外界热辐射作用下木材热解和着火的预测模型

在突发的火灾过程中，可燃性固体材料的着火时间和热解特性是非常关键的参数。建立了预测木材在外界热辐射作用下热解和着火时间的数值模型，可以给出固体的质量损失速率、内部温度场随时间的变化，并根据模型计算得到的表面温度随时间的变化对着火时间进行预测。计算结果和实验测量值相符较好。     

Effects of thermal radiation heat transfer on flame acceleration and transition to detonation in particle-cloud hydrogen flames

The current work examines regimes of the hydrogen–oxygen flame propagation and ignition of mixtures heated by radiation emitted from the flame. The gaseous phase is assumed to be transparent for the radiation, while the suspended particles of the dust cloud ahead of the flame absorb and reemit the radiation. The radiant heat absorbed by the particles is then lost by conduction to the surrounding unreacted gaseous phase so that the gas phase temperature lags that of the particles. The direct numerical simulations solve the full system of two phase gas dynamic time-dependent equations with a detailed chemical kinetics for a plane flames propagating through a dust cloud. It is shown that depending on the spatial distribution of the dispersed particles and on the value of radiation absorption length the consequence of the radiative preheating of the mixture ahead of the flame can be either the increase of the flame velocity for uniformly dispersed particles or ignition either new deflagration or detonation ahead of the original flame via the Zel'dovich gradient mechanism in the case of a layered particle-gas cloud deposits. In the latter case the ignited combustion regime depends on the radiation absorption length and correspondingly on the steepness of the formed temperature gradient in the preignition zone that can be treated independently of the primary flame. The impact of radiation heat transfer in a particle-laden flame is of paramount importance for better risk assessment and represents a route for understanding of dust explosion origin.     

热颗粒和热辐射共同作用阴燃点燃松针燃料床：数值研究

建立了热颗粒和热辐射共同作用下阴燃点燃松针燃料床的二维数值模型,计算得到了热颗粒和热辐射单独点燃以及共同作用点燃的临界条件,分析了燃料床的点燃过程。与前人实验数据对比表明数值模型能够较好预测临界点燃条件。金属热颗粒单独作用时点火所需的临界温度与颗粒直径呈双曲线关系。金属热颗粒与热辐射共同作用时,临界辐射热通量呈现出随颗粒温度和直径的增加而显著减小的趋势。热辐射持续供热能有效维持表层炭氧化反应,两者共同作用下点燃危险性增加。研究同时发现,燃料含水率对共同作用下的临界辐射热通量有较大影响。     

A numerical model for the prediction of the minimum ignition temperature of dust clouds

This paper presents a numerical model for the prediction of the minimum ignition temperature (MIT) of dust clouds. First, a physical model is developed for the dust cloud ignition in the Godbert-Greenwald furnace. A numerical approach is then applied for the MIT prediction based on the physical model. The model considers heat transfer between the air and dust particles, the dust particle reaction kinetics, and the residence times of dust clouds in the furnace. In general, for the 13 dusts studied, the calculated MIT data are in agreement with the experimental values. There is also great accordance between the experimental and numerical MIT variation trends against particle size. Two different ignition modes are discovered. The first one consists in ignition near the furnace wall for bigger particles characterized by rather short residence times. In the second mode, the ignition starts from the center of the furnace by self-heating of the dust cloud for smaller particles with longer residence times. For magnesium, as dust concentration increases, the lowest ignition temperature of the dust cloud IT(conc) decreases first, then transits to increase at a certain point. The transition happens at different dust concentrations for different particle sizes. Moreover, the MIT of the magnesium dust cloud generally increases as particle size increases, but the increasing trend stagnates within a certain medium particle size range.     

飞火和热辐射耦合作用点燃松针的实验研究

飞火颗粒点燃和火焰辐射引燃是森林-城镇交界域火灾两种主要引燃方式。已有很多研究关注单种引燃方式,但缺乏二者耦合作用下的引燃机理研究。实验研究了热颗粒与热辐射耦合作用下松针燃料床的点燃行为,分析了热颗粒尺寸、温度及辐射热通量对点燃特性的影响。实验结果表明:热颗粒和热辐射耦合作用的点燃危险性远远高于热颗粒或者热辐射单独作用下的点燃危险性。耦合作用时,点燃概率随颗粒尺寸、温度和辐射热通量增大而增大,热颗粒临界点燃温度以及临界辐射热通量均随热颗粒尺寸增大而降低;阴燃向明火转变的点燃时间随辐射热通量增大而降低,但与颗粒状态呈现出较为复杂的关系。     

