The influence of shock bifurcations on shock-flame interactions and DDT

Time-dependent, multidimensional, reactive Navier-Stokes fluid-dynamics sim ulations are used to examine the effects of bifurcated shock structures on shock-flame interactions and deflagration-to-detonation transition (DDT) in shock-tube experiments. The computations are performed for low-pressure (1 00 torr) ethylene-air mixtures using a dynamically adapting computational m esh to resolve flames, shocks, boundary layers, and vortices in flow. Resul ts of the simulations show a complex sequence of events, starting from the interactions of an incident shock with an initially laminar flame, formatio n of a flame brush, DDT, and finally the emergence of a self-sustained deto nation with the type of transverse-wave structure that forms detonations ce lls. An important process, studied here in detail, is the interaction of th e reflected shock with the boundary layer formed by the incident shock. Thi s interaction leads to bifurcation of the reflected shock and the formation of a complex structure containing a leading oblique shock followed by a re circulation region. If the flame is close enough to the bifurcated structur e, it becomes entrained in the recirculation region and attached to the bif urcated shock. This changes the nature of the shock-flame interaction both qualitatively and quantitatively. The reactive bifurcated structure, contai ning an attached flame, appears as a shock-flame complex propagating at app roximately one half of the CJ velocity. The presence of a bifurcated struct ure leads to an increase in the energy-release rate, the formation of Mach stems in the middle of the shock tube, and creation of multiple hot spots b ehind the Mach stem, thus facilitating DDT. (C) 2001 by The Combustion Inst itute.