二维导流管喷动床离散颗粒动力学模拟

采用离散单元法(DEM)-计算流体力学(CFD)双向耦合数值方法对二维导流管喷动床进行了模拟,颗粒的运动通过DEM模型描述,而气体的运动用Navier-Stokes方程进行求解,气体和固体颗粒之间的相互作用通过曳力形式传递。文中将DEM和边界元方法(BEM)结合起来解决颗粒在具有复杂边界设备内的运动。通过采用BEM+DEM-CFD相结合的方法进行模拟计算,得到了喷动床的最小喷动速度,研究了不同表观气速下床内的流型,得到了二维导流管喷动床的床层压降与表观气速的关系,统计分析了喷射区、环隙区内颗粒的运动速度和相应的空隙率,全面地描述了二维导流管喷动床内的气固流动特征。     

矩形导流管喷动床下行区的床层压降

带导流管的矩形喷动床是传统喷动床的改进型式,矩形床内设置的与床同厚的垂直导流管,可以控制固体颗粒的内循环速率,同时使下行区中的气固移动床维持平推流.本文实验测定了不同表观气速、床层重量、不同固体颗粒与气体入口形式与尺寸时,矩形导流管喷动床下行区的床层压降,以考察其流动特征.实验结果表明,下行区存在床层压降的轴向分布,气固流动处于负压差下移上流区,且气固滑移速度自下而上是逐渐下降的.下行区颗粒床层的压降以及颗粒的移动下输,受到喷动床表观气速、床高、喷嘴尺寸、物料种类和颗粒直径的不同影响.     

双喷嘴矩形导流管喷动床脱硫性能的研究

针对半干法烟气脱硫方法,提出了一种新型的双喷嘴矩形导流管喷动床半干法烟气脱硫装置。在不同的钙硫摩尔比、静床层高度、表观气速、绝热饱和温差下,以消石灰为脱硫剂,研究了该装置的脱硫性能,并与未加导流管的双喷嘴矩形喷动床的脱硫性能进行了比较;研究了不同脱硫剂流量下导流管喷动床的喷动压降。实验结果表明:脱硫率与钙硫摩尔比、静床层高度成正比,与绝热饱和温差、表观气速成反比;喷动压降与脱硫剂流量成反比。并最终得出了实验条件下双喷嘴矩形导流管喷动床的最佳操作范围和脱硫率关联式。     

小波分析方法在喷动床流型识别中的应用

在直径150 mm、静床层高280 mm的柱锥形喷动床中,采用平均粒径为1 mm、密度为2 770 kg/m~3的刚玉球为实验固相,采集不同气速下无导流管和带开孔导流管喷动床中的压力信号,通过小波分析与功率谱相结合的方法表征柱锥形喷动床的流型转变。首先利用平均压力法测得本实验条件下无导流管和带开孔导流管喷动床的最小喷动速度分别为0.57,0.50 m/s,然后采用Daubechies2小波对压力信号进行7层小波分解得出能反映喷动床流动特性的主频信号,再对主频信号进行功率谱分析。结果表明:无导流管和带开孔导流管喷动床的主频分别出现在D3(6.25—12.5 Hz)和D2(12.5—25 Hz)频段,且主频信号的功率谱图可以表征喷动床的3种流型:固定床、稳定喷动和不稳定喷动;开孔导流管喷动床的操作弹性(1相似文献   

双喷嘴矩形喷动床流动性能实验研究

在120 mm×240 mm的双喷嘴矩形不锈钢床内,对新型双喷嘴矩形导流管喷动床的最小喷动速度和喷动高度进行了研究,考察了喷动气速、粒径、静床层高度、导流管直径、导流管安装位置对最小喷动速度和喷动高度的影响。结果表明:最小喷动速度随颗粒直径、导流管直径、导喷距的增大而增大,随静床层高度的增大而减小;喷动高度随喷动气速的增大而增大,随导流管直径的增大而减小,受静床层高度和导喷距的影响不大,并得出了最小喷动速度的关联式。     

导流管的不同形式对喷动床颗粒循环量的影响

对于导流管喷动床,导流管的形式对颗粒物料的循环量影响不能忽略.研究了四种不同形式的导流管在导流管不同的安装高度、不同的床层料位高度以及不同的吹松气量对颗粒循环量的影响.结果表明:增加床层料位高度,在一定范围内增加导流管安装高度和吹松气量,会增大颗粒循环量;导流管形式对颗粒循环量影响至关重要.     

