The role of low temperature chemistry in combustion mode development under elevated pressures

Negative Temperature Coefficient (NTC) behavior is an essential feature of low-temperature oxidation for large hydrocarbon fuels, which is of particular relevance to cool flame and auto-ignition. In this study, using n-heptane as a typical fuel exhibiting NTC, combustion phenomena involving both auto-ignition and flame propagation are computationally studied at initial temperatures within and above NTC regime under elevated pressures in a one-dimensional planar constant-volume configuration, with detailed kinetics and transport. Multi-staged flame structures representing cool flame and hot flame are observed, and consequently, different types of auto-ignition are identified during two-staged and single-staged flame propagation scenarios by varying initial temperature. Specially, as the initial temperature increases, the behavior of cool flame is gradually suppressed and auto-ignition position is transferred from the location ahead of flame front to end-wall region, leading to different combustion modes and peak pressure magnitudes. Moreover, attributed to the chemical reactivity processed by cool flame, the flame propagation of the cases within NTC regime is even faster than those beyond NTC regime. A recently developed two-staged Livengood-Wu integral is further utilized to predict these auto-ignition scenarios, yielding good agreement and further demonstrating the significant role of NTC chemistry in modifying the thermodynamic state and chemical reactivity at upstream of a reaction front. Finally, different combustion modes and knocking intensity for these detailed calculations are summarized in non-dimensional diagrams, which suggest that a higher initial temperature does not guarantee a higher knocking intensity, instead, the developing and developed detonation wave initiated by an auto-ignition occurring within NTC regime could even induce higher knocking intensity in comparison to the thermal explosion under the temperatures beyond NTC regime.