气固两相流介尺度LBM-DEM模型

LBM-DEM耦合方法通常是指一种颗粒流体系统直接数值模拟算法,即是一种不引入经验曳力模型的计算方法,颗粒尺寸通常比计算网格的长度大一个量级,颗粒的受力通过表面的粘性力与压力积分获得,其优点是能描述每个颗粒周围的详细流场,产生详细的颗粒-流体相互作用的动力学信息,可以探索颗粒流体界面的流动、传递和反应的详细信息及两相相互作用的本构关系,但其缺点是计算量巨大,无法应用于真实流化床过程模拟。本文针对气固流化床中的流体以及固体颗粒间的多相流体力学行为,建立了一种稠密气固两相流的介尺度LBMDEM模型,即LBM-DEM耦合的离散颗粒模型,实现在颗粒尺度上流化床的快速离散模拟。该耦合模型采用格子玻尔兹曼方法(LBM)描述气相的流动和传递行为,离散单元法(DEM)用于描述颗粒相的运动,并利用能量最小多尺度(EMMS)曳力解决气固耦合不成熟问题,以提高其模拟精度。通过经典快速流态化的模拟,验证了介尺度LBM-DEM耦合模型的有效性。模拟结果表明介尺度LBM-DEM模型是一种探索实验室规模气固系统的有力手段。     

3D full-loop simulation and experimental verification of gas-solid flow hydrodynamics in a dense circulating fluidized bed

Because of their advantages of high efficiency and low cost, numerical research methods for large-scale circulating fluidized bed （CFB） apparatus are gaining ever more importance. This article presents a numer- ical study of gas-solid flow dynamics using the Eulerian granular multiphase model with a drag coefficient correction based on the energy-minimization multi-scale （EMMS） model. A three-dimensional, full-loop, time-dependent simulation of the hydrodynamics of a dense CFB apparatus is performed. The process parameters （e.g., operating and initial conditions） are provided in accordance with the real experiment to enhance the accuracy of the simulation. The axial profiles of the averaged solid volume fractions and the solids flux at the outlet of the cyclone are in reasonable agreement with experimental data, thereby verifying the applicability of the mathematical and physical models. As a result, the streamline in the riser and standpipe as well as the solids distribution contours at the cross sections is analyzed. Computational fluid dynamics （CFD） serves as a basis for CFB modeling to help resolve certain issues long in dispute but difficult to address experimentally. The results of this study provide the basis of a general approach to describing dynamic simulations of gas-solid flows.     

Effects of grid size on predictions of bed expansion in bubbling fluidized beds of Geldart B particles: A generalized rule for a grid-independent solution of TFM simulations

Numerical simulations of gas–solid fluidized beds based on the kinetic theory of granular flow exhibit a significant dependence on domain discretization. Bubble formation, bubble size and shape all vary greatly with the discretization, and the use of an inappropriate scale resolution leads to inaccurate predictions of fluidization hydrodynamics. In this study, grid-independent solutions of the two fluid model were examined by comparing the bed expansions obtained from numerical simulations with experimental results and empirical predictions, based on bubbling fluidized beds of Geldart B particles. Grid independence was achieved with a grid resolution equal to 18 times the particle diameter. The simulation results were compared with previously published data for verification purposes. The results of this work should provide a guideline for choosing the appropriate grid size and thereby minimize the time and expense associated with large simulations.     

鼓泡流化床中流动特性的多尺度数值模拟

鼓泡流化床因其较高的传热特性以及较好的相间接触已经被广泛应用于工业生产中,而对鼓泡流态化气固流动特性的充分认知是鼓泡流化床设计的关键.在鼓泡流化床中,气泡相和乳化相的同时存在使得床中呈现非均匀流动结构,而这种非均匀结构给鼓泡流化床的数值模拟造成了很大的误差.基于此,以气泡作为介尺度结构,建立了多尺度曳力消耗能量最小的稳定性条件,构建了适用于鼓泡流化床的多尺度气固相间曳力模型.结合双流体模型,对A类和B类颗粒的鼓泡流化床中气固流动特性进行了模拟研究,分析了气泡速度、气泡直径等参数的变化规律.研究表明,与传统的曳力模型相比,考虑气泡影响的多尺度气固相间曳力模型给出的曳力系数与颗粒浓度的关系是一条分布带,建立了控制体内曳力系数与局部结构参数之间的关系.通过模拟得到的颗粒浓度和速度与实验的比较可以发现,考虑气泡影响的多尺度曳力模型可以更好地再现实验结果.通过A类和B类颗粒的鼓泡床模拟研究发现,A类颗粒的鼓泡床模拟受多尺度曳力模型的影响更为显著.      

