Counterflow diffusion flame oscillations induced by ns pulse electric discharge waveforms

Repetitive ns pulse, dielectric barrier discharge voltage waveforms, combined with a tail several ms long, are used to induce oscillations of a counterflow atmospheric pressure diffusion flame. A baseline ns pulse discharge operated at 10 Hz results in a relatively modest oscillatory response of the flame, which becomes more pronounced in burst mode operation, at the same burst repetition rate of 10 Hz. This effect is most likely caused by the residual electric field after the discharge pulse, producing the electrohydrodynamic (EHD)force (“ion wind”)on the charges generated during the discharge, although plasma chemistry and Joule heating by the discharge may also contribute. Manipulating the external circuit to add a variable duration tail to the discharge pulse, without changing the pulse shape during breakdown or the pulse repetition rate, considerably enhances the impulse of the EHD force and increases the amplitude of the flame oscillations. To quantify this effect, the electric field distribution between the electrodes during and after the discharge pulse is measured by ps Electric Field Induced Second Harmonic (E-FISH)diagnostic. The results show that the electric field is maintained during the voltage tail, although it is lower compared to the Laplacian field due to the charge accumulation on the dielectric sleeves covering the electrodes. The time scale of the flame oscillations at the present conditions, of the order of ∼10 ms, is limited by the relatively slow momentum transfer from the ions to the neutral species. The present results demonstrate feasibility of enhancing the flame control authority, by combining a high peak ionization fraction generated by a ns pulse discharge with the EHD force applied on a long time scale, using a single plasma generator.