虹吸管气液两相流过流能力影响因素分析

基于系列工况下虹吸管的过流量、压降和含气率的试验结果,分析了虹吸管内分别出现气泡流、过渡流和气团流时影响其过流能力的主要因素。结果表明,当虹吸管内为气泡流时,气体存在对流动沿程阻力系数的影响可忽略不计,不同安装高度时管道实际过流量小于计算值的原因是由于实际过水断面面积小于液相满流过水断面面积,面积减小率与流量减小率相等;当虹吸管内为气团流和过渡流时,过流断面面积减小率与流量减小率相差较大,实际过流能力减小不仅与面积减小有关,气体存在对虹吸管道沿程阻力系数的影响也不可忽略,且含气率的增大使气液两相流动阻力增大、沿程阻力系数增大、流量减小。     

逆向气流条件下半椭圆管外水平降膜厚度分布研究

为研究半椭圆管水平降膜厚度的分布规律,搭建逆向气流条件下水平降膜实验平台,并结合数字图像处理技术,研究逆流风速(0～5 m/s)和喷淋流量(0.025～0.221 L/min)对液膜厚度的影响。研究表明:逆向气流会对管外水膜产生影响,并存在临界速度;当逆流风速低于临界速度,液膜厚度沿圆周方向先减小后增大,与无空气流动时相似;当逆流风速超过临界速度时,液膜分布严重不均甚至被吹飞;随着逆流风速增大,平均液膜厚度先增大后减小;随着喷淋流量增大,平均液膜厚度持续增大;当喷淋流量减小、逆流风速增大时,平均液膜厚度减小。     

厌氧发酵液中气泡生成特性的数值研究

为了探究带有屈服应力性质的厌氧发酵液中的气泡生成特性,采用VOF法对带有屈服应力性质的厌氧发酵液中的气泡生成过程进行了数值研究。研究发现:带有屈服应力性质的厌氧发酵液中的气泡生成过程可分为膨胀阶段和脱离阶段。在气泡膨胀阶段,气泡中心的起始纵向速度较大,然后速度下降,并维持在一个较小的速度下膨胀;此时,气泡周围的流体主要沿气泡的外法线方向流动,使气泡加速膨胀,直至膨胀成球状。在气泡脱离阶段,气泡的侧端产生了对称的左右漩涡,促使其顶部继续向外扩张,纵向速度有较明显的增加;同时,气泡的"长颈"开始加速生成,最后与喷嘴脱离。在气泡的生成过程中,屈服区域逐渐变大。对于不同的屈服应力的流体而言,气泡的纵横比随着屈服应力的增大而减小,其生成时间随着屈服应力的增加而增加。文章的研究结果将有助于禽畜粪便厌氧发酵过程的优化设计。     

考虑气体压缩性的液力透平内气液两相流场分析

为研究在气液两相条件下液力透平内部的流动规律,选择比转速为55.7的单级单吸离心泵反转作液力透平(Pumps as Turbines,PAT),在考虑气体可压缩的基础上对该模型在不同流量、不同含气率下进行数值计算。分析含气率对液力透平外特性和液力透平各过流部件内流场的影响规律,总结不同工况下液力透平内气液两相流动规律。研究发现:随着含气率的增大,液力透平的效率和功率逐渐减小、扬程逐渐增加,气体的存在对液力透平效率影响较大;液力透平叶片进口有明显的旋涡,随着流量和含气率的增大,混合介质的相对速度均增加;含气率从液力透平进口到出口逐渐增大,叶片背面的含气率要比工作面大,过流部件内的气体分布不对称,随着含气率的变大,气体分布的均匀性变差。     

外波纹管管外降膜流动过程液膜厚度及速度分布特性

针对外波纹管管外降膜流动过程,采用实验结合数值模拟的方法,考察了液体喷淋密度、管间距和管径变化对液膜厚度周向分布的影响,并与光滑管进行了比较,同时分析了外波纹管管外液膜速度分布特性。结果表明:光滑管外液膜厚度由上至下沿周向呈先减小、后增加的趋势,在90°～120°之间液膜最薄;外波纹管去除波纹间凹槽内的液体后,波纹外的液膜厚度数值及其周向分布规律与相同直径的光滑管相似,周向平均液膜厚度随着液体喷淋密度的增加、管间距及管径的减小而增大;液膜沿周向分布的均匀程度及流动速度大小均与液膜厚度有关,波纹外液膜沿周向分布的不均匀性随着液膜厚度的增加而增加,气液界面处的液体速度沿周向分布规律与液膜厚度分布规律相反;相邻两波峰间凹槽内的液体存在局部循环流动。     

