A Transition of Ignition Kernel Delay Time at the Early Stages of Lean Premixed n-Butane/Air Turbulent Spherical Flame Propagation

This paper explores the effects of root-mean-square turbulence fluctuation velocity (u^′ ) and ignition energy (E_ig ) on an ignition kernel delay time (τ_delay ) of lean premixed n-butane/air spherical flames with an effective Lewis number Le ≈ 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. τ_delay 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_L and MIE_T ) are measured at 50% ignitability, where MIE_L = 3.4 mJ and the increasing slopes of MIE_T with u^′ change from gradual to drastic when u^′ > 0.92 m/s (MIE transition). In quiescence, a transition of τ_delay is observed, where the decrement of τ_delay becomes rapid (modest) when E_ig is less (greater) than MIE_L . For turbulent cases, when applying E_ig ≈ MIE_T, the reverse trend of MIE transition is found for τ_delay versus u^′ results with the same critical u^′ ≈ 0.92 m/s. These results indicated that the increasing u^′ could reduce τ_delay on the one hand, but require higher E_ig (or MIE_T ) on the other hand. Moreover, the rising of E_ig in a specific range, where E_ig ≤ MIE, could shorten τ_delay, but less contribution as E_ig > 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.