森林阴燃火排放特性：研究前沿与展望

森林阴燃火是世界上最大尺度的燃烧现象之一,其排放在全球大气碳循环中扮演着重要的角色。然而其产生并累积的复杂排放物会导致区域性的雾霾灾害,严重威胁居民和消防人员的健康安全。尽管林火排放给社会经济生活带来诸多不良影响,但目前对森林阴燃火排放特性的科学认识还不足。本文总结了阴燃与明火排放的差异、影响阴燃排放的因素,以及在不同尺度研究中用于不同森林可燃物排放的测量方法。此外,本文还统计汇编了文献中包含主要火灾气体和颗粒物产物在内的排放因子数据,分析发现除CO₂及CO外,森林阴燃火产物的排放因子随分子质量的增加而逐渐减少。本文有助于提高对阴燃排放的科学认知,为防治由阴燃林火导致的雾霾灾害提供了丰富的数据基础。     

木质材料火灾研究：前沿与展望

古建筑保护和现代木结构建筑发展受制于木质材料的火灾安全问题。本文综述了木材火灾动力学的燃烧现象与理论,总结其热解、点火、火蔓延和熄火的燃烧行为,并分析了材料特性、系统属性和火灾环境对燃烧行为的影响。同时,总结了近年木材火灾研究的常用实验方法及其燃烧数值模型的主控方程。最后,探讨了木材的火灾动力学与林火和生态学、材料学、结构力学的交叉研究,以期对未来木质材料火灾的多学科交叉研究提供必要的参考和指导。     

火灾科学概论

 一、引言火是人类的朋友,也是人类的敌人。从远古时期,人类发现了火,并利用它取暖、熟食,在不断地学习和掌握用火的过程中,实现了人类生活由野蛮到文明的飞跃.半个多世纪来,为了给研制和设计高效、净化的燃烧装置提供科学依据、燃烧学应运而生.近20年来,人们开展了对火灾发生、发展和防治机理的研究,逐步形成了和发展了一门新的学科一火灾科学.二、发展简史失去控制的火,给人类生命财产和自然资源带来损失,则成为火灾.火灾现象遍及城市与乡村,包括森林、建筑、油类,装备设施和交通工具等火灾,以及可燃气体和粉尘的爆炸等.     

不同燃烧状态飞火颗粒自由燃烧规律研究

飞火是开放空间中大尺度火灾非连续性蔓延的主要形式。本文通过不同热流下的木质飞火颗粒自由燃烧实验,揭示不同燃烧状态飞火颗粒的结构变形、质量损失及温度分布的变化规律。研究表明,颗粒结构变形受材料化学反应机制和热机械力作用共同影响;颗粒燃烧反应易造成热解气体的内部积聚,以致内压激增、诱发喷射或喷溅细小颗粒的现象;阴燃过程颗粒表面温度变化较小但持续时间很长,明火状态的颗粒持续高温并且温度与质量变化剧烈。     

森林火行为计算机模拟的尝试

森林火灾是灾害性的复杂燃烧现象,通常分为地下火、地表火和树冠火。地下火在地下可燃物诸如泥煤、有机土壤中以焖烧极缓慢地蔓延;树冠火是火焰锋在树木顶部推进,特点是火强度大,蔓延迅速,它是大火灾环境中活跃的火行为;地表火则是在森林地表植被诸如荒草、灌木上蔓延,是最常见的林火现象。     

生物质内部燃烧特性的阴燃实验研究

根据生物质颗粒内部燃烧过程和生物质粉自然向下阴燃过程的共性,采用阴燃实验的方法研究了生物质内部燃烧特性。考查了物料种类、含水率、孔隙尺寸对生物质内部燃烧温度,燃烧中干燥前沿、炭氧化前沿的移动速度,裂纹、气体成分等的影响。实验结果为生物质燃烧和阴燃过程模拟以及深入理论分析提供了依据。     