激冷室内气体穿越液池过程气液固三相的数值模拟

水煤浆气化炉激冷室内合成气穿越液池过程是复杂的气液固三相的流动过程,该过程起到合成气的进一步冷却及其所含凝渣捕集的作用。将欧拉和拉格朗日方法相结合,用VOF模型跟踪气液界面,用直接模拟蒙特卡罗方法(DSMC)计算颗粒碰撞。对含渣气体穿越液池的气液固三相流动过程进行了数值模拟,探讨了不同粒径、气流速度以及下降管出口淹没深度对气液流场以及对固体颗粒分离的影响。研究表明:气体出下降管后做流动方向急剧突变的流动,穿越过程气液形成具有一定周期的波状流动;含尘气体穿越液池过程对颗粒具有较高的捕集效率;颗粒的捕集效率随着粒径的增大而提高;随着气流出流速度以及下降管出口淹没深度的增加,液体的扰动加强,产生更多的液滴,有助于颗粒捕集效率的提高,但气流速度及淹没深度对颗粒捕获效率的影响逐渐减弱。     

水平PDMS微通道内气-液弹状流特性观察

对以空气、甘油为工作流体在水平聚二甲基硅氧烷(polydimethylsiloxane,PDMS)微通道内产生的弹状流进行了可视化实验研究.实验结果表明,微通道中弹状流的气柱速度随着体积含气率增大而阶梯状减小.气柱长度在体积含气率小于0.06时几乎不发生变化;当体积含气率大于0.06时,随体积含气率的增大而呈线性增大.气柱间距随体积含气率的变化与气柱长度相反.气柱和微通道壁面之间存在稳定的薄液膜,当地薄液膜压力稳定,气柱流经当地时所造成的压力波动很小;当地静压随体积含气率的增大而减小.     

消声器压力损失与气流再生噪声映射转化模型

提出了消声器压力损失与气流再生噪声的湍动能分布转化系数,建立了二者的映射转化模型.对气流再生噪声的影响因素进行分析,气流再生噪声随进口气流速度和压力损失的增大而增大,总声功率级与压力损失呈类对数变化关系,在不同消声器和不同气流速度下,计算值和试验值相对误差分别小于6%和4%.以偏置式简单扩张消声单元为例,提出了湍动能分布转化系数,建立了映射转化模型,运用该模型进行了结构参数对气流再生噪声的影响讨论.结果表明:气流再生噪声总声功率级随着扩张腔直径的增加而增大,随着进口管直径、出口管直径的增大而减小;气流再生噪声总声功率级随着腔体长度的增大而急速增大,随着进/出口管偏置距离的增大而快速下降.     

基于高速图像采集的管束间气液两相绕流旋涡特性研究

采用高速图像采集方法,研究了不同节距比时以旋转正方形排列的错列管束间气液两相流旋涡的演化特性,通过动态图像重建了旋涡生成、卷吸及聚合过程,分析了旋涡演化机理,并运用时间序列图像相关性对比分析了旋涡脱落的周期特征.结果表明:随着节距比的增大,斯特罗哈数增大;随着含气率的增大,斯特罗哈数减小;当含气率大于0.14时,不存在周期性旋涡脱落现象;随着雷诺数的增大,旋涡脱落频率增加.     

小流量气泡雾化喷嘴试验研究

设计了一个1.2 kg/h的小流量气泡雾化喷嘴,利用粒子动态分析仪(PDA),对喷嘴下游流场进行测量,分析了液雾粒径和速度的分布规律及其相关因子,考察了气液质量流量比、进气压力、混合室长度对雾化特性的影响。结果表明,液雾粒径沿径向呈非轴对称分布,轴线下方平均粒径大于上方平均粒径,液滴粒径随轴向距离增加呈先减小后增大的趋势;液雾轴向平均速度呈钟形分布,喷嘴出口区域液滴轴向平均速度和均方根速度都比较大,两者值均随轴向距离增加而逐渐减小。喷嘴出口区域,液滴粒径与速度间负相关性很强,随轴向距离的增加,其相关性可以忽略。气液比增大液雾粒径减小;在相同的气液质量流量比(ALR)下,进气压力增大,雾化效果变差;混合室长度为其直径的2.5倍时,雾化效果较好。     