导向管直径对喷动床流动特性影响的计算颗粒流体力学数值模拟

导向管喷动床是较为常见的一种喷动床改进床型,通过阻断喷动区与环隙区气固接触来提高颗粒循环的规律性与稳定性。本文采用计算颗粒流体力学（CPFD）方法对于直径150mm的柱锥式导向管喷动床进行了数值模拟研究,考察了导向管直径对于喷动床内颗粒流动特性的影响,从环隙区死区分布、颗粒速度分布、固体循环量等方面分析了具有不同直径导向管喷动床的运行状态。结果表明,加入导向管在减少床内死区的同时也降低了运行时的固体循环量,对于本次采用的喷动床结构尺寸与运行参数,只有在导向管直径为40～60mm时才能保证床内具有良好喷动状态,综合考虑各因素,选用直径50～55mm的导向管最为合适。对于具有类似结构与运行条件的柱锥喷动床,导向管直径可考虑选为无导向管运行时喷动区直径的1.2～1.375倍。     

喷动床反应器气固流动模型的研究进展

首先对喷动床流动特征参数关联式进行了扼要介绍,继而着重总结了近年来喷动床内气固流动模型(喷射区气固稀相流模式、环隙区气体渗流模式以及固体颗粒流模式)、以及喷动床放大规律等基础理论研究的进展。最后,提出了未来喷动床流动模型工作的突破点和相应思路。     

喷动流化床流动特性数值模拟

为了捕捉喷动流化床中微观层次上的颗粒运动信息,建立了基于CFD的二维非稳态喷动流化床欧拉-欧拉两相流模型。分析了不同流化气速对喷动流化床气固流动特性的影响,即不同工况下的炉内压力降、颗粒浓度、床内空隙率分布、气体速度分布和固体颗粒速度分布。数值模拟研究结果表明:随流化气速的增大,压降和炉内平均空隙率逐渐增大,密相床层高度逐渐增加,沿着轴向方向的气体流量增大,喷动气的射流深度逐渐增加,同时射流半径也逐渐增加。     

新颖环形喷动床颗粒喷动特性实验研究

一种新颖的环形喷动床由内外两个不同内径、同心的垂立圆筒组成,在环形空间底部设置多个喷口,在喷口两侧布置倾斜的导流板.研究颗粒在这种喷动床内的流动特性,探讨喷口结构、颗粒种类以及床内载料量对环形喷动床颗粒喷动特性的影响.实验结果表明:颗粒在环形喷动床内分为三个明显不同的区域,即颗粒填充移动区、密相喷动流化区以及稀相夹带区.当颗粒出现分区喷动后,随床内载料量的增多,填充移动区的高度维持不变,始终等于导流板的高度,而密相喷动区的高度不断增加.风量和颗粒种类对床层最大喷动量、密相喷动高度以及床层压力分布规律有着十分重要的影响.采用不同的喷口结构时,在相同的载料量下,直向喷口的密相喷动区高度更大,而且床内各测点的平均压力大于采用斜向喷口时的相应测点压力.     

Flow Characteristics in Slot‐Rectangular Spouted Beds with Draft Plates

An experimental investigation was conducted of slot‐rectangular spouted beds with air entry slots spanning the full thickness of the column and vertical draft plates intended to help control the solids circulation rate. With increasing superficial gas velocity, the flow between the draft plates changed from bubbling to slugging and then to spouting with dilute pneumatic between the plates and moving‐bed downward motion on both sides. However, there was difficulty maintaining stability and symmetrical flow on the two sides. Once spouting is established, pressure drops and local voidages vary with gas velocity, particle size and gas entry size in broadly similar manners as for conventional spouted beds     

Simulations of flow behavior of gas and particles in spouted bed with a porous draft tube

Flow behaviors of particles in a two-dimensional spouted bed with a porous draft plates are studied using a fluid dynamic computation with kinetic-frictional stresses models. Gas flow through the porous draft tube is simulated by Brinkman-Darcy-Forchheimer formulation. The distributions of concentration and velocities of particles are predicted. Simulated results predict the solid circulation rate and gas flux rate measured in the spouted bed with a porous draft tube (Ishikura et al., 2003). The solid circulation rate in the spouted bed with a porous draft tube is larger than that with a non-porous draft tube. The predicted bed pressure drop with the porous draft tube is high in comparison to the spouted bed with non-porous draft tube. The effect of the porosity of the porous draft tube on distributions of gas flux rate through the annulus and solid circulation rate are discussed.     

HEAT TRANSFER IN SPOUTED BEDS

Experiments were carried out in order to analyse the wall-to-bed and fluid-to-particle heat transfer coefficients in spouted Beds. wall-to-bed heat transfer coefficients were determined in cylindrical-conical and conical spouted beds for various gas flow rates, particle sizes and bed heights for spouted beds with and without draft tubes.

A new definition for wall-to-bed transfer coefficient was proposed baaed on experimental observations.

The heat tranefer area was also studied to ensure that a physically significant fluid-to-particle heat transfer coefficient was achieved.     

