Autoignition-cascade in the windward mixing layer of a premixed jet in hot vitiated crossflow

Abstract

This numerical study builds upon the experimental work of Wagner et al., e.g. [Combust. Flame 176 (2017)], and focuses on the flame stabilization mechanism of a reactive jet in crossflow (RJICF). A 300 K premixed ethylene-air jet (ϕj=1.2) is injected into a hot vitiated crossflow at 1500 K. A compressible three-dimensional (3-D) large eddy simulation (LES) was performed with an analytically reduced chemistry mechanism. Comparisons between LES and experiments are in good agreement in terms of jet trajectories and flow velocities. The LES captures the flame-flow interactions that were experimentally observed in the central plane of the RJICF by means of particle image velocimetry and OH planar laser-induced fluorescence. It gives access to the spatio-temporal evolution of the autoignition process along the windward mixing layer of the RJICF. Chemical explosive mode analysis shows that autoignition is the dominant flame stabilization mechanism on the windward side of the jet. “Low-heat-release” autoignition starts at the front root of the jet, at the “very lean” most reactive mixture fraction, which is the spot where the autoignition time of the mixture is the smallest. Immediately downstream, the released heat is advected and diffuses, contributing to a decrease of autoignition times in the neighboring richer inner side of the jet. Autoignition in this richer layer of the jet is characterized by higher heat release rate that again diffuses toward even richer regions until the stoichiometric layer ignites. This autoignition-cascade mechanism determines the position of the “visible” windward flame. Moreover, a 3-D flame analysis shows that the resulting “high-heat-release” autoignition patches expand within the stoichiometric layer and ultimately merge with the leeward propagating flame while being advected toward the flame tip. It is also shown that the latter layer, and therefore the growing autoignition patches, get wrinkled by the coherent structures that originate from the hydrodynamically unstable front root of the jet in crossflow.