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。

Characterization of burning behaviors of plant foliages under external radiant heat flux

In the present work, burning behaviors of plant leaf samples were studied using a cone calorimeter, with a focus on exploring the reasons behind the phenomena observed. Piloted ignition measurements were performed at the radiant heat flux of 35, 55, 70 and 85 kW m-2, respectively. Coniferous and broadleaved plant species of 13 kinds in total were used, with their moisture content varying from 45% to 79%. Measurement results show that the samples underwent ignition in two major modes, flaming or smoldering, depending on the plant species and the radiant heat flux. The critical peak value of heat release rate (PHRR) for flaming combustion was found to fall in the range between 22.3 and 35.6 kW m-2, reflecting the minimum volatile mass flow rate required for establishing a flame. Excellent linear correlation was observed between the values of PHRR and the peak rates of CO2 produced, indicating the similarity in the gas-phase combustion reactions and oxidation on the solid surfaces for various leaf samples. Further analysis suggested that the unique pattern in the HRR curves is the compromised result affected by the layered feature of the samples in conjunction with their physical and thermal properties. At the beginning of a test, the surface layer of a sample with thermally-thin property intercepts external radiant heat, and its pyrolysis products are responsible for the formation of a flame in the gas phase. After its transforming into a char layer, it plays a role in blocking the external heat flowing into the internal layers. The established understanding provides guidance for the assessment of the combustibility of layered samples.