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气体多火源燃烧的实验和理论研究
中文摘要

多火源燃烧是指多个非连续火源在相同时间及相近空间下燃烧的一种特殊火现象,常见于森林、城市及工业火灾之中。不同于单个火源燃烧,多火源燃烧火焰之间存在相互作用,并极有可能引发火焰合并、飞火及火旋风等特殊火行为,使得火灾的危害性及复杂性增加。在单个火源燃烧研究中,火源可理想化地分为轴对称火源及线火源。对于多火源火焰合并,则可能存在轴对称火源合并为轴对称火源、线火源合并为线火源及轴对称火源向线火源转变的物理情形。对应地,本文采用火方阵、双长方形火源及双正方形火源作为研究对象模拟上述情形,对多火源燃烧进行了系统性的研究。 本文的研究目标是,发展判断多火源火焰合并的方法,定量地描述火焰合并过程,探讨火焰合并过程中物理机制的变化,并基于火焰行为特征划分火焰合并状态;对应于不同的火焰合并状态,研究多火源火焰高度的变化规律,并发展相应的火焰高度表征模型;研究多火源燃烧火焰的温度分布及速度分布规律,比较多火源与单个火源浮力火羽流热场及流场分布特征的异同点。本文具体的研究工作包括: (1)以火焰合并判据及合并火焰高度为主要研究点,开展了偶数火方阵形式的多火源燃烧实验。改变了火源数目、火源间距及火源热释放速率,获得了不同火焰合并状态的火方阵火焰。通过火方阵中心线温度的分布规律的分析及火焰燃烧图像的对比,发现随着火源间距的减小,火方阵中心线底部区域发生物理机制的改变,由受加热的空气转变成火焰,再转变成富燃料区。随着火方阵火焰由不合并转化为完全合并,对应的火方阵中心线温度的最大值先增加再降低,因此完全合并火焰的中心线最高温度可以作为该变化过程中的特征温度。基于特征温度发展了火方阵火焰合并的判断方法,并得到了由火源间距比、火源数目及火源无量纲热释放速率组成的火焰合并判据。发现可以采用火方阵整体尺寸作为特征尺寸,对火焰高度及热释放速率进行无量纲化分析,并且从Heskestad火焰高度模型出发,发展了描述火方阵合并火焰高度的经验模型。通过对中心线温度及速度在尺度化高度下的幂指数的分析,发现随着火焰由未合并转变为合并,火方阵的火羽流温度及速度分布逐渐趋近于单个轴对称火源火羽流的分布规律。 (2)以火焰合并过程及对应的燃烧特性变化规律为主要研究点,开展了双平行长方形火源燃烧实验,改变了火源长宽比、火源间距及火源单位长度热释放速率,获得了不同火焰合并状态的双长方形火焰。通过对双火焰内侧压差的理论分析,得到随着火源长宽比及热释放速率的增加,火源间距的减小,火焰之间的相互作用增强,火焰趋于合并,并且通过实验图像分析验证了这一结论。基于观察将火焰合并过程划分成三个合并阶段:独立阶段、间歇阶段及稳定阶段,并进一步细分为五个合并状态:不存在相互作用的状态Ⅰ、存在相互作用但不合并的状态Ⅱ、间歇合并的状态Ⅲ、部分合并的状态Ⅳ及完全合并的状态Ⅴ。采用了火焰合并概率对火焰合并状态进行量化描述,发展了火焰合并概率的无量纲表征模型。研究了不同合并状态下火焰高度的变化规律,耦合了火源间距及热释放速率,建立了两种火焰高度的无量纲表征模型。研究发现:在状态Ⅰ下,火焰高度变化规律与单个火源一致;在状态Ⅱ下,由于火焰两侧压力不平衡与火焰空气卷吸受限的相互竞争,双火源火焰高度与单火源火焰高度之比在一定范围内可采用常数描述;在状态Ⅲ与状态Ⅳ下,双火源火焰高度变化规律一致,且无量纲火焰高度随整体无量纲热释放速率的增长指数大于单个火源的增长指数2/3;在状态Ⅴ下,双火焰可以整体视为单一火焰,高度变化规律与单个火源一致。研究了不同合并状态下火羽流温度二维分布及中心线速度分布特征,发现在水平方向上的温度分布并非自相似分布,在竖直方向上的中心线温度及速度与尺度化高度的幂指数关系随着合并程度加深逐渐趋近于单个线火源火羽流。 (3)以火焰合并形态及零间距下火焰高度变化规律为主要研究点,开展了双正方形火源燃烧实验,改变了火源尺寸、火源间距及火源热释放速率,获得了不同火焰合并状态的双正方形火焰。通过观察火焰合并图像,发现合并发生时,火源间距及火焰倾斜角度较小,并且火焰有较大概率先从底部开始碰触,说明了双正方形火焰问的相互作用较弱,火焰较难发生稳定合并。针对共合并过程,发展了火焰合并概率的无量纲表征。研究了不同合并状态下火焰高度与单个火焰高度之比的变化规律。发现随着火源无量纲热释放速率增加,火焰空气卷吸形态由正四棱锥转换成长方体,对应的零间距下的火焰高度比由0.85增加到1.15。 关键词:多火源 火方阵 长方形火源 火焰合并 合并概率 火焰高度 温度分布

