Thermodynamic and chemical kinetic characteristics of NTC turnover states

This work investigates the thermodynamic and chemical kinetic behavior of the upper and lower turnover states of the Negative-temperature Coefficient (NTC) regime, in which the ignition delay exhibits a local maximum and then a local minimum with increasing temperature. It is shown that the ignition delays vary Arrheniusly with the temperature, while the pressure can also be correlated Arrheniusly with temperature for both turnover states. These two behaviors have been investigated using detailed mechanisms for typical n-alkanes exhibiting NTC, namely n-butane, n-heptane and iso-octane. Further theoretical analyses demonstrate that the upper turnover states result from the competition between the low-temperature chain branching reactions and the decomposition of the intermediate species, which corresponds to a critical value, α, of the ratio of OH production from low-temperature chemistry, and that the pressure dependence of the ignition delay mainly results from the H₂O₂ decomposition reaction. It is further shown that, for the lower turnover state the first and second stage delays are approximately equal and the change of the second-stage ignition delay due to both pressure and temperature variation is balanced, with the change of the first-stage ignition delay due to its temperature sensitivity. Analytical expressions are derived and are shown to well reproduce the lower turnover states. These thermodynamic and kinetic insights related to both turnover states also achieve quantitative agreement with the literature experimental data of n-heptane ignition delay time.