INVESTIGATION INTO MALDISTRIBUTION IN A CIRCULATING FLUIDIZED BED

1. Introduction The design of gas distributor has a major influence on gas flow patterns, dumping and dead zones. It has been demonstrated that maldistribution will occur if the distribu-tor has a low pressure drop, that is, some parts of the bed will receive much less gas than others, and may be tem-porarily or permanently defluidized, while the gas forms semi-permanent spouts or channels in other parts. There-fore, maldistribution is undesirable in industry. For example, the temperature in a…     

MULTIFRACTAL ANALYSIS OF PARTICLE-FLUID SYSTEM IN A CIRCULATING FLUIDIZED BED

In this paper, multifractal analysis together with wavelet transform modulus maxima (WTMM) method is used to analyze the fluctuating signals of circulating fluidized bed (CFB). Singularity spectrum shows that the gas-particle flow in CFB has multifractal character. Motion behavior of the particle-fluid system of CFB can be described by singularity spectrum. Intermittency index can be used to determine the transition of flow regime from fast fluidization to pneumatic conveying.     

SIMULATION OF PARTICLE COATING IN THE SUPERCRITICAL FLUIDIZED BED

Fluidized bed technology using supercritical carbon dioxide both as a fluidizing gas and as a solvent for the coating material makes possible the production of thin, uniform and solvent-free coatings. But operation at low fluidizing velocities, which is favorable to facilitate gas cleaning under the high pressure conditions, may lead to uneven distribution of the coating in the fluidized bed and to unstable operation due to agglomeration. Therefore a model has been developed which describes local fluid dynamics within the high pressure fluidized bed. Based on this model, the coating process is described and the distribution of the coating inside the fluidized bed is calculated. Furthermore a submodel for the calculation of local concentrations of liquid paraffin has been set up, which may be used as a basis for the prediction of agglomeration and thus stability of operation.     

MODELING NONLINEAR DYNAMICS OF CIRCULATING FLUIDIZED BEDS USING NEURAL NETWORKS

In the present work, artificial neural networks (ANNs) were proposed to model nonlinear dynamic behaviors of local voidage fluctuations induced by highly turbulent interactions between the gas and solid phases in circulating fluidized beds. The fluctuations of local voidage were measured by using an optical transmittance probe at various axial and radial positions in a circulating fluidized bed with a riser of 0.10 m in inner diameter and 10 m in height. The ANNs trained with experimental time series were applied to make short-term and long-term predictions of dynamic characteristics in the circulating fluidized bed. An early stop approach was adopted to enhance the long-term prediction capability of ANNs. The performance of the trained ANN was evaluated in terms of time-averaged characteristics, power spectra, cycle number and short-term predictability analysis of time series measured and predicted by the model.     

DEVELOPMENT OF COAL DRY BENEFICIATION WITH AIR-DENSE MEDIUM FLUIDIZED BED IN CHINA

In China more than two-thirds of available coal reserves are in arid areas, where, to beneficiate the run-of-mine coal,there is not enough water resource required by conventional processing. Developing efficient dry beneficiation technology is of great significance for efficient coal utilization in China, notably the clean coal technology (CCT). The dry coal beneficiation technology with air-dense medium fluidized bed utilizes air-solid suspension as beneficiating medium whose density is consistent for beneficiation, similar in principle to the wet dense medium beneficiation using liquid-solid suspension as separating medium. The heavy portion in feedstock whose density is higher than the density of the fluidized bed will sink, whereas the lighter portion will float, thus stratifying the feed materials according to their density.In order to obtain efficient dry separation in air-dense medium fluidized bed, stable fluidization with well dispersed micro-bubbles must be achieved to ensure low viscosity and high fluidity. The pure buoyancy of beneficiation materials plays a main role in fluidized bed, and the displaced distribution effect should be restrained. The displaced distribution effects include viscosity displaced distribution effect and movement displaced distribution effect. The former is caused by viscosity of the fluidized bed. It decreases with increasing air flow velocity. Movement displaced distribution effect will be large when air flow rate is too low or too high. If medium particle size distribution and air flow are well controlled, both displaced distribution effects could be controlled effectively. A beneficiation displaced distribution model may be used to optimize beneficiation of feedstock with a wide particle size distribution and multiple components in the fluidized bed. The rheological characteristics of fluidized beds were studied using the falling sphere method. Experimental results indicated that the fluidized bed behaves as a Bingham fluid. The plastic viscosity and yield stress can be obtained by measurement of the terminal settling velocity of spheres and linear regression of the experimental data. Both plastic viscosity and yield stress increase with increasing size of the fluidized particles. The drag coefficient can be calculated with favorable agreement with experimental data.The first dry coal beneficiation plant with air-dense medium fluidized bed was established by CUMT with an output of 320000 t.a-1 and a probable error Ep value up to 0.05 was achieved. The plant was accepted by the Chinese government in June, 1994. Since then, new applications have been found including a 700000 t.a-1 dry coal beneficiation plant put up for commercial testing. To realize coal dry beneficiation of full size range of 300～0 mm, further research on dry coal beneficiation of different size fractions has been under way at the Mineral Processing Research Center of CUMT, leading to the following results:● Dry beneficiation technology with a vibrated air-dense medium fluidized bed for fine coal of size fraction 6～0.5 mm Ash content was reduced from 16.57% to 8.35%, with yield up to 80.20% and Ep value up to 0.065.● Coal dry beneficiation technology with a deep air-dense medium fluidized bed for ＞50 mm coal An Ep value up to 0.02 was achieved. This technology is of great value for waste removal from 300～50 mm large feedstock, especially for big surface mines in China.● Coal triboelectric cleaning technology for ＜1 mm pulverized Coal Coal is comminuted down to 320 mesh (0.043 mm) to fully liberate the embedded minerals, yielding an ultra-low ash coal (less than 2%). Currently a pilot system with triboelectric cleaning has successfully passed technical appraisal.● Three-product beneficiation technology with dual-density fluidized bedThis technology yields three products: clean coal, middling and tailing, with the following typical results: Ep value of 0.06～0.09 for the upper layer with a density of 1.5～1.54 g.cm-3 and Ep value of 0.09～0.11 for the lower layer with density of 1.84～1.9 g.cm-3.     

