Model validation for low and high superficial gas velocity bubble column flows

In the present work, gas-liquid flow dynamics in a bubble column are simulated with CFDLib using an Eulerian-Eulerian ensemble-averaging method in a two-dimensional Cartesian system. The two-phase flow simulations are compared to experimental measurements of a rectangular bubble column performed by Mudde et al. [1997. Role of coherent structures on Reynolds stresses in a 2-D bubble column. A.I.Ch.E. Journal 43, 913-926] and a cylindrical bubble column performed by Rampure et al. [2003. Modeling of gas-liquid/gas-liquid-solid flows in bubble columns: experiments and CFD simulations. The Canadian Journal of Chemical Engineering 81, 692-706] for low and high superficial gas velocities, respectively. The objectives are to obtain grid-independent numerical solutions using CFDLib to reconcile unphysical results observed using FLUENT with increasing grid resolutions [Law, D., Battaglia, F., Heindel, T.J., 2006. Numerical simulations of gas-liquid flow dynamics in bubble columns. In: Proceedings of the ASME Fluids Engineering Division, IMECE2006-13544, Chicago, IL], and to validate computational fluid dynamics (CFD) simulations with experimental data to demonstrate the use of numerical simulations as a viable design tool for gas-liquid bubble column flows. Numerical predictions are presented for the local time-averaged liquid velocity and gas fraction at various axial heights as a function of horizontal or radial position. The effects of grid resolution, bubble pressure (BP) model, and drag coefficient models on the numerical predictions are examined. The BP model is hypothesized to account for bubble stability, thus providing physical solutions.     

THEORETICAL PREDICTION OF GAS HOLDUP IN BUBBLE COLUMNS WITH NEWTONIAN AND NON-NEWTONIAN LIQUIDS

A simple model based on an energy balance which takes into account the friction losses at the gas-liquid interface and the slip velocity of single bubble is used to simulate the gas holdup in bubble columns containing Newtonian and non-Newtonian liquids which circulate in both laminar and turbulent flows. Experimental data available from the literature for bubble columns up to 7 m height and 1 m diameter with water and glycerol as Newtonian liquids and different solutions of CMC in a wide range of concentrations as non-Newtonian liquids are simulated with good agreement despite the simplifications made to describe the gas liquid flow regimes. Most of the differences between experimental and calculated gas holdup are justified on the basis of the simplifying assumptions.     

Flow regimes and mass transfer in counter-current bubble columns

Counter current bubble columns have the feature that specific gas-liquid interfacial area and gas holdup are larger than those for standard and cocurrent bubble columns. In this study, three different flow regimes, churn-turbulent flow, bubble flow and bubble down-flow, have been observed in a counter-current bubble column and correlations of gas holdup and volumetric liquid-phase mass transfer coefficient have been proposed as functions of operating variables such as the superficial velocities of gas and liquid, the gas-liquid slip velocity and the liquid properties.     

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...     

Experimental and numerical investigations of scale-up effects on the hydrodynamics of slurry bubble columns

Experiments and simulations were conducted for bubble columns with diameter of 0.2 m(180 mm i.d.), 0.5 m(476 mm i.d.) and 0.8 m(760 mm i.d.) at high superficial gas velocities(0.12–0.62 m·s-1) and high solid concentrations(0–30 vol%). Radial profiles of time-averaged gas holdup, axial liquid velocity, and turbulent kinetic energy were measured by using in-house developed conductivity probes and Pavlov tubes. Effects of column diameter, superficial gas velocity, and solid concentration were investigated in a wide range of operating conditions. Experimental results indicated that the average gas holdup remarkably increases with superficial gas velocity, and the radial profiles of investigated flow properties become steeper at high superficial gas velocities. The axial liquid velocities significantly increase with the growth of the column size, whereas the gas holdup was slightly affected. The presence of solid in bubble columns would inhibit the breakage of bubbles, which results in an increase in bubble rise velocity and a decrease in gas holdup, but time-averaged axial liquid velocities remain almost the same as that of the hollow column. Furthermore, a 2-D axisymmetric k–ε model was used to simulate heterogeneous bubbly flow using commercial code FLUENT 6.2. The lateral lift force and the turbulent diffusion force were introduced for the determination of gas holdup profiles and the effects of solid concentration were considered as the variation of average bubble diameter in the model. Results predicted by the CFD simulation showed good agreement with experimental data.     

