Deflagrations, hot spots, and the transition to detonation

A series of multidimensional numerical simulations were used to investigate how, through a series of shock-flame interactions, a turbulent flame may s uddenly evolve into a detonation, the process of deflagration-to-detonation transition (DDT). The reactive Navier-Stokes equations were solved on an a daptive mesh that resolved selected features of the flow including the stru cture of the laminar flame. The chemical and thermophysical models used rep roduced the flame and detonation properties of acetylene in air over a rang e of temperatures and pressures. The interactions of an incident shock with the initially laminar flame led to the formation of secondary shocks, rare factions, and contact surfaces that continued to distort the flame surface, eventually creating a turbulent flame brush. Pressure fluctuations, genera ted by shock-flame interactions in the flame brush, were the seeds for hot spots in unreacted material. The simulations showed that these hot spots un derwent transition to a detonation when the gradients in induction time in the hot spot allowed the formation of spontaneous waves. An unsuccessful ex plosion in hot spots formed a shock with a flame left behind it. As the str ength of the initial incident shock was increased, the location of DDT shif ted from outside the flame brush to inside the flame brush. The main featur es of the simulated DDT process show trends similar to those observed in ex periments.