无人机视角下的林木爆燃火特性研究（英文）

森林火灾"爆燃"现象的特征是突然发生的高强度、高蔓延速度的燃烧。目前为止,关于"爆燃火"的原因还没有达成共识。以无人机视角下对林木爆燃火特性研究,以四川木里特大森林火灾为研究对象,通过分析凉山州某森林扑火部队3月31日木里森林火灾当天KWT(科卫泰)无人机航拍火场画面,结合无人机实时影像及实地调研数据,分析了峡谷地形林火蔓延时空特征,探讨了峡谷中风向风速变化时空分布规律,研究了地形变化条件下,不同海拔高度风速特征,建立了无人机倾角测量风速模型(其中为风速m·s~(-1),为无人机倾角°)。结果表明,高山峻岭特殊地形环境下每天4:00—12:00时间段为静风期,为峡谷林火扑救最佳时期;午后15:00—17:00和晚上20:00—22:00为山谷地形风速活跃期;仿真软件数据显示山顶、谷底与山腰不同海拔位置的风速风向不统一,谷底会产生乱流现象,且风速与海拔不存在正相关关系,小气候在复杂地形中占主导影响地位;在谷口至山谷深处的中间位置会出现气流速度的波峰状态,并易形成乱流,为爆燃火发生提供了客观必要条件。该研究可为复杂地形环境下,森林草原火灾扑救安全提供数据和技术支撑。     

无人机视角下的林木爆燃火特性研究

森林火灾“爆燃”现象的特征是突然发生的高强度、高蔓延速度的燃烧。目前为止，关于“爆燃火”的原因还没有达成共识。以无人机视角下对林木爆燃火特性研究，以四川木里特大森林火灾为研究对象，通过分析凉山州某森林扑火部队3月31日木里森林火灾当天KWT（科卫泰）无人机航拍火场画面，结合无人机实时影像及实地调研数据，分析了峡谷地形林火蔓延时空特征，探讨了峡谷中风向风速变化时空分布规律，研究了地形变化条件下，不同海拔高度风速特征，建立了无人机倾角测量风速模型（其中为风速m·s-1，为无人机倾角°）。结果表明，高山峻岭特殊地形环境下每天4:00—12:00时间段为静风期，为峡谷林火扑救最佳时期；午后15:00—17:00和晚上20:00—22:00为山谷地形风速活跃期；仿真软件数据显示山顶、谷底与山腰不同海拔位置的风速风向不统一，谷底会产生乱流现象，且风速与海拔不存在正相关关系，小气候在复杂地形中占主导影响地位；在谷口至山谷深处的中间位置会出现气流速度的波峰状态，并易形成乱流，为爆燃火发生提供了客观必要条件。该研究可为复杂地形环境下，森林草原火灾扑救安全提供数据和技术支撑。     

燃池内的阴燃过程的实验分析研究

为了分析燃池内的燃烧过程,设计制造了水平和竖直的纤维质燃料阴燃实验台,进行了 阴燃过程气体成分的实验分析。实验结果给出了在不同的阴燃阶段ＣＨ４、Ｈ２、ＣＯ和ＣＯ２等 气体的生成规律,以及含水量、空气量和阴燃传播方向等对生成气体的影响,为深入研究阴燃的机 理,提高燃池的用能水平提供了必要的依据。     

隧道内固体与液体燃料火灾特征对比试验研究

本文建立了1/9的缩尺寸隧道火灾模型,对隧道内固体和液体燃料火灾特征的差异及两者之间的相互影响进行了试验研究,分析了隧道内固液体燃料着火时的火场温度、烟气层高度、燃料质量损失速率等特征参数的变化情况,试验结果表明:虽然甲醇与木垛质量相同,且平均火源功率相等,但木垛起火阶段发展较慢,其热释放速率峰值明显较甲醇火的高,且相应的该阶段所引起的隧道顶棚温度也较高,而甲醇起火阶段燃烧较迅速,且到达峰值热释放速率后维持稳定燃烧的时间比木垛要长.同时,木垛火燃烧时隧道内的烟气温度分层较甲醇火的更加明显;当两种火源同时存在时,隧道内温度的最高点偏向于木垛火上方一侧,且此时甲醇油盘火源上方温度下降时间明显提前,而木垛火的燃烧受到的影响较小.     

