Gas heating by nanosecond repetitively pulsed glow discharges applied to a methane–air flame

In this work, the thermal effect induced by non-equilibrium plasma produced by nanosecond repetitively pulsed (NRP) glow discharges applied across a lean premixed methane–air flame is investigated. The flame is laminar, stationary, and axis-symmetric. The discharges are applied on the symmetry axis of the flame, crossing the fresh reactants, flame front, and burned gases. The obtained plasma-assisted flame is stable and reproducible, allowing phase-locked averaged diagnostics. The thermal effect is investigated by acquiring spatially and temporally resolved temperature measurements in the plasma discharges using optical emission spectroscopy (OES) of the second positive system of nitrogen. The results show that for an applied voltage of 10 kV, NRP glow discharges increased the gas temperature by up to 130 K within 10 ns. However, this heating was location-dependent, i.e., high near the anode tip and not detectable 0.5 mm above the anode. It was also shown that reducing the applied voltage down to 9 kV caused this ultra-fast heating to disappear. On the other hand, the reactants near the anode tip had a constant temperature of 480 K, between discharges, while the reactants were injected at 293 K. The reason for this slow heating could be due to (1) heating by the flame which is pulled down to the anode when NRP discharges are applied, (2) slow gas heating induced by relaxation of vibrational excited states of nitrogen, or (3) post-discharge oxidation chemistry. Based on temperature measurements performed on NRP corona discharges, heating by the flame could be ruled out. Zero-dimensional numerical simulations’ results suggest that post-discharge oxidation chemistry can heat the gas by 110–130 K, indicating that this is an important heating mechanism. Further investigation will be necessary to assess the significance of the vibrational–translational relaxation of nitrogen.