Laminar Burning Velocities of Methanol/Ammonia/ <i>N</i> -Heptane Fuels

Methanol/ammonia blends are promising zero-carbon fuels. However, their poor autoignition tendency limits engine applications. To overcome this, n-heptane is introduced as a high-reactivity additive to enhance combustion. In this work, the laminar burning velocities of methanol/ammonia/n-heptane blends were investigated, and a detailed chemical kinetic mechanism for the ternary fuel system was developed and validated to clarify the associated combustion chemistry. The results show that partial replacement of methanol by n-heptane significantly enhances flame propagation, especially under lean and near-stoichiometric conditions. At ϕ = 1.0, increasing the n-heptane substitution ratio from 0% to 10% increases the laminar burning velocities of M20A80, M40A60, M60A40, and M80A20 by 50%, 31.97%, 22.54%, and 4.68%, respectively, indicating that the promoting effect weakens as the methanol fraction increases. The proposed mechanism reproduces the measured burning velocities well and captures the main variation trends for different fuel compositions. Kinetic analysis further reveals that n-heptane addition enhances H/O radical chain-branching, accelerates the oxidation of ammonia and hydrocarbon intermediates, shifts the reaction zone upstream, and strengthens radical circulation. These effects collectively improve flame reactivity and explain the observed increase in laminar burning velocity of methanol/ammonia/n-heptane blends. Moreover, as the equivalence ratio increases from 0.8 to 1.2, the dominant sensitivity contributions progressively shift toward reactions involving CH2OH, HCO, and NH3. These effects collectively improve flame reactivity and explain the observed increase in laminar burning velocity of methanol/ammonia/n-heptane blends.