Autoignition Characteristics of NH₃/C₂H₆ Mixtures over Wide Pressure Ranges

Ignition delay times of (IDTs) NH3/C2H6 fuel mixtures are measured using a heated ultrahigh-pressure rapid compression machine, with ethane blending ratios ranging from 10 to 50% by mole fraction. The experimental conditions span a wide range, including temperatures from 875 to 1075 K, pressures from 20 to 100 bar, equivalence ratios from 0.5 to 2.0, and dilution ratios of 3.76 and 8.52. The influence of pressure and ethane blending ratio on IDTs progressively diminishes as the pressure and ethane concentration increase. To quantify the combined effects of pressure and other variables on IDTs, a new parameter named the IDT ratio is introduced. A higher IDT ratio indicates a more significant pressure effect on the IDTs. The results showed that lower temperatures and lower ethane blending ratios lead to higher IDT ratios. A blending mechanism is developed based on the Aramco 3.0 submechanism and the ammonia submechanism from POLIMI. The new blending mechanism shows good prediction on the experimental data and can well-predict the effect of different parameters. Sensitivity analysis shows the competitive reactions NH2 + NO2 = H2NO + NO and NH2 + NO2 = N2O + H2O have significant effects on IDTs. As the ethane concentration increases, reactions related to ethane oxidation gain predominance. Higher reactivity of the fuel mixtures under higher pressure should be attributed to the increasing rates of H2O2(+M) = 2OH(+M) and CH2CHO + O2 = CH2O + CO+ OH reactions, which are pressure-dependent channels and can produce OH radicals. However, the sensitivity of H2O2(+M) = 2OH(+M) on ignition decreases with increasing temperature. Oxygen concentration significantly affects the IDTs for mixtures with a 10% mole fraction of ethane, while it becomes negligible for mixtures with a 50% mole fraction of ethane, and the reason behind this phenomenon requires further investigation. Additionally, a higher ethane blending ratio inhibits the oxidation of ammonia because the proportion of the ammonia consumption pathway decreases as the ethane blending ratio increases, with ethane being preferentially consumed in the initial stages. However, the overall effect of ethane addition promotes ammonia ignition due to the enrichment of radical pools.