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。     

Horizontal air–water flow in a square cross‐section channel: Pseudo‐slug flow regime

New experimental data for air–water flow in a horizontal square cross‐section channel (H = 24.25 mm) is presented, including data on liquid hold‐up, gas and liquid velocities, and wave velocities and frequencies. For the majority of gas and liquid flow rates studied, the regime observed was pseudo‐slug. Using visualization studies it was possible to identify wavy‐stratified and pseudo‐slug flows. For the pseudo‐slug regime new correlations were obtained for liquid hold‐up, for gas and liquid velocities as a function of the ratio between gas and liquid mass flow rates, and for the frequency of roll‐waves as a function of gas and liquid mass flow rates.     

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.     

利用一维波和段塞稳定性模型预测分层流向段塞流的转变

利用气液二相流一维波模型和段塞稳定性模型,对直径2.54 cm水平管内空气-水二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。计算中发现,2种模型分别适用于不同的流速区域,在较低的气相流速下,一维波模型的预测结果比较理想,但是在较高的气速条件下不太适合,而利用段塞稳定性模型可以较好地获得高流速下分层流向段塞流的流型转变条件。因此,结合这2种模型对发生流型转变时的临界参数作了分析,并且应用于40 mm和50 mm水平管道的油气二相流实验。将理论计算的结果和实验测得的流型数据进行了对比,并且对影响流型的管径、流速等因素作了分析,结果表明计算得到的特征参数和实验数据比较吻合。     

Effect of tube diameter on two‐phase flow patterns in mini tubes

The effect of tube diameter on two‐phase flow patterns was investigated in circular tubes with inner diameters of 0.6, 1.2, 1.7, 2.6, and 3.4 mm using air and water. The gas and liquid superficial velocity ranges were 0.01–50 and 0.01–3 m/s, respectively. The gas and liquid flow rates were measured and the two‐phase flow pattern images were recorded using high‐speed CMOS camera. The flow patterns observed were dispersed bubbly, bubbly, slug, slug‐annular, wavy‐annular, stratified, and annular flows. These flow patterns were not observed in all the test diameters, but were found to be unique to particular tube diameters, confirming the effect of tube diameter on the flow pattern. The data obtained were compared to existing experimental data and flow regime transition maps which show generally reasonable overall agreement at the larger diameters, but significant differences were observed with the smaller diameter tubes.     

3‐D numerical simulations on flow and mixing behaviors in gas–liquid–solid microchannels

Three‐dimensional (3‐D) gas‐liquid–solid flow and mixing behaviors in microchannels were simulated by coupled volume of fluid and discrete phase method and simulations were validated against observations. The detachment time and length of gas slug are shortened in liquid–solid flow, compared with those in liquid flow due to higher superficial viscosity of liquid–solid mixture, which will move the bubble formation toward the dripping regime. Solid particles mainly distribute in liquid slug and particle flow shows obvious periodicity. With the increase of contact angle of the inner wall, gas slug (0–50°), stratified (77–120°), and liquid drop (160°) flows are observed. The residence time distributions of solid and liquid phases are similar because particles behave as tracers. The backmixing of solid and liquid phases in liquid drop flow is the weakest among the three flow patterns, and the backmixing of gas phase in slug flow is weaker than that in both stratified and liquid drop flows. The results can provide a theoretical basis for the design of microreactors. © 2013 American Institute of Chemical Engineers AIChE J, 59: 1934–1951, 2013     

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。     

Design and operation of gas–liquid slug flow in miniaturized channels for rapid mass transfer

The influences of operating parameters such as channel size, flow rate, and void fraction on the mass transfer rate in the gas–liquid slug flow are investigated to establish a design method to determine the parameters for rapid mass transfer. From the experimental results, the turnover index, including the slug linear velocity, its length, and the channel size that represents the turnover frequency of the internal circulation flow, is proposed. For PTFE tube in which no liquid film exists in slug flow, a master curve is derived from the relationship between the mass transfer coefficient and the turnover index. For each channel material, the Sherwood number is also roughly correlated with the Peclet number. These correlations make it possible to arbitrarily determine a set of operating parameters to achieve the desired mass transfer rate. However, the turnover index and the Peclet number include the slug length, which cannot be controlled directly. The relationship between the slug length and the operating parameters is also investigated. The slug volume mainly depends on the inner diameter (i.d.) of a union tee. At a fixed union tee i.d., the slug length is controlled through the exit i.d. of the channel connected to the union tee and the void fraction. Thus, the final slug length depends on the union tee and exit channel inner diameters. At low flow rates, the gas and liquid collision angle is significant in determining the slug length.     

