房间开口的相对位置对影响轰燃发生时间的研究

利用FDS对室内火灾轰燃现象进行了数值模拟,主要从改变房间开口的相对高度与水平位置,模拟室内火灾的变化情况,探讨了不同通风开口位置条件对轰燃发生时间和破坏程度的影响。     

建筑火灾中回燃对灭火救援的影响及对策

在简要介绍回燃现象理论的基础上,分析了回燃现象的危害性及回燃对灭火救援的影响,提出了扑救建筑火灾过程中如何避免回燃的发生以及回燃后如何减少人员伤亡.回燃前对室内喷水降温,打开排烟口排放可燃烟气,运用高压细水雾预防回燃发生.回燃后重新部署力量,堵截火势,防止轰燃发生.冷却承重构件,防止建筑物倒塌.     

通风条件对受限空间回燃的影响研究

利用FDS对不同通风条件下受限空间的回燃过程进行数值模拟。通过对比不同通风口高度、宽度及位置等条件下的热释放速率、热烟气层温度等参数的发展变化,分析通风条件对受限空间回燃的影响。结果表明:通风口宽度对受限空间回燃基本没有影响,但通风口高度对回燃的影响显著,当高度不大于0.6m时回燃难以形成;当受限空间存在阻碍空气流动的障碍物时,通风口位于中部位置不易形成回燃,若障碍物不阻碍空气流动则角部位置不易形成回燃。建议消防员在对卧室类房间进行灭火救援时,在中部位置破拆宽度较大但高度不大于0.6 m的孔洞。     

特殊火灾燃烧现象仿真优化方法研究

为了提高特殊火灾燃烧现象仿真的逼真程度,提出了以火灾动力学模型为支撑,利用FDS进行二次开发的技术路线。在建立轰燃、回燃、火旋风的数学模型基础上,将其耦合到FDS子程序当中,编译出新的程序。利用新的FDS程序模拟了三种燃烧现象,并与实验现象进行了比对。从四种输出文件的格式、意义和内部结构出发,解析出了燃烧现象的数据,为特殊火灾处置虚拟仿真训练系统的开发提供了新的思路。     

回燃现象与气体爆炸的相似性分析

利用火灾理论分析回燃与轰燃两种特殊现象的产生过程,区分两种现象的差异,明确回燃现象的特性,阐述了气体爆炸的分类、定义及特点,并与回燃现象进行了对比分析,最后,通过燃烧热和气体流动特性两方面研究讨论能够产生回燃现象的最低可燃气体组分高于其气体爆炸极限的原因,同时提出了一些减少回燃现象危害的建议。     

装饰材料对室内回燃特性的影响分析

选取几种典型建筑装饰材料,测试不同内衬材料热惯性下热烟气层温度、温升速率和回燃延迟时间等参数的变化,研究其对室内回燃特性的影响。     

建筑室内火灾温度的预测分析

为了得到室内火灾温度,采用理论推导和数值模拟相结合的方法对其进行研究。首先建立了室内火灾温度场的数值模拟方法,以Steckler开口流动试验数据作为参考,对比软件FDS模拟室内火灾开口流动的准确性及适用性;其次通过火灾室内热烟气层的热平衡得到了室内火灾温度的基本公式,公式体现了影响室内温度的相关因素;最后采用火灾场分析软件FDS建立多种火灾情况模型,从而得到火灾室内热烟气层温度,进而得到基本公式中相关参数的取值。分析表明:室内火灾温度随着火源热释放速率的增加而增大,随着围护结构的有效热传递系数和通风口尺寸的增加而减小。     

密闭腔室火灾温度的大涡模拟

应用大涡模拟方法模拟全尺寸的密闭腔室火灾场景。提出应用平均燃烧效率的概念模拟密闭腔室火灾的方法,选取平均燃烧效率为0.75进行大涡模拟,得到密闭腔室室内温度场以及烟气运动的模拟数据。通过分析密闭腔室内竖直方向上的温度分布,发现在竖直方向上存在温度分层现象。将模拟结果与实验数据进行对比可以看出,大涡模拟可以较好地模拟密闭腔室内火灾的发生发展情况。     

基于FDS的室内火灾回燃现象数值模拟分析

用FDS对一次火灾现场进行数值模拟,通过分别设置不同火源功率、不同火源位置,以及加入不同位置排烟送风进行对比分析,探究室内火灾回燃现象中的一些通性,结合消防员灭火救援过程中的常见行为,得出安全建议。     

