Experimental and numerical study of nanosecond pulsed water-containing discharges

Abstract

Water-containing discharges have been extensively studied for their potential in environmental remediation and biomedical applications. Highly reactive OH radicals are abundantly produced in the presence of water discharges enabling the oxidation and dissociation of harmful compounds and pollutants or the deactivation of bacteria and viruses. Although water-containing discharges can be highly effective for these applications, the energy efficiency is often a bottleneck, particularly for direct liquid discharges, for the wide industry-level applications of the water-containing discharges for environmental remediation. The goal of this study is to understand the OH production and destruction mechanism in the water-containing discharges within the framework of improving the energy efficiency of the water-containing discharges, especially for discharges directed generated in the liquid water. The OH production and destruction mechanism in the low electron density (ne ~ 10^18 m^-3) gas phase discharges have been extensively studied. The electron-induced water dissociation is responsible for OH production and three-body recombination of OH with OH and H are the major loss mechanisms. However, the OH production and destruction mechanisms in the high electron density (> 10^22 m^-3) gas phase discharges is not well understood, let alone the OH production and destruction mechanisms in liquid water in which electron density up to 10^25 m^-3 has been observed. These higher electron density discharges are important for many applications as most applications involve filamentary discharges with often high specific energies and electron densities. A plasma jet with gas shielding has been implemented to achieve a stable plasma filament with a high electron density in argon with added water vapor. For this study, the water concentration was kept within 1%. The discharge was stabilized in time and space using a pin-to-plate geometry. The time resolved OH density was measured using laser induced fluorescence (LIF). The measured OH radical densities together with previously measured H densities by two-photon laser induced fluorescence (TaLIF) were used to validate a reaction set with a global 0-D plasma kinetics model. The model indicates that at high energy density the increase of atomic H and O densities lead to rapid radical recombination mainly producing H2, O2 and H2O. A higher selectivity and energy efficiency for H2O2 production occurs at lower discharge energies for concentrations of water ranging from 0.1% to 100%. Total H and O radical production increase with increasing power (or energy) deposition regardless of conditions until H2O is depleted, nonetheless for OH radical there is a transition point observed at higher power densities. When the power density exceeds the corresponding power at the transition point, the H and O radical can be significantly larger than the OH density. This is due to the large electron density leading to electron-induced dissociation of OH during the voltage pulse. The radial distribution of the OH fluorescence intensity in and near the plasma filament was also obtained by LIF measurement. A local minimum of OH fluorescence intensity at the position of the plasma filament was observed. This is ascribed to high O and H density leading to a much shorter lifetime of the OH radical in the filament compared to the surrounding of the filament. The H and O, present at significantly higher densities than the OH densities, diffuse out from the discharge core during the afterglow and produce OH in the immediate surrounding of the discharge filament. The reaction set was further extended to enable its use for nanosecond pulsed discharges in pure water vapor by adapting reaction rates for high temperature and including thermal dissociation reactions. Fast gas heating leads to significant pressure build up within tens of nanosecond and a two-stage simulation was used to calculate the kinetics during pressure build up and the kinetics after pressure relaxation due to shock wave relaxation. The pure water vapor discharges kinetics was compared with previously reported measured electron density, gas temperatures and H2 production. In the case of high electron density pure water vapor kinetics, the H and O density even exceed the electron density during the discharge. As a result, OH radical is produced during the discharge by H and O reaction in good approximation balancing the electron-induced dissociation of the OH radicals leading to similar densities of H, O and OH. The OH density is not so significantly consumed during the high electron density pure water vapor discharges as in the high electron density Ar discharges with small water vapor concentrations. Although a key interest is understanding the kinetics of direct liquid phase discharges, the small length scales, often random and irreproducible formation of discharge filaments and changes in refractive index make laser diagnostics extremely challenging. To enable future advanced optical diagnostics for discharge in bubbles and direct liquid phase discharges in water we developed two experimental setups. We developed a bubble generation system synchronized with multi-pulse discharge capabilities and performed imaging and electrical measurements. Surprisingly, due to the surface discharge accumulation at the quartz tube wall a memory effect is observed which leads to a reduction of the discharge intensity with increasing discharge repetition rate. We have also developed a reactor enabling the formation of controlled nanosecond pulse discharge in liquid water. A major challenge for this setup is to measure the power deposition accurately since an accurate power waveform is a crucial input parameter to plasma kinetics models. We developed and manufactured an embedded power measurement system, including a voltage and current sensor. The home-made sensors can achieve a nanosecond resolution measurement and provide a clear waveform till tens of microsecond. Even the nanosecond voltage is usually applied in the generation of the liquid water discharge, the oscillation and reflection can last for several microsecond which produces re-ignitions of the plasma. The developed bubble and direct liquid water discharge reactor with preliminary characterization will allow for the development of more accurate and detailed measurements in the future which will without doubt enhance further our understanding of these discharges.