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。     

不同T型微通道内弹状流相分离规律的实验研究

为获得不同形式顺流T型、冲击T型跨尺度通道对两相流流型的不同调控作用,设计跨尺度-并行多T型管-单晶硅方形通道,增大分液量的同时获得更接近实际应用的多分支口最长气弹伸缩长度;利用高速压力监测系统及高速摄像监测系统分别监测T型分支通道的压力及两相流界面运动,获得不同形式微通道相分离弹状流的压力波动及分液规律,建立微通道内气液界面运动与宏观调控参数两相流流速、压差之间的相关性,同时证明冲击型跨尺度微通道具有更高分液能力。     

壁面润湿性对蛇形微通道内两相流流动特性影响的格子Boltzmann模拟

在90°Y形汇流的矩形截面蛇形微通道内,采用格子Boltzmann方法对不同接触角的蛇形微通道内气液两相流动进行了数值计算。首先以空气和水为工作流体对气液两相流动进行模拟研究并通过实验进行验证。验证模型合理性后,根据模拟计算结果,以气液相流速为坐标绘制了不同接触角下的流型图并分析其差异性及原因;同时深入研究了液相黏度和接触角对于弹状流流体力学性质的综合影响;比较了具有不同接触角壁面的蛇形微通道内两相流压降、摩擦因子、壁面摩擦系数和剪切应力的分布规律,并讨论了蛇形微通道内气液两相流动的影响因素。研究表明疏水壁面即接触角大于90°时,微通道内两相流压降、摩擦因子、壁面摩擦系数和剪切应力均低于亲水壁面微通道内相关参数,更利于流体流动。     

微通道气液两相流型及界面面积测定

以氢氧化钠水溶液及纯二氧化碳气体为工质,研究了石英玻璃微通道中的气液两相流流型及两相界面面积.实验中可清晰分辨的流型为弹状流(Slug),弹状-环状流(Slug-annular)及搅拌流(Churn),由实验观测的结果建立了相应的流型图并与Triplett实验结果进行了比较,结果表明高液相表观流速对流型转换影响较大.在微通道中可以实现比较高的气液两相界面面积,实验范围内气液两相界面面积高达5 070 m2/m3.两相界面面积随气相表观流速的增加而增加,而液相表观流速对两相界面面积的影响则不显著.相对于弹状流区域,在弹状-环状流及搅拌流区域可以实现比较高的传质界面面积.搅拌流区域气液两相界面面积可以采用气液表观雷诺数进行很好关联,其绝对平均偏差仅为3.76%.     

入口角度及壁面性质对蛇形微通道内弹状流流动特性影响的格子Boltzmann模拟

在Y型汇流的矩形截面蛇形微通道内,采用格子Boltzmann方法对不同壁面性质的蛇形微通道内弹状流流动进行了数值计算。首先以空气和水为工作流体对气液两相流动进行模拟研究并通过实验进行验证。通过验证实验后,模拟计算了气相速度,Y型夹角和壁面性质对气泡长度的影响,以及Y型夹角对微通道内弹状流压降和流动阻力的影响;探讨了粗糙度与壁面润湿性对流动阻力的影响;同时,针对蛇形微通道弯管部分,分析了角度和壁面性质对弹状流流动的影响。通过计算,发现当壁面接触角及Y型夹角为90?时,气泡长度最大;当直微通道为亲水性光滑壁面,回转弯道为粗糙度较大的疏水壁面时,Po数较小。     

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。     

微通道内气液(液液)二相流的实验研究进展

综述了微通道内气液(液液)二相流的流型特征.微通道内气液二相流常见的流型为泡状流、弹状流、环状流和翻腾流;液液二相流常见的流型为液滴流、塞状流、平行流及环状流.分析了不同操作条件对气液(液液)二相流行为的影响.介绍了微通道内气液(液液)二相流流型判别谱图,对常用的弹状流、液滴流和塞状流进行了重点介绍.指出了微通道内气液...     

