Ignition delay times of ram accelerator CH4/O2/diluent mixtures

An experimental study was performed to determine ignition delay times for CH4/O-2/diluent mixtures and conditions relevant to forebody combustion on ram accelerator projectiles, All measurements,were performed in the reflected-shock region of a high-pressure shock tube. Temperatures from 1040 to 1600 K and pressures between 35 and 260 atm were studied, and the CH4/O-2/diluent mixtures had an equivalence ratio of 0.4, 3.0, or 6.0 with either N-2, Ar, or He as the bath gas. Reaction progress mas monitored primarily via piezoelectric pressure transducer and visible emission. For each mixture and condition, the ignition developed as a strong ignition front beginning at the endwall with little or no preignition deflagration, Ignition delay time (tau(ign)) correlations were generated for each mixture and the entire data set; the latter correlation indicates that ignition delay is dependent only on the fuel and oxidizer concentrations and, therefore, not on the diluent species or concentration. At temperatures below approximately 1300 K for the Fuel-rich mixtures, the Arrhenius temperature dependence of tau(ign) changes from an average activation energy of 32.7 kcal/mol, at higher temperatures, to approximately 19.0 kcal/mol, at lower temperatures. The transition occurs at higher temperatures as the pressure is increased, and is indicative of a shift in chain-branching kinetics between the high- and intermediate-temperature regimes.