Investigating Autoignition Characteristics of Ammonia/Heptamethylnonane Mixtures Over Wide Pressure Ranges: Rapid Compression Machine Measurements and Kinetic Modeling Study

Ammonia (NH3) blending combustion with high-reactivity
fuel has garnered substantial attention in terms of decarbonization
potential in internal combustion engines. 2,2,4,4,6,8,8-Heptamethylnonane,
denoted as HMN, an important large-molecular weight component for
diesel and jet fuel surrogates, was selected to be blended with NH3 in this study. The ignition delay times (IDTs) of NH3/HMN mixtures were measured using a heated rapid compression
machine (RCM) over an extensive range of conditions (temperature of
680–1025 K, pressure of 20–100 bar, equivalence ratios
of 0.5–1.0, and NH3 energy ratio (NER) of 50–90%).
Experimental results show that increasing the pressure, equivalence
ratio, and oxygen concentration reduces both the total and first-stage
IDTs, while an increase in the NH3 energy ratio prolongs
the IDTs. For the mixture with the lowest NH3 energy ratio
of 50%, non-Arrhenius-type behavior was observed at a pressure of
20 bar, while it transfers to a monotonic decrease of IDTs with increasing
temperature at a pressure of 40 bar. An NH3/HMN blending
mechanism was developed by merging individual NH3 and HMN
submechanisms, updating NH3 submechanism, and adding C–N
cross-reaction subset. Simulation results show that under most experimental
conditions, the blending mechanism exhibits reasonable prediction
on the measured NH3/HMN IDTs. Kinetic analysis shows that
the discrepancy in the first-stage ignition between experiments and
simulations may be associated with the inaccurate OH consumption proportion
between HMN and NH3, while at the intermediate-temperature
region, it may be related to the core C0–C4 mechanism and the NH3-related reactions. Further experimental
or quantum calculations are needed in the future to refine the NH3/HMN blending mechanism on the basis of this work.