Combustion characteristics of ammonia near-limit laminar flame

[Objective] Ammonia fuel is a promising alternative fuel. Research on the near-limit combustion characteristics of ammonia is the basis for the efficient use of ammonia fuel and provides guidance for the construction and optimization of the detailed and comprehensive chemical reaction kinetic models of ammonia. Herein, the near-limit combustion characteristics of ammonia gas were investigated experimentally and computationally. [Methods] The constant volume combustion bomb method was used to experimentally measure the laminar burning velocity (SL) of the near-limit premixed ammonia/air/dilution mixture. A high-speed camera with a frame rate of 10000 fps was employed to record images of the outwardly propagating spherical flame. MATLAB code was used to process the images and extract the flame radius. In addition, a corresponding computational study, including the prediction of the SL of the near-limit ammonia/air/dilution premix and analysis of chemical reaction kinetics, was conducted with the CHEMKIN code package. During the simulation, CURV and GRAD values were set to 0.02 and at least 500 grid points were used. [Results] The SL values and flame images of NH₃/air, 95% (NH₃/air)/5% N₂and 95% (NH₃/air)/5% Ar with an equivalence ratio (φ) range of 0.8—1.2 were obtained. A buoyancy effect on the flame of NH₃/air plus dilution gas (N₂/Ar) was observed, and N₂suppressed flame propagation more than Ar. As φ increased, SL values first increased and then decreased, and the SL peak value (maximum) was at φ=1.1. For the NH3/air premixed mixture, the experimental values were in overall good agreement with nine chemical reaction kinetic mechanisms of NH3, and the predicted values of the Han 2020 mechanism were in the best agreement with the experimental values. For NH₃/air/dilution(N₂/Ar), the predicted values of the Mei 2021 mechanism were in the best agreement. Sensitivity and reaction pathway and flux analyses were performed under different dilution conditions by using the Mei 2021 mechanism. The reaction H+O₂⇌O+OH had a strong promoting effect on S_Lwhereas the reaction NH₂+NO⇌N₂+H₂O had a strong inhibiting effect. The sensitivity coefficients, as well as the fluxes of each reaction branch, were slightly different under the two dilution conditions. By comparing adiabatic reaction temperatures, the increase in S_Lfor Ar dilution compared with that for N₂dilution was found to be mainly due to the difference in thermophysical properties between Ar and N₂[Conclusions] N₂has a stronger inhibitory effect on flame propagation than Ar, a buoyancy effect on the flame of the NH₃/air/dilution (N₂/Ar) premix exists, and a minimum SL of approximately 2.9cm/s is obtained. Numerical simulation results indicate that the experimental and simulated values are generally in good agreement. The Mei 2021 mechanism predicts the SL of NH3/air/dilution(N₂/Ar) well. A slight difference exists between the reaction pathways and sensitivity reaction coefficients for N2 and Ar dilution, and the concentrations of the active free radicals O, OH, and H significantly affect laminar burning velocities.