Created on Wednesday, September 19, 2018 and posted in Fire Fighting Articles
Fire Behaviour: Heat, fuel and air are necessary for fires to start and continue to burn. Normally in the case of fires in buildings the fuel is separate, for example in the form of a television or a sofa, and the air is also separate (if we disregard the air in the upholstery). The fuel and the air are not mixed until during the actual combustion, or just before. The type of flame in this situation is called a diffusion flame, and a good example of this is the flame on a normal candle. The fuel (the stearin) melts, is transported up through the wick, and gasifies. Air presses in towards the fuel from the sides, i.e. the surrounding air, and fuel in the form of gasified stearin diffuses into the combustion zone. Combustion takes place in the interface between the fuel and the oxygen, i.e. in the combustion zone.
A fire in a building or in a room is generally characterized by four phases, which taken together constitute the fire behavior in a building or in a room.
- The initial stage of the fi re, ignition and growth (pre-fl ashover)
- The fully developed fi re (post-fl ashover)
- The cooling phase (decay)
In the initial stage of the fire only one or a few objects will be burning, for example a television or a sofa (the so-called initial fire, i.e. the first fire in the room). Fire gases are formed by the fire, rise upwards, and forms a hot layer of fire gases close to the ceiling. This grows in volume and be comes hotter and hotter as more fire gases are supplied. This part of the scenario is relatively calm and undramatic.
It is often possible to stay in the room without any major problems, and the fire is relatively easy to put out with a hand-held fire extinguisher, a blanket or a rug etc. Gradually, however, it becomes impossible to stay in the room because of the fire gases and the heat. The fire and the hot gases emit thermal radiation in the immediate vicinity of the fire and in the rest of the room, which causes the fire to subsequently increase in size at an ever increasing rate.
When the initial fire has developed to such an extent that the ceiling, walls, floor and interior furnishings in the room have reached a certain temperature, what is called a flashover takes place. The flashover is the transitional stage bet ween the initial stage of the fire and the fully developed fire.
In this stage the fire changes from continuing to burn in one or a few separate objects to envelop, in a few seconds, all the objects and surfaces in the room. The whole room becomes
engulfed in flames. One prerequisite for this to happen, however, is that there is sufficient air in the room, or that a sufficient amount of air is supplied to the room. Otherwise there will be no flashover and the fire will diminish in intensity, and become ventilation controlled.
Flashover results in a fully developed fire. The flamesrush out through openings and the heat has a powerful effect on the surrounding structures, through radiation and convection (transport of heat through movements in the air) and through conduction (transport of heat inside
materials). The risk of the fire spreading to adjoining rooms or buildings is imminent at this stage. The fire also has a powerful effect on the surrounding structure.
Irreversible chemical decomposition of a material due to heat.
During an enclosure fi re a stage can be reached where the thermal radiation from the fire, the hot gases and the hot enclosing surfaces cause all the combustible surfaces in the fire room to
pyrolyse. Sudden transition to a state of total surface involvement in a fire of combustible materials within a compartment.
When the fuel begins to be used up, the fire starts to subside. It enters into its cooling phase and the temperature gradually drops. The intensity of the fire is determined by the volume of the fuel and the air, and the ratio between these two components. The fire can therefore be characterised on the basis of its access to air. A fire that burns with a deficit of air, for example in a closed room or a room with small openings, is called a ventilation-controlled fire.
If it is sufficiently hot in the room more pyrolytic gases, i.e. combustible gases, will be produced than what the fire can consume. These gases are contained in the room. If a door is opened more air will be supplied to the fire, more fuel can then be combusted (burn), and the intensity of the fire will increase. In certain conditions the uncombusted gases that have collected can be ignited and start to burn, either outside or in the room, or both inside and outside the room.
A fire that burns with a surplus of air, for example in the open or in a room with very large openings, is called a fuel controlled fire. Additional openings (for example if a door is opened) will not noticeably increase the combustion, and therefore neither will they influence the intensity of the fire.
In the initial stage of a fire the fire is normally fuel controlled, while the fully developed fire is normally ventilation controlled. The flashover is therefore the transition stage from fuel control to ventilation control. What can often happen during fire ventilation is that the conditions change from ventilation control to fuel control, or from a stage with powerful ventilation control to a stage with not so fully developed ventilation control.