Reducing the plasma-induced thermal NOX production in lean flames assisted by NRP discharges

This work studies the toroidally shaped recirculation cells induced by Nanosecond Repetitively Pulsed (NRP) discharges in lean premixed methane-air flames and their effect on the production of NO X in plasma-assisted methane-air flames. Using OH planar laser-induced fluorescence, the recirculation cell structures are observed for the first time in a plasma-assisted combustion experiment. The gas temperature between the electrodes inside the flame is measured by optical emission spectroscopy of the N 2 C 3 Π u -B 3 Π g transition. At fixed pulse energy and repetition frequency, reducing the inter-electrode gap enhances post-discharge recirculation. In conditions without recirculation cells, the gas temperature reaches about 3500 K at the beginning of each pulse in the steady-state pulsing regime. With recirculation cells (typically when the deposited lineic energy is above 0.6 mJ/mm), the gas temperature remains around 2300 K, close to the adiabatic flame temperature (2000 K at ϕ = 0.8). This lower temperature is beneficial to reduce thermal NO formation in plasma-assisted flames. The measured NO X density increases linearly with the plasma power up to a certain threshold, and then continues increasing at a lower rate. In contrast with prior studies that showed a linear dependence of NO X on plasma power, we report a novel two-slope behaviour of NO X evolution with plasma power. The break in slope coincides with a transition from homogeneous nonequilibrium discharges to discharges with bright spots near the electrode tips. Spectroscopic studies show that these bright spots are thermal sparks, reaching the high temperatures required to trigger the formation of toroidal recirculation cells via flow dynamic effects. Exploiting NRP-induced post-discharge recirculation cells is thus an effective strategy to reduce NO X penalty for a given plasma power.