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

Featured Application A transition of ignition kernel delay time at the early stages of lean premixed n-butane/air turbulent expanding flame propagation is found, which may be relevant to spark ignition engines operated at lean-burn turbulent conditions. This paper explores the effects of root-mean-square turbulence fluctuation velocity (u ') and ignition energy (E-ig) on an ignition kernel delay time (tau(delay)) of lean premixed n-butane/air spherical flames with an effective Lewis number Le approximate to 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. tau(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 (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 tau(delay) is observed, where the decrement of tau(delay) becomes rapid (modest) when E-ig is less (greater) than MIEL. For turbulent cases, when applying E-ig approximate to MIET, the reverse trend of MIE transition is found for tau(delay) versus u ' results with the same critical u ' approximate to 0.92 m/s. These results indicated that the increasing u ' could reduce tau(delay) on the one hand, but require higher E-ig (or MIET) on the other hand. Moreover, the rising of E-ig in a specific range, where E-ig <= MIE, could shorten tau(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.