Characterization of OH species in kHz He/H2O atmospheric pressure dielectric barrier discharges

Abstract

This work investigates experimentally and numerically the chemical kinetics of OH species generated in kHz helium atmospheric-pressure dielectric barrier discharges with different H2O concentrations. The OH densities of cases are determined from measurements of the ultraviolet absorption spectroscopy (UVAS) system with transition parameters evaluated by LIFBASE. The 1D plasma fluid model (PFM) with compiled chemistry covering chemical kinetics of helium, H2O, and interaction reactions of reactive species including the formation of charged cluster ions is employed to capture the discharge dynamics for analyzing mechanisms of OH species properly. The simulated current densities of cases with 0.8% and 2.0% H2O concentrations increase from 29 to 51 A m−2, agreeing with experimental measurements. The simulated OH density increases from 1.7 × 1019 m−3 to 2.4 × 1019 m−3 as the H2O concentration increases from 0.4% to 2.0%, agreeing with OH densities interpreted from the UVAS system for cases with various H2O concentrations. In general, the 1D PFM developed captures the discharge behavior and predicts the OH densities of cases with different H2O concentrations. The simulated results reveal that the electron and H2O+ are essential species for OH production through dissociative reactions (e + H2O → e + OH + H) and (H2O+ + H2O → H+(H2O) + OH) contributing to 61% and 23% of OH production, respectively. It is observed that the OH self-recombination reaction (OH + OH + He → H2O2 + He) contributes to 32% of OH consumption as the dominant reaction. Moreover, the stepwise recombination reactions (OH + H2O2 → H2O + HO2 and OH + HO2 → O2 + H2O) initiated by OH species contribute to the overall 27% of OH consumption as another dominant mechanism. The Penning ionization reactions involving water molecules are the dominant reactions for electron production in the present discharge with H2O up to 0.8%, then the electron-impact ionization becomes the dominant reaction for electron production in cases with higher H2O concentrations. As the essential species contributing to the OH consumption, H species is produced majorly from the dissociation reaction (e + H2O → e + OH + H). As one of the dominant OH production reactions, the dissociative attachment (e + H2O → H− + OH) is the dominant electron consumption reaction, resulting in the reactive H− species as the source of negative cluster ions. The high average reaction rates of dissociative attachment reactions consume most of the electrons after the breakdown, which leads to the variation of discharge current density. The saturation of OH species, which is observed experimentally and numerically, is attributed to the transition of electron chemical kinetics and the recombination reactions of OH species.