Application of emission spectroscopy in plasma-assisted NH3/air combustion using nanosecond pulsed discharge

Abstract

This work investigates the effects of atmospheric-pressure nanosecond (ns) pulsed plasma discharges on NH3/air flames using optical emission spectroscopy (OES). Firstly, spatially-resolved OES in the 200–800 nm range of NH3/air flames with and without discharges are presented to identify the plasma-induced excited species (such as N2* and H*). Then, time-resolved OES with ns timescale resolution are achieved to illustrate the discharge dynamics using a statistical imaging strategy, which is specifically designed to overcome the timing jitter of the discharge. From the time-resolved spectra of excited N2, the first and second emission events in NH3/air flames with a time interval of 80 ns are revealed, and each exhibits a profile with two exponential decays. Furthermore, spatial- and time-resolved rotational and vibrational temperatures are derived from the spectral fitting of N2(C→B) to quantify the thermal effects of ns plasmas, thus revealing the ultrafast and slow gas heating processes. In addition, the electron number density along the flame is obtained from the intense Hα line, which locally reaches up to 1 × 1017 cm−3. Besides, the spatial-resolved Hα line intensity suggests that the flame chemistry does not directly affect, but the discharge dominates the production of the atomic hydrogen. It provides insight into the kinetic effects of ns pulsed discharge on NH3 combustion enhancement.