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。     

垂直上升气液弹状流中含气率分布的实验研究

彩和光导纤维探针和美国柯达公司生产的ＥＫＴＡＰＲＯ１０００型高速动态分析仪研究了生趣直上升管内气液两相弹状流中含气率分布,获得了有关弹状流入口区及充分发展区相分布的实验数据。     

用统计方法研究未充分发展弹状流动

在一内径19 mm、长2 m的垂直有机玻璃管内,采用自制的电导探针对未充分发展的气-液二相弹状流中的弹状气泡上升速度、液塞上升速度、弹状气泡长度和液塞长度进行了测量。得到了各自随表观气速或表观液速的变化规律。结果表明:在未充分发展的弹状流状态下,弹状气泡的上升速度略高于液塞的上升速度:弹状气泡长度随表观气速的增大而增大,随表观液速的增大而减小。文章对弹状气泡长度进行了统计分析。未充分发展弹状流中弹状气泡长度符合正态分布律。     

脉动条件下旋流场内气液两相流流型及其转变机理

运用高速摄像技术对流量脉动条件下旋流分离器内气泡动力学行为及气液两相流流型展开研究。研究发现,在完整脉动周期内,流量在3.62~4.18 m³/h范围内波动,流量增大段的气核整体向溢流口方向运移,流量减小段的气核整体向底流口方向运移,气核大小及形态变化呈现周期往复性。通过脉动周期内特殊帧的分析,得出流量脉动条件下旋流场内气液两相流流型主要包括：气泡流、塞状流、弹状流、丝状流及波状流等五种形式。根据实验得出的气液两相折算速度,确定了脉动条件下气液两相流流型转换界限图,而气泡间的聚并破碎行为是产生气液两相流型的主要原因,最终构建了表征截面含气量和分离效率之间关系的评价模型。     

脉动条件下旋流场内气液两相流流型及其转变机理

运用高速摄像技术对流量脉动条件下旋流分离器内气泡动力学行为及气液两相流流型展开研究。研究发现,在完整脉动周期内,流量在3.62~4.18 m³/h范围内波动,流量增大段的气核整体向溢流口方向运移,流量减小段的气核整体向底流口方向运移,气核大小及形态变化呈现周期往复性。通过脉动周期内特殊帧的分析,得出流量脉动条件下旋流场内气液两相流流型主要包括：气泡流、塞状流、弹状流、丝状流及波状流等五种形式。根据实验得出的气液两相折算速度,确定了脉动条件下气液两相流流型转换界限图,而气泡间的聚并破碎行为是产生气液两相流型的主要原因,最终构建了表征截面含气量和分离效率之间关系的评价模型。     

矩形截面螺旋通道内气液两相流局部含气率分布实验研究

应用电导探针测量技术,对矩形截面螺旋通道内气液两相流局部含气率进行实验研究。在不同的气相折算速度下,应用电导探针测量了弹状流弹单元的长度,并与可视化方法进行对比,验证了电导探针的可靠性,并为信号处理选择合适的阈值。分别在泡状流、弹状流及环状流三种流型的条件下,分析了气相与液相折算速度对局部含气率分布的影响。实验结果发现,螺旋通道气液两相局部含气率呈非对称的抛物线形分布,这种非对称性受流型和液相折算速度的影响。     

垂直管内向上气液流动的分析(Ⅰ)——弹状气泡与下落液膜流动

弹状流是垂直管内气液两相上升流中的一种重要流型。本文从相对坐标系的质量与动量衡算出发,对弹状气泡与下落液膜流动进行分析,建立了相应的计算公式。本文也是完整的弹状流数学模型的一部分。     

垂直上升管中的液弹长度分布

根据气液相不同的电导性质,通过实验测得了气液二相弹状流中气液弹的长度及速度。利用统计方法研究了在垂直上升管内不同位置以及不同气液表观速度下的液弹长度分布。结果表明该分布能够与对数正态分布关系式符合得很好,气液表观速度对液弹长度影响并不显著。同时还研究了气泡上升速度与前方液弹长度的关系,该关系可以用R.van Hout得到的经验关系式来描绘。     

微通道分流弹状流的界面过程及压力演变规律

弹状流分流不仅能调控两相流流型从而强化传热,同时也是生物化工、制药行业的传统过程。针对壁面微通道分流液相、调控两相流型的过程进行数值模拟,获得局部参数变化规律,是获得两相流流型演变机理的基础。采用VOF模型耦合动态自适应网格精准追踪气液界面,模拟气液界面在分液口的界面运动;获得轴向及壁面静压、动压的演变规律。通过模拟可知微通道分流弹状流的关键是气弹在分液口的类活塞运动;同时由于界面拉普拉斯压力差的存在,弹状流压降具有不连续性;且此不连续压力随气弹在分液口的类活塞运动具有周期波动性。而弹状流液桥部分的局部压降是影响总压降的关键;近气弹头部的液相区压降显著,近气弹尾部的液相区域由于液速降低其压降明显衰弱;此为弹状流有别于其他两相流流型的压降特点。     

垂直上升弹状流中液弹长度分布

提出了计算垂直上升弹状流中尾随Taylor气泡上升速度的关系式;并基于气泡追赶合并机理,建立了预测沿管路任意位置液弹长度分布模型,提供了弹状流发展过程中弹长分布的变化情况。     

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.     

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.     

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。     

Hydrodynamic characteristics of gas-liquid slug flow in a downward inclined pipe

Gas-liquid slug flow in a downward inclined pipe was studied experimentally by employing a wire-mesh sensor that enables quantitative measurements of the cross-sectional void fraction distribution. Processing of the wire-mesh sensor data was applied to carry out a statistical analysis of characteristic parameters of downward slug flow, such as bubble and liquid slug length distributions, as well as to determine the ensemble-averaged shapes of the bubble nose, liquid film and bubble tail. It was found that the pipe inclination affects mainly the bubble length, while variation in the gas flow rate affects both bubble and slug length. The bubble nose shape is more sensitive to the flow conditions than the bubble tail. The 3D structure of an elongated bubble in downward slug flow was reconstructed from the wire-mesh sensor data.     

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.     

垂直管内气液两相弹状流含气率研究

In order to investigate the influence of the entrance effect on the spatial distribution of phases, the experiments on gas-liquid two-phase slug flow in a vertical pipe of 0.03m ID were carried out by using optical probes and an EKTAPRO 1000 high speed motion analyzer. It demonstrates that the radial profile of slug flow void fraction is parabolic. Influenced by the falling liquid film, the radial profile curve of liquid slug void fraction in the wake region is also parabolic. Since fully turbulent velocity distribution is built up in the developed region,the void fraction profile in this region is the saddle type. At given superficial liquid velocity, the liquid slug void fraction increases with gas velocity. The radial profiles of liquid slug void fraction at different axial locations are all saddle curves, but void fraction is obviously high around the centerline in the entrance region. The nearer the measuring station is from the entrance, the farther the peak location is away from the wall.