STUDY ON THE OVERALL PRESSURE BALANCE OF A DOWNFLOW CIRCULATING FLUIDIZED BED SYSTEM

A pressure balance model for a circulating fluidized bed unit that incorporates a downer has been proposed. The model predictions were validated with the experimental data obtained from a special cold-model circulating fluidized bed. Comparison of the operation stability between a CFB downer and a CFB riser has been carried out. Only one critical gas velocity exists in the CFB-riser for a given riser solids flux, while there can be many critical gas velocities for the operation of a CFB downer. Therefore, it is possible to achieve high solids concentration in a CFB downer if appropriate operating conditions are used.     

计算流体力学在炼钢中的应用和发展

介绍了计算流体力学应用到炼钢过程中所面临的边界条件和材料物性数据等问题．并着重评述了钢包精炼和连铸结晶器过程中流动及相关现象研究概况,指出炼钢过程中数值模拟的目标不是获得描述某给定过程的精确数值,而是获取过程知识,特别是获取实验无法得到的信息．数值模拟不能代替实验,相反是扩大实验的内容．将来的炼钢过程数值模拟工作将面临更复杂的三维、时间相关的多相反应系统或气金渣等自由表面问题的挑战．      

COLLISION PROCESS DETAILS CONTROL THE HYDRODYNAMICS AND HEAT TRANSFER IN FLUIDIZED BED REACTORS

In the ‘90s DEM simulation research was successful in exploring its potential in the simulation of fluidization phenomenaand in its application to the design of fluidized-bed processes. Nevertheless, not much progress has been made regardingthe realistic treatment of collision processes that are critical in determining macroscopic mode of fluidization. All throughthe second half of the ‘90s, the author investigated/demonstrated the issue by introducing different surface interactionscaused by formation of liquid and/or solid bridges, van der Waals force and the existence of surface roughness. In the firstpart of the presentation these are to be summarized and the tasks remaining are discussed. In the second part, are pre-sented the results from a newly developed force-deformation meter to demonstrate the significance of the surfaceroughness and its elasto-plastic characteristics.     

数值模拟方法在体液流动研究中的应用与展望

计算流体力学的数值模拟技术产生于20世纪60年代,现已成功地应用到众多科学研究与工程设计领域. 从20世纪80年代开始,虽已有不少研究工作开始利用CFD方法与计算机技术对人体的血流、气管中的气流、胆汁流等流动进行数值模拟与分析,但并没有与器官功能分析和病理分析及临床应用紧密联系. 近十几年来,这方面的工作已经取得了长足的进步和发展,模拟出了譬如脑血管、颈动脉、心血管等复杂血管流动,甚至整个肺泡的活动,得到了非常有病理意义和临床价值的图像.本文介绍和评述计算人体流体动力学的历史,发展和已经取得的成果.      

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.      

VERIFICATION OF SMOM AND QMOM POPULATION BALANCE MODELING IN CFD CODE USING ANALYTICAL SOLUTIONS FOR BATCH PARTICULATE PROCESSES

For many processes of industrial significance, due to the strong coupling between particle interactions and fluid dynamics, the population balance must be solved as part of a computational fluid dynamics （CFD） simulation. In this work, a CFD based population balance model is tested using a batch crystallization reactor. In this CFD model, the population balance is solved by the standard method of moments （SMOM） and the quadrature method of moments （QMOM）. The results of these simulations are compared to analytical solutions for the population balance in a batch tank where 1） nucleation, 2） growth, 3） aggregation, and 4） breakage are taking place separately. The results of these comparisons show that the first 6 moments of the population balance are accurately predicted for nucleation, growth, aggregation and breakage at all times.     

