Modeling of gas-solid flow in a CFB riser based on computational particle fluid dynamics

A three-dimensional model for gas-solid flow in a circulating fluidized bed（CFB） riser was developed based on computational particle fluid dynamics（CPFD）.The model was used to simulate the gas-solid flow behavior inside a circulating fluidized bed riser operating at various superficial gas velocities and solids mass fluxes in two fluidization regimes,a dilute phase transport（DPT） regime and a fast fluidization（FF） regime.The simulation results were evaluated based on comparison with experimental data of solids velocity and holdup,obtained from non-invasive automated radioactive particle tracking and gamma-ray tomography techniques,respectively.The agreement of the predicted solids velocity and holdup with experimental data validated the CPFD model for the CFB riser.The model predicted the main features of the gas-solid flows in the two regimes;the uniform dilute phase in the DPT regime,and the coexistence of the dilute phase in the upper region and the dense phase in the lower region in the FF regime.The clustering and solids back mixing in the FF regime were stronger than those in the DPT regime.

Mechanisms for Particle Clustering in Upward Gas-Solid Flows

Two-dimensional unsteady cocurrent upward gas-solid flows in the vertical channel are simulated and the mechanisms of particles accumulation are analyzed according to the simulation results. The gaseous turbulent flow is simulated using the large eddy simulation (LES) method and the solid phase is treated using the Lagrangian approach, and the motion of the gas and particles are coupled. The formation of clusters and the accumulation of particles near the wall in dense gas-solid flows are demonstrated even if the particle-particle collisions were ignored. It is found that a cluster grows up by capturing the particles in the dilute phase due to its lower vertical velocity. By this way the small size clusters can evolve to large-scale clusters. Due to the interaction of gas and particles, the large-scale vortices appear in the channel and the boundary layer separates from the wall, which results in very high particle concentration in the near wall region and a very large-scale cluster formed near the separation point.

Solid Particles Injection in Gas Turbulent Channel Flow

This paper represents a review of the recent researches that investigate the behavior of the gas turbulent flow laden with solid particles. The significant parameters that influence the interactions between the both phases, such as particle size, loading ratio and the gas velocity, have been extensively reviewed. Those parameters are presented in dimensionless numbers in which the applicability of studying its effect in terms of all circumstances of the gas turbulent channel flow at different condition is possible. The represented results show that the turbulence degree is proportional to the particle size. It was found that at the most flow conditions even at low mass ratio, the particle shape, density and size significantly alter the turbulence characteristics. However, the results demonstrate that the particle Reynolds number is a vital sign: the turbulence field becomes weaker if particle Reynolds number is lower than the critical limit and vies verse. The gas velocity has a strong effect on the particles settling along the channel flow and as a result, the pressure drop will be affected.

Experimental study on the spatial distribution of particle rotation in the upper dilute zone of a cold CFB riser

Particle rotation plays an important role in gas-solid flows. This paper presents an experimental investigation on the spatial distribution of average rotation speed for glass beads in the upper dilute zone of a cold circulating fluidized bed(CFB) riser. It is shown that in the horizontal direction,the average rotation speed in the near-wall area is larger than that in the center area,while in the vertical direction,it decreases as the height increases. The reason resulting in this distribution is analyzed by considering several factors including particle size,particle shape,particle number density,particle collision behavior,and the surrounding flow field,etc. The effects of CFB operation conditions on the spatial distribution of average rotation speed are also studied. The results show that the increasing superficial gas velocity increases the average rotation speed of particles in the near wall area but takes nearly no effect on that in the center area. The external solids mass flux,however,takes the opposite effect. It is found that the average rotation speeds of particles in both areas are increased as the total amount of bed material increases.

高速条件下细长旋成体背风区流动特性试验研究

导弹全方向攻击时会产生由旋涡主导的复杂流动,发生转捩和流动分离,对导弹的机动性和控制能力具有重大影响。为探索攻角及马赫数对弹体流动形态的影响,针对某细长旋成体在FD-12风洞开展高速粒子图像测速及荧光油流试验,来流Ma分别为0.4、0.6、0.8,攻角范围α为0°~180°,获取了细长体背风区的流场特性演化规律。研究结果表明:当攻角α<90°时,随着攻角的增大,模型背风区流场从附着流变化为分离流,且分离涡从对称变为非对称,最终演化为非定常流动;当攻角α>90°时,情况有所不同,α=100°及α=120°时背风区存在非定常涡脱落现象,时均流场具有明显的非对称性;当攻角α=150°时,迎风面分离区出现不对称偏移,初始分离区下边界已越过模型端面,在模型中后部形成分离线;当攻角α=180°时,时均流场没有明显的特征,在模型头部位置出现了环形分离区及再附区,表明由于底部扰动的影响,该截面流场呈现局部的非定常、非线性流动状态;在攻角相同情况下,增大流场的来流Ma,不但会使分离涡的影响范围变大,也会导致分离涡的位置抬高。

