【作者】 张孝春；

【导师】 杨立中； 胡隆华；

【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】 现实世界中发生的火灾可以呈现不同的火源形状,例如：燃气管道阀门损坏引起的气体泄漏可以抽象为轴对称火源；输电电缆的燃烧以及森林地表的火蔓延均可以抽象为线性火源;我国近年来屡屡发生的高层建筑火灾中的火溢流现象,也可以视火焰根部形状抽象为轴对称、矩形或者线性火源诱导的火灾。从火源形状上来讲大部分可以近似处理为矩形火源。当火源的长边与短边基本一致时,可以抽象为轴对称火源；而当火源的长边比短边大很多时,则可以抽象为线性火源。前人研究大都针对上面两种火源类型给出相应的规律,相比之下,对于轴对称火源进行了较多研究,而对线性火源引发的火灾则研究的相对较少；对于介于两种火源之间的更为常见的矩形火源,还未有系统研究。本文针对不同火源形状条件下的火羽流及顶棚射流特征参数演化行为进行了较为系统的研究,揭示其火焰高度、中心线温度及其诱导的顶棚射流相关规律。本文首先研究了矩形射流火的火焰高度和中心线温度规律；然后研究了线性火源与轴对称火源诱导的顶棚射流规律,并揭示了二者的异同点；最后将部分研究结论向高原低压环境进行了推广。具体的工作包括：利用自制的截面面积相等的一系列矩形喷嘴作为燃烧器开展了火焰高度实验研究。首先利用1：1宽长比的喷嘴进行实验,验证了Quintiere和Grove的理论分析结果对本实验的适用性,同时验证了本实验数据结果的可信性。其次,在宽长比在12到1：71的范围内对每个喷嘴的火焰高度数据进行拟合,得到与每个喷嘴对应的重要参数C1的值。最后,在得出C1的基础上给出矩形火焰高度的显式数学预测模型。研究结果表明：对于轴对称的燃烧器,本研究得出的C1值与Quintiere和Grove给出的结果完全一致；C1值在1：1到1：71的范围内基本不随宽长比的变化而改变,并不存在从0.179至0.444的转折,证明了线性火源的卷吸系数与其他种类的火源的卷吸系数基本相等；显式火焰高度预测模型在实验火焰功率范围内能较好的预测实际火焰高度值。研究揭示了自由边界条件下浮力羽流区中心线温度分布规律。由于火焰区的温度分析目前只能基于数据拟合或者进行数值计算得出,所以我们主要关注浮力羽流区的温度分布规律。首先,假设羽流中心线弗洛德数为与出口速度相关的常数,从而推导出浮力羽流区中心线温度的分布应满足的规律。其次,给出了羽流中心线弗洛德数随火源弗洛德数的变化关系并给出了基于此关系所推出的理论公式的适用范围。最后,通过实验研究来验证理论关系式,结果表明基于此假设得出的理论模型能很好的描述羽流中心线温度。研究揭示了不同火源形状下火羽流诱导的顶棚射流相关规律。首先通过实验研究,得到了轴对称火源顶棚射流情况下的顶棚温度分布及火焰扩展长度规律,提出了新的顶棚温度和火焰扩展长度关系式。其次研究了线性火源诱导的顶棚射流顶棚温度分布规律。最后,通过对比揭示了两种不同的火灾情况下二者之间的规律差异。值得注意的是同样规模的火源功率下,线性火诱导的顶棚射流最高温升和高温区域都比轴对称情况要小。通过实验与理论分析相结合的方式研究了轴对称火源强羽流驱动的顶棚射流的温度分布规律、火焰扩展长度和线性火源诱导的顶棚射流的温度分布规律,完善了Heskestad和Delichatsios的强羽流驱动的顶棚射流理论,揭示了线性火弱羽流诱导的顶棚射流温度分布规律。最后本文研究了高原低压低氧特殊环境(西藏拉萨地区)下矩形射流火火羽流的火焰高度、中心线温度及其诱导的顶棚射流行为特性。通过对比研究合肥和拉萨两地实验数据,验证了拉萨地区与合肥地区卷吸强度差异。基于拉萨地区卷吸强度减弱的特点,给出了矩形火焰高度统一预测模型中对应的Cf值,并进一步给出了重要参数n值(卷吸进火焰的氧气量与所需氧气量的比值)。同样基于卷吸强度减弱的特点,修正了矩形火源中心线温度统一预测模型。最后,我们建立了线性火源弱羽流驱动条件下顶棚射流温度分布在不同压力下的统一预测模型。这些研究结论对认识高原地区的火灾发展规律和火灾防治都有较大的理论指导意义。更多还原

