IGNITION IN NONPREMIXED COUNTERFLOWING HYDROGEN VERSUS HEATED AIR - COMPUTATIONAL STUDY WITH DETAILED CHEMISTRY

Forced ignition in counterflowing jets of N-2-diluted H-2 versus heate d air has been investigated over a wide range of temperature, pressure , and strain rate by numerical modeling with detailed chemistry and tr ansport. Ignition temperatures calculated at constant strain rates are seen to exhibit a Z-shaped pressure dependence similar to that observ ed in explosion limits of homogeneous H-2/air mixtures. As with the co rresponding explosion limits, the first and second ignition limits are governed by the competition for hydrogen radicals between chain branc hing (H + O-2 --> O + OH) and termination (H + O-2 + M --> HO2 + M) pa thways, and the third ignition limit involves additional propagation ( 2HO(2) --> H2O2 + O-2 --> 2OH + O-2) and branching (HO2 + H-2 --> H2O2 + H --> 2OH + H) pathways that compete with chain termination. Igniti on in this inhomogeneous, diffusive system is found to involve a spati ally 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 reacti ons within the kernel, particularly in the first and third limits wher e the dominant ignition chemistry is relatively slow. Ignition tempera tures in these limits are found to be much more sensitive to aerodynam ic straining than in the second limit. By controlling the width of the ignition kernel and thus the characteristic residence time of key rad icals within it, the strain rate is found to determine the dominant ch emistry and the relevant ignition limit at any given pressure.