Ignition of hydrogen-enriched methane by heated air

This study investigates experimentally and computationally the effects of hydrogen addition on ignition in nonpremixed, counterflowing methane vs. heated air jets for ambient pressures between 0.2 and 8.0 arm, hydrogen concentrations in the range 0-60% by volume, and pressure-weighted strain rates of 150, 300, and 350 s^-1. The effect of flow strain rate was further investigated computationally for strain rates between 100 and 10,000 s^-1. Hydrogen addition was found to significantly improve methane ignition through a mechanism of increased radical production and weakening of kinetic inhibition by diffusive separation of branching and termination reactions. Three ignition regimes were identified, depending on the H₂ concentration: 1) hydrogen-assisted methane ignition, 2) transition, and 3) hydrogen- dominated ignition. Both experiments and modeling indicated two-stage ignition within the first two regimes, with the first stage controlled by radical runaway, and the second stage involving thermal feedback. The controlling chemistry within the three ignition regimes was investigated using the Computational Singular Perturbation (CSP) method applied to conditions within an ignition kernel identified similar to previous studies on counterflow ignition. Chemical heat release was shown to be indispensable at ignition in the first two regimes, but negligible within the third, kinetically dominated regime, except at high pressures. Similarly, transport effects were found to be significant in regimes 1) and 2), but the ignition temperatures were largely insensitive to strain within the third regime. Methane addition to the H₂/N₂/air system was found to inhibit ignition at low and moderate pressures, while facilitating it at pressures greater than ~ 5 atm, primarily because of the interaction with the HO₂/H₂O₂ chemistry which is dominant in these regimes. A CSP-derived skeletal mechanism was found to represent, within a 3% deviation, the ignition temperatures and species concentrations calculated using the full mechanism.