Abstract
Using a detailed chemical kinetic model of normal heptane, a constant-volume ignition process of a highly-diluted mixture was analyzed. When the N2 concentration increases or the fuel concentration decreases to a considerable extent, an ignition process starting with the LTO phase indicates double peaks of heat release in the thermal ignition preparation phase. The H2O2 loop reactions mainly contribute to heat release in a low-temperature region of the thermal ignition preparation phase, and H + O2 + M = HO2 + M mainly contributes to heat release in a 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, H2O2 is consumed in a lower-temperature region, and the heat release by the H2O2 loop reactions stagnates at a lower temperature. Thus, a gap of heat release between the low-temperature and high-temperature regions of the thermal ignition preparation 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 proceeds slowly with H + O2 + M = HO2 + M in the final stage of the ignition process rather than with the branching chain reaction.
