网格尺寸对循环流化床内气固两相流动的影响

将基于能量最小多尺度方法(EMMS)的曳力模型耦合到双流体模型中,并针对循环流化床内的气固两流动进行了模拟研究。采用全滑移壁面边界条件处理颗粒相,考察了3种网格尺度对轴向空隙率和出口颗粒循环量等气固流动特性的影响。计算结果表明,应用EMMS曳力模型处理相间作用力,同时在采用全滑移壁面边界条件处理颗粒相时,双流体模型能够正确预测轴向空隙率分布。采用网格尺寸为2.325 mm×20 mm时,模拟结果和实测数据吻合较好,表明在循环流化床的数值模拟中选择恰当的网格尺度是极为重要的。     

耦合EMMS曳力与简化双流体模型的气固流动模拟

提出了一种耦合EMMS曳力的简化双流体模型,该模型忽略固相黏度,用简单的经验关联式来计算固相压力,并且耦合考虑了介尺度结构的EMMS曳力模型来计算气固相间作用力。采用简化双流体模型成功模拟一个三维实验室尺度鼓泡流化床,数值模拟结果与完整双流体模型以及实验测量结果进行了比较,结果表明耦合EMMS曳力的简化双流体模型模拟结果与完整双流体模型耦合EMMS曳力的模拟结果基本相当,并且都与实验结果吻合良好,然而简化双流体模型的计算速度是完整双流体模型的两倍以上。这表明曳力模型在气固模拟中起着主导作用,而固相应力的作用是其次的,耦合EMMS曳力的简化双流体模型在实现工业规模气固反应器快速模拟中具有巨大潜力。     

颗粒相壁面条件对提升管内气固流动模拟的影响

引言提升管是非均匀结构显著的气固两相流动体系,其流动特性主要表现为轴向空隙率的"S"形分布、径向的"环-核"结构以及团聚物的生成和破碎等。近年来,国内外学者致力于数值计算方法的研究增多,其中双流体模型的应用最为广泛;颗     

Acceleration of CFD simulation of gas-solid flow by coupling macro-/meso-scale EMMS model

Gas-solid flow features significant dynamic multi-scale structure; multi-scale modeling is therefore in order. In this article, the macro-scale EMMS model was coupled with a two-fluid method (TFM) elaborated by the meso-scale EMMS model resolving sub-grid scale heterogeneity to simulate the hydrodynamics of circulating fluidized bed (CFB) risers. The overall flow distribution under the steady state was approximately predicted by the macro-scale EMMS model, which serves as the initial condition for meso-scale TFM simulations reproducing the dynamic behavior of heterogeneous gas-solid flows. Using the solid circulation flux as criterion, it was shown that this coupling approach can significantly reduce the time required to reach the statistically steady state, as compared to the packed bed or homogeneously dispersed initial condition. It also suggests a general approach to speedup dynamic simulation in the multi-scale paradigm of computation.     

边界条件和结构尺寸对提升管内气固流动特性的影响

引言提升管是非均匀结构显著的气固两相流动体系,其流动特性主要表现为轴向空隙率的S形分布、径向的"环-核"结构以及团聚物的生成和破碎等。近年来,有关数值计算方法的研究增多,其中双流体模型的应用最为广泛,颗粒动理学则是目前封闭控制方程的最合理有效的方法。     

Simulation of gas–solid flows in riser using energy minimization multiscale model: Effect of cluster diameter correlation

Reduced effective drag is observed in gas–solid riser flows due to formation of clusters. Thus cluster diameter correlation has direct impact on the calculated drag and the hydrodynamics predictions. However, its effect has not been studied. Therefore in this study, the effect of cluster diameter correlations on the drag coefficient and simulation predictions is evaluated. A structure-based drag is derived using the EMMS model, and is used to carry out computational fluid dynamics (CFD) simulations for low solid flux fluid catalytic cracking (FCC) risers. The results are compared with those using the Gidaspow drag model, as well as experimental data and previous simulation results. The time-averaged axial and radial profiles of voidages are compared with the experimental data. The comparison shows that only EMMS model is able to capture the axial heterogeneity with the dense bottom and dilute top sections. The radial profiles using both drag models shows only qualitative agreement with the experimental data. The results using the EMMS and Gidaspow drag model show a reasonable agreement near the wall and the centre, respectively. In order to improve the quality of the results obtained by the EMMS model, simulations are conducted using calculated drag coefficients from different cluster diameter correlations. The cluster diameter correlation proposed by Harris et al. (2002) gives reasonable qualitative and quantitative agreement with the experimental data for axial voidage profile, particularly in the dense bottom section; however, the quantitative disagreements in the radial profiles persists.     

