Effect of nanosecond repetitively pulsed discharges on the dynamics of a swirl-stabilized lean premixed flame

The effects of nanosecond repetitively pulsed (NRP) plasma discharges on the dynamics of a swirl-stabilized lean premixed flame are experimentally investigated. Voltage pulses of 8 kV in amplitude and 10 ns in duration are applied at a repetition rate of 30 kHz. The average electric power deposited by the plasma is limited to 40 W, corresponding to less than 1% of the thermal power of 4 kW released by the flame. The investigation is carried out with a dedicated experimental setup that allows for studies of the flame dynamics with applied plasma discharges. A loudspeaker is used to acoustically perturb the flame and the discharges are generated between a central pin electrode and the rim of the injection tube. The velocity and CH^* chemiluminescence signals are used to determine the flame transfer function, assuming that plasma discharges do not affect the correlation between the CH_* emission and heat release rate fluctuations. Phase-locked images of the CH_* emission show a strong influence of the NRP discharges on the flame response to acoustic perturbations, thus opening interesting perspectives for combustion control. An interpretation of the modifications observed in the transfer function of the flame is proposed by taking into account the thermal and chemical effects of the discharges. It is then demonstrated that by applying NRP discharges at unstable conditions, the oscillation amplitudes can be reduced by an order of magnitude, thus effectively stabilizing the system.