【作者】 赵伟涛；

【导师】 刘乃安； 陈海翔；

【作者基本信息】 中国科学技术大学 ， 安全科学与工程， 2014， 博士

【摘要】 森林地下火是森林地表腐殖质阴燃形成的一种火灾,对森林生态系统和大气循环有巨大破坏作用。前人的工作重点是研究森林地表可燃物的点燃概率经验模型,缺乏对腐殖质阴燃规律的深入研究。随着全球气候的变化,世界各地森林地下火频发,森林腐殖质阴燃机理的研究开始受到关注和重视,已逐渐成为阴燃研究的前沿热点领域。腐殖质类可燃物由森林凋落物质经过生物化学作用形成,有机成分种类繁多、结构复杂,水分含量、无机物含量不均匀,且阴燃过程受到腐殖质深度、风速等条件影响,因而腐殖质阴燃过程复杂,研究面临诸多挑战。在阴燃过程中,腐殖质中有机物在高温条件下与氧气发生热解氧化反应,释放能量,维持阴燃传播。有机物成分和含量将影响能量释放的功率。腐殖质中水分在高温下蒸发而吸收能量,延缓阴燃蔓延速度。腐殖质孔隙率则影响燃烧的能量密度、氧气流通性及材料导热性能。本文选择与腐殖质材料热解过程类似但结构和成分相对均匀的泥炭作为研究材料,研究各种参数对阴燃蔓延规律的影响,并从化学反应和能量方程出发,建立阴燃数值模型,探讨揭示泥炭阴燃蔓延的内在规律。本文首先实验研究了森林泥炭的元素组成、导热系数、比热容、颗粒真密度、灰烬表观密度等重要参数,界定了孔隙率和无水无灰烬的研究标准,为后续实验和模拟研究提供可靠数据支持。进而,通过热分析实验研究了微观尺度不同氧气浓度下泥炭的化学反应机理,建立了化学反应模型。研究发现泥炭在氮气条件下热解过程由三个失重过程组成,从而建立了三组分叠加反应模型,这与有氧条件下热解反应机理不同。对有氧但不同氧气浓度下,泥炭热解呈现两步失重过程,从而建立了两步连续反应模型。应用Kissinger、FWO、Starink、Friedman、Gyulai等五种不受机理函数影响的等转化率方法求解了热解过程的活化能,活化能随氧气浓度的变化规律与两步反应模型的结果一致,验证了两步反应机理假设及模型的恰当性。发现并分析了DSC和DTG峰不重合现象,完成了两种曲线的联立模拟,从而建立了同时模拟化学组分质量和热释放速率的化学反应模型。自呼中保护区和加格达奇自然保护区采集了经历过森林地表火和未经受森林地表火的不同深度处原始森林腐殖质和土壤,对五种腐殖质试样进行在空气气氛下热分析实验。利用热解阶段(第一个失重峰)的失重质量作为基准进行数据标准化处理,发现了不同样品在热解阶段表现一致的失重速率,但在氧化阶段清晰分成了受地表火和未受地表火影响的两种失重速率类型,进而提出了采用热分析技术衡量火灾消耗地表可燃物比例的计算方法。此方法可作为评估森林火灾严重程度的一种方法。通过自行设计的垂直向下自然阴燃实验装置,研究了不同水分含量、不同孔隙率、不同粒径的泥炭阴燃蔓延规律。实验过程精确控制泥炭样品粒径、孔隙率、含水率等参数。对不同水分含量和粒径的泥炭阴燃系列实验,保证泥炭孔隙率相同,从而保证可燃有机物表观密度一致。发现并解释了阴燃热解前锋速度不变而焦炭氧化区域不断增厚的现象,发现并分析了水分含量和孔隙率对阴燃蔓延的强化和抑制的双向影响效应,分析了阴燃点燃和熄灭过程中出现的炭黑挂壁、点燃明火、锥形熄灭、底部持续高温等现象,丰富了对阴燃现象及规律的认识。通过对微观尺度泥炭热解DTG曲线和阴燃实验中阴燃区域特征的对比分析,分析了阴燃前锋的四个区域结构和阴燃蔓延速率。提出利用曲面插值平滑方法对阴燃过程中温度-时间-位置三维数据进行处理的方法,并由此得到阴燃四个区域的前锋速度和厚度。此方法减小了阴燃实验中数据采集不均匀等因素产生的误差,为后续阴燃蔓延实验研究提供了更快速的解决方案。依据微观尺度化学反应模型和能量守恒方程,建立了泥炭阴燃蔓延的速率模型和阴燃蔓延的移动边界模型,解决了阴燃蔓延模拟中求解边界移动和灰烬累积的问题。更多还原

