On the crossover temperature and lower turnover state in the NTC regime

This work investigates the thermodynamic and chemical kinetic behavior of the lower stationary point, denoted as the lower turnover state, of the NTC curve near the low temperature regime, where the ignition delay exhibits a local minimum. It is found that the ignition delay varies Arrheniusly with the turnover temperature under various pressures, and that the turnover temperature, Tlower, and the initial pressure, P0, are related and can be correlated by another Arrhenius dependence, as P0 ∼ exp(−Ea/Tlower). These two behaviors have been systematically investigated using detailed mechanisms for typical n-alkanes exhibiting NTC, namely n-butane and n-heptane. It is found that the first and second stage ignition delays are approximately equal to each other at the lower turnover states under various pressures, such that the total ignition delay shares similar temperature dependence with the first stage ignition delay. To further characterize the second ignition stage, the cool flame temperature rise at the end of the first stage is investigated and shown to decrease linearly with increasing initial temperature. A crossover temperature is then defined at the state where the cool flame temperature rise disappears. This state can be correlated with a constant α, which denotes the ratio between OH production from the low-temperature chemistry and the total fuel consumption. It is shown that the second stage ignition delay correlates well with the reaction rate constant for the H2O2 decomposition. Analytical expressions are then derived based on these kinetic insights on the two stages, which well reproduce the lower turnover temperature under different pressures and provide further insights into the two-stage ignition at the turnover states. Comparison of the present results with the literature experimental data of the n-heptane ignition delay time shows good agreement.