Ignition delay times of ram accelerator CH4/O2/diluent mixtures
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An experimental study was performed to determine ignition delay times for CH4/O2/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/O2/diluent mixtures had an equivalence ratio of 0.4, 3.0, or 6.0 with either N2, Ar, or He as the bath gas. Reaction progress was 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 (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 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. 1998 by the American Institute of Aeronautics and Astronautics, Inc.
