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...     

管壳式鼓泡降膜塔的流体力学研究

本文提出了管壳式鼓泡阵膜塔的结构构思与操作工况.该塔的列管具有两种流体分布器,上端口的液体分配器与降膜塔相似,下端口的气体分布器与鼓泡塔相似,以实现列管内上半段降膜、下半段鼓泡的鼓泡降膜联合操作.文中阐述了在φ16—36mm单管和多管(塔径φ200mm)中水-空气系统的流体力学实验结果.实验表明,该塔具有良好的操作性能:气液负荷大,操作弹性大,两相流状态稳定,鼓泡层返混较小,兼具鼓泡塔和降膜塔的特点.实验并为这种新型反应器提供了必要的设计数据.     

矩形鼓泡塔流体动力学数值模拟

鼓泡塔反应器内流体动力学特性预测对于鼓泡塔的设计和发展具有重要意义。文中应用CFX4.4商业软件,采用k-ε模型中气泡诱发湍流模型,对较高鼓泡塔中湍流弥散力影响和Tomiyama模型的作用进行了数值模拟。研究发现,目前通用的湍流弥散力对所研究的气泡流体动力学影响很小,引入带有气泡诱发湍流模型的Tomiyama模型能获得较好的速度分布。在这种情况下,Pfleger&Becker的模型要优于Troshko&Hassan模型,原因在于后者高估气泡诱发湍流时间尺度。     

管壳式鼓泡降膜塔新技术的应用

管壳式鼓泡降膜塔是降膜塔和鼓泡塔的发展。该塔具有良好的操作性能:气液负荷大,操作弹性大,两相流状态稳定,鼓泡层返混较小,兼具鼓泡塔和降膜塔的特点。现已成功地应用于生产中,本文介绍该装置的结构形式,鼓泡降膜段传质、传热系数和经济效益的核算,可供设参考。     

散射管气体鼓泡塔流场分布特性及优化

鼓泡塔因其较好的气-液传质性能具有高污染物脱除效率,被广泛应用于生物化工和烟气处理等领域。鼓泡塔散射管气体分布器的几何尺寸和结构是影响相间传质效率的关键因素,优化塔内流场对于提高鼓泡塔内气-液两相间的传质效率至关重要。采用Fluent软件对有内构件散射管横向进气口式的鼓泡塔进行模拟研究,基于双流体方法和群体平衡模型(PBM)模型对鼓泡塔三维建模,采用一阶迎风差分格式离散,使用Phase Coupled Simple算法进行压力速度耦合。研究了散射管所在圆环直径d分别为0. 375D、0. 5D、0. 625D、0. 75D时(D为鼓泡塔直径),散射管进气口的布置对整体和局部气含率、液速和气泡尺寸等的影响。研究结果表明,随着散射管分布环直径的增大,整体气含率先增大后减少,平均气泡直径先减小后增大;当散射管所在圆环直径d=0. 5D时,鼓泡塔整体气含率和液相循环速度最大,平均气泡直径最小,鼓泡塔流场综合性能最好。     

鼓泡塔出口边界条件的数值模拟

鼓泡塔反应器内流体动力学特性预测对于鼓泡塔的设计和发展具有重要意义。应用CFX4.4商业软件,分别采用"压力"和"开口"出口边界,数值模拟研究出口边界条件对鼓泡塔里气液泡状流动结果的影响。结果表明,当出口应用"开口"边界条件,自由表面是一个动态表面,CFX可以捕捉塔顶部整个自由表面特性。由"压力"出口边界和"开口"出口边界条件求得的数值结果差异很小。     

不同分布器对鼓泡塔气液两相流影响的CFD模拟

对不同分布器的鼓泡塔反应器内的气液两相流体动力学行为进行了三维瞬态数值模拟.模型采用欧拉-欧拉双流体模型以及RNG κ-ε湍流模型,研究了不同分布器设计对鼓泡塔反应器气液两相流的影响.模拟得到了鼓泡塔内整体气含率、液相速度矢量分布、时均气含率以及时均轴向液速的径向分布等结果,并对部分模拟结果与文献实验结果进行了比较,其结果吻合得较好,证实了随着分布器开孔越均匀则有利于加强流形的非对称性,从而加剧了气液两相径向混合,但对整体气含率的影响不大.     

不同类型鼓泡塔气液并流时液相轴向扩散系数

在相同尺寸的鼓泡塔、筛板鼓泡塔、振动筛板鼓泡塔中采用脉冲示踪法对空气-水体系进行轴向返混实验研究,考察了水和空气流速对鼓泡塔、鼓泡筛板塔液相轴向扩散系数(EZ)和水、气体流率、筛板振动强度对振动筛板鼓泡塔液相轴向扩散系数的影响.结果表明,鼓泡塔EZ与uL0.696～1.158成正比(uL为液速);筛板鼓泡塔EZ与uL和气速(uG)关系式为EZ=0.798uG0.77uL1.252;振动筛板鼓泡塔的EZ与uL0.8354～1.2740成正比.     

