Experimental and kinetic study on the laminar burning velocity of TRF/NH₃-air premixed flame at elevated pressure

The application of NH₃ to transportation is a practical measure to drive the automotive industry towards a more environmentally friendly and sustainable future. Thus, this study focuses on an experimental and modeling study of the laminar burning velocity (S_L) of ternary reference fuel/ammonia (TRF/NH₃) mixtures under specific operating conditions. TRF blend was meticulously formulated with a specific volumetric composition, comprising 17 vol% n-heptane (nC₇H₁₆), 69 vol% iso-octane (iC₈H₁₈), and 14 vol% toluene (C₇H₈). The laminar burning velocity (S_L) of TRF/NH₃ mixtures was measured using the constant-volume method, with an initial temperature (Ti) of 400 K, initial pressures (P) from 1 atm to 3 atm, equivalence ratio (ϕ), ranging from 0.8 to 1.3, and ammonia (NH₃) mole fraction (X_NH3) from 0 % to 70 %. By integrating TRF model developed by Cai and Pitsch [42] and NH₃ model by Zhang et al. [26], a kinetics model for TRF/NH₃ mixtures was constructed. Subsequently, the model was extensively validated for iC₈H₁₈/NH₃, nC₇H₁₆/NH₃, and TRF/NH₃ mixtures, focusing on S_L, ignition delay time (IDT), and stable species concentrations. Subsequently, employing sensitivity analysis combined with reaction pathway, the impact of NH₃ addition on TRF mixture was investigated. The results indicate that the addition of NH₃ competes with TRF for OH radicals, leading to a reduction in the proportion of most initial oxidation reaction pathways of TRF. Simultaneously, in TRF/NH₃ blend, the oxidation of NH₃ precedes that of TRF and has a longer oxidation duration. NH₂ radicals generated during the initial oxidation of NH₃ can become integrated into the H-abstraction oxidation of TRF. Additionally, radicals produced during TRF oxidation can enhance the oxidation of NH₃, particularly in the later stages of combustion. Finally, the effect of initial pressure on the mole fraction of NO in TRF/NH₃ flames is analyzed. For TRF/NH₃ mixture, an increase in initial pressure results in a reduction in the concentration of NO. The concentration of NO decreases with the increase of initial pressure mainly because (1) the concentration of OH radicals in the flame is abundant and (2) the concentration of OH radicals decreases with the increase of initial pressure. Additionally, this study found that a high equivalence ratio and high pressure can effectively reduce the increase of NO concentration caused by the addition of NH₃.