Ammonia and Hydrogen are attractive alternative fuels for a zero-carbon combustion solution that can rapidly decarbonize the transportation industry. Understanding the chemical behavior and combustion characteristics of these fuels individually, as well as blended together, is pivotal to ensuring their widespread adoption and utilization. Furthermore, in the era of computer-aided engineering, it is critical to evaluate our ability to computationally model the chemical reactivity of these two fuels and validate predictions of experimentally observed phenomena using multi-dimensional simulations. In this study, ammonia/hydrogen chemical kinetics mechanisms available from the research literature are investigated through 0-D, 1-D, and 3-D simulations. The 0-D and 1-D simulations were carried out to understand the ignition delay and laminar flame speeds, respectively, at different operating pressures and temperatures. 3-D simulations were also performed to test the fuels’ behavior in a closed volume combustion chamber. The multi-dimensional computational results were compared against optically measured experimental data available in recent publications. Specifically, a comparison of unstretched flame speeds determined from stretched flame speeds of post-processed computational results is made. Lean and rich combustion limits have been computationally evaluated as well. Lastly, observed physical buoyancy effects were reproducible in a quiescent computational environment leading to increased confidence in using the evaluated chemical kinetics mechanisms for high-fidelity reciprocating piston engine computational research and development.
Lean-burn spark ignition engines can reduce emissions, increase efficiencies, and mitigate knocking conditions. Several factors can affect the lean flammability limit of natural gas engines, including the fuel composition, temperature, pressure, and spark characteristics. It has recently been shown that spark plugs with a nanostructured central electrode, treated using pulsed laser irradiation and effectively increasing the surface area, extend the lean flammability limit (LFL) of methane/air mixtures in a constant volume combustion chamber (CVCC). In this study, the effect of varying levels of surface modifications is experimentally examined for two different power configurations of femtosecond laser. These spark plugs are tested by igniting methane/air mixtures at different equivalence ratios in a CVCC coupled with high-speed Z-type Schlieren visualization. The durability of the nanostructures on the electrode surfaces is tested by repeating the evaluations after 6,000, 66,000 and 666,000 spark events. Scanning Electron Microscope (SEM) images at different magnification rates and the root mean square (RMS) surface roughness derived from optical profilometry are used to examine the degradation of the electrode surfaces. The results point towards the existence of an optimized value of surface roughness in terms of the LFL (phi = 0.55 for 5.89 μm and phi = 0.58 for 13.68 μm). Performance degradation was particularly pronounced for electrodes with a high level of initial surface roughness (13.68 μm) whereas the electrode with a lower initial surface roughness (5.89 μm) held a superior LFL (phi = 0.57) compared to the standard spark plug (phi = 0.61) even after going through 666,000 sparks.
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