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 (dgap=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 (Pig) decreases significantly with increasing u′ for most PRFs, except at PRF=60 kHz having turbulent Pig,T = 37% at u′=0.5 m/s > laminar Pig,L = 34%, showing possible TFI. Using the same methodology, this paper presents two new Pig datasets at dgap=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 Etot≈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 Etot≈MIEL≈23 mJ at dgap=0.8 mm as a baseline for comparison; MIEL is the laminar minimum ignition energy measured by CSSD at 50% ignitability and MIEL=26.3 mJ/3.4 mJ at dgap=0.6 mm/2.0 mm. While TFI is noticeably observed at dgap=0.6 mm when PRF=60 kHz with Pig,T = 92% at u′=0.5 m/s > Pig,L = 78%, turbulence reclaims its dominating role when u′ > 1.4 m/s with Pig,T << Pig,L. At dgap=2.0 mm, there is no TFI; a Pig,T transition is found where the decrease of Pig,T with u′ changes from gradually to rapidly when u′ > u′c (critical u′ depending on PRF). We also discover that the curves of Pig,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 dgap, PRF, u′, and differential diffusion.
|期刊||Combustion and Flame|
|出版狀態||已出版 - 9月 2021|