【Abstract】 Ground fire in the forest is a kind of smoldering fire of humus and is a great threat to the forest ecosystem and atmospheric circulation. Previous research mainly focuses on the empirical model of ignition probability of humus, and there is much lack of the mechanism research of humus smoldering. As the change of global climate, ground fires have occurred over the world and much attention begins to be paid on forest humus smoldering, which has gradually become a frontier and hot research topic in the field of fire science.The humus material is formed by forest litter through biochemical degradation. It contains many different kinds of organic compounds with complicated structure, nonuniform contents of water and mineral matters. The smoldering process is influenced by the duff depth, wind speed and so on, and the research on this process faces great challenges. In this thesis, the peat is selected as the research material because of its similar thermal decomposition behavior with that of forest humus. The uniform structure and compounds of peat will benefit the mechanism research. The organic matter of peat reacts at high temperature with oxygen by pyrolysis and oxidation reactions and releases heat to maintain the smoldering spread. The composition and content of organic matter will affect the energy released. Water in the peat will evaporate by absorbing energy. The porosity of peat influences the energy density, the flow of gas and heat conduction. The influcence of many factors on peat smoldering was investigated experimentally. From the perspective of the chemical reactions and energy converasion, the numerical model is set up to discuss the inherent law of peat smoldering.This thesis firstly measured the elements of forest peat, the coefficient of thermal conductivity, the specific heat coefficient, the true density of particles and the apparent density of ash. The porosity and the standard mass without water and ashes were defined. It provided reliable data for the experiments and simulations of peat smoldering.The micro-scale chemical reaction mechanism of peat was studied by thermal analysis experiments under different oxygen concentrations, and the chemical reaction model was established. It was found that the pyrolysis process of peat in nitrogen atmosphere was composed of three weight loss processes, which was different from those in the aerobic conditions. The three component superposition reaction scheme was established for peat pyrolysis in nitrogen. For peat decomposition under the aerobic with different oxygen concentrations, a two-step consecutive reaction model was established. The methods of Kissinger, FWO, Starink, Friedman and Gyulai were applied to obtain the activation energy of peat decomposition process. Results were in accordance with the two-step reaction model, which verified the rationality of this reaction model. The inconsistent phenomenon of DSC and DTG peaks was discovered and analyzed. The simultaneous simulation method of two kinds of curves was proposed and the method could obtain the chemical composition and heat release at the same time for peat smoldering simulation.The humus and soil samples were collected layer by layer from the forests experiencing surface fire and the forests not subjected to fire, from HuZhong and JiaGeDaJi nature reserves in China where wild fires often occur. The thermal analysis experiments of five kinds of duff samples were conducted in air condition. The DTG cures were standardized by using the loss mass of the pyro lysis stage. The results showed that there was almost the same weight loss rate of five samples in the pyro lysis stage and DTG cures was clearly divided into two groups of mass loss rate curves, corresponding to the samples affected by surface fire and not affected by surface fire. The burned ratio of peat due to forest fire was evaluated which could be used as a severity evaluation of forest fire.The smoldering experiment device was designed to study the natural vertical downward smoldering fire. Series of smoldering experiments of peat with different water content, porosity and particle size were conducted. The particle size, porosity and water content of samples were controlled carefully in the experiments. The porosity was kept same during the series smoldering experiments of peat with different water content and particle size. So the apparent density of organic matter of peat was same. The reasons of the stable speed of pyro lysis frontier and the continuously thickening oxidation zone were illustrated. The dual effect of strengthening and restraining smoldering spread of water and porosity parameters was discovered. Many special phenomenon were also discovered and explained, such as black carbons on the wall of device, the conical shape after extinguish and the continuous high temperature on the furnace bottom. These results will be helpful to the cognition of smoldering.By the comparison of the DTG curves and the character of smoldering zones in the experiments, four zones of smoldering frontier and smoldering spread rate were defined. The temperature-time-position data of smoldering experiments were dealt with the surface interpolation smooth method. And the frontier spread rates and thicknesses of four zones were obtained. This smooth method can decrease the influence of experiment error, such as non-uniform position of thermalcouples inserted in the device.Based on the micro-scale chemical reaction model and the equation of energy, a smoldering spread rate model and a moving boundary smoldering model were established. The moving boundary and the accumulation of ashes problems were solved.更多还原