Propagation modes and statistics of near-limit hydrocarbon detonations

Abstract

Universal propagation mechanisms and structural dynamics of hydrocarbon detonations near the detonation limit are investigated using MHz-rate shadowgraph and CH* chemiluminescence imaging to capture the coupled shock and reaction front. A total of 310 realizations of methane-oxygen and ethylene-oxygen mixtures with varied nitrogen dilution were conducted in a fully automated thin-channel detonation facility. Spatial wavefront velocity fields were extracted from high-speed imagery to compute ensemble statistics, including probability density functions, and the mean, variance, skewness, and kurtosis. The transition from freely propagating detonations to the onset of spin is distinctly separated into four propagation regimes: CJ multiheaded, transitional multiheaded, weak multiheaded, and singleheaded. These regimes are defined by the emergence and eventual dominance of transverse detonation waves, which compensate for weakening cellular instabilities by driving local velocities significantly above the Chapman-Jouguet velocity. This shift in mechanism is shown to be clearly linked to the lack of prompt autoignition by the leading shock indicated by the increasing induction length. The modes are shown to be separable statistically with the combination of the mean velocity and the coefficient of variation (standard deviation scaled by the mean). Transverse detonations are observed to form through the autoignition of large unreacted gas pockets in transverse wave collisions. The induction length is shown to govern the transition, with increasing length correlating to broader velocity distributions (higher variance and skewness) and a shift toward sub-Chapman-Jouguet velocities. Elevated kurtosis values mark the presence of overdriven transverse detonations as statistical outliers. The product of the activation energy and the Von Neumann density ratio, which effectively captures the effects of the activation energy and molecular collisions, were shown to differentiate the modes.