Particle size distribution in CPFD modeling of gas-solid flows in a CFB riser

A computational particle fluid dynamics(CPFD) numerical method to model gas-solid flows in a circulating fluidized bed(CFB) riser was used to assess the effects of particle size distribution(PSD) on solids distribution and flow.We investigated a binary PSD and a polydisperse PSD case.Our simulations were compared with measured solids concentrations and velocity profiles from experiments,as well as with a published Eulerian-Eulerian simulation.Overall flow patterns were similar for both simulation cases,as confirmed by experimental measurements.However,our fine-mesh CPFD simulations failed to predict a dense bottom region in the riser,as seen in other numerical studies.Above this bottom region,distributions of particle volume fraction and particle vertical velocity were consistent with our experiments,and the simulated average particle diameter decreased as a power function with riser height.Interactions between particles and walls also were successfully modeled,with accurate predictions for the lateral profiles of particle vertical velocity.It was easy to implement PSD into the CPFD numerical model,and it required fewer computational resources compared with other models,especially when particles with a polydisperse PSD were present in the heterogeneous flow.     

Scaling laws for gas-solid riser flow through two-fluid model simulation

Scale up of gas-solid circulating fluidized bed (CFB) risers poses many challenges to researchers.In this paper,CFD investigation of hydrodynamic scaling laws for gas-solid riser flow was attempted on the basis of two-fluid model simulations,in particular,the recently developed empirical scaling law of Qi,Zhu,and Huang (2008).A 3D computational model with periodic boundaries was used to perform numerical experiments and to study the effect of various system and operating parameters in hydrodynamic scaling o...     

Development and assessment of algorithms for DEM-LES simulations of fluidized bed

The use of high-fidelity Discrete Element Method (DEM) coupled with Computational Fluid Dynamics (CFD) for particle-scale simulations demands extensive simulation times and restricts application to small particulate systems. DEM-CFD simulations require good performance and satisfactory scalability on high-performance computing platforms. A reliable parallel computing strategy must be developed to calculate the collision forces, since collisions can occur between particles that are not on the same processor, or even across processors whose domains are disjoint. The present paper describes a parallelization technique and a numerical verification study based on a number of tests that allow for the assessment of the numerical performance of DEM used in conjunction with Large-Eddy Simulation (LES) to model dense flows in fluidized beds. The fluid phase is computed through solving the volume-averaged four-way coupling Navier-Stokes equations, in which the Smagorinsky sub-grid scale tensor model is used. Furthermore, the performance of Sub-Grid Scale (SGS) turbulence models applied to Fluidized Bed Reactor (FBR) configurations has been assessed and compared. The developed numerical solver represents an interesting combination of techniques that work well for the present purpose of studying particle formation in fluidized beds.     

Flow regime diagrams for gas-solid fluidization and upward transport

Flow regime maps are presented for gas-solids fluidized beds and gas-solids upward transport lines. For conventional gas solids fluidization, the flow regimes include the fixed bed, bubbling fluidization, slugging fluidization and turbulent fluidization. For gas solids vertical transport operation, solids flux must be incorporated in the flow regime diagrams. The flow regimes then include dilute-phase transport, fast fluidization or turbulent flow, slug/bubbly flow, bubble-free dense-phase flow and packed bed flow. In practical circulating fluidized beds and transport risers, operation below the fast fluidization regime is commonly impossible due to equipment limitations. Practical flow regime maps are proposed with the flow regimes, including homogeneous dilute-phase flow, core-annular dilute-phase flow (where there are appreciable lateral gradients but small axial gradients) and fast fluidization (where there are both lateral and axial gradients). The boundary between fast fluidization and dilute-phase pneumatic transport is set by the type A choking velocity, at which the uniform suspension collapses and particles start to accumulate in the bottom region of the transport line, while the mechanism of transition from fast fluidization to dense-phase flow depends on the column and particle diameters.     

