Numerical simulation of gas-solid flows in fluidized bed gasification reactor

A numerical model for simulating a fluidized bed gasifier should include appropriate parameters to capture the dynamics of gas-solid flows, gasification kinetics and the interaction between these two. The focus of the present study is to analyze the effects of coal gasification chemistry models reported in literature on the prediction of product gas composition in a fluidized bed gasification reactor. Numerical results are validated against the experimental data available in literature. The validated model is used to examine the available chemical kinetics schemes for water gas shift reaction, steam methane reforming reaction and char heterogeneous reactions. It is also used to assess the effects of hydrodynamic models parameters such as drag model, particle-particle restitution coefficient and specularity coefficient on exit gas composition. Results show that the predictions of product gas composition are notably affected by the choices of the kinetics schemes for water gas shift and steam methane reforming reactions. Systematic analysis using the available choices to simulate initial processes such as moisture removal, volatile and tar cracking is reported. Drag models and the value of specularity coefficient are shown to have no effect on product gas composition, and the particle-particle restitution coefficient slightly influences the predicted gas composition.     

Numerical simulation of hydrodynamic behaviors in a gas-solids dense downer reactor

A Eulerian-Eulerian model incorporating the kinetic theory of granular flow was adopted to simulate the gas-solids flow behaviors in a dense downer below a conventional downer, which could be used for the further pyrolysis of coal and/or decomposition of tar on the generated char before the char and tar are completely separated in a triple-bed combined circulating fluidized bed (TBCFB) system. The high solids holdup in the dense downer can enhance the heat transfer to completely pyrolyze coal as well as decompose the heavy tar, avoiding the negative impact of pyrolysis products on char gasification. In order to obtain the optimal structural parameters and operating conditions and evaluate the performance of this dense downer, the influences of downer diameter, cone angle and solids mass flux on the hydrodynamic behaviors were investigated in details. The results demonstrate that the solids holdup in the dense downer can be increased, however, the maximum solids holdup is limited to approximately 0.4 owing to the ultimate carrying capacity. Moreover, it is found that there is a peak solids holdup in the annular region near the wall whereas many particles concentrate at the center in the high-density operation states. Meanwhile, the unique solids radial distribution could be caused by the radial movement of particles. Moreover, the intense collisions and turbulence caused by high velocity could inhibit agglomerates, which should be benefit for the heat transfer. It is expected that these results could offer a guidance for the design of such a dense downer for effective improvement of the efficiency of the pyrolyzer.     

The simulation of a gas-liquid chemical reactor with dispersed medium

It is suggested to develop a gas-liquid reactor in which heterogeneous exothermal reactions will proceed, with cumene selected as fuel. The temperature field and the yield of reaction product are given as functions of flow rate and initial temperature of mixture. The case is considered of the mass content of fuel components, at which the detonation occurs. Calculation is preformed of the cooling system for preventing the ignition and maintaining the temperature conditions of reaction. The calculation of chemical reaction is performed in the approximation of model of eddy dissipation which is valid at high values of Reynolds number.     

仿生表面减磨特性的数值模拟研究

为探究颗粒冲蚀作用下的仿生表面减磨特性,基于沙漠蝎表面抗蚀结构,确定凹槽、凸包和凹槽+凸包3种形式的仿生表面,采用数值模拟方法计算其磨损率分布规律,并分析仿生结构对流场和颗粒场的影响.结果表明:在颗粒入射角小于70°时,3种仿生表面均有减磨效果,其中凹槽结构减磨效果显著,但凹槽表面两侧磨损率略微增大,引入凸包结构后磨损...     

Numerical simulation of smoke clearing with nanoparticle aggregates

Smoke reduction processes in an indoor room‐scale chamber are generated by injecting nanoparticle aggregates. A numerical model, with a flow solver implemented with a particle collision model, is used to simulate the smoke‐reduction effect. The collision model, developed particularly for simulating collisions among particles with significantly different sizes, enables real‐time simulations of three‐dimensional, two‐phase flow when flow/particle interactions need to be considered. The accuracy of the collision model is estimated by comparing with the exact solution from the Smoluchowski equation. The simulated smoke reduction results are compared with measured data with good agreement. Optimized particle size distributions are studied using the simulation. Copyright © 2007 John Wiley & Sons, Ltd.     

等轴晶移动对宏观偏析影响的数值模拟

建立了自由等轴晶移动对宏观偏析影响的数学模型,对铸锭凝固过程中的对流和溶质分布进行了数值模拟.在模型中按照临界固相分数将糊状区分为紧密树枝晶和自由等轴晶两个不同的区域.对带冒口铸钢锭的宏观偏析进行了数值模拟,并同实验结果进行了比较.与假设糊状区内固相静止的模型相比,考虑等轴晶移动的模型得到的溶质分布结果与实验结果更接近.在凝固过程中,等轴晶随液体流动并在铸锭的底部中心聚集,在凝固后的铸锭中形成锥形的负偏析.还发现,在铸锭的中心靠上的区域形成正偏析,在铸锭的外部区域形成负偏析.     

