A phenomenological model for the impact of nanosecond repetitively pulsed discharges on a laminar methane-air flame*

Nanosecond repetitively pulsed (NRP) discharges are a promising technique to enhance combustion efficiency and control. Numerical studies are essential to improve the understanding of complex plasma-combustion interaction. Limited by the prohibitive computational cost of fully coupled detailed plasma mechanism and combustion chemistry, a phenomenological model taken from literature is further developed to study the behavior of a laminar methane-air flame under NRP discharges. The phenomenological model focuses on two channels through which the electric energy is deposited: 1) the ultrafast heating and ultrafast dissociation of O₂ coming from the relaxation of electronically excited N₂; and 2) the slow gas heating coming from the relaxation of vibrational states of N₂. The energy fraction deposited to these two channels is governed by the reduced electric field (E/N) which cannot be accurately predicted without resolving ion transport. Electric field is instead determined by solving the static poisson equation between two pin electrodes with three tested geometries. The predicted flame displacement under plasma qualitatively matches the experimental result. The roles played by chemical and thermal effects are strongly dependent on the E/N profile. Higher prediction of E/N magnitude at the preheating zone results in stronger dissociation effect and modifies the flame morphology more than what a lower E/N prediction concludes.