连续进出料鼓泡流化床停留时间分布的相似准则研究

以连续进出料鼓泡流化床为研究对象,在Glicksman相似准则的基础上引入了扩散准则数,获得了具备颗粒停留时间分布（RTD）相似性的流化床相似放大准则,并明确了颗粒RTD的相似转换关系。对缩尺流化床模型与原型的流动行为及颗粒RTD进行了数值模拟分析,发现在给定的几何相似常数范围内（1相似文献   

鼓泡塔反应器分布板的开孔率和液体返混

考察鼓泡塔内鼓泡现象,发现气流经过小孔的速度（称为小孔气速）需大于某一临界值。计算并比较了气流通过多孔板的干板压力降和表面能损耗,提出计算此临界气速的公式和确定最大开孔率的方法。 用脉冲注入法测定鼓泡塔液体停留时间分布,提出非整数串连混合器模型,推导出其停留时间分布曲线方程式,计算结果与实验数据相符。并验证了Towell的轴向分散系数的计算方法。     

鼓泡塔中Reynolds剪切力及涡扩散系数的局部分布

根据鼓泡塔内的两相湍流特性,提出估算塔内局部Ｒｅｙｎｏｌｄｓ剪切力的方法,由尼导出新的局部涡扩散系数关系式,讨论其与关系式的异同并与湍流实测数据比较。结果表明,新关系式给出与实测值更为一致的结果,而既有的一些关系式的计算值偏高,较之单相管流关系式,新关系式具有物理意义。     

Numerical Simulation of Dispersed Gas/Liquid Flows in Bubble Columns at High Phase Fractions using OpenFOAM®. Part II – Numerical Simulations and Results

The design of industrial gas/liquid reactors such as bubble columns requires detailed information with respect to the flow structure and characteristics of two‐ or multiphase systems in the reactor. The contribution is focused on the evaluation of the simulation results obtained by model selection. The results are further compared with those reported in literature. The simulation has been performed with the CFD software OpenFOAM®. The main focus of the numerical simulation was set on capturing the characteristic process and design parameters of bubble columns.     

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.     

Numerische Simulation disperser Gas/Flüssig‐Strömungen in Blasensäulen bei hohen Gasgehalten mit OpenFOAM®. Teil 2 – Numerische Simulation und Ergebnisse

The design of industrial gas/liquid reactors such as bubble columns requires detailed information with respect to the flow structure and characteristics of two‐ or multiphase systems in the reactor. The contribution is focused on the evaluation of the simulation results obtained by a selection of models. The results are further compared with those reported in literature. The simulations have been performed with the CFD software OpenFOAM®. The main focus of the numerical simulation was set on capturing the characteristic process and design parameters of bubble columns.     

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．     

Experimental detection of bubble-wall interactions in a vertical gas-liquid flow

Bubble motions and bubble-wall interactions in stagnant liquid were experimentally investigated by high-speed CCD and PIV technique with the main feature parameters such as E(o)tv(o)s numbers Eo =0.98-1.10,Morton number Mo =3.21 × 10-9 and Reynolds numbers Re =180 ～ 190.The effect of bubble injecting frequency and the distance S between the gas injection nozzle and the wall on the statistical trajectory of bubbles,average velocity distribution of flow field and Reynolds shear stress were studied in detail.It was shown that the combination of bubble injecting frequency and the distance S caused different bubble motion forms and hydrodynamic characteristics.When the normalized initial distance was very little,like S* ≈ 1.2 (here S* =2S/de,and de is the bubble equivalent diameter),bubbles ascended in a zigzag trajectory with alternant structure of high and low speed flow field around the bubbles,and the distribution of positive and negative Reynolds shear stress looked like a blob.With the increase of distance S*,bubbles' trajectory would tend to be smooth and straight from the zigzag curve.Meanwhile,with the increase of bubble injecting frequency,the camber of bubble trajectory at 20 ＜ y ＜ 60 mm had a slight increase due to the inhibitory effect from the vertical wall.Under larger spacing,such as S* ≈ 3.6,the low-frequency bubbles gradually moved away from the vertical plane wall in a straight trajectory and the high-frequency bubbles gradually moved close to the vertical wall in a similar straight trajectory after an unstable camber motion.Under the circumstances,high-speed fluid was mainly distributed in the region between the wall and the bubbles,while the relative large Reynolds shear stress mainly existed in the region far away from the wall.     