Simulation of the L-valve in the circulating fluidized bed with a coarse-grained discrete particle method

Stable and controllable solid flow is essential in circulating fluidized bed (CFB) systems. The L-valve is a typical non-mechanical valve that can provide flexible solid feeding. The investigation of the solid circulation rate and the hydrodynamic characteristics of the L-valve is crucial to its design and operation. The gas-solid flow in the L-valve of a full-loop CFB is studied with the coarse-grained discrete particle method (EMMS-DPM). Good agreements on the solid circulation rate and the pressure drop through the L-valve are achieved between the simulated and experimental data. The solid circulation rate increases linearly with the aeration velocity until the stable particle circulation of the CFB is destroyed. The flow patterns in the horizontal section of L-valve are gas-solid slug flow above the stationary solid layer and the moving solid layer, respectively. The effects of L-valve geometric parameters on the solid flow characteristics are also investigated. The results indicate that reducing the diameter and length of the horizontal section of L-valve can improve the solid transport efficiency, especially at low aeration velocity. Besides, the solid conveying capacity and flow stability are improved when the sharp bend of L-valve is modified to be a gradual bend.

Numerical analysis on the transport properties and residence time distribution of ribbon biomass particles in a riser reactor based on CFD-DEM approach

A bended ribbon biomass particle model was developed to explore the dynamic transport properties inside a riser reactor.Residence time distribution(RTD)of the particles was analyzed by using the Eulerian-Lagrange method.The effects of sampling height,particle density,particle size and gas-to-solid mass ratio on RTD were investigated.The coupled Computational Fluid Dynamics and Discrete Element Method(CFD-DEM)model was verified firstly by experimental data on pressure drop and residence time distribution density function.The simulation results demonstrated that the ribbon biomass particles display a typical annular-core spatial distribution during transportation.The RTD of particles exhibit an approximate single-peaked normal distribution.The mean residence time(MRT)can reach up to 0.7 s when the particle density is 1200 kg/m^(3).Particle with higher density has longer mean residence time.The flow patterns are closer to plug flow if particle length over 12 mm.The particle flow pattern is not sensitive to changes in particle density and size,while the gas-to-material mass ratio has a significant impact on it.

Experimental Investigation on the Hydrodynamic Characteristics of Fluidized Bed Particle Solar Receiver with Gas-Solid Countercurrent Flow Pattern

To design a particle solar receiver(PSR),a vital energy conversion system,is still a bottleneck for researchers.This study presents a novel PSR based on countercurrent fluidized bed(CCFB)technology,named CCFB receiver.In this design,downward-moving particles are subjected to the action of an up-flow gas to reduce the falling speed and enhance the radial disturbance,and hence increase the residence time of particles and improve the heat transfer.A cold-mold visual experimental setup is established.The influence factors are investigated experimentally,including the superficial gas velocity,solid flux,aeration gas,particle size and transport tube diameter.The results indicate that the maximum solid holdup can exceed 9%or so with fine particles of diameter d_(p)=113.5 μm and a tube diameter of 40 mm.It is proved that the CCFB can operate stably and adjust the solid flux rapidly.The results of this study provide a new structure for PSRs in the concentrated solar power field and could fill the research insufficiency in the gas-solid counterflow field.

Gas-solid flow in a high-density circulating fluidized bed riser with Geldart group B particles

We carried out experiments to explore and characterize the gas-solid flow dynamics of Geldart group B particles in a dense circulating fluidized bed riser. By reducing the pressure drop across the solid control valve and increasing the solid inventory in the storage tank, a high solid circulation rate and a solid holdup above 0.075 throughout the riser were simultaneously achieved. At a solid-to-gas mass flux ratio of approximately 105, flow transitioned from fast fluidization to a dense suspension upflow. In the axial direction of the riser, solid holdup had an exponential profile, increasing with increasing solid circulation rate and Jot decreasing superficial gas velocity. From the riser＇s center to its wall, the solid holdup increased markedly, exhibiting a steep parabolic profile. Increasing the solid circulation rate increased the radial non-uniformity of the solid concentration, while increasing the superficial gas velocity had the opposite effect, In our dense circulating fluidized bed riser, Geldart group B particles had similar slip characteristics to Geldart group A particles,

Tribo-charging of binary mixtures composed of coarse and fine particles in gas-solid pipe flow

Experiments were conducted to investigate the effect of adding fines on the tribo-charging of coarse glass beads. Four types of fines, i.e., copper, stainless steel, uncoated and silver-coated fine glass beads, mixed w让h 240-830 μm glass beads were conveyed by air through a stainless-steel spiral pipe acting as a tribo-charger. Regardless of the type or electrical conductivity of the fine particles tested, adding small amounts of fines (up to 10wt%) to coarse glass beads resulted in a sharp increase in the mass and surface charge densities of the particles. In gen eral, the profiles of the mass and surface charge den sides of the fine-coarse particle mixtures as a function of the mixture composition were determined by the relative magnitude of contact potential differences and the total surface areas of all the comp on ents. The domi nant particle tri bo-electrification mecha nism switched from coarse particle-wall con tacts to fine particle-wall contacts when the fines weight percentage in the mixture exceeded 10%. A model was developed to predict the mass charge density of bi nary mixtures as a function of the mixture composition.

