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'''Deflagration to detonation transition''' (DDT) refers to a phenomenon in [[Combustion|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.
==Description==
A [[deflagration]] is characterized by a [[Speed of sound|subsonic]] flame [[propagation velocity]], typically far below 100 [[m/s]], and relatively modest [[overpressure]]s, say below 0.5 [[Bar (unit)|bar]]. 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 of heat and mass. In its most benign form, a deflagration may simply be a [[flash fire]]. In contrast, a [[detonation]] is characterized by [[supersonic]] flame propagation velocities, perhaps up to 2000 m/s, and substantial overpressures, up to 20 bars. The main mechanism of combustion propagation is of a powerful [[pressure]] wave that compresses the unburnt gas ahead of the wave to a [[temperature]] above the [[autoignition]] temperature. In technical terms, the reaction zone (chemical combustion) is a self-driven [[shock wave]] where the reaction zone and the shock are coincident, and the chemical reaction is initiated by the compressive heating caused by the shock wave.▼
A [[deflagration]] is characterized by a [[Speed of sound|subsonic]] flame [[propagation velocity]], typically far below {{convert|100|m/s|mph}}, and relatively modest [[overpressure]]s, say below {{convert|0.5|bar|psi}}. 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]] of [[mass transfer|heat and mass]]. In its most benign form, a deflagration may simply be a [[flash fire]].
▲
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 current]]s, a subsonic flame may accelerate to supersonic speed, transitioning from deflagration to detonation. The exact mechanism is not fully understood,<ref name=GexCon>{{cite web |title=Gas explosion handbook |url= http://www.gexcon.com/index.php?src=handbook/GEXHBchap6.htm|work= |publisher= Gexcon AS, Norway |accessdate=}}</ref>▼
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.▼
▲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 current]]s), a subsonic flame may accelerate to supersonic speed, transitioning from deflagration to detonation. The exact mechanism is not fully understood,<ref name=GexCon>{{cite web |title=Gas explosion handbook |url= http://www.gexcon.com/index.php?src=handbook/GEXHBchap6.htm|work= |publisher= Gexcon AS, Norway |accessdate=}}</ref>
▲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.{{cn|date=February 2017}}
==Examples==
A deflagration to detonation transition has been a feature of several major [[industrial accident]]s:
* [[1970 Propane vapour cloud explosion in Port Hudson]]
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==Applications==
The phenomenon is exploited in [[pulse detonation engine]]s, 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]] to facilitate the deflagration to detonation transition.<ref>{{cite conference | first = TH | last = New | authorlink = |author2=PK Panicker |author3=FK Lu |author4=H M Tsai | year = 2006| title = Experimental Investigations on DDT Enhancements by Schelkin Spirals in a PDE | conference = 44th AIAA Aerospace Sciences Meeting and Exhibit 9–12 January 2006, Reno, Nevada | url = http://arc.uta.edu/publications/cp_files/aiaa-2006-7958.pdf | format = | accessdate = | doi = | id = | oclc =}}</ref><ref>{{cite conference | first = E | last = Schultz | authorlink = |author2=E Wintenberger |author3=J Shepherd | year = 1999 | title = Investigation of Deflagration to Detonation Transition for Application to Pulse Detonation Engine Ignition Systems | conference = Proceedings of the 16th JANNAF Propulsion Symposium | url = http://www.galcit.caltech.edu/EDL/publications/reprints/jannaf99_paper.pdf | format = | accessdate = | doi = | id = | oclc =}}</ref>
The mechanism has also found military use in
==Related phenomena==
An analogous deflagration to detonation transition (DDT) has also been proposed for thermonuclear reactions responsible for [[supernovae]] initiation.<ref>{{cite conference | first = Vadim N. | last = Gamezo | authorlink = |author2=Oran ES | year = 2008 | title = Mechanisms for Detonation Initiation in Type Ia Supernovae | conference = American Astronomical Society, AAS Meeting #211, #162.08 | url = http://adsabs.harvard.edu/abs/2008AAS...21116208G | format = | accessdate = | doi = | id = | oclc =}}</ref> This process has been called a "[[carbon detonation]]".
==See also==
*[[Pressure piling]]
*[[Boiling liquid expanding vapor explosion]] (BLEVE)
==References==
{{reflist|30em}}
* {{cite book
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