【作者】 阳东；

【导师】 霍然；

【作者基本信息】 中国科学技术大学 ， 安全技术及工程， 2010， 博士

【摘要】 狭长受限空间是指长宽比较大的建筑结构,包括交通隧道、走廊、地下通道等具体形式。由于狭长受限空间火灾的危害性,它引起越来越多火灾科学工作者的关注。统计表明,有毒有害的烟气是火灾中最主要的致死因素,而狭长受限空间的结构特点和通风条件决定,其火灾产生的烟气难以立即排出,因此,狭长受限空间中火灾烟气的输运特性是值得关注的问题。其中,火灾烟气的分层与卷吸特性是与消防安全设计密切相关的科学问题。本文围绕狭长受限空间火灾烟气分层与卷吸特性,采用试验、数值模拟和理论分析相结合的方法开展研究。为了开展本文的相关研究,建立了66m(长)×1.5m(宽)×1.3m(高)的隧道火灾试验台,发展了用于测量火灾烟气分层形态的试验方法和适于火灾烟气速度场测量的Particle Image Velocimetry (PIV)试验系统,并且在火灾流场相干结构的理论分析方法上做了一些探索。本文考查了纵向强迫气流对狭长受限空间火灾烟气微观流场结构的影响,通过试验揭示了影响火灾烟气层对下层空气卷吸量的物理机制,获得了烟气层维持稳定性的无量纲判据,发现了CO浓度竖向分布曲线和温升竖向分布曲线的不一致性并揭示了造成二者差别的物理因素。具体的工作包括：分别利用数值模拟和粒子图像测速技术(PIV)获取狭长受限空间中火灾浮力驱动的分层流流场信息,利用特征正交分解(POD)方法对其流场相干结构和能量谱进行提取,分析发现,狭长受限空间中的纵向气流使流场中大尺度流场结构的能量向小尺度结构转移,并使得剪切层附近竖向流动的能量增加。为揭示影响狭长受限空间火灾烟气层对下层空气卷吸的物理机制,在隧道火灾试验台中开展试验,并与全尺寸隧道火灾试验进行对比,结果发现,火灾烟气层对下层冷空气的卷吸,除受到分层流经典卷吸模型中提到的Richardson数的影响外,还受到Reynolds数的影响。当Re＜104时,火灾烟气分层流没有达到足够的湍流强度,火灾烟气对下层空气的卷吸可以忽略；当Re数达到Re～105时,火灾烟气层对下层空气产生的卷吸加强,其卷吸系数接近Ellison和Turner模型的预测值。通过试验获得狭长受限空间火灾烟气层维持稳定性的无量纲判据。结果表明,当Ri＞0.9或Fr＜1.2时,浮力起主导作用,烟气分层能维持稳定;当0.3＜Ri＜0.9或1.2＜Fr＜2.4时,惯性力的作用开始变得明显,分层界面上不稳定的涡旋数量增多,导致部分烟颗粒向下部空间扩散；当Ri＜0.3或Fr＞2.4时,惯性力起主导作用,烟颗粒与冷空气大量掺混,烟气层的稳定性被破坏。通过数值模拟发现,在弧形隧道中,当近测壁火和纵向通风联合作用时,烟气出现螺旋状的分层形态,其“螺旋”长度与火源功率和纵向风速有关,火源功率越大,“螺旋”长度越小；纵向风速越大,“螺旋”长度越大。通过试验发现了CO浓度竖向分布曲线和温升竖向分布曲线的不一致性。通过对无量纲输运方程及其边界条件的分析,揭示了造成二者差别的物理因素。结果发现,当采用自然通风时,CO浓度随高度降低而衰减的趋势明显弱于温升：当纵向通风强度较大时,二者的竖向分布曲线趋于一致。理论分析和试验结果均表明,壁面传热的强度是影响CO浓度竖向分布与温度竖向分布相似性的主要物理因素,壁面传热的强度越小,二者的相似性越好。更多还原

