逆流鼓泡塔气含率分布的试验测量与CFD模拟

利用热膜测速仪测得了气液逆流鼓泡塔内不同表观气速、表观液速和径向位置下的气液信号,采用改进的阈值法进行分析,得到塔内气含率的径向分布。结果表明气含率在各个截面上都是从塔中心到塔壁逐渐减小;同时利用计算流体力学方法对气液逆流的鼓泡塔内的气液两相流动进行了模拟,计算了不同气速和不同液速下的气含率,计算结果与试验数据吻合较好。     

多级鼓泡塔内液体速度分布的实验研究

液体循环流动是多级鼓泡塔重要流体力学特征之一,文中在内径为282 mm,高2000 mm的鼓泡塔内,采用不同类型的筛板将普通鼓泡塔分割成双级气液鼓泡塔.采用Pavlov管测液速的方法考察了不同筛板、不同表观气速下该鼓泡塔中上下二侧的液体速度分布.根据实验结果得出了液体速度在塔中心处最大,且与表观气速有关,随着表观气速的...     

喷淋散射塔鼓泡区域的气液两相流动特性

为了探究喷淋散射脱硫塔鼓泡区域的气液两相流动特性,搭建了鼓泡塔实验台,在不同表观气速和气体分布器浸没深度的条件下进行了实验。实验结果表明:气体分布器表面形成的气泡直径约12 mm;气泡溢出液面时有小气泡形成,并随着返混的液相运动,且液相的返混剧烈程度与表观气速和气体分布器浸没深度正相关;液相在散射管管壁和鼓泡池壁面间形成大尺度循环;随着表观气速增大,气泡直径和气泡运动速度均增大,使得气含率增大,气液两相湍动加剧;随着气体分布器浸没深度增大,鼓泡床床层气含率降低,表观气速对气含率的影响效果减弱。实验结果对喷淋散射脱硫塔的设计和运行有一定参考价值。     

鼓泡塔内气液两相湍流实验研究

介绍了研究鼓泡塔气液两相流的实验装置、实验方法。液相用激光多普勒测速技术(LDV)测量,气相用粒子示踪测速技术(PIV)测量。实验表明,轴向液相速度的径向分布呈塔中心峰值、壁面附近倒流形式,且与气相表观速度大小有关,当液相表观速度一定时,随气相表观速度增大而愈加陡峭,返混也剧烈。当表观液速与表观气速之比小于19．6时,返混区总是存在,且返混区大小与高度有关：当表观液遣与表观气速之比大于19．6时,返混消失,含气率分布由塔中心峰值转向壁面峰值。径向液相速度既与气相表现速度有关又与位置高度有关,在塔底部呈现负值,这意味着向塔轴心方向流动。随着塔高增加。流动方向逐渐转变为向塔壁方向,且又有明显的峰值。     

气液两相单孔鼓泡过程的混沌分析

运用确定性混沌分析技术 ,研究了气液两相单孔鼓泡过程的混沌机理 .结果表明 ,单孔鼓泡过程是由周期及拟周期鼓泡通向混沌的 .鼓泡过程随气体流量增加可分为 3个动力学流区 :周期鼓泡区、混沌鼓泡区及喷射区 .     

流场径向分割对鼓泡塔液相返混特性的影响

在直径为50 cm,总高为550 cm的鼓泡塔中,采用脉冲示踪技术考察了流场径向分割对鼓泡塔内液相返混特性的影响.实验中气液两相分别为空气和水,气相流速为3.0～8.6 cm/s,液相流速为0.30～1.00 cm/s.实验结果表明:在考察条件下,随着表观液速的增加,停留时间分布曲线的拖尾情况得到改善,鼓泡塔内液相返混...     

多级鼓泡塔中液相返混系数与交换速度的研究

采用示踪方法对高2 000 mm,内径282 mm多级筛板鼓泡塔内液相返混系数进行测量研究,并通过扩散-返混模型以及RTD曲线给出鼓泡塔内筛板上下二侧液体交换速度,同时考查了表观气速、开孔率等因素对轴向扩散系数与液体交换速度的影响.根据实验得出鼓泡塔内轴向返混系数以及液体交换速度与表观气速、开孔率有很大关系,均随表观气...     