SUPERSONIC DUSTY-GAS FLOWS WITH KNUDSEN EFFECT IN INTERPHASE MOMENTUM EXCHANGE

On the basis of the two-continuum model of dilute gas-solid suspensions,the dynamic behavior of inertial particles in supersonic dusty-gas flows past a blunt body is studied for moderate Reynolds numbers,when the Knudsen effect in the interphase momentum exchange is significant. The limits of the inertial particle deposition regime in the space of governing parameters are found numerically under the assumption of the slip and free-molecule flow regimes around particles.As a model problem,the flow structure is obtained for a supersonic dusty-gas point-source flow colliding with a hypersonic flow of pure gas.The calculations performed using the full Lagrangian approach for the near-symmetry-axis region and the free-molecular flow regime around the particles reveal a multi-layer structure of the dispersed-phase density with a sharp accumulation of the particles in some thin regions between the bow and termination shock waves.

颗粒间碰撞对气固两相流中细长颗粒流化取向分布的影响

细长颗粒循环流化在工业生产中具有广泛的应用背景,如生物质秸杆在循环流化床中的燃烧,烟丝在循环流化床中的干燥、加湿等。细长颗粒在流化床内流化运动过程中,不可避免地存在颗粒之间的碰撞。文中根据直接模拟蒙特卡罗方法、刚体动力学原理及动量守恒定律,建立起气固两相流中细长颗粒间相互碰撞的三维数学模型。通过对比实验结果发现,考虑颗粒间碰撞的模拟结果比不考虑颗粒间碰撞的模型的模拟结果更加接近实验结果。研究发现,细长颗粒的取向分布随径向位置变化很大,颗粒间碰撞对细长颗粒的取向分布随高度及长径比的变化规律没有明显影响。

细长颗粒与流场双向耦合的气固两相流数值模拟研究

细长颗粒气–固两相流中湍流流变特性研究是细长颗粒气固两相流研究的重要内容之一。基于离散单元法、刚体动力学,并引入拉格朗日时间尺度与?-ε模型的耦合关联式,构建起细长颗粒?湍流场气–固双向耦合模型,并采用此模型对某一实际流化床内的细长颗粒气-固两相流场进行了数值研究。研究发现,多数细长颗粒以其轴与流场主流速度方向近于平行的姿态在流场中向上运动;在高度上,一方面细长颗粒在此存在会导致此高度上湍流速度梯度的增加从而导致整个高度上湍动能值的增加,另一方面细长颗粒的存在又会消耗一部分当地湍流场的湍动能;水平方向上有细长颗粒存在的区域,当地湍动能值会有明显的下降;细长颗粒的存在会导致整个湍流场的压强梯度增加。

提升管内细长颗粒流化运动的气固多向耦合模型的构建

细长颗粒气固两相流研究已成为气固两相流研究的重点问题之一。而各种细长颗粒在流场中的受力、运动模型构建以及细长颗粒/流场间的耦合关系的构建是细长颗粒两相流数值模拟研究的难点之一。在已经基于刚体动力学原理构建了细长粒子气固两相流单向耦合模型的基础上,结合基于κ-ε模型的球形粒子-流场气固间耦合关联式,并改进细长颗粒间碰撞模型,从而构建了细长颗粒气固两相流动多向耦合数值模拟平台。并且采用此平台对某一实际流化床内的细长粒子气固两相流场进行了数值模拟研究。研究表明,不同时间细长颗粒在流场内的停留位置具有较强的随机性;细长颗粒在流化运动过程中伴随有较明显的取向选择性;细长颗粒的存在会导致当地流场的速度、压强均有较为明显的下降。

圆锥前体涡的等离子体控制特性及机理

在半顶角为10°的圆锥前体尖端处布置一对等离子体激励器,通过占空循环控制技术实现了对圆锥前体力和力矩的近似比例控制,并对流动控制机理进行了讨论。采用静态和动态压力测量实验技术对圆锥前体各测量截面轴向压力分布进行测量,并对流场旋涡结构变化进行了二维粒子图像测速。实验在3.0m×1.6m低速低湍流度风洞中进行,迎角45°,基于圆锥段底面直径的雷诺数为5×104。实验结果表明,等离子体激励器关闭时圆锥前体涡流场非对称,当左、右舷等离子体激励器开启时,涡流场在双稳态之间转换;在占空循环控制下,不同周期同一相位的平均压力结果表明涡流场具有一定的周期性并且在2个稳态之间平缓变化,但变化幅度较原来的双稳态小。