二维鼓泡床内气液流动特性实验与数值模拟

采用高速摄像法测量了0.20 m×0.02 m×2.00 m拟二维床内气泡尺寸分布和流型等变化规律,结果表明,随着表观气速的增大,鼓泡床内依次呈现均匀鼓泡区、过渡区和湍动区3种形式,以气泡个数概率表示的气泡尺寸分布呈对数正态分布。以计算流体力学软件ANSYS CFX 10.0为平台,采用k-ε湍流模型和GRACE曳力模型对气液鼓泡床内流体动力学行为展开了数值模拟,其结果与实验值比较吻合。研究表明,从多相流理论出发的计算流体力学模拟方法可以用来预报鼓泡床内流型过渡等流体动力学特性。     

Gepackte Aufstrom-Blasensäulen

Packed upflow bubble columns . Packed upflow bubble columns are used in the chemical industry, in biotechnology, and in waste-water purification. They are usually operated in the co-current mode and have various advantages, but also disadvantages, compared with empty bubble columns. This survey reports the current state-of-the-art in fluid dynamics (flow states, pressure drop, holdup and dispersion, bubble size and bubble rise velocity, interfacial area), mass and heat transfer (mass transfer coefficients at the gas/liquid and liquid/solids interface, heat transfer coefficients for fluid/solids and fluid/wall interfaces, effective thermal conductivity of the bulk solids through which flow occurs), flow models (continuum models, stage models, zone models), and other aspects of this type of multiphase reactor; gaps in our knowledge are also indicated.     

气液两相逆流-错流撞击洗涤器内两相流动与传质特性

针对一种能实现气液两相逆流-错流撞击的洗涤喷嘴,采用溶氧法、摄像法和电导探针法等考察了操作参数对洗涤器内两相流型、传质性能、局部气含率的影响,并分析了洗涤器内流型、气含率与传质特性的关系. 结果表明,由于气液两相沿洗涤器径向分布不均,文献中通过直观观察描述流型的方法明显欠准确. 在直观观察的基础上,依据局部气含率分布将两相流型分为液柱型、环流型和泡沫型三类,其中泡沫型分为环状泡沫型和碗状泡沫型两种. 为了定量分析不同流型的传质效果,定义了有效传质区(气液接触、传质过程中形成的两相流中气含率75%≤eg≤85%的区域),得出了有效传质体积的计算表达式. 气液逆流撞击的洗涤器内有效传质体积越大,传质效果越好.     

加压鼓泡塔反应器中气液两相流CFD数值模拟

对加压气液鼓泡塔反应器内的气液两相流进行了二维数值模拟,模拟的压力为0.5～2.0 MPa,表观气速为0.120～0.312 m/s;模拟采用了Euler-Euler模型,并耦合了气泡群平衡模型（PBM）预测气泡尺寸,该模型考虑了气泡聚并与破碎对气泡的影响。液相湍流采用标准k-ε模型,两相间的作用力只考虑曳力。模拟获得了局部气含率、局部气/液相时均轴向速度及其径向分布等数据,并与实验结果进行比较。结果表明,局部气含率、局部气相速度模拟结果与实验结果吻合较好,局部液相速度径向分布特征模拟结果与文献结果相符。     

基于EMMS方法的鼓泡塔反应器CFD及群平衡模拟

能量最小多尺度（energy-minimization multi-scale,EMMS）方法已经被应用于气液体系中群平衡（population balance model,PBM）模型的改进。EMMS模型可计算气泡破碎聚并过程的能量,进而获得聚并速率的修正因子。应用这一模型对高气速鼓泡塔进行了模拟计算,并进一步对比了均一尺径模型、CFD-PBM模型以及CFD-PBM-EMMS模型的模拟结果与实验数据。结果表明,在高表观气速条件下,基于EMMS方法的群平衡模型可以更加准确地预测鼓泡塔中不同高度的气泡尺径分布和轴向液速,同时提高了对整体气含率和局部气含率的模拟准确性。在表观气速为0.16 m·s^-1和0.25 m·s^-1时,CFD-PBM-EMMS模型对气泡尺径分布的预测精度更高,同时整体气含率模拟的相对误差下降为5%和15%,局部气含率模拟平均相对误差下降为8%和17%。     

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。     

Experiment and lattice Boltzmann simulation of two-phase gas-liquid flows in microchannels

The two-phase flows in microchannels have many advantages in heat and mass transfer compared to single-phase flows. In particular, segmented flows such as bubbly and slug flows are often used in microfluidic devices. In the present study, experiments and Lattice Boltzmann simulations were carried out to study the gas-liquid flow in microchannels under various conditions. Two types of mixer geometries were used, including the cross-shape and the converging shape channels. The bubble shape, bubble size, and formation mechanism were investigated for different flow rates and different mixer geometries. The simulation results and the experimental results were compared based on dimensionless numbers, and good agreement was found in general. Different flow regimes with different bubble shapes were found depending on the Capillary number of the flow. The simulation data confirmed that the breakup was induced by the pressure difference in the two phases for small Capillary numbers. The geometry of the mixing section was also observed to have an impact on the size of the gas and liquid slugs.     

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.     

