An Experimental and Detailed Kinetics Modeling Study of Norbornadiene in Hydrogen and Methane Mixtures: Ignition Delay Time and Spectroscopic CO Measurements

Abstract

High-energy-density compounds such as norbornadiene (NBD) are being considered as potential cost-effective fuel additives, or partial replacements, for high-speed propulsion applications. To assess the ability of NBD to influence basic fuel reactivity enhancement and to build a database for developing future NBD kinetics models, ignition delay times were measured in two shock-tube facilities at Texas A&M University for H2/O2, CH4/O2, H2/NBD/O2, and CH4/NBD/O2 mixtures (ϕ = 1) that were highly diluted in argon. The reflected-shock temperatures ranged from 1014 to 2227 K, and the reflected-shock pressures remained near 1 atm for all of the experiments, apart from the hydrogen mixtures, which were also tested near 7 atm, targeting the second-explosion limit. The molar concentrations of NBD were supplemented to the baseline mixtures representing 1–2% of the fuel by volume. A chemiluminescence diagnostic was used to track the time history of excited hydroxyl radical (OH*) emission, which was used to define the ignition delay time at the sidewall location. Spectroscopic CO data were also obtained using a tunable quantum cascade laser to complement both the ignition and the chemiluminescence data. The CH4/O2 mixtures containing NBD demonstrated reduced ignition delay times, with a pronounced effect at lower temperatures. Conversely, this additive increased the ignition delay time dramatically in the H2/O2 mixture, which was attributed to changes in the fundamental chemistry with the introduction of molecules containing carbon bonds, which require stronger activation energies for ignition. Correlations were developed to predict the ignition delay time, which depends on species concentration, temperature, and pressure. Additionally, one tentative mechanism was tested, combining base chemistry from NUIGMech 1.1 with pyrolysis and oxidation reactions for NBD using the recent efforts from experimental and theoretical literature studies. The numerical predictions show that the rapid decomposition of NBD provides a pool of active H-radicals, significantly increasing the reactivity of methane. This study represents the first set of gas-phase ignition and CO time-history data measured in a shock tube for hydrogen and methane mixtures containing the additive NBD.