Ignition in nonpremixed counterflowing hydrogen versus heated air: Computational study with detailed chemistry

Forced ignition in counterflowing jets of N₂-diluted H₂ versus heated air has been investigated over a wide range of temperature, pressure, and strain rate by numerical modeling with detailed chemistry and transport. Ignition temperatures calculated at constant strain rates are seen to exhibit a Z-shaped pressure dependence similar to that observed in explosion limits of homogeneous H₂/air mixtures. As with the corresponding explosion limits, the first and second ignition limits are governed by the competition for hydrogen radicals between chain branching (H + O₂ → O + OH) and termination (H + O₂ + M → HO₂ + M) pathways, and the third ignition limit involves additional propagation (2HO₂ → H₂O₂ + O₂ → 2OH + O₂) and branching (HO₂ + H₂ → H₂O₂ + H → 2OH + H) pathways that compete with chain termination. Ignition in this inhomogeneous, diffusive system is found to involve a spatially localized ignition 'kernel,' identified as the region near the point of maximum temperature where the rate of hydrogen radical chain branching is maximized. Mass transport of radicals out of the ignition kernel affects the ignition process by competing with chemical reactions within the kernel, particularly in the first and third limits where the dominant ignition chemistry is relatively slow. Ignition temperatures in these limits are found to be much more sensitive to aerodynamic straining than in the second limit. By controlling the width of the ignition kernel and thus the characteristic residence time of key radicals within it, the strain rate is found to determine the dominant chemistry and the relevant ignition limit at any given pressure.