Ignition enhancement and deterioration by nanosecond repetitively pulsed discharges in a randomly-stirred lean n-butane/air mixture at various inter-electrode gaps

At 38th Symposium, Shy and co-workers investigated a turbulence-facilitated-ignition (TFI) phenomenon via both conventional-single-spark-discharge (CSSD) and nanosecond-repetitively-pulsed-discharge (NRPD) at various pulsed-repetitive-frequency (PRF=5–70 kHz) using a fixed inter-electrode gap (d_gap=0.8 mm) in a randomly-stirred lean n-butane/air mixture with Lewis number Le≈2.2 ≫ 1 over a range of r.m.s. turbulent fluctuating velocity (u′=0–2.1 m/s). It was found that the ignition probability (P_ig) decreases significantly with increasing u′ for most PRFs, except at PRF=60 kHz having turbulent P_ig,T = 37% at u′=0.5 m/s > laminar P_ig,L = 34%, showing possible TFI. Using the same methodology, this paper presents two new P_ig datasets at d_gap=0.6 mm and 2.0 mm over wider ranges of PRF=1–90 kHz and u′=0–2.8 m/s. All NRPD experiments apply the same total ignition energy E_tot≈23 mJ via a train of 11 pulses, each pulse having about 2.2 mJ except for the first pulse having about 1 mJ when using sufficiently high open-circuit-voltage of 28 kV. Note that E_tot≈MIE_L≈23 mJ at d_gap=0.8 mm as a baseline for comparison; MIE_L is the laminar minimum ignition energy measured by CSSD at 50% ignitability and MIE_L=26.3 mJ/3.4 mJ at d_gap=0.6 mm/2.0 mm. While TFI is noticeably observed at d_gap=0.6 mm when PRF=60 kHz with P_ig,T = 92% at u′=0.5 m/s > P_ig,L = 78%, turbulence reclaims its dominating role when u′ > 1.4 m/s with P_ig,T << P_ig,L. At d_gap=2.0 mm, there is no TFI; a P_ig,T transition is found where the decrease of P_ig,T with u′ changes from gradually to rapidly when u′ > u′_c (critical u′ depending on PRF). We also discover that the curves of P_ig,T versus PRF are highly non-monotonic with peaks around PRF=40 kHz for u′ > 1.4 m/s; too high PRFs are detrimental for spherical flame ignition. These results are explained by coupling effects of d_gap, PRF, u′, and differential diffusion.