of gas/liquid spaces in the cylinder, the optimal expanding diameter ratio and inclination angle values can be determined.     

Non-Newtonian multi-phase flows: On drag reduction, pressure drop and liquid wall friction factor

The present work focuses on the non-Newtonian liquid drag reduction by gas injection. Two regimes are taken in consideration: fully stratified gas shear-thinning liquid flow and gas shear-thinning liquid slug flow regimes.Predictions of drag reduction ratio and holdup are presented for the stratified flow of gas and non-Newtonian Ostwald-de Waele liquid. Fully stratified flow is considered and the approach developed in Taitel and Dukler (1976) is used. For these regimes, CMC (CarboxyMethyl Cellulose) solution is used in order to investigate the behaviour of the gas and non-Newtonian liquids in horizontal pipes. Results have been reformulated and an extension to interfacial Andreussi and Persen (1987) correlation has been carried out for stratified regimes.For slug flow regimes, the mechanistic slug unit model is adopted in order to estimate the pressure gradients along the slug unit. The slug unit model is rearranged and reinterpreted as inviscid Burgers's equation for incompressible phases.For both stratified and slug flow regimes, three dimensional CFD (computational fluid dynamics) simulations were performed in order to compare the drag reduction ratio and pressure gradients. In stratified flows, CFD is also used in an attempt to evaluate the liquid wall friction factor and to compare the obtained values with those given by empirical standard correlations.