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

Abstract

Abstract Number densities of oxygen atoms, n O , in Ar-O 2 mixtures with small initial O 2 fractions, $${x}_{{O}_{2}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>x</mml:mi> <mml:msub> <mml:mi>O</mml:mi> <mml:mn>2</mml:mn> </mml:msub> </mml:msub> </mml:math> &lt; 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 $${x}_{{O}_{2}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>x</mml:mi> <mml:msub> <mml:mi>O</mml:mi> <mml:mn>2</mml:mn> </mml:msub> </mml:msub> </mml:math> = 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.