Impact of Noble Gas Working Fluids on Laminar Burning Velocity and Intrinsic Instability of Premixed Methane Flames

Abstract Hydrodynamic and diffusional-thermal instabilities affect flame dynamics, resulting in flame fronts with self-accelerating cellularities and wrinkles. In premixed flames, the primary driving mechanism for perturbations is hydrodynamic instability, which is associated with thermal expansion and can affect flame behavior and morphology. This study focuses on the replacement of nitrogen with a noble gas (argon or krypton) as the working fluid in the premixed combustion of methane to investigate its effect on flame stability and dynamics. From a practical perspective, the utilization of noble gases as working fluids in thermodynamic cycles can markedly reduce NOx emissions and enhance the ideal thermal efficiency. The constant volume combustion chamber (CVCC) experiments are conducted at several equivalence ratios (Φ=0.8, 1.0, 1.1, 1.2, and 1.3), at atmospheric and elevated initial pressures, and at a constant initial temperature. The spherically propagating flames are analyzed in terms of laminar flame burning velocity, cellular instability, unburned gas Markstein length, and flame morphology through high-speed Z-type schlieren visualization, in situ pressure diagnostics, and image analysis. The laminar burning velocity increases when argon and krypton are used instead of nitrogen as the working fluid, and the increase with argon is higher compared to krypton. Spherical flames with krypton in the mixture are the most prone to wrinkling due to intrinsic instabilities, followed by argon, with nitrogen mixtures not showing significant wrinkling.