Phenomenological modeling of plasma-assisted methane ignition using plasma energy fraction manifolds

The present work advances a phenomenological plasma-assisted combustion model by implementing spatiotemporally varying plasma energy fractions for fast gas heating, vibrational energy excitation, and fast chemical dissociation of gas species. This is achieved through a 2D manifold of plasma energy fractions generated for varying reduced electric fields (E/N) and the thermochemical state of the gas mixture, represented by a progress variable, forming a 2D manifold of energy fractions. The manifold is generated using 0D calculations with a detailed chemical mechanism for plasma-assisted combustion of methane at E/N ranging from 100 to 500 Td and progress variable ranging from 0 (fresh gas mixture) to 1 (burned gas mixture), and is integrated into the phenomenological model. Additionally, a spatiotemporally evolving plasma power density (PPD) obtained from 2D detailed plasma simulations for an air discharge is accommodated in the phenomenological model to describe the evolution of the plasma streamer during discharge. This enhanced model is validated against experiments representing plasma discharge and post-discharge ignition kernel evolution. Specifically, the present model is compared against experimental ultra-fast gas heating and O2 dissociation during plasma discharge, in addition to experimental pressure wave and heated channel radius. The present model is then utilized to investigate ignition kernel evolution for a methane-air discharge across a pin-pin discharge configuration. Specifically, the plasma evolution and the evolution of the post-discharge ignition kernel for a stoichiometric methane-air plasma discharge that utilizes the accommodated spatiotemporal PPD are compared against the results obtained using the original phenomenological model.