Spark Ignition Transition of Premixed Turbulent Expanding Flames using Nanosecond Repetitively Pulsed Discharges
Department of Mechanical Engineering, National Central University, Jhong-li District, Taoyuan City 32001, Taiwan
This talk presents the coupling effects of pulsed repetitive frequency (PRF = 0-90 kHz), r.m.s turbulent fluctuating velocity (u = 0-2.8 m/s), and inter-electrode gap (dgap = 0.6-2.0 mm) on the enhancement and deterioration of laminar and turbulent ignition probabilities (Pig,L and Pig,T) by nanosecond-repetitively-pulsed-discharges (NRPD) in a randomly-stirred lean n-butane/air mixture with large Lewis number Le 2.2 >> 1. All centrally-ignited NRPD experiments are conducted in a dual-chamber fan-stirred cruciform burner using the same total ignition energy (Etot 23 mJ) via a burst of 11 pulses, each pulse having about 2.2 mJ except for the first pulse having about 1 mJ. Here Etot MIEL 23 mJ at dgap = 0.8-mm (MIEL=26.3-mJ/3.4-mJ at dgap = 0.6-mm/2.0-mm), where MIEL is the laminar minimum ignition energy measured by the conventional-single-spark-discharge (CSSD) at 50% ignitability. At small dgap, we discover a synergistic coherence ignition enhancement (SCIE) regime having very high Pig,L = 80-90% within certain PRFs, i.e. 20-40 kHz at dgap = 0.8-mm and 40-60 kHz at dgap = 0.6-mm. This is attributed to the coherence between PRF and an inward reactant flow recirculation frequency (fRC) inside the torus-like kernel induced by the discharges. Outside the SCIE regime when PRF < fRC and/or PRF > fRC, the deterioration of Pig,L is found. When PRF 5-kHz/10-kHz at dgap=0.8-mm/0.6-mm, Pig,L=0 even using 5,000 pulses with Etot 11 J. Moreover, Pig,T decreases with increasing u for all three dgap and for most PRFs, except for small dgap = 0.6-mm/0.8-mm at higher PRF 60 kHz and at lower u = 0.5 m/s where a turbulence-facilitated-ignition (TFI) with Pig,T > Pig,L is observed. There is no TFI at dgap = 2-mm; the decrease of Pig,T with u changes from gradually to rapidly when u is greater than some critical value depending on PRF, showing a Pig,T transition. 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. Finally, the aforesaid NRPD ignition transition (IT) is compared with previous CSSD IT results using various fuels, such as hydrogen, methane, propane, n-butane, iso-octane, and the primary reference fuel of gasoline, which should be important for achieving a better ignition strategy in spark ignition engines.