Direct numerical simulation of ammonia/n-heptane dual-fuel combustion under high pressure conditions

Ammonia (NH 3 ) combustion has received increasing attention for its carbon -free nature. However, the utilization of ammonia in engines is constrained by its relatively low reactivity. To overcome this shortcoming, ammonia blending with high -reactivity fuels has been used. In the present work, direct numerical simulation (DNS) of ammonia/n-heptane dual -fuel combustion was performed to understand the effects of ammonia addition on the combustion behavior of n-heptane sprays. Three DNS cases with NH 3 energy ratios of 0%, 20%, and 40% were considered. Two -stage combustion was observed in all cases. It was found that both the first -stage and second -stage ignition delay times increase with increasing NH 3 energy ratio, and the peak of mean heat release rate (HRR) reduces with increasing NH 3 energy ratio. For low -temperature combustion, ignition kernels were observed in the case with pure n-heptane while ignition occurs almost volumetrically in the cases with NH 3 addition. The reaction zone of the cases with NH 3 addition is thicker compared to that of the case with pure n-heptane. The reactions of NH 3 involve radicals, such as OH, which affects the low -temperature combustion of n-heptane. In the stage of high -temperature combustion, individual ignition kernels with significant heat release rate were observed in the cases with NH 3 addition, where the reaction zones are thin. In contrast, for the case with pure n-heptane, large heat release rate occurs simultaneously over the entire domain with thick reaction zones. Ignition occurs in regions characterized by low scalar dissipation rate for both low -temperature and high -temperature combustion in all cases. It was found that, though the contributions of NH 3 -related reactions to the total HRR are noticeable, the contributions of important elementary reactions to the total HRR in pure n-heptane combustion still prevail in the cases with NH 3 addition, which indicates that n-heptane chemistry dominates that of NH 3 in this work.