Oxygen containing plasmas produce several species that have a greater oxidation potential than molecular oxygen in its 3 Σ ground state. These species include O, O 3 and O 2 in metastable excited states, namely 1 Δ and 1 Σ. In experiments that explore the enhancement of combustion processes with plasma, it has been difficult to isolate the various enhancement mechanisms. In this study, two oxygen containing plasma-produced species, O 3 and O 2 (a 1 Δ g ), have been produced successfully in a microwave plasma, isolated in the afterglow, quantified and transported to C 3 H 8 and C 2 H 4 lifted flames. Significant kinetic enhancement by O 3 and O 2 (a 1 Δ g ) were observed for each flame by comparing flame stabilization locations with and without the plasma generated species. Atmospheric pressures were utilized to investigate the effects of O 3 and showed up to a 10% enhancement in the flame speed for 1300 ppm of O 3 addition to the O 2 /N 2 oxidizer of lifted C 3 H 8 flames. Numerical simulations showed that the O 3 decomposition early in the preheat zone of the flame produced O which rapidly reacted with C 3 H 8 to abstract an H, which led to OH production. The subsequent reaction of the OH with fuel fragments produced H 2 O and other stable species, yielding chemical heat release to enhance the flame speed. The effect of O 2 (a 1 Δ g ) was studied at low pressure (27 Torr) and was isolated by adding NO to the plasma afterglow to eliminate O 3 . For transport times on the order of one second in the presence of NO, the only remaining oxygen species were O 2 (X 3 Δ g ) and O 2 (a 1 Δ g ). Under these conditions, the enhancement of O 2 (a 1 Δ g ) could be studied in isolation, becoming an ideal source for combustion experiments. It was found that O 2 (a 1 Δ g ) was a better oxidizer than O 2 by significantly enhancing the propagation speed of C 2 H 4 flames. The present experimental results will have a direct impact on the development of elementary reaction rates with O 2 (a 1 Δ g ) and O 3 at flame conditions to establish detailed plasma-flame kinetic mechanisms.
Nomenclature
A= pre-exponential factor E = activation energy eV = electron volts P = pressure ppm = parts per million S lifted = lifted flame speed S laminar = laminar flame speed T = temperature ρ unburned = density of unburned gas ρ burned = density of unburned gas ω = reaction rate