环境风与飞火颗粒耦合作用下可燃物阴燃及其诱发明火的点燃机理研究

The spotting ignition by firebrands is an important pathway by which wildland and urban (including WUI) spot fires are started. Upon impacting with combustible materials, such as natural fuels or building materials, firebrands can initiate spot fires by direct flaming or smoldering ignition，and self-sustained smoldering can transition to flaming. However, available studies on the firebrand ignition process mainly concentrate on the flaming ignition behavior and the knowledge on the smoldering and transition to flaming ignition behavior is limited. The current project aims to systematically investigate the smoldering and transition to flaming ignition of the various recipient fuels with the coupling effects of wind and firebrands, and capture the critical conditions for firebrand ignition. Analyses of typical characteristic parameters (ignition delay time, temperature distribution around particle, critical ignition parameters) and theories based on heat/mass transfer, pyrolysis kinetics and fluid mechanics, are to be used to uncover the heat transfer during the ignition process and clarify the effects of the particle temperature/energy, the recipient fuel properties and wind on the control mechanism of the smoldering and transition to flaming ignition. Then, a numerical model for the smoldering and transition to flaming ignition of recipient fuel, associated with the coupling the pyrolysis kinetics of recipient fuel and the oxidation reaction of pyrolysis gases, will be proposed based on the nature of firebrand ignition. Finally, the heat/mass transfer mechanism between the firebrands, atmosphere and recipient fuel during the ignition process will be revealed. The research of this project will promote the scientific understanding of spot fire phenomenon.

飞火点燃是导致森林与城市及其交界域火灾发生的重要潜在路径。飞火颗粒接触森林或建筑可燃物，可引发其直接明火或阴燃点燃行为，而自维持阴燃可转化为明火。然而，现有飞火点燃研究关注直接明火点燃行为，缺乏对阴燃点燃及其诱发明火点燃行为的研究。本项目系统研究飞火颗粒与环境风耦合作用下，不同热解动力学特性可燃物阴燃点燃及其诱发明火点燃行为，建立可燃物飞火点燃的临界条件；通过分析典型特征参数（点燃延滞时间、颗粒周围温度分布、点燃临界参数），采用传热传质、热解动力学和流体力学理论，揭示可燃物点燃过程的热量输运机制，阐明颗粒温度/能量、可燃物特性及环境风对可燃物阴燃点燃及其诱发明火点燃控制机理的影响；基于飞火点燃的气相化学反应本质，发展耦合可燃物热解反应和热解气氧化燃烧的可燃物阴燃点燃及其诱发明火点燃数值模型；最终揭示飞火颗粒、环境及可燃物交互作用中的传热传质机理。项目成果将促进对飞火颗粒点燃行为的科学认识。

飞火点燃是导致森林与城市及其交界域火灾发生的重要潜在路径。飞火颗粒接触森林或建筑可燃物，可引发其直接明火或阴燃点燃行为，而自维持阴燃可转化为明火。本项目系统研究飞火颗粒对不同热解动力学特性可燃物点燃行为。主要研究进展：（1）开展4种尺寸11种空心率热颗粒点燃聚苯乙烯泡沫的模拟实验，探究颗粒温度/能量对点燃的影响。研究首次观察到热颗粒诱发的自燃点火现象，其由混合时间和冷却时间控制。相比质量和能量，颗粒尺寸是衡量其点火能力的特征参数。氧气供应可将热颗粒点燃从“闪点”转变为“火点”，半开放边界意味着更大的火灾风险。（2）木质飞火颗粒的点燃过程复杂，热辐射实验首先探究木质飞火颗粒的点燃临界条件且观察到其燃烧过程的复杂的动态形变。细化研究发现，飞火颗粒动态形变存在四个阶段：(Ⅰ) 干燥收缩形成∪型，(Ⅱ)外部辐射引发热膨胀形成∩型，(Ⅲ)热解收缩形成∪型，(Ⅳ) 炭氧化引发热膨胀形成∩型。建立了耦合三步非均相反应和热力学模型的二维有限元数值模型。模型结果进一步验证了每个变形阶段的化学反应（脱水、热解和氧化）和热机械应力（热膨胀和预应力）的作用机制。（3）可燃物特性重要影响其点燃特性，尤其多孔易诱发阴燃向明火转变。研究开展与加热锥不同距离的等热通量燃料（PMMA（非炭化）、松木（炭化）和松针（多孔离散炭化））点燃实验。PMMA出现加热锥表面点火及不点火两种实验现象，松木出现加热锥表面点火/阴燃燃烧及不点火三种实验现象。松针出现加热锥表面点火/燃料床表面阴燃点火/阴燃燃烧/不点火四种实验现象。点火时间在6 cm左右的位置出现最小点火时间，且点火现象从不点燃、加热锥先导点燃、热解气体自发点火到不点燃转变。（4）基于飞火点燃的气相化学反应本质，建立自然对流条件下飞火颗粒点燃的数值模型。模拟初步预测实验结果，后期将考虑颗粒与燃料床的接触关系及点火临界条件的优化。项目成果将促进对飞火颗粒点燃行为的科学认识。