并列矩形微通道内两相流流型和压降试验研究

以空气-水、空气-酒精为工质,在矩形截面为100μm×800μm的并列微通道(三通道)中进行了气液两相流动的流型和压降试验。观察到了泡状流、弹状流、波状流和分层流,而且微通道Ⅰ内流型变化最为复杂,微通道Ⅲ内的流型变化只出现了弹状流。通过与均相流计算模型预测值的对比发现:Mecadam粘度计算式预测的压降值在低气相速率时能较好地预测压降,但是预测范围非常小,全液相粘度预测的压降值在低液速时能够较好地预测本试验中的整体压降;分相流模型中Chishlom预测效果较好。     

微通道内醇胺水溶液吸收CO2的压力降研究

采用高速摄像仪对400μm×400μm T型微通道内N-甲基二乙醇胺(MDEA)与单乙醇胺(MEA)混合醇胺水溶液(含0.2%SDS)吸收CO_2过程的气液两相流压力降进行了实验研究。观测到了泡状流、泡状-弹状流、弹状流、弹状-环状流流型。考察了弹状流型下气液两相流流量、醇胺溶液浓度、气液雷诺数与增强因子对压力降的影响。结果表明:压力降随着气液两相流量、气液雷诺数及增强因子的增大而增大。当Q_G﹥80 mL/h时,压力降随醇胺浓度的增大而减小。以分相流模型为基础,提出了微通道内伴有化学吸收的气液两相流压力降预测模型,平均偏差为8.46%,模型计算值与实验值吻合良好。     

绝热状态下板壳式换热器壳侧的流型与压降

实验研究了板壳式换热器波纹通道内垂直向上气-液两相流动的流型和压降特性，讨论了圆形波纹通道内流型特征及转变机理，根据相界面形态特征将流型划分为泡状流、弹状流、膜状流和搅混流；同时分析了流型与压降之间的关系，发现泡状流中的压降波动幅值最小，弹状流与膜状流次之，搅混流中压降波动幅值最大；获得了波纹通道内单相以及气-液两相压降的分布规律，拟合了单相压降关联式，并基于Lockhart-Martinelli理论，通过分析两相摩阻系数与Martinelli参数的关系拟合了波纹通道内两相流动压降关联式，发现Chisholm参数C的值与Chisholm最初建议的光滑管内层流-层流的值接近。     

气液两相流差压测量波动信号的符号序列统计分析

将符号时间序列分析方法应用到两相流测量波动信号分析，并讨论了关键参数对符号统计量影响问题。在此基础上，利用垂直上升管中采集到的80组气液两相流差压动态波动信号，提取了时间不可逆转性Tfb及χ2fb统计量。当气相表观速度小于002 m·s-1时，随着气相表观速度增加，泡状流随机可变的运动特征逐渐加剧，其动力学特性变得相对复杂；当气相表观速度大于002 m·s-1时，在流型从泡状流向段塞流转变过程中，随着气相表观速度增大，流型演化的动力学特性逐渐变得相对简单；在流型从段塞流向混状流转变过程中，随着气相表观速度增加，混状流的动力学特性逐渐变得愈加复杂。研究结果表明，时间不可逆转性Tfb及χ2fb统计量两个符号是表征气液两相流流型的敏感特征量，考察这两个统计量随两相流流动参数变化规律有助于更好地理解两相流流型动力学特性。     

对称分支并行微通道中气液两相流的均匀性规律

采用高速摄像系统研究了对称分支形并行微通道内气液两相流及弹状气泡均匀性规律。实验中分别采用含0.3% SDS的甘油-水溶液与氮气作为液相和气相。观察到弹状流和泡状流两种流型,作出了由两相操作条件构成的流型图及流型转变线。结果表明,气泡非均匀性主要由两微通道内流体之间的相互作用、下游通道中流体动力学的反馈作用以及通道制造误差造成。随液相黏度增大,气泡均匀性变好;在高液相流量以及低气相压力下操作,气泡尺寸分布更易达到均匀。基于压力降守恒原理和微通道内气液两相流阻力模型,构建了两通道中气泡尺寸的预测模型。     

