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 detonation type of explosion.
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 contrast, a detonation is characterized by supersonic flame propagation velocities, perhaps up to 2,000 metres per second (4,500 mph), and substantial overpressures, up to 2 megapascals (290 psi).
Such engines typically employ a Shchelkin spiral in the combustion chamber to facilitate the deflagration to detonation transition.
An analogous deflagration to detonation transition (DDT) has also been proposed for thermonuclear reactions responsible for supernovae initiation.