封闭空间火灾——轰燃

The growth of a fire confined to a room is usually constrained by the ventilation-limited flow of air, smoke, and hot gases into and out of the enclosure. Confining variables may include the ceiling height, ventilation openings formed by windows and doors, room volume, and location of the fire in the room or compartment. While the fire gases are constrained, flashover may occur as a consequence of the heat release rate of the combustible materials contained within the room.

    局限在室内的火灾，其发展通常受到有限空气流动、烟气和进出房间热气流的影响。限制变量包括：顶棚高度、门窗构成的通风开口、房间体积、房间或隔间内的起火位置。如果火灾烟气被限制在房间内，就有可能因为室内可燃物燃烧产生的热释放速率达到某个值而发生轰燃。

In the growth of a compartment (room) fire, flashover is defined by NFPA 921 (2017d) as the transition phase in the development of a compartment fire in which surfaces exposed to thermal radiation reach ignition temperature more or less simultaneously, and fire spreads rapidly throughout the space, resulting in full room involvement, or total involvement, of the compartment or enclosed space. This transition may occur in just a few seconds in small, heavily fueled rooms, or over a period of minutes in large rooms, or may not take place at all due to an insufficient heat release rate of the fire. A useful reference on this subject is NFPA 555: Guide on Methods for Evaluating Potential for Room Flashover (NFPA 2017d).

    在室内火灾发展过程中，NFPA 921（2017版）将轰燃定义为在室内火灾发展中，房间内物体表面受到热辐射作用，所有可燃物几乎同时达到着火温度，火势快速蔓延至整个房间，造成整个房间发生燃烧或全面燃烧的过渡阶段。这种过渡阶段在体积小、可燃物多的房间里可能只需几秒，对于较大房间，则需经过几分钟，或者因为火灾的热释放速率过低，根本就不会发生。关于这个问题，《NFPA 555：室内轰燃评估方法指南》（NFPA 2017版）中提供了参考。

Fire researchers have documented other observations during this transition phase. Defining observations often include one or more of the following criteria that address the effects of flashover: ignition of floor targets, high heat flux (>20 kW/m2) to the floor, and flame extension out of the compartment’s vents (Babrauskas, Peacock, and Reneke 2003; Milke and Mowrer 2001; Peacock et al. 1999).

    火灾调查员还记录了在该过渡阶段观察到的其他现象。典型的现象包含下列一个或多个表征轰燃影响效果的标准：地面可燃物被引燃、地面接受到高的热通量（＞20kW/m2），火焰从通风开口处窜出。

The investigator may observe the following during flashover:

调查人员在轰燃过程中可能会观察到以下现象：

Flames are emitted from openings in the compartment.

火焰从房间通风口处窜出。

Upper-layer gas temperature rises to ≥ 600℃.

上层气体温度达到600℃及以上。

Heat flux at floor level reaches ≥ 20 kW/m2

地面受到的热通量达到20kW/m2以上。

Oxygen level in upper portions of the room decreases to approximately 0 to 5 percent.

房间上部的氧气含量下降到0~5%左右。

There is a small, short-term pressure rise of approximately 25 Pa (0.0036 psi).

存在短暂小幅度的压力升高，约25Pa（0.0036psi）。

There are two fundamental definitions for the occurrence of flashover. The first looks at flashover as a thermal balance in which critical conditions are produced when the compartment exceeds its ability to lose heat. The second definition views the compartment as being in a mechanical fluid-filling process. In this approach, the point at which the compartment’s cool air layer is replaced with hot fire gases is flashover.

    对轰燃的发生有两个基本定义。第一个是将轰燃视为热平衡，当房间内部产生的热量超过其散热能力时，会达到轰燃发生的临界条件。第二个定义是认为房间处于流体充填的机械过程中，在这一定义中，室内冷空气被火灾热烟气取代的点就是轰燃的发生。

Those who examine a fire scene after extinguishment or burnout should be aware of the importance of recognizing whether flashover did actually occur and the impact it may have had on the burning of a room’s contents.

    火灾调查员在灭火或可燃物烧尽后进入现场时，应该意识到是否发生轰燃以及轰燃对房间内物品燃烧造成的影响的重要性。

Post-flashover burning in a room produces numerous effects that were once thought to be produced only by accelerated arson fires involving flammable liquids such as gasoline. A fireball of burning gasoline vapors can sometimes produce floor-to-ceiling damage. The brief duration of such fires will result in shallow penetration. No matter how the fire began, walls scorched or charred from floor to ceiling can be due to post-flashover burning as well as from a fireball of burning gasoline vapors.

