Spread of Fire in Rooms :

In the early stages of a fire in a building, when only a small amount of  fuel is involved, the rate of spread of fire is independent of the size of the enclosure. A fire may start for example in an upholstered chair and a flame will spread to involve most of its surface. As radiation from the flames increases , other items of furniture which are sufficiently close will become involved. If the fire is burning freely (in an adequate supply of air ) about one-third of the heat generated by the flames is radiated to the surroundings and will cause this initial fire spread. The remainder of the heat is removed by convection in a buoyant plume which rises to the ceiling. This smoke layer increases in concentration, defth and temperature as  the fire progresses , and radiates an increasing amount of heat back into the room. This enhances burning rates , and promotes the spread of flame. Heat reaching uninvolved fuel increases its temperature until volatiles are released, and these can often be seen as white smoke. As soon as critical flow of volatiles has been achieve from this fuel there will be a very rapid spread of fire to involve the whole room. This is a flashover .

Variation of average temperature with time in a room fire.

• the pre-flashover or growth stage in which the average temperature is low and the fire is still burning quite close to its origin.

• the post-flashover or fully-developed fire in which all the fuel is involved and the compartment appears to be filled with flames.

• the delay period during which both the burning rate and the temperature are falling.

It should be noted that in a room fire involving modern foamed plastic upholstery, the time to flashover may be only a few minutes. The radiation reaching the floor at that time is  likely to be 20kW/m2, with a ceiling temperature of 600°C.

Flashover is not an event , like ignition, but it is a transition, frequently a very rapid one, from the growth stage to a fully developed fire. It also marks the transition from a potentially survivable fire situation to one from which there is no escape.

Not all fires will progress to flashover. The item first involved may be sufficiently isolated that fire cannot spread to other items. Even when the potential for spread is present , it may not occur if there is not sufficient air . If there is inadequate ventilation the fire may continue to burn very slowly, or may self-extinguish.

If the initial fire is so large that the unrestricted flame height would be greater than the ceiling height , flames as well as hot gases will spread beneath the ceiling. The temperature there will rapidly increase, and so will the radiation down to the room. The result will be reduction in the time to flashover. The height of the fuel may be important in determining whether or not the flames will reach the ceiling. The mode of ignition will also have an effect. A large source in the centre of a mass of fuel will cause a rapid initial growth and a reduction in the flashover time. The presence of combustible lining materials in a room will also reduce this time, but the effect is not significant unless the lining is involved in the early stages. The physical properties of the ceiling and walls will also affect the amount of radiation into the room. However the most significant factors affecting the time to flashover Concern the properties of the fuel and the available air.

Fuel :

The nature, bulk density and distribution of the fuel all significantly affect both the spread and severity of a fire. For example wood shavings will ignite more readily and burn more fiercely than the solid block from of books will behave very differently from the same materials when they are used to package goods in cardboard cartons. The burning rates of these materials , estimated from their behaviour in large fires, are 7 g/s/m2 of fire for books and 48g/s/m2 for cardboard cartons. Items of furniture burn at about the same rate as books, at between 25 and 60g/s, although one modern chair made from a block of polyurethane achieved a maximum rate in a test of 151g/s. A chair burning at 25g/s would release heat at about 370kW, but this latter one developed about 2500kW . Item burning at the same rate as cardboard cartons, at 100 to 250g/s, include stacked polyurethane foam, plastic goods and drums of flammable liquids. The maximum rate of heat release from them could be between 1500 and 4000 kW. Intermediate between these lower and higher ranges are fuels like sawn timber , and items packed in solidly made wooden crates.

The height to which the fuel is stacked affects not only the total fire load but also the rate of burning . This is of course flames will spread much more rapidly vertically than horizontally. The seperation between stacks of fuel greatly affects the horizontal spread of fire. However these factors have no effect on the rate of burning once flashover has occurred.

Ventilation:

A fully ventilated fire is fuel – controlled, in the sense that the rate of burning is determined by the rate at which flammable vapours are released from the fuel. Such fires have minimum flashover times and achieve maximum burning rates and temperature ( up to 800 and 1000° C in the hot gases) . If insufficient air is available to achieve these maxima, the fire is air- controlled: the burning rate is dependent on the rate at which air is flowing into the room. It has been found that the burning rate is strongly related to both the shape and area of the ventilating opening, which is usually a window or a door. The height of the opening has an important effect. The results of a number of large – scale tests, in which ventilation was through a single opening, showed that the relationship between the burning rate and the size of the opening could be given approximately by:

R= 5.5 AO H

Where R = burning rate in kg/min, A = area in m2 , and H = height in m.

An air – Controlled fire, although of limited intensity , may cause additional problems. The heated fuel continues to release flammable vapours , only part of which can react with the available air. The remaining hot flammable gases may then spread a considerable distance.

Before they can find an additional supply of air. They may do this for example by melting through a plastic roof light. The hog gases emerging into the air may still be sufficiently hot to ignite . The flames , remote from the original fire, may cause additional fire spread.

The sudden ventilation of an air – controlled fire can cause a dangerous flashback , unfortunately this effect is sometimes called a flashover, which may be confusing.

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Bapi Sarkar.