Ignition and deflagration-to-detonation transition modes in ethylene/air mixtures behind a reflected shock

Abstract

Dynamics of ethylene autoignition and deflagration-to-detonation transition (DDT) are first numerically investigated in a one-dimensional shock tube using a reduced chemistry including 10 species and 10 reactions. Different combustion modes are investigated through considering various premixed gas equivalence ratios (0.2 - 2.0) and incident shock wave Mach numbers (1.8 -3.2). Four ignition and DDT modes are observed from the studied cases, i.e., no ignition, deflagration combustion, detonation after reflected shock, and deflagration behind the incident shock. For detonation development behind the reflected shock, three autoignition hot spots are formed. The first one occurs at the wall surface after the re-compression of the reflected shock and contact surface, which further develops to a reaction shock because of "the explosion in the explosion"regime. The other two are off the wall, respectively, caused by the reflected shock/rarefaction wave interaction and reaction induction in the compressed mixture. The last hot spot develops to a reaction wave and couples with the reflected shock after a DDT process, which eventually leads to detonation combustion. For deflagration development behind the reflected shock, the wave interactions, wall surface autoignition hot spot as well as its induction of reaction shock are qualitatively similar to the mode of detonation after incident shock reflection, before the reflected shock/rarefaction wave collision point. However, only one hot spot is induced after the collision, which also develops to a reaction wave but cannot catch up with the reflected shock. For deflagration behind the incident shock, deflagration combustion is induced by the incident shock compression whereas detonation occurs after the shock reflection. The chemical timescale increases after the reflected shock/contact surface collision, whereas decreases behind the incident and reflected shocks, as well as after the reflected shock/rarefaction wave interaction. Therefore, mixture reactivity behind the reflected shock is weakened by the contact surface, but is intensified by the rarefaction wave. The multi-dimensionality characteristics, including reflected shock/boundary layer interactions, reflected shock bifurcation, destabilization, and detonation, are further present in a two-dimensional configuration. Planar autoignition occurs because of reflected shock compression and detonation combustion is formed first in the central region due to the collision of the reflected shock wave/reflected compression wave. The left and right bifurcations of the separation region in the wall boundary layer are then sequentially ignited.