LIQUID CIRCULATION PATTERNS AND THEIR EFFECT ON GAS HOLD-UP AND AXIAL MIXING IN BUBBLE COLUMNS

Liquid velocity profiles in a bubble column were measured with the aid of a hot-film anemometer. Two different types of profiles were detected that were characterized by the formation of a boundary layer at the column wall. Under slow flow conditions, the boundary layer is large and controls the liquid velocity profile, but in turbulent flow the boundary layer is very small and has no significant effect on the velocity profile. Microscopic and macroscopic balances were used to predict both the liquid velocity profile and the average liquid velocity.

The effect of the liquid velocity profile and average velocity on axial dispersion and gas-hold-up are analyzed and design procedures are recommended.     

Radial liquid distribution in cocurrent two-phase downflow in packed beds

Radial liquid distribution was measured experimentally for cocurrent, two-phase downflow in packed beds. The effects of bed length, water and air flow rates, and type of packing were determined. The experimental data were obtained in the gas-continuous, transition and pulsing trickling-flow regimes. For all finite air rates, the liquid velocity profiles were approximately flat with the maximum average velocity occurring at the center of the packed column. Increasing the air rate increased the center liquid velocity. The gas rate effect was more pronounced in shorter beds. At higher gas rates the liquid rate had less effect on the radial liquid distribution than at lower gas rates. Operation at higher liquid rates resulted in a flatter radial liquid veilocity profile. It was observed that the bed of pellets operated at high liquid rate and low gas rate was unstable. Increasing the bed height increased the stability of the system and a better liquid distribution was obtained. The effects of water flow rate, bed length, and packing type on the shape of the liquid velocity profiles were minor.     

PROFILE OF LIQUID FLOW IN BUBBLE COLUMNS

Equations for the liquid velocity profile and the average gas hold-up in bubble columns including cocurrent flow are proposed. It is shown that the inversion point of liquid flow can be used as the characteristic parameter for calculating the liquid flow profiles and gas hold-up. A tracer method was developed to measure the inversion point of liquid flow in bubble column reactors. For water as the liquid phase this inversion point was found at a distance from the column axis of 0·70 0·73 times the column radius. Besides, bubble velocities and bubble diameters in water and methanol-water solutions were determined, using a 5-point conductivity microprobe. It was found that in dilute solutions of methanol the bubble velocity is lower than in pure water. With increasing superficial gas velocity, the bubble velocity steadily increases in pure water, whereas in methanol-water solution it first decreases and, after reaching a minimum, increases too.     

ON OPTIMUM TEMPERATURE OPERATIONS IN DEACTIVATING FIXED-BED REACTORS

Equations for the liquid velocity profile and the average gas hold-up in bubble columns including cocurrent flow are proposed. It is shown that the inversion point of liquid flow can be used as the characteristic parameter for calculating the liquid flow profiles and gas hold-up. A tracer method was developed to measure the inversion point of liquid flow in bubble column reactors. For water as the liquid phase this inversion point was found at a distance from the column axis of 0·70 0·73 times the column radius. Besides, bubble velocities and bubble diameters in water and methanol-water solutions were determined, using a 5-point conductivity microprobe. It was found that in dilute solutions of methanol the bubble velocity is lower than in pure water. With increasing superficial gas velocity, the bubble velocity steadily increases in pure water, whereas in methanol-water solution it first decreases and, after reaching a minimum, increases too.     

THERMAL DISPERSION AND HEAT TRANSFER IN NONISOTHERMAL BUBBLE COLUMNS†

Local axial and radial temperatures were measured at steady-state conditions in a 0.078-m-I.D. bubble column heat exchanger. Nitrogen and water superficial velocity ranges were 0-0.6 m/s and 0-0.02 m/s, respectively. Average column pressures were 3.0, 5.1, and 7.1 atm. The axial temperature profile varied significantly with all conditions encountered. Radial temperature profiles were found to be nearly constant, indicating very good radial mixing.

