Experimental and kinetic modeling study on auto-ignition of ammonia/n-heptane mixtures at intermediate temperatures

Abstract

Ignition delay times (IDTs) were measured in a shock tube facility for NH₃/n-heptane mixtures with NH₃ concentrations in the blending fuel ranging from 0.3 to 0.95 by molar fraction. The measurements were conducted under low pressure of 2 atm and intermediate temperatures of 1350–1500 K at equivalence ratios of 0.5, 1, and 2. With the increase of n-heptane content or equivalence ratio, there is a decrease in the IDTs of NH₃/n-heptane mixtures at intermediate temperatures. A detailed mechanism was updated in this study based on the mechanism of Dong et al. Subsequently, the proposed mechanism was compared to existing blending mechanisms of ammonia and n-heptane in terms of laminar burning velocities (LBVs), IDTs, and species profiles reported in literature. The present model improved the predictions in reproducing the performed experimental measurements compared to previous mechanisms. Finally, rate of production (ROP), sensitivity analysis, and instantaneous and cumulative reaction path analysis were performed to interpret the experiment observations and deepen the understanding of auto-ignition kinetics of ammonia and n-heptane. The results indicate that intermediate species, such as C₂H₄ and C₃H₆, characterized by long lifespans and high concentrations during n-heptane decomposition, play a crucial role at elevated temperatures, while the significance of n-heptane dehydrogenation by NH₂ diminishes with increasing temperature.