Numerical and experimental studies on minimum ignition energies in primary reference fuel/air mixtures

Understanding flame initiation is of great interests to combustion research. For induced ignition, a minimum ignition energy (MIE) must be provided by an external ignition source for the formation of a flame kernel. In this paper, the minimum ignition energy in lean primary reference fuel (PRF) /air mixtures is investigated by simulations and experiments. The simulations feature a detailed treatment of chemical kinetics and molecular transport to solve the conservation equations for a flame evolving from an ignition source. In the experiments, a cruciform burner is used for the ignition of the fuel/air mixture. After applying the heat source, three qualitatively different types of evolution are observed: ignition failure, flame kernel formation with subsequent flame extinction, and flame propagation. We define two kinds of MIEs: one for flame initiation, and one for flame propagation. The dependence of MIE for flame propagation on the research octane number (RON) in both experiments and simulations exhibits an upturn at a certain RON: Below this RON, the MIE for flame propagation increases weakly, and above this RON, the MIE for flame propagation increases rapidly with increasing RON. The existence of the upturn can be explained by different reaction pathways of the n-heptane and iso-octane flame. At smaller RON, the massive production of C₂H₅ during the oxidation of n-heptane helps to sustain the critical early phase of flame with strong diffusion. The results in this study highlight the importance of the interaction between chemical reactions and transport for understanding behaviours of practical ignition processes.