Dust ignition of pure and encapsulated paraffin phase change materials

In this study, the dependence of the flammable concentration on particle size is investigated for Phase Change Material (PCM) and Encapsulated Phase Change Material (EPCM) particles using a novel continuous particle dispersion apparatus into which a propane flame is introduced creating a test akin to the flash-point test for liquids. The results show that the threshold concentration is a strong function of particle size. For tested particles with size ranging from 290 μm to 750 μm, the threshold concentration is above the predictions based on an instantaneous heat transfer limit, and is approximately linear with the particle size, following a heat transfer limited ignition model. For sizes above ≈1 mm, the particles behave like the bulk material, and ignition is not observed for the concentrations tested. The results obtained here are important for the safe construction, handling, and operation of systems using PCM and other particles.     

固体可燃物易点燃性标准的探讨

可燃性固体的引燃和熄灭与辐射到其表面的临界热能量和产生挥发分所必需的表面温度有关。探讨固体可燃物易点燃性标准,燃怀标准,对防火和灭火在理论上有一定的指导意义。本文在理论分析的基础上,提出了一种用固体挥发分的临界质量通量量化解释引燃和熄灭条件的方法,给出了易点燃性的标准。     

木材在线性增长热流下着火的复合判据研究

为探索木材在热流变化时的着火特点并提出着火判据,通过试验研究了木材在线性增长热流条件下的自发着火性能,测定了泡桐、椿木、榆木和刺槐4种木材的点燃时间、木材表面的入射热流以及试样内部的温度.结果表明.当热流增长率大于等于0.199kw/(m~2·s)时,试验的所有木材均可以被点燃,点燃这些木材的临界热流增长率介于0.065～0.103 kw/(m~2·s).建立了木材着火的计算模型,计算得出木材自发着火时的表面温度约为500℃.根据试验和计算的结果提出了-结合表面温度和临界热流增长率的木材自发着火复合判据.     

A theoretical model for the prediction of the minimum ignition energy of dust clouds

In this study, a physical model of the dust cloud ignition process is developed for both cylindrical coordinates with a straight-line shaped ignition source and spherical coordinates with a point shaped ignition source. Using this model, a numerical algorithm for the calculation of the minimum ignition energy (MIE) is established and validated. This algorithm can evaluate MIEs of dusts and their mixtures with different dust concentrations and particle sizes. Although the average calculated cylindrical MIE (MIE_cylindrical) of the studied dusts only amounts to 63.9% of the average experimental MIE value due to reasons including high idealization of the numerical model and possible energy losses in the experimental tests, the algorithm with cylindrical coordinates correctly predicts the experimental MIE variation trends against particle diameter and dust concentration. There is a power function relationship between the MIE and particle diameter of the type MIE ∝ d_p^k with k being approximately 2 for cylindrical coordinates and 3 for spherical coordinates. Moreover, as dust concentration increases MIE(conc) first drops because of the decreasing average distance between particles and, at fuel-lean concentrations the increasing dust cloud combustion heat; however, after the dust concentration rises beyond a certain value, MIE(conc) starts to increase as a result of the increasingly significant heat sink effect from the particles and, at fuel-rich concentrations the no longer increasing dust cloud combustion heat.     

Parametric study of the explosivity of graphite-metals mixtures

The explosivity of dust clouds is greatly influenced by several parameters which depend on the operating conditions, such as the initial turbulence, temperature or ignition energy, but obviously also on the materials composition. In the peculiar case of a mixture of two combustible powders, the physical and chemical properties of both dusts have an impact on the cloud flammability and on its explosivity. Nevertheless, no satisfactory ‘mixing laws’ predicting the mixture behavior are currently available and the composition variable to be considered for such models greatly depend on the safety parameters which have to be determined: from volume ratios for some thermal exchanges and ignition phenomena, to surface proportions for some heterogeneous reactions and molar contents for chemical reactions. This study is mainly focused on graphite/magnesium mixtures as they are encountered during the decommissioning activities of UNGG reactors (Natural Uranium Graphite Gas). Due to the different nature and reactivity of both powders, these mixtures offer a wide range of interests. Firstly, the rate-limiting steps for the combustion of graphite are distinct from those of metals (oxygen diffusion or metal vaporization). Secondly, the flame can be thickened by the presence of radiation during metal combustion, whereas this phenomenon is negligible for pure graphite. Finally, the turbulence of the initial dust cloud is modified by the addition of a second powder. In order to assess the explosivity of graphite/magnesium clouds, a parametric study of the effects of storage humidity, particle size distribution, ignition energy, and initial turbulence has been carried out. In particular, it was clearly demonstrated that the turbulence significantly influences the explosion severity by speeding up the rate of heat release on the one hand and the oxygen diffusion through the boundary layer surrounding particles on the other hand. Moreover, it modifies the mean particle size and the spatial dust distribution in the test vessel, impacting the uniformity of the dust cloud. Thus, the present work demonstrates that the procedures developed for standard tests are not sufficient to assess the dust explosivity in industrial conditions and that an extensive parametric study is relevant to figure out the explosive behavior of solid/solid mixtures subjected to variations of operating conditions.     

