Experimental Investigation of Effects of Electric Operating Parameters on Pulsed Corona Discharges in Humid Air at Atmospheric Pressure

The present work is devoted to electrical and optical study of a point-plane atmospheric pressure corona discharge reactor in humid air powered by pulsed high voltage supply. The corona current and the injected energy are analyzed as a function of several parameters such as applied voltage and humidity rate. Then, investigations based on emission spectroscopy analysis were used in UV range (from 200 nm to about 400 nm). The main observed excited species were the second positive (SPS), the first negative (FNS) systems and OH(A-X) rotational bands. The latter band was used to simulate the rotational temperature (Tr), whereas the N2+ (FNS) band was used to determine the vibrational temperature (Tv). The electron temperature (Te ) is determined from the ratio of line intensities of the spectral bands of both N2+ FNS at 391.4 nm and N2SPS at 394.4 nm. The rotational, vibrational and electronic temperatures are analyzed as a function of above parameters (applied voltage, frequency and hygrometry rate) near the anodic tip. As well we study the axial variation of electronic temperature for a fixed applied voltage at 6.4 kV, frequency at 10 kHz and 100% of humidity. It is found that the rotational, vibrational and electronic temperatures increased with increasing applied voltage, frequency and humidity rate. The increase of rate hygrometry for an inter-electrode distance fixed at 10 mm causes an increase in both the amplitude of the corona current discharge and the energy injected in corona discharge. This is indicative of more intense reactive plasma while increasing hygrometry rate.

Measurements of Absolute Atomic Nitrogen Density by Two-Photon Absorption Laser-Induced Fluorescence Spectroscopy in Hot Air Plasma Generated by Microwave Resonant Cavity

For the first time, absolute densities of atomic nitrogen in its ground state (N4S) have been measured in hot dry and humid air plasma columns under post-discharge regime. The determination of space-resolved absolute densities leads to obtain the dissociation degrees of molecular nitrogen in the plasma. The hot plasmas are generated inside an upstream gas-conditioning cell at 600 mbar when the air gas flow is directly injected at 10 slm in a microwave resonant cavity (2.45 GHz, 1 kW) placed in the downstream side. Density measurements based on laser induced fluorescence spectroscopy with two-photon excitation (TALIF), are more particularly performed along the radial and axial positions of the plasma column. Calibration of TALIF signals is performed in situ (i.e. in the same gas-conditioning cell but without plasma) using an air gas mixture containing krypton. Optical emission spectroscopy is considered to estimate the rotational gas temperature by adding a small amount of H2 in dry air to better detect OH (A-X) spectra. The rotational temperatures in humid air plasma column (50% of humidity) are larger than those of dry air plasma column by practically 30% near the nozzle of resonant cavity on the axis of the plasma column. This is partly due to attachment heating processes initiated by water vapor. A maximum of the measured absolute nitrogen density is also observed near the nozzle which is also larger for humid air plasma column. The obtained dissociation degrees of molecular nitrogen in both dry and humid air plasma along the air plasma column are lower than the cases where only thermodynamic equilibrium is assumed. This is characteristic of the absence of chemical and energetic equilibria not yet reached in the air plasma column dominated by recombination processes.

Electric and Spectroscopic Studies of Pulsed Corona Discharges in Nitrogen at Atmospheric Pressure

This paper is mainly dedicated to the experimental electric and spectroscopic analysis of positive corona discharges in the case of point to plane configuration, generated in nitrogen at atmospheric pressure. The maximum corona current (a few hundreds of mA), the average current (a few μA) and the average propagation velocity (a few 107 cm/s) are analyzed with the variation of the applied voltage (a few kV) and the gap distance (not exceeding 16 mm). By using an ICCD camera, the dynamics of the discharge during the propagation of primary and secondary streamers across the gap distance was analyzed. Spectroscopic study is emphasized in a spectral range from 200 nm up to 500 nm, to determine the important excited species present in the gaseous environment such as the second positive and the first negative systems (SPS and FNS respectively). The identification of the quenching NOγ emission bands is also emphasized.