粒径分布对有机粉尘爆炸中火焰结构的影响

采用高速摄影及带通滤波片相结合的方法,记录并分析了火焰在开放空间两种不同粒径分布硬脂酸粉尘云中的传播特性.实验中,通过控制喷嘴压力形成两种典型粒径分布的硬脂酸粉尘云.实验发现,火焰在两种不同粒径粉尘云中传播时,具有明显不同的火焰前锋结构特征,即连续火焰前锋和离散火焰前锋.在较小粒径分布粉尘云中传播时,火焰前锋形状规则、连续,燃烧反应区均匀一致,类似于预混燃烧现象；而在较大粒径分布粉尘云中传播时,火焰前锋黄色发光区周围分布着离散的蓝色亮点.在此基础上,进一步利用CH辐射放大图像揭示了粉尘云离散火焰前锋的形成机理.理论模型的分析结果证明了粒径小于58 μm的粒子质量浓度是决定能否出现离散火焰前锋的关键参数.     

锆粉尘云的火焰传播特性

使用高速摄影、超细热电偶和纹影技术,对方形开口管道中锆粉尘云火焰传播特性进行了实验研究.锆粉尘通过压缩空气喷散到管道中形成粉尘云,随后,锆粉尘云被电火花点燃.实验结果表明,低质量浓度下,火焰传播速度和温度随质量浓度增加而增加.当粉尘云质量浓度为0.627 kg/m3时,火焰传播速度和温度达到最大值,分别为30.41 m/s和1716 K,其后随着质量浓度的继续增加,火焰传播速度和温度开始略微下降.锆粉尘云中粒子与空气形成气-固表面燃烧体系,一些粒子在燃烧结束前发生微爆.预热区厚度在0.39～2.52 cm,随粉尘云质量浓度的增加先减小后增加.     

玉米粉火焰在开口垂直管道中的传播

以对粉尘云状态参数的定量测定为基础,对玉米粉尘火焰在开口垂直管道中向上传播的过程进行了实验研究.在情形A中,火焰从管道的封闭端向开口端传播,在情形B中,从开口端向封闭端传播.实验中,观察到两种粉尘火焰,即湍流火焰和层流火焰,火焰形态转变对应的点火延迟时间约等于1.1 s,即粉尘云湍流运动强度为10cm/s.情形A中,层流火焰的传播出现周期性振荡现象,湍流火焰在传播过程中不断加速;情形B中,两种火焰都匀速传播,湍流火焰传播速度明显大于层流火焰.在所考察的实验条件下,粉尘浓度对于玉米粉尘火焰传播速度的影响不大.     

煤粉复合旋流火焰燃烧特性研究（1）：实验研究

本文实验研究了煤粉复合旋流火焰燃烧特性，进行了包括３个典型煤种：褐煤、烟煤和无烟煤，在不同煤粉浓度下的燃烧试验，测量了火焰场的温度分布、气体组份分布、火焰周界辐射和煤颗粒燃烧参数，对火焰场特性、不同煤种影响和煤粉浓度作用做了较系统的试验研究，揭示了该燃烧方式的稳定着火、强化燃烧机理，获得了这种煤粉火焰的燃烧特性。     

5个待定系数的修正韦伯函数在发动机多火焰区燃烧模型中的应用

提出了含5个待定系数的修正韦伯函数，并应用多元非线性回归技术确定这些系数，模拟了具有双火焰区燃烧模式和稀燃－速燃特性的汽油机射流燃烧系统燃烧室中混合气的燃烧特性，从而成功地借助示功图分离出对分析整个燃烧过程至关重要的各火焰区中的质量燃烧率和火焰速比等参数，构成了实用的附有双火焰区的三区准维燃烧分析模型。应用实例表明，该函数与多区模型相结合，将在火花点火发动机燃烧模拟中具有强大生命力。     

