Effect of NO2 addition on flame dynamics and chemical kinetics of near-limit n-dodecane cool and warm diffusion flames

Abstract

The present study is a continuation of the previous study by Zhou et al. (2021), in which kinetic effects of NO addition on n-dodecane cool and warm diffusion flames are investigated via counterflow flames and a skeletal n-dodecane/NOx model is developed by performing jet-stirred reactor experiments. Here, effects of NO2 addition on the flame dynamics and kinetics of n-dodecane diffusion flames are studied. The focus is on understanding the kinetic effects of NO2 on cool flame extinction limits and the transitions between cool, warm, and hot flames and elucidating the differentiated effects of NO and NO2 on low- and intermediate-temperature chemistry of n-dodecane. NO2 also plays different roles in cool and warm flames. Specifically, NO2 addition reduces the cool flame temperature and strain rates at cool flame extinction, indicating that NO2 inhibits the low-temperature chemistry of n-dodecane. NO2 addition increases the warm flame temperature, delays the warm flames extinction to cool flames, and promotes the warm flames re-ignition to hot flames, demonstrating NO2 enhances the intermediate-temperature oxidation of n-dodecane. However, reaction pathways and sensitivity analyses reveal that reactions responsible for NO2 and NO affecting the low- and intermediate-temperature oxidation of n-dodecane diffusion flames are differentiated. For cool flames, reaction R + NO2 <=> RO + NO for NO2 addition and reaction RO2 + NO <=> RO + NO2 for NO addition are the major corresponding reactions, respectively, which compete with the low-temperature chain-branching reaction pathway. For warm flames, the kinetic effects of NO on the intermediate-temperature oxidation are due to NO converting the relative inactive HO2 to reactive OH radicals via the reaction NO + HO2 <=> OH + NO2, which simultaneously accelerates radical production of QOOH => QO + OH to the increase of warm flame temperature, while the effect of NO2 is mainly induced by the conservation of NO2 to NO. Moreover, comparisons of the effects of NO and NO2 addition on cool flame extinction limits are performed, and results show that the kinetic effects of NO2 addition are weaker than those of NO addition since the effects of NO2 are partially contributed by NO from the conservation of NO2.