Mechanism of transition to detonation in unconfined volumes

The paper is aimed at numerical study of one of the most hazardous events at a launch place: open space explosion of fuel air mixtures due to accidental loss of containments. A mechanism of transition to detonation in the process of unconfined flame propagation is proposed. It is shown that the detonation onset takes place as a result of local exponential growth of unstable short-wavelength perturbations. Exactly the same mechanism is known to be responsible for the self-similar flame acceleration, however the detonation can arise only in the case of extremely high reaction rate. High reaction rate defines a coupling of the accelerating flamelets with diverging shock waves that leads to the detonation onset. •Deflagration-to-detonation transition (DDT) in unconfined volume is studied.•The mechanism of DDT is related to the flame instability development.•The rise of flame surface disturbances leads to the shock waves generation.•Local compression coupled with flame sheet acceleration causes detonation onset.