Numerical Study on the Ignition and Flame Propagation of Ammonia/<i>n</i>-Heptane Dual Fuels

Abstract

Ammonia is carbon-free and thus is a promising renewable fuel for carbon neutrality. However, because ammonia has low reactivity and narrow flammability range, blended fuels of ammonia/n-heptane are often used to trigger high-temperature combustion in internal combustion engines. In this study, the ignition and flame propagation of ammonia/n-heptane dual-fuels were investigated in a specified mixing layer using a skeletal mechanism via a high-fidelity simulation. The results showed that auto-ignition was initiated at the locations of the most reactive mixture fraction ξmr calculated by the zero-dimensional homogeneous reactor model. Two cool and hot flame fronts were observed to propagate both toward the fuel-lean and fuel-rich regions. A stabilized hot flame front identified by the temperature isoline was observed in the fuel-rich region as a result of the low reaction rates and adiabatic flame temperatures. The temperature of the pilot fuel (TFuel) significantly affected the flame structures in the presence of compositional stratification. When TFuel was increased, the dual-fuel flame structure transitioned from quadruple peaks in the heat release rate (HRR) profiles at 800 K to double peaks at 900 K and to a single peak at 1000 K. The propagation of the flame front was controlled by the cool flame during the early stage and the gradient of the ignition delay times. Subsequently, it transformed into a diffusion-driven flame. Finally, the average HRR profiles had double peaks at 800–1000 K owing to the presence of cool and hot flames. Ammonia is expected to promote the oxidization of n-heptane via the active reaction NO+HO2 ⇌ NO2 + OH at 800 K, causing a high peak of HRR.