【Abstract】 Most of the fires occurred in the real world can be approximately divided into three types from the dimensions of fire source:axisymmetric fire, rectangular fire and line fire. It can be considered as axisymmetric fire when the long side is equal to the short side of the fire source while it can be dealt with line fire when the long side is much larger than the short side of the fire source. The fire dimensions fall between these two types are the general rectangular cases. For example:The fire due to gas leakage from a broken valve of pipeline can be considered as a axisymmetric fire; The feeder cable fire can be considered as a line fire; the forest surface fire spreading can also be dealt with the line fire theory; the flame ejecting behavior from a building facade opening can be considered as an axisymmetric fire or a line fire according to different real situations. A lot of research results about axisymmetric fire have been reported in the past years because it is easy to analyze. However, the study on the line fire is relatively rarer than that about axisymmetric fire. In addition, the investigation about the fire driven by rectangular fire sources are also few. So, it is necessary to study the fire characteristics of fires driven by difform nozzle fire sources and give the unified rules of them.Firstly, we study the flame height and axial temperature distribution of jet fires driven by rectangular sources with aspect ratio ranging from1:1to1:71. Secondly, we research the flame extension length and the temperature distribution of ceiling jet flow induced by axisymmetric fire and the temperature distribution of ceiling jet driven by weak line fire flume. Then, we reveal the differences between these two types of ceiling jets. Finally, we expand the research results to the hypobaric hypoxia environment in high altitude. The major findings include:Comprehensive experimental work was carried out to study the flame height of jet fire produced by rectangular equal-area nozzles with different aspect ratios. First of all, we verify the applicability of the theoretical results deduced by Quintiere and Grove using the experimental data of rectangular nozzle with aspect ratio of1:1. Meanwhile, this also can prove the credibility of our experimental results. Then, the value of C1is obtained through the data fitting of experimental data for each rectangular nozzle whose aspect ratio varied from1:2to1:71. Finally, an explicit model is established to predict the flame height based on the implicit correlation. Results show that:For the axisymmetric fire, the value of C1in this work are in good agreement with Quintiere and Grove have proposed; However, the value of C1needs not to transit from0.179to0.444with decreasing of aspect ratio from1:1to1:71. The value of C1can be taken as a constant, which implies that the entrainment coefficient is nearly the same for both axisymmetric fire source and line cases; the flame height can be well collapsed by the proposed explicit model.The axial temperature profiles of buoyant plume driven by fires produced by rectangular sources are investigated in this paper. We assume that the centerline Froude number of a buoyant plume can be presented by source Froude number which is related to the fuel exit velocity. Then, we theoretically deduced the axial temperature profiles correlations based on this assumption. Immediately following, the correlation to predict centerline Froude number as well as the range of its application is given based on experimental data. In the end, the comparison of experimental data and the theoretical correlation shows that the axial temperature profiles of buoyant plume can be well described by the new proposed model.The evolution behaviors of smoke movement of the ceiling jet flow induced by a fire plume impinging upon a ceiling are studied in this thesis. Experiments have been conducted to investigate the ceiling temperature profiles and the flame extension length under the ceiling for ceiling jets driven by strong axisymmetric fire plume. New predictive model for flame extension length is established and a modified model about temperature profiles based on weak plume driven ceiling jets’correlations is proposed. The line source plume driven ceiling jets have also been studied and the temperature distribution profiles are discovered. Finally, the differences between these two kinds of fire sources induced ceiling jets are discussed. It should be noted that the ceiling temperature induced by the line fire is usually lower than that of the axisymmetric fire under the same fire situation. This means that the fire produced by a line source is more difficult to be detected by the traditional fire detectors equipped at the roof of the buildings.At last, we investigate the flame height, the axial temperature and the ceiling flow characteristics of rectangular jet fire under a hypobaric hypoxia environment in plateau (Tibet Lhasa city at a high altitude). The experimental data in both Hefei and Lhasa have been compared. Based on the phenomenon of weakened entrainment in Tibet Lhasa, the correlations about flame heights, centerline temperature profiles and ceiling jets established in normal atmosphere are expanded to this hypobaric hypoxia environment. It is found that the coefficient Cf in the explicit model for flame heights and C/β in temperature correlations are both higher in Lhasa than that in Hefei In addition, this paper gives the value of an important coefficient n (ratio of oxygen entrained into fire and oxygen needed for combustion) in Lhasa. Finally, a global model to predict temperature distribution of ceiling flow driven by line source weak buoyant plume in these two ambient atmospheric pressures are proposed. The conclusions would be significant for the regularity and prevention of fire in high latitudes.更多还原