Combustion and Flame Propagation Characteristics of Decomposed Ammonia Fuel Mixtures in a Rapid Compression Machine

Abstract

Rising fuel demands and the need to address climate change have created an increasing need for carbon-free energy sources. Ammonia (NH3) presents a promising alternative fuel solution, as it is readily available, functions as a dense hydrogen carrier, and has an existing global production/transportation network. However, neat ammonia exhibits unfavorable combustion characteristics, including slow flame speeds and long ignition delay times. The decomposition (or cracking) of ammonia, a process that breaks ammonia into its components, hydrogen (H2) and nitrogen (N2) is known to improve the combustion characteristics, enhancing laminar flame speed, turbulent flame speed, and its utilization in both experimental devices and engines. In this study, the first of their kind combustion and flame propagation characteristics of decomposed ammonia fuel blends were measured using a laser-ignited dual-piston rapid compression machine at engine-relevant condition. Experimental measurements of decomposed ammonia (0%, 10%, 20%, and 42.5%, Φ=1) flame speeds were conducted across a range of elevated pressure/temperature conditions (20-30 bar, 760-850 K). Stoichiometric methane (CH4) was also investigated as a comparative baseline to the 42.5% decomposition case provided by preliminary flame speed calculations. Stoichiometric fuel blends were ignited using a 1064 nm Nd:YAG laser producing spherically expanding flames. Complementing these experiments, 1-D and 3-D simulations were performed to evaluate eight current chemical kinetic mechanism against the collected dataset. Measured flame speeds revealed significant discrepancies in chemical kinetic mechanisms. High-speed schlieren imaging revealed the presence of hydrogen-inducted cellular instabilities at higher decomposition percentages and pressures. 3-D computational-fluid-dynamics (CFD) simulations of the RCM revealed that current mechanisms and numerical models struggle to model combustion phasing and transport. Furthermore, diesel-ammonia dual-fuel ignition delay times were measured presenting a balance needed between the diffusion of hydrogen and the available oxidizer. These results highlight the need for the development of mechanisms and simulation approaches at elevated/engine-relevant conditions for decomposed ammonia blends. This work provides a crucial dataset and a foundation for further high temperature/pressure flame speed measurements, accelerating the development of energy conversion devices capable of leveraging reformed ammonia mixtures as a carbon-free fuel.