Effect of bubble deformation on stability and mixing in bubble columns

The paper deals with hydrodynamics in bubble columns. The objective of the paper is to study stability and mixing in a bubble column. The modeling of parameters such as stationary drag and added mass is addressed. In addition, the effect of bubble deformation in terms of eccentricity is highlighted. In a previous paper, the transition between homogeneous and heterogeneous regimes in bubble column without liquid flow has been shown to be driven by the deformation of the bubbles associated to drag and added mass. In the present paper, this work is generalized to bubble column with liquid flow and to the transition from bubble flow to slug flow in a vertical pipe. Numerical simulations of gas-liquid reactors are presented. The numerical simulations are validated in the case of gas plume after the Becker et al. data (Becker, S., Sokolichin, A., & Eigenberg, G. (1994) Gas-liquid flow in bubble columns and loop reactors: Part II. Comparison of detailed experiments and flow simulations. Chemical Engineering Science, 49 (24B), 5747-5762. The numerical simulations are finally applied to a bubble column. The simulations of residence time distribution coupled to transient hydrodynamics are shown to be very sensitive to the modeling of interfacial transfer of momentum from the bubbles to the liquid in terms of drag and added mass, including the effect of bubble deformation.     

Gas displacing liquids from tubes: high capillary number flow of a power law liquid including inertia effects

Computer simulation has been used as a virtual experimental tool to investigate the displacement of a shear thinning Power Law liquid from a cylindrical tube by a gas, in the limit of high capillary number and in the absence of gravity effects. Two scenarios have been considered. In the first, gas enters at a steady rate, and the gas penetration velocity and residual wall layer thickness attain steady values. In the second, a constant gas pressure is applied at the inlet, and the gas penetration rate accelerates as the column of liquid ahead of it becomes shorter. The first set of experiments confirm that the developed wall layer thickness falls with increased degrees of shear thinning and with increased Reynolds Number, and quantifies the latter effect for the first time. The relationship is summarized by a correlation formula for dimensionless layer thickness as a function of Power Law index, n, and an appropriately defined Reynolds group in the range 0.1?n?1.0,0.001?Re?100. The flow pattern ahead of the gas bubble throughout the range of these experiments was always of the ‘by-pass’ type, consistent with a generalized criterion for the transition between by-pass and re-circulating flow which is derived for a Power Law liquid. In the second set of experiments, where a constant gas inlet pressure is applied, giving accelerating gas penetration, a comparison of layer thickness values at various axial positions, with those obtained at corresponding Reynolds number in steady flow, showed close agreement, though a small discrepancy for the highest Reynolds numbers could indicate some influence of inertia in the accelerating liquid column. At higher Reynolds number, in both steady and accelerating flow, the gas bubble near to the inlet shows a concave region on the axis, with re-circulation in the liquid ahead of it. As the bubble moves down the tube, the radius of this concavity decreases and a steady convex profile is eventually attained, with reversion of the flow to by-pass type. We show that the origin of this is inertial.The results have applications in a number of technologies, including gas-assisted injection moulding of plastics and certain gas liquid reactors.     

中等Reynolds数线性剪切泡状流的液相雷诺应力

Analytical investigation of liquid Reynolds stress in shear bubbly flow with intermediate Reynolds numbers is absent. In this paper, the velocity field around a sphere bubble in linear shear liquid is assumed to be the linear superposition of the velocity field of uniform liquid passing a sphere bubble and the linear shear velocity field. The formula of shear liquid Reynolds stress was derived by averaging the velocity field in the cell-ensemble averaging method, and the formula was corrected under conditions of intermediate Reynolds number. The formula was compared with that of Sato, and the predicted results of local liquid velocity of the fully developed upward bubbly flow in pipes were compared with the experimental data. The results show that the formula is valid and accurate in prediction.     

Mass transfer and shear in an airlift bioreactor: Using a mathematical model to improve reactor design and performance

Several studies have shown a strong relationship between morphology and agitation ( [Cui et al., 1997] and [Berzins et al., 2001] ). The shear stress distribution and mass transfer are the important parameters which can improve the performance of bioreactor. In this work, a mathematical model using computational fluid dynamics (CFD) techniques is used to study the gas–liquid dispersion in an airlift reactor. Multiple rotating frame (MRF) technique is used to approximate the movement of the impeller in the stationary reactor. Population balance modeling (PBM) is used to describe the dynamics of the time and space variation of bubble sizes in the reactor. The PBM equation is solved using an approximate method known as the class method (CM) and the bubble sizes are approximated through a discrete number of size ‘bins’, including transport, and different bubble phenomena. These equations of the CM are then written as scalar transport equations and added to the multiphase fluid mechanical equations describing the dynamics of the flow. All these equations are solved using control volume formulation through the use of an open-source CFD package OpenFOAM. The model is used to analyze an existing geometry of an airlift bioreactor and validate the modification on the initial design. The new design of airlift gives a clear performance by the increase of the global and local mass transfer and the decrease of the shear stress.