Understanding the coupling between nanosecond repetitively pulsed discharges and the thermoacoustic behavior of a swirl flame at 2 bar

Abstract

This study experimentally investigates the coupling between nanosecond repetitively pulsed (NRP) discharges and the thermoacoustic behavior of a methane-air swirl flame at 2 bar for different power ratios (ratio of plasma power to flame thermal power), at pulse repetition frequencies of 10 kHz, 30 kHz, and 70 kHz, and applied voltages in the range 7 to 14 kV. The NRP discharges are created in the annular gap between a bluff body and the metal ring located at the tip of the injection tube. The results show that when NRP discharges are applied, the thermoacoustic behavior and the discharges are strongly coupled. At each pulse repetition frequency studied, for low power ratios, a strong thermoacoustic instability (TAI) is induced by the discharges until a critical power ratio beyond which reduction in amplitude of turbulent induced noise (TIN) of the flame is observed. This nonlinear behavior can be attributed to different levels of coupling between the discharge regimes and the acoustic fluctuations. At low power ratios, the discharge regimes oscillate between no discharges and glows/sparks at the frequency of the TAI. These oscillations are in phase with the pressure fluctuations leading to the strong TAI. However, this modulation of the discharges transits to continuous NRP sparks when the power ratio is increased. This helps in reducing the amplitude of TIN. By explaining how different plasma strategies can be beneficial or detrimental for the control of flame dynamics, best practices for the development of plasma actuators are also discussed.