Pressure drop and bubble-liquid interfacial shear stress in a modified gas non-Newtonian liquid downflow bubble column

A functional form of equation for predicting pressure drop in a modified non-Newtonian downflow bubble column has been formulated. The equation has been developed based on the bubble formation, drag at interface and the wettability effect of the liquid. Also the bubble-liquid interfacial shear stress in two-phase flow is analyzed and correlated with the dynamic, geometric and physical variables. The functional form of equation appears to predict the pressure drop satisfactorily for two-phase dispersed flow in the co-current modified downflow bubble column with carboxy methyl cellulose (CMC) solution in water with different concentrations.     

Analysis of the nonlinear dynamic characteristics of two-phase flow based on an improved matrix pencil method☆

Gas–liquid two-phase flow is complex and has uncertainty in phase interfaces, which make the two-phase flow look very complicated. Even though the flow behavior(e.g. coalescence, crushing and separation) of single bubble or bubble groups in the liquid phase looks random, combining some established characteristics and methodologies can find regularities among the randomness. In order to excavate the nonlinear dynamic characteristics of gas–liquid two-phase flow, the authors developed an improved matrix pencil(IMP) method to analyze the pressure difference signals of the two-phase flow. This paper elucidates the influence of signal length on MP calculation results and the anti-noise-interference ability of the MP method. An IMP algorithm was applied to the fluctuation signals of gas–liquid two-phase flow to extract the mode frequency and damping ratio, which were combined with the component energy index(CEI) entropy to identify the different flow patterns. It is also found that frequency, damping ratio, CEI entropy and stability diagram together not only identify flow patterns, but also provide a new way to examine and understand the evolution mechanism of physical dynamics embedded in flow patterns. Combining these characteristics and methods, the evolution of the nonlinear dynamic physical behavior of gas bubbles is revealed.     

垂直上升绝热管内汽、液两相流的压降

针对蒸发过程中沸腾层内汽、液两相流的流动状态,对垂直上升绝热管内汽、液两相流流动状态(泡状流和弹状流)中压降的主要部分采用与实际情况相近的模型进行分析,对压降的次要部分采用均相模型和分相模型进行分析,通过积分推出两相流体在区间[Z_1,Z_(1+1)]内总压降的计算式。本文理论计算值与实验结果符合较好。     

Hydrodynamics and mass transfer of oscillating gas‐liquid flow in ultrasonic microreactors

Ultrasonic microreactors were used to intensify gas‐liquid mass‐transfer process and study the intensification mechanism. Fierce surface wave oscillation with different modes was excited on the bubble. It was found that for slug bubbles confined in smaller microchannel, surface wave oscillations require more ultrasound energy to excite due to the confinement effect. Cavitation microstreaming with two toroidal vortices was observed near the oscillating bubble by a streak photography experiment. Surface wave oscillation at the gas‐liquid interface increases the specific surface area, while cavitation microstreaming accelerates the interface renewal and thus improves the individual mass‐transfer coefficient. With these two reasons, the overall mass‐transfer coefficient was enhanced by 3–20 times under ultrasonication. As for gas‐liquid flow hydrodynamics, ultrasound oscillation disturbs the bubble formation process and changes the initial bubble length and pressure drop. © 2015 American Institute of Chemical Engineers AIChE J, 62: 1294–1307, 2016     

大输量多相混输管路气液两相流实验研究

为了研究大输量条件下多相混输管路的流动特性,以水和空气为实验介质,在长江大学多相流实验平台上进行了水平状态的高气液量两相流模拟实验研究。实验采用内径为60 mm、长9.4 m的透明有机玻璃管,并利用高速摄像仪记录实验过程中的流型。通过对实验流型进行整理,将水平管内的气液两相流流型划分为分层流、泡状流、段塞流和环状流,并与典型的Mandhane流型图进行对比分析。另外,对实验范围内的几种典型流型下的压降梯度变化规律进行了研究,泡状流区域压降梯度随气流速的增大而减小,段塞流区域压降梯度随气流速的增大而缓慢增大,环状流区域压降梯度随气流速的增加而继续增大。     

