鼓泡塔反应器气液两相流CFD数值模拟

对圆柱形鼓泡塔反应器内的气液两相流动进行了三维瞬态数值模拟,模拟的表观气速范围为0.02～0.30 m•s^-1; 模拟采用了双流体模型,并耦合了气泡界面密度单方程模型预测气泡尺寸,该模型考虑了气泡聚并与破碎对气泡尺寸的影响。液相湍流采用考虑气相影响的修正k-ε模型,两相间的动量传输仅考虑曳力作用。模拟获得了轴向气/液相速度分布、气含率分布、湍流动能分布以及气泡表面面积密度等,对部分模拟结果与实验值进行了定量比较,结果表明模拟结果与实验结果吻合较好。     

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。     

鼓泡塔气液两相流不同曳力模型的数值模拟

采用欧拉-欧拉双流体模型对圆柱形鼓泡塔内气液两相流动进行了三维数值模拟. 通过UDF自定义程序对气相出口边界进行了速度修正,解决了模拟中自由区域内有漩涡的问题;分别使用单一气泡尺寸模型和群体平衡模型(PBM)计算气泡尺寸,并比较其对气含率分布的预测结果,分别采用Schiller-Naumann, Grace和Tomiyama曳力系数模型进行模拟. 结果表明,在全塔径均匀进气的简化条件下,单一气泡尺寸模型不适用,在合适的Hamaker数下,PBM模型中原用于颗粒计算的Abrahamson模型可计算气泡聚并速率;Tomiyama曳力模型耦合PBM模型可更好地描述塔内流动情况,并与文献值吻合良好. Schiller-Naumann模型所得平均气含率与实验值相差约40%,而Grace模型所得湍动耗散比Tomiyama的结果高14.5%,气含率分布与文献值相差16.3%.     

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 .     

鼓泡塔反应器内两相流动态模拟研究

采用双流体模型以及考虑气相影响的湍流模型,对矩形鼓泡反应器中气液两相流行为进行了两维和三维数值模拟.结果表明,两维模拟不能准确预测气液两相流动态行为,而三维数值模拟则能准确模拟两相流动态行为.通过比较湍流粘度,发现两维模拟所得湍流粘度要比三维模拟高一个数量级.比较了三维模拟中湍流模型对模拟结果的影响.发现RNGK-E模型所模拟的气泡流摆动周期明显小于K-E模型的模拟结果,模拟结果更接近实验结果.考察了升力和虚拟质量力在多相流动态行为模拟中的作用,发现它们对模拟结果产生不同程度的影响,其中升力的影响比虚拟质量力的影响显著.     

双气泡相群平衡模型模拟鼓泡塔气液两相流

基于对鼓泡塔气泡行为的现有认识,把气泡分成大、小气泡. 首次建立了完整的双气泡相群平衡模型(TBPBM),以预测气泡尺寸,并耦合TBPBM与CFD双流体模型对D=440 mm鼓泡塔进行数值模拟,获得了气泡尺寸体积概率分布、时均气含率与液相速度径向分布、大小气泡相尺寸分布,对部分模拟结果与实验值及文献模拟结果进行了比较. 结果表明,TBPBM-CFD模型预测的时均气含率和液相速度分布与实验结果吻合最好,较SBPBM、平均气泡尺寸模型的模拟结果有明显改善. 与实验值相比,TBPBM模型的整体气含率模拟误差为5.7%,而SBPBM模型和平均气泡尺寸模型的误差分别为27.2%和17.3%.     

气液两相单孔鼓泡流体动力学行为混沌预测

应用混沌预测方法 ,对气液两相单孔鼓泡系统的压力波动时间序列进行了短期预测 .结果表明 ,混沌预测方法是预测气液两相鼓泡系统压力波动等流体动力学行为的有效新途径 .     

气－液两相湍流流动的双流体模型与模拟

按双流体模型的基本思想，分别将鼓泡流中的气液两相视作连续介质，用Ｅｕｌｅｒｉａｎ坐标系中多流体模型统一描述两相各自的运动，提出包括气相（气泡）湍动动能及其耗散率的ｋ＿１－ε＿１－ｋｂ－ε＿ｂ两相湍流模型。以鼓泡塔内两相湍流流动为例进行的数值模拟结果表明：预报结果与实测数据符合良好，在预示湍流输运性质上较之已有模型有明显改进。为鼓泡流研究提供了一种新的途径。     

CFD simulation of effects of the configuration of gas distributors on gas-liquid flow and mixing in a bubble column

Numerical simulations of gas-liquid flow in a cylindrical bubble column of 400 mm in diameter at the superficial gas velocity were conducted to investigate effects of the configuration of gas distributors on hydrodynamic behaviour, gas hold-up and mixing characteristics. Eight different gas distributors were adopted in the simulation. The simulation results clearly show that the configuration of gas distributor have an important impact on liquid velocity and local gas hold-up in the vicinity of the gas distributor. Comparisons of the overall gas holdup and mixing time among different gas distributors have demonstrated that none of the adopted gas distributors was able to produce the highest interfacial area and also yield the shortest mixing time. The CFD modelling results reveal that an increase in the number of gas sparging pipes used in gas distributors is beneficial in improving the gas hold-up but is disadvantageous in reducing bubble size due to a decrease in turbulent kinetic dissipation. It has been demonstrated from the simulations that the appearance of asymmetrical flow patterns in the bubble column and the adoption of smaller gas sparging pipes for gas distributors are effective in improving the mixing characteristics.     