A new drag model for TFM simulation of gas-solid bubbling fluidized beds with Geldart-B particles

In this work, a new drag model for TFM simulation in gas-solid bubbling fluidized beds was proposed, and a set of equations was derived to determine the meso-scale structural parameters to calculate the drag characteristics of Geldart-B particles under low gas velocities. In the new model, the meso-scale structure was characterized while accounting for the bubble and meso-scale structure effects on the drag coefficient. The Fluent software, incorporating the new drag model, was used to simulate the fluidization behavior. Experiments were performed in a Plexiglas cylindrical fluidized bed consisting of quartz sand as the solid phase and ambient air as the gas phase. Comparisons based on the solids hold-up inside the fluidized bed at different superficial gas velocities, were made between the 2D Cartesian simulations, and the experimental data, showing that the results of the new drag model reached much better agreement with exoerimental data than those of the Gidasoow dra~ model did.     

Particle size distribution in CPFD modeling of gas–solid flows in a CFB riser

A computational particle fluid dynamics (CPFD) numerical method to model gas–solid flows in a circulating fluidized bed (CFB) riser was used to assess the effects of particle size distribution (PSD) on solids distribution and flow. We investigated a binary PSD and a polydisperse PSD case. Our simulations were compared with measured solids concentrations and velocity profiles from experiments, as well as with a published Eulerian-Eulerian simulation. Overall flow patterns were similar for both simulation cases, as confirmed by experimental measurements. However, our fine-mesh CPFD simulations failed to predict a dense bottom region in the riser, as seen in other numerical studies. Above this bottom region, distributions of particle volume fraction and particle vertical velocity were consistent with our experiments, and the simulated average particle diameter decreased as a power function with riser height. Interactions between particles and walls also were successfully modeled, with accurate predictions for the lateral profiles of particle vertical velocity. It was easy to implement PSD into the CPFD numerical model, and it required fewer computational resources compared with other models, especially when particles with a polydisperse PSD were present in the heterogeneous flow.     

CFD–DEM study of effect of bed thickness for bubbling fluidized beds

The effect of bed thickness in rectangular fluidized beds is investigated through the CFD–DEM simulations of small-scale systems. Numerical results are compared for bubbling fluidized beds of various bed thicknesses with respect to particle packing, bed expansion, bubble behavior, solids velocities, and particle kinetic energy. Good two-dimensional (2D) flow behavior is observed in the bed having a thickness of up to 20 particle diameters. However, a strong three-dimensional (3D) flow behavior is observed in beds with a thickness of 40 particle diameters, indicating the transition from 2D flow to 3D flow within the range of 20–40 particle diameters. Comparison of velocity profiles near the walls and at the center of the bed shows significant impact of the front and back walls on the flow hydrodynamics of pseudo-2D fluidized beds. Hence, for quantitative comparison with experiments in pseudo-2D columns, the effect of walls has to be accounted for in numerical simulations.     

RADIAL PROFILE OF THE SOLID FRACTION IN A LIQUID—SOLID CIRCULATING FLUIDIZED BED

In a liquid-solid circulating fluidized bed, lateral forces acting on the particles determine the movement of the particles in the radial direction, and this creates a radial profile for the solid fraction. This work proposes a model to calculate the radial profile of the solid fraction in a liquid-solid circulating fluidized bed based on the balance of the lateral force and the turbulent dispersion force.     

Micro/meso simulation of a fluidized bed in a homogeneous bubbling regime

Due to the wide range of spatial scales and the complex features associated to fluid/solid and solid/solid interactions in a dense fluidized bed, the system can be studied at different length scales, namely micro, meso and macro. In this work, we select a flow configuration relevant of a homogeneous liquid/solid fluidization and compare computed results from Particle Resolved Simulation (PRS) with those from locally averaged Euler/Lagrange simulation. PRS at the micro-scale is carried out by a parallel Distributed Lagrange Multiplier (DLM) solver in the framework of fictitious domain methods (Wachs, 2011a, 2015). For meso-scale simulations, the set of mass and momentum conservation equations is averaged in control volumes encompassing few particles and momentum transfer between the two phases is modeled using appropriate drag laws. Both methods are coupled to a Discrete Element Method (DEM) combined with a soft-sphere contact model to solve the Newton–Euler equations with collisions for the particles in a Lagrangian framework (Wachs et al., 2012). A test case of intermediate size with 2000 spheres is chosen as a sensible compromise between size limitations of the meso-scale model for an appropriate averaging process and computational resources required to run micro-scale simulations. These two datasets yield new insight on momentum transfer at different spatial scales in the flow, and question the validity of certain approximations adopted in the meso-scale model. Results demonstrate an acceptable agreement between the micro- and meso-scale predictions on integral measures as pressure drop and bed height. Investigating more detailed features of the flow, it has been shown that particles fluctuations are considerably suppressed in meso-scale simulations and in particular the particles transverse motion is underestimated, regardless of the selected drag law. The origin of these dependencies is carefully investigated by reconstructing the closure laws based on PRS results and comparing them to the closure laws proposed in the literature.     