A bubble-based EMMS model for gas–solid bubbling fluidization

An EMMS/bubbling model for gas–solid bubbling fluidized bed was proposed based on the energy-minimization multi-scale (EMMS) method (Li and Kwauk, 1994). In this new model, the meso-scale structure was characterized with bubbles in place of clusters of the original EMMS method. Accordingly, the bubbling fluidized bed was resolved into the suspending and the energy-dissipation sub-systems over three sub-phases, i.e., the emulsion phase, the bubble phase and their inter-phase in-between. A stability condition of minimization of the energy consumption for suspending particles (N_s→min) was proposed, to close the hydrodynamic equations on these sub-phases. This bubble-based EMMS model has been validated and found in agreement with experimental data available in literature. Further, the unsteady-state version of the model was used to calculate the drag coefficient for two-fluid model (TFM). It was found that TFM simulation with EMMS/bubbling drag coefficient allows using coarser grid than that with homogeneous drag coefficient, resulting in both good predictability and scalability.     

基于EMMS的循环流化床流域研究

流化床的设计、放大和优化需要对流域有基础的认识，然而气固系统的流域划分至今仍存在诸多争议。总结了气固流化系统流域划分的研究现状，并分析了流域划分的主要争议，发现文献中对快速床的界定存在分歧。通过耦合基于稳态EMMS的曳力模型开展双流体模拟，对不同气速和颗粒浓度下的循环流化床进行了数值研究。模拟结果捕捉到了颗粒回流、节涌等现象，据此确定了快速床的操作边界并绘制了流域图，该流域图能够展示循环床中的各流域形态。     

垂直并流向上气固两相流中流动结构的分析及曳力系数的计算

This study investigates the heterogeneous structure and its influence on drag coefficient for concurrent up gas-solid flow.The energy-minimization multi-scale (EMMS) model is modified to simulate the variation of structure parameters with solids concentration,showing the tendency for particles to aggregated to form clusters and for fluid to pass around clusters.The global drag coefficient is resolved into that for the dense phase,for the dilutephase and for the so-called inter-phase,all of which can be obtained from their respective phase-specific structure parameters.The computational results show that the drag coefficients of the different phases are quite different,and the global drag coefficient calculated from the EMMS approach is much lower than that from the correlation of Wen and Yu.The simulation results demonstrate that the EMMS approach can well describe the heterogeneous flow structure,and is very promising for incorporation into the two-fluid model or the discrete particle model as the closure law for drag coefficient.     

初始分布对气固提升管动力学模拟的影响

近期研究表明,合理的初始分布能够大幅缩短气固提升管瞬态模拟从初始到达稳定状态所需的过渡时间。以此为基础,研究发现,若以时均统计的流场信息作为模拟的初始分布,则模拟能够很快达到稳定。然后,对影响过渡时间的各个时均流场参数(如压力、气固相速度、空隙率分布等)进行了敏感性分析,发现提升管的空隙率轴向分布是其中最为关键的因素。最后,利用EMMS(energy-minimization multi-scale)模型预测的空隙率轴向分布作为气固提升管动力学模拟的初始条件进行模拟,结果表明,计算很快能达到稳定,且预测的颗粒通量等参数与实验吻合较好。     

Modeling of drag with the Eulerian approach and EMMS theory for heterogeneous dense gas-solid two-phase flow

Combined with the Eulerian approach, energy minimization multi-scale (EMMS) theory was used to develop a new theoretical model for the drag between the gas and solid phases in dense fluidized systems. The energy minimization was used in the solution procedure as an additional stability condition to close the conservation equations. The model was derived without introducing any empirical factors, so it can be used for more flow conditions in circulating fluidized beds (CFBs) than empirical models, especially for heterogeneous gas-solid two-phase flows that include cluster formation. Non-uniform particle distribution in computational cells, which is usually not described by the differential equations, is also considered in the new drag model. Both the drag values given by the model and simulation results for real systems agree well with experimental data. The results show that the model reasonably describes the interactions between the gas and particle phases in dense flows.     

