Deflagration

A log in a fireplace.

Deflagration (Lat: de + flagrare, "to burn down") is a term describing subsonic combustion that usually propagates through thermal conductivity; hot burning material heats the next layer of cold material and ignites it. Most "fire" found in daily life, from flames to explosions, is deflagration. Deflagration is different from detonation, which is supersonic and propagates through shock compression.

Applications

In engineering applications, deflagrations are easier to control than detonations. Consequently, they are better suited when the goal is to move an object (a bullet in a gun, or a piston in an internal combustion engine) with the force of the expanding gas. Typical examples of deflagrations are the combustion of a gas-air mixture in a gas stove or a fuel-air mixture in an internal combustion engine, and the rapid burning of gunpowder in a firearm or of pyrotechnic mixtures in fireworks.

Oil/wax fires and water

Addition of water to a burning hydrocarbon such as oil or wax produces a deflagration. The water boils rapidly and ejects the burning material as a fine spray of droplets. A deflagration then occurs, as the fine mist of oil ignites and burns extremely rapidly. These are particularly common in chip pan fires, which are responsible for one in five house fires in Britain.^[1]

Flame physics

We can better understand the underlying flame physics by constructing an idealized model consisting of a uniform one-dimensional tube of unburnt and burned gaseous fuel, separated by a thin transitional region of width $\delta\;$ in which the burning occurs. The burning region is commonly referred to as the flame or flame front. In equilibrium, thermal diffusion across the flame front is balanced by the heat supplied by burning.

There are two characteristic timescales which are important here. The first is the thermal diffusion timescale $\tau_d\;$, which is approximately equal to

$\tau_d \simeq \delta^2 / \kappa$,

where $\kappa \;$ is the thermal diffusivity. The second is the burning timescale τ_b that strongly decreases with temperature, typically as

$\tau_b\propto \exp[\Delta U/(k_B T_f)]$,

where $\Delta U\;$ is the activation barrier for the burning reaction and $T_f\;$ is the temperature developed as the result of burning that can be found from thermodynamics (the so-called "flame temperature").

For a stationary moving deflagration front, these two timescales are equal: The heat generated by burning is equal to the heat carried away by heat transfer. This lets us find the characteristic width $\delta\;$ of the flame front:

$\tau_b = \tau_d\;$,

thus

$\delta \simeq \sqrt {\kappa \tau_b}$.

Now, the thermal flame front propagates at a characteristic speed $S_l\;$, which is simply equal to the flame width divided by the burn time:

$S_l \simeq \delta / \tau_b \simeq \sqrt {\kappa / \tau_b}$.

This simplified model neglects the change of temperature and thus the burning rate across the deflagration front. Also this model neglects the possible influence of turbulence. As a result, this derivation gives the laminar flame speed -- hence the designation $S_l\;$.

Damaging deflagration events

Damage to buildings, equipment and people can result from a large-scale short-duration deflagration. The potential damage is primarily a function of the total amount of fuel burned in the event (total energy available), the maximum flame velocity that is achieved, and the manner in which the expansion of the combustion gases is contained.

In free-air deflagrations, there is a continuous variation in deflagration effects relative to maximum flame velocity. When flame velocities are low, the effect of a deflagration is the release of heat. Some authors use the term flash fire to describe these low-speed deflagrations. At flame velocities near the speed of sound, the energy released is in the form of pressure and the results resemble a detonation. Between these extremes both heat and pressure are released.

When a low-speed deflagration occurs within a closed vessel or structure, pressure effects can produce damage due to expansion of gases, as a secondary effect. The heat released by the deflagration causes the combustion gases and excess air to try to expand thermally as well. The net result is that the volume of the vessel or structure needs to either expand/fail to accommodate the hot combustion gases, or build internal pressure to contain them. The risks of deflagration inside waste storage drums is a growing concern among storage facilities.

See also

• Pressure piling

References

1. ^ UK Fire Service advice on chip pan fires