Ammonia/hydrogen laminar flame speed measurements at elevated temperatures

Laminar flame speeds of ammonia/hydrogen blends diluted in a mixture of 79% Ar and 21% O2 (referred to as “airgon”) were measured for the first time at high temperatures exceeding 500 K and atmospheric pressure using the shock tube flame speed method. Lean (ϕ = 0.8), stoichiometric (ϕ = 1) and rich (ϕ = 1.2) mixtures, with hydrogen fuel fractions xH2 = 0.2, 0.4, and 0.6 were studied in the temperature range of 500–820 K behind reflected shock waves. A significant enhancement in flame speeds and improved flame morphology was observed with increasing concentrations of hydrogen. Equivalent fuel/air data were computed from fuel/airgon measurements using a mixture-scaling approach and compared against available literature data. Although current data do not overlap with the temperatures of data in the literature, the full 300–800 temperature range of experimental data remain consistent with a non-Arrhenius correlation, while a power-law fit does not capture the full data trend. 1-D flame speed simulations were conducted using five recent chemical kinetic models of ammonia combustion available in the literature. Large disagreement was found among the models, with the model by Zhou et al. agreeing well with experiments at all conditions studied in this work. Sensitivity analyses were performed, and the most relevant reactions involved in the computation of flame speed, according to the models, were identified. Overall, the experimental data and modeling demonstrate the value of using blends of ammonia and hydrogen to improve the combustion characteristics of ammonia as a carbon-free fuel.