Ammonia as a fuel: Optical investigation of turbulent flame propagation of NH3/Air and NH3/H2/N2/Air flames at engine conditions

Ammonia is a promising future energy carrier because of its carbon-free nature and high volumetric energy density compared to hydrogen. However, implementing ammonia as a fuel appears challenging due to its low reactivity. This can be improved, inter alia, by cofiring with a highly reactive fuel like hydrogen. A fuel mixture of ammonia, hydrogen, and nitrogen with favorable thermochemical properties can be produced by partially cracking ammonia. To assess the combustion behavior of ammonia and partially cracked ammonia at engine conditions, this study performs experiments on an optical engine test rig. Ammonia cracking ratios of 0, 7.5, and 10%, fuel-air equivalence ratios of 0.7 to 1.2, and different turbulence conditions at variable engine speeds are investigated at a compression pressure of 7 MPa. A turbulent flame speed approach is determined from high-speed schlieren imaging in the combustion chamber. The corresponding laminar flame properties and effective Lewis number are calculated numerically and the combustion regimes are assessed. The results show that ammonia/air flames propagate significantly faster under turbulent, engine-like conditions than expected from results at laminar, ambient conditions. Additionally, the partial cracking of ammonia further improves the turbulent combustion behavior. With lean fuel/air mixtures, a cracking ratio of 10% is sufficient to achieve flame speeds close to that of methane under highly turbulent flow conditions. The observed stronger influence of turbulence on the flame speed of ammonia and partially cracked ammonia compared to methane is due to the lower effective Lewis numbers and higher Karlovitz numbers of these fuels.