Effect of spark gap on turbulent facilitated ignition through differential diffusion

This note is based on a recent brief communication published in Combustion and Flame (Shy et al. 2107) to discuss an unexpected result discovered by Wu et al. (2014). Contrary to traditional views, Wu et al. found that turbulence can facilitate ignition through differential diffusion when the effective Lewis number (Le) of mixtures is sufficiently greater than unity, where ignition experiments were conducted in a near-isotropic turbulence generated by a fan-stirred burner by applying small spark electrode gaps (d_gap ≤ 0.8 mm). This implied that the associated minimum ignition energy (MIE) in intense turbulence could be smaller than that in quiescence. In this work, the effect of d_gap on the turbulent facilitated ignition (TFI) for Le > 1 is measured and discussed. 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 with the rms turbulent fluctuating velocity u′ = 5.4 m/s. When d_gap is increased from 0.58 mm to about 6 mm, a drastic fall of values of laminar and turbulent MIE (MIE_L and MIE_T) is found. TFI only occurs for Le > 1 (φ = 5.1) in support of Wu et al.’s finding, but it is restricted at smaller d_gap = 0.58 mm. 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.