循环流化床内细长颗粒取向分布的多向耦合数值模拟

采用拉格朗日时间尺度与k-ε模型的耦合关联式,构建三维多向耦合的细长颗粒气-固两相流模型,并采用此模型对某一实际流化床内的细长颗粒两相流场进行了数值模拟.研究发现,在提升管内径向方向上,由中心区域到近壁区,细长颗粒的取向性越发趋于明显,除近壁区受壁面约束影响外,其他区域均以15°~30°章动角姿态流化运动的细长颗粒最多;在高度方向上,由床层底部到提升管出口处,细长颗粒的取向性越发趋于明显并在提升管出口处达到最大值.

气/固双向耦合的细长颗粒流化运动特征的数值研究

在已有三维的单向耦合的细长颗粒-气/固两相流模型基础上,引入拉格朗日时间尺度与κ-ε模型的耦合关联式,构建细长颗粒-湍流场气/固双向耦合模型,并采用此模型对某一实际流化床内的细长颗粒气-固两相流场进行数值研究。研究表明,径向中心区域的细长颗粒的数量浓度低于近壁处的数量浓度。细长颗粒水平方向上的速度小于高度方向的速度,且水平方向速度大小及方向分布随机性较强。水平方向的细长颗粒角速度大于高度方向上的细长颗粒角速度,且分布随机性较强;高度方向细长颗粒角速度在近壁区随高度上升逐渐下降,而在径向中心区域随高度上升逐渐上升并在出口处略有下降。在中心区域与近壁区中间的区域细长颗粒间的碰撞机率最高。

含细长颗粒的洗涤冷却室内的多相分布特性

采用改进的直接取样法,在按几何尺寸缩小的工业气化炉洗涤冷却室冷模装置内,同时测量不同操作条件下的轴径向局部固含率和气含率,对细长颗粒在洗涤冷却室内的多相分布特性进行研究。结果表明:以下降管出口截面为界,洗涤冷却室可分为上部气液固混合区和下部固液流动区,其中上部区域由下降管出口区、破泡板作用区和气垫层区组成,下部区域由气相湍动作用区、回流区和二次流动区组成;在颗粒阻碍效应减速沉降和团聚效应加速沉降的共同作用下,轴向固含率呈现波动分布;环隙气速、固相体积分数和长径比的增加均会增强床层的湍动,促进气体的径向扩散;操作条件的改变使颗粒的漂移速度发生改变,径向固含率分布出现波动;在气相扰动和回流作用下,二次流动区呈现环状流动,流体和颗粒的"壁面效应"使该区域的固含率呈现中心高边壁低的特点。

20°圆锥分离流动的发展特性

文章对20°顶角的圆锥-圆柱组合体在3.0×1.6 m低速低湍流度风洞进行了表面压力分布测量和二维粒子图像测速。基于圆锥底面直径的雷诺数0.3×106,迎角0°-35,°包含40个滚转角,以9°为等间隔,滚转角范围0°-351°。文中主要分析了不同迎角下,圆锥段9个测量截面上的压力分布以及积分得到的截面当地侧向力随滚转角的变化特性,由压力分布对测量截面的流动分离状态进行了推断,推断结果与二维粒子图像测速实验进行了比较。结果表明：迎角从0°增大到35°的过程中,当地侧力系数随滚转角的变化曲线分别呈现：①零值线;②连续波曲线;③方波曲线。当侧力系数变化呈现连续波曲线和方波曲线时,所有测量截面上波曲线的周期和相位都近似相同。圆锥上非对称力的产生和发展是由于分离涡的不稳定性造成的。

EXPERIMENTAL RESEARCH OF FLOW STRUCTURE IN A GAS-SOLID CIRCULATING FLUIDIZED BED RISER BY PIV

Particle Imaging Velocimetry （PIV） techniques were applied to investigate the particle motion and cluster properties in a gas-solid two-phase flow in a circulating fluidized bed riser. Visual images and micro-structure of various clusters were captured. After the boundary of clusters was determined by the gray level threshold method, clusters were classified by the distance between particles and the shape and position of clusters. In addition, the process of clusters forming and breaking up was described, and the sizes of clusters were also obtained. With the Minimum Quadric Difference （MQD） cross-correlation algorithm suitable for high-density particles, the axial velocities of the particles were obtained in the dilute phase section. The features of particle motion were revealed by investigating statistically the magnitude and distribution of particle axial velocity in the radial direction. At most radial cross-sections, there exists a parabola-shaped distribution of upward axial velocity of particles, namely, the magnitude of axial velocity in the core region is higher than that near the wall region of the riser.