Kinetic effects of n-propylbenzene on n-dodecane counterflow nonpremixed cool flames

The development of alternative drop-in fuels and fuel blends with petroleum-derived fuels necessitate an understanding of the kinetic interactions between aromatics and alkanes for the development of advanced low-temperature combustion engines. In the present study, the role of aromatic chemistry on n-alkane low-temperature chemistry is investigated by using nitrogen-diluted nonpremixed cool flames of n-dodecane/n-propylbenzene blends in an atmospheric counterflow burner. Effects of aromatic addition on n-alkane reactivity are examined by fixing the concentration of n-dodecane while varying the ratio of nitrogen/n-propylbenzene. The cool flame structure is probed using formaldehyde laser-induced fluorescence and thermocouple thermometry, and measurements of the cool flame and hot flame extinction limits are obtained. Fundamental differences between the response of cool flame and hot flame heat release and radical production to stretch are also discussed. The results show that, while the specific heat release rate characterizes the burning intensity of a hot flame, the same is not true for a cool flame due to the competition between reactant leakage and negative temperature coefficient reactivity. It is also found that, contrary to hot flames, in cool flames the addition of n-propylbenzene promotes n-dodecane cool flame extinction. Kinetic analysis reveals that aromatics act as a sink of hydroxyl and hydroperoxyl radicals in cool flames by forming stable oxygenated aromatics due to the low flame temperatures. As a result, the low-temperature reactivity of n-dodecane is significantly inhibited by the addition of aromatics, even relative to inert substitution. The present experimental results are valuable for comprehensive chemical kinetic model development and validation.