| Summary: | This paper explores the effects of root-mean-square turbulence fluctuation velocity (<i>u</i>′) and ignition energy (E<sub>ig</sub>) on an ignition kernel delay time (<i>τ</i><sub>delay</sub>) of lean premixed n-butane/air spherical flames with an effective Lewis number <i>Le</i> ≈ 2.1 >> 1. Experiments are conducted in a dual-chamber, fan-stirred cruciform burner capable of generating near-isotropic turbulence with negligible mean velocities using a pair of cantilevered electrodes with sharp ends at a fixed spark gap of 2 mm. <i>τ</i><sub>delay</sub> is determined at a critical flame radius with a minimum flame speed during the early stages of laminar and turbulent flame propagation. Laminar and turbulent minimum ignition energies (MIE<sub>L</sub> and MIE<sub>T</sub>) are measured at 50% ignitability, where MIE<sub>L</sub> = 3.4 mJ and the increasing slopes of MIE<sub>T</sub> with <i>u</i>′ change from gradual to drastic when <i>u</i>′ > 0.92 m/s (MIE transition). In quiescence, a transition of <i>τ</i><sub>delay</sub> is observed, where the decrement of <i>τ</i><sub>delay</sub> becomes rapid (modest) when E<sub>ig</sub> is less (greater) than MIE<sub>L</sub>. For turbulent cases, when applying E<sub>ig</sub> ≈ MIE<sub>T</sub>, the reverse trend of MIE transition is found for <i>τ</i><sub>delay</sub> versus <i>u</i>′ results with the same critical <i>u</i>′ ≈ 0.92 m/s. These results indicated that the increasing <i>u</i>′ could reduce <i>τ</i><sub>delay</sub> on the one hand, but require higher E<sub>ig</sub> (or MIE<sub>T</sub>) on the other hand. Moreover, the rising of E<sub>ig</sub> in a specific range, where E<sub>ig</sub> ≤ MIE, could shorten <i>τ</i><sub>delay</sub>, but less contribution as E<sub>ig</sub> > MIE. These results may play an important role to achieve optimal combustion phases and design an effective ignition system on spark ignition engines operated under lean-burn turbulent conditions.
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