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 ≤ 0.8 mm) 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 ϕ = 5.1 (Le ≈ 2.3) and ϕ = 0.18 (Le ≈ 0.3) 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 (MIE_L and MIE_T) with increasing d_gap is observed. TFI only occurs for Le > 1 (ϕ = 5.1) and it is restricted at smaller d_gap = 0.58 mm, where MIE_L = 61.5 mJ >> MIE_T = 26 mJ (0.25-mm tungsten electrodes) and MIE_L = 255.5 mJ >> MIE_T = 36.8 mJ (2-mm tungsten electrodes) in support of Wu et al.’s finding. However, we discover that the MIE_L and MIE_T 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 MIE_L = 0.52 mJ << MIE_T = 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.