高压乙烷火焰传播特性的试验研究

对乙烷气体在高压环境中的火焰传播特性进行了试验研究。试验压力从0．1MPa到1．5MPa，在相同的氧气／隋性气体比例下，通过调整氮气与氩气的比例，保持不同反应当量比时的理论绝热燃烧温度不变，得到了等绝热燃烧温度条件下的层流火焰形态及传播速度。试验表明：在相等的绝热燃烧温度条件下，火焰传播特性曲线与非等温条件下有明显的不同；高压环境下的火焰传播特性与常压条件下的火焰传播特性也有较大的不同。     

层流预混滞止火焰结构及传播速度实验研究

火焰传播速度反映了火焰和燃料燃烧的基本特性,但火焰传播比较复杂,受流动,传热以及化学反应等众多因素的影响,在实际的系统中,无法得到理想化的平面燃烧波,而利用平面滞止火焰可外推得到零位伸率下的火焰传播速度,即理想层流火焰传播速度。在建立稳定的层流预混滞止火焰的基础上,用激光多普勒仪测定了滞止流动的速度分布,得到了火焰结构的一些特征,并根据同一空燃比下不同位伸率与当地火焰传播速度的关系,获得了理想层流     

微尺度甲烷扩散火焰特性的数值解析

以均匀空气流中圆管形成的甲烷扩散火焰为对象，用数值解析的方法研究了微尺度扩散火焰的火焰结构和燃烧特性．燃烧反应采用甲烷／空气一步总括反应，喷管壁面绝热．在Re一定的情况下，改变喷口尺寸和喷口流速，考察了微扩散火焰的结构和火焰熄灭的尺度效应．计算结果表明，Re=12条件下，喷口直径为0．07mm时达到熄灭极限；稳定燃烧区的最小总放热率约为0．5W；微尺度条件下，Da对火焰结构和火焰熄灭有显著影响，熄火附近的Da的数量级在0．01．     

9E燃气轮机低氮改造后火焰筒燃烧特性的数值计算

针对改造后的LEC-Ⅲ低NOx燃烧室,运用FLUENT软件对该火焰筒的燃烧流场进行数值模拟。在模拟过程中采用了标准k-ε湍流模型,用SIMPLE算法进行求解,分析了一次模式下火焰筒内速度分布、温度分布、组分分布。结果表明:在一次模式时,火焰筒通过旋流和第一排射流孔形成的回流稳定火焰;文丘里喉道高速气流可以防止预混模式时二级燃烧区的火焰回火,避免一级燃烧区气体再点火燃烧;在一次模式时,火焰筒头部的圆柱形腔室的工作特性相当于六个小型的火焰筒;NOx排放的数值模拟结果与运行值接近。     

汽油机双焰区准维燃烧分析模型

本文针对具有双火焰区燃烧方式的汽油机射流燃烧过程提出了准维分析模型.本模型不仅可以计算出燃烧放热率,燃气平均温度,NO生成规律,还可提供各火焰区的紊流燃烧速度、紊流传播速度和火焰速比等重要参数.计算结果表明,运用本模型可成功地对双焰区汽油机射流燃烧过程进行多方面的综合研究.     

Concentration profile of particles across a flame propagating through an iron particle cloud

The number density profile of particles across a flame propagating through an iron particle cloud has been examined experimentally. The iron particles were suspended in air and ignited by an electric spark. Measurements were performed using high-speed photomicrography combined with laser light scattering technique. It is shown that for relatively large (agglomerated) particles the number density of iron particles changes in the range of x smaller than 11.0 mm, where x is the distance from the leading edge of the combustion zone. The number density increases with the decrease of x in the range 0.6 ≤ x ≤ 11.0 mm, reaches a maximum at x ≈ 0.6 mm, and then decreases. The maximum value of the number density is about 2.6 times larger than that at the region far ahead of the flame (x >11.0 mm). This increase in the number density of particles must cause a change of the lower flammability limit. By assuming that the increase in the number density is caused by the velocity difference of particles from surrounding gas flow, the profile of the number density of particles has been estimated on the basis of measured velocities of particles. The estimated number density profile of particles agrees well with that of the measured profile. The increase in the number density of particles just ahead of the flame will appear not only in iron particle cloud but also in any two-phase combustion systems, such as combustible particle cloud, combustible spray and so on.     

