Numerical modeling of H2–O2 flames involving electronically-excited species O2(a1Δg)O2(a1Δg),O(1D)O(1D) and OH(2Σ+)OH(2Σ+)

Abstract The promoting action of electrical discharges on combustion processes results in reduction of the ignition delay, improvement of flame stability as well as extension of the flammability limits. These features are key technical issues for combustion improvement. One particularly promising approach consists of plasma-enhanced activation of the oxidizing substance, such as when transforming molecular oxygen into its electronically excited singlet delta O 2 ( a 1 Δ g ) and singlet sigma O 2 ( b 1 Σ g + ) states. In contrast with non-excited reactants, singlet oxygen molecules display a higher chemical activity and can affect reaction kinetics due to a decrease of the energy barrier associated with endoenergetic reactions. Previous publications have presented results of recent feasibility experiments on singlet oxygen generation in non-equilibrium electric discharges, from reduced pressures up to atmospheric pressure and for different discharge powers, using a high voltage pulse generator. It was possible to produce O 2 ( b 1 Σ g + ) and O 2 ( a 1 Δ g ) molecules and to detect them by optical emission spectroscopy. As a complement, the present study is devoted to the numerical study of ignition and combustion in subsonic flows of diluted hydrogen–air mixtures including electronically-excited O 2 molecules, as they appear in the experiments. All computations rely on a detailed reaction scheme implemented within the package CHEMKIN. The reaction mechanism involves the excited species O 2 ∗ ( = O 2 ( a 1 Δ g ) ) , O ∗ ( = O ( 1 D ) ) and OH ∗ ( = OH ( 2 Σ + ) ) . Results show that in the presence of excited oxygen in the initial mixture, a reduction of the ignition delay and of the minimum temperature for inflammation is observed, together with an increase of the laminar flame speed, the thermal flame thickness and of the maximum concentration of all main radicals. The extinction strain rate increases with oxygen excitation.