煤焦破碎成灰模型研究

本文从颗粒破碎的角度,建立了燃烧过程中单颗煤焦的逾渗破碎及其成灰模型,并深入研究了初始大孔隙率对颗粒破碎程度以及残灰粒径分布的重要影响。模拟结果再现了无大孔存在(φ=0)情况下,单颗煤焦生成单颗残灰的实验事实;研究结果还表明,在所考察的取值范围内,初始大孔隙率φ越大,煤焦颗粒破碎越剧烈,燃烧前期的破碎程度也随之升高,同时,φ越大,燃烧过程中单颗煤焦生成残灰的数目越多,平均粒径越小,而且残灰主要集中在粒径较小的范围内。     

磁性纳米颗粒膜的微磁学模拟

用微磁学方法对磁性纳米颗粒膜的磁特性进行了模拟，采用的模型是由122个磁性纳米颗粒组成的面心立方(fcc)结构体系.结果表明：在该体系中，偶极相互作用对体系的静态磁结构的影响显著，而交换相互作用的影响表现不明显.在此基础上，本文还采用有效媒质理论计算分析了磁性合金颗粒不同体积比时颗粒膜的磁谱和表征电磁参量发生显著变化的逾渗现象和逾渗阈值.并完成了对高磁损耗磁性纳米颗粒膜的材料设计. 关键词： 微磁学 纳米颗粒膜 逾渗阈值 磁导率 材料设计     

煤粉燃烧过程中灰膜形成比例实验研究

煤焦颗粒燃烧过程中,灰膜形成显著影响其燃烧特性。因此,本文借助高温沉降炉研究了61~75,75~90和90~125μm三种粒径黄陵烟煤在1273和1673 K温度下的燃烧特性与灰膜形成比例;借助扫描电镜(SEM)详细观测空心微珠颗粒内部结构,提出灰膜比例计算公式,并分析温度,粒径和碳转化率对灰膜比例的影响。结果表明,高温下大部分灰分在焦炭烧尽阶段以灰膜形式存在。灰膜比例随温度和碳转化率增加而增加,随煤粉粒径增大而减小。高温下灰分用于形成灰膜比例相对较高,这为煤焦燃尽阶段的低反应性提供了合理的解释。煤焦颗粒动态燃烧过程中灰膜形成比例随燃烧工况变化而变化。该研究为煤焦颗粒燃烧动力学模拟灰膜比例选择提供了关键数据支撑。     

煤焦O_2/CO_2燃烧反应性及结构演化的分子模拟研究

本文通过自编程序Fringe3D和Vol3D构建了具备一定孔隙结构特征的大尺度煤焦分子模型(碳原子数~40000),通过简化的分子模拟方法编程实现了煤焦O_2/CO_2燃烧微观反应过程的模拟。模拟结果表明:随着燃烧反应的进行,煤焦结构逐渐"缩核"变小,而煤焦结构尺寸的变化主要发生在燃烧后期。随着碳转化率的增大,低活性碳原子的数量逐渐减少,而高活性碳原子的数量先增加后减少。与空气燃烧相比,煤焦在O_2/CO_2气氛下的燃烧过程明显延迟,而煤焦比表面积的改变对煤焦反应性的变化有紧密关联。     

感染生长模型的逾渗模拟

在规则格子点阵中，活跃点逐步动态地以可变概率感染附近空缺点而生成系综.利用感染概率替代系综温度，给粒子划分能级，可以用巨正则系综配分函数表征体系.蒙特卡洛方法模拟验证了该体系在逾渗阈值处的相变行为.提出了一种新的较为普适的估算规则点阵逾渗阈值的方法.对介质基底上金属薄膜的实验研究验证了该感染生长模型的合理性.由此给出了格子点阵的固有属性(逾渗)如何在粒子聚集成团簇这一动态过程中体现出来的物理模型. 关键词： 逾渗 系综 蒙特卡洛方法 生长模型     

粒径均匀散体导热系数的分形描述

本文以分形理论为工具,采用填充率、散体颗粒直径等宏观易等测参数较准确地描述了颗粒散体的几何结构;运用导热与逾渗的类比,建立了均匀散体导热系数的逾渗模型,模型的预测值与实验测量结果基本一致。     

