不同曳力模型及颗粒碰撞恢复系数对短接触旋流反应器内气固流场的影响

为考察曳力模型和颗粒碰撞恢复系数对短接触旋流反应器内流动特性的影响,基于双流体模型,结合颗粒动力学理论,对反应器内气固两相流场进行模拟研究。分别采用Gidaspow、WenYu和Syamlal-O’Brien 3种曳力模型,考察颗粒速度特性以及固含率径向分布。对比分析不同曳力模型的计算结果表明,Syamlal-O’Brien模型计算结果与实验结果误差较大,WenYu模型在反应器边壁附近区域的计算结果误差较大,Gidaspow模型计算结果与实验结果最为吻合。此外,颗粒碰撞恢复系数较小时,所得计算值小于实验测量值,当恢复系数为0.95时颗粒扩散效果最好,计算结果与实验数据吻合度最高。     

散料颗粒密相气力输送动力参数优化及压降特性分析

为了降低管内压降,减小散料颗粒群非均匀悬浮输送所产生的湍流强度,以欧拉双流体模型为基础,应用流体力学Fluent仿真软件,建立粘度系数μ_s,k_(in)、曳力系数β在Syamlal-O’Brien和Gidaspow两种模型下形成的4种组合模型,并与实验结果进行对比,选出Gidaspow与Gidaspow组合模型作为气力输送计算模型,进而分析影响输送过程中压降变化的主要动力参数:粘度系数、曳力系数和碰撞恢复系数e_(ss),探讨前两者在该模型组合时管内压力的分布规律,得出碰撞恢复系数和入口风速v_g对管内压降变化特性的影响。     

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.     

浆态床内固含率轴向分布的数值模拟

采用DBS曳力模型计算气液相间作用,分别采用Gidaspow曳力模型、经Brucato修正的Gidaspow曳力模型和Schiller?Naumann曳力模型计算液固相间作用,忽略气固间的直接作用,对比了浆态床内不同颗粒粒径体系轴向固含率的模拟和实验结果. 结果表明,不同液固相间曳力模型对气含率的预测影响不大;在颗粒粒径较大(140 ?m)的体系中,较低表观气速下气液DBS与液固Schiller?Naumann曳力模型组合模拟的固含率随床高度增加而减小,与实验结果吻合,而其它曳力模型组合的模拟结果较差,轴向分布较均匀;在颗粒粒径较小(35 ?m)的体系中,几种曳力模型组合的模拟结果均与实验结果吻合较好,轴向分布较均匀.     

液固散式流态化CFD模拟中曳力模型的影响

基于无黏性双流体简化模型在商业化软件平台上,通过增加用户自定义子程序考察了Gidaspow、Syamlal-O’Brien、Di Felice、Gibilaro、Dallavalle和BVK曳力模型对液固散式流态化CFD模拟结果的影响行为,探讨了相应的影响机制。经与文献中不同颗粒Reynolds数的代表性实验数据对比后发现：BVK和Dallavalle曳力模型对床层膨胀高度和整体固含率的预测精度较高;BVK、Syamlal-O’Brien以及Dallavalle曳力模型给出的床内固含率径向分布较为准确;BVK曳力模型较为准确地再现了颗粒轴向速度的径向分布特征。BVK曳力模型的影响机制与液固散式流态化中颗粒动力学特性相符合,在所考察范围内其预测性能最优;Dallavalle曳力模型在其余5个传统模型中预测性能较优且形式简洁在程序中易于实现。     

循环流化床脱硫反应器气固两相流动模拟

为加深对循环流化床脱硫反应器的流动特性的认识,本研究采用双流体模型,耦合非均匀曳力模型和颗粒动力学理论,使用非均匀的修正的Syamlal O?Brien曳力模型考虑颗粒团聚现象对气固流动的影响,对循环流化床脱硫反应器中的流体力学特性进行了数值模拟。模拟得到的时均颗粒浓度和压降值与实验数据具有较好的一致性,验证了模拟方法的可靠性。颗粒速度的时间序列和概率分布函数显示,在反应器壁面附近区域均存在颗粒的向上、向下运动,在床层内颗粒总体上向上运动,同时还存在微观的内循环运动。模拟结果为颗粒速度的径向非均一分布提供了合理的解释,较Gidaspow模型有更好的适用性。     

