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Chemical Kinetics Study on Two-Stage Main Heat Release in Ignition Process of Highly Diluted Mixtures
Abstract:
Some experimental data indicate that an HCCI process of a highly
diluted mixture is characterized with a two-stage profile of heat
release after the heat release by low-temperature oxidation, and
with slow CO oxidation into CO₂ at a low temperature. In the
present paper, these characteristics are discussed using a detailed
chemical kinetic model of normal heptane, and based on an
authors' idea that an ignition process can be divided into five
phases.
The H₂O₂ loop reactions mainly contribute to heat release in a
low-temperature region of the TI (thermal ignition) preparation
phase. However, H+O₂+M=HO₂+M becomes the main contributor to heat
release in a high-temperature region of the TI preparation phase.
H₂O₂ is accumulated during the LTO (low-temperature oxidation) and
NTC (negative temperature oxidation) phases, and drives the H₂O₂
loop reactions to increase the temperature during the TI
preparation phase. When the heat capacity of a mixture increases by
dilution, H₂O₂ is consumed in a lower-temperature region. Thus, the
heat release by the H₂O₂ loop reactions stagnates at a lower
temperature, causing a gap of heat release between the
low-temperature and high-temperature regions of the TI preparation
phase.
When a mixture is diluted to a considerable extent, the rate of
a branching chain reaction, H+O₂=OH+O, cannot overtake the rate of
H+O₂+M=HO₂+M to the end of an ignition process. Thus, the CO
oxidation into CO₂, CO+OH=CO₂+H, slowly proceeds along with
H+O₂+M=HO₂+M rather than with the branching chain reaction.
Conventional combustion control ways cannot be useful for
activating low-temperature combustion.