地形起伏管路气液两相段塞流模型研究

段塞流是气液两相流动中的一种常见流型,由于地形原因,管路多处于起伏状态,而目前国内外对起伏诱发的气液两相管路段塞流研究尚不成熟。针对实际气液两相管路中频繁出现的地形起伏段塞流,首先利用历史数据对现有段塞流模型的适用性进行了比较,建立了地形起伏状态下段塞流的液塞追踪修正模型,最后利用FLUENT软件进行了模拟,研究了管路起伏诱发状况对段塞流段塞分布、拐角处持液率、液塞长度和压降的影响,并将模拟压降与计算压降进行对比,结果表明建立的模型具有一定的精度,对于实际的地形起伏诱发段塞流管道的安全高效运行有一定的指导意义。     

气液两相流的多尺度混沌特性分析

Using the high-speed camera the time sequences of the classical flow patterns of horizontal gas-liquid pipe flow are recorded, from which the average gray-scale values of single-frame images are extracted. Thus obtained gray-scale time series is decomposed by the Empirical Mode Decomposition (EMD) method, the various scales of the signals are processed by Hurst exponent method, and then the dual-fractal characteristics are obtained. The scattered bubble and the bubble cluster theories are applied to the evolution analysis of two-phase flow patterns. At the same time the various signals are checked in the chaotic recursion chart by which the two typical characteristics (diagonal average length and Shannon entropy) are obtained. Resulting term of these properties, the dynamic characteristics of gas-liquid two-phase flow patterns are quantitatively analyzed. The results show that the evolution paths of gas-liquid two-phase flow patterns can be well characterized by the integrated analysis on the basis of the gray-scale time series of flowing images from EMD, Hurst exponents and Recurrence Plot (RP). In the middle frequency section (2nd, 3rd, 4th scales), three flow patterns decomposed by the EMD exhibit dual fractal characteristics which represent the dynamic features of bubble cluster, single bubble, slug bubble and scattered bubble. According to the change of diagonal average lengths and recursive Shannon entropy characteristic value, the structure deterministic of the slug flow is better than the other two patterns. After the decomposition by EMD the slug flow and the mist flow in the high frequency section have obvious peaks. Anyway, it is an effective way to understand and characterize the dynamic characteristics of two-phase flow patterns using the multi-scale non-linear analysis method based on image gray-scale fluctuation signals.     

An experimental investigation of gas-liquid two-phase flow in single microchannel contactors

This paper describes two-phase flow pattern and pressure drop characteristics during the absorption of CO₂ into water in three horizontal microchannel contactors which consist of Y-type rectangular microchannels having hydraulic diameters of 667, 400 and , respectively. With the help of a high-speed photography system, flow patterns such as bubbly flow, slug flow (including two sub-regimes, Taylor flow and unstable slug flow), slug-annular flow, churn flow and annular flow were observed in these microchannels. The applicability of the currently available correlations for describing flow pattern transitions in microchannels has been examined. Generally, the predicting performance of these correlations deteriorates as the channel diameter further reduces. Toward solving this discrepancy, an empirical correlation based on the superficial Weber numbers was developed to interpret the transition from Taylor flow to unstable slug flow in three microchannels. Taylor bubble formation process in microchannels was found to be in the squeezing regime at lower superficial liquid velocities (Ca ranging from 0.0019 to 0.029) while the transition to the dripping regime was observed at the highest superficial liquid velocity of 1.0 m/s. Lengths of Taylor bubbles formed in the squeezing regime can be well represented by the scaling relation proposed by Garstecki et al. [Formation of droplets and bubbles in a microfluidic T-junction—scaling and mechanism of break-up. Lab on a Chip, 6, 437-446]. For flow patterns including slug-annular flow, annular flow and churn flow, a simple analysis based on the separated flow model has been performed in order to reveal the observed effect of the superficial liquid velocity on two-phase frictional multiplier in the present microchannels. Then, reasonable correlations for the prediction of two-phase frictional pressure drop under these flow patterns were suggested.