Long-term prediction of nonlinear hydrodynamics in bubble columns by using artificial neural networks

An artificial neural network (ANN) model was proposed for the long-term prediction of nonlinear dynamics underlying holdup fluctuations in bubble columns with three different diameters of 200, 400 and 800 mm. Local holdup fluctuations were measured with an optical probe in the bubble columns. The superficial gas velocity was varied in the range of 33–90 mm/s. The time intervals between successive bubbles were extracted from the time series of holdup fluctuations to represent hydrodynamic behaviors in the system and used as training and validation data sets. The effect of data preprocessing as well as the numbers of nodes in input and hidden layers on the ANN training behavior was systematically investigated. The prediction capability of the ANN was evaluated in terms of time-averaged characteristics, power spectra and Lyapunov exponents. It was observed that: the ANN model, which was trained with experimental time series and gas velocity, can be used for the long-term prediction of dynamic characteristics in bubble columns by using random data as the initial input. The results indicate that the trained ANN models have the potential of modeling nonlinear hydrodynamic behaviors in bubble columns.     

CFD simulation of bubble columns incorporating population balance modeling

A computational fluid dynamics (CFD)-code has been developed using finite volume method in Eulerian framework for the simulation of axisymmetric steady state flows in bubble columns. The population balance equation for bubble number density has been included in the CFD code. The fixed pivot method of Kumar and Ramkrishna [1996. On the solution of population balance equations by discretization—I. A fixed pivot technique. Chemical Engineering Science 51, 1311-1332] has been used to discretize the population balance equation. The turbulence in the liquid phase has been modeled by a k-ε model. The novel feature of the framework is that it includes the size-specific bubble velocities obtained by assuming mechanical equilibrium for each bubble and hence it is a generalized multi-fluid model. With appropriate closures for the drag and lift forces, it allows for different velocities for bubbles of different sizes and hence the proper spatial distributions of bubbles are predicted. Accordingly the proper distributions of gas hold-up, liquid circulation velocities and turbulence intensities in the column are predicted. A survey of the literature shows that the algebraic manipulations of either bubble coalescence or break-up rate were mainly guided by the need to obtain the equilibrium bubble size distributions in the column. The model of Prince and Blanch [1990. Bubble coalescence and break-up in air-sparged bubble columns. A.I.Ch.E. Journal 36, 1485-1499] is known to overpredict the bubble collision frequencies in bubble columns. It has been modified to incorporate the effect of gas phase dispersion number. The predictions of the model are in good agreement with the experimental data of Bhole et al. [2006. Laser Doppler anemometer measurements in bubble column: effect of sparger. Industrial & Engineering Chemistry Research 45, 9201-9207] obtained using Laser Doppler anemometry. Comparison of simulation results with the experimental measurements of Sanyal et al. [1999. Numerical simulation of gas-liquid dynamics in cylindrical bubble column reactors. Chemical Engineering Science 54, 5071-5083] and Olmos et al. [2001. Numerical simulation of multiphase flow in bubble column reactors: influence of bubble coalescence and breakup. Chemical Engineering Science 56, 6359-6365] also show a good agreement for liquid velocity and gas hold-up profiles.     

Dynamics of gas–liquid flow in a rectangular bubble column: experiments and single/multi-group CFD simulations

Several flow processes influence overall dynamics of gas–liquid flow and hence mixing and transport processes in bubble columns. In the present work, we have experimentally as well as computationally studied the effect of gas velocity, sparger design and coalescence suppressing additives on dynamics of gas–liquid flow in a rectangular bubble column. Wall pressure fluctuations were measured to characterize the low frequency oscillations of the meandering bubble plume. Bubble size distribution measurements were carried out using high-speed digital camera. Dispersed gas–liquid flow in bubble column was modelled using Eulerian–Eulerian approach. Bubble population was represented in the model with a single group or multiple groups. Bubble coalescence and break-up processes were included in the multi-group simulations via a suitable population balance framework. Effect of superficial gas velocity and sparger configurations was studied using single-group simulations. Model predictions were verified by comparison with the experimental data. Role of bubble size in determining plume oscillation period was studied. Multi-group simulations were carried out to examine evolution of bubble size distribution. An attempt is made to understand the relationship between local and global (over all the dispersion volume) bubble size distribution. The models and results reported here would be useful to develop and to extend the applications of multi-group CFD models.     

气液两相鼓泡塔流区及其过渡的混沌分析

应用确定性混饨分析技术,以气液两相鼓泡塔内的压力波动时间序列为分析对象,系统研究了鼓泡塔系统的混饨特性．结果表明,鼓泡塔内气液两相流动系统为混饨动力学系统,混饨特征参数最大Ｌｙａｐｕｎｏｖ指数、Ｋｏｌｍｏｇｏｒｏｖ熵和关联维数Ｄ２等可以有效地表征鼓泡塔的流区及其过渡．混沌分析为定量判别鼓泡塔的流区及其过渡提供了新途径．操作条件对鼓泡塔内气液两相流动的混沌特性影响显著,表现为混沌特征参数值随表观气速增加而增加,随表观液速增加而减小,但是,混沌特性随空间位置的变化不显著．