Ozone-Assisted Deflagration to Detonation Transition of Lean C2H2/Ox Mixtures in Microchannels

Abstract

The deflagration to detonation transition (DDT) of acetylene/O2 mixtures at lean conditions in a 1 mm square microchannel are investigated using high-speed imaging. Ozone was introduced into a mixture at φ=0.4 in 0.5%, 1%, and 1.5% oxidizer concentrations to study the effects of long lived plasma species on flame acceleration and detonation processes. The times and run-up distances for detonation were reduced by 56.7% and 41.8%, respectively, and CJ velocities were increased by 6.7%. Statistical modeling using a third-degree Gaussian distribution revealed that intrinsic combustion behavior of DDT was not affected by ozone; the process was simply accelerated. A two-sample t-test comparing ozone and non-ozone populations revealed statistically significant results, with a p-value of 0.012. Numerical simulations corroborate these experimental findings, which showed that the presence of ozone significantly onsets mixture ignition and detonation at earlier times. These improvements in detonation performance could possibly be attributed to the thermal decomposition of ozone into free radicals, which play a significant role in attacking unreacted fuel and initiating chain branching reactions. Repeated DDT tests at the same equivalence ratios showed slight increases in recorded detonation times and run-up distances, which may be caused by the build-up of combustion products in the tube that may render mixtures leaner than expected. Flame quenching and pulsating flame propagation were experienced at φ=0.3-0.4, possibly a result of the struggle between sufficient energy release from combustion and significant heat losses within the microchannel.