Ignition delay times of ram accelerator mixtures
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1996, American Institute of Aeronautics and Astronautics, Inc. An experimental and analytical study was performed to determine ram accelerator ignition delay times at conditions relevant to forebody combustion. All measurements were performed in the reflectedshock 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 3.0 or 6.0 and either N2, Ar, or He as the bath gas. 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. Therefore ram accelerator bath gas combinations can, for specified CH4 and O2 mole fractions, be selected for their gas dynamic properties with little or no effect on the ignition delay time. At temperatures near 1300 K, the Arrhenius temperature dependence of ign changes abruptly from an average activation energy of 30-40 kcal/mol to approximately 20 kcal/mol. The transition occurs at higher temperatures as the pressure is increased. This trend is characteristic of a shift from high- to low-temperature chain branching as reactions involving HO2 become important. A detailed CH4/O2 kinetics mechanism designed for high-temperature combustion increasingly overpredicts ign at higher pressures and cannot model the transition to low temperatures. Improved agreement between model and experiment may require additional reactions important in low-temperature methane combustion, including reactions containing higher hydrocarbons.
