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(a¹δ_g), and O(¹D) are analyzed and their implications to other systems are discussed.