Is turbulent facilitated ignition through differential diffusion independent of spark gap?

In 2014, Wu et al. discovered an unexpected result. Turbulence can facilitate ignition through differential diffusion when the effective Lewis number (Le) of mixtures is sufficiently larger than unity using small electrode gaps (d(gap) <= 0.8mm) in near-isotropic turbulence generated by a fan-stirred burner. This suggested that the required minimum ignition energy (MIE) in intense turbulence can be smaller than that in quiescence (Wu et al. did not measure MIE). This work explores whether the aforesaid turbulent facilitated ignition (TFI) for Le > 1 is independent of d(gap). We apply the same hydrogen mixtures at the equivalence ratio phi = 5.1 (Le approximate to 2.3) and phi = 0.18 (Le approximate to 10.3) as Wu et al. in our large fan-stirred cruciform bomb capable of generating near-isotropic turbulence to measure values of MIE as a function of d(gap) at both quiescence and intense turbulence (the rms turbulent fluctuating velocity u'=5.4 m/s) conditions. A drastic fall of values of laminar and turbulent MIE (MIEL and MIET) with increasing d(gap) is observed. TFI only occurs for Le > 1 (phi = 5.1) and it is restricted at smaller d(gap) = 0.58 mm, where MIEL = 61.5 mJ > > MIET = 26 mJ (0.25-mm tungsten electrodes) and MIEL = 255.5 mJ > > MIET = 36.8 mJ (2-mm tungsten electrodes) in support of Wu et al.'s finding. However, we discover that the MIEL and MIET curves versus d(gap) can cross each other at larger d(gap), at which no TFI for Le > 1 at d(gap) = 2 mm where MIEL = 0.52 mJ MIET = 17.3 mJ (2-mm tungsten electrodes). This interesting result depending on d(gap) should be disseminated in our combustion community for stimulating further research. (C) 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.