Kinetic modeling and sensitivity analysis of plasma-assisted oxidation in a H2/O2/Ar mixture

A numerical scheme describing the oxidation process induced by single and multiple dielectric barrier discharges in a H2/O2/Ar mixture is developed. Path flux and sensitivity analyses are performed in order to identify important reaction paths. The analyses are conducted for an initial mixture of 2000 ppm H2 and 3000 ppm O2 balanced in Ar at a constant pressure of 1 atm and temperatures ranging from 474 K to 1008 K, covering both below and above the second explosion limit. Results with only one discharge as well as with repetitive discharges at rates between 1 and 5 kHz are discussed. Three sets of reaction schemes leading to H2O formation are identified: two short-time-scale schemes involving negative ions and O(1D), respectively, and a long-time-scale scheme involving ground state radicals, which is responsible for the majority of the H2O formation and driven by plasma dissociation of H2 and O2. At temperatures below the second explosion limit, the last scheme drives the straight chain propagation mechanism without causing exponential growth of radicals, where the reactions of HO2 play important roles on the overall fuel oxidation. At temperatures above the second explosion limit, H and O atoms produced from the dissociation of reactants by deactivation of excited argon subsequently trigger ignition significantly reducing the ignition delay. The consumption of H2 and O2 which follows is governed by conventional high temperature chain branching kinetics and occurs at a rate equivalent to that of the purely thermal reaction. Increasing the pulse repetition rate of the discharge at low temperatures results in a shift of the rate-limiting steps from reactions of H atoms to those of the OH radical. Furthermore, the kinetics of metastable Ar*m, HO2, O2(a1Δg), and O(1D) are analyzed and their implications to other systems are discussed.