液固散式流态化CFD模拟中曳力模型的影响

基于无黏性双流体简化模型在商业化软件平台上,通过增加用户自定义子程序考察了Gidaspow、SyamlalO’Brien、Di Felice、Gibilaro、Dallavalle和BVK曳力模型对液固散式流态化CFD模拟结果的影响行为,探讨了相应的影响机制。经与文献中不同颗粒Reynolds数的代表性实验数据对比后发现:BVK和Dallavalle曳力模型对床层膨胀高度和整体固含率的预测精度较高;BVK、Syamlal-O’Brien以及Dallavalle曳力模型给出的床内固含率径向分布较为准确;BVK曳力模型较为准确地再现了颗粒轴向速度的径向分布特征。BVK曳力模型的影响机制与液固散式流态化中颗粒动力学特性相符合,在所考察范围内其预测性能最优;Dallavalle曳力模型在其余5个传统模型中预测性能较优且形式简洁在程序中易于实现。     

稠密同轴气固射流的CFD模拟

同轴气固射流在能源领域具有广泛地应用,但大多数研究集中在颗粒浓度较低的工况。为了研究稠密同轴气固射流的流动特性,采用了稠密离散相模型(DDPM)耦合离散元模型(DEM)的方法对该体系进行计算流体动力学(CFD)模拟,该方法同时考虑了孔隙率对气固曳力的影响和颗粒间的碰撞作用。由于射流过程中,气体对颗粒的作用占主导,分别考虑了不同环形气体速度和气固曳力模型对气固流动的影响。模拟结果表明,该模型能合理地模拟在不同气速下稠密气固两相射流的颗粒弥散特性,与实验现象定性一致。在较高气速下,引入湍流模型对预测结果有显著影响,模拟得到的颗粒弥散程度较大。不同气固曳力模型对颗粒弥散的预测有明显影响,WenYu曳力模型下颗粒弥散程度较大,Gidaspow模型次之,SyamlalO’Brien模型给出的颗粒弥散程度较小。     

磷石膏颗粒湍动流化特性实验及模拟

针对磷石膏颗粒湍动流化体系曳力变化的问题,在实验的基础上,考虑非均匀结构对曳力的影响,引入修正因子φ修正Gidaspow曳力模型,对2D流化床进行了数值模拟研究。通过将Gidaspow模型在不同φ值下的模拟结果与实验结果进行对比,研究φ值的改变对模拟结果的影响规律及一定气速范围内磷石膏颗粒湍动流化体系曳力变化特性。结果表明,Gidaspow模型高估了实验体系曳力,对体系流化特性的预测效果较差;适当φ值的引入能明显提高Gidaspow模型对床层膨胀、压降及体系非均匀度的模拟精度。模拟结果反映出φ值越小,床层膨胀高度越低,床内颗粒浓度分布越不均匀,床层压降波动性越大。随着气速的升高(0.144~0.240 m/s),颗粒沿水平方向上的聚集程度加剧,φ值呈非线性减小(0.31~0.24)。流化体系的非均匀度随着气速增加而增大,颗粒浓度沿径向存在较大梯度,两侧边壁处附近出现环-核结构且流场分布对称性较差。     

液固流态化动态过程中相间作用力的数值模拟及实验验证

选择恰当的相间作用力模型是液固流态化动态特性CFD建模的关键。首先采用Richardson-Zaki关联式验证了稳态操作条件下整体固含率的实验结果,然后在基于颗粒动理学理论的欧拉-欧拉双流体模型中比较了Wen-Yu、Gidaspow、Syamlal-O’Brien、Dallavalle和TGS 5个曳力计算公式对液固流化床收缩和膨胀特性的数值模拟结果,进而探讨了Moraga等提出的升力模型影响行为及主要相间作用力影响机制。与实验测量数据比较结果表明：收缩过程中Syamlal-O’Brien和TGS曳力模型对响应时间预测较为准确,TGS曳力模型对整体固含率的预测精度较高;膨胀过程中TGS曳力模型对响应时间和整体固含率的预测优于其他模型。整体而言,基于静止颗粒群绕流直接模拟得到的TGS曳力模型忽略了颗粒-颗粒相互作用,与液固散式体系中颗粒动力学特性相符合。升力模型对动态特性模拟结果影响较小,CFD模拟时根据选择体系可予以适当忽略。     

