Temporal and spatial evolution of OH concentration in a lean premixed propane-air flame assisted by nanosecond repetitively pulsed discharges

Planar laser-induced fluorescence (LIF) has been used to study the evolution of the OH concentration in a weakly turbulent lean premixed flame both spatially and temporally, when applying Nanosecond Repetitively Pulsed (NRP) discharges. A 2-kW turbulent leanpremixed propane-air flame is stabilized by a cylindrical bluff-body. The discharge is created by voltage pulses of amplitude 7 kV, duration 10 ns, applied at a frequency of 30 kHz between two pin electrodes placed in the recirculation zone downstream of the bluffbody. The average electric power deposited by the plasma is up to 20 W, typically less than 1% of the thermal power of the flame. LIF images of the recirculation zone are recorded starting from 15 microseconds after discharge initiation. CH* chemiluminescence images of the flame are also recorded to trace the location of the flame. The results show that OH is produced in the plasma region and is then convected towards the shear layer. The key mechanism of the reduction of flame lift-off height by NRP discharges is the continuous ignition of the fresh combustible mixture in the shear layer, where it comes in contact with the active species produced by the NRP discharges.