The mathematical simulation of a fluidized-bed reactor

The authors examine the problem of the mathematical simulation of heterogeneous processes in a fluidized-bed reactor.     

构件跌落碰撞的数值模拟研究

针对构件跌落碰撞问题,将Hertz-damp模型与有限元方法结合,给出了结构碰撞位移突加约束条件,建立了碰撞结构体系的动力方程。该方法能考虑碰撞体质量分布对碰撞过程的影响,同时可描述结构碰撞过程中的刚度非线性和能量损失。建立了构件跌落碰撞模型,初步分析了跌落构件质量、质量分布以及跌落高度等参数对碰撞过程的影响。研究表明:结构碰撞模拟中碰撞体质量沿接触面法线轴向不对称时应考虑其质量分布,否则会高估碰撞反应;碰撞荷载峰值和单次碰撞持续时间与跌落构件质量和跌落高度均成指数关系,碰撞荷载峰值随跌落构件质量和跌落高度增大而增大,而单次碰撞持续时间则随跌落构件质量增大而增大,随跌落高度增大而减小。这些规律能为减轻结构碰撞破坏研究提供参考。     

含有柔性电缆的复杂系统装配仿真

将装配单元划分为“刚性组件”和“柔性电缆”,从装配对象建模、装配工艺规划和装配过程模拟三个方面对复杂系统的装配仿真方法进行系统的研究。首先,根据刚性组件和柔性电缆的不同特点,给出相应的建模方法,即针对刚性组件的CAD三维建模方法和针对柔性电缆的虚拟布线方法;然后,将装配过程视为拆卸过程的逆过程,提出含有柔性电缆的复杂系统的装配工艺规划方法,其中考虑了电缆形变过程及相应的模拟方法;最后,以装配工艺为基础对仿真对象进行装配仿真。实例表明,所提出的方法可以用于含有柔性电缆的复杂系统装配仿真。     

考虑地面积尘时地板送风的数值模拟

将室内的积尘简化为具有一定直径的球形颗粒,利用CFD数值模拟软件对考虑地面积尘时地板送风房间的室内空气品质进行了模拟,结果表明即使在考虑地面积尘的情况下,室内空气品质也是可以满足设计要求的。     

分段装药结构爆破效果的数值模拟研究

基于大型非线性动力分析软件LS-DYNA,选用弹塑性模型*MAT_PLASTIC_KINEMATIC和JWL方程,建立无限岩体介质中中深孔爆破时连续装药结构和分段装药结构三维模型,并进行了数值模拟,再现了两种装药结构爆破过程中的应力发展过程,对比分析了它们在Von Mises有效应力分布、自由面有效应力峰值、孔壁压力分布及加速度等衰减上的差异。结合工程现场爆破实践验证了分段装药结构可以明显改善爆破效果。     

PSTM中不同探针扫描成像的模拟分析

金属纳米粒子或薄膜在可见光的激励下产生表面增强效应。尝试在光子扫描隧道显微镜(PSTM)中应用带铝的光纤探针来提高成像的分辨率和灵敏度。采用二维时域有限差分法(FDTD)模拟计算了裸光纤、尖端镀铝颗粒光纤、孔径镀铝膜光纤和全镀铝膜光纤探针的成像。结果显示,采用带铝探针后的灵敏度均比裸光纤提高两个数量级;尖端镀铝颗粒光纤探针的分辨率最好,可优于20nm;全镀铝膜和孔径镀铝膜光纤探针的分辨率分别可达到20nm和60nm。结果可供探针制备及实验参数设置参考。     

Numerical analysis of ferrite recording heads with complex permeability

Numerical methods based on finite differences are applied to the analysis of ferrite recording heads in which the permeability is complex and flux leakage effects are significant. The methods described allow computation of head efficiency, inductance, loss angle, and phase shift. As a specific example, the analysis is performed for a reduced track width version of the Memorex 3650 head for a wide range of the real and imaginary parts of the permeability. Using published values for the frequency dependence of the complex permeability, the frequency dependence of the head performance is estimated. Some experimental data are presented and compared to the theoretical predictions.     

Numerical simulation of multi-species diffusion

A numerical model has been developed to simulate the transport of several ionic species across a saturated concrete or mortar sample. The chloride binding as well as the electrical coupling between the different ionic fluxes are included in the model by using the Nernst-Planck system of equations. This model highlights which parameters affect substantially chloride penetration into reinforced concrete structures and then shows that the use of Fick’s first law in a predictive model for chloride penetration is strongly challenged. The simulations are in good agreement with diffusion-cell experiments and membrane potential measurements.