CFD modelling of a liquid-solid fluidized bed

A multifluid Eulerian computational fluid dynamics (CFD) model with granular flow extension is used to simulate a liquid-solid fluidized bed. The numerical simulations are evaluated qualitatively by experimental data from the literature and quantitatively by comparison with new experimental data. The effects of mesh size, time step and convergence criteria are investigated. Varying the coefficient of restitution did not alter the results significantly. The Gidaspow drag relationship predicted a higher voidage than the Wen and Yu drag law. Two different liquid distributors (uniform and non-uniform) were simulated and compared, but a better representation of the geometry of the distributor plate did not greatly influence the results. Qualitatively, the simulations show trends similar to experimental trends reported by various authors. The predictions are also compared with new experimental results for 1.13 mm glass spheres at a wide variety of superficial liquid velocities (0.0085-0.110 m/s) and two different temperatures (12 and ) significantly affecting the liquid viscosity. The CFD model predictions are within 5% of the steady-state experimental data and show the correct trend with variation in viscosity.     

Cluster-based drag coefficient model for simulating gas-solid flow in a fast-fluidized bed

Drag coefficient is of essential importance for simulation of heterogeneous gas-solid flows in fast-fluidized beds, which is greatly affected by their clustering nature. In this paper, a cluster-based drag coefficient model is developed using a hydrodynamic equivalent cluster diameter for calculating Reynolds number of the particle phase. Numerical simulation is carried out in a gas-solid fast-fluidized bed with an Eulerian-Lagrangian approach and the gaseous turbulent flow is simulated using large eddy simulation (LES). A Lagrange approach is used to predict the properties of particle phase from the equation of motion. The collisions between particles are taken into account by means of direct simulation Monte Carlo (DSMC) method. Compared with the drag coefficient model proposed by Wen and Yu, results predicted by the cluster-based drag coefficient model are in good agreement with experimental results, indicating that the cluster-based drag coefficient model is suitable to describe various statuses in fast-fluidized beds.     

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.     

Comparison and evaluation of interphase momentum exchange models for simulation of the solids volume fraction in tapered fluidised beds

An Eulerian computational fluid dynamics (CFD) model with granular flow extension was used to simulate a gas–solid fluidised bed in a tapered reactor. Various drag coefficient models were evaluated, which are used to calculate the drag force, describing the momentum transfer between the gas and solid phases. Comparison and evaluation between time-averaged solids volume fractions obtained from experiments and from simulations with several drag coefficient models were made. The predicted results obtained by the different drag models were verified using experimental data of Depypere et al. (2009). Initial results using a 2-phase Eulerian model showed poor agreement with experimental results. However, extending the Eulerian model to include 3 solid phases—with different mean particle diameter per phase in order to account for the particle size distribution of the fluidised solid material—yielded good agreement with experimental results. Furthermore, quantitative analyses showed that the modified Gidaspow drag model gave the best agreement between CFD simulations and experimental data.     

CFD simulation of gas–solid bubbling fluidized bed: A new method for adjusting drag law

