Kinetic effects of NO addition on n-dodecane cool and warm diffusion flames

The kinetic effects of NO addition on the flame dynamics and burning limits of n-dodecane cool and warm diffusion flames are investigated experimentally and computationally using a counterflow system. The results show that NO plays different roles in cool and warm flames due to their different reaction pathway sensitivities to the flame temperature and interactions with NO. We observe that NO addition decreases the cool flame extinction limit, delays the extinction transition from warm flame to cool flame, and promotes the ignition transition from warm flame to hot flame. In addition, jet-stirred reactor (JSR) experiments of n-dodecane oxidation with and without NO addition are also performed to develop and validate a n-dodecane/NO_x kinetic model. Reaction pathway and sensitivity analyses reveal that, for cool flames, NO addition inhibits the low-temperature oxidation of n-dodecane and reduces the flame temperature due to the consumption of RO₂ via NO+RO₂↔NO₂+RO, which competes with the isomerization reaction that continues the peroxy radical branching sequence. The model prediction captures well the experimental trend of the inhibiting effect of NO on the cool flame extinction limit. For warm flames, two different kinds of warm flame transitions, the warm flame extinction transition to cool flame and the warm flame reignition transition to hot flame, were observed. The results suggest that warm extinction transition to cool flame is suppressed by NO addition while the warm flame reignition transition to hot flame is promoted. The kinetic model developed captures well the experimentally observed warm flame transitions to cool flame but fails to predict the warm flame reignition to hot flame at similar experimental conditions.