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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. The effects of a detonation are usually devastating.
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, the flame may accelerate to supersonic speed. Then a transition may take place 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|date= |work= |publisher= Gexcon AS, Norway |accessdate=}}</ref>
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