In computational fluid dynamics modelling of gas–solid two phase flow, drag force is one of the dominant mechanisms for interphase momentum transfer. Despite the profusion of drag models, none of the available drag functions gives accurate results in their own original form. In this work the drag correlations of Syamlal and O'Brien (Syamlal and O'Brien, Int. J. Multiphase Flow. 1988; 14(4):473–481), Gidaspow (Gidaspow, Appl. Mech. Rev. 1986; 39:1–23), Wen and Yu (Wen and Yu, Chem. Eng. Prog. Symp. Ser. 1966; 62(2):100–111), Arastoopour et al. (Arastoopour et al., Powder Technol. 1990; 62(2): 163–170), Gibilaro et al. (Gibilaro et al., Chem. Eng. Sci. 1985; 40:1817–1823), Di Felice (Di Felice, Int. J. Multiphase Flow. 1994; 20(1):153–159), Zhang‐Reese (Zhang and Reese, Chem. Eng. Sci. 2003; 58(8):1641–1644) and Hill et al. (Hill et al., J. Fluid Mech. 2001; 448:243–278) are reviewed using a multi‐fluid model of FLUENT V6.3.26 (FLUENT, 2007. Fluent 6.3 User's Guide, 23.5 Eulerian Model, Fluent, Inc.) software with the resulting hydrodynamics parameters being compared with experimental data. The main contribution of this work is to propose an easy to implement and efficient method for adjustment of Di Felice drag law which is more efficient compared to the one proposed by Syamlal‐O'Brien. The new method adopted in this work showed a quantitative improvement compared to the adjusted drag model of Syamlal‐O'Brien. Prediction of bed expansion and pressure drop showed excellent agreement with results of experiments conducted in a Plexiglas fluidized bed. A mesh size sensitivity analysis with varied interval spacing showed that mesh interval spacing with 18 times the particle diameter and using higher order discretization methods produces acceptable results.     

Numerical Simulation of Particle Segregation in Bubbling Gas‐Fluidized Beds

A multi‐fluid Eulerian model incorporating the kinetic theory of granular flow is used for the simulation of bubbling fluidized beds containing a binary mixture of Geldart B particles at low gas velocities. The cases of density, size and combined density/size segregation are investigated using computational fluid dynamic simulations. Various expressions for the drag force are evaluated for predicting different segregations. The simulation results show that summation of the particle‐particle drag force, i.e., the “hindrance effect” term, and the Stokes drag of particles, which is modified based on the Wen‐Yu drag model can be used for accurate simulation of a binary mixture of particles differing in size, density, or both. Bed expansion and dimensionless axial segregation profiles of CFD results are compared with the experimental data and good agreement is found.     

Fluidization of micronic particles in a conical fluidized bed: Experimental and numerical study of static bed height effect

The numerical simulations and experimental data of bed hydrodynamics in a conical fluidized bed unit are compared. Experimental studies have been carried out in a bed containing TiO₂ particles belonging to A/C boundary of Geldart's classification with a wide particle‐size distribution. Thus, pressure measurements and an optical fiber technique allowed determining the effect of static bed height on the fluidization characteristics of micronic particles. Numerical simulations have then been performed to evaluate the sensitivity of gas‐solids drag models. The Eulerian multiphase model has been used with different drag models and three boundary conditions (BC) consisting of no‐slip, partial‐slip, and free‐slip. The numerical predictions using the Gidaspow drag model and partial‐slip BC agreed reasonably well with the experimental bed pressure drop measurements. The simulation results obtained for bed expansion ratio show that the Gidaspow model with the free‐slip BC best fit with the experimental data. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

65 t/h高低差速循环流化床流动特性模拟

对一台65 t/h高低差速循环流化床炉内流动特性进行二维数值模拟。采用基于颗粒动力学理论的欧拉双流体模型来描述气固流动,湍流模型、气固曳力模型和不同粒径颗粒间曳力模型分别采用RNG k-ε per phase模型、Gidaspow模型和Schiller-naumann模型,并应用商业计算流体力学软件Fluent进行数值计算,得到炉内颗粒速度分布、压力分布和颗粒浓度分布,并将压力分布与实测值进行对比。在欧拉双流体模型中分别采用单粒径固相模型和多粒径固相模型,并对模拟结果进行对比分析。结果表明,单粒径固相模型能够较好预测高低差速循环流化床炉内流动特性,为其优化设计、运行及大型化提供了理论依据。     

喷动床内气固二相流动行为的计算模拟

采用双流体模型结合颗粒动理学理论对喷动床内气固二相流体流动行为进行了计算模拟研究。模型中运用颗粒动理学理论描述颗粒相应力封闭流体控制方程,使用Gidaspow曳力模型描述气固相间作用。喷动床内颗粒在浓相区的体积分数很大,采用Schaeffer′s模型描述颗粒间的摩擦应力。模拟计算结果表明,喷动床内分喷射区、喷泉区、环隙区3个区域,在射流入口处形成一个瓶颈。模拟计算得到的颗粒速度和空隙度分布与实验数据进行比较,计算结果与实验结果吻合较好。