B类颗粒在鼓泡流化床中流动特性的数值模拟

采用欧拉双流体模型对鼓泡流化床中气-固两相流动行为进行数值模拟。模拟结果表明,采用结构曳力模型能够较好地预测B类颗粒在鼓泡流化床中的流动行为。对比初始流态化颗粒浓度图和完全流态化颗粒浓度分布图,可以发现结构曳力模型能够较好地展现鼓泡流化床中气泡的运动特性。当比较不同曳力模型下的模拟结果时,结构曳力模型比传统曳力模型能够更好地预测颗粒的径向分布。     

鼓泡塔中气泡群运动的实验与模拟

采用高速摄像法测量了均匀鼓泡流状态下,水以及体积分数分别为20%和40%的甘油-水体系中气泡群的浮升运动,考察了气含率、雷诺数和分布器孔径对气泡尺寸、形状、浮升速率和曳力系数的影响。构建了立方体单元胞模型,并根据雷诺数的不同选取层流和湍流模型,模拟得到气泡的浮升速率和曳力系数与实验值吻合较好。结果表明:随着气含率及液体黏度的提高,气泡群浮升阻力增大,浮升速率减少;随着雷诺数和气泡直径的增加,曳力系数减小,气泡浮升速率增大。单元胞模型能较好地反映气泡群浮升过程中各因素的影响,是处理气泡群运动的有效工具。     

分区曳力模型的2D鼓泡流化床流动特性模拟

曳力对两相流动特性具有很大影响,由于颗粒团聚的影响,传统曳力模型不能准确预测气固间曳力.为了提高两相流动特性的模拟精度,文中综合考虑不同曳力模型在特定浓度的曳力系数特性,分区确定曳力模型,并在模型断点处使用光滑函数,基于CFD开源平台OpenFOAM耦合TFM模型对2D鼓泡流化床进行流动特性模拟.结果表明:分区曳力模型...     

基于格子Boltzmann方法的单颗粒绕流数值模拟

采用格子Boltzmann方法（LBM）研究了单颗粒绕流流动过程。通过使用LBM中的LBGK（lattice Bhatnagar-Gross-Krook）模型和二阶精度的曲线边界条件处理方法,实现了对单颗粒绕流问题的定常及非定常流动过程中涡结构的模拟。采用动量交换法分别计算了Reynolds数在0.1～200范围内27个不同Reynolds数时的曳力系数,并将计算结果拟合得到基于LBM数值模拟的曳力曲线。计算结果表明,LBM在气固两相流的模拟计算中具有精确、可靠的优点,使用LBM模拟计算曳力曲线的方法经济、易行,并且可以克服由传统实验方法获得曳力曲线的局限性。     

锥形分布板射流流化床CFD模拟及参数分析

采用双流体模型结合颗粒动力学理论,对锥形分布板射流流化床内气固流动行为进行了三维的计算流体力学(CFD)模拟研究,系统分析了曳力模型、恢复系数和颗粒间摩擦力对射流流化床膨胀高度和气泡动力学行为的影响.结果表明,Syamlal-O'Brien和Gidaspow曳力模型低估了床内实际曳力,从而导致模拟膨胀高度低于实验值,而Modified Syamlal-O'Brien曳力模型能更好地预测床内实际曳力,计算结果与实验值吻合得较好;恢复系数对于射流流化床内气泡动力学行为有着重要影响,气泡的大小、上升速率和产生频率均随着恢复系数的增加而减小;颗粒间的摩擦力也是影响床层膨胀高度和床内气泡产生的重要因素.     

采用改进的曳力模型模拟2D鼓泡流化床的流化特性

针对特定颗粒浓度范围颗粒团聚作用导致的曳力下降问题,基于传统曳力模型在不同颗粒浓度段的特性分析,选择适用于不同颗粒浓度区间的曳力模型,通过光滑函数得到改进的曳力模型,并耦合欧拉双流体模型对2D鼓泡流化床进行数值模拟. 结果表明,与Gidaspow和Syamlal-O'Brien模型相比,改进的曳力模型对床层局部压降的预测结果更好;随表观气速增加,改进的曳力模型能更准确地预测床层膨胀;当表观气速Ug=0.46 m/s时,改进的曳力模型对径向颗粒浓度分布的模拟结果明显好于Syamlal-O'Brien模型.     

