分层燃料可燃性和点燃条件研究

发布时间：2018-03-10 10:32

本文选题：分层燃料　切入点：可燃性　出处：《中国科学技术大学》2017年博士论文　论文类型：学位论文

【摘要】：针对树冠火发生和蔓延规律的研究是森林火灾预防和控制技术中的重要研究课题。生物防火林带作为林火防控的关键技术,针对其阻火机理的研究目前停留在树冠叶片可燃性层面,没有形成有说服力的认识并建立相应的技术理论体系。阔叶树叶在树冠中呈现显著分层特性,针对分层树叶燃料可燃性的研究不仅可以加深对树冠火蔓延规律的认识,同时也增进对生物防火林带阻火机理的科学认识,从而推动该项技术的完善与发展。对典型阔叶树树叶的热解特性及燃烧热值进行测试和分析,一方面可以直观呈现阔叶树叶形成明火的潜力,另一方面也为后续工作奠定数据基础。利用综合热分析仪开展热解实验,并利用单组分反应和多组分平行反应模型分别进行动力学解析。结果表明,不同种类树叶热解反应的表观活化能范围为43～80kJmol-1,而茎(枝)的在84～110kJmol-1之间。分析表明,表观活化能在很大程度上反映叶片中纤维素含量,并不表征特定组分热解反应的能垒信息。实验测得失重峰处对应转化率越低,单组分反应模型对植物热解反应过程描述的贴近程度越高。选取了 27种不同科属的木本植物叶样,开展元素分析、工业分析及热值测试。结果表明,植物叶样的干基高位热值HHV_d在17.48～24.01 MJkg-1之间变化,平均值为21.30MJkg-1。其中,同种科的植物叶片热值大小一般较为接近。干基高位热值受样品自身灰分含量影响可以忽略。分析表明,叶样干基高位热值波动与其挥发分和固定碳的含量分布密切相关。叶片中挥发分和固定碳间含量比值随着样品中氧碳摩尔比增加而增大,并且固定碳在可燃组分中的相对含量对应样品中木质素含量。样品氧碳摩尔比和氢碳摩尔比的增加意味着其木质素含量降低,使得其干基高位热值减小。植物叶片的干基高位热值可以分别由经验式HHV_d=18.93VM_d+36.95FC_d 和 HHV_d=0.4478C_d+1.4072H_d-0.2837O_d来准确计算获得,据此还可以迅速确定样品收到基热值。利用锥形量热仪开展具有分层特性的树叶样品的着火实验,以模拟野外林火环境下树冠层叶片的辐射引燃过程。实验结果表明,在特定辐射热流强度下,新鲜树叶样品主要呈现两种典型着火模式:一种是在点燃初期出现短时间气相火焰厚转为阴燃;另一种是在测试过程中仅出现阴燃。随着外部辐射热流强度提高或挥发分含量增加,样品在点燃初期的着火模式会由纯阴燃向有焰燃烧转变。理论分析表明,样品表层初始挥发分含量及表层温升速率是影响样品产生挥发分质量通量水平的主要因素,进而决定样品在点燃过程中是否出现明火。根据树叶样品的分层特性,将样品分离成具有热薄特性的表层和热厚特性的内层,由此建立起两部分的能量守恒和质量守恒的积分模型。计算结果表明,表层在样品点燃过程中接受的辐射能量主要用于表层水分蒸发和内能增加,向内层的热量输运占份额相当小。这导致能量分配关系呈现出典型的表层效应,进而使得内层对点燃现象的影响被严重削弱。水分含量降低会削弱样品表层吸收和遮挡热量的效应,相应导致内层传热项的量阶显著增加。构建考虑热解反应的分层样品辐射引燃模型,并以临界挥发分质量通量为点燃判据,可以对分层燃料出现明火的临界条件作进一步分析。结果表明,树叶样品产生挥发分主要来自于表层,并且表层贡献比例会随着辐射热流强度提高而增加。树叶含水量越低,表层对样品挥发分质量通量的贡献比例相应越小,意味着树叶样品分层特性被明显削弱。分析表明,随着树叶样品含水量减少,内层产生挥发分质量通量会增加,这导致样品出现明火的临界辐射热流强度降低。作为比对,构建出针对连续性可燃物的点燃模型,以直观呈现致密性材料与分层燃料间点燃条件和现象的差异。计算结果表明,样品点燃过程中实际吸收净能量会因外部辐射热流强度提高或环境风速减小而相应降低,并随着样品含水量增加而增加。净吸收能量与表观能量比值Eig/Ea主要取决于外部辐射热流强度和点燃时刻表面热损速率,基本与样品种类和含水量无关。分析表明,比值Eig/Ea可以表示为表征点燃条件优劣程度的参数γig的函数。建立的认识不仅可以由表观能量以及经验式来精确计算出点燃过程中固体净吸收能量,同时也为构建基于"最小所需能量"的着火判据奠定基础。
[Abstract]:Study on regularity and spread of crown fire is an important research topic in the prevention and control technology in forest fires. Biological fire belt is the key technology of prevention and control of forest fire, the research on the mechanism of fire resistance of the stay in the leaf canopy combustible level, there is no convincing understanding and establish the corresponding theoretical system technology. The leaf showed significant stratification characteristics in the canopy, based on hierarchical leaves fuel flammability can not only deepen our understanding of the spread regularity of crown fire, but also to promote bio Fuelbreak fire resistance mechanism of scientific understanding, so as to promote the perfection and development of the technology. The pyrolysis characteristics of typical broad-leaved tree leaves and combustion the calorific value of test and analysis, one can visualize the leaf form fire potential, on the other hand also lay the foundation for the follow-up work data. By using the comprehensive Carry out the pyrolysis and kinetics of thermal analyzer, respectively analysis by using single component reaction and multicomponent parallel reaction model. The results show that different kinds of leaves of pyrolysis reaction apparent activation energy range is 43 ~ 80kJmol-1, and the stem (Branch) between 84 ~ 110kJmol-1. The analysis shows that the apparent activation energy of cellulose the content of leaves in a large extent, does not characterize the specific components of the pyrolysis reaction energy barrier information. The experimental peaks corresponding to the conversion rate and the lower the higher the single component reaction model to describe plant pyrolytic reaction degree. Selected 27 different genera of woody plant leaf samples, carry out elemental analysis, industrial analysis and calorific value test. The results showed that the plant leaf samples of dry basis perch calorific value HHV_d varies between 17.48 ~ 24.01 MJkg-1, the average value of 21.30MJkg-1. the same section of the plant leaves are generally small high calorific value Close. Dry basis perch calorific value is affected by the sample itself can be ignored. The ash content analysis showed that the leaf like stem base with high calorific value fluctuation of volatile matter and fixed carbon content is closely related to the distribution. In the leaves of volatile matter and