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Deflagration to detonation transition (DDT) refers to a phenomenon in ignitable mixtures of a
flammable A combustible material is something that can burn (i.e., ''combust'') in air. A combustible material is flammable if it ignites easily at ambient temperatures. In other words, a combustible material ignites with some effort and a flammable mat ...
gas and air (or
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...
) when a sudden transition takes place from a
deflagration Deflagration (Lat: ''de + flagrare'', "to burn down") is subsonic combustion in which a pre-mixed flame propagates through a mixture of fuel and oxidizer. Deflagrations can only occur in pre-mixed fuels. Most fires found in daily life are diff ...
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. Combus ...
to 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 propagate supersonically through shock waves with s ...
type of explosion.


Description

A
deflagration Deflagration (Lat: ''de + flagrare'', "to burn down") is subsonic combustion in which a pre-mixed flame propagates through a mixture of fuel and oxidizer. Deflagrations can only occur in pre-mixed fuels. Most fires found in daily life are diff ...
is characterized by a subsonic flame
propagation velocity The phase velocity of a wave is the rate at which the wave propagates in any medium. This is the velocity at which the phase of any one frequency component of the wave travels. For such a component, any given phase of the wave (for exampl ...
, typically far below , and relatively modest
overpressure Overpressure (or blast overpressure) is the pressure caused by a shock wave over and above normal atmospheric pressure. The shock wave may be caused by sonic boom or by explosion, and the resulting overpressure receives particular attention when m ...
s, typically below . The main mechanism of combustion propagation is of a flame front that moves forward through the gas mixture - in technical terms the reaction zone (chemical combustion) progresses through the medium by processes of
diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical ...
of heat and mass. In its most benign form, a deflagration may simply be a flash fire. In contrast, 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 propagate supersonically through shock waves with s ...
is characterized by
supersonic Supersonic speed is the speed of an object that exceeds the speed of sound ( Mach 1). For objects traveling in dry air of a temperature of 20 °C (68 °F) at sea level, this speed is approximately . Speeds greater than five times ...
flame propagation velocities, perhaps up to , and substantial overpressures, up to . The main mechanism of detonation propagation is of a powerful
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country a ...
wave that compresses the unburnt gas ahead of the wave to a
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...
above the autoignition temperature. In technical terms, the reaction zone (chemical combustion) is a self-driven
shock wave In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a me ...
where the reaction zone and the shock are coincident, and the chemical reaction is initiated by the compressive heating caused by the shock wave. The process is similar to ignition in a
Diesel engine The diesel engine, named after Rudolf Diesel, is an internal combustion engine in which ignition of the fuel is caused by the elevated temperature of the air in the cylinder due to mechanical compression; thus, the diesel engine is a so-cal ...
, but much more sudden and violent. Under certain conditions, mainly in terms of geometrical conditions (such as partial confinement and many obstacles in the flame path that cause turbulent flame eddy currents), a subsonic flame front may accelerate to supersonic speed, transitioning from deflagration to detonation. The exact mechanism is not fully understood, and while existing theories are able to explain and model both deflagrations and detonations, there is no theory at present which can predict the transition phenomenon.


Examples

A deflagration to detonation transition has been a feature of several major industrial accidents: * 1970 Propane vapour cloud explosion in Port Hudson * Flixborough disaster * Phillips disaster of 1989 in Pasadena, Texas * Damage observed in the Buncefield fire *
2020 Beirut Explosions On 4 August 2020, a large amount of ammonium nitrate stored at the Port of Beirut in the capital city of Lebanon exploded, causing at least 218 deaths, 7,000 injuries, and US$15 billion in property damage, as well as leaving an estimated 300 ...


Applications

The phenomenon is exploited in pulse detonation engines, because a detonation produces a more efficient combustion of the reactants than a deflagration does, i.e. giving a higher yields. Such engines typically employ a Shchelkin spiral in the
combustion chamber A combustion chamber is part of an internal combustion engine in which the fuel/air mix is burned. For steam engines, the term has also been used for an extension of the firebox which is used to allow a more complete combustion process. Intern ...
to facilitate the deflagration to detonation transition. The mechanism has also found military use in
thermobaric weapon A thermobaric weapon, also called an aerosol bomb, a vacuum bomb or a fuel air explosive (FAE), is a type of explosive that uses oxygen from the surrounding air to generate a high-temperature explosion. The fuel–air explosive is one of the be ...
s.


Related phenomena

An analogous deflagration to detonation transition (DDT) has also been proposed for thermonuclear reactions responsible for
supernovae A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when ...
initiation. This process has been called a " carbon detonation".


See also

* Zeldovich spontaneous wave * Dust explosion * Pressure piling * Boiling liquid expanding vapor explosion (BLEVE)


References

* {{cite book , last = Lea , first = CJ , author2=HS Ledin , title = A Review of the State-of-the-Art in Gas Explosion Modelling, HSL/2002/02 , publisher = UK Health and Safety Laboratories , year = 2002 , url = http://www.hse.gov.uk/research/hsl_pdf/2002/hsl02-02.pdf Combustion Industrial fires and explosions Explosives engineering