Group combustion of char/carbon particles

Extensive literature exists for the experimental data on coal/char ignition and combustion. While most of the experiments are performed with a cloud or stream of particles, the theoretical modeling used to compare and interpret the experimental data is based on the individual particle combustion (IPC) model. As opposed to individual particle modeling, a group combustion (GC) theory is proposed for the combustion modeling of char/carbon particles. For a cloud of liquid drops, the group behavior implies the formation of a flame (group flame) around a large number of drops rather than a flame around each drop. More generally, the group behavior for a cloud of particles represents the change in the burning characteristics due to collective behavior of particles with or without a group flame. To gain a basic understanding of the group behavior, a model such as the analysis of a spherically symmetric cloud of particles burning in quiescent air is presented here. Each particle within the cloud produces CO, due to both the oxidation of C to CO and the reduction of CO₂ to CO which subsequently oxidizes to CO₂ in the homogeneous gas phase.

Generalized results for the burning rate and the flame structure are given as a function of group combustion number (G). Predicted results show unexpected results including the independence of the burning rate of CO kinetics. Quantitative results for both the cases of frozen and fast CO kinetics are given. There is a group flame for the case of fast CO kinetics. It is shown that the group flame occurs at G > 5 while for a cloud of liquid drops, the group flame occurs at G > 0.1. The higher critical group combustion number is attributed to the lower burning rate of particle inside the cloud compared to the burning rate of liquid drops inside the cloud. The results show that there exists mainly three modes of combustion: (i) Individual Particle Combustion (IPC, low G), (ii) Group Combustion (GC, intermediate G) and (iii) Sheath Combustion (SC, high G). Criteria are given for identifying the mode of combustion from the experimental conditions. The criteria and the establishment of modes of combustion are independent of the extent of CO kinetics. It is found that the experimental data, obtained with a stream of particles and commonly interpreted with the IPC model, indicate the combustion modes to vary from IPC to SC modes. These data are now reinterpreted with the group theory.     

两相流动对流化床燃烧行为的影响

循环床锅炉沿床高的烟气浓度及燃烧份额分布测试结果证明,鼓泡流化床和循环流化床的重要差异表现为密相区燃烧行为的根本不同,由于床料平均粒径较低,循环床密相区的流动不同于鼓泡床,导致气固两相之间的传质阻力增加,从而影响燃烧反应,密相区的燃烧行为表现为欠氧。循环床锅炉沿床高乃至分离器都有燃烧反应发生,建立了考虑气固相间传质阻力的流化床密相区燃烧模型,并与实际循环流化床锅炉的测试数据比较,计算结果与测试值比较吻合。     

The effect of Lewis and Damk?hler numbers on the flame propagation through micro-organic dust particles

In this study, the role of Lewis and Damköhler numbers on the premixed flame propagation through micro-organic dust particles is investigated. It is presumed that the fuel particles vaporize first to yield a gaseous fuel, which is oxidized in the gas phase. In order to simulate the combustion process, the flame structure is composed of four zones; a preheat zone, a vaporization zone, a reaction zone and finally a post flame zone, respectively. Then the governing equations, required boundary conditions and matching conditions are applied for each zone and the standard asymptotic method is used in order to solve these differential equations. Consequently the important parameters on the combustion phenomenon of organic dust particles such as gaseous fuel mass fraction, organic dust mass fraction and burning velocity with the various numbers of Lewis, Damköhler and the onset of vaporization are plotted in figures. This prediction has a reasonable agreement with experimental data of micro-organic dust particle combustion.     