辐射热流作用下树叶样品的燃烧特征研究

研究植物叶样在外部热辐射和值班火源作用下的燃烧现象特征并探索其成因和物理及化学本质。采用锥形量热仪开展实验,热辐射强度分别设定为35kW m-2、55kW m-2、70kW m-2和85kW m-2。实验样品为针阔叶树种共13种,其含水率在45%至79%间变化。实验表明,样品表现出不同的着火模式,有的为明火,有的则为阴燃,取决于树种和设定的辐射强度。样品出现有焰燃烧现象的最低热释放速率峰值(PHRR)在22.3kW m-2至35.6kW m-2之间,反映了形成气相火焰所需最低挥发分质量流率。气相产物CO2产生速率的峰值与PHRR呈高度线性性,表明了不同样品间气相燃烧或固相表面氧化(阴燃燃烧)反应的相似性。进一步分析表明,出现的独特热释放速率尖锐峰形是样品分层特性以及树叶的物理属性和热物性共同作用的结果。具有热薄特性的样品表层,在实验初期是接受外部辐射热的主体,其热解的产物是形成气相火焰的物质来源;在其转变为焦炭层后,对辐射热流向内层的渗透具有阻挡作用。建立起来的认识对于评估分层样品的燃烧性有一定的指导意义。     

Modeling quenching distance and flame propagation speed through an iron dust cloud with spatially random distribution of particles

In this research combustion of iron dust particles in a medium with spatially discrete sources distributed in a random way has been studied using a numerical approach. A new thermal model is generated to estimate flame propagation speed and quenching distance in a quiescent reaction medium. The flame propagation speed is studied as a function of iron dust concentration and particle diameter. The predicted propagation speeds as a function of these parameters are shown to agree well with experimental measurements. In addition, the minimum ignition energy has also been investigated as a function of equivalence ratio and particle diameter. The quenching distance has been studied as a function of particle diameter and validated by the experiment. Considering random distribution of particles, the obtained results provide more realistic and reasonable predictions of the combustion physics compared to the results of the uniform distribution of particles.     

Simulation of the flow of an explosive atmosphere exposed to a hot surface

The accidental ignition of combustible atmospheres by hot surfaces is of great concern for chemical and process plant safety. In this paper, we present our research regarding the evolution of thermal plumes originating from hot hemispheres and discs. In particular, we focus on the effect of the orientation of the surface on the ignition process. The auto-ignition temperatures and ignition locations were studied experimentally. To get further insight, we conducted detailed numerical simulations and validated them with measurements. Three-dimensional simulations were performed on hot hemispheres and hot discs for different orientations ranging from 0° to 180°. The solver employs a transient, implicit scheme which is based on the coupled heat transfer and flow equations. The mesh in the vicinity of the hot surfaces is refined to resolve the steep temperature gradients and to capture the boundary layer separation. The influence of the orientation on critical hot spots in the gas mixture is analysed by examining the flow structures and the temperature evolution of the buoyancy-driven flow. Using the obtained results, we discuss the change of the onset and location of the ignition.     

不同点火模式下软质聚氨酯泡沫燃烧的实验研究*

为研究软质聚氨酯泡沫（FPUF）的燃烧行为对火灾的影响,采用锥形量热仪（CONE）分析FPUF在强制点火和非强制点火模式下的燃烧行为、热释放速率HRR、质量损失速率MLR和燃烧效率η。研究结果表明:当外部辐射≥40 kW/m2时,FPUF在2种点火模式下燃烧的HRR和MLR均出现2个明显的峰值且η约为88%;当外部辐射<40 kW/m2时,FPUF在非强制点火模式下燃烧的HRR和MLR曲线趋于单峰形式;FPUF在强制点火模式下燃烧前期为异氰酸酯（TDI）和多元醇的混合燃烧,在非强制点火模式下为TDI的挥发。根据研究结果修正现有燃料组分的热值计算方法,获得FPUF和多元醇的热值分别为（20±2）,（28±3） kJ/g。     