气液并流向上填充鼓泡塔液体混合特性的研究

在内径为165 mm气液并流向上填充鼓泡塔中采用示踪法考查了表观气速、表观液速、液体黏度、填料大小等因素对液体轴向混合特性的影响。根据实验结果得出:Peclet准数随表观液速的增加而增大,轴向返混程度减小,而随表观气速或液相黏度的增加而Peclet准数略有减小,轴向混合程度增加。同时,填料的引入明显减少轴向混合程度,随着填料直径的增加,Peclet准数明显减少,轴向混合程度增大。根据实验结果,关联了实验范围内Peclet准数的计算式,为填充鼓泡塔反应器的设计与放大提供了一定的指导作用。     

气液鼓泡塔液体流动状态的声发射检测

提出一种基于声发射气液鼓泡塔液相流动状态检测方法,将多通道声发射传感器等距地置于鼓泡塔轴向壁面,提取不同表观气速下的各通道声发射信号,对其做经验模态分解(EMD),根据每个imf的Hurst指数值,提取和重构相应的imf,求其每个信号的平均能量。实验结果表明:鼓泡塔壁处液体轴向流速随着表观气速的增加而增大,当表观气速为14 m/min时,塔壁处液体轴向环流速度最大,当表观气速大于18 m/min时,液体轴向完全变为湍流,出现严重返混。声发射检测法能够有效检测出液体流动状态。     

鼓泡塔反应器的两级气泡模型参数研究

为了研究鼓泡塔反应器两级气泡模型在高黏度下的适用性,采用动态气体逸出法,在内径为286 mm,总高为7 200 mm的鼓泡塔中考察了液体黏度(1.2×10-3—210.4×10-3Pa·s)和操作条件对塔内总气含率,大、小气泡相含率和大、小气泡上升速度等两级气泡模型参数的影响。结果表明:床层总气含率随表观气速的增加而增大,大气泡相含率受液体黏度的影响较小,受表观气速的影响较大;小气泡相含率随黏度的增加而迅速下降,在高气速时受表观气速的影响较小。大、小气泡上升速度均随液体黏度的增加而降低,但随表观气速的升高有着不同的变化关系:前者明显升高,后者略有降低。大气泡直径随着黏度增大而稍有增大,小气泡直径随着黏度增大急剧减小。     

MULTI-SCALE CHARACTERISTICS OF CHAOS BEHAVIOR IN GAS-LIQUID BUBBLE COLUMNS

The multi-value phenomenon of correlation dimension appearing in chaos analysis of time series of pressure fluctuation obtained from gas-liquid bubble columns was studied. Its relationship with multi-scale flow behavior and possible application in the identification of flow regime and regime transition in bubble columns were investigated. The results indicated that the multi-value phenomenon of correlation dimension results from the multi-scale behavior existing in the heterogeneous churn flow regime in bubble columns. When a bubble column is in the homogeneous flow regime, only one correlation dimension is found at a specified superficial gas velocity, indicating that single-scale behavior is dominant in the system. When a bubble column is in the heterogeneous churn flow regime, multi- (generally three) correlation dimensions can be obtained, showing the appearance of multi-scale behavior. Therefore, the formulation of an effective flow model depends on an appropriate multi-scale analysis for bubble columns. Flow regime and regime transition can be characterized by the structure and structure variation of the plot of the correlation integral versus radius of the hyper-sphere. On the basis of the above analysis, a complementary potential methodology called correlation integral analysis for the identification of flow regime and regime transition in gas-liquid bubble columns is recommended.     

MULTI-SCALE CHARACTERISTICS OF CHAOS BEHAVIOR IN GAS-LIQUID BUBBLE COLUMNS

The multi-value phenomenon of correlation dimension appearing in chaos analysis of time series of pressure fluctuation obtained from gas-liquid bubble columns was studied. Its relationship with multi-scale flow behavior and possible application in the identification of flow regime and regime transition in bubble columns were investigated. The results indicated that the multi-value phenomenon of correlation dimension results from the multi-scale behavior existing in the heterogeneous churn flow regime in bubble columns. When a bubble column is in the homogeneous flow regime, only one correlation dimension is found at a specified superficial gas velocity, indicating that single-scale behavior is dominant in the system. When a bubble column is in the heterogeneous churn flow regime, multi- (generally three) correlation dimensions can be obtained, showing the appearance of multi-scale behavior. Therefore, the formulation of an effective flow model depends on an appropriate multi-scale analysis for bubble columns. Flow regime and regime transition can be characterized by the structure and structure variation of the plot of the correlation integral versus radius of the hyper-sphere. On the basis of the above analysis, a complementary potential methodology called correlation integral analysis for the identification of flow regime and regime transition in gas-liquid bubble columns is recommended.     

