含水木材热解和着火试验研究

在自行研制的火灾可燃物热解和着火早期特性试验台上,针对建筑装潢中常用的木材进行了热解和着火特性试验,试验的辐射热流选择20～70 kW/m2,试验结果表明,水分对木材的热解和着火特性影响很大,相同辐射热流下,随着含水率的增加,木材的着火时间延长;辐射热流强度对木材的热解和着火也有很大影响,随着外加辐射热流的增加,木材着火时间逐渐缩短。     

含水木材热解和着火试验研究

在自行研制的火灾固体可燃物热解和着火早期特性试验台上,针对建筑装潢中常用的水曲柳木材进行热解和着火特性试验。辐射热流选择20～70kW／m^2。试验结果表明,水分对木材的热解和着火特性影响很大。相同的辐射热流下,随着含水率的增加,木材的着火时间延长;辐射热流强度对木材的热解和着火也有很大影响,随着外加辐射热流的增加,木材的着火时间逐渐缩短。提出一个考虑水分影响的木材热解模型,通过模型计算的木材表面温度和失重与试验结果对比,两者吻合较好。     

热辐射下木材的热解与着火特性

在大量的实验基础之上,深入分析了木材热解与着火过程的机理及其中的物理与化学变化,建立了考虑水分蒸发以及表面辐射与对流热损失影响的一维湿木热解与着火微分模型。模型对木材热解与着火过程中的温度变化、失重以及着火时间进行了预测,且预测值与实验值吻合得较好,最后分别将该微分模型、一个经典的热解与着火积分模型的预测值与实验值对比,发现考虑热解与着火过程复杂物理与化学变化的微分模型的预测结果与实验结果吻合较好。     

变热流下透明可燃物热解着火的影响机理分析

着火时间是可燃物热解着火过程的重要特征参数.辐射热流直接影响可燃物的着火时间,为了简化解析求解,前人往往认为辐射热流为恒定常数且不进入样件内部,但火灾发生早期,透明可燃物接收到的辐射热流可能是随着时间上升的变化热流.针对此问题,本文以适用于变化辐射热流的透明可燃物热解数值和解析模型为基础,系统研究了辐射热流上升速度、表面吸收、内部吸收等因素对着火时间的影响,比较了两种模型的结果差异并探讨了环境与物性参数对解析模型准确性的影响.结果表明:在上升热流早期,解析法与数值法求解结果符合较好,随着热流与表面温度增加,表面对流换热、辐射、热解等因素开始作用,导致解析与数值结果出现偏差,需根据此偏差修正给定可燃物着火时间解析预测结果.     

热辐射下常用木材热解的动力学与燃烧特性

在热重分析仪上进行了空气气氛、不同升温速率下几种木材的热解动力学过程的研究.对不同升温速率以及木材种类对失重过程的DTG曲线的影响进行了深入分析,在热重实验的基础上建立了"双组分两阶段反应"模型来描述空气气氛下木材的热解动力学过程,并利用C-R方法求得了对应失重阶段的动力学参数.在火灾早期特性实验台上进行了4种木材在空气气氛、不同辐射热流下的热解与燃烧过程的特性研究,通过对高、低热流下几种木材表面温度变化、失重率以及着火时间等参数的对比,发现在较高热流下,固定碳含量相似的几种木材的反应机理及其表观热行为趋于一致.     

变热流条件下木材点燃时间与热流变化率关系的实验研究

实验研究了外加线性变化热流条件下木材点燃时间与热流变化率之间的关系,测量了不同种类木材在不同热流变化率下的点燃时间。通过分析实验数据发现,点燃时间和热流变化率之间符合很好的幂函数关系,而密度是影响木材着火的一个关键因素。     

几种常用木材热解(燃烧)后的烟气析出规律试验研究

在火灾可燃物热解与着火实验台上，对白桦、红皮云杉、蒙古栎、山杨、樟子松五种木材进行了热解燃烧试验，测定了各种木材在20～60kW／m^2辐射热流下的主要烟气成分，得到了在不同辐射热流下，各种木材的烟气析出规律，分析了辐射热流和样品密度对烟气析出过程的影响，探讨了样品表面温度、失重速率和烟气析出的内在联系。     

木材在热解与燃烧中几何特性的变化

通过对木材热解与燃烧过程中几何结构变化共性的分析,建立相应的几何物理模型,提出了试样的两个几何结构特性参数(体积收缩系数与开裂系数).选择了3种木材(杉木、桦木与实木地板)试样(尺寸为100,mm×100,mm×15,mm),分别在15,k W/m2、30,k W/m2与50,k W/m2的热流下进行了热解与燃烧过程的试验.定量分析了过程中木材几何结构变化的特性,讨论了热流与试样种类对上述两个特性参数的影响,最后研究了几何结构变化对木材热解与燃烧特性的影响.     

不同气氛和尺寸下可燃物热解与着火特性的试验研究

对生产和生活中常用的白松、椴木、桦木三种建筑装潢材料，在自行研制的火灾可燃物热解与着火特性试验台上进行了着火特性试验。在40kW／m^2外加辐射热流、5％、15％和18％氧浓度及空气气氛条件下，得到了三种材料的表面温度随时间变化的规律，同时测量了三种材料在缺氧环境下的质量损失速率随时间的变化，给出了气氛对材料热解与着火的影响。对桦木和白松进行了不同尺寸的着火性能试验，给出了其表面温度及质量损失随时间变化的曲线，得出了材料尺寸对着火性能的影响规律。图10表3参15     

变热流条件下木材点燃的实验研究

为研究木材在变化的热流条件下的点燃性能,对泡桐、榆木、红椿木和刺槐等4种木材进行了小尺寸的实验研究.测量了木块表面接受到的热辐射强度、点燃时间、质量损失和木块的内部温度等参数.在实验数据的基础上,得出木材点燃的临界热流增长率约为0.07kW/(m~2·s),临界质量损失速率约为20～30g/(m~2·s).利用PDE模型计算出木块温度变化情况,并与实验数据进行了比较.计算结果表明,在自然着火情况下,木块点燃时的表面温度约为500℃左右.     

