Flame Velocity and Combustion Behaviour of Ammonia–Hydrogen Blends: A Combined Experimental and Numerical Study

Ammonia is gaining importance as a viable energy vector for decarbonising the maritime sector. However, ammonia flame speed is much slower than that of traditional fuels, and this can result in incomplete burning, decreased engine efficiency, and increased undesirable emissions such as unburned ammonia (NH₃). While blending hydrogen with ammonia has been shown to mitigate some of ammonia's combustion drawbacks, the underlying mechanisms driving these improvements are still not fully understood. This study explores these processes to inform better optimisation. A 1.2 L optically accessible constant volume combustion chamber equipped with a wall-mounted surface spark plug was used to investigate the combustion characteristics and apparent flame velocity of ammonia-hydrogen blends. Chamber pressure and temperature were measured using high-frequency piezoresistive pressure transducers and K-type thermocouples. Flame propagation was captured at 9,000 frames per second using a Photron FASTCAM NOVA S9 high-speed camera. Flame velocity was calculated through post-processed flame images using a custom MATLAB script. To complement the experiments, Computational Fluid Dynamics (CFD) simulations were conducted using CONVERGE software with the C3MechV3.5 chemical mechanism to analyse flame propagation and development. Both experimental and numerical studies were performed at pressures of 0.1, 0.3, and 0.5 MPa, with a hydrogen substitution ratio of 30% and an equivalence ratio of 1.0. The numerical results showed good agreement with the experimental data. The addition of hydrogen led to a noticeable increase in flame velocity. Cellular instabilities were observed in the flame structure, and the study found that slower-burning flames were more susceptible to buoyancy-induced distortions.