Deflagration - Wikipedia, The Free Encyclopedia
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A log in a fireplace.
DeflagrationFrom Wikipedia, the free encyclopedia
Deflagration[1] (Lat: de + flagrare, "to burn down") is a term
describing subsonic combustion propagating through heat transfer; hot
burning material heats the next layer of cold material and ignites it.
Most "fire" found in daily life, from flames to explosions, isdeflagration. Deflagration is different from detonation, which is
supersonic and propagates through shock.
Deflagration is a rapid high energy release combustion event that
propagates through a gas or an explosive material at subsonic speeds,
driven by the transfer of heat.
Contents
1 Applications
2 Oil/wax fires and water
3 Flame physics
4 Damaging deflagration events
5 See also
6 References
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. 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 fires and water
Adding 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 household fires in Britain.[2]
Flame physics
The underlying flame physics 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 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.
https://en.wikipedia.org/wiki/Physicshttps://en.wikipedia.org/wiki/Physicshttps://en.wikipedia.org/wiki/United_Kingdomhttps://en.wikipedia.org/wiki/Deflagration#cite_note-2https://en.wikipedia.org/wiki/Gas_stovehttps://en.wikipedia.org/wiki/Deflagration#Referenceshttps://en.wikipedia.org/wiki/Deflagration#Damaging_deflagration_eventshttps://en.wikipedia.org/wiki/File:Log_in_fireplace.jpghttps://en.wikipedia.org/wiki/Supersonichttps://en.wikipedia.org/wiki/Shock_wavehttps://en.wikipedia.org/wiki/File:Log_in_fireplace.jpghttps://en.wikipedia.org/w/index.php?title=Flame_front&action=edit&redlink=1https://en.wikipedia.org/wiki/Flamehttps://en.wikipedia.org/wiki/Physicshttps://en.wikipedia.org/wiki/Deflagration#cite_note-2https://en.wikipedia.org/wiki/United_Kingdomhttps://en.wikipedia.org/wiki/Chip_panhttps://en.wikipedia.org/wiki/Hydrocarbonhttps://en.wikipedia.org/w/index.php?title=Gas_pressure_blasting&action=edit&redlink=1https://en.wikipedia.org/wiki/Fireworkhttps://en.wikipedia.org/wiki/Gunpowderhttps://en.wikipedia.org/wiki/Internal_combustion_enginehttps://en.wikipedia.org/wiki/Gas_stovehttps://en.wikipedia.org/wiki/Combustionhttps://en.wikipedia.org/wiki/Internal_combustion_enginehttps://en.wikipedia.org/wiki/Bullethttps://en.wikipedia.org/wiki/Deflagration#Referenceshttps://en.wikipedia.org/wiki/Deflagration#See_alsohttps://en.wikipedia.org/wiki/Deflagration#Damaging_deflagration_eventshttps://en.wikipedia.org/wiki/Deflagration#Flame_physicshttps://en.wikipedia.org/wiki/Deflagration#Oil.2Fwax_fires_and_waterhttps://en.wikipedia.org/wiki/Deflagration#Applicationshttps://en.wikipedia.org/wiki/Shock_wavehttps://en.wikipedia.org/wiki/Supersonichttps://en.wikipedia.org/wiki/Detonationhttps://en.wikipedia.org/wiki/Explosionhttps://en.wikipedia.org/wiki/Flamehttps://en.wikipedia.org/wiki/Firehttps://en.wikipedia.org/wiki/Heat_transferhttps://en.wikipedia.org/wiki/Combustionhttps://en.wikipedia.org/wiki/Deflagration#cite_note-1https://en.wikipedia.org/wiki/File:Log_in_fireplace.jpg -
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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 diffusivity. 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 o
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 are equal: The heat generated by burning is
equal to the heat carried away by heat transfer. This makes it possible to calculate 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.
This model also neglects the possible influence of turbulence. As a result, this derivation gives only the laminar
flame speed -- 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 iscontained.
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 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 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.
https://en.wikipedia.org/wiki/Detonationhttps://en.wikipedia.org/wiki/Flash_firehttps://en.wikipedia.org/wiki/Laminar_flame_speedhttps://en.wikipedia.org/wiki/Turbulencehttps://en.wikipedia.org/wiki/Heat_transferhttps://en.wikipedia.org/wiki/Thermal_diffusivity -
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See also
Pressure piling
References
1. ^ http://www.hutchisonrodway.co.nz/Auckland/Technical/Glossary-D-H2. ^ UK Fire Service advice (http://www.fireservice.co.uk/safety/chip-pans) on chip pan fires
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