非常压条件下木质材料炭化层缩聚的动力学机制及缩裂现象产生机理研究

Char layer patterns significantly affect the combustion processes of wood materials in fires. Meanwhile, it is regarded as the most useful tool for fire investigation. Therefore it is worthwhile studying the char layer patterns from scientific and practical points of view. Char layer of wood materials shrinks and cracks with pattern changing while pyrolysis. However the mechanisms of shrinkage and cracking are still unknown so far. The current project studies the char layer shrinkage and cracking behaviors by varying the ambient pressure and oxygen concentration. The shrinkage dynamics as well as the shrinkage cracking mechanisms are investigated in depth. The current study is expected to reveal the developing process of char pattern at different ambient pressures and oxygen concentrations. The shrinkage ratio and rate are experimentally measured and then correlated with incident heat flux, ambient pressure, sample size and so on to develop a model expressing the shrinkage dynamics. The model should be able to reveal the effect of pressure on char layer shrinkage. The transient state between uncracking and cracking is determined while the fissure system is associated with the experimental conditions. The effects of flame and oxidization on the char layer patterns are studied. The effects of char layer patterns on the flame height and shape are also investigated by combining the experiments with and without oxygen. A one dimensional model is developed to predict the shrinkage cracking behavior by involving shrinkage dynamics, pyrolysis kinetics, heat and mass transfer processes. The internal tension due to unbalance shrinkage is calculated and compared with the tensile strength to establish the shrinkage cracking criterion. The transient state between uncracking and cracking is used to validate the model and the criterion.

木质材料炭化层的几何形态是材料燃烧的重要影响因素及火灾痕迹鉴定的首要依据，研究其演化过程具有重要的科学意义与应用价值。炭化层几何形态是由热解过程中的缩聚和龟裂特征决定的，但目前二者的产生和发展机理尚未得到系统揭示。本项目以木质材料为对象，开展变氧压热解实验，重点解决两个科学问题：①引起炭化层缩聚的内在动力学机制，②龟裂（或缩裂现象）的产生机理。通过本项目的研究，可望得到不同压力和氧浓度下炭化层几何形态演变规律；建立收缩率和收缩速率随压力、辐射强度、试件尺寸等变化的非线性动力学模型，揭示压力对炭化层缩聚的驱动作用；发展裂纹形态参数与环境条件变量的无量纲关系式，确定炭化层从未裂向龟裂转化的临界环境条件；明确火焰及表面氧化对几何形态的影响，以及炭化层对火焰形态的反作用关系；构建缩裂模型，计算不均匀缩聚度、内应力和抗拉强度，揭示缩裂现象的产生机理。

木质材料炭化层的几何形态是材料燃烧的重要影响因素及火灾痕迹鉴定的首要依据，研究其演化过程具有重要的科学意义与应用价值。本项目以木质材料为对象，开展变氧压热解实验，重点解决两个科学问题：①引起炭化层缩聚的内在动力学机制，②龟裂（或缩裂现象）的产生机理。项目的主要研究内容包括：①惰性气氛下炭化层缩聚龟裂规律及缩聚动力学模型，②不同氧浓度条件下炭化层缩聚龟裂规律及对燃烧的影响，③木质材料炭化缩裂现象的产生机理。针对项目的研究内容，研究小组首先利用自行研发的变氧压实验箱开展热解实验，获取木质材料炭化层缩聚及开裂的演变规律、裂纹数量和形态、及相应的温度失重数据。进而，建立了裂纹形态与缩聚速率、炭化速率的相互关系，确立了从未裂向龟裂转化的临界环境条件，提出“屈曲+缩聚”效应产生内应力的裂纹发生机制，揭示燃烧导致的炭化层屈曲缩聚及炭化裂纹的产生机理。在此基础上，开展了热重、锥形量热仪及大尺度燃烧房实验，获取了材料热解曲线，有焰燃烧下炭化、温度和失重，轰燃条件下炭化层几何形态分布规律等基础数据。建立了火焰热反馈的四阶段模型，提出了可用于火灾调查的炭化深度云图法。发展了快速获取材料热解动力学参数的“K-K”方法，解决微尺度热分析动力学中单-多温升速率不相容问题，与传统方法相比，计算效率提高20倍以上。因而，本项目主要成果包括：①提出了材料炭化的屈缩理论，②总结了火焰热反馈边界及轰燃条件下炭化开裂规律，③发展了材料热解动力学的“K-K”方法。项目的研究成果揭示了木质材料炭化层缩裂现象的产生机理，提高了热解过程模拟的效率，推进了现有热解模型的发展，同时，为火灾调查的理论体系提供了基础实验数据和一定的理论支撑。项目提出的利用纤维板作为典型炭化材料研究炭化热解模型的理念在国际上获得了一定的认可。目前，国际上已有多个研究机构利用纤维板研究炭化材料在火灾过程中的行为。