Fire ventilation

chapter 2Fire behaviourHeat, fuel and air are necessary for fires to start and continueto burn. Normally in the case of fires in buildings thefuel is separate, for example in the form of a television or asofa, and the air is also separate (if we disregard the air inthe upholstery). The fuel and the air are not mixed untilduring the actual combustion, or just before. The type offlame in this situation is called a diffusion flame, and a goodexample of this is the flame on a normal candle. The fuel(the stearin) melts, is transported up through the wick, andgasifies. Air presses in towards the fuel from the sides, i.e.the surrounding air, and fuel in the form of gasified stearindiffuses into the combustion zone. Combustion takes placein the interface between the fuel and the oxygen, i.e. in thecombustion zone.A fire in a building or in a room is generally characterizedby four phases, which taken together constitute the firebehavior in a building or in a room.1. The initial stage of the fire, ignition and growth(pre-flashover)2. Flashover3. The fully developed fire (post-flashover)4. The cooling phase (decay)In the initial stage of the fire only one or a few objects willbe burning, for example a television or a sofa (the so-calledinitial fire, i.e. the first fire in the room). Fire gases are formedby the fire, rise upwards, and forms a hot layer of firegases close to the ceiling. This grows in volume and be comeshotter and hotter as more fire gases are supplied. This partof the scenario is relatively calm and undramatic.ABCDThe flame from acandle is an example ofa diffusion flame.A. Air (oxygen) diffusesinto the flame andreacts with gaseous fuel.B. Transport andvaporisation of fuel inthe wickC. Liquid fuelD. Stearin (solid fuel)in solid state.17