An axial thermal dispersion heat transfer model, capable of representing nonisothermal systems, was employed to characterize the measured bubble column temperature profiles. Thermal dispersion was apparent from large temperature changes in the entrance of the bubble column. Heat transfer coefficients depended on the gas and liquid flow rates. However, the thermal dispersion coefficients depended on linear gas velocity and were a weak function of liquid flow rates. The thermal dispersion coefficients obtained in this study were found to be consistent with other investigations. In addition, they were compared to the mass dispersion coefficients obtained by other studies and found to be in good agreement     

竖直窄矩形通道内环状流的流动传热特性

为了研究竖直窄矩形通道内环状流的流动传热特性,建立了窄矩形通道内环状流的数学物理模型,并进行了实验验证。通过数值求解环状流的数学物理模型得到了环状流区域的压降梯度、沸腾传热系数和液膜内的速度分布。结果表明窄矩形通道内的环状流模型能够很好地预测环状流区域的压降梯度和沸腾传热系数,而且环状流液膜内速度在法向的分布是非线性的,在层流边界层区速度梯度较大。热通量和窄矩形通道的尺寸对液膜的流速有很大影响,随热通量的增加和窄矩形通道尺寸的减小液膜的流速逐渐增加,然而质量流速对液膜流速的影响较小,而且随质量流速的增加液膜的速度逐渐减小。     

湍动浆态床流体力学研究(Ⅴ)-带多层水平网格的浆态床流体力学模型

水平布置的多层筛网是最简单的阻尼型内构件,用于改善流动分布。在安装多层筛网的?500×5000鼓泡塔冷模装置中进行了流速分布与气含率分布测定。实验表明,网格型内构件可以有效抑制塔中心区过快上升的流速,改善其分布。网孔越小,流动阻尼效果越明显,同时平均气含率也会有所增大。提出了与列管内构件相似的一维流体力学模型,将网格内构件对流体的阻尼效果作为体积源来考虑,湍流耗散方程中的参数值c3=1.0,c4=1.3与列管束内构件略有不同。模型计算与实验数据符合良好,能够定量描述网格型阻尼内构件对流场的影响。     

Studies on hydrodynamics of turbulent slurry bubble column (Ⅴ)：Modeling of the bubble column with multi-layer screens

水平布置的多层筛网是最简单的阻尼型内构件,用于改善流动分布。在安装多层筛网的?500×5000鼓泡塔冷模装置中进行了流速分布与气含率分布测定。实验表明,网格型内构件可以有效抑制塔中心区过快上升的流速,改善其分布。网孔越小,流动阻尼效果越明显,同时平均气含率也会有所增大。提出了与列管内构件相似的一维流体力学模型,将网格内构件对流体的阻尼效果作为体积源来考虑,湍流耗散方程中的参数值c3=1.0,c4=1.3与列管束内构件略有不同。模型计算与实验数据符合良好,能够定量描述网格型阻尼内构件对流场的影响。     

Flow in the nose region and annular film around a Taylor bubble rising through vertical columns of stagnant and flowing Newtonian liquids

The flow in the nose region and in the annular film around individual Taylor bubbles rising through stagnant and co-current vertical columns of liquid were studied, employing particle image velocimetry (PIV) and pulsed shadowgraphy techniques (PST) at the same time. The combined techniques enabled simultaneous determination of the bubble shape and the velocity profiles in the liquid film. Experiments were performed with water and aqueous glycerol solutions in a wide range of viscosities , in an acrylic column of 32 mm ID.Values for the distance ahead of the nose in which the flow is disturbed by the presence of the bubble are presented for the conditions studied. The bubble shapes in the nose region are compared with Dumitrescu's shape for potential flow. The velocity profiles show that after the nose region the liquid begins to accelerate downwards, and at a certain distance from the bubble nose the velocity profile and the liquid film thickness stabilise. The liquid film acquires characteristics of a free-falling film. Values of the developing length and film thickness are reported for the experimental conditions studied. Average velocity profiles in the fully developed film are also presented. A critical Reynolds number of around 80 (based on the mean absolute velocity in the liquid film and on the film thickness) is reported for the transition from laminar to turbulent regime. Shear stress profiles (in the fully developed film) are also provided.The data reported are relevant for the validation of numerical codes in slug flow.     

