Flame dynamics and kinetic coupling of ammonia and dimethyl-ether in non-premixed cool and warm flames at elevated pressure

Developing advanced low-temperature combustion engines with ammonia-biofuel blends requires a comprehensive understanding of low-temperature flame dynamics and kinetic interactions between ammonia and oxygenated fuels at elevated pressures. This work aims to study the dynamics and kinetics of non-premixed Dimethyl Ether (DME)/Ammonia (NH₃) cool and warm flames, and their reignition to hot flames. A counterflow burner is employed to establish DME/NH₃ cool/warm flames at pressures up to 5 atm. The extinction limits of cool flame and the reignition limits of warm flame to hot flame are measured by varying NH₃ concentrations and compared to simulations to quantitatively examine the effects on DME/NH₃ flames. It is found that NH₃ inhibits low-temperature DME oxidation and results in lower cool flame extinction limits. Warm flames in the presence of NH₃ are observed for the first time, revealing a non-monotonic effect of NH₃ addition: a small amount of NH₃ presence enhances warm flame chemistry and promotes reignition to hot flames, while a high NH₃ concentration weakens the warm flame. This trend is further explained by 0-D PSR kinetic simulations and 1-D S-curve flame dynamic calculations. Three flame transition regimes between cool flames (CF), warm flames (WF), and hot flames (HF) by different levels of NH₃ additions at a specific strain rate are identified, namely WF-HF reignition, WF-CF transition, and WF extinction. Reaction sensitivity analyses of OH at low temperatures show that NH₃ inhibits DME oxidation through OH consumption via H-abstraction and the kinetic couplings of RO₂/NH₂, RO₂/NO_x, R/NO_x, and O₂QOOH/NO_x further suppress the low-temperature branching. At intermediate-temperatures, NH₂/NO_x/HO₂ coupling promotes warm flames via the pathway NH₂ → H₂NO → HNO → NO by converting O₂ → HO₂ → OH. At even higher NH₃ concentrations, radical termination reactions of NH₂ + NO/NO₂ and excessive OH consumption via H-abstraction inhibit the flame. The insights into the kinetic coupling between NH₃ and low-temperature chemistry at elevated pressure and its impact on the dynamics of cool-warm-hot flame transitions will contribute to advancing combustion technologies with reduced emissions and improved energy-efficiency.