不同曳力模型对鼓泡床内气固两相流的模拟研究

基于双流体模型,应用Fluent商业软件包对带有单喷嘴的二维鼓泡床中气固两相流进行模拟研究。采用三种基于不同机理的曳力模型;半经验的Gidaspow模型、由格子波尔兹曼方法导出的Koch-Hill模型与修正的半经验的McKeen模型,通过模拟气泡的形成、上升及破裂过程,气泡形状和颗粒运动特征,计算气体泄漏率、气泡直径及气泡上升速度,对不同曳力模型进行比较研究,其中编程实现了Koch-Hill和McKeen曳力模型模块。通过与文献中的实验结果进行对比发现,Gidaspow模型对气泡形状的模拟效果较好,与实验数据的误差介于其他两种模型之间;Koch-Hill模型捕捉到的气泡特征最逼真,且床层膨胀效果明显,但定量计算的误差最大;而McKeen模型与实验数据的误差最小,但对气泡形状的模拟效果较差。     

非稳态载粒多相流曳力研究进展

介绍不稳定曳力的理论研究现状,并形成了初步的理论体系。实验研究方面,利用激波管和高速摄像法进行了曳力系数可视化研究,但仍需要更全面、更精确的数据补充;在实验研究的基础上又进行了模拟研究,有助于揭示非稳态多相流的瞬时微观特性,但由于模型本身的局限性,仍无法准确描述非稳态流场和颗粒的复杂耦合作用。最后指出DNS模型修正和可视化实验技术将是后续研究需要突破的方向。     

颗粒团绕流曳力系数的LBM计算

采用格子Boltzmann方法(Lattice Boltzmann method, LBM) 中的LBGK(Lattice Bhatnagar- Gross-Krook)模型和二阶精度的曲线边界条件处理方法对二维颗粒团绕流现象进行了数值模拟,并同时使用动量交换法计算了两种颗粒团构型中不同颗粒的曳力系数。结果表明：颗粒团曳力系数与颗粒聚团的构型有着密切联系,颗粒聚团的形成将导致颗粒团曳力系数大幅度减小。除颗粒团构型因素外,颗粒间距和流动Reynolds数也是导致颗粒团曳力系数发生改变的主要因素。当颗粒聚团存在时,颗粒团中不同颗粒的受力有较大差异,若忽略颗粒聚团效应,则颗粒团曳力系数的计算必然将产生偏差。     

基于气泡群相间作用力模型的加压鼓泡塔流体力学模拟

目前,多数文献报道了冷态加压湍动鼓泡塔内流动特征,并且通过实验数据回归相关经验关联式。然而,此类关联式适用范围有限,难以直接外推到工业鼓泡塔反应器条件。因此,在FLUENT平台上建立了基于气泡群相间作用力的、动态二维加压鼓泡塔计算流体力学模型。通过数值模拟考察了操作压力为0.5~2.0 MPa,表观气速为0.20~0.31 m·s^-1,内径0.3 m鼓泡塔内流场特性参数分布,并且与冷态实验数据进行比较。结果表明,采用修正后的气泡群曳力模型、径向力平衡模型以及壁面润滑力模型描述气泡群相间作用力,能够较为准确地反映平均气含率和气含率径向分布随操作压力和表观气速变化的规律。     

用单元胞模型数值模拟气泡群中气泡的运动

用数值方法模拟了气泡群中Reynolds数为50的气泡的运动,以单元胞模型体现气泡群中气泡间的相互作用。数学模型用完全的流体力学方程、以流函数-涡度为变量来描述变形气泡周围的液体流动,控制方程在贴体正交坐标系中、以有限差分法离散后数值求解。模拟结果表明,气含率对气泡阻力系数和气泡偏心率有很大的影响,与理论分析较符合。但单元胞模型预测的阻力系数与基于爬流中球形气泡的理论值相比,在数值上偏高很多。采用无剪切为外边界条件比用无涡度为外边界条件的模型预测更接近理论公式。预计单元胞模型经进一步改进后可成为处理气泡群的有效工具。     

