Conventional spark versus nanosecond repetitively pulsed discharge for a turbulence facilitated ignition phenomenon

Abstract

This work applies both conventional-single-spark-discharge (CSSD) at 500-µs pulse duration time and nanosecond-repetitively-pulsed-discharge (NRPD) at various pulsed-repetitive-frequency PRF = 5–70 kHz to explore a turbulence facilitated ignition (TFI) phenomenon using a pair of pin-to-pin electrodes at an inter-electrode gap of 0.8 mm in randomly-stirred lean n-butane/air mixture with Lewis number ≫ 1. For CSSD, measured laminar and turbulent minimum ignition energies (MIEL and MIET) at 50% ignitability show that MIEL≈ 23 mJ > the smallest MIET≈ 19.7 mJ at u′ = 0.9 m/s (TFI) and then MIET≈ 28.6/30.8/36.8 mJ at u′ = 1.4/2.1/2.8 m/s (no TFI), where u′ is the r.m.s turbulent fluctuating velocity. For comparison, all NRPD experiments apply the same total ignition energy Etot≈ 23 mJ via a fixed train of 11 pulses, each pulse with 2.2 mJ except for the first pulse with 1 mJ. NRPD results show a cumulatively synergistic effect depending on the coherence between PRF and an inward reactant flow recirculation frequency (fRC) inside the torus-like kernel induced by the discharge that could enhance ignition. When PRF is approximately synchronizing with fRC, the synergistic effect is most profound at PRF = 20-kHz/40-kHz with very high ignition probability Pig = 90%/85% > 50% in quiescence, whereas lower values of Pig = 42%/34% are found at PRF = 10-kHz/60-kHz. Further, Pig = 0 at PRF = 5-kHz even when 5000 pulses (Etot≈ 10 J) are applied. We discover that Pig decreases significantly with increasing u′ for most PRFs (no TFI) except at higher PRF ≥ 60 kHz showing possible TFI. These results are attributed to the interactions between turbulent dissipation, differential diffusion, and synergistic influence, which are substantiated by Schlieren images of initial kernel development and the ignition time determined at one half of the flame critical radius that leads to a self-sustained spherical flame propagation.