Combustion dynamics of large-scale wildfires

A fact often overlooked is that large-scale wildfires, although occurring infrequently, are responsible for the overwhelming majority of fire-related suppression costs, economic losses, and natural resources damages. Fortunately, the increasingly severe problems of large-scale wildfires worldwide have been receiving ever-growing academic attention. The high-intensity burning behaviors in wildfires stem from the significant interaction of combustion with heat transfer and atmospheric flow under complicated fuel, meteorology, and topography conditions. Therefore, mitigating measures against large-scale wildfire disasters have grown into a challenging research focus for combustion scientists. Research over the past century has resulted in incrementally enhanced insights into the mechanisms of combustion dynamics underlying the various erratic behaviors in large-scale wildfires, with theories and models of fire accelerations developed and validated. These advances are expected to improve the efficacy of large-scale wildfire predictions significantly. Nevertheless, the physical interpretation of the acceleration of large-scale wildfires is far from adequate and complete. This paper intends not to make a comprehensive review of the entire wildfire research field, but to depict an overall pattern of the essential factors that lead an initial small-scale spreading flame to a large-scale wildfire beyond control. It is outlined that the complicated transformation of fuel preheating mechanisms determines the growth of surface fire spread, while varied large-size flame fronts and unique spread modes induced in specific fire environments play an essential role in fire spread acceleration. Additionally, multiple fires burning and merging often act as crucial steps for accelerating surface fire spread, generating large-size flames, and triggering unique spread modes. These major potential factors strike the energy balance of a low-intensity wildfire and push it to a high-intensity state. Several issues regarding intensely burning behaviors in large-scale wildfires are selected for in-depth discussions, for which an overview of the progress and challenges in research is presented. It is concluded that the fundamental exploration targeted at developing application tools capable of dealing with large-scale wildfires remains at its early stages. Opportunities for innovation are abundant, yet systematic and long-term research programs are required.     

Modeling of one-dimensional smoldering of polyurethane in microgravity conditions

Results are presented from a model of forward smoldering combustion of polyurethane foam in microgravity. The transient one-dimensional numerical-model is based on that developed at the University of Texas at Austin. The conservation equations of energy, species, and mass in the porous solid and in the gas phases are numerically solved. The solid and the gas phases are not assumed to be in thermal or in chemical equilibrium. The chemical reactions modeled consist of foam oxidation and pyrolysis reactions, as well as char oxidation. The model has been modified to account for new polyurethane kinetics parameters and radial heat losses to the surrounding environment. The kinetics parameters are extracted from thermogravimetric analyses published in the literature and using Genetic Algorithms as the optimization technique. The model results are compared with previous tests of forward smoldering combustion in microgravity conducted aboard the NASA Space Shuttle. The model calculates well the propagation velocities and the overall smoldering characteristics. Direct comparison of the solution with the experimental temperature profiles shows that the model predicts well these profiles at high temperature, but not as well at lower temperatures. The effect of inlet gas velocity is examined, and the minimum airflow for ignition is identified. It is remarkable that this one-dimensional model with simplified kinetics is capable of predicting cases of smolder ignition but with no self-propagation away from the igniter region. The model is used for better understanding of the controlling mechanisms of smolder combustion for the purpose of fire safety, both in microgravity and normal gravity, and to extend the unique microgravity data to wider conditions avoiding the high cost of space-based experiments.     

