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。     

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.     

热辐射下木材热解与着火特性实验

对几种木材在不同辐射热流下的热解与着火特性进行了研究,且研究了含水率对木材热解与着火过程的影响.分析热解与着火过程中木材试样不同深度的温度变化与失重变化,对木材的着火时间与着火温度进行记录,并着重分析水分对木材温度分布、着火时间与着火温度的影响.通过间接的方法从实验的着火数据中获得了木材的临界着火热流值约为25 kW/m2.     

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.     

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。     

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．     

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。     

庚烷池火辐射及其对相邻油池的引燃特性

油库罐区时常发生由着火油罐的强烈热辐射而引燃相邻油罐的火灾事故,扩大燃烧范围。因此,需要对储油罐火焰辐射及其对相邻油池的引燃特性进行研究。本文采用内径分别为74 mm、114 mm、150 mm和200 mm的4种圆形石英玻璃油盘,开展不同油盘间距条件下的庚烷池火和点火实验。结果表明,根据圆柱体模型计算得到的辐射热流预测值与实验值吻合较好.当油盘尺寸一定时,随着油盘间距增大,待引燃油盘点接收的辐射热流逐渐减小;而当油盘间距相同时,油盘直径越大,待引燃油盘中心点接收的辐射热流越大。点火过程按照时间顺序划分为3个阶段:加热升温阶段、蒸气积聚阶段和点火阶段。随着油盘尺寸变大,庚烷的临界辐射热流随之变大且作为低闪点燃料,庚烷的临界辐射热流比高闪点燃料小得多.     

室内火灾条件下木材点燃的判据研究

在室内火灾中,木材是最为常见的可燃物,因此室内火灾条件下木材点燃问题的研究引起了广泛关注.建立了室内火灾热烟气和挥发份的填充模型,对热烟气层的稳定高度和木材点燃时的可燃挥发份临界质量通量进行了理论推导;在此基础上,对不同条件下木材的点燃进行了小规模实验研究,对点燃判据的有效性进行了验证.理论分析结果和实验研究结果具有较...     

热流强度测试数据采集方法的改进

本文通过分析热电偶型温度记录仪工作原理,实验得到不同温差范围内的HOBO-U12型温度记录仪的温差电动势曲线及函数关系.并用于现场实测热流数据的采集,结果与实验室确定的函数关系符合较好,完全能够满足建筑热工检测的热流数据采集要求.解决目前建筑热工测试中热流强度数据长时间无人值守采集的问题.     

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.     

Calculating transient wall heat flux from measurements of surface temperature

Wall heat fluxes can be derived from time resolved measurements of the surface temperature. This paper describes an analytical approach to calculate the heat flux from an analytical solution of the one-dimensional transient energy equation with transient boundary conditions using the Laplace transformation. The results are compared to simple test cases for which the heat fluxes are given in literature. The method is used to calculate the heat flux from a fuel spray to a wall at diesel engine conditions.     

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。     

Numerical investigation of fluid flow and heat transfer under high heat flux using rectangular micro-channels

A 3D-conjugate numerical investigation was conducted to predict heat transfer characteristics in a rectangular cross-sectional micro-channel employing simultaneously developing single-phase flows. The numerical code was validated by comparison with previous experimental and numerical results for the same micro-channel dimensions and classical correlations based on conventional sized channels. High heat fluxes up to 130 W/cm² were applied to investigate micro-channel thermal characteristics. The entire computational domain was discretized using a 120 × 160 × 100 grid for the micro-channel with an aspect ratio of (α = 4.56) and examined for Reynolds numbers in the laminar range (Re 500–2000) using FLUENT. De-ionized water served as the cooling fluid while the micro-channel substrate used was made of copper. Validation results were found to be in good agreement with previous experimental and numerical data [1] with an average deviation of less than 4.2%. As the applied heat flux increased, an increase in heat transfer coefficient values was observed. Also, the Reynolds number required for transition from single-phase fluid to two-phase was found to increase. A correlation is proposed for the results of average Nusselt numbers for the heat transfer characteristics in micro-channels with simultaneously developing, single-phase flows.     

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.     

热流计测量精度影响因素的数值分析

本文通过建立二维非稳态导热的数学模型 ,模拟了瞬态热流计测量低温金属表面与高温流体换热时影响测量精度的五种主要因素。计算得出 ,由于热流计测头的出现 ,对流换热系数的变化会给热流测量带来较大误差。其次 ,热流计测头自身因素的变化也会给测量带来误差。测头越薄 ,测量误差越小 ,稳定越迅速 ;测头边长越长 ,测量误差越小。但当被测物表面积较大时 ,测头边长存在一个最优值 ,能达到测量精度与测头尺寸的最佳结合 ,这个值约为 2 0mm ;测头与被测物粘贴越紧密 ,误差越小 ,稳定越迅速 ;被测物导热系数越小 ,则测量误差越小。模拟计算的结果能够为热流计的设计制作以及实际应用提供指导和参考     

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.