Atomic-Oxygen Number Densities in Ar-O2 DBDs and Post-discharges with Small Initial O2 Fractions: Plug-Flow Model and Experiments

Number densities of oxygen atoms, n(O), in Ar-O-2 mixtures with small initial O-2 fractions, x(O2). < 1%, flowing through a dielectric-barrier discharge (DBD), are calculated using a plug-flow reactor model, presuming that dissociation and excitation of oxygen species are solely driven by energy-transfer from long-lived excited Ar species, collectively denoted as Ar*. The rate by which Ar* species are generated is calculated from the volume density of power dissipated in the DBD. To obtain extended post-discharge (PD) regions with large n(O), experiments were performed with x(O2)( )=100 ppm. For such low O-2 fractions, the time-dependence of n(O) in the DBD and the early PD can be calculated by a closed equation. Calculations are compared with optical emission spectroscopic (OES) results, utilizing the proportionality of O-atom emission intensity at 777.4 nm to n(O). O-atom densities in the PD are made accessible to OES using a tandem setup with a second DBD as sensing discharge. Model testing by experiment is based on the functional dependence of n(O) on DBD-residence time and PD-delay time, respectively. Wall losses of O atoms in asymmetrical DBD reactors are calculated by an alternative to Chantry's equation. The agreement between O-atom densities attained at the DBD exit and experimental results is generally good while the speed of rise of n(O) in the discharge is overestimated, due to the assumption of a constant wall-loss frequency, k(W) Compared with literature data, k(W) is orders of magnitude higher in the DBD and at least one order of magnitude lower in the PD.

Automated summary

The number densities of oxygen atoms in Ar-O-2 mixtures with small initial O-2 fractions flowing through a dielectric-barrier discharge were calculated using a plug-flow reactor model. The calculations were compared with experimental results and optical emission spectroscopic measurements.