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

This paper reports laminar/turbulent ignition probability (P_ig,L/P_ig,T) of single- and dual-channel nanosecond-discharge (SN and DN) at small and large inter-electrode gaps (d_gap = 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 (u_rms = 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 E_tot≈18 mJ via a train of 9 pulses/6 pulses for SN/DN experiments, each pulse having E_ig≈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 d_gap. Unlike the common notion that the larger ignition kernel leads to higher P_ig,L, it is found that the DN configuration at small d_gap = 0.8 mm is totally detrimental for spark ignition (SI) with P_ig,L = 0% for all PRFs = 1–100 kHz, while the SN configuration at small d_gap = 0.8 mm has its maximum P_ig,L = 58% at PRF = 40 kHz showing the synergy effect. But the reverse is found at large d_gap = 2.0 mm with P_ig,L(DN) > P_ig,L(SN). Moreover, when 1 m/s < u_rms < 4 m/s at PRF = 40 kHz and d_gap = 2.0 mm, although both P_ig,T(DN) and P_ig,T(SN) decrease with increasing u_rms, P_ig,T(DN) is noticeably higher than P_ig,T(SN) showing ignition enhancement of dual-channel sparks at large d_gap, P_ig,T(DN) = P_ig,T(SN) = 100% when u_rms < 1 m/s, and P_ig,T(DN) = P_ig,T(SN) = 0 % when u_rms ≥ 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.