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

Abstract

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