Thermal and chemical effects of nanosecond repetitively pulsed glow discharges applied to an ammonia–hydrogen–air flame

Abstract

This work investigates the thermal and kinetic effects of non-equilibrium plasma produced by nanosecond repetitively pulsed (NRP) glow discharges applied across an ammonia–hydrogen–air flame. The examined flame is stationary, laminar, and axis-symmetric. The discharges are applied across the symmetry axis of the flame crossing the fresh reactants, flame front, and burned products, resulting in a stable flame with reproducible discharges allowing for phase-locked averaged diagnostics. The thermal effects of the plasma are investigated by temporally resolved thermometry, based on optical emission spectroscopy of the second positive system of nitrogen. Kinetic effects are investigated by spatially and temporally resolved measurements of the imidogen radical (NH), amidogen radical (NH₂), and ammonia (NH₃) with planer laser-induced fluorescence. The temperature measurements reveal the presence of ultra-fast heating by up to 380 K within 14 ns during a discharge but only near the anode and not detectable in other locations of the inter-electrode gap. Although reactants are injected at 293 K, their temperature near the anode is stable at 480 ± 50 K. This slow heating was not caused by proximity to the flame front, but rather by the discharges themselves. An enhancement of the flame propagation speed by the plasma is shown by a 1.60-mm shift of the position of the flame closer to the nozzle, i.e., a shift of the NH, NH₂, and NH₃ fluorescence signals. Each discharge dissociates less than 10% of NH₃ upstream of the flame and produces less than $1.85\times 10^4$  ppm of NH and NH₂. NH and NH₂ upstream of the flame are initially produced during the discharges, then their intensity increases and peaks 400-800 ns after each discharge, revealing post-discharge chemistry induced by the plasma. Although the ammonia in the reactants is subjected to around 60 pulses before it reaches the flame front, its consumption by the plasma is minimal compared to the consumption of ammonia in the preheat zone of the flame. These results illustrate that NRP glow discharges induce various thermal and kinetic effects while enhancing ammonia flames.

Novelty and significance This work presents the first temperature and important chemical species measurements for NRP discharges in the glow regime applied to an ammonia–hydrogen–air flame at atmospheric conditions highlighting different thermal and kinetic interactions induced by the plasma such as ultra-fast/slow heating, and prolonged post-discharge chemistry. In addition, this work demonstrates the ability of imaging imidogen radical (NH), amidogen radical (NH₂), and ammonia (NH₃) in plasma assisted ammonia combustion experiments using a single laser.