基于浓度/速度场测量的吸收过程液相传质系数

针对矩形流道内气、液流体的并流吸收传质过程,分别应用实时激光全息干涉术和激光多普勒速度仪对不同气、液流速下液相内近界面浓度分布与速度分布进行了实验观测.结果表明,边界层内浓度分布呈指数下降,流速越大梯度越陡,且速度边界层厚度要大于浓度边界层厚度.建立了通过物料衡算求算液相传质系数的方法.得到了不同条件下平均液相传质系数,并与Whiteman’s双膜理论的计算结果进行了比较.     

Heat and mass transfer in non-newtonian fluid flow with power function velocity profiles

Heat and mass transfer in laminar and turbulent non-Newtonian fluids is investigated in this work using the power function velocity profiles. Analytical solutions are presented for cases of mass transfer in laminar non-Newtonian fluid flows, namely for a flat velocity profile (plug flow), for the case of a constant velocity gradient at the solid boundary (Couette flow), and for the velocity distribution within a laminar boundary layer on a flat plate, and these are illustrated by rotating disks and cylinders in laminar Ostwald-de Waele fluids. Further, turbulent mass transfer processes (tubular flow, rotating disk, and rotating cylinder) in non-Newtonian fluids (Ostwald-de Waele fluid and drag-reducing fluid) at low and large Schmidt numbers are also discussed using the solutions of mass transfer in flows with power function velocity profiles. Reasonable agreement is found between the predictions of this work and the available experimental data and correlations.     

A numerical study on unsteady natural convection of air with variable viscosity over an isothermal vertical cylinder

The present study deals with the boundary layer flow and heat transfer of unsteady laminar free convection flow past a semi-infinite isothermal vertical cylinder immersed in air. The fluid viscosity is assumed to vary with the temperature. An implicit finite-difference method has been employed to solve the governing non-dimensional boundary layer equations. A parametric study is performed to illustrate the influence of variable viscosity on the velocity and temperature profiles. The numerical results reveal that the viscosity has significant influences on the transient velocity and temperature profiles, average skin-friction coefficient and the average heat transfer rate. The results indicate that as the viscosity parameter increases, the temperature and the skin-friction coefficient increase, while the velocity near the wall and the Nusselt number decrease.     

Experimental determination of the random lump-age distribution in the boundary layer of the turbulent pipe flow using laser-doppler anemometry

Local velocities in the boundary layer of a fully-developed turbulent water flow in a horizontal, straight, smooth, 50 mm dia. tube were measured at 15, 20 and 30°C, and at Re = 5000, Re = 9500 and Re = 20000 with a laser-Doppler velocimeter, accessory optics and a data processing system. Velocities in axial and radial directions at fixed positions near the wall were measured and their fluctuations were attributed to bursts and background turbulence.Each burst is considered as a spontaneous replacement of a liquid lump on the wall by a fluid entity from the bulk; background turbulence is considered as noise. For the velocity fluctuations in the axial direction an adequate procedure was applied to discriminate between bursts and noise. The values of a large number of time intervals between successively detected bursts were measured and presented in histograms. Further the time-average velocity profile in the boundary layer and the velocity profile near the wall just after the moment that a burst occurs, were determined experimentallyThe results of these measurements are compared with those predicted by the RSRP-model, based on Danckwerts' concept and recently applied to momentum transfer by Fortuin and Klijn [20].In the present paper the RSRP-theory is verified by comparing: measured mean values of the time intervals and predicted mean values of the lump ages; the measured time-interval distribution and the predicted random lump-age distribution; the measured velocity profile in a “new” fluid entity on the wall and the predicted profile in a “young” lump; the measured and predicted time-averaged velocity profiles in the boundary layer.Moreover, the behaviour of the fluid near the wall, reconstructed from measured values of the velocities in axial and radial directions around the moment that a burst occurs, agrees with the concept of surface renewal.     