A MODEL FOR PULSING FLOW IN COCURRENT DOWN-FLOW TRICKLE-BED REACTORS

Based on macroscopic mass and momentum balances, a model is developed to characterize the hydrodynamic behavior of pulsing flow in cocurrent down-flow trickle-bed reactors. Predictions of the model regarding liquid saturation, length and velocity of both the liquid rich slug and gas rich pulse are reported. Also presented are experimental data of an air-water system for the effects of varying gas and liquid flow rates on pulse frequency, total pulse length and apparent slug velocity. To confirm the theory, predictions are compared with measured overall pressure gradient. Agreement is reasonably good.     

Aerobic oxidation of benzyl alcohol in a slug flow microreactor: Influence of liquid film wetting on mass transfer

Homogeneous Co/Mn/Br catalyzed aerobic oxidation of benzyl alcohol in acetic acid to benzaldehyde was performed in polytetrafluoroethylene microreactors operated under slug flow at temperatures up to 150°C and pressures up to 5 bar. Depending on the bubble velocity and length, a wetted or dewetted slug flow was observed, characterized typically by a complete or partially wetting liquid film around the bubble body. The latter flow suffered from a limited interfacial area for mass transfer. Experiments at temperatures up to ca. 90°C were under kinetic control given no product yield difference under wetted and dewetted slug flows and were used to establish a simplified kinetic expression (first order in benzyl alcohol and zero order in oxygen). This allows to develop a mass transfer model combined with an instantaneous reaction regime that well described the experimental results at higher temperatures where mass transfer was limiting in the dewetted slug flow.     

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.     

水平管道气液两相段塞流参数计算的精确模型

分析了水平管内气液两相段塞流的运动特性和形态特征以及段塞单元内部的速度分布规律,建立了水平管路气液两相段塞单元的物理模型。将一个完整的段塞流单元分为液相段塞区和液层/气泡区,建立了液相段塞区的质量和动量守恒方程,计算了其压降和持液率;对液层区,模型考虑液层厚度分布不均(坡状液层)对参数计算的影响,通过建立局部控制方程,推导了液层高度随流动方向坐标变化的表达式,并将持液率和湿周写成液层高度的函数。通过与实验和其他模型的计算结果对比,本文建立的模型可以对压降和持液率有更准确的预测结果。     

A Hydrodynamic Modei for Slug Frequency in Horizontal Gas-Liquid Two-Phase Flow

The prediction of slug frequency has important significance on gas-liquid two-phase flow. A hydrody-namic modei was put forward to evaluate slug frequency for horizontal two-phase flow, based on the dependence of slug frequency on the frequency of unstable interfacial wave. Using air and water, experimental verification of the modei was carried out in a large range of flow parameters. Six electrical probes were installed at different positions of a horizontal plexiglass pipe to detect slug frequency development. The pipe is 30m long and its inner diameter is 24 mm. It is observed experimentally that the interfacial wave frequency at the inlet is about l to 3 times the frequency of stable slug. The slug frequencies predicted by the modei fit well with Tronconi (1990) modei and the experimental data. The combination of the hydrodynamic modei and the experimental data results in a conclusion that the frequency of equilibrium liquid slug is approximately half the miniraum frequency of interfacial wave.     

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。     

水平管内气液段塞流液塞长度发展规律的实验和模拟研究

In petroleum industry, the slug flow is a fre-quently encountered flow regime in multiphase flowpipeline. For pipeline designers, the liquid slug lengthdistribution is important for the proper design ofdownstream facilities, such as slug catcher and sepa-ration system. However, for its transient and unsteadynature, it is a great challenge for engineers to correctlypredict the flow parameters of slug flow, especiallythe maximum liquid slug length. The unit cell model for slug flow in horizontal…     

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。     

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。     

水平管段塞流气弹区液膜特性研究

为了研究气液段塞流相界面的结构特征,采用了双平行电导探针技术对水平管内段塞流气弹区的液膜特性进行了实验测量,并基于一维双流体模型导出液膜厚度的控制方程,该方程对液膜厚度的预测结果与实验结果吻合良好。结合液膜厚度计算模型提出了计算段塞流的机理模型,该段塞流机理模型的计算结果表明对于液相表观速度较高而气相表观速度较低的段塞流,机理模型中忽略液膜非平衡性时得到的平均液膜厚度和气弹区长度明显偏低。     

Void fraction in vertical gas-liquid slug flow: Influence of liquid slug content

In this study we develop a model for computing the mean void fraction and the liquid slug void fraction in vertical upward gas-liquid intermittent flow. A new model for the rate of gas entrained from the Taylor bubble to the liquid slug is formulated. It uses the work done by the pressure force at the rear of the Taylor bubble. Then an iterative approach is employed for equating the gas entrainment flux and the gas flux obtained via conservation equations. Model predictions are compared with experimental data. The developed iterative method is found to provide reasonable quantitative predictions of the entrainment flux and of the void fraction at low and moderate liquid slug void fraction conditions. However, with an increased liquid slug void fraction experimental data indicate that the flow in the liquid slug transits to churn-heterogeneous bubbly flow thus gas entrainment flux tends to zero. Considering this effect in the iterative model significantly improved the predictions for large liquid slug void fraction conditions.