我国口腔生物力学研究进展与展望

本文综述了我国口腔生物力学研究现状、研究手段、达到的水平以及临床应用状况、并作了展望。      

空间机械臂动力学与控制研究进展

综述了近年来国内外在空间机械臂动力学与控制研究领域内的最新进展．包括路径规划的逆动力学、挠性影响、自适应控制及最优控制,以及工作空间等方面的研究状况和展望      

计算流体力学中的验证与确认

计算流体力学(CFD)在航空航天等诸多领域的应用越来越广泛.特别是近年来, CFD在实际飞行器的设计中扮演着越来越重要的角色, 许多设计参数直接来源于CFD的计算结果.由此, 飞行器设计师对CFD提供结果的可信度提出了更高的要求.验证(verification)与确认(validation)是评价数值解精度和可信度的主要手段.本文综述了国内外开展CFD验证与确认研究的进展.在引言中论述了开展CFD验证与确认的重要性和必要性, 简述了国内外CFD验证与确认研究的历史和发展现状.第2节中讨论了CFD验证与确认的一些基本概念,以及这些概念定义的形成过程, 并指出了进行CFD验证与确认的基本步骤.第3节和第4节分别讨论了CFD验证与确认的方法, 如CFD验证中的精确解比较方法, 制造解比较方法, 网格收敛性研究;CFD确认中的层次结构, 流动分类法, 确认实验指南.在第5节中我们列举了几个CFD验证与确认的应用实例.最后, 对我国开展CFD验证与确认研究工作提出了若干建议, 包括:(1)开展流动分类法研究,(2)推行软件质量工程方法,(3)开展规范精细的实验, 建立国内的网络数据库.      

NO_x生成湍流反应率数值模拟的进展

利用计算机数值模拟来优选燃烧器或炉内降低氮氧化物的各种措施,如空气分级,燃料分级（再燃烧）,烟气再循环,煤和生物质混烧等,预测其效果,受到国际上越来越大的重视．如何建立合理而经济的数学模型,是其中的关键问题之一．根据近年来国际上发表的不同研究者的研究结果,对湍流燃烧氮氧化物生成湍流反应率数值模拟的研究现状进行了评述,并且讨论了有待解决的问题和今后的发展方向．      

输液管模型及其非线性动力学近期研究进展

综述了输液管系统的各类物理模型及其相应的数学模型,在流体满足基本假设条件下,对于管道内径远远小于管道长度的直管和曲管,详细叙述了梁模型管动力学数学模型的建模过程以及建模方法,针对在水动压力作用下以及管道短而且薄的情形,综述了壳模型的输液管道的动力学方程.在此基础上,概述了近几年来输液管道的非线性振动、稳定性、分岔与混沌、特别是管道控制的研究现状,并对今后的发展趋势作了分析和预测.综观非线性动力学理论的发展历程可以发现选取研究对象和典型的数学模型是至关重要的.对于低维的非线性系统,常常选用van der Pol、Duffing、Mathieu、Lorenz等典型系统来进行研究工作的.通过本文可以看出,对于研究高维非线性系统动力学,流诱发输液管的动力学问题是非常典型的模型之一,它有着容易理解的工程背景、包含了梁和壳的振动问题,并且它的数学模型相对简单,然而却能包含非常复杂的非线性动力学现象,同时容易解释数学方法得到的结果易对应到工程中的实际现象.本文希望通过对输液管动力学模型及其非线性动力学和控制研究现状的综述,建立高维非线性动力学的分析模型,以便发展高维非线性动力学的分岔与混沌理论,同时建立相应的控制理论基础.      

A CORRELATION EQUATION FOR CALCULATING INCLINED JET PENETRATION LENGTH IN A GAS-SOLID FLUIDIZED BED

Numerical simulation of gas-solid flow in a two-dimensional fluidized bed with an inclined jet was performed. The numerical model is based on the two-fluid model of gas and solids phase in which the solids constitutive equations are based on the kinetic theory of granular flow. The improved ICE algorithm, which can be used for both low and high-velocity fluid flow, were used to solve the model equations. The mechanism of jet formation was analyzed using both numerical simulations and experiments. The emergence and movement of gas bubbles were captured numerically and experimentally. The influences of jet velocity, nozzle diameter, nozzle inclination and jet position on jet penetration length were obtained. A semi-empirical expression was derived and the parameters were correlated from experimental data. The correlation equation, which can be easily used to obtain the inclined jet penetration length, was compared with our experimental data and published correlation equations.     

欧洲微重力研究和用户支持的现状及展望

本文综述了欧洲在微重力状态下对材料科学、流体科学和生物科学的研究以及用户支持的主要情况。从简短的历史回顾开始,分析了欧洲主要的空间飞行试验,以及以长远目标为重点的未来发展情况。叙述了进行地面研究的指导思想,分析了所使用的地面实验设施(落塔,抛物线飞行,探空火箭等)。此外,还简要地介绍了微重力下的实验操作技术(遥科学,机器人等)。     

超高速撞击动力学及航天器防护研究进展

概述了超高速撞击动力学的国内外研究背景;从超高速撞击厚板成坑、中厚板侵彻、薄板形成碎片 云、模拟实验技术和数值模拟技术5个方面总结了超高速撞击动力学的研究进展情况;分析 了超高速撞击对航天器的毁伤效应;给出了航天器的防护要求;讨论了航天器防护结构的方 案设计和性能表征. 为超高速撞击动力学和航天器防护研究提供技术参考.