Experimental and kinetic studies of extinction limits of counterflow cool and hot diffusion flames of ammonia/n-dodecane

Abstract

The present study focuses on the effects of ammonia on the extinction limits of n-dodecane cool and hot diffusion flames and the kinetic interactions between ammonia and n-dodecane at low and high temperatures. The extinction limits of cool and hot diffusion flames for ammonia/n-dodecane blends are measured in an atmospheric counterflow burner. The results show that the extinction limits of both cool and hot flames decrease with the increase of the ammonia blend ratios and an obvious non-linear tendency can be found for the hot flames. Moreover, the laminar flame speeds and ignition delay times of ammonia/n-dodecane blends are also measured and a detailed ammonia/n-dodecane model is developed and validated with the measured data. With the assistance of kinetic modeling, the physical and chemical effects of ammonia on the extinction limits of ammonia/n-dodecane hot and cool diffusion flames are examined. It is found that the physical effects of ammonia suppress the extinction limits of both cool and hot diffusion flames, relative to the nitrogen dilution. Chemical effects of ammonia play a leading role and increase the extinction limits of hot diffusion flames but decrease the extinction limits of cool diffusion flames, indicating that the ammonia promotes the high-temperature oxidation of n-dodecane but inhibits the low-temperature oxidation of n-dodecane. Kinetic analyses reveal that ammonia addition inhibits the low-temperature chemistry of n-dodecane via NH3 + OH <=> NH2 + H2O (NH3 directly involves the reactions slowing down the n-dodecane H-abstraction reaction) and NO + RO2 <=> RO + NO2 (NO produced from NH3 and NO competes with low-temperature chain-branching reaction pathways for RO2). Moreover, different major reaction pathways of ammonia oxidation at low and high temperature are summarized: NH3 → NH2 → H2NO → HNO → NO is responsible for the ammonia consumption at low temperature, while NH3 → NH2 → NH → N → NO is the one at high temperature.