Plasma-assisted methane combustion in swirled-flow reactors: Experiments and multiphysics simulations

Recent experimental investigations of methane plasma-assisted combustion (PAC) using coaxial microwave reactors have offered an opportunity to validate multiphysics simulation techniques. The experimental work investigates the coupling of microwave power into air:CH4 mixtures using a coaxial torch along with swirl-stabilization provided by tangential air jets. The PAC assembly is enclosed in a quartz channel, allowing optical access to the flame for various techniques such as planar laser-induced fluorescence (PLIF) to determine two-dimensional density distributions of radicals (e.g. OH, CH, NO), Rayleigh scattering thermometry (RST) for temperature profiles, and particle image velocimetry (PIV) to characterize the flame flow-field. The degree of plasma non-equilibrium can be determined via probe measurements and spectroscopic emission measurements. In addition, the acoustic modes resulting from combustion processes within the enclosure can be evaluated using a high-temperature probe-type microphone. Introduction of plasma power deposition to the combustion process has the impact of rapid decomposition of flow constituents through electron impact mechanisms and interactions with plasma-excited species, as well as Ohmic heating which can increase reaction rates and influence transport. To capture these important dynamics in modeling efforts, a robust multiphysics approach is being developed, one that allows coupled simulations which merge fluid dynamics, turbulence modeling, electric field coupling, and transport models for neutrals and charged species, while also incorporating detailed reaction mechanisms for both combustion and air-plasma. The current work has identified swirl-stabilized and PAC cases to be used for validation of simulations. Various characterizations of the PAC apparatus are discussed, including OH PLIF, PIV, and acoustic measurements. Cold flow simulations are provided, and next steps in development of the PAC model are overviewed.