A detailed experimental investigation of the mechanism of ultrafast heating and oxygen dissociation produced by nanosecond repetitively pulsed discharges in atmospheric pressure air preheated at 1000 K is presented. The ultrafast mechanism creates excited electronic states of nitrogen, which then dissociate molecular oxygen through quenching reactions, with the remaining energy dissipated as heat. Optical and electrical diagnostic techniques have been applied to provide a self-consistent set of experimental data for a reference test-case with well-defined discharge and gas conditions. The pulses have a duration of 10 ns, an amplitude of 5.7 kV, a repetition frequency of 10 kHz and the pin-to-pin electrodes are separated by 4 mm. We present measurements of the gas temperature during and after the discharge using optical emission spectroscopy of the second positive system of nitrogen, spatially resolved profiles of the absolute densities of excited electronic states, determined using Abel-inverted spectra of the first and second positive systems of nitrogen, as well as the temporal evolution of the absolute densities of N2(A), N2(B), N2(C), electrons and atomic oxygen. These measurements are synchronized with electrical measurements of pulse current, voltage, and energy. The discharge is found to dissociate about 50% of molecular oxygen and to produce a temperature increase of about 900 K within 20 ns, corresponding to an ultrafast heating rate of about 5 × 1010 K s−1. Comparisons with numerical simulations show good agreement with the measurements and validate the ultrafast mechanism. About 35% of the electric energy deposited into the gas goes into O2 dissociation, and about 21% into gas heating. Finally, the dissociative quenching rates of N2(B) and N2(C) with O2 at 2200 K were measured and found to be 2.8(±0.6) × 10−10 cm3 s−1 and 5.8(±0.9) × 10−10 cm3 s−1, respectively. Combining these measurements with the literature values at 300 K, we propose a functional temperature dependence in the range 300–2200 K of 3.0 × 10−10(T/300)0.3 cm3 s−1 for the C state, and a constant value of 3.0 × 10−10 cm3 s−1 for the B state.

A nanosecond repetitively pulsed plasma (NRPP) produced by electric pulses of 10 kV during 10 ns at a frequency of up to 30 kHz has been used to stabilize and improve the efficiency of a 25-kW lean turbulent premixed propane/air flame (Re_D=30000) at atmospheric pressure. We show that, when placed in the recirculation zone of the flow, the plasma significantly increases the heat release and the combustion efficiency, thus allowing to stabilize the flame under lean conditions where it would not exist without plasma. Stabilization is obtained with a very low level of plasma power of about 75 W, or 0.3% of the maximum power of the flame. In addition, they find that at high flow rates, where the flame should normally blow out, the NRPP allows the existence of an intermittent V-shaped flame with significant heat release, and at even higher flow rates the existence of a small dome-shaped flame confined near the electrodes that can serve as a pilot flame to reignite the combustor. Optical emission spectroscopy measurements are presented to determine the temperature of the plasma-enhanced flame, the electron number density, and to identify the active species produced by the plasma, namely O, H, and OH

Results of an experimental study of the efficiency of the ignition of propane-air mixtures by a high voltage repetitively pulsed nanosecond gas discharge (10 kV, 10 ns, 30 kHz) are presented for the pressure range 0.35-2.0 bar. The measured minimal energy for ignition is found to decrease with the pressure. A significant reduction of the ignition delay and a decrease of the overall combustion duration were obtained by using a train of high-voltage pulses. Spectroscopic measurements in a 1-bar air just after a 10-pulse train (300 mus) of about 10 mJ in total energy show the presence of N, N⁺, O, and O⁺ atomic species and a gas temperature increase up to 3000 K

In atmospheric pressure air preheated from 300 to 1000 K, the nanosecond repetitively pulsed (NRP) method has been used to generate corona, glow, and spark discharges. Experiments have been performed to determine the parameter space (applied voltage, pulse repetition frequency, ambient gas temperature, and interelectrode gap distance) of each discharge regime. In particular, the experimental conditions necessary for the glow regime of NRP discharges have been determined, with the notable result that there exists a minimum and maximum gap distance for its existence at a given ambient gas temperature. The minimum gap distance increases with decreasing gas temperature, whereas the maximum does not vary appreciably. To explain the experimental results, an analytical model is developed to explain the corona-to-glow (C-G) and glow-to-spark (G-S) transitions. The C-G transition is analyzed in terms of the avalanche-to-streamer transition and the breakdown field during the conduction phase following the establishment of a conducting channel across the discharge gap. The G-S transition is determined by the thermal ionization instability, and we show analytically that this transition occurs at a certain reduced electric field for the NRP discharges studied here. This model shows that the electrode geometry plays an important role in the existence of the NRP glow regime at a given gas temperature. We derive a criterion for the existence of the NRP glow regime as a function of the ambient gas temperature, pulse repetition frequency, electrode radius of curvature, and interelectrode gap distance.