fixed carbon content ratio increases with the mole ratio of oxygen and carbon in the sample, and the relative content in combustible in the corresponding fixed carbon in the sample. The content of lignin samples and the molar ratio of hydrogen to carbon ratio of carbon and oxygen increase means that reduce the lignin content, the dry high calorific value decreased. Leaf dry high calorific value can be measured by the experience of HHV_d=18.93VM_d+36.95FC_d and HHV_d=0.4478C_d+1.4072H_d-0.2837O_d to obtain accurate calculation, it also can quickly determine samples basic calorific value. Fire experiments carried out with hierarchical properties of leaf samples using a cone calorimeter, to simulate the field of fire The tree canopy radiation environment of ignition process. The experimental results show that the specific heat radiation intensity, fresh leaf samples showed two typical fire models: one is in the early short time light gas flame thickness to smolder; the other is a smoldering appears only in the testing process with external radiation. The content of volatile heat flux intensity increase or increase in sample mode by pure smoldering to flaming combustion transformation ignited early. Theoretical analysis shows that the sample surface initial volatile content and the surface temperature rise rate is essential factors mainly cause the sample mass flux level, and then decide whether the sample in the ignition process in fire. According to the layered characteristics of leaf samples, the samples were separated into the inner surface and the thermal characteristics with thermal characteristics of thin thickness, thereby establishing the mass and energy conservation of the two part The integral model. The calculation results show that the sample surface in the ignition process of the radiant energy received mainly for surface water evaporation and can increase, to the inner layer of the heat transport accounted for a relatively small share. This leads to energy distribution showing a typical surface effect, which makes the effect of inner layer on kindling phenomenon has been seriously weakened moisture. The content of reducing will weaken the effect of sample surface absorption and heat shielding, resulting in heat transfer of the inner order amount increased significantly. The layered sample pyrolysis reaction and radiation ignition model, based on the critical quality of volatile flux for ignition criterion, the critical condition for further analysis of layered fuel flame appears. The results showed that the leaves the sample volatiles mainly from the surface, and the surface contribution ratio will increase with the increased heat radiation intensity. The water content is low, the surface of the sample The volatile products of mass flux ratio with corresponding smaller mean leaf samples was attenuated by the hierarchical characteristics. The results show that, with the leaf moisture content of the samples decreased, the inner produced volatile mass flux will increase, which leads to the decrease of the critical heat flux of radiation samples. As the flames appear for comparison, construct the continuous flammable kindling model the difference to show the density of material and Fuel Stratified between igniting conditions and phenomena. The calculation results show that the samples of the ignition process of the actual absorption of net energy due to external heat radiation intensity increase or decreases while the wind speed decreased, and increased with the increase of the moisture content of the sample. The net energy absorption and apparent energy ratio Eig/Ea mainly depends on the external heat radiation intensity and light time of surface heat loss rate, and sample type and content. Analysis shows that the ratio of Eig/Ea Can express the function parameter Ig lit conditions. The degree for the characterization of knowledge can not only establish energy and experience to accurately calculate the ignition process of the solid net energy absorbed by the apparent, is also based on the "minimum energy required for ignition criterion" lay the foundation.

【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：S762.1

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