【作者】 谢小冬；

【导师】 刘乃安；

【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】 本文开展上坡地表火蔓延的实验和理论研究,重点研究燃烧过程中火蔓延速率的变化规律和燃料的预热机制。本文首先研究常规的上坡火,在此基础上研究另外三种受坡度控制的火行为：峡谷火、爆发火和跳跃火,在变坡度火蔓延实验台展开对四种火行为的实验研究,分析不同环境下的燃烧行为、火焰几何参数、流场分布和热流分布等特性,来揭示控制火蔓延速率变化的物理机制。在线火源引燃的上坡火蔓延实验中,坡度角范围为0°-35°。当坡度角小于等于20°时,燃烧过程中火线轮廓维持近似线形；当坡度角大于等于25°时,燃烧过程中火线轮廓从线形逐渐转变成一个稳定的V形,之后火线夹角不再随时间变化。对于两种不同的火线形状,火蔓延速率在整体上均是稳定的。本文通过对线火源引燃的上坡火开展实验和理论研究,来分析火线轮廓对燃料预热机制的影响。对燃料床中心线上的辐射热流的数值计算表明,控制单一变量的条件下,火线夹角仅在一定的范围内影响辐射热流分布；辐射热流主要来自火焰宽度方向上的中间部分；火焰倾斜角和火焰长度在整个范围内都会影响辐射热流分布。利用实验测量的火线夹角,与经验公式计算的火焰倾斜角和火焰长度,来计算上坡火蔓延实验中,火头前方燃料床中心线上的辐射热流分布。结果表明,当坡度角从20°上升至25°,火线轮廓从线形转变成v形时,辐射热流的最大值降低。计算未燃区燃料面上的辐射热流分布,发现线形火线辐射热流的最大值在中心线上,而V形火线辐射热流的最大值在燃料床两侧。从计算的辐射热流分布、测量的流场分布、能量方程三个角度,研究不同的火线轮廓对对流传热机制的影响,得出一致的结论：对于线形火线,火头前方中心线上的燃料受到对流冷却的作用；对于V形火线,火头前方中心线上的燃料受到对流加热的作用。概括分析上坡火蔓延过程中燃料的预热机制,对于线形火线,辐射传热起主导作用；对于V形火线,辐射传热和对流传热共同作用。对于峡谷地形,受到两个重要的几何参数——中间坡度角和侧面坡度角的控制。本文通过对峡谷火蔓延开展实验和模型研究,来揭示峡谷的两个坡度角对火线发展的影响。在峡谷火中,侧面坡度角为零时,相当于常规的上坡火蔓延；对于点火源或线火源引燃的上坡火,火头均沿着最大坡度线方向蔓延。在峡谷火中,当侧面坡度角大于零,使用点火源引燃,火头蔓延的方向偏离最大坡度线,偏向燃料床的中心线；当中间坡度角上升至一定值(本文中为30°),火头仅沿着燃料床的中心线蔓延。在峡谷火中,固定侧面坡度角,不同的中间坡度角可导致火线形状的显著差异；对于较大的中间坡度角,两侧燃料床上火焰之间的相互作用会增强火焰的辐射传热,同时会促使热气体流过燃料床中心线上,对燃料产生强烈的对流加热作用。受到对流加热引起的正反馈效应,峡谷中心线上的火蔓延速率和热释放速率都会急剧上升,甚至产生爆发火。爆发火是通常发生在峡谷、长的陡坡、沟壑等特殊地形,以短时间内火蔓延速率和热释放速率突变式地增长为特征的森林火灾现象,是一种极端火行为。本文回顾了爆发火的研究现状,总结出爆发火的理论和实验研究都非常匮乏,缺少爆发火形成机制的动力学模型或物理模型。本文分析了四川草原火灾案例中的爆发火现象。本文从火焰加速的角度来初步研究爆发火的形成机制：在上坡火蔓延中,火焰附壁是形成火焰加速的关键因素；在峡谷火蔓延中,对流加热引起的正反馈效应,是产生火焰加速的主要原因。当两条火线相交,它们的交点在强烈的辐射与对流加热作用下,会产生一个很快的火蔓延速率,形成跳跃火。本文基于上坡火和跳跃火的实验研究,分析两种火行为的火线夹角、火蔓延速率、无量纲辐射热流分布的变化规律,对比分析了两种火行为的异同点。点火源引燃的上坡火中,一旦形成明显的火线夹角,其大小不再随时间变化：跳跃火中,火线夹角不断增加直至1800。线火源引燃的上坡火蔓延速率在整体上是稳定的；点火源引燃的上坡火蔓延速率随时间缓慢增大至峰值,后下降；跳跃火蔓延速率在初始时急剧上升至峰值,之后逐渐减小。在跳跃火中,坡度角对于火线夹角和火蔓延速率变化的影响远大于初始火线夹角带来的影响。无量纲的辐射热流分布可以有效地解释线火源引燃的上坡火与跳跃火的火蔓延速率变化规律。更多还原