Drag force and clustering in bubble swarms

In this article, results of detailed numerical simulations are reported meant to provide a closure relation for the drag force acting on bubbles rising in a dense swarm. The formation of clusters of bubbles in a periodic domain and the effect thereof on the rise velocity and effective drag coefficient on the bubbles are studied. Using smaller bubble sizes than presented in our earlier work, we are also able to refine our correlation for the drag coefficient acting on bubbles rising in a swarm, such that it is applicable for a large range of bubble sizes. The simulations are performed with an advanced Front‐Tracking model in which Lagrangian marker points are used to track the gas–liquid interface, while accounting for surface tension and substantial interface deformation. Simulations were performed using periodic domains to simulate rising air bubbles in water from 1.0 mm up to 6.0 mm in diameter. The effect of liquid phase viscosity was also studied to extend the range of validity of the drag correlation. For the 1.0 and 1.5 mm cases, strong horizontal clustering effects are observed. Especially, at high gas fractions, the bubbles tend to form rigid horizontal arrays, which have been shown to strongly increase the drag force acting on the bubbles in the cluster. For viscous liquids, the tendency to form horizontal clusters is lower, and even vertical clustering is observed. The bubble slip velocity was compared with the experimental results of Zenit et al., which agree very well taking into account the differences between simulations and experiments. Based on our simulations, a new drag correlation was proposed, taking into account Eötvös numbers ranging from 0.13 to 4.9, and Morton numbers in the range 3.8 ≤ ? log Mo < 6.6, and gas hold‐ups up to 40% (30% for Eo < 0.3). At lower values for ?log Mo, the Reynolds number drops to the order of unity, and the correlation overpredicts the drag coefficient, which defines the range of applicability of the currently proposed drag correlation. The correlation itself describes a linear increase of the normalized drag coefficient as a function of the gas hold‐up. The strength of linear increase is stronger at lower Eötvös numbers. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1791–1800, 2013     

拟泡-乳曳力模型在大型高温费托流化床反应器中的CFD模拟研究

以带冷却盘管的大型高温费托流化床反应器为研究对象,开展三维计算流体力学模拟研究。传统双流体模型基于局部平均的假设,认为单位控制体内气固两相均匀分布,网格尺寸必须足够小才能正确揭示局部非均匀结构的所有细节。采用双流体模型模拟大型工业化流化床装置时,将导致网格数量过于庞大,远超现有计算能力。为提高计算效率的同时不损失模拟精度,提出了基于局部非均匀假设、适用于粗网格的拟泡-乳三相非均匀曳力(PBTD)模型。该模型将流化床分为乳化相气体、乳化相颗粒以及气泡三相,分别建立守恒方程,体现气泡的非均匀特性对气固曳力的影响。乳化相内气固曳力以及气泡相与乳化相内颗粒的曳力分开考虑。采用PBTD模型耦合传质和反应模型,建立基于局部非均匀假设的高温费托合成反应器三维流动-传递-反应模型,包括各相守恒控制方程、气泡尺寸模型、相间物质和动量交换模型、高温费托合成反应动力学模型以及初始和边界条件,预测反应器内的流场和组分浓度分布。研究结果表明:在粗网格条件下,非均匀曳力模型可以预测床层内相含率的分布情况,预测的床层膨胀高度与经验公式计算值接近,偏差为1.2%。反应器出口气体组分的质量分数与试验测量值相近,偏差在1.5%~16.0%。模拟结果证实,基于非均匀假设的PBTD模型适用于模拟工业规模的鼓泡流化床反应器,对其设计开发和工业运行具有指导价值。     

Unsteady motion of a single bubble in highly viscous liquid and empirical correlation of drag coefficient

The unsteady motion of single bubbles rising freely in a quiescent liquid with high viscosity was measured using a CCD (charge coupled device) camera. Sequences of the recorded frames were digitized and analyzed using image analysis software and the measurements of the acceleration and steady motion of bubbles were obtained. The total drag coefficient was calculated from the accelerating motion to the steady motion with the added mass force and history force included. In virtue of dimensional analysis, the total drag coefficient of single bubbles is correlated as a function of the acceleration number, Archimedes number and Reynolds number based on the equivalent bubble diameter. The proposed correlation represents very well the experimental data of the total drag force in a wide range covering both unsteady accelerating motion and steady motion. The combined added mass and history force coefficient accounting for the accelerating effect on single bubbles was evaluated and correlated.     

