基于双流体模型的流化床模拟

从单相流体力学中描述气体流动的Navier-Stokes方程和单颗粒运动的Newton方程出发,使用比较严格的体积平均法推导出描述气固两相宏观流动的模型方程组并设法确定了模型参数,把该方程组加以简化,能得到Gidaspow、Blake和Davidson等人的模型方程,初步证明了模型的正确性,求解使用了经本文改进的基于控制容积有限差分的IPSA方法,编制了通用程序,模拟了二维射流流化床中浓密气固两相的流动,并计算出射流的穿透深度。     

射流流化床中锥形分布板对流动的影响

给出了具有锥形分布板的射流流化床中浓密气固两相流动的多相流体力学基本方程组. 采用二维正交曲线坐标并生成了数值网格,用改进的IPSA方法求解二维正交曲线坐标中的多相流基本方程组,并编制了大型通用程序,流场可视化使用Tecplot软件. 对于给定的模拟计算,计算结果与实验值吻合. 模拟计算中改变了锥形筛板的角度、射流管的直径、床层高度、分布板开孔率的分布、射流气速、床层表观气速等,通过模拟得到床内的流动图像,考察了射流高度及颗粒循环的影响.     

流化床中射流机制和双射流相互作用

在文献的基础上, 通过实验和模拟的方法研究了射流机制和双射流的相互作用. 采用一个300 mm′51 mm的两维气固流化床,内置两个垂直射流, 使用多路毕托管系统测量射流穿透深度. 使用描述气固流态化的双流体模型进行模拟,用改进的IPSA求解模型方程,通过数值模拟, 讨论了射流产生的机理, 再现了双射流, 并发现双射流的相互作用可分为三类：孤立射流、过渡射流和互作用射流,提出了相应的射流间距判据. 发现影响双射流穿透深度最主要的因素是射流动量、两相间曳力、射流间距和床层表观气速, 建议使用Froude数、Reynolds数、床层表观气速、射流间距和喷口直径来关联不同区域的射流穿透深度. 得到了一个关联式并与文献中的关联式或实验数据做了比较.     

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。     

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。     

循环流态化：（Ⅲ）气,固流动模型

根据气、固两相流动的基本特征,循环流态化气、固两相流动模型可分为局部流动结构模型及整体流动结构模型两类。局部流动结构模型主要描述床内稀相及絮状物相的局部分布及变化,而整体流动结构模型主要描述床中空隙率。颗粒速度以及气体速度轴、径向分布及变化,其中根据实用层次不同,整体流动结构模型可分为一维、一维两区及两维模型。本文对循环流态化气、固流动模型进行了分类评述,供循环流化床反应器设计时参考。     

基于图像法的稀相气／固两相流混合比及空隙率分析

通过使用高速摄影系统获得稀相气／固两相流流动图像,并利用图像处理技术对获得的图像进行均匀光照、二值化及粘连颗粒分割等预处理,统计出图像中颗粒尺寸参数。利用数学公式,计算出稀相气／固两相流流混合比与空隙率的纵向分布和横向分布,以及不同截面积的混合比与空隙率分布,并对混合比与空隙率的分布进行比较分析。最终得出在稀相气／固两相流中,混合比比空隙率能更好地描述气／固两相流流动特性的结论。     

气液固三相湍流流动的E/E/L模型与模拟

采用基于双流体模型与粒子分散模型相结合的方法 ,建立了一个用于描述气液固三相湍流流动的Eulerian/Eulerian/Lagrangian模型 (简称E/E/L模型 ) .在Euler坐标系中考虑了气液两相 ,利用双流体模型来表述气液两相的相互关系 ;同时在Lagrange坐标系中考察了颗粒的运动 ,并把颗粒对气液两相的影响耦合于双流体模型中 .以流化床内气液固三相湍流流动为例进行的数值模拟结果与实验结果吻合良好 .所提出的模型及其模拟具有很好的准确性和可靠性 ,为研究气液固三相湍流流动提供了一种新的途径     

线性变气速下的下行床内轴向流动结构分析

在气固两相连续方程和动量方程的基础上,考虑到下行床内表观气速的变化,建立了适用于变气速条件下的一维轴向流动模型,并着重考察了气固速度和空隙率的轴向分布情况. 计算结果表明,线性变气速条件下的轴向流动情况与常气速下有明显的不同,也更接近于热态反应器的实际情况.     

