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Chemical Kinetics Study on Ignition Process of Highly Diluted Mixtures
Published October 03, 2012 by Society of Automotive Engineers of Japan in Japan
Using a detailed reaction model of normal heptane, the constant-volume ignition process of highly diluted mixtures was analyzed. When the N₂ concentration increases or the fuel concentration decreases too much, an ignition process starting with the LTO phase indicates two peaks of the heat release in the thermal ignition preparation phase. The H₂O₂ loop reactions mainly contribute to the heat release in the low-temperature region of the thermal ignition preparation phase, and H + O₂ + M = HO₂ + M mainly contributes to the heat release in the high-temperature region of the phase. H₂O₂ is accumulated during the LTO and NTC phases, and then drives the H₂O₂ loop reactions to increase the temperature in the thermal ignition preparation phase. When the heat capacity of a mixture per unit fuel concentration increases, the H₂O₂ is consumed out in the middle of the thermal ignition preparation phase, and the heat release by the H₂O₂ loop reactions weakens, so that a gap between the heat release rates in the low-temperature and high-temperature regions of the thermal ignition phase is generated. Because the rate of H + O₂ = OH + O cannot overtake the rate of H + O₂ + M = HO₂ + M, CO + OH = CO₂ + H slowly proceeds with H + O₂ + M = HO₂ + M, not with the branching chain reaction, in the final stage of the ignition process.