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/* '''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... |
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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 combustion
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.
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.
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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
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| 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 =
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}}</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
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}}</ref>
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| first = Vadim N.
| last = Gamezo
| authorlink =
|author2=Oran ES
| year = 2008
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| conference = American Astronomical Society, AAS Meeting #211, #162.08
| url = http://adsabs.harvard.edu/abs/2008AAS...21116208G
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}}</ref> see also ''[[carbon detonation]]''.
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