Flame propagation enhancement by plasma excitation of oxygen. Part I: Effects of O3

The thermal and kinetic effects of O3 on flame propagation were investigated experimentally and numerically by using C3H8/O2/N2 laminar lifted flames. Ozone produced by a dielectric barrier plasma discharge was isolated and measured quantitatively by using absorption spectroscopy. Significant kinetic enhancement by O3 was observed by comparing flame stabilization locations with and without O3 production. Experiments at atmospheric pressures showed an 8% enhancement in the flame propagation speed for 1260 ppm of O3 addition to the O2/N2 oxidizer. Numerical simulations showed that the O3 decomposition and reaction with H early in the pre-heat zone of the flame produced O and OH, respectively, from which the O reacted rapidly with C3H8 and produced additional OH. The subsequent reaction of OH with the fuel and fuel fragments, such as CH2O, provided chemical heat release at lower temperatures to enhance the flame propagation speed. It was shown that the kinetic effect on flame propagation enhancement by O3 reaching the pre-heat zone of the flame for early oxidation of fuel was much greater than that by the thermal effect from the energy contained within O3. For non-premixed laminar lifted flames, the kinetic enhancement by O3 also induced changes to the hydrodynamics at the flame front which provided additional enhancement of the flame propagation speed. The present results will have a direct impact on the development of detailed plasma-flame kinetic mechanisms and provided a foundation for the study of combustion enhancement by O2(a1Δg) in part II of this investigation.