The Dynamics and Kinetics of Dimethyl-Ether/Ammonia Non-Premixed Cool and Warm Flames at Elevated Pressures

The development of advanced low-temperature combustion engines utilizing ammonia fuel blends requires a comprehensive understanding of the kinetic interaction between ammonia and reactive fuels. This work aims to study experimentally and numerically the addition of ammonia and dimethyl ether (DME), and its subsequent effect on low- temperature chemistry (LTC) and intermediate-temperature chemistry (ITC) governing cool and warm flames, respectively. A high-pressure counterflow burner is employed to establish self-sustained DME/NH₃ cool flames and measure the extinction limits. The low-temperature peroxy branching reactions are promoted at elevated pressures and therefore enhance the cool flames. However, NH₃ oxidation suppresses the isomerization of RO2↔QOOH, and opens the channel O₂QOOH→CH₂O+HO₂ which prevents O₂QOOH→ketohydroperoxide+OH. As a result, NH₃ addition inhibits the low-temperature DME oxidation and weakens the cool flames. Numerical modeling is performed to analyze the kinetic coupling of NH₃ with ITC and warm flames. While the low-temperature DME oxidation becomes monotonically weaker as NH₃ concentration increases, the intermediate-temperature oxidation is enhanced as NH₃ is increased above 40% in the fuel mixture, followed by inhibition at higher fractions. The promoting effects can be mainly attributed to two mechanisms. First, the reactions NO+HO₂→NO₂+OH, NO₂+NH₂→H₂NO+NO, NO₂+CH₂O→HCO+HONO and HONO(+M)→OH+NO(+M) forms an evolution cycle of NO₂→HONO→NO→NO₂ that converts CH₂O and HO₂ into active OH radicals. Second, NH₃ oxidation opens the channels R→RO→CH₂O and O₂QOOH→CH₂O+HO₂, where the released CH₂O radicals promote the evolution cycle. The calculated S-curve warm flame branches with varying NH₃ concentrations confirm that warm flames can be enhanced with small NH₃ concentrations. However, as NH₃ concentrations exceed 8000 ppm, the promotion effects become excessive and the stable warm flame branch disappears from the S-curve. This work will provide insights into the effects of NH₃ on the dynamics and chemistry of multi-staged flames under engine-like conditions.