耦合反应器中化学链燃烧的模拟

基于颗粒动理学和化学动力学理论,建立化学链燃烧计算模型,数值模拟了耦合反应器内化学链燃烧过程,获得了反应器内流场特性和各组分分布规律,并很好地捕捉到了空气反应器中颗粒呈现出的非均匀环核流动结构。模拟结果同时给出了反应器中温度分布规律以及各出口颗粒质量流率和各组分浓度随时间的变化,为耦合反应器的设计优化提供了一定的依据。     

灰对煤焦燃烧的动力学研究

基于表观动力学灰膜缩核模型,本文发展并利用耦合的灰膜与灰渗透本征动力学模型,详细研究焦炭颗粒燃烧特性。氧化反应与气化反应耦合能够高精度预测煤焦颗粒燃烧特性。随灰膜形成比例下降,焦炭颗粒燃烧温度升高,碳燃尽时间缩短;当碳转化率低于80%时,碳转化率主要受灰膜影响,灰渗透影响甚微;但当碳转化率大于80%时,灰膜与灰渗透同时起作用。     

裂隙多孔介质双重逾渗模型的算法实现

简要介绍基于孔隙逾渗和裂隙逾渗叠加的双重逾渗模型,阐述模型的原理、算法及其实现过程.初步研究模型的分形特性,认为分形维数D是能够衡量模型连通性的重要参数.最后探讨模型的蒙特卡洛数值计算方法,兼顾计算精度与计算耗时,提出可操作的计算规模.     

等离子体对含硼两相流扩散燃烧特性的影响

为排除来流空气对含硼燃气的掺混效应, 研究等离子体对含硼富燃料推进剂在补燃室二次燃烧过程的影响, 建立了含硼两相流平行进气扩散燃烧物理模型. 利用高速摄影仪拍摄了含硼燃气在补燃室二次燃烧的火焰图像, 分析了该物理模型的扩散燃烧特性和硼颗粒的二次点火距离. 采用硼颗粒的King点火模型、有限速度/涡耗散模型、颗粒轨道模型和RNG k-ε模型以及等离子体模型, 模拟了一定条件下等离子体对含硼两相流扩散燃烧过程的影响. 结果表明, 依据含硼燃气二次燃烧图像得到的硼颗粒二次点火距离, 与数值模拟结果基本一致, 保证了该物理模型和计算方法的可靠性. 含硼两相流经过等离子体区域后, 硼颗粒在运动轨迹上颗粒温度明显增加, 颗粒直径明显减小, B₂O₃的质量分数分布区域明显扩增, 70%的硼颗粒在到达补燃室2/3尺寸前燃烧效率已达到100%, 硼颗粒充分燃烧释放出更多热量导致中心流线区域温度增加近1/2, 可见等离子体可以明显强化含硼两相流的燃烧过程, 提高硼颗粒的燃烧效率.     

Dynamic fractal structure of emulsions due to motion and interaction of the particles: Numerical simulation

The method of numerical simulation is used to study the geometrical structure of micro-emulsions in the plane. It is found that the interaction between the particles leads to the formation of a dynamic homogeneous fractal structure of the micro-emulsion. In the absence of any interaction between the particles the structure of the emulsion is homogeneous. The interaction energy of the particles at which the fractal inhomogeneity arises is close in magnitude to the interaction energy of the particles in real (e.g., aqueous) micro-emulsions. It is also found that the size of the inhomogeneities (correlation radius) depends on the particle density in the system and is largest for the density of the percolation transition. The numerical simulation data qualitatively coincide with the results of measurements in real micro-emulsions. Zh. éksp. Teor. Fiz. 111, 1314–1319 (April 1997)     

煤多相燃烧分形增长模型的初步研究

本文运用分形理论对煤的多相燃烧过程进行了探索性研究。根据分形动力学的概念认为；煤的多相燃烧中，反应面积的分形增长有DLC模式和KLC模式，一般情况是这两种模式的迭加。模型还引入了孔洞的合并及煤种的影响因素，初步提出了较有通用意义的燃烧模型。该模型把对碳焦的表面分形与燃烧速率的计算结合起来初步揭示了碳焦结构对燃烧过程的影响。     

