Abstract

With climate change and the ever-drier climate, the issue of wildfires is becoming increasingly prominent, generating growing interest in the study of wildfires. The majority of the ongoing research is focused on surface wildland fuels with particular emphasis on dead and usually dry fuel. These insights are difficult to transpose to live fuels, particularly to crown fires. The flammability properties of dead and dry forest fuels are of little significance for understanding the onset and spread of crown fires. Hence, research regarding the flammability properties of fresh forest vegetation is very sparse. The same observation applies to crown fires, despite the fact that this type of wildfires is devastating, difficult to suppress, and usually having dramatic consequences. The aim of this paper is to determine how moisture dynamics of live crown samples (terminal parts of basal branches) of two coniferous species, Abies alba and Picea abies, influence their flammability properties. Experiments were performed in an adapted mass loss calorimeter with a custom-made sample holder in order to mimic the scenario of initiation of crown fires (surface to crown fire interface). Tests were performed with heat flux values of 50, 60, and 70 kW/m² and with different moisture levels. At all heat flux values, the results show an increasing trend for the peak heat release rate when moisture content is reduced. A. alba samples reach higher peak release rates in comparison with P. abies samples. At heat fluxes of 50 kW/m² and 60 kW/m², fresh A. alba samples take longer to ignite than the P. abies samples. At the heat flux of 70 kW/m², for the set of analyzed moisture contents, the ignition time interval for the A. alba samples is shorter than for the P. abies samples. The results of the principal component analysis (PCA) show that variables such as time to ignition (TTI), peak heat release rate (PHRR), and mean heat release rate (mean HRR) best describe the ignitability of the analyzed conifer samples.

中文翻译：

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从地表火灾到地面火灾的转变：水分含量的影响

摘要

随着气候变化和气候日益干燥，山火问题日益突出，人们对山火研究的兴趣日益浓厚。大多数正在进行的研究都集中在地表荒地燃料上，特别是死燃料和通常干燥的燃料。这些见解很难转化为活燃料，特别是树冠火灾。死亡和干燥森林燃料的可燃性对于了解树冠火灾的发生和蔓延意义不大。因此，关于新鲜森林植被可燃性的研究非常少。同样的观察结果也适用于树冠火灾，尽管这种类型的野火具有毁灭性、难以扑灭，并且通常会产生严重后果。本文的目的是确定两种针叶树种冷杉和云杉的活树冠样本（基部枝的末端部分）的湿度动态如何影响其可燃性。实验在带有定制样品架的改良质量损失热量计中进行，以模拟冠火引发的场景（表面到冠火界面）。测试的热通量值为 50、60 和 70 kW/m ²以及不同的湿度水平。在所有热通量值下，结果显示当含水量降低时峰值放热率呈增加趋势。与P. abies样品相比，A. alba样品达到了更高的峰值释放率。在50 kW/m ²和60 kW/m ²的热通量下，新鲜的A. alba样品比P. abies样品需要更长的时间才能点燃。^{在70 kW/m 2}的热通量下，对于所分析的水分含量组，A. alba样品的点燃时间间隔比P. abies样品短。主成分分析 (PCA) 结果表明，点火时间 (TTI)、峰值放热率 (PHRR) 和平均放热率 (平均 HRR) 等变量最能描述所分析的针叶树样品的可燃性。