Fire behaviour

SONY DSCWhen we talk about fires, we often use the term fire behaviour, especially, when the fire is in a building or in a room. Then, fire behaviour refers to how fire “behaves”, i.e. how it develops and what happens with fire and objects or materials exposed to fire.

Imagine an ordinary living room, i.e. a room with a television, a sofa, two armchairs, a small table with a cloth, a carpet on the floor, a few pictures on the walls, and a lamp hanging from the ceiling. It is Christmas Eve and someone forgets a candle on the table. After a few hours the candle has burnt down to the wooden candel stick (with paper and pastic decorations, of course). The decoration catches fire and the fire spreads to the candle stick. This is what is refered to as the initial fire.

From this initial fire, a plume of hot gases and particles, fire gases, raises to the ceiling. Even though the fire gases can be very dark and very hot, they consists mainly of air that has been heated by the flames. Of course, the fire gases containes gases such as carbon monoxide, carbon doixide and a number of various hydrocarbons. The fire gases also contains particles, soot.

The plume raises upwards, hits the ceiling, and forms a ceiling jet which spreads out beneath the ceiling. Eventually, the fire gases reaches the walls – a warm upper layer of fire gases is formed. This upper layer of fire gases may in many cases be more or less invisible at at start. But as more and more particles (soot) is formed, the layer gets darker and darker and of course also hotter and hotter. In an ordinary room fire, this process takes just about the same time as it takes to read to hear. However, it should be noted that this is a very complicated process which is controlled by a large number of factors.

If there is a sufficient amount of air (oxygen) in the room for the initial fire to grow, the hot upper layer of fire gases will grow in depth (measured from the ceiling and down) becoming hotter as it develops. Now, the upper layer together with the flames will, through radiation, start to heat up objects and surfaces in the room. When such other surfaces reaches a few hundred degrees Celsius (depending on the material), they will start to decompose into flammable gases and particles. This process, from start up to this point, is refered to as the early stages of the fire. Usually, the early stages of the fire is fuel controlled, i.e. the size of the fire, how fast it grows etc. is mainly controlled by the amount of available fuel.

The next stage of the fire is called a flashover. This is a transition between the early stages of the fire and the fully develoed fire. This is in many cases also where the fire changes from being fuel controlled to being ventilation controlled, i.e. controlled by the available amount of air, in many cases the number and sizes of openings. The flashover is usually a very slow and undramatic event. It is where the fire ”jumps” from one or a few objects in the room, to where all of the room is involved in the fire. If you are inside the room, you can identify an upcoming flashover by a rapid increase in temperature, and a fast descent of the upper layer towards the floor. Unless you are wearing GOOD protective equipment, you are in great danger at this point! And even with protective gear, you should leave the room unless you have a nozzle and know how to use it…

The flashover is followed by the fully developed fire. The fully developed fire is usually ventilation controlled, i.e. the rate of heat release, temperature in the room, etc. is mainly controlled by the size of ventilation openings (generally, the available amount of air/oxygen). The fully developed fire can last from a few minutes to serveral hours, depending on the available amount of fuel, the quality and type of building, etc.

The fully developed fire is followed by the cooling phase. At this point, the fire is running out of fuel. In addition, if it comes this far, the building is lost…