强化气固两相并流干燥传递过程的模型与计算

对气固两相并流流动的干燥过程的强化方式进行了研究，对脉冲流动中气固两相的运动与传递方程提出了模型并进行了数值计算，同时对相同条件下直管型干燥器的干燥过程进行了计算与比较。实际应用结果表明，该计算模型能较好地反应气固两相并流流动干燥过程的规律。     

Effect of agitation on the fluidization behavior of a gas–solid fluidized bed with a frame impeller

The effect of agitation on the fluidization performance of a gas–solid fluidized bed with a frame impeller is experimentally and numerically investigated. A 3‐D unsteady computational fluid dynamics method is used, combining a two‐fluid model and the kinetic theory of granular flow. The rotation of the impeller is implemented with a multiple reference frame method. The numerical model is validated using experimental data of the bed pressure drop and pressure fluctuation. Although the minimum fluidizing velocity and bed pressure drop are independent of the impeller agitation, a sufficiently high agitation speed yields higher fluidization performance with reduced bubble diameters and internal circulations of particles. The fluidized bed can be divided into three zones: inlet zone where the gas distribution plays a major role, agitated fluidization zone where the impeller agitation has a positive effect on fluidization, and free fluidization zone where the impeller agitation has no effect on fluidization. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1066–1074, 2013     

提升管内气固流动行为的数值模拟

应用计算流体力学软件Fluent,对空气为连续相、固相为催化裂化反应催化剂的循环流化床提升管内的气固流动行为进行模拟。采用用户自定义函数引入颗粒与壁面的恢复系数和颗粒的镜面反射系数,对颗粒在边壁处的部分滑移运动进行描述。采用不同的计算动力学模型及参数,数值模拟了径向颗粒浓度、轴向床层压降的空间分布,以及用以描述颗粒脉动动能的颗粒温度与固含率的关系,并与文献报道的实验和数值模拟结果进行对比分析。结果表明,选取的颗粒动力学理论模型及参数、颗粒部分滑移边界条件及气固曳力模型,可计算得到合理的颗粒轴向及径向分布,验证了提升管中存在典型的径向环核流动结构和轴向压降分布。进一步分析表明固含率显著影响颗粒温度,当固含率为0.05~0.1,颗粒温度存在转折区。     

Particle‐scale simulation of the flow and heat transfer behaviors in fluidized bed with immersed tube

A kind of new modified computational fluid dynamics‐discrete element method (CFD‐DEM) method was founded by combining CFD based on unstructured mesh and DEM. The turbulent dense gas–solid two phase flow and the heat transfer in the equipment with complex geometry can be simulated by the programs based on the new method when the k‐ε turbulence model and the multiway coupling heat transfer model among particles, walls and gas were employed. The new CFD‐DEM coupling method that combining k‐ε turbulence model and heat transfer model, was employed to simulate the flow and the heat transfer behaviors in the fluidized bed with an immersed tube. The microscale mechanism of heat transfer in the fluidized bed was explored by the simulation results and the critical factors that influence the heat transfer between the tube and the bed were discussed. The profiles of average solids fraction and heat transfer coefficient between gas‐tube and particle‐tube around the tube were obtained and the influences of fluidization parameters such as gas velocity and particle diameter on the transfer coefficient were explored by simulations. The computational results agree well with the experiment, which shows that the new CFD‐DEM method is feasible and accurate for the simulation of complex gas–solid flow with heat transfer. And this will improve the farther simulation study of the gas–solid two phase flow with chemical reactions in the fluidized bed. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Experimental Investigation and Modeling of Plug-Flow Fluidized Bed Drying Under Steady-State Conditions

In this study, a model for a plug-flow fluidized bed dryer under steady-state conditions was presented. The model was based on differential equations; thus the bed of the dryer was divided horizontally and vertically into major and minor control volumes, respectively. Each control volume was composed of two thermodynamic systems: solid and gas. The mass and energy balances of the particles in the major control volume based on the axial dispersion were developed to derive the axial profiles of solid moisture content and temperature. To derive the variation of gas humidity and temperature along the bed height and hence the axial profiles of outlet gas humidity and temperature, the mass and energy balances in the gas over the minor control volume, considering the plug flow of gas through the bed, were developed. The model was validated by comparing the simulation results with the experimental data obtained by drying the long-grain rough rice under steady-state conditions in a laboratory-scale, plug-flow fluidized bed dryer. A very satisfactory agreement between the simulation and the experimental data of solid moisture content, solid temperature, and outlet gas humidity and temperature was achieved. Also, the effects of inlet gas temperature, weir height, and inlet dry solid mass flow rate on the simulated axial profiles of solid moisture content and temperature, humidity difference between inlet and outlet gas, and outlet gas temperature were investigated.     

A CFD‐PBM‐PMLM integrated model for the gas–solid flow fields in fluidized bed polymerization reactors