非均匀气固两相系统中多尺度传质模型

建立了适用于气固循环流化床的多尺度传质模型 .从过程与尺度的角度出发 ,将非均匀气固两相流中的传质过程分解为静态与动态的过程 ,并将前者分解为稀相内、密相内以及稀密相间 3个尺度下的传质 ;在用多尺度能量最小 (EMMS)模型求解已知物系性质和操作条件的非均匀气固两相流体动力学参数的基础上 ,借助于前人的研究结果 ,利用相对滑移速度、空隙率等参数求解传质系数 ,求得轴向的浓度场分布 ,并讨论非均匀两相流动结构对传质效率的影响     

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     

气固垂直并流向上两相流流体动力学模型

运用能量最小多尺度 (EMMS)模型 ,推导出滑移速度及颗粒速度的径向分布 ,并进一步计算了径向空隙率、流体速度、团聚物直径、流体对颗粒的悬浮输送能量和颗粒的质量流率等相关的动力学参数 .模型计算结果反映了气固垂直两相流的径向不均匀性 (稀、密两相共存 )及环 /核结构的存在 .计算值与实验结果相吻合 .滑移速度的计算结果表明在核心区内也存在着团聚物 ,并不是呈现单一颗粒运动形态 ,这与实验现象一致 .该模型不仅克服了原有EMMS模型求解困难的问题 ,而且其计算不依赖于核心区半径等任何待定参数 ,只需物性参数、床层截面平均空隙率和操作参数即可 ,方便了实际应用 .     

EMMS曳力模型及其颗粒团模型的构建和检验

准确描述颗粒团聚特性是发展完善基于多尺度最小能量原理（EMMS）的曳力模型的重要方向之一。提出描述颗粒团聚特性的数学模型,不仅符合物理判断,而且与实验结果吻合。采用颗粒团模型,改进EMMS曳力模型,与实验及直接数值模拟结果吻合较好。改进的曳力模型与欧拉-欧拉双流体方法耦合,实现了不同工况下A、B类颗粒流化床流动特性的数值模拟。成功预测了颗粒非均匀分布特性、局部滑移速度、局部非均匀度以及噎塞状态。     

流态化模拟:基于介尺度结构的多尺度CFD

介尺度结构是研究气固流态化多尺度行为的关键。传统的基于平均化处理方式的双流体模拟不能准确描述流化床中的多尺度流动和传递行为。相较而言,基于能量最小多尺度(EMMS)方法的结构多流体模型(SFM)基于局部空间(网格)内的非均匀介尺度结构流动特征,其宏观预测结果与网格分辨率基本无关,因而可以大幅降低模拟计算量。基于SFM模拟得到的流动结构,EMMS多尺度传质模型进一步成功解释了传统传质文献中的数据差异。集成上述模型,形成了一整套模拟流化床流动-传递-反应耦合过程的多尺度计算流体力学(CFD)方法,并将其应用于预测循环流化床中典型的S型轴向分布、揭示噎塞转变的机理以及流化床放大困难的原因。多尺度CFD使工业规模循环床的三维、全系统、动态流动-反应耦合过程的准确模拟成为可能,并为实现从模拟向实时虚拟过程转变的目标打下基础。     

GAS-SOLIDS FLOW PATTERNS IN A CONCURRENT DOWNFLOW FAST FLUIDIZED BED(CDFFB)

Radial profiles of solid concentration and velocity for concurrent downward gas-solid suspension in a140mm inside diameter fast fluidized bed were investigated.The influence of gas velocity,solid circulating rateand axial position on radial profiles of solid concentration and particle velocity has been examined.It hasbeen found that an annular region of high solid concentration exists at r/R=0.94.At both the center and wallregion,the solid concentration and the particle velocities are relatively low.The shape of radial solid con-centration profile curves is mainly dependent on the cross-section averaged voidage,and the shape of radialparticle velocity profile is mainly affected by the gas velocity and cross-section averaged voidage.Based on the radial profiles of solid concentration and particle velocity,the solid mass flux profile and thenonuniformity of solids flow are discussed in this paper.It is shown that solids flow in CDFFB is much moreuniform than that in UFFB.     

气固流化系统多尺度跨流域EMMS建模

气固流化床反应器是典型的具有多尺度非均匀动态结构的复杂系统。实现对该类反应器定量描述和定向调控的关键是深入了解系统内介尺度结构的形成和演化特征。能量最小多尺度（EMMS）方法为气固非均匀系统的量化表征提供了一种通用的建模思路。首先回顾了EMMS理论在构建曳力本构关系方面的应用,重点介绍了本课题组在EMMS曳力模型普适化方面所做的部分工作;随后对介尺度结构时空动态演化行为的群平衡建模方法进行了论述,并给出了群平衡和结构曳力模型相耦合的连续介质模拟框架;最后讨论了EMMS原理在预测反应器宏尺度动力学方面的应用,包括模型在不同流域的拓展、操作相图的绘制以及循环流化床的全回路稳态建模方法等。