水雾与可燃多孔介质火焰相互作用的实验研究

采用非传播扩散火焰形式,研究了固体可燃多孔介质中水雾-火焰的相互作用,探讨了床层厚度、多孔介质粒度以及燃料预燃时间对水雾灭火效果的影响。结果表明,随着预燃时间增长,水雾停止后床层内部可能发生闷烧现象,随着燃料粒度的减小,这种闷烧几率反而降低。这表明,固体火焰与水雾的相互作用有其独特之处。     

On the importance of the heat release rate in numerical simulations of fires in mechanically ventilated air‐tight enclosures

A fire in a compartment with limited ventilation can cause a significant pressure rise, up to hundreds of Pascal. This is important in practice, as the pressure rise can cause damage or hinder evacuation, but also from the perspective of fire safety science. From the energy balance, taking into account the interaction between compartment pressure, fire dynamics and mechanical ventilation, the importance of the net heat gained per unit time in the gas phase is well recognized. This leads to the need to accurately quantify the heat release rate inside the compartment as a function of time. It is explained that scaling of the transient phenomena is not straightforward. The paper then focuses on numerical simulations, in particular on CFD in the gas phase. An overview is presented of different existing approaches for turbulent combustion modelling in turbulent buoyancy-driven flames with low values of scalar dissipation rate, typical for fire flames. A dynamic approach for modelling turbulent combustion, and the coupling with radiation modelling, is briefly discussed. Extinction and re-ignition are discussed extensively, in the context of reduced ventilation conditions. Finally, low-frequency oscillatory behaviour in mechanically ventilated air-tight compartments is addressed. It is argued that CFD simulations are a very valuable tool to gain further insight in this phenomenon. Suggestions for future research are formulated.     

基于红外热成像的林地余火死灰复燃点诊断方法研究

林地余火阴燃特性具有隐蔽性强、持续时间长、目测难度大、且具有死灰复燃等特点,从而一直困扰森林火灾的彻底扑灭。为了及时、高效地发现林地余火阴燃点,探索林地余火死灰复燃的特征及规律,在南京森林警察学院点烧基地里进行测试实验,以无人机搭载热红外成像系统、气象采集系统等为工具,把火源点设置在杨树林内,人为干预进行点烧、熄灭、复燃等重复实验,实验包括白天和夜晚两个时间段,用安装于无人机上的红外热成像仪对火源进行观测。实验表明：林地余火死灰复燃的温度在500～600℃,离散程度较大;林地余火死灰复燃在白天的时间普遍短于夜晚时间,表明外界温度越高,越会促进林地余火死灰复燃的速率;在森林中,不同地点、不同时间段的森林背景温度标准差比较稳定,主要处于1～9 之间;林地余火红外图像的温度数据的标准差值分布在30～85之间;红外图像的温度数据的标准差值分布在55～85 之间,则可定为死灰复燃可疑阶段。该方法量化了死灰复燃的火环境及温度参数阈值,明确林地余火阴燃点引燃特征值。该研究成果将推动森林防火技术的发展,为安全扑火提供重要的方法和资料。     

Intelligent computing based forecasting of deforestation using fire alerts: A deep learning approach

Deforestation is depletion of the forest cover and degradation in forest quality mainly through repeated fires, over-exploitation, and diseases. In a forest ecosystem, occurrence of wildfires is a natural phenomena. The curse of global warming and man-made interventions have made the wildfires increasingly extreme and widespread. Though, extremely challenging due to rapidly changing climate, accurate prediction of these fire events can significantly improve forestation worldwide. In this paper, we have addressed this issue by proposing a deep learning (DL) framework using long short term memory (LSTM) model. The proposed mechanism accurately forecasts weekly fire alerts and associated burnt area (ha) utilizing historical fire data provided by GLOBAL FOREST WATCH. Pakistan is taken as a case study since its deforestation rate is among the highest in the world while having one of the lowest forest covers. Number of epochs, dense layers, hidden layers and hidden layer units are varied to optimize the model for high estimation accuracy and low root mean square error (RMSE). Simulation results show that the proposed method can predict the forest fire occurrences with 95% accuracy by employing a suitable hyperparameter tuning.     

Scaling-Up fire

The role of combustion research in fire safety is revisited through the process of Scaling-Up fire. Scaling-Up fire requires the adequate definition of all the building blocks and couplings associated with the construction of a fire model. The model then has to deliver predictions of the evolution of a fire and its environment with the precision, completeness and robustness relevant to fire safety. Areas of combustion research relevant to the development of fire models emerge from an assessment of methodology, complexity, incompatibility and uncertainty associated to the Scaling-Up process.