振动状态下水平管内气液两相流流型转变的实验研究

对振动状态下水平管内气液两相流流型进行了实验研究,分析了不同振动频率、振动幅度对流型转变的影响。研究发现:随着振动频率或振动幅度的提高,在液相折算速度相同的情况下,环状流的形成需要更高的气相折算速度,而在气相折算流速相同时,泡状流的形成需要更高的液相流量;振动频率或幅度的提高使各个流型的走势分布发生变化,整体看是以弹状-波状流为中心向外有不同程度的扩张;振动频率与振动幅度两者对流型转变的影响基本相同。     

Characteristics of Two-Phase Flows in a Rectangular Microchannel with a T-Junction Type Gas-Liquid Mixer

In this study, gas–liquid two-phase flows in a horizontal rectangular microchannel have been investigated. The rectangular microchannel has a hydraulic diameter of 0.235 mm, and a width and depth of 0.24 mm and 0.23 mm, respectively. A T-junction-type gas–liquid mixer was used to introduce gas and liquid in the channel. In order to know the effects of liquid properties, distilled water, ethanol, and HFE7200 were used as the test liquids, with nitrogen gas was used as the test gas. The flow pattern, the bubble length, the liquid slug length, and the bubble velocity in two-phase flow were measured with a high-speed video camera, and the void fraction was determined from the bubble velocity data and the superficial gas velocity data. In addition, the pressure drop was also measured with a calibrated differential pressure transducer. The bubble length data were compared with the calculation by the scaling law proposed by Garstecki et al. [7]. The bubble velocity data and/or the void fraction data were well correlated with the well-known drift flux model [12] with a new distribution parameter correlation developed in this study. The frictional pressure drop data were also well correlated with the Lockhart-Martinelli method with a correlation of the two-phase friction multiplier.     

Bubble generation in quiescent and co-flowing liquids

A numerical simulation has been accomplished to analyze the problem of dynamic bubble formation from a submerged orifice in an immiscible Newtonian liquid under the condition of constant gas inflow. We have considered two cases for the surrounding liquid, namely the liquid in a quiescent condition and the liquid as a co-flowing stream with the gas. The full cycle, from formation to detachment of the bubbles and the corresponding bubble dynamics, was simulated numerically by using a coupled level-set and volume-of-fluid (CLSVOF) method. The role of the liquid to gas mean velocity ratio, the Bond number and the Weber number in the bubble formation process was studied and the order of magnitude of forces involved in bubble dynamics are presented. Our simulation results show that the minimum radius of the neck decreases with a power law behavior and the power law exponent in a co-flowing liquid is less than 1/2 as predicted by the Rayleigh–Plesset theory for quiescent inviscid liquids. Single periodic and double periodic bubbling (with pairing and coalescence) regimes are observed in the present investigation. It is identified that a moderate co-flowing liquid may inhibit the bubble coalescence. The volume of the bubble and the bubble formation time decrease with increasing liquid to gas mean velocity ratios. For small Bond numbers, significant differences pertaining to bubble dynamics are observed between the co-flowing liquid and the quiescent liquid. Furthermore, the generation and breakup of the Worthington jet after bubble pinch-off and formation of tiny drops inside the detached bubbles are observed.     

Adiabatic two-phase flow in rectangular microchannels with different aspect ratios: Part II – bubble behaviors and pressure drop in single bubble

One of the major flow patterns in a microchannel is an elongated bubble flow, which is similar to a long slug bubble. Behaviors and pressure drop for a single bubble in a rectangular microchannel were studied. Based on the experiments in Part I of this paper, data for liquid superficial velocities of 0.06–0.8 m/s, gas superficial velocities of 0.06–0.66 m/s and AR of 0.92, 0.67, 0.47 and 0.16 were analyzed. The velocity, length, number, and frequency of the single bubble in the rectangular microchannel were obtained from image processing based on a unit cell model. The bubble velocities were proportional to total superficial velocity. As the aspect ratio decreased, the portion of the bubble area increased due to the corner effect. New correlation of the bubble velocity for different aspect ratio was proposed. Also, bubble and liquid slug length, the number of the unit cell and bubble frequency were analyzed with different aspect ratios. The pressure drop for the single bubble in the rectangular microchannels was evaluated using the information of the bubble behavior. The pressure drop in the single elongated bubble was proportional to the bubble velocity. The pressure drop in the single elongated bubble in the rectangular microchannel increased as the aspect ratio decreased.     

