Spectral element modeling of plasma-assisted ignition using a phenomenological model*

Non-equilibrium plasma presents a promising tool to reduce ignition delay and extend flame stability limits. To study its influence on turbulent reacting flows, the spectral element code, Nek5000, is extended to include a phenomenological model of energy deposition into gas mixtures. This model assumes ultra-fast gas heating and dissociation via a 2-step mechanism to account for excitation and relaxation of electronically excited N₂. Extra source terms are introduced in the governing equations to incorporate this model. Nek5000 is then used to perform 2D simulations of nano-second repetitively pulsed plasma in a premixed CH₄/air mixture. First, a comparison of the effect of thermal and non-equilibrium plasma on ignition delay is performed for the same amount of deposited energy. This comparison confirms that the production of active radicals through non-equilibrium plasma greatly enhances ignition compared to thermal pathways. Second, different Reynolds numbers are tested to study the competition between the kinetic effects of plasma and the convection of heat and active radicals. It is found that stronger turbulence tends to reduce the overall reactivity during the transient ignition processes. Last, chemical explosive mode analysis is performed to demarcate ignition zones.