Investigation of the hydrodynamic effect of nanosecond repetitively pulsed discharges on a laminar stagnation flame

There has been increased interest in plasma-assisted combustion over the last two decades due to its ability to improve flame stability and combustion performance. However, the effect of nanosecond repetitively pulsed (NRP) discharges is still not well understood since the plasma can act on the thermal, kinetic and hydrodynamic properties of combustion. This study investigates the hydrodynamic effect of NRP discharges produced upstream of a lean premixed methane-air flame at atmospheric pressure. The effect of the plasma pulse repetition frequency (PRF) on the flame was studied with time-resolved imaging and shows that, as the PRF increases, the flame stabilizes further upstream closer to the plasma source. Time-resolved imaging also shows that the flame remains steady and does not relax between consecutive NRP discharges for PRFs ranging between 1 to 5 kHz and that the full relaxation of the flame takes ? 100 ms. Particle image velocimetry is used to assess the effect of the plasma on both a cold flow and a stagnation flame. The results show that the NRP discharges reduce the flow velocity upstream of the flame front by up to 20%, which is the main cause of the displacement of the flame. The stretch rate of the flame was also studied, and the stretch induced by the plasma was found to be the main cause of the 12% increase in flame speed. The mechanism through which the plasma causes the observed hydrodynamic effect remains unknown. ? 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Automated summary

There has been an increased interest in plasma-assisted combustion due to its ability to improve flame stability and combustion performance over the past two decades. This study investigates the hydrodynamic effect of nanosecond repetitively pulsed discharges on a lean premixed methane-air flame at atmospheric pressure, finding that the plasma pulse repetition frequency impacts flame stability and upstream movement. The plasma reduces flow velocity upstream of the flame front and induces stretch in the flame, resulting in an increase in flame speed.