矿井下着火点位置数值模拟研究

矿井煤炭自燃是煤矿五大自然灾害之一,而煤炭自燃隐蔽着火点位置的确定是解决煤炭自燃问题的关键。本文结合实际情况将火源位置的确定问题归结为热传导方程的寻源反问题。在理想状态下,把煤矿井下热传导的三维模型简化为二维模型,建立热传导方程及初始条件和边界条件,在matlab中编制了有限差分程序对井下着火点的温度场进行了正反演模拟。数值模拟结果表明:该方法能够较准确地反演出火源的特性,并随着离散化程度的提高,离散解逐渐逼近真实解。通过本文的数值模拟我们得知有限差分法求解热传导寻源反演的方法是解决矿井隐蔽火源发火点位置的有效途径之一,对矿井防、灭火研究具有较高的理论和实际应用价值。     

Detonation flows in aluminium particle gas suspensions,inhomogeneous in concentrations

A physical and mathematical model of the reduced kinetics is presented describing heterogeneous detonation in suspensions non-uniform in particle concentration. The model is based on the heterogeneous media approaches, semi-empirical laws of ignition and combustion, and data on the dependence of the detonation velocity on particle concentration. Formation of suboxides and incomplete combustion of aluminum are taken into account integrally. The dependence of the heat release of chemical reactions and the fraction of unburnt particles on the initial composition is determined from the solution of the stationary problem of the structure of the detonation wave. In the calculations of unsteady detonation flows, it is supposed to solve an additional equation for the spatial distribution of initial concentrations. The problems of initiation and development of cellular detonation in flat channels in suspensions of micron-sized aluminum particles are studied. Dependences of the cell size on particle concentration in uniform suspensions are determined. The flow patterns of cellular structures, the forms of the leading front, and the propagation velocities in channels with longitudinal or transversal gradients of particle concentration are analyzed.     

基于正交试验的谷物粉尘燃烧特性影响因素研究

为了研究谷物粉尘燃烧特性影响因素，以可食用玉米淀粉为研究对象，通过ZY6243锥形量热仪（CONE）进行了单因素和正交试验，研究了粉尘粒径、惰性介质CaCO3质量分数、热辐射通量对玉米淀粉燃烧特性的影响效应。应用直观分析法和方差分析法定性地研究了各因素对玉米淀粉热释放速率峰值的影响程度并进行了排序，2种方法得出的结论基本一致。研究结果表明:各因素对玉米淀粉热释放速率峰值的影响程度依次为:CaCO3质量分数＞粉尘粒径＞热辐射通量；其中惰性介质CaCO3对热释放速率峰值和引燃时间的影响最为显著。     

球形压力容器中甲烷-氢气-空气爆炸过程数值模拟及实验研究

为研究受限空间内甲烷-氢气-空气混合气体爆炸特性参数分布规律,在20 L球形压力容器装置内开展甲烷-氢气-空气混合气体爆炸实验,探究掺氢比变化对当量比为1的甲烷-氢气-空气混合气体爆炸过程的影响;运用Fluent数值模拟软件,采用标准k-ε湍流模型,结合层流有限速率燃烧模型,探究混合气体爆炸过程中燃烧特性(爆炸温度、压力、密度等)与反应时间的变化规律。研究结果表明：爆炸过程中,添加一定氢气时爆炸压力峰值、爆炸压力上升速率峰值增大,而到达峰值时间缩短;反应初期,中心点火处密度下降,反应釜各处密度持续上升;距离点火点越远,密度变化越大,反应釜中压力分布基本相同。研究结果可为甲烷-氢气-空气混合燃料的安全使用提供相关参考。     

About stability of the ignition process of small solid particle

The distributed mathematical model of magnesium particle ignition is developed taking into account the heterogeneous chemical reaction and the domain of particle thermal influence on gas. The problem solvability in the stationary case has allowed one to expand classification of regular modes of heating as well as the modes of extinction and ignition of a particle. The limiting size of a gas layer near a particle is found that determines an ignition mode. It is shown that the ignition delay time grows if we take into account the gas layer near the particle. Stability of some heating modes within the framework of finite and infinitesimal perturbations is studied. An opportunity to control the ignition process by high-frequency thermal action on the unstable particle gas states is shown.