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Deflagration (Lat: ''de + flagrare'', "to burn down") is
subsonic Subsonic may refer to: Motion through a medium * Any speed lower than the speed of sound within a sound-propagating medium * Subsonic aircraft, a flying machine that flies at air speeds lower than the speed of sound * Subsonic ammunition, a type of ...
combustion Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion ...
propagating through
heat transfer Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, t ...

heat transfer
; hot burning material heats the next layer of cold material and ignites it. Most "
fire BBQ. Fire is the rapid oxidation of a material in the exothermic chemical process of combustion, releasing heat, light, and various reaction products. Fire is hot because the conversion of the weak double bond in molecular oxygen, O2, to the ...

fire
s" found in daily life, from
flame A flame (from Latin ''flamma'') is the visible, gaseous part of a fire. It is caused by a highly exothermic chemical reaction taking place in a thin zone. Very hot flames are hot enough to have ionized gaseous components of sufficient density to ...
s to
explosion An explosion is a rapid expansion in volume associated with an extremely vigorous outward release of energy, usually with the generation of high temperatures and release of high-pressure gases. Supersonic explosions created by high explosive ...

explosion
s such as that of
black powder Gunpowder, also known as the retronym black powder to distinguish it from modern smokeless powder, is the earliest known chemical explosive. It consists of a mixture of sulfur (S), carbon (C), and potassium nitrate (saltpeter, KNO3). The sulfur a ...
, are deflagrations. This differs from
detonation Detonation () is a type of combustion involving a supersonic exothermic front accelerating through a medium that eventually drives a shock front propagating directly in front of it. Detonations occur in both conventional solid and liquid explosi ...
, which propagates
supersonic F/A-18F Super Hornet in transonic flight Image:FA-18 Hornet breaking sound barrier (7 July 1999) - filtered.jpg, 275px, U.S. Navy F/A-18 Hornet, F/A-18 approaching the sound barrier. The white cloud forms as a result of the Prandtl–Meyer expan ...
ally through
shock wave of an attached shock on a sharp-nosed supersonic body firing a broadside during training exercises in Puerto Rico, 1984. Circular marks are visible where the expanding spherical atmospheric shockwaves from the gun firing meet the water surface. ...
s, decomposing a substance extremely quickly.


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 ''cartridge'' consisting of the following:''1.'' ''bullet'', as the projectile; ''2.'' ''metallic'' ''case'', which holds all parts together; ''3.'' ''propellant'', for example gunpowder or cordite;''4.'' ''rim'', which provides the extractor on ...

bullet
in a firearm, or a piston in an
internal combustion engine An internal combustion engine (ICE) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combustion engine, t ...
) with the force of the expanding gas. Typical examples of deflagrations are the combustion of a gas-air mixture in a
gas stove A gas stove is a stove that is fuelled by combustible gas such as syngas, natural gas, propane, butane, liquefied petroleum gas or other flammable gas. Before the advent of gas, cooking stoves relied on solid fuels such as coal or wood. The first ga ...

gas stove
or a fuel-air mixture in an internal combustion engine, and the rapid burning of
gunpowder Gunpowder, also known as the retronym black powder to distinguish it from modern smokeless powder, is the earliest known chemical explosive. It consists of a mixture of sulfur (S), carbon (C), and potassium nitrate (saltpeter, KNO3). The sulfur a ...
in a firearm or of pyrotechnic mixtures in
firework Fireworks are a class of low explosive pyrotechnic devices used for aesthetic and entertainment purposes. The most common use of a firework is as part of a fireworks display (also called a fireworks show or pyrotechnics), a display of the eff ...
s. Deflagration systems and products can also be used in mining, demolition and stone quarrying via gas pressure blasting as a beneficial alternative to high explosives.