CFD simulation of gas–solid flow in a spouted bed with a non-porous draft tube

A multi-fluid Eulerian–Eulerian approach incorporating the kinetic theory of granular flow was used to simulate a spouted bed containing non-porous draft tube. Drag function and coefficient of restitution were investigated. Solid and gas velocity vector, gas flow rate in annulus and spout regions and longitudinal pressure distribution were evaluated. In addition, the effects of the entrainment height and the draft tube diameter were studied. Simulation indicates the formation of three regions namely, annulus, spout and fountain; similar to a conventional spouted bed. Current model predicts acceptable results in both spout and annulus regions. Simulation results indicate that the model can be employed for both mono-size and multi-size particles reasonably. This paper provides useful basis for further works on understanding gas–solid flow mechanism in spouted beds containing a non-porous draft tube.     

BATCH DRYING OF SHELLED CORN IN TWO-DIMENSIONAL SPOUTED BEDS WITH DRAFT PLATES

Geometrically similar spouted beds with draft plates were used to obtain the drying characteristics of freshly harvested shelled corn with 0.28 to 0.31 kg/kg initial moisture content at different air inlet temperatures and bed heights. Thermal equilibrium between air and grains was achieved at minimum spouting conditions. The drying kinetics of shelled corn in a drafted two-dimensional spouted bed was found to be of the "thin layer" type. Expressions for the model parameters in Page's equation accounting for the bed geometry, grain moisture content, and drying conditions were developed.     

Effect of a Draft Tube on the Fluid Dynamics of a Spouted Bed: Experimental and CFD Studies

Knowledge about the gas and particle dynamics in spouted beds is important in the evaluation of particle circulation rates and the efficiency of gas-solid contacts. In this work, the mechanism of transition from a static bed to a spouted bed was numerically simulated using a Eulerian multiphase model. This model was applied to two distinct spouted bed geometries: a conventional device and a spouted bed with draft tube. The radial voidage and particle velocity profiles along the longitudinal position in the annular and spout regions were simulated for the geometries under study. The characteristic simulated curves were congruous with the experimental data.     

Effect of a Draft Tube on the Fluid Dynamics of a Spouted Bed: Experimental and CFD Studies

Knowledge about the gas and particle dynamics in spouted beds is important in the evaluation of particle circulation rates and the efficiency of gas-solid contacts. In this work, the mechanism of transition from a static bed to a spouted bed was numerically simulated using a Eulerian multiphase model. This model was applied to two distinct spouted bed geometries: a conventional device and a spouted bed with draft tube. The radial voidage and particle velocity profiles along the longitudinal position in the annular and spout regions were simulated for the geometries under study. The characteristic simulated curves were congruous with the experimental data.     

Discrete element method simulation of three-dimensional conical-base spouted beds

A discrete element method (DEM) simulation of three-dimensional conical-base spouted beds is presented. The overall height and diameter of the vessel are 0.5 and 0.15 m, respectively, and the nozzle diameter is 0.02 m. The inclined angle of the conical section varies from 0 to 60 degrees. The gas flow is described by the continuity and Navier-Stokes equations and solved by a finite difference method of second order accuracy in space and time. For gas-particle interaction, the Ergun equation (for void fraction smaller than 0.8) and the Wen-Yu model (for void fraction of 0.8 and above) are employed. A new method for treatment of the boundary condition for 3-D gas flow along the cone surface is proposed. This boundary condition satisfies both the continuity and momentum-balance requirements for the gas phase. Usefulness of the present simulation for studying gas flow pattern and particle motion in conical-base spouted beds is demonstrated. The effects of the inclined angle and draft tube on gas and particle flow in spouted beds are discussed.     

Hydrodynamic simulations of gas–solid spouted bed with a draft tube

Flow behavior of particles in a two-dimensional spouted bed with a draft tube is studied using a continuous kinetic-friction stresses model. The kinetic stress of particles is predicted from kinetic theory of granular flow, while the friction stress is computed from a model proposed by Johnson et al. (1990). A stitching function is used to smooth from the rapid shearing viscous regime to the slow shearing plastic regime. The distributions of concentration and velocities of particles are predicted in the spouted bed with a draft tube. Simulated results compare with the vertical velocity of particles (Zhao et al., 2008) measured and in the spout bed with draft plates and solid circulation rate (Ishikura et al., 2003) measured in the spouted bed with a draft tube. The impact of the friction stress of particles on the spout, annulus, fountain and entrancement regions is analyzed in gas–solid spouted bed with a draft tube. Numerical results show that the gas flow rate through the annulus increases with the increase of the entrainment zone. The solids circulation rate decreases with the decrease of inlet gas velocity and the height of the entrainment zone. The effect of spouting gas velocity on distributions of concentration, velocity and particle circulation is discussed.