Statistical modeling and optimization of a multistage gas-solid fluidized bed for removing pollutants from flue gases

The present paper describes the statistical modeling and optimization of a multistage gas-solid fluidized bed reactor for the control of hazardous pollutants in flue gas.In this work,we study the hydrodynamics of the pressure drop and minimum fluidization velocity.The hydrodynamics of a three-stage fluidized bed are then compared with those for a single-stage unit.It is observed that the total pressure drop over all stages of the three-stage fluidized bed is less than that of an identical single-stage system.However,the minimum fluidization velocity is higher in the single-stage unit.Under identical conditions,the minimum fluidization velocity is highest in the top bed,and lowest in the bottom bed.This signifies that the behavior of solids changes from a well-mixed flow to a plug-flow,with intermediate behavior in the middle bed.     

Experimental and computational studies on flow behavior of gas-solid fluidized bed with disparately sized binary particles

This paper presents experimental and computational studies on the flow behavior of a gas-solid fluidized bed with disparately sized binary particle mixtures. The mixing/segregation behavior and segregation efficiency of the small and large particles are investigated experimentally.Particle composition and operating conditions that influence the fluidization behavior of mixing/segregation are examined. Based on the granular kinetics theory, a multi-fluid CFD model has been developed and verified against the experimental results. The simulation results are in reasonable agreement with experimental data. The results showed that the smaller particles are found near the bed surface while the larger particles tend to settle down to the bed bottom in turbulent fluidized bed. However, complete segregation of the binary particles does not occur in the gas velocity range of 0.695--0.904 m/s. Segregation efficiency increases with increasing gas velocity and mean residence time of the binary particles, but decreases with increasing the small particle concentration. The calculated results also show that the small particles move downward in the wall region and upward in the core. Due to the effect of large particles on the movement of small particles, the small particles present a more turbulent velocity profile in the dense phase than that in the dilute phase.     

Study of fluid cell coarsening for CFD-DEM simulations of polydisperse gas–solid flows

Particle polydispersity is ubiquitous in industrial fluidized beds, which possesses a significant impact on hydrodynamics of gas–solid flow. Computational fluid dynamics-discrete element method (CFD-DEM) is promising to adequately simulate gas–solid flows with continuous particle size distribution (PSD) while it still suffers from high computational cost. Corresponding coarsening models are thereby desired. This work extends the coarse-grid model to polydisperse systems. Well-resolved simulations with different PSDs are processed through a filtering procedure to modify the gas–particle drag force in coarse-grid simulations. We reveal that the drag correction of individual particle exhibits a dependence on filtered solid volume fraction and filtered slip velocity for both monodisperse and polydisperse systems. Subsequently, the effect of particle size and surrounding PSD is quantified by the ratio of particle size to Sauter mean diameter. Drag correction models for systems with monodisperse and continuous PSD are developed. A priori analysis demonstrates that the developed models exhibit reliable prediction accuracy.     

Numerical simulation of gas-solid flow with two fluid model in a spouted-fluid bed

The flow characteristics in a spouted-fluid bed differ from those in spouted or fluidized beds because of the injection of the spouting gas and the introduction of a fluidizing gas. The flow behavior of gas-solid phases was predicted using the Eulerian-Eulerian two-fluid model （TFM） approach with kinetic theory for granular flow to obtain the flow patterns in spouted-fluid beds. The gas flux and gas incident angle have a significant influence on the porosity and particle concentration in gas-solid spouted-fluid beds. The fluidizing gas flux affects the flow behavior of particles in the fountain. In the spouted-fluid bed, the solids volume fraction is low in the spout and high in the annulus. However, the solids volume fraction is reduced near the wall.     

Multi-scale analysis on the pulverized coal flow behaviors under high pressure dense-phase pneumatic conveying

To deeply knowledge of the flow behaviors of pulverized coal particles in dense gas-solid two-phase flow,a multi-scale analysis method based on electrostatic se...