Effects of initial mixture temperature and pressure on laminar burning velocity and Markstein length of ammonia/air premixed laminar flames

Ammonia is attractive not only as a hydrogen energy carrier but also as a carbon-free fuel. The goal of the present work is to study the laminar burning velocities and Markstein lengths of ammonia/air under a broad range of conditions including high-temperature and high-pressure because high-temperature and high-pressure conditions are relevant in internal combustion engines. The experiments were conducted in a constant volume chamber for equivalence ratios ranging from 0.8 to 1.2, initial mixture temperatures of 400 and 500 K, and initial mixture pressures ranging from 0.1 to 0.5 MPa. The temperature and pressure exponents were experimentally obtained, and it was clarified that the temperature exponents of the ammonia/air flame were larger than those of the methane/air flame. To evaluate the temperature and pressure effects on ammonia/air flames, these effects were compared with the effects on methane/air flames. Numerical simulations using the detailed reaction mechanisms showed that the effect of the initial mixture temperature on the reaction rate of H + O₂ = O + OH was larger for ammonia/air flames than that for methane/air flames. This could be related to the difference in the effects of initial mixture temperature on the H₂ and H radical formation reactions. The results also show that the pressure exponent of the ammonia/air flame was closer to zero compared with that of the methane/air flame. This observation can be explained from the standpoint of the effects of pressure on the reaction path of ammonia flames.