Combustion of particles in a large pulverized brown coal flame

A model of the ignition of a polydisperse cloud of brown coal particles, in a known gas environment, is presented and used to predict the behavior of the particles in a burner jet of a utility boiler. The model allows for drying, devolatilization, and char combustion of the particles. It is assumed that the volatiles burn in the free stream so that char combustion can occur during volatiles evolution, the diffusion of oxygen to the particle surface being inhibited due to the net outflow of volatiles. The model is used to calculate the behavior of a cloud of p.f. size particles along the centerline of a brown coal burner jet in which the gas temperature and composition have been measured. Rates of volatile release and char combustion are calculated and shown to be in agreement with measurements of volatile material in the flame. It is found that particles smaller than about 80 μm contribute most to the ignition of the jet and that they closely follow the local gas temperature. The unique character of brown coal of combustion, its high volatile evolution on rapid heating, the high activity of its char at low temperature, and the demonstrated ignition of its char without a jump in temperature make the overlap of devolatilization and char combustion more likely than with other coals. The mathematical formulation that allows this overlap gives oxygen consumption levels consistent with measurement. An analysis is made of the relative importance of radiation from the flame front to the particle, and entrainment of hot combustion gases into the jet. It is found that the radiation is of secondary importance compared to the effect of entrainment which is the controlling mechanism in the initial heating of the particles. Also, the significance of the assumption that the volatiles burn in the free stream is discussed.     

A model of the combustion of a single small coal particle using kinetic parameters based on thermogravimetric analysis

A mathematical model for the combustion in air of a single entrained spherical coal particle, 30 μm in diameter, has been developed incorporating thermogravimetric analysis data of Whitwick coal. The model is based on a set of ordinary differential equations, describing the reaction rates and the mass and heat transport processes. The system of equations was solved numerically. The combustion mechanism of the particle was described by locating the reaction zone at the solid surface, where gas-phase combustion of volatiles and heterogeneous reaction between gaseous oxygen and the carbon and hydrogen in the solid occurred in parallel. The combustion process was chemical-reaction-rate-controlled, with the oxygen partial pressure at the surface almost that of the surrounding bulk gas. The simulation results using this model, with the kinetic parameters for devolatilization and combustion derived from the experimental thermogravimetric data, are consistent with previously reported combustion lifetimes of approximately 1 s, for particles of this size and rank. They are also consistent with the anticipation that higher ambient gas temperatures should result in shorter burn-out times. The use of thermogravimetric data in the modelling of the combustion of small particles of these low-rank coals is a potentially valuable method for characterization of feedstocks for pulverized coal-fired boilers. © 1998 John Wiley & Sons, Ltd.     

Interactive processes in gasification and combustion—Part III: Coal/char particle arrays, streams and clouds

A comprehensive review is presented on the interactive transport processes in the gasification and combustion of a cloud of drops and solid particles. The review is divided into three parts. Part I is concerned with the interactive processes for arrays, streams and clouds of drops, Part II presents a review of isolated coal, carbon and porous char particles pertaining to interactive processes, and finally Part III deals with the interactive processes for solid particle arrays, streams and clouds. Isolated particle gasification (pyrolysis and heterogeneous) and combustion were briefly reviewed in Part II. Because of strong analogy of the group ignition and combustion, to porous char ignition and combustion, the literature on porous char combustion was also included in Part II and new results were presented on the internal ignition of porous char particle using Frank-Kamanetsky type of analysis. Part III presents an integrated approach starting from arrays to clouds and gasification to combustion. The interactions occur through processes ranging from pure diffusive to convective transport processes. Approximate criteria for interactive processes are given.

As opposed to liquid drop arrays and clouds, there is no systematic study for arrays of char or coal particles. Due to the similarity between droplet evaporation and char combustion, new results are presented for the combustion of char arrays in quiescent atmosphere. Convective effects are also briefly discussed. Expanding the Frank-Kamenetsky analysis to ignition of clouds, simple solutions are presented for cloud ignition temperatures. A comparison of results between different techniques and between theory and experiment is given.