【Abstract】 This paper presents experimental and theoretical researches on upslope surface fire spread. The aim is to investigate the spreading behaviors of flame fronts and the mechanisms of fuel preheating in the fire spread process. The research scope covers the general upslope fire, canyon fire, eruptive fire and jump fire. All the experiments were performed on an upslope fuel bed bench. The burning behaviors, flame geometrical characteristics, heat flux distribution and flow velocities are examined. This work tries to reveal the governing physical mechanism for the evolution of fire spread under different conditions.Upslope fire tests (slope angles:0°～35°) for line ignition were carried out, by using pine needles as the fuels. Experimental results indicate that when slope angles were less than20°, the fire line contour remained nearly linear in the tests. When slope angles were over25°, the fire line contour underwent a transition from linear shape to V-shape, with a steady fire line angle. The rate of fire spread remained steady in global sense for the two types of fire lines.In this paper, the impact of fire line contour on the burning behaviors and fuel preheating is investigated by experiments on pine needle fuel bed and computational analyses. Numerical results show that the radiation heat flux in the centerline has minor dependence on fire line angle within certain ranges, and is mainly attributable to the central fire line segment of a certain size. The radiation heat flux depends on the fire tilt angle and flame length in the whole range. The fire line angle is extracted from experimental results, while the flame tilt angle and flame length are evaluated respectively by the empirical equations combined with experimental data. When the fire line changes from linear shape to V-shape with increasing slope angle, the radiation heat transfer along the centerline undergoes a reverse decrease. For the calculation of the heat radiation distribution on the unburnt fuel bed area, the maximum heat flux appears on the centerline of the fuel bed for a linear fire line, while the maximum value appears at a certain lateral position for a V-shape fire line. Further, analyses on the calculated radiant heat flux distribution, the measured velocity data, and the energy conservation equation justify the convective heating in V-shape fire lines. It is revealed that radiant heating is the dominant mechanism for a linear fire line spreading over pine needle fuel beds, while the fire spread for a V-shape fire line is dominated by the combined effects of radiant and convective heating.In canyon topography, central slope angle and lateral slope angle are the two key parameters. This paper investigates the effect of canyon slope on the fire line evolution by a geometrical model and experimental data. For zero lateral slope angle, which corresponds to normal upslope fires, both line ignition and point ignition were used. Fire head spreads along the maximum slope angle line. For canyon fire tests with non-zero lateral slope angles, dead pine needles were used as the fuels, and the fires were all initiated by a point ignition. It is found that in canyon fire, the trace of fire head deviates from the line of the maximum slope and approaches the centerline of the fuel bed. When the central slope angle is over a certain value (30°in this work), the fire head would spread just along the centerline of the fuel bed. For a fixed lateral slope angle, different central slope angles may cause distinct fire line contours. For higher central slope angles, the significant interaction between the two lateral fire lines enhances both the heat radiation from the two lateral fires and the forward hot gas flow, leading to significant enhancement in convective heating. Due to the positive feedback mechanism of heat convection, both the fire spread rate and the heat release rate increase sharply, which may cause an eruptive fire.Eruptive fires usually occur in special terrains of canyon, long and steep slope, or trench. Eruptive fires are characterized by the sudden increase of fire spread rate and the heat release rate. This paper presents an overview on the current status and challenges for the research of eruptive fires. However, there is obvious lack of experimental data and theoretical models for interpretation of the physical mechanisms and dynamics of eruptive fires. One eruptive fire accident in Sichuan grassland fire is analyzed. The theory of fire acceleration is introduced to interpret the generation of eruptive fire in upslope fires or canyon fires. The flame attachment is a key factor for eruptive fire in upslope fire. The positive feedback mechanism of heat convection in canyon fire is suggested to be the potential mechanism for eruptive fire.When two fire lines intersect with each other, the crossing point spreads very quickly under the enhanced convective and radiative heat transfer mechanism, which could induce jump fire. This paper presents an elementary analysis on the difference and similarity between upslope fire and jump fire, by a series of experiments performed in laboratory. The rate of fire spread, fire line angle and non-dimensional radiant heat flux for the two kinds of phenomena are investigated. For upslope fire with point ignition, the initially generated fire line angle remains steady. For jump fire, the fire line angle increases with time. For upslope fire, it is found that ROS remains almost steady for line ignition, while for point ignition, the ROS increases with time to a peak value and then decreases. For jump fire, the ROS first increases sharply and then decreases gradually. For jump fire, both the fire line angle and fire spread rate depend on the slope angle more significantly than the initial fire line angle. Non-dimensional radiant heat flux for fuel preheating is calculated which effectively explains the ROS development in upslope fire with line ignition and that in jump fire.更多还原