Experimental and numerical study on the laminar burning velocities of n-dodecane/ammonia/air mixtures

Laminar burning velocity (LBV) is a crucial parameter in describing the engine combustion characteristics, such as flame stability and thermal efficiency. In this study, n-dodecane was used as a surrogate fuel for diesel. Unique LBV data for n-dodecane/NH3/air mixtures at various initial temperatures (Tu = 488–650 K), ammonia blending ratios (XNH3 = 0∼0.9), and equivalence ratios (ϕ = 0.6–1.4) were measured by the Externally heated diverging channel (EHDC) method. Additionally, an improved mechanism incorporating a subset of C-N interaction reactions was developed, which predicts LBV of n-dodecane/NH3/air flames with high accuracy. The results show that LBV increases with initial temperature, while it decreases with increasing XNH3. When XNH3 increased from 0.1 to 0.9 (ϕ = 1), the LBV decreased by 24.1 cm/s (from 81.3 cm/s to 57.2 cm/s) at an initial temperature of 488 K, and by 35.0 cm/s (from 139.4 cm/s to 104.4 cm/s) at an initial temperature of 650 K. This study also investigated the sharp decline in LBV at high XNH3 from the perspectives of chemical, thermal, and diffusion effects. The results indicate that at high ammonia concentrations, the influence of the adiabatic temperature increase becomes more significant, with the thermal effect proportion increasing by nearly 20 % as XNH3 rises from 70 % to 90 %. During the combustion of NH3 with large n-alkanes, key C/N compound intermediates play a critical role in explaining both LBV changes and nitrogen-containing product formation. C-N interaction reactions, such as R8403(CH3 + H2NO = CH3O + NH2), R8392(CH3 + NH2 = CH3NH2), R8414(CH + NO = HCN + O), and R8433(C2H + NO = HCN + CO), play a crucial role in optimizing nitrogen product formation.