Laminar and turbulent minimum ignition energies of lean n-butane/air and lean iso-octane/air (le >> 1) mixtures at small and modest electrode spark gaps

The laminar and turbulent minimum ignition energies (MIEL and MIET) of lean n-butane/air and lean iso-octane/air pre-mixtures with Le ? 2.1 and 2.98 >> 1 are measured at two electrode spark gaps, i.e. small dgap = 0.8 mm (n-C4H10) or 0.95 mm (i-C8H18) < 1 mm and modest dgap = 2 mm > 1 mm, in near-isotropic turbulence generated by a dual-chamber fan-stirred explosion facility, where Le is the effective Lewis number. Depending on the coupling effects of Le, dgap, and the turbulent fluctuating velocity (u), two contrary phenomena have been observed: Turbulent Facilitated Ignition (TFI) where MIET/MIEL < 1 and MIE Transition beyond which log(MIET/MIEL) ~ log(u). High-speed Schlieren imaging shows that the embryonic spark kernel in quiescence is ball (rod) like when dgap < 1 mm (dgap > 1 mm), demonstrating positive (negligible) curvature stretch. TFI only occurs when dgap is sufficiently small (< 1 mm) as well as Le is sufficiently greater than unity, because large positive curvature stretch weakens reaction rate due to differential diffusion. No TFI when dgap > 1 mm, and MIE transition is observed at dgap = 2 mm. These results are important to lean burn spark ignition in gasoline engines.