Ignition and extinction characteristics of n-alkane cool flames on a heated wall with different surface reactivity

Abstract

Cool flames play essential roles in the ignition process of hydrocarbon fuels, so that understanding their dynamics is highly crucial to develop advanced engines with high thermal efficiency. There have been extensive studies to elucidate the combustion characteristics of cool flame and the low-temperature chemistry governing its kinetics. However, wall chemical effects on the cool flame ignition and extinction are still not fully clarified, although our recent related works have shown their significant impact on the cool flame ignition temperatures of DME.In this study, ignition and extinction characteristics of n-alkanes with carbon numbers from 7 to 10 are examined systematically, with particular attention paid to the effects of fuel carbon number, oxygen concentration, and surface reactivity. An experimental setup enabling the ignition and extinction of cool flames in the vicinity of a wall is newly built. The surface reactivity is controlled by depositing thin films of specific materials, namely SiO2, Fe, and Ru, on the wall surface. The cool flame ignition and extinction behaviors of each fuel on each wall are identified by measuring HCHO molecules with laser-induced fluorescence (LIF). In addition, numerical simulations assuming one-dimensional axisymmetric impinging flows with detailed gas-phase reactions are performed in order to further investigate the effect of fuel carbon number on the cool flame ignition and extinction. The results indicate that the cool flame ignition and extinction behaviors are highly dependent on the surface reactivity, showing completely different aspects according to the fuel.