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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 (dgap ≤ 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 dgap. 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 dgap 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 dgap is observed. TFI only occurs for Le > 1 (ϕ = 5.1) and it is restricted at smaller dgap = 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 dgap can cross each other at larger dgap, at which no TFI for Le > 1 at dgap = 2 mm where MIEL = 0.52 mJ << MIET = 17.3 mJ (2-mm tungsten electrodes). This interesting result depending on dgap should be disseminated in our combustion community for stimulating further research.
|Number of pages||3|
|Journal||Combustion and Flame|
|State||Published - 2017|