Interesting results for the ignition characteristics of coal dispersions are obtained in that the particles with relatively small or low volatile matter which ignite heterogeneously when isolated are found to ignite homogeneously under cloud conditions. The minimum ignition temperature is found to increase with decrease in size under isolated mode while the opposite is true under interactions. The problems of the gasification, ignition, and combustion of clouds in confined and unconfined volumes are reviewed.

Experiments conducted with streams (laminar flow reactors, LFR) and clouds (TGA, heated grids, shock tubes, batch of particles in LFR, Hertzberg Ignition apparatus) are reviewed. Following the drop literature, the relation between array and group combustion is presented. Finally, the relevance of the reviewed literature to pollutants' formation and destruction and spray combustion modeling is briefly discussed.     

On the burning behavior of pulverized coal chars

A model that predicts the physical changes that pulverized coal char particles undergo during combustion has been developed. In the model, a burning particle is divided into a number of concentric annular volume elements. The mass loss rate, specific surface area, and apparent density in each volume element depend upon the local particle conditions, which vary as a consequence of the adsorbed oxygen and gas-phase oxygen concentration gradients inside the particle. The model predicts the particle's burning rate, temperature, diameter, apparent density, and specific surface area as combustion proceeds, given ambient conditions and initial char properties. A six-step heterogeneous reaction mechanism is used to describe carbon reactivity to oxygen. A distributed activation energy approach is used to account for the variation in desorption energies of adsorbed O-atoms on the carbonaceous surface. Model calculations support the three burning zones established for the oxidation of pulverized coal chars. The model indicates two types of zone II behavior, however. Under weak zone II burning conditions, constant-diameter burning occurs up to 30% to 50% conversion before burning commences with reductions in both size and apparent density. Under strong zone II conditions, particles burn with reductions in both size and apparent density after an initial short period (<2% conversion) of constant-diameter burning. Model predictions reveal that early in the oxidation process, there is mass loss at constant diameter under all zone II burning conditions. Such weak and strong burning behavior cannot be predicted with the commonly used power-law model for the mode of burning employing a single value for the burning mode parameter. Model calculations also reveal how specific surface area evolves when oxidation occurs in the zone II burning regime. Based on the calculated results, a surface area submodel that accounts for the effects of pore growth and coalescence during combustion under zone I conditions was modified to permit the characterization of the variations in specific surface area that occur during char conversion under zones II conditions. The modified surface area model is applicable to all burning regimes. Calculations also indicate that the particle's effectiveness factor varies during conversion under zone II burning conditions. With the adsorption/desorption mechanism employed, a near first-order Thiele modulus-effectiveness factor relationship is obeyed over the particle's lifetime.     

Ignition and combustion of boron particles

The successful development of highly energetic boron-based propellants will require a thorough knowledge of the chemical and physical processes controlling ignition and combustion of single boron particles. Significant recent progress on various problems in the field of boron particle combustion makes our goal prospective.

This paper reviews our current understanding of the ignition and combustion processes of single boron particles, including a comprehensive survey of the previous experimental work, theoretical models, and chemical kinetics studies. In addition, this paper presents up-to-date research findings which represent two major research needs strongly recommended by previous researchers.

An experimental diagnostics was developed to study the ignition and combustion of fine-size single particles. The measured ignition delay and two-stage combustion times of single boron particles with sizes of 3 μm are presented. Effects of gas temperature, oxygen concentration, and magnesium coating on boron combustion were examined.

An investigation was performed to resolve long-term contradicting theories regarding the mechanisms which governed the species diffusion across the liquid B₂O₃ layer covering a single boron particle at elevated temperatures. Observations showed that the diffusion of dissolved boron into molten B₂O₃₍₁₎ dominates the diffusion process through the liquid layer. Subsequently, a polymeric vitreous (BO)_n complex is formed through the reaction between dissolved boron and molten B₂O₃₍₁₎. Based upon these experimental findings, an appropriate boron reaction mechanism was obtained by deduction. A theoretical model was then developed to simulate boron particle combustion in two consecutive stages.