变热流条件下木材点燃临界判据的实验研究

对无点火源条件下4种木材在一系列变热流情况下的点燃特性进行了实验研究,实验结果近似地给出了点燃试样的临界热流变化率,采用传热模型计算了木材点燃时的平均表面温度,在实验数据及理论计算的基础上提出了变热流条件下木材点燃的复合判据,并通过小尺寸轰燃实验中的计算点燃时间与实验数据的比较进行了初步的验证。     

Mass flux at ignition in reduced pressure environments

Ignition of solid combustible materials can occur at atmospheric pressures lower than standard either in high altitude environments or inside pressurized vehicles such as aircraft and spacecraft. NASA’s latest space exploration vehicles have a cabin atmosphere of reduced pressure and increased oxygen concentration. Recent piloted ignition experiments indicate that ignition times are reduced under these environmental conditions compared to normal atmospheric conditions, suggesting that the critical mass flux at ignition may also be reduced. Both effects may result in an increased fire risk of combustible solid materials in reduced pressure environments that warrant further investigation. As a result, a series of experiments are conducted to explicitly measure fuel mass flux at ignition and ignition delay time as a function of ambient pressure for the piloted ignition of PMMA under external radiant heating. Experimental findings reveal that ignition time and the fuel mass flux at ignition decrease when ambient pressure is lowered, proving with the latter what earlier authors had inferred. It is concluded that the reduced pressure environment results in smaller convective heat losses from the heated material to the surroundings, allowing for the material to heat more rapidly and pyrolyze faster. It is also proposed that a lower mass flux of volatiles is required to reach the lean flammability limit of the gases near the pilot at reduced pressures, due mainly to a reduced oxygen concentration, an enlarged boundary layer, and a thicker fuel species profile.     

The pyrolysis and ignition of charring materials under an external heat flux

Experimental measurements have tested the effect of an external heat flux on the pyrolysis and combustion of charring materials using a cone calorimeter and a radiation platform, particularly with a small heat flux. Differences in the pyrolysis and burning of wood under strong and weak heat fluxes are discussed. Also, a modified model of pyrolysis for charring materials in a fire is proposed. In this model some special factors that affect pyrolysis, such as heat loss by convection and radiation caused by the surface temperature rising and also shrinkage of the char’s external surface, are considered. The pyrolysis of wood and the time to ignition is predicted using the model, which is also used to explain the mechanism of the pyrolysis of wood exposed to weak external fluxes for long times.     

Autoignition of solid combustibles subjected to a uniform incident heat flux: The effect of distance from the radiation source

The ignitability of solid combustibles by a radiation source has been investigated experimentally as a function of distance between the solid and the source, for heating with a uniform incident heat flux. Unlike piloted ignition, the critical incident heat flux for autoignition increases with the increase of radiant distance, which is a result that has not been reported before. It is found that the generation rate of combustible pyrolysis volatiles is a necessary but not sufficient condition for autoignition. By two dimensionless parameters, it is clearly indicated that the occurrence of autoignition needs both sufficient gas-phase temperature and sufficient concentration of combustible pyrolysis volatiles simultaneously, especially for the larger radiant heating distance condition which is also closer to realistic fire conditions.     

Experiments on the influence of intake conditions on local instantaneous heat flux in reciprocating internal combustion engines

The present study tries to be a contribution for the development of more precise theoretical models for predicting the dissipation of heat through the combustion chamber walls of reciprocating (internal combustion) IC engines. A fast response thermocouple was embedded in the combustion chamber of a single cylinder engine to measure instantaneous wall temperatures. The heat flux was obtained by solving the one-dimensional transient energy equation with transient boundary conditions using the Fast Fourier Transform. The engine was tested under different operating conditions to evaluate the sensitivity of the measurement procedure to variations of three relevant combustion parameters: injection pressure, air temperature and oxygen concentration at the intake. The local heat flux obtained was compared with other relevant parameters that characterize the thermal behaviour of engines, showing, in most of the cases, correlation among them. The results showed that the instantaneous heat flux through the walls and hence the local wall temperatures are strongly affected by the ignition delay and the start of combustion.     

聚氨酯泡沫材料阴燃的点燃过程

阴燃材料的点燃系指材料在受到外热源加热后,依靠其自身异相氧化反应所放出的热量实现阴燃自维持传播的前期过程,如果导致熄灭则称不能点燃．聚氨酯泡沫材料在受到加热时,热流密度和加热时间是实现点燃的两大因素．在自然对流条件下,热流密度在1．6～6．8kw／m^2之间能实现阴燃材料的点燃．通过积分模型分析,得出了实现点燃的临界表达式．该式表明在一定的外部条件下,点燃成功表现为需要一个确定厚度的初始高温反应区（炭层）．在不同热流值的实验中,实现点燃的临界炭层厚度大约为40mm,实验结果与理论分析能够较好吻合．