Characterization of Synthetic-Coal Char Particles using fractal dimension analysis

The development and change of surface ruggedness in chars was studied at conditions typical in a pulverized coal furnace. The fractal dimension, a measure of surface ruggedness, of chars was measured using physisorption techniques. By adjusting the temperature encountered (1173 to 1773 K) and residence time (0.1 to 1.5 s) of the synthetic coal (sized to 46–106 μm diameter), chars at different stages of combustion were prepared in a laminar flow (drop-tube) furnace. The particles were quickly cooled and quenched in an inert atmosphere. The samples were examined using a scanning electron microprobe, and their fractal dimensions were determined using gas physisorption. The adsorption data were used to test if the char surface was fractal on a molecular scale, to determine the fractal dimension, and to quantify changes in the fractal dimension during combustion. The fractal dimension of the unburned synthetic coal was approximately 2. The fractal dimension increased as high as 2.85 as the carbon matrix burned away and exposed mineral moieties. However, as combustion continued the carbon burned completely away leaving a mineral fly ash particle with a fractal dimension as low as 2.47.     

Characterizing char particle fragmentation during pulverized coal combustion

A particle population balance model was developed to predict the oxidation characteristics of an ensemble of char particles exposed to an environment in which their overall burning rates are controlled by the combined effects of oxygen diffusion through particle pores and chemical reactions (the zone II burning regime). The model allows for changes in particle size due to burning at the external surface, changes in particle apparent density due to internal burning at pore walls, and changes in the sizes and apparent densities of particles due to percolation type fragmentation. In percolation type fragmentation, fragments of all sizes less than that of the fragmenting particle are produced. The model follows the conversion of particles burning in a gaseous environment of specified temperature and oxygen content. The extent of conversion and particle size, apparent density, and temperature distributions are predicted in time.Experiments were performed in an entrained flow reactor to obtain the size and apparent density data needed to adjust model parameters. Pulverized Wyodak coal particles were injected into the reactor and char samples were extracted at selected residence times. The particle size distributions and apparent densities were measured for each sample extracted. The intrinsic chemical reactivity of the char to oxygen was also measured in experiments performed in a thermogravimetric analyzer. Data were used to adjust rate coefficients in a six-step reaction mechanism used to describe the oxidation process.Calculations made allowing for fragmentation with variations in the apparent densities of fragments yield the type of size, apparent density, and temperature distributions observed experimentally. These distributions broaden with increased char conversion in a manner that can only be predicted when fragmentation is accounted for with variations in fragment apparent density as well as size. The model also yields the type of ash size distributions observed experimentally.     

Pore-Resolving Simulations of Biomass Char Particle Combustion

Biomass char morphology affects combustion behavior at the particle scale for zone II conditions, in which both heterogeneous reaction and intra-particle diffusion govern the overall rate. Furthermore, particle-scale processes affect reactor-scale outputs, and reactor-scale simulations are sensitive to particle-scale models. However, most char particle combustion models employ coarse-grained, effective-continuum approaches, which treat all porosity at the subgrid-scale. Effective-continuum approaches are not valid or accurate in the presence of large, irregular pores which can approach the size of the particle. A 3-D, pore-resolving CFD simulation approach using real biomass char particle geometries obtained from X-ray micro-computed tomography (micro-CT) is therefore used to examine the impact of morphology on zone II combustion for pulverized (∼100 µm) biomass char particles for the first time. In contrast to larger, millimeter to centimeter sized particles, the sub-millimeter, high aspect ratio biomass char particles exhibited localized reactant penetration into the innermost regions of the particles, facilitated by the presence of large pores connected to the external surface. The oxygen mole fraction distributions were governed by the large pore morphology, were non-monotonic with distance from the surface, and achieved minima in thick microporous char regions surrounding the large pores. A comparison between the pore-resolving simulation and an equivalent, spatially resolved, effective-continuum simulation revealed that even in the microporous char, the effective-continuum model underpredicted reactant penetration. A careful comparison was then performed between 30 pore-resolving particle simulations and several effectiveness factor models that employed particle-specific parameters. Commonly used uniform cylinder models significantly underpredicted effectiveness factors for these real pulverized pine char particles, while accessible hollow cylinder models achieved less than 10% relative error when averaged over all 30 particles.     

