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Deflagration to Detonation Transition (DDT)
The transition phenomenon resulting from the acceleration of a deflagration flame to detonation via flame-generated turbulent flow and compressive heating effects. At the instant of transition a volume of pre-compressed, turbulent gas ahead of the flame front detonates at unusually high velocity and overpressure.
Detonation
A release of energy caused by the propagation of a chemical reaction in which the reaction front advances into the unreacted substance at greater than sonic velocity in the unreacted material.
Detonation Flame Arrester
A flame arrester used to prevent the transmission of a detonation.
Detonation Momentum Attenuator
A mechanical device inside of a detonation flame arrester whose purpose is to reduce both the high pressure and the dynamic energy of a detonation and to split the flame front before it reaches the actual flame arrester element, thus avoiding structural damage to the element. (This device is also called a shock absorber device by some manufacturers.)
Galloping Detonation
A detonation that periodically fails and reinitiates during propagation. This type of detonation is typically observed in near-limit mixtures. Since it reinitiates via DDT, a galloping detonation is periodically overdriven and results in large overpressures at periodic distances along a pipe.
Overdriven Detonation
The unstable condition that exists during a deflagration-to-detonation transition (DDT) before a state of stable detonation is reached. Transition occurs over the length of a few pipe diameters and propagation velocities of up to 2000 m/s have been measured for hydrocarbons in air. This is greater than the speed of sound as measured at the flame front. Overdriven detonations are typically accompanied by side-on pressure ratios (at the pipe wall) in the range 50-100. A severe test for detonation flame arresters is to adjust the run-up distance so the DDT occurs at the flame arrester, subjecting the device to the overdriven detonation impulse.
Stable Detonation
A detonation that progresses through a confined system without significant variation of velocity and pressure characteristics. For atmospheric conditions, typical velocities range between 1600 and 2200 m/s for standard test mixtures and test procedures.
Unstable Detonation
See Overdriven Detonation.