Although the use of computational fluid dynamics (CFD) model coupled with population balance (CFD‐PBM) is becoming a common approach for simulating gas–solid flows in polydisperse fluidized bed polymerization reactors, a number of issues still remain. One major issue is the absence of modeling the growth of a single polymeric particle. In this work a polymeric multilayer model (PMLM) was applied to describe the growth of a single particle under the intraparticle transfer limitations. The PMLM was solved together with a PBM (i.e. PBM‐PMLM) to predict the dynamic evolution of particle size distribution (PSD). In addition, a CFD model based on the Eulerian‐Eulerian two‐fluid model, coupled with PBM‐PMLM (CFD‐PBM‐PMLM), has been implemented to describe the gas–solid flow field in fluidized bed polymerization reactors. The CFD‐PBM‐PMLM model has been validated by comparing simulation results with some classical experimental data. Five cases including fluid dynamics coupled purely continuous PSD, pure particle growth, pure particle aggregation, pure particle breakage, and flow dynamics coupled with all the above factors were carried out to examine the model. The results showed that the CFD‐PBM‐PMLM model describes well the behavior of the gas–solid flow fields in polydisperse fluidized bed polymerization reactors. The results also showed that the intraparticle mass transfer limitation is an important factor in affecting the reactor flow fields. © 2011 American Institute of Chemical Engineers AIChE J, 58: 1717–1732, 2012     

Fictitious particle method: A numerical model for flows including dense solids with large size difference

Large solids coexist with small solids in a number of dense gas‐solid flow applications such as fluidized beds and pneumatic conveyers. A new numerical model that is based on the discrete element method–computational fluid dynamics mesoscopic model and extended by introducing an idea appearing in volume penalization method is presented. In computational cells including large and small solids, the amount of momentum exchange between the fluid and the solids is estimated by assuming that a large solid consist of small, dense fictitious particles. We describe the proposed model in detail and show the optimal model parameters found through a number of parameter‐dependency studies. Validation study is performed for the motion of a large sphere in a bubbling fluidized bed and good agreements are confirmed for floating and sinking motions of the sphere between the present model and the experiment. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1606–1620, 2014     

Numerical and experimental investigation of a fluidized bed chamber hydrodynamics with heat transfer

The hydrodynamics and heat transfer of a gas-solid fluidized bed chamber was investigated by computational fluid dynamic (CFD) techniques. A multifluid Eulerian model incorporating the kinetic theory for solid particles was applied to simulate the unsteady state behavior of this chamber. For momentum exchange coefficients, Syamlal-O’Brien drag functions were used. A suitable numerical method that employed finite volume method was applied to discretize the equations. The simulation results also indicated that small bubbles were produced at the bottom of the bed. These bubbles collided with each other as they moved upwards forming larger bubbles. Also, the solid particle temperature effect on heat transfer and hydrodynamics was studied. Simulation results were compared with the experimental data in order to validate the CFD model. Pressure drops and mean gas temperature predicted by the simulations at different positions in the chamber were in good agreement with experimental measurements at gas velocities higher than the minimum fluidization velocity. Furthermore, this comparison showed that the model could predict hydrodynamics and heat transfer behaviors of gas solid fluidized bed reasonably well.     

EFFECTS OF PARTICLE CHARACTERISTICS ON THE PERFORMANCE OF A FAST FLUIDIZED BED REACTOR

A heterogeneous model for the fast fluidized bed reactor which carries out a gas-solid non catalytic reaction is presented. The hydrodynamics of the fast fluidized bed is characterized by the model of Kwauk et al. (1985) which assumes the existence of two phases; a dense phase and a dilute pneumatic transport phase. For a given solid flowrate, the length of the reactor occupied by each phase depends on gas velocity, particle diameter and density and average voidage within the reactor. The gas-solid reaction is assumed to follow the shrinking core model. The solids are assumed to be completely backmixed in the dense phase and move in plug How in the dilute pneumatic transport phase. The gas phase is assumed to be in plug flow in both phases

For given gas and solid flowrates, the transition from the dense phase flow to the fast fluidized bed (containing two regions) as functions of particle size and density is determined using the model of Kwauk et al. (1985). The numerical solution of the governing mass balance equations show that for given solid and gas flowrates, (and average voidage) the gas phase conversion shows an unusual behavior with respect to particle diameter and density. Such behavior is resulted from the effects of particle diameter and density on the reactor volume occupied by each phase and the effect of particle diameter on the apparent reaction rate. The numerical results show that a fast fluidized bed gives the best conversion at large particle density and for the particle diameter which results the fast fluidized bed to be operated near the pure dense phase flow.     

A spatially‐averaged two‐fluid model for dense large‐scale gas‐solid flows

We present a spatially‐averaged two‐fluid model (SA‐TFM), which is derived from ensemble averaging the kinetic‐theory based TFM equations. The residual correlation for the gas‐solid drag, which appears due to averaging, is derived by employing a series expansion to the microscopic drag coefficient, while the Reynolds‐stress‐like contributions are closed similar to the Boussinesq‐approximation. The subsequent averaging of the linearized drag force reveals that averaged interphase momentum exchange is a function of the turbulent kinetic energies of both, the gas and solid phase, and the variance of the solids volume fraction. Closure models for these quantities are derived from first principles. The results show that these new constitutive relations show fairly good agreement with the fine grid data obtained for a wide range of particle properties. Finally, the SA‐TFM model is applied to the coarse grid simulation of a bubbling fluidized bed revealing excellent agreement with the reference fine grid solution. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3544–3562, 2017