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。     

水平气液两相流流型空间图像信息复杂性测度分析

为了考察从图像灰度序列提取的复杂性测度与气液两相流流型变化之间的关系,本文首先从高速摄影系统拍摄的60种流动工况下水平管内气液两相流流型图像中提取了三种复杂性测度（Lempel-Ziv复杂度,分形盒维数,Shannon信息熵）,在此基础上研究了不同表观气速下三种复杂性测度的混沌动力学特性,以及对气液两相流流型的表征能力。实验结果表明：三种复杂性测度均能敏感地指示出流型的变化;通过对三种复杂度随两相流流动参数变化规律分析,可以得到气液两相流动力学结构反演特征, 为揭示气液两相流流型转化机理和定量识别流型提供了一种有效的辅助诊断工具。     

A CONTRIBUTION TO THE MATHEMATICAL MODELING OF BUBBLY/SLUG FLOW REGIME TRANSITION

A new mathematical modeling approach has been applied to the analysis of bubbly vapor/liquid flows. In particular, an integro-differential equation has been formulated which describes the bubble size distribution function. Various moments of this equation yield important two-phase flow parameters, such as the bubble number density, the mean bubble radius, and the interfacial area density. The steady-state distribution function has been numerically evaluated and an approximate analytical solution has been constructed. It was found that the model appears to be inherently capable of predicting the bubble to slug flow regime transition.     

CFD Simulations of a Bubble Column Operating in the Homogeneous and Heterogeneous Flow Regimes

Bubble columns are operated either in the homogeneous or heterogeneous flow regime. In the homogeneous flow regime, the bubbles are nearly uniform in size and shape. In the heterogeneous flow regime, a distribution of bubble sizes exists. In this paper, a CFD model is developed to describe the hydrodynamics of bubble columns operating in either of the two flow regimes. The heterogeneous flow regime is assumed to consist of two bubble classes: “small” and “large” bubbles. For the air‐water system, appropriate drag relations are suggested for these two bubble classes. Interactions between both bubble populations and the liquid are taken into account in terms of momentum exchange, or drag‐, coefficients, which differ for the “small” and “large” bubbles. Direct interactions between the large and small bubble phases are ignored. The turbulence in the liquid phase is described using the k‐ϵ model. For a 0.1 m diameter column operating with the air‐water system, CFD simulations have been carried out for superficial gas velocities, U, in the range 0.006–0.08 m/s, spanning both regimes. These simulations reveal some of the characteristic features of homogeneous and heterogeneous flow regimes, and of regime transition.     

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.     

Theoretical prediction of flow regime transition in bubble columns by the population balance model

Theoretical prediction of flow regime transition in bubble columns was studied based on the bubble size distribution by the population balance model (PBM). Models for bubble coalescence and breakup due to different mechanisms, including coalescence due to turbulent eddies, coalescence due to different bubble rise velocities, coalescence due to bubble wake entrainment, breakup due to eddy collision and breakup due to large bubble instability, were proposed. Simulation results showed that at relatively low superficial gas velocities, bubble coalescence and breakup were relatively weak and the bubble size was small and had a narrow distribution; with an increase in the superficial gas velocity, large bubbles began to form due to bubble coalescence, resulting in a much wider bubble size distribution. The regime transition was predicted to occur when the volume fraction of small bubbles sharply decreased. The predicted transition superficial gas velocity was about 4 cm/s for the air-water system, in accordance with the values obtained from experimental approaches.     

Characterizing regime transitions in a bubble column with internals

Effects of internals on flow regimes are systematically investigated in a lab-scale bubble column (BC) through six approaches: total gas holdup, drift-flux, standard deviation, fractal analysis, chaos analysis and wavelet analysis. With increasing internals-covered cross-sectional area (CSA), these methods give various value of the first transitional superficial gas velocity, as different scales of structures are detected to characterize regime transition. While internals are found to have marginal effect on total gas holdup, significant change in gas-holdup structure is discovered, showing a decrease in large-bubbles holdup and an increase in small-bubbles holdup in transitional and churn-turbulent regimes. We propose a novel approach based on wavelet analysis to demarcate the boundaries among micro-, meso-, and macro-scales. The energy fraction of these representative scales identifies two transitional velocities, suggesting that internals advance the advent of first transition and delay the second one. The operating window of transitional regime is, therefore, remarkably extended.