A comprehensive study on dynamics of flames in a nanosecond pulsed discharge. Part I: Discharge formation and gas heating

Abstract

Nanosecond pulsed discharges (NPD) have been extensively used in plasma-assisted combustion to stimulate combustion kinetics. Experimental measurement of the energy transfer processes in non-equilibrium plasma-assisted processes is extremely difficult, as the non-equilibrium plasma discharge involves numerous different species with transient and complex three-dimensional structures across various time scales. This paper is Part I of a systematic study of the dynamics of flat flames in a pin-to-pin NPD (4 ns FWHM, 30–50 kV, 1–5 Hz) at atmospheric pressure. For a comprehensive study of one single discharge, the plasma source is running at low frequencies to avoid pulse-to-pulse interactions. The plasma/flame interactions are accessed using laser-based diagnostics, combined with current/voltage measurements, optical emission spectroscopy, and high-speed videography. Particularly, Rayleigh scattering with Structured Laser Illumination Planar Imaging (SLIPI-RS) is applied with a spatial lock-in algorithm to minimize the interference from plasma emission and stray light problem. The current paper (Part I) details SLIPI-RS measurements and focuses on the discharge dynamics and gas temperature in a lean CH₄/air flame within the first 500 μs after the discharge stimulation. For a methane/air flame, a luminous and hot discharge channel was observed between the two electrodes with a shockwave on its edge. The plasma emission is dominated by the second positive band of nitrogen emission (C-B) and dies out within tens of nanoseconds, while the hot channel expands outwards to its maximum at 5 μs, when the shockwave is also observed to detach from the hot channel. Two-dimensional gas temperature map of the flame is calculated using SLIPI-RS until 500 μs after the discharge stimulation when discharge-induced turbulence starts dominating, while gas heating by shockwave is also analyzed using classical Rankine-Hugoniot relation. Temperatures acquired by both methods indicate that much more energy is deposited in the unburnt region of the flame. The dynamics from microseconds to milliseconds, with an emphasis on plasma effects on combustion and ignition enhancement, will be presented in Part II, for both CH₄/air flames and NH₃/air flames.