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Résumé Un modèle numérique a été développé afin de simuler le transport de plusieurs espèces ioniques dans une éprouvette de béton ou de mortier saturée. La fixation des ions chlorures ainsi que le champ électrique créé par les divers flux ioniques sont modélisés à l’aide d’un système d’équations de Nernst-Planck. Le modèle permet, premièrement, de mettre en évidence les paramètres qui influencent le plus la pénétration des ions chlorures et, deuxièmement, il montre que l’utilisation de la 1^e loi de Fick dans un modèle de prédiction de la pénétration des ions chlorures est fortement remise en cause. Les résultats sont en accord avec les expériences de diffusion et les mesures expérimentales du potentiel de membrane.

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Editorial Note Prof. Jean-Pierre Ollivier is a RILEM Senior Member. He works at the LMDC, a RILEM Titular Member and participates in RILEM TCs 170-CSH (The structure of C-S-H) and 178-TMC (Testing and modelling chloride penetration in concrete). Prof. Lars-Olof Nilsson is a RILEm Senior Member.     

Numerical simulation of monsoon circulations

Short period variations in monsoon rainfall are caused by the westward passage of low pressure systems (depressions) from the northern sector of the Bay of Bengal. A primitive equation model was used to predict the movement of one such depression (20 August 1977). Four research ships from the USSR, which formed a part of the recently concluded Monsoon Experiment, provided additional meteorological data within the field of this depression. This enabled us to fix the depression’s initial position with greater accuracy than would have been possible without the ships’ data. The model used a co-ordinate system in which the lower boundary coincided with the earth’s surface, while the upper boundary was placed at 200 mb. It resembled a three-dimensional channel with side walls at 0° and 140°E, and northern and southern boundaries at 60°N and the equator. The grid spacing was 250 km. Numerical integration was performed upto 5 days of model time. Gravity waves and other forms of ‘noise’ were filtered out every 24 hr by a process of adjustment referred to as ‘initialisation’. Initialisation after every 24 hr was necessary because the boundary conditions in this regional model did not permit sufficiently rapid dispersal of gravity waves. In the first experiment only orographic features were included, but the second experiment considered the main features of atmospheric radiation in addition to orography. The paper presents a statement of deviations between the predicted and actual movement of the depression, and discusses reasons for such deviations.     

钢坯加热的数值模拟

借助CFD仿真手段,模拟蓄热式加热炉内钢坯加热的实际状况,研究了钢坯长度、宽度方向上下表面和中心温度的温度分布,提出钢坯存在上下温差、四角边缘温度高和靠近出钢口钢坯温度低等问题。同时,研究了炉内不同厚度钢坯与加热时间的基本关系,比较了仿真计算结果与实际炉内加热时间,通过数值拟合,得出钢坯厚度与加热时间的拟合公式,可在生产实际中使用。上述研究结果可为加热炉钢坯加热制度的制定和优化提供依据。     

Numerical simulation of electroosmotic flow

We have developed a numerical scheme to simulate electroosmotic flows in complicated geometries. We studied the electroosmotic injection characteristics of a cross-channel device for capillary electrophoresis. We found that the desired rectangular shape of the sample plug at the intersection of the cross-channel can be obtained when the injection is carried out at high electric field intensities. The shape of the sample plug can also be controlled by applying an electric potential or a pressure at the side reservoirs. Flow induced from the side channels into the injection channel squeezes the streamlines at the intersection, thus giving a less distorted sample plug. Results of our simulations agree qualitatively with experimental observations.     

Numerical simulation of electrodeposition problems

The mathematical modelling of the electrodeposition phenomenon leads to a linear elliptic partial differential equation subject to non-linear mixed boundary contitions. This is complicated further by the fact that the domain under consideration is evolving with time. Owing to the nature of the mathematical model, the boundary element method is a very efficient numerical technique for the solution of such problems. The authors report their numerical simulation of the above problem which also has applications in the cathodic protection systems for corrosion prevention.     

不同尺寸装药烤燃特性的数值模拟研究

为研究装药尺寸和升温速率对装药烤燃的点火位置、点火温度和点火时间的影响,该文利用有限元商业软件LS-DYNA对不同尺寸装药在不同升温速率热环境下的烤燃特性进行数值模拟研究。研究发现,不同升温速率下,装药烤燃时的点火位置随着装药直径的增加其变化路径相似,均是从装药中心沿着中心轴向两端移动,在距离上下端约1/4处离开中心轴,沿着近似直线向边缘移动。不同升温速率下,装药的点火温度和点火时间均随着装药直径的增大先减小后增大,存在一个最小值。装药直径不变时,装药的点火温度均随着升温速率的增大而增大,装药的点火时间随着升温速率的增大而缩短。