Deflagration is a term describing subsonic
combustionCombustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...
that usually propagates through
thermal conductivityIn physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....
; hot burning material heats the next layer of cold material and ignites it. Most "
fireFire is the rapid oxidation of a material in the chemical process of combustion, releasing heat, light, and various reaction products. Slower oxidative processes like rusting or digestion are not included by this definition....
" found in daily life, from
flameA flame is the visible , gaseous part of a fire. It is caused by a highly exothermic reaction taking place in a thin zone...
s to
explosionAn explosion is a rapid increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases. An explosion creates a shock wave. If the shock wave is a supersonic detonation, then the source of the blast is called a "high explosive"...
s, is deflagration. Deflagration is different from
detonationDetonation involves a supersonic exothermic front accelerating through a medium that eventually drives a shock front propagating directly in front of it. Detonations are observed in both conventional solid and liquid explosives, as well as in reactive gases...
, which is
supersonicSupersonic speed is a rate of travel of an object that exceeds the speed of sound . For objects traveling in dry air of a temperature of 20 °C this speed is approximately 343 m/s, 1,125 ft/s, 768 mph or 1,235 km/h. Speeds greater than five times the speed of sound are often...
and propagates through
shockA shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...
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
bulletA bullet is a projectile propelled by a firearm, sling, or air gun. Bullets do not normally contain explosives, but damage the intended target by impact and penetration...
in a gun, or a piston in an
internal combustion engineThe internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
) with the force of the expanding gas. Typical examples of deflagrations are the
combustionCombustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...
of a gas-air mixture in a
gas stoveIn cooking, a gas stove is a cooker which uses natural gas, propane, butane, liquefied petroleum gas or other flammable gas as a fuel source.-History:...
or a fuel-air mixture in an
internal combustion engineThe internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
, and the rapid burning of
gunpowderGunpowder, also known since in the late 19th century as black powder, was the first chemical explosive and the only one known until the mid 1800s. It is a mixture of sulfur, charcoal, and potassium nitrate - with the sulfur and charcoal acting as fuels, while the saltpeter works as an oxidizer...
in a firearm or of pyrotechnic mixtures in fireworks.
Oil/wax fires and water
Addition of water to a burning
hydrocarbonIn organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
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 panA chip pan is a deep-sided cooking pan used for deep-frying. Chip pans are named for their traditional use in frying chips ....
fires, which are responsible for one in five house fires in
BritainThe United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
.
Flame physics
We can better understand the underlying flame
physicsPhysics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
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
in which the burning occurs. The burning region is commonly referred to as the
flameA flame is the visible , gaseous part of a fire. It is caused by a highly exothermic reaction taking place in a thin zone...
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
, which is approximately equal to
,
where
is the
thermal diffusivityIn heat transfer analysis, thermal diffusivity is the thermal conductivity divided by density and specific heat capacity at constant pressure. It has the SI unit of m²/s...
. The second is the burning timescale
that strongly decreases with temperature, typically as
,
where
is the activation barrier for the burning reaction and
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 transferHeat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase-change transfer...
. This lets us find the characteristic width
of the flame front:
,
thus
.
Now, the thermal flame front propagates at a characteristic speed
, which is simply equal to the flame width divided by the burn time:
.
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
turbulenceIn fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic and stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time...
. As a result, this derivation gives the
laminar flame speedLaminar flame speed is a property of a combustible mixture. It is the speed at which an un-stretched laminar flame will propagate through a quiescent mixture of unburned reactants. Laminar flame speed is given the symbol sL...
-- hence the designation
.
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 fireA flash fire is a sudden, intense fire caused by ignition of a mixture of air and a dispersed flammable substance such as a solid , flammable or combustible liquid , or a flammable gas...
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
detonationDetonation involves a supersonic exothermic front accelerating through a medium that eventually drives a shock front propagating directly in front of it. Detonations are observed in both conventional solid and liquid explosives, as well as in reactive gases...
. 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.