中庭建筑邻室及开口尺寸对烟气运动规律影响研究

利用FDS模拟了中庭建筑不同邻室及开口尺寸条件下的烟气运动。讨论了模拟主要现象及各工况下中庭和邻室烟气温度随时间分布特点,得到烟气运动与邻室尺寸及开口尺寸的关系,并对两种因素对烟气运动影响程度进行了分析。研究结果表明,开口尺寸一定时,相同体积的邻室对中庭内烟气运动影响可忽略;由邻室溢出到中庭的烟气量随邻室体积增加不断减小,随开口尺寸减小不断减小;相对于邻室尺寸,开口尺寸对邻室内烟气运动影响更为明显。     

Subgrid scale laminar flamelet model for partially premixed combustion and its application to backdraft simulation

Flame spread after air is suddenly introduced to a vitiated compartment in backdraft is between non-premixed and premixed flame spread under a ventilation-controlled condition. And it is necessary, but difficult to numerically simulate it. In this paper, an attempt of backdraft simulation is introduced. Numerical models including a subgrid scale laminar flamelet model and a partially premixed model are imbedded in FDS3.0 source code for backdraft simulation. Some significant fire characteristics reported in previous backdraft experiments can be seen in the numerical results. It is also indicated that these combined models can be used to predict the partially premixed combustion and fire phenomena under ventilation-controlled conditions.     

Large eddy simulation of the backdraft phenomenon

A sub-grid scale model for partially premixed combustion has been adapted and applied to simulate the backdraft phenomena. A fast deflagration or backdraft is produced when into a hot, fuel-rich compartment an inflow of fresh air is suddenly allowed through an opening. It is essentially a violent combustion process involving both premixed and non-premixed regimes. The present model is based on the coupling of independent approaches for non-premixed and premixed turbulent combustion. The ‘flame index’ concept was used to separate the two different combustion regimes. This index describes the structure of the flame based on fuel and oxygen gradients. Due to the lack of detailed experimental measurements, the results were largely analysed qualitatively. The predictions have provided valuable insight into the backdraft phenomenon suggesting that the development of backdraft can be divided into five phases, i.e. initial condition, free “spherical propagation, “plane” front propagation, stretching of the flame front through the opening and fireball outside the container. Quantitatively, the experimentally measured and predicted lapse of time between the maximum over- and under-pressure at the opening of the container is found to be in reasonably good agreement.     

自然排烟口对机场航站楼排烟效果的影响

采用 FDS 模拟与理论计算相结合的方法，设定 6 种上、下开口面积之比的工况进行模拟，研究自然排烟口对机场航站楼排烟效果的影响。结果表明：当上、下开口面积之比不大于 1.8时，随着排烟口面积增加，室内平均温度下降，烟气层高度抬升，排烟效果越来越好；当火灾发展至 720 s 时，室内平均温度仍在安全范围内，对于人员疏散影响不大。建立理论模型，与模拟结果进行对比，得出当上、下开口面积之比为 1.8 时理论模型适用性较好。     

Influence of Obstacles on the Development of Gravity Current Prior to Backdraft

The phenomenon of backdraft is closely linked to the formation of a flammable region due to the mixing process between the unburned gases accumulated in the compartment and the fresh air entering the compartment through a recently created opening. The flow of incoming fresh air is called the gravity current. Gravity current prior to backdraft has already been studied, Fleischmann (1993, Backdraft phenomena, NIST-GCR-94-646. University of California, Berkeley) and Fleischmann (1999, Numerical and experimental gravity currents related to backdrafts, Fire Safety Journal); Weng et al. (2002, Exp Fluids 33:398–404), but all simulations and experiments found in the current literature are systematically based on a perfectly regular volume, usually parallelipedic in shape, without any piece of furniture or equipment in the compartment. Yet, various obstacles are normally found in real compartments and the question is whether they affect the gravity current velocity and the level of mixing between fresh and vitiated gases. In the work reported here, gravity current prior to backdraft in compartment with obstacles is investigated by means of three-dimensional CFD numerical simulations. These simulations use as a reference case the backdraft experiment test carried out by Gojkovic (2000, Initial Backdraft. Department of Fire Safety Engineering, Lunds Tekniska Högskola Universitet, Report 3121). The Froude number, the transit time and the ignition time are obtained from the computations and compared to the tests in order to validate the model.     