Order-disorder transitions in atactic polystyrene films

The results of the electron microscopy investigation of the surface topology of the films obtained from solutions of linear atactic polystyrene in chloroform are presented. It has been shown that the distribution of density fluctuations in the films can be described using the model of a fractal percolation cluster of macromolecular coils. A decrease in the local packing density of particles upon going from θ-coils to blobs is associated with mutual penetration of the coils. An increase in density fluctuations and a decrease in the relative area and fractal surface of the cluster of the particles are associated with a decrease in the short-range order caused by the formation of the percolation cluster, which reflects portions of the chains not involved in the blobs.     

Microscopic description of an Ising spin glass near the percolation threshold

Monte Carlo results using a microscopic model to describe FexZn(1-x)F2 indicate that its spin-glass phase at x=0.25 and zero magnetic field is characterized by the presence of antiferromagnetic fractal domains, separated by random vacancies and strongly correlated in time. The effective local random-field distribution corroborates this glassy behavior, which emerges irrespective of ab initio competing interactions and is a consequence of the fractal domain structure near the percolation threshold, x(p)=0.24. The aging properties of the system are in agreement with predictions of short-range stochastic spin-glass models and with the droplets model for spin glass close to percolation.     

多相燃烧分形模型及其实验研究

本文提出了多相燃烧的分形模型．模型中认为在多相燃烧中内部孔洞体积与表面积存在分维指数关系，而且反应面积的增长为两种分形增长模式的叠加．结合孔洞合并的因素得到了描述煤多相反应速率的分形模型．该模型描述的反应速率先增加后减少的规律与实验结果十分相符．对五个煤种和其中两个煤焦样品进行了试验研究，在两个不同升温速率下得出的试验数据与理论计算相符．     

Flame characterisation of gas-assisted pulverised coal combustion using FPV-LES

A multiphase flamelet/progress variable (FPV) model for the large eddy simulation (LES) of gas-assisted pulverised coal combustion (PCC) is developed. The target of the simulation is the Darmstadt turbulent gas-assisted swirling solid fuel combustion chamber. The coal particles are treated as Lagrangian point particles, the position, momentum and energy of which are tracked. The gas phase is described by the low-Mach Navier-Stokes equations alongside the Eulerian transport equations of the governing variables for the FPV model. The set of chemical states of the PCC flame is pre-tabulated in a six-dimensional flamelet table and determined by the mixing of the primary fuel stream, volatiles and char off-gases with the oxidising air, the progress of chemical reactions, the interphase heat transfer, as well as sub-grid scale variations. A presumed $\beta$-PDF approach for the total mixture fraction is applied to capture sub-grid scale effects. The discrete ordinate method (DOM) with the weighted sum of grey gases model (WSGGM) is employed to model radiation. The FPV-LES results are validated against the experimental evidence and a good agreement of the predicted mean and RMS velocities, as well as the mean gas temperature between experiments and simulations is obtained. The contributions of the pilot, volatile and char off-gas fuel streams to the coal flame are analysed. It is found that most regions of the furnace are dominated by either pilot or volatile combustion, while char conversion only occurs in the far downstream and outer furnace regions. The pilot gas dominates the near-wall region inside the quarl, whereas the volatile gas mainly released from small particles dominates a first volatile combustion zone in the interior of the internal recirculation zone. Larger particles heat up more slowly and release their volatile content further downstream, leading to a secondary volatile combustion zone.     

Size dependence of self-diffusion in the hard-square lattice gas

A striking size dependence of the mean-square displacement of diffusing particles in the two-dimensional lattice gas of hard squares has been observed by Monte Carlo simulation. It is shown that the size effect is due to the formation of a stable cage structure in small lattices when the particle concentration is high. The formation of cages is governed by a new type of percolation problem related to bootstrap percolation.