An experimental study of the statistical parameters of gas–liquid two-phase slug flow in horizontal pipeline

Experiments were performed in a horizontal test loop with inner diameter 50 mm to study the gas–liquid slug flow. The translational velocities of elongated bubbles, lengths of liquid slugs and elongated bubbles, and slug frequencies were measured using two pairs of conductivity probes. Correlations are presented for elongated bubble translational velocity, length of elongated bubble and slug frequency, respectively. It was found that the translational velocity of elongated bubble is not only dependent on Froude number, but also is significantly affected by the distance from the entrance of pipeline in the higher mixture velocity range. Mean liquid slug length is relatively insensitive to the gas and liquid flow rates in the higher mixture velocity range, however in the lower mixture velocity range, the mean liquid slug length is affected by the mixture velocity. Mean slug frequency clearly increases as the liquid superficial velocity increases but it weakly depends on the gas superficial velocity.     

Gas Properties on Crown Behavior and Drop Coalescence

A single drop impact on a liquid film is numerically investigated using the coupled level set and volume of fluid (CLSVOF) method. Influences of gas properties including gas density and viscosity on the crown liquid sheet behavior and the drop-film coalescence process are mainly discussed. It is found that the decrease in the liquid–gas density ratio will result in incurve of the crown, especially when the density ratio is less than 100, due to the large gas velocity difference between the crown upper and lower positions. When the liquid–gas viscosity ratio is decreased to a value of less than 0.5, both splashing occurrence and crown expansion are suppressed, caused by the combination effect of the pressure difference and the gas velocity in the drop-film-contacting neck region. The density ratio has only minor influence on the drop coalescence process. However, when the viscosity ratio is decreased, the capillary wave on the drop surface gradually disappears. It is considered that the gas eddy reduction on the drop surface should be responsible for the capillary wave disappearance during drop coalescence.     

Application of a two-phase flow model for natural convection in an electrochemical cell

A two-phase mathematical model is applied to natural convection in an electrochemical cell. The model solves transport equations for both phases with an allowance of interphase mass and momentum exchange. The effect of current density and bubble size on the gas release rate, velocity field and void fraction distribution are investigated in a range of parameter. The flow in the system was generated due to the density difference between gas and liquid phases. It is found that both current density and bubble size significantly affect the gas release rate and velocity field. At an intermediate current density two circulation patterns form at the vicinity of the free surface. The circulations rotating opposite directions enhance lateral diffusion of gas phase. The gas evolution is enhanced with higher current density and lower bubble diameters.     

Numerical Study of Bubbly Flow Using the Second-Order Moment Turbulence Model and the Population Balance Method

A comprehensive computational fluid dynamics (CFD) turbulence model, coupled with a population balance method (PBM), is presented to simulate the two-phase gas–liquid bubbly flow. The simulation results are in good agreement with the experimental data. Furthermore, considering the bubble size distribution including the drag force and lift force, the results showed that the calculated results are in good agreement with the experimental measurements. The studies reveal the liquid recirculation and bubble uprising flow patterns, and anisotropic liquid and bubble normal Reynolds stresses. Moreover, both the liquid velocity gradient and the bubble–liquid interaction are important for the generation of liquid turbulence.     

Numerical study on the formation of Taylor bubbles in capillary tubes

Numerical simulations have been carried out for the transient formation of Taylor bubbles in a nozzle/tube co-flow arrangement by solving the unsteady, incompressible Navier–Stokes equations. A level set method was used to track the two-phase interface. The calculated bubble size, shape, liquid film thickness, bubble length, drift velocity, pressure drop and flow fields of Taylor flow agree well with the literature data. For a given nozzle/tube configuration, the formation of Taylor bubbles is found to be mainly dependent on the relative magnitude of gas and liquid superficial velocity. However, even under the same liquid and gas superficial velocities, the change of nozzle geometry alone can drastically change the size of Taylor bubbles and the pressure drop behavior inside a given capillary. This indicates that the widely used flow pattern map presented in terms of liquid and gas superficial velocities is not unique.     

Synchronous observation of rising soluble bubble through quiescent solution

An experimental method using computer image processing technology (CIPT) was proposed to observe and investigate the velocity, deformation, heat and mass transfer, etc. of a rising soluble gas (CO₂) bubble through a quiescent hot water. A model was set up to describe the behavior of the bubble in a visual experimental system in which a high-speed camera rose instantaneously with the movement of the bubble. A series of trajectory videos about the bubble were recorded by a computer linked to the camera. The trajectory, volume changes and rate of mass transfer of the bubble were obtained by the CIPT. It is found that the single bubble follows a rolling trajectory at the initial stage when there is mass transfer. With the volume decreasing, the disturbed behavior of the bubble becomes tempered. When the rising velocity of the bubble reaches the maximum, the velocity is nearly at a constant. The experimental and analysis results show that this method is useful for the research on the mass transfer and the movement of rising bubbles in liquid.