Flame acceleration and detonation

The detonation of a gas cloud occurs either directly by detonative ignition (explosive, turbulent jet) or starting from a deflagration by flame acceleration. During the transition from deflagration to detonation (DDT), two phases are distinguished  

Occurrence of a local explosion (hot spot) leading to a shock wave The area in front of the reaction zone is ideally prepared for autoignition. Hot spots are being created by focussing of weak waves behind obstacles. Amplification of the shock wave until autoignition and transition to detonation by means of an effective mechanism 

One DDT mechanism is SWACER ( Shock Wave Amplification by Coherent Energy Release ) based on a stimulated release of the chemical energy which goes synchronous with the shock wave. 

Critical tube diameter is the minimum diameter of a tube that will peimit a detonation wave to continue propagating into an open volume as a spherically expanding wave. 

If the boundary layer between two fluids of different densities is accelerated towards the higher density fluid (like reaction products towards unburot mixture), disturbances are amplified causing an enhanced reaction rate (and vice versa). 

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