【Abstract】 A fire occurs in a confined space with relatively large ratio of length to width can be defined as a channel fire, which includes tunnel fire, corridor fire, underground passage fire and so on. Since channel fire brings about tremendous fire safety problems, it attracts increasing attentions from both fire safety engineers and fire researchers. Statistics have shown that the toxic fire smoke is the most hazardous factor in a fire. Due to the characteristic of structure and that of ventilation condition, it is difficult to exhaust fire smoke out of a channel immediately. Therefore, the characteristics of smoke transport in a channel should be concerned. Entrainment and stratification of fire smoke are closely related to fire safety design, the physics of which should be understood first.Experiments, numerical simulation and theoretical analysis were carried out to investigate the characteristics of entrainment and stratification in a channel fire. To carry out experimental study, a reduced-scale tunnel with dimensions of 66m (Length)×1.5m (Width)×1.3m (Height) were designed and constructed. Experimental techniques for stratification pattern measurement were developed. Experimental system for Particle Image Velocimetry (PIV) of fire-induced flows was also developed. Some new analytical techniques for fire-induced flow structure were proposed. This thesis focused on four issues:Effects of longitudinal air flow on the structure of smoke flow; Entrainment physics of smoke layer; the dimensionless criteria for stability of smoke layer; the differences between the vertical profiles of temperature rise and those of CO concentration. The main contents include:Vortex fields or velocity fields of the fire-induced flows were obtained from numerical simulation and PIV measurements. Proper Orthogonal Decomposition (POD) was used to extract coherent structures from the fire-induced flows. Energy spectrums of these structures were also obtained. Results indicate that, with the increase in longitudinal ventilation velocity, the energies of the large-size structures decrease but those of the small-size structures increase. Further, the ratios of energy of vertical flow pattern to that of horizontal flow pattern increase with the increase in longitudinal ventilation velocity.Experiments were conducted to investigate the physics that control air entrainment into smoke layer. Results indicate that, besides Richardson number, Reynolds number is also an important dimensionless parameter that affects the amount of air entrainment into smoke layer. Smoke flows of reduced-scale experiments have the orders of Reynolds numbers lower than 105, which are demonstrated to be lower than the critical Reynolds number that is necessary to sustain fully inertial turbulent fluctuations, and thus hardly entrain the fresh air. Whereas smoke flows of full-scale experiments have Reynolds numbers on the order of 105, which can sustain fully inertial turbulent fluctuations and the entrainment coefficients were close to the laws of Ellison and Turner.Dimensionless criteria for sustainability of stability of fire-induced smoke layer were obtained. The stratification pattern was found to fall into three regimes. Buoyancy force and inertia force, as the two dominant factors that affect the buoyant flow stratification, were correlated through the Froude number and the Richardson number. At RegionⅠ(Ri>0.9 or Fr<1.2), the buoyant flow stratification was stable, where a distinct interface existed between the upper smoke layer and the lower air layer. At RegionⅡ(0.3<<Ri<0.9 or 1.2<Fr<2.4), the buoyant flow stratification was stable but with interfacial instability. At Range III (Ri<0.3 or Fr>2.4), the buoyant flow stratification becomes unstable, with a strong mixing between the buoyant flow and the air flow and then a thickened smoke layer.Differences between vertical profiles of CO volume concentration and those of temperature rise were found. By analyzing dimensionless transport equations and boundary conditions, the factors that are attributed to these differences can be revealed. Results indicated that, under the conditions with natural ventilation or relatively weak longitudinal ventilation, CO volume concentration decays much more slowly than temperature rise in the vertical direction, whereas, under the conditions with strong longitudinal ventilation, the vertical profiles of CO volume concentration and those of temperature rise show similarity. The intensity of heat transfer from smoke flow to wall boundaries is demonstrated to be closely related to the correlation between vertical profiles of CO volume concentration and those of temperature rise. A small amount of heat loss from smoke flow to wall boundaries lead to a higher similarity between vertical profiles of CO volume concentration and those of temperature rise.更多还原