英文摘要

Multiple fires burning is a special fire phenomenon when multiple flames from separate fuel beds bum simultaneously in a certain space range. It is commonly observed in wildland, urban and industrial fires. In contrast to single fire burning, the flames of multiple fires have interaction with each other. What's more, multiple fires burning may result in flame merging, spot fire and fire whirl and these make the fire more hazardous and complcx. In the researches of single fire, based on the shape of fire source, the fire could be classified into axisymmetric fire and line fire. For the flame merging of multiple fires, there may exist different merging cases, for example, axisymmetric fires merging into axisymmetric fire, line fires merging into line fire and axisymmetric fires merging into line fire. In this paper, we use fire array, two rectangular fires and two square fires to stand for these merging cases and to study multiple fires burning systematically. This paper presents experimental investigation and theoretical analysis on flame merging behavior and combustion characteristics of multiple fires. New methods to identify flame merging are proposed. The flame merging process is described quantificationally and the change of physical mechanism in the flame merging process is discussed. For different flame merging states, the variation trends of flame height are studied and several scaling laws, empirical models and theoretical models are developed. The similarities and differcncc between multiple fires and single fire are examined from flow field structure and heat field structure. The detailed work and results of this thesis are summarized as follows. (1)Focusing on the flame merging criterion and flame height of merged flames, the experiments of fire array with even number of flames are conducted. By changing the number of flames, the spacing between burners and the heat release rate, flames with different merging states are obtained. By analyzing the variation trends of centerline temperature, it is found that a physical transition occurs with the decrease of fire spacing. The lower region in the centerline of fire array changes from the heated air region to the flame region and then to the fuel-rich region. When the flames change from unmerged state to completely merged state, the maximum centerline temperature first increases and then decreases. Thus the temperature of completely merged flames could be used as a reference flame merging temperature. Based on these, A new method is proposed to determine the flame merging states and a new flame merging criterion is developed. It is found that the fire array size could be used as a characteristic length scale to nondimensionalize flame height and heat release rate. Based on the flame height model of Heskestad, an empirical model to describe the flame height of merged fire array is established. It is also found that the distributions of centerline temperature and velocity of fire array are getting close to that of single axisymmetric fire plume, when the flames change from unmerged state to merged state. (2)Focusing on the flame merging process and the change rule of combustion characteristic, the experiments of two rectangular fires are conducted. By changing the length-width ratio of burner, the spacing between burners and the heat release rate, flames with different merging states are obtained. By theoretical analysis and experimental observation, it is found that the flames trend to get merged with increasing the length-width ratio of burner and the heat release rate and decreasing the spacing between burners. Based on the observation, the flame merging process is divided into three flame merging stages: independent stage, intermittent stage and consistent stage. These stages can be further divided into five flame merging states: State I with no flame interaction, State II with flame interaction but no flame merging, State III with intermittent flame merging, State IV with partly flame merging and State V with completely flame merging. Flame merging probability is used to describe the level of flame merging and a correlation with Boltzmann function is used to describe the variation trend of flame merging probability. The variation trends of flame height in different merging states are also studied. Coupling burner spacing and heat release rate, two empirical models are established to describe flame height. It is found that the change rule of flame height of two rectangular fires is the same as that of single fire in State I. Due to the competition between the restriction of air entrainment and pressure drop in the space among the flames, the ratio of flame height to single flame height trends to be a constant in State II. The dimensionless flame height increases with the dimcnsionlcss heat release rate under the same power law in State III and State IV and the power exponential is larger than 2/3. It is also found that the two flames behave as one single larger flame in State V. Moreover, the distribution characteristics of temperature and velocity are discussed. It is found that the distributions of centerline temperature and velocity of two rectangular fires are getting close to that of single line fire plume, when the flames are going to get merged. (3)Focusing on the flame merging morphological characteristics and the change rule of flame height, the experiments of two square fires are conducted. By changing the size of burner, the spacing between burners and the heat release rate, flames with different merging states are obtained. It is found that nearly no flame tilt is observed when the flames get merged and the flames may first get touched with each other at the base of flames. So the flame interaction between two square fires is weak and it is hard for flames to get merged. Dimensionless model is also developed to describe the flame merging probability. The variation trends of the ratio between flame height of two flames and single flame height are discussed. It is found that the flame height ratio increases from 0.85 to 1.15 with the increase of dimensionless heat release rate when the burner spacing is zero and this could be explained by the change of air entrainment model. Key Words: Multiple fires burning; Fire array; Rectangular fire; Flame merging; Merging probability; Flame height; Temperature distribution

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