Fire Engineering - Chapter 03. Flame and its type and Transmission of heat

1. Flame and its type

A flame is zone in which chemical reaction takes place between gases with the evolution of heat and light accompanied by vigorous combustion.

i) Luminous flame-

Producing heat and light, e.g. the flame of a burning oil, an oil lamp, a candle.

ii) Non-Luminous flame-

Producing heat and very little light, e.g. burning hydrogen or burning of carbon monoxide.

2. Diffusion Flames

All combustion processes are vapour – phase reaction and the happen when the mixture of fuel vapour and air is within the limits of flammability. In an ordinary fire, the fuel vapour and air meet and are mixed by the process called diffusion within the reaction zone.

At high temperature, the mixing of the fuel with air takes place quickly and the mixing within the reaction zone is very efficient. Flames in which the reactants are mixed by diffusion in this way are called diffusion flames.

3. Premixed Flames

Unlike the diffusion flame, where the flame flammable vapour and air enter the reaction zone in suitable concentration and mix by diffusion and burn, a pre-mixed flame is one where the flame would occur due to the flammable vapour having been already mixed with air in the requisite concentration before the ignition source is applied.

4. Burning velocity

If an explosive mixture in the vapour space of methanol tank is ignited, a pre-mixed flame will begin to travel away from the source at a characteristic speed called the burning velocity.

If the source is small the flame will be roughly spherical and it will also be very thin probably with a bright blue colour.

As it spreads through the explosive mixture there will be a rise in pressure.

The rate at which pressure rises will increase during an explosion.

5. Transmission of heat by-

Conduction, Convection & Radiation.

i) Conduction

If an iron bar is heated at one end, the other end will gradually become hot. What happens here is that heat travels from particle to particle in the direction of decreasing temperature, till it reaches the other end. In the process, the particles do not change their position. They convey the heat from the hot side to the cold side. Transmission of heat by this process takes place only in solids and it is called conduction.

ii) Convection

In the process the heated particles go up and move in the air or the liquid in a cycle it is called convection. Their heat with them from a hotter to a colder place and by frequent collisions with the colder particles; the rise temperature is diffused throughout the whole mass. Convection currents can occur only in liquids and gases.

iii) Radiation

In conduction and convection heat is propagated through the intervention of particles of matter. In radiation, heat travel trough space from which all matter might be removed. There is no necessity of any material medium unlike the other two processes from transmission of heat. Radiated heat does not heat the intervening medium through which it passes.

E.g. Sun’s rays