Influence of Nanosecond Repetitevely Pulsed Plasma Discharges on the Stability Limits of Premixed Methane Swirl Flames at Elevated Pressures

Abstract

High-pressure premixed lean combustion is widely used in gas turbine engines to reduce pollutant emissions. One of the main challenges associated with this technology is the flame stabilization under lean conditions. Non-thermal plasma discharges have been extensively investigated as a way of stabilizing premixed flames in extremely-lean conditions at atmospheric pressure1. However, there is still a lack of understanding about the ability of this type of discharges to stabilize swirl flames at elevated pressures. This study investigates the influence of nanosecond repetitively pulsed (NRP) plasma discharges on the stability limits of premixed methane-air swirl flames at pressures up to 5 bar. The effects of two discharge regimes, NRP glows and NRP sparks, were analyzed. The voltage and the current of the discharge were measured and the influence of the discharge on the flame stabilization was assessed through direct images of the flames at 60 Hz. Carbon oxide (CO) emissions were also measured with a flue gas analyzer, with and without plasma discharges. Results demonstrated that NRP discharges improved stabilization of premixed swirl flames at elevated pressures even if the ratio of NRP discharge power to flame thermal power equal was 0.7% or less. It was also observed that the peak voltage required to maintain this very low power ratio did not increase linearly with increasing pressure. This was contrary to expectations because increasing pressure should result in a linearly decreasing reduced electric field. Moreover, the relative effectiveness of NRP glow and the NRP spark discharges to extend the flame stability limits changed by increasing pressure. Furthermore, the CO concentration in flue gases was reduced when NRP discharges were used to stabilize the flames. Improved flame stability and reduced CO emissions demonstrated the strong potential of utilizing NRP discharges in gas turbines.