Combustion mechanism study of ammonia/n-dodecane/n-heptane/EHN blended fuel

Abstract

Due to the challenges of ammonia, such as high ignition energy and slow flame propagation speed, the utilization of ammonia in engines might necessitate the implementation of dual-fuel combustion modes along with the use of cetane improvers. To optimize the performance of ammonia-fueled engines, and achieve efficient and clean combustion, three-dimensional computational fluid dynamics (CFD) simulations are imperative. However, prior to conducting a three-dimensional CFD study, an accurate reaction mechanism for each component is essential. In this study, the blending mechanisms of ammonia, n-dodecane, and 2-ethylhexyl nitrate (EHN) were constructed using CHEMKIN software, including 243 species and 1293 reactions. The calculated results of ignition delay, laminar flame velocity, and the concentration of significant species agreed well with experiment results. The ignition delay, laminar flame velocity, adiabatic flame temperature, and the concentration of major products were analyzed in different blending ratios. The use of high cetane fuel shortens ignition delay and increases laminar flame speed. As the equivalence ratio of ammonia increases, the concentration of NO decreases while the concentration of H₂ increases. The combustion process is also analyzed based on optical diagnostic results, and the impact of the ammonia/n-dodecane mixtures blended with EHN on the combustion process is investigated. The addition of EHN proved to shorten the ignition delay of the mixture. Furthermore, the introduction of EHN exhibits marginal influence on NOx generation, with the predominant augmentation of NOx emissions stemming from fuel-derived NOx resulting from the fuel's inherent nitrogen content. Due to the small amount of EHN added, the overall impact is relatively small.