On the drag force of bubbles in bubble swarms at intermediate and high Reynolds numbers

An accurate and fast simulation of large-scale gas/liquid contact apparatusses, such as bubble columns, is essential for the optimization and further development of many (bio)chemical and metallurgical processes. Since it is not feasible to simulate an entire industrial-scale bubble column in full detail from first principles (direct numerical simulations), higher-level models rely on algebraic closure relations to account for the most important physical phenomena prevailing at the smallest length and time scales, while keeping computational demands low. The most important closure for describing rising bubbles in a liquid is the closure for the drag force, since it dominates the terminal rise velocity of the bubbles.Due to the very high gas loadings used in many industrial processes, bubble–bubble (or ‘swarm’) interactions need to be accounted for in the drag closure. An advanced front-tracking model was employed, which can simulate bubble swarms up to 50% gas hold-up without the problem of (numerical) coalescence. The influence of the gas hold-up for mono-disperse bubble swarms with different bubble diameters (i.e. Eötvös numbers) was quantified in a single drag correlation valid for the intermediate to high Reynolds numbers regime . Also the physical properties of the liquid phase were varied, but the simulation results revealed that the drag force coefficient was independent of the Morton number. The newly developed correlation has been implemented in a larger-scale model, and the effect of the new drag closure on the hydrodynamics in a bubble column is investigated in a separate paper (Lau et al., this issue).     

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&#8226;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。     

Numerical investigation for the suitable choice of bubble diameter correlation for EMMS/bubbling drag model

Mesoscale bubbles exist inherently in bubbling fluidized beds and hence should be considered in the constitutive modeling of the drag force. The energy minimization multiscale bubbling(EMMS/bubbling) drag model takes the effects of mesoscale structures(i.e., bubbles) into the modeling of drag coefficient and thus improves the coarse-grid simulation of bubbling and turbulent fluidized beds. However, its dependence on the bubble diameter correlation has not been thoroughly investigated. The hydrod...     

Assessment of gas-particle flow models for pseudo-2D fluidized bed applications

The aim of this work is to provide more insight into the general modeling criteria for simulating pseudo-2D bubbling fluidized beds. For this purpose, two experimental-based problems are studied. First, a fluidized bed with a high-speed central jet problem is analyzed. A qualitative study of the first bubble indicates that the bubble shape prediction is highly sensitive to the frictional model adopted. The most accurate results in terms of bubble shape and detachment time are given by a frictional model that relates the strain-rate fluctuations with the granular temperature. Second, a uniformly fluidized bed problem in bubbling regime is considered. For this case, the drag models and boundary conditions for the particulate phase are investigated. Time-averaged solid phase velocity profiles are compared with the results of the literature where it is found that no-slip conditions (or partial slip with a high specularity coefficient) are more appropriate than slip conditions at the walls for these regimes. Regarding the drag force, although none of the models presented could match the experimental velocity predictions for low gas velocities at the lower region of the bed, the Di Felice model produces the most accurate results for the whole range of regimes considered.     

Hydrodynamic Model for Horizontal Two‐phase Flow Through Porous Media

A unidirectional, two‐fluid model based on the volume‐average mass and momentum balance equations was developed for the prediction of two‐phase pressure drop and external liquid hold‐up in horizontally positioned packed beds experiencing stratified, annular and dispersed bubble flow regimes. The so‐called slit model drag force closures were used for the stratified and annular flow regimes. In the case of dispersed bubble flow regime, the liquid‐solid interaction force was formulated on the basis of the Kozeny‐Carman equation by taking into account the presence of bubbles in reducing the available volume for the flowing liquid. The gas‐liquid interaction force was evaluated by using the respective solutions of drag coefficient for an isolated bubble in viscous and turbulent flows. The proposed drag force expressions for the different flow patterns occurring in the bed associated with the two‐fluid model resulted in a predictive method requiring no adjustable parameter to describe the hydrodynamics for horizontal two‐phase flow in packed beds.