Nature and behavior of fire

Flame is the visible part of an exothermic vapor phase reaction. Sufficient heat is released to raise the reactants and their products to a temperature at which they are incandescent. Reaction rates at these temperatures (commonly 1300 to 2000 °C) are extremely high, and a great deal of energy is released. This is the energy that drives your car or takes a spaceship to the moon.

Like most chemical reactions, those in a flame can’t proceed to completion unless the conditions (of temperature, pressure and mixture ratio) are right. Combustion reactions can be maintained in a flame only when sufficient heat is being transferred to the unburned gases to bring them up to flame temperature. This fresh gas entering the flame can then react, and the process continuous, according to the Fire Protection Association, U.K.

Taking a match to the candle wick heats up, melts up, and eventually ignites, the wax (the ignition process itself is discussed in greater detail below). The resulting flame melts more wax, which is fed up the wick, and sustains the flame. In most cases, it is the heat from the chemical reactions in the flame itself, which provides the energy required to generate more fuel for the reaction.

Although radiated heat is the primary source of energy for the generation of fuel in most fires, it is not the primary method by which energy from the flame is dissipated.

Only about 30% of the energy of the flame is lost through radiated heat, most of the rest is lost by convection, added, Commissioner Mbando of the Firefighting Company in Dar-es-Salaam.

The oxygen, which is drawn into the combustion process, is converted to CO2, CO (soot is pure carbon which has not reacted with any oxygen), and water vapor, in the flame, but is also heated up. About 70% of the heat of the flame is carried away by these ‘products of combustion’, in convention currents.

In the case of a ‘tea light candle’, some energy will conduct into the surface on which the candle stands once all of the wax has melted. Although technically this energy has only got into the wax by radiation in the first place, it is not contributing to the flame because it is being conducted away.

Question: Why does a flame spread upwards more quickly than it spreads horizontally, and much more quickly than it spreads downwards?

Answer: Because flame spread relies on heat braking down the fuel into flammable vapors. Spreading vertically, the convected heat in the flame, and the direct heat of the flame, combine to create a lot of ‘pre heating’. Flame spread horizontally or downwards relies only on radiated heat from the flame doing the ‘pre heating’.

Notice in the candle flame, that the fuel (the flammable vapors) enters the flame from the inside, while the oxygen from the air enters the flame from the outside. The flame reaction occurs where the fuel and the oxygen have diffused together in just the right proportions for a reaction to take place. For this reason, this type of flame is called a diffusion flame.

Premixed flames

Premixed flames are far less frequently seen than diffusion flames, but they are of considerable importance because a propagating premixed flame is an explosion.

In combustion technology the word explosion is sometimes understood to mean just this- a propagating premixed flame. The rupture of a vessel that has been over pressurized is called a pressure burst. There is also a specific meaning of the word detonation, and this is explained later.

Consider a house which has experienced a gas leak. Flammable vapor spreads throughout the house, mixing with air as it does so. The concentration of gas will vary widely from being too rich to ignite close to the leak, to being too weak to ignite a long way from the leak. But is the correct gas and air concentration drifts into a pilot light for example, a flame will burn through the whole house.

Of course, once the premixed flame has burnt out, you may be left with a diffusion flame burning from the initial leak.

The big difference between a diffusion flame and a premixed flame, is that (relative to stationary observer), the diffusion flame is stationary, while the premixed flame is moving. The speed at which a premixed flame moves varies according to the mixture, which is burning, but for any given vapor/air mixture, a premixed flame will move at the same speed. This is called the ‘characteristic burning velocity’ of the mixture.

If the premixed flame is within an enclosure like a vessel, a pipe, or a building, it will actually travel faster than the ‘characteristic burning velocity’ of the mixture. As the flame burns through the mixture, it leaves behind itself large volumes of fire products like CO, and CO2.Not only can there be more product gas than there was mixture in the first place, but the product gas is hot, so it takes up a large volume. These effects push the flame front above its ‘characteristic burning velocity’, and cause the increase in pressure which can burst vessels, and blow out doors and windows.

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