γ射线透射技术测量气液鼓泡塔内气含率的轴向分布

引　言鼓泡床反应器以其具有良好的传热、传质、相间充分接触和高效的可连续操作等特点在许多领域得到了广泛的应用 ,如氧化、加氢、烷基化、污水处理等工业过程 .在鼓泡反应器中气体以分散相的形式存在 ,轴向气含率分布是反映气泡在鼓泡塔反应器内运动行为的重要参数之一 .虽然采用了各种测量方法对流体力学行为进行了大量研究[1～ 3] ,取得了一定的研究成果 ,但工业过程往往是在高温高压的条件下操作 ,大部分的测量方法难以适应这种条件 ,所以探求新的测试技术尤为重要 .　　γ射线透射技术不仅用于塔设备的故障检测 ,而且用于其他的石油…     

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。     

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.     

CFD modeling of pervaporative mass transfer in the boundary layer

Modeling mass transfer in the liquid boundary layer accounting for concentration polarization in pervaporation (PV) is particularly challenging since there is no practical way of experimentally determining solute concentration at the membrane surface. We have developed a computational fluid dynamics (CFD) approach to describe not only velocity distribution but also concentration profile in the liquid boundary layer of a slit membrane channel. The satisfactoriness of the numerical methodology used in CFD for obtaining concentration profiles were verified using a classic diffusion problem with its known analytical solution. The overall mass transfer coefficients from the numerical study were also compared with those from the experiment.     

Liquid and gas flow patterns in random packings

Large-scale liquid and gas flow patterns were measured in a 0.5-m diameter column dumped with 25- and 50-mm plastic Pall rings and 38-mm ceramic Intalox saddles. Liquid gas velocity and profiles measured for these packings were shown as a function of the initial distribution of the liquid and gas, packed height and liquid and gas flow rates. There was an emphasis in the experiments to study the development of liquid wall flow as well as the interaction between countercurrent liquid and gas. In addition, the effect of maldistribution was studied by means of water cooling experiments. It was of a particular interest to see the extent to which liquid temperature profiles, or analogously concentration profiles, were affected by known velocity profiles.     

A Simple Model for Turbulent Boundary Layer Momentum Transfer on a Flat Plate

A simple model is presented for turbulent momentum transfer on a flat plate. The proposed model is based on some polynomial velocity profiles in a laminar sublayer as well as in a fully developed boundary layer and two integral boundary layer equations. The model could be used for the calculation of boundary layer thickness, velocity profile and skin friction factor on the flat plate. The calculated results are in very good agreement with other proposed empirical correlations.     

Solids motion and holdup profiles in liquid fluidized beds

Non-invasive gamma rays-based techniques, computer tomography and computer-aided radioactive particle tracking, were used to measure solid holdup and solid velocity profiles in liquid-solid fluidized beds. The time averaged velocity measurements indicated that multiple circulation cells exist in the column. The solid motion was upward in the center and downward near the walls in the fully developed part of the column. The flow pattern is reversed in the entry region of the column. The solid holdup values increase slightly with the increase in radial position in the fully developed region. The average values of holdup in the column were in agreement with other measurements and with the modified Richardson-Zaki equation. The solids mean velocities and eddy diffusivities increase with increase in liquid superficial velocity, column size, particle size and density. Distributor-type affects the mean velocity and turbulence parameters while the column height has a relatively minor effect. The solids motion and turbulence parameters presented here are useful for validation of CFD models.