kHz-rate particle-image velocimetry of induced instability in premixed propane/air flame by millisecond pulsed current-voltage

Particle-image velocimetry (PIV) measurements were performed at 6 kHz repetition rate in a premixed propane/air flame to examine the effects caused by applied millisecond-wide pulsed voltage-current below self-sustained breakdown. We have demonstrated significant structural changes to a burner-stabilized downward-propagating atmospheric pressure propane/air flame with overall flow speeds near 2 m/s with +3 kV pulsed applied voltages over 30 mm gaps. Phase-locked, 2 kHz broadband emission measurements of flame structure were also collected to support the PIV velocity data. The combined high-speed PIV and flame emission measurements were both capable of capturing changes from a single applied voltage pulse rather than using a phase matching approach requiring a highly repeatable disturbance as done previously [1]. The measured reductions in flame height, increases in local flow speeds, generation of large velocity gradients, and rapid oscillations in flame front are suggestive of an induced turbulence in an otherwise laminar flame. Taylor microscale lengths were calculated from the kHz PIV data and structures comparable to the reaction zone thickness were shown to increase during the applied voltage pulse. The timescale under which the flame flow changes combined with the accompanying flame emission measurements suggest that flame fluidics are modified by ion drift current induced net body force in or near the cathode fall at the base of the flame. The reduction in overall flame height and increase in speed near the base of the flame is suggestive of a 'virtual' bluff-body present in the flow. These fluidic changes force the flame to transition from a laminar to a highly unstable, transitioning to turbulence regime.