Oil/wax fire and water

Adding water to a burning
hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons from which one hydrogen atom has been removed are functional groups called hydrocarb ...
such as oil or wax produces a
boilover upright=1.5, Fire fighters simulating boilover to demonstrate the risks. Length of the sequence 2.4 seconds. it was 1 kg cooking oil and 1 litre of water. A boilover (or boil-over) type of fire refers to an extremely hazardous situation where an ...
, where water boils rapidly and ejects the material as a fine spray of droplets. A deflagration then occurs as the droplets ignite and burn extremely rapidly. These are particularly common in
chip pan A chip pan is a deep-sided cooking pan used for deep-frying. Chip pans are named for their traditional use in frying chips (called "French fries" in the United States). Today, they are made from either aluminium or stainless steel, although in t ...
fires, which are responsible for one in five household fires in Britain.


Flame physics

The underlying flame
physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its motion and behavior through space and time, and the related ent ...
can be understood with the help of 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 A premixed flame is a flame formed under certain conditions during the combustion of a premixed charge (also called pre-mixture) of fuel and oxidiser. Since the fuel and oxidiser—the key chemical reactants of combustion—are available throughout ...
. In equilibrium, thermal diffusion across the flame front is balanced by the heat supplied by burning.Zeldovich, I. A., Barenblatt, G. I., Librovich, V. B., & Makhviladze, G. M. (1985). Mathematical theory of combustion and explosions. Two characteristic timescales 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 diffusivityIn heat transfer analysis, thermal diffusivity is the thermal conductivity divided by density and specific heat capacity at constant pressure. It measures the rate of transfer of heat of a material from the hot end to the cold end. It has the SI deri ...
. The second is the
burning timescale
burning timescale
\tau_b that strongly decreases with temperature, typically as :\tau_b\propto \exp Delta U/(k_B T_f)/math>, where \Delta U\; is the activation barrier for the burning reaction and T_f\; is the temperature developed as the result of burning; the value of this so-called "flame temperature" can be determined from the laws of thermodynamics. For a stationary moving deflagration front, these two timescales must be equal: the heat generated by burning is equal to the heat carried away by
heat transfer Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, t ...

heat transfer
. This makes it possible to calculate the characteristic width \delta\; of the flame front: :\tau_b = \tau_d\;, thus : \delta \simeq \sqrt . 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 . This simplified model neglects the change of temperature and thus the burning rate across the deflagration front. This model also neglects the possible influence of
turbulence In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between th ...
. As a result, this derivation gives only the
laminar flame speedLaminar flame speed is an intrinsic characteristic of premixed combustible mixtures that plays a key role in understanding a mixture’s reactivity, diffusivity, and exothermicity. It is the speed at which an un-stretched laminar flame will propagat ...
—hence the designation S_l\;.


Damaging 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 the maximum flame velocity. When flame velocities are low, the effect of a deflagration is to release heat. Some authors use the term
flash fire A flash fire is a sudden, intense fire caused by ignition of a mixture of air and a dispersed flammable substance such as a solid (including dust), flammable or combustible liquid (such as an aerosol or fine mist), or a flammable gas. It is characte ...
to describe these low-speed deflagrations. At flame velocities near the
speed of sound The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elastic medium. At , the speed of sound in air is about , or a kilometre in or a mile in . It depends strongly on temperature as well as th ...
, the energy released is in the form of pressure and the results resemble a
detonation Detonation () is a type of combustion involving a supersonic exothermic front accelerating through a medium that eventually drives a shock front propagating directly in front of it. Detonations occur in both conventional solid and liquid explosi ...
. Between these extremes, both heat and pressure are generated. 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 expand thermally. The net result is that the volume of the vessel or structure must expand to accommodate the hot combustion gases, or the vessel must be strong enough to withstand the additional internal pressure, or it fails, allowing the gases to escape. The risks of deflagration inside waste storage drums is a growing concern in storage facilities.


See also

*
Conflagration A conflagration is a large and destructive fire that threatens human life, animal life, health, and/or property. It may also be described as a blaze or simply a (large) fire. A conflagration can begin accidentally, be naturally caused (wildfire), ...
*
Deflagration to detonation transition Deflagration to detonation transition (DDT) refers to a phenomenon in ignitable mixtures of a flammable gas and air (or oxygen) when a sudden transition takes place from a deflagration type of combustion to a detonation type of explosion. Descripti ...
* Pressure piling


References

{{Authority control Explosives