CFD simulation of gas–liquid flow in a high-pressure bubble column with a modified population balance model

In this study, based on the Luo bubble coalescence model, a model correction factor C_e for pressures according to the literature experimental results was introduced in the bubble coalescence efficiency term. Then, a coupled modified population balance model (PBM) with computational fluid dynamics (CFD) was used to simulate a high-pressure bubble column. The simulation results with and without C_e were compared with the experimental data. The modified CFD-PBM coupled model was used to investigate its applicability to broader experimental conditions. These results showed that the modified CFD-PBM coupled model can predict the hydrodynamic behaviors under various operating conditions.     

鼓泡塔气液两相流内部流场的流体力学特性

采用计算流体力学CFD软件对鼓泡塔内部4种表观气速下内部流场的流体力学行为进行模拟。分析了鼓泡塔内部整体气含率及轴截面处（X=0）液相速度随时间的变化情况;并且对比了在不同表观气速下局部气含率和液相速度在不同高度处的径向分布情况。模拟结果表明,随着时间的增大,整体气含率增大速度比较快,到达稳定时间,整体气含率不再增大。在同一高度处局部气含率随着表观气速的增大而增大。H/D<3时,液相循环流动表现为单相循环流;当H/D=3时,表现为双循环流,流型较单相复杂。     

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.     

二维鼓泡床内气泡尺寸分布的实验与CFD模拟

在有机玻璃制成的二维鼓泡床(0.20m×0.02m×2.00m)内,采用摄像法对空气－自来水的气液两相体系的气泡尺寸分布进行了考察。以商业计算流体力学软件ANSYS CFX 10.0为平台,在双流体模型的基础上,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进行了多相流CFD数值模拟。结果表明 MUSIG(Multiple Size Group)模型实现了对多气泡体系内气泡尺寸分布特性的考察,气泡尺寸分布的模拟结果与实验结果吻合得较好,从而说明了考虑了气泡聚并破碎的MUSIG模型能很好地反映出鼓泡床内气泡尺寸分布特性。     

Mixing in mechanically agitated gas-liquid contactors,bubble columns and modified bubble columns

Mixing time measurements were made in 300 and 1000 mm i.d. mechanically agitated contactors with different types of impellers, located at different heights from the bottom of the vessel. Mixing time measurements were also made in 150, 200, 385 and 1000 mm i.d. bubble columns with varying liquid heights. Transient pH measurement and conductivity measurement were used to measure the mixing times. Impeller speed was varied in the range of 3.33–20 r/sec in the case of mechanically agitated contactors and gas superficial velocity was varied in the range of 10–250 mm/sec in bubble columns. Effect of physical properties of the fluid (surface tension, ionic strength, liquid viscosity) and that of the non-Newtonian behavior on mixing time was studied. Mixing time in the presence of drag reducing agents was also investigated.In the range of variables covered in this work mixing time in mechanically agitated contactors and bubble columns was found to be in the range of 4–6Mixing time predictions based on the longest loop length and circulation velocity are made in the presence and absence of a gas for mechanically agitatA procedure is given for the prediction of the critical impeller speed for gas phase dispersion in mechanically agitated contactors.     

气液鼓泡床内的液体流速分布

引言 鼓泡床是一种重要的气液或气液固多相反应器.液体循环流动是鼓泡床的一个重要流体力学特征,从20世纪50年代人们就开始对此进行了比较系统的实验研究[1-6].这个特征对鼓泡床的流体返混行为、气含率、气液界面积以及传热传质系数都有很大影响,特别是液体返混行为可以由液体循环特性直接决定.如何准确地描述和预测鼓泡床中的液体流速沿径向的分布,关系到鼓泡床反应器的设计、放大和优化.因此,许多年来它一直是人们致力探讨的重要课题之一[7-8].     

Multi-scale analysis of gas–liquid interaction and CFD simulation of gas–liquid flow in bubble columns

The ratio of effective drag coefficient to bubble diameter is of critical importance for CFD simulation of gas–liquid flow in bubble columns. In this study, a novel model is proposed to calculate the ratio on the basis of the Dual-Bubble-Size (DBS) model. The motivation of the study is that a stability condition reflecting the compromise between different dominant mechanisms can serve for a closure in addition to mass and momentum conservative constraints, and the interphase momentum transfer should be related to different paths of energy dissipation. With the DBS model, we can first offer a physical interpretation on macro-scale regime transition via the shift of global minimum point of micro-scale energy dissipation from one potential trough to the other. Then the proposed drag model is integrated into a CFD simulation. Prior to this integration, we investigate the respective effects of bubble diameter and correction factor and found that the effect of bubble diameter is limited, whereas the correction factor due to the bubble swarm effect is eminent and appropriate correction factor has to be selected for different correlations of standard drag efficient to be in accord with experiments. By contrast, the DBS drag model can well predict the radial gas holdup distribution, the total gas holdup as well as the two-phase flow field without the need to adjust model parameters, showing its great potential and advantage in understanding the complex nature of multi-scale structure of gas–liquid flow in bubble columns.