The Use of CFD Calculations to Evaluate Fire-Fighting Tactics in a Possible Backdraft Situation

This paper is an attempt to integrate theoretical Computational Fluid Dynamics (CFD) calculations with practical fire-fighting tactics commonly used when arriving at the scene of an underventilated fire. The paper shows that CFD has a great potential in improving understanding and creating better effectiveness in the estimation of fire-fighting tactics. If burning has occurred in a lack of oxygen for a long time, excessive pyrolysis products may have accumulated in the fire compartment. If air is suddenly introduced in the compartment a backdraft may occur. The CFD code used for the simulations is fire dynamics simulator (FDS). In this paper, we focus on the conditions that can lead to backdraft, and not the deflagration or rapid combustion in itself. Therefore, the simulations focus on the gravity current and the mixing process between cold fresh air and hot smoke gases by considering a uniform temperature inside the building as initial condition. The different tactics studied include natural ventilation, positive pressure ventilation (PPV) and dilution by water mist. Their effectiveness is observed comparing them with a reference scenario, where no action is taken. The main objective of natural ventilation is to find the fire source, and the venting is more effective with several openings. Tactics involving PPV are very effective in evacuating the unburnt gases, but increases the mixing, and consequently the probability of backdraft during the early stage of operation. On the other hand, the addition of water mist can reduce the danger of backdraft by reducing the concentration of unreacted combustible gases below the critical fuel volume fraction (CFVF), where ignition cannot occur. If the dilution level is insufficient the danger of backdraft is increased, mainly because the process of gases evacuation is longer due to cooling, which reduces the density difference between hot and cold gases. During a fire-fighting operation, the choice of tactic depends mainly on whether there are people left in the building or not, but also on the fire-fighters’ knowledge of the building’s geometry and the fire conditions. If the situation shows signs of strongly underventilated conditions, the danger of backdraft has to be considered and the most appropriate mitigation tactics must be applied.     

地铁火灾中回燃现象研究初探

回燃是在通风受限的建筑火灾进入缺氧燃烧甚至闷烧后,由于新鲜空气的突然大量补充引起热烟气急剧燃烧的现象.目前腔体回燃的研究已取得很多有价值的成果.在总结前人研究成果的基础上,结合地铁火灾的特点,指出地铁火灾中也存在回燃现象,并简略地提出了针对地铁火灾中回燃现象应进行的研究内容.     

通风条件对单侧开口室内火灾起火点偏移影响研究

以一起出现“起火点偏移”现象的单侧开口室内火灾为研究对象,利用FDS进行仿真,通过改变通风口数量和通风口位置,对比分析不同通风条件下火势蔓延发展、温度分布、火焰和烟气分布和热释放速率等的差异。结合火灾现场痕迹特征,探讨通风口数量和位置与单侧开口室内火灾发展蔓延及现场燃烧痕迹的关系,以及通风条件变化导致“起火点偏移”的原因,为类似场所火灾调查排除干扰痕迹,准确判定起火部位（点）提供参考。     

Numerical simulation of pressure changes in closed chamber fires

Pressure rising in closed chamber fires will be studied numerically in this paper with Computational Fluid Dynamics (CFD). The Fire Dynamics Simulator (FDS) developed at the National Institute of Standards and Technology (NIST) in USA is taken as the simulation tool. Experiments on closed chamber fires were reported in the literature by the Defence Research Establishment (FoA) in Sweden. Scenarios similar to those in that experimental study were taken as examples for simulation. Air flow pattern, pressure and temperature distribution are predicted. Functional analysis is used to compare predicted values with experiments. This gives some indication on how good CFD can be when applied for studying closed chamber fires. It is observed that input heat release rate to FDS is a key point. Predicted results on air velocities, temperature and pressure agree reasonably well with experimental results. Care should be taken in applying the fast reaction mixture fraction combustion model for simulating under-ventilated fires.     

Exploratory backdraft experiments

This study is a qualitative exploration of backdraft phenomena. Backdraft is defined as a rapid deflagration following the introduction of oxygen into a compartment filled with accumulated excess pyrolyzates. A scenario describing the physical and chemical fundamentals underlying backdraft phenomena is presented. A half-scale apparatus, designed to avoid dangerous overpressures, was used to obtain data from backdraft experiments. A gas burner supplied a 150 kW natural gas fire in a 1.2 m high, 1.2 m wide, 2.4 m long compartment with a small, 25 mm high, 0.3 m wide vent to ambient at floor level. Significant unburnt fuel accumulates in 180 seconds, when a hatch covering a 0.4 m high, 1.2 m wide vent, centered on a short wall, is opened, simulating a window breaking due to thermal stresses. The propagation across the compartment of the cold density-driven flow, which enters through the new opening, is called a gravity current. This gravity current carries a flammable mixed layer to a spark located near the burner on the opposite wall. The rapid deflagration that results upon ignition of the mixed layer is the backdraft. A compartment fire model is used to calculate conditions in the compartment before the vent opens. The hypothesized scenario appears to be confirmed by the deflagrations and exterior fire balls observed in these preliminary experiments.