Single- and dual-channel nanosecond repetitively pulsed discharges at small and large spark gaps for turbulent premixed spherical flame initiation

Abstract

This paper reports laminar/turbulent ignition probability (Pig,L/Pig,T) of single- and dual-channel nanosecond-discharge (SN and DN) at small and large inter-electrode gaps (dgap = 0.8 mm and 2.0 mm) using a lean n-butane/air mixture in a double-chamber fan-stirred explosion facility capable of generating near-isotropic turbulence over wide ranges of pulsed repetitive frequency (PRF = 1–100 kHz) and r.m.s. turbulence fluctuation velocity (urms = 0–4.9 m/s). The new DN configuration consists of two parallel anode electrodes with 2.6 mm apart and one central cathode electrode on the same vertical plane, which can generate two spatially separated and temporally synchronized discharges having an almost twice larger initial kernel size than the SN configuration. We apply the same total ignition energy Etot≈18 mJ via a train of 9 pulses/6 pulses for SN/DN experiments, each pulse having Eig≈2.2 mJ (SN) and/or 3.3 mJ (DN) except for the first pulse with the same 0.8 mJ regardless of PRF and dgap. Unlike the common notion that the larger ignition kernel leads to higher Pig,L, it is found that the DN configuration at small dgap = 0.8 mm is totally detrimental for spark ignition (SI) with Pig,L = 0% for all PRFs = 1–100 kHz, while the SN configuration at small dgap = 0.8 mm has its maximum Pig,L = 58% at PRF = 40 kHz showing the synergy effect. But the reverse is found at large dgap = 2.0 mm with Pig,L(DN) > Pig,L(SN). Moreover, when 1 m/s < urms < 4 m/s at PRF = 40 kHz and dgap = 2.0 mm, although both Pig,T(DN) and Pig,T(SN) decrease with increasing urms, Pig,T(DN) is noticeably higher than Pig,T(SN) showing ignition enhancement of dual-channel sparks at large dgap, Pig,T(DN) = Pig,T(SN) = 100% when urms < 1 m/s, and Pig,T(DN) = Pig,T(SN) = 0 % when urms ≥ 4.2 m/s. Finally, high-speed schlieren images of SN/DN kernel evolutions are used to comprehend these results that may be useful for the ignition strategy in lean-burn SI engines.