Cellular Properties of Unstable Detonations from Mixtures of Ethylene with Nitrous Oxide

Detonation cell width and length were recorded for homogeneous mixtures of ethylene with nitrous oxide, using sooted foils in a 2.21cm x 2.21cm cross-section linear detonation combustor for equivalence ratios (Φ) from Φ=0.5 to Φ=2.2. These mixtures have a moderate polytropic index, a high stability parameter, and produce high Mach number detonations. These factors lead to highly unstable detonations with small irregular cells. Detonations from homogeneous mixtures were compared to detonations from a direct injected (DI) configuration. Detonation velocities recorded for the homogeneous mixtures confirm that the reported length scales correspond to steady detonations near the Chapman-Jouget velocity. Premixed detonations were steady and repeatable, with velocity deficits of ≈1% for most cases. Direct injected detonations were more variable, and had velocity deficits near 3%. Box and whisker plots are used to show the large variability in cell size and. A slight U-shaped dependence was found for both as a function of Φ for the premixed cases. Cell size and variability rise faster on the lean side than on the rich side. The range of Φ represents the boundaries for steady, CJ detonations achievable for this setup. Although the premixed lean (Φ=0.5) and rich (Φ=2.2) limits resulted in unsteady detonations, a longer combustor may have allowed an eventual relaxation towards. None of the DI detonations achieved a steady velocity, but well-developed detonations were achievable over a wide range of Φ.