    房间发生轰燃后燃烧产生的许多痕迹，曾被人们认为是只有汽油等易燃液体作为助燃剂实施放火时才会出现的。汽油蒸气燃烧形成的火球有时会造成从地板到顶棚的整体性破坏。这种火灾持续时间较短，形成浅层烧灼。无论火灾是如何引发的，轰燃后的燃烧和汽油蒸气燃烧的火球一样，都会使地板到顶棚的墙体烧焦炭化。

Fire damage under tables and chairs, once thought to be the result of the burning of flammable liquids at floor level, can be produced by the flaming combustion of the carpet and pad as they ignite during flashover. High temperatures and total room involvement, at one time thought to be linked to flammable liquid accelerants, are produced during post-flashover burning without accelerants. The extremely high heat fluxes produced in post-flashover fires can char wood or burn away other material at rates up to 10 times the rate at lower heat fluxes (Babrauskas 2005; Butler 1971).

    过去曾认为桌椅下方的火灾破坏是由地面易燃液体燃烧造成的，但事实上轰燃时被引燃的地毯和垫子的有焰燃烧也会造成这类破坏。高温和整个房间参与燃烧，曾被认为与使用易燃液体助燃剂有关，但事实上轰燃过程中即使没有助燃剂参与也会产生这两种现象。轰燃后所产生的极高热通量，会于低热通量条件下的木材炭化速率和其他物质燃烧速率造成10倍

Prolonged post-flashover burning can make the reconstruction of the fire very difficult as post-flashover or full room involvement fires typically ignite all exposed fuel surfaces from floor to ceiling. Localized patterns of smoke and fire damage that can help locate fuel packages, characterize their flame heights, and reveal the direction of flame spread can be destroyed. Some larger rooms sometimes exhibit progressive flashover, with floor-to-ceiling ignition at one end and undamaged floor covering at the other. Brief exposure (less than five minutes) to post-flashover fires does not necessarily compromise fire patterns on walls (see Hopkins, Gorbett, and Kennedy 2007).

    轰燃后的燃烧时间越长，火灾重建工作的难度就越大，因为轰燃后或全面发展阶段的一个特征就是房间内从地板到顶棚所有暴露的可燃物表面都被引燃着火。因此烟气和火灾破坏形成的局部痕迹会被销毁，而这些痕迹的存在能够帮助调查人员认定可燃物位置、确定火焰高度以及判断火势蔓延方向。在某些较大房间里，有时会出现逐步发展的轰燃——房间一端从地板到顶棚全部发生燃烧，而另一端的地板却保持完好。短时间（15min以下）暴露于轰燃后的火灾，墙壁上的火灾痕迹并不一定会被破坏（见Hopkins，Gorbett和Kennedy 2007）。

In pre-flashover burning, the most intense thermal damage will be in areas immediately around or above burning fuel packages. the higher temperatures of the hot gas layer will produce more thermal damage in the upper half or upper third of the room. In post-flashover fires, all fuels are involved, the fires may become ventilation limited, and the most efficient (highest temperature) combustion occurs in turbulent mixing around the ventilation openings, where the oxygen supply is greatest. Oxygen-depleted burning occurs in many post-flashover rooms where the available fuel exceeds the air supply.

    在轰燃前的火灾中，热破坏最严重的位置位于燃烧可燃物周围或正上方。热烟气层温度越高，对房间上部二分之一或三分之一部分造成的热破坏就越为严重。轰燃发生后，所有可燃物参与燃烧，火灾变为通风控制型，效率最高的燃烧（温度最高）发生在通风口附近的湍流混合区，此处氧气供应最为充足。许多轰燃后的室内火灾，可燃物超过空气的供应量，进而发生缺氧燃烧。

Interviews of first-in firefighters may reveal whether a room was fully involved, floor to ceiling, when entry was first made. Investigators must be aware of the conditions that can lead to flashover and how to recognize whether it has occurred. A thorough understanding of heat release rates and fire spread characteristics of modern furniture and the dynamics of flashover will be of great assistance (Babrauskas and Peacock 1992).

    通过询问第一个进入现场的消防员，可以了解进入房间时，是否发生了从地板到顶棚的全室性燃烧。调查人员必须熟知轰燃发生的条件，并且能够辨认是否发生过轰燃。全面理解现代家具火灾蔓延、热释放速率以及轰燃动力学的特征，对于火灾调查工作大有帮助（Babrauskas 和Peacock 1992）。