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Premixed and diffusion flames

A flame is the region in which chemical reactions take place and turn unburnt fuel vapours into burnt gases. The combustion products, for example methane and oxygen, react to give carbon dioxide and water vapour.

A certain amount of heat energy is required to start this reaction but more heat is produced by the reaction than it takes to initially start it, so the burning process is self-sustaining.

Premixed flames occur when a fuel is well-mixed with an oxidant, (e.g. 10% methane mixed with air). For ignition to occur, energy must be supplied in the form of a spark or small flame. A self-sustaining flame will then be established around the ignition source and will propagate outwards in all directions.

The flame consists of a zone where cold, unburnt gas (reactants) is transformed into hot burnt gas (products). The flame zone of a premixed flame may be less than 1 mm thick. As the volume of the hot burnt gas is greater than that of the same mass of cold unburnt gas, the flame front is pushed outwards from the ignition point, like the skin of an inflating balloon.

Not every mixture of air and fuel will burn. Depending on the type of fuel and oxidant involved (air or pure oxygen, for example), a mixture initially at room temperature and pressure will only burn if the concentration of fuel lies between certain well-defined limits, called flammability limits. For example, mixtures of methane and air will only burn if the concentration of methane in air lies between 5% and 15%, whereas hydrogen will burn in air at concentrations between 4% and 76%.

Table 10 Flammability limits
Gas Lower limit Upper limit
Hydrogen 4.0 76.0
Carbon monoxide 12.5 74.2
Methane 5.0 15.0
Butane 1.5 9.0
Ethylene 2.7 28.6
Acetylene 2.5 100.0
Propane 2.4 9.5

The figures quoted for limits of flammability may vary as a number of factors may slightly alter the value: pressure, temperature, dimensions of the test apparatus, direction of flame propagation and moisture content of the mixture all have some effect.

In general, the range between limits widens with increased temperature.

For each mixture of fuel and air between the flammability limits, there is a characteristic burning velocity at which a premixed flame will propagate through a stationary gas. Burning velocity is dictated by the chemical processes involved: how quickly the fuel reacts with the oxygen. The methane and oxygen molecules do not simply combine instantaneously to form carbon dioxide and water vapour, but form free radicals and intermediates such as formaldehyde and carbon monoxide along the way to completing the reaction.

If the premixture flows into a flame with a laminar flow whose velocity is equal to the burning velocity of the mixture, the flame can be held stationary. This is how premixed flames on Bunsen burners, domestic gas rings, etc. are held steady.

Local air currents and turbulence caused by obstacles can cause a flame to move at speeds much higher than the burning velocity. The speed at which a flame moves relative to an observer is the flame speed, which is different to the burning velocity. For example, the burning velocity of a methane-air flame is about 0.45 m/s. If the unburnt gases are no longer stationary, the flame propagates at the local flow speed plus the burning velocity. As the flame gets faster, the flame front wrinkles as turbulence is produced in the unburnt gas, increasing the surface area of the flame front. This increases the reaction rate, increasing the rate at which burnt gas is produced, so pushing the flame front forward faster.

In explosions, flame speeds of hundreds of metres per second can be achieved in gas-air mixtures, though the burning velocity of the mixture will be much lower than this. It is possible to achieve supersonic flame speeds, in which the combustion region is strongly coupled to a shock wave; this phenomenon is called detonation.

Diffusion flames occur at the interface where fuel vapour and air meet. Unlike premixed flames, the fuel vapour and the oxidant are separate prior to burning. The dominant process in the diffusion flame is the mixing process. The fuel vapour and oxygen mix with each other by molecular diffusion, which is a relatively slow process, though the high temperatures associated with flames increase the rate at which diffusion occurs.

Because diffusion flames exist only at the fuel-air interface, there is no equivalent of burning velocity, and no equivalent to rich or lean mixtures, or flammability limits.

Diffusion flames themselves fall into two broad types.

  • Laminar diffusion flame – in slow-burning diffusion flames, such as candle flames, the fuel vapour rises slowly from the wick in a laminar flow and molecular diffusion dominates
  • Turbulent diffusion flame – in industrial burners, fuel is injected into the air at high velocity, as a spray or jet. Turbulence is induced at the interface where mixing takes place. This gives the flame an extremely large surface area in comparison to the relatively small surface area of the smooth fuel/air interface of the candle flame. In this turbulent case, it is the large interface area, rather than the rate of molecular diffusion, that determines the rate of mixing.

In a large fire (more than 1m in diameter), the flames are turbulent diffusion flames, the turbulence generated by the strong buoyancy of the flames themselves. Inside the flame, there are regions of high temperature and low oxygen concentration where the fuel vapour is subjected to a mixture of pyrolysis (chemical decomposition in the absence of oxygen) and partial oxidation, leading to the formation of soot particles and products of incomplete combustion, in particular carbon monoxide (CO). These are the source of smoke, and of the gaseous species that render the fire products toxic.