The Effect of 2,5-Norbornadiene on the Ignition of Hydrogen and Methane Behind Reflected Shock Waves

Abstract

Ignition delay times were measured for stoichiometric H2/O2, CH4/O2, H2/ C7H8/O2, and CH4/C7H8/O2 mixtures highly diluted in argon using the High-Pressure Shock Tube and Aerospace Shock Tube Facilities at Texas A&M University. Reflected-shock temperatures ranged from 1014 to 1459 K in the hydrogen mixtures and 1778 to 2227 K in the methane mixtures. Reflected-shock pressures remained near 1 atm for all experiments apart from the hydrogen mixtures which were also tested at intermediate pressures near 7 atm, targeting the second explosion limit. A chemiluminescence diagnostic was used to track the time history of excited hydroxyl radical (OH*) emission at 307 nm, which was used to define ignition delay time at the sidewall location. Molar concentrations of 2,5-norbornadiene were supplemented to the baseline mixtures representing 1-2% of the fuel by volume. CH4/O2 mixtures containing 2,5-norbornadiene demonstrated reduced ignition delay times, with a pronounced effect at lower temperatures. These mixtures exhibited similar Arrhenius behavior and possessed comparable activation energies. The larger departure in ignition delay time at lower temperatures suggests alternative reaction pathways were taken to complete the chain-branching kinetic sequence. Conversely, this additive increased the ignition delay time dramatically in the H2/O2 mixture. The observed reduction in reactivity is attributed to changes in the fundamental chemistry with the introduction of molecules containing carbon bonds which require stronger activation energies for ignition. At intermediate pressures, the presence of 2,5-norbornadiene in the H2/O2 mixture suppresses the chain- terminating reactions and allows ignition to be extended to higher temperatures. Lastly, correlations were developed to predict the ignition delay time which depend on species concentration, temperature, and pressure. To the author’s knowledge, this study represents the first set of gas-phase ignition data measured in a shock tube for hydrogen and methane mixtures containing the additive 2,5-norbornadiene.