On the NTC behaviors in explosion limits of C1 to C3 n-alkane/air mixtures

In this study, the explosion limits of methane, ethane and propane with air under various equivalence ratios were investigated. For the methane-air mixture, a Negative Temperature Coefficient (NTC) response progressively appears with an increase in equivalence ratio. The NTC response for ethane-air mixture remains relatively constant, and the explosion boundary shifts to high pressure in the high-temperature region. In propane-air mixtures, both the explosion limit curve in the NTC response section and the high-temperature section shift to high pressure, causing a double NTC response under high equivalence ratios. Detailed reaction path analysis confirmed that the primary reaction path of methyl radicals shifted from oxygen addition reactions to recombination reactions, leading to the NTC response and reduced reactivity. The decomposition of C2H5O2H promotes the reactivity of the ethane-air mixture, thereby generating the NTC response. Under high equivalence ratios, both the reaction path of the C3H7 radical via secondary oxygenation at low temperatures and the oxygen addition reactions of small alkane at high temperatures are hindered, leading to the generation of a double NTC response. Temperature and species profiles are compared under the NTC region for these fuels. These findings provide valuable insights and guidance for applications involving fuel-rich hydrocarbon combustion processes.