Effects of electrode spark gap, differential diffusion, and turbulent dissipation on two distinct phenomena: Turbulent facilitated ignition versus minimum ignition energy transition

This paper reports laminar and turbulent minimum ignition energies (MIEL and MIET) of hydrogen/air mixtures at two equivalence ratios (ϕ = 0.18 and 5.1) where Lewis numbers Le ≈ 0.3 and 2.3, respectively, over wide ranges of the electrode spark gap (dgap = 0.3–6.5 mm) and the r.m.s. turbulent fluctu...

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Bibliographic Details
Published in:Combustion and flame 2019-07, Vol.205, p.371-377
Main Authors: Shy, S.S., Nguyen, Minh Tien, Huang, Shih Yao
Format: Article
Language:English
Subjects:
Aerodynamics
Curvature
Differential diffusion
Diffusion rate
Effects of electrode gap
Electrodes
Kernels
Lewis numbers
Minimum ignition energy transition
Spark ignition
Spark ignition in premixed turbulent combustion
Turbulence
Turbulent combustion
Turbulent dissipation
Turbulent facilitated ignition
Variation
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Summary:This paper reports laminar and turbulent minimum ignition energies (MIEL and MIET) of hydrogen/air mixtures at two equivalence ratios (ϕ = 0.18 and 5.1) where Lewis numbers Le ≈ 0.3 and 2.3, respectively, over wide ranges of the electrode spark gap (dgap = 0.3–6.5 mm) and the r.m.s. turbulent fluctuating velocity (u′ = 0–8.3 m/s). Depending on the coupling effects of Le, dgap, and u′, we explain what causes two distinct phenomena: Turbulent Facilitated Ignition (TFI) meaning MIEL >> MIET and MIE Transition meaning a change from MIET ≥ MIEL to MIET >> MIEL when u′ is greater than some critical value. High-speed Schlieren imaging shows that the embryonic spark kernel in quiescence is ball (rod) like when dgap < 1 mm (dgap > 1 mm), demonstrating large (very small or negligible) positive curvature. This explains why TFI, an unusual phenomenon, only occurs at sufficiently small dgap < 1 mm and at sufficiently large Le >> 1 because large positive curvature stretch weakens reaction rate due to differential diffusion, making successful ignition in quiescence very difficult to achieve. At dgap = 0.58 mm and Le ≈ 2.3, a non-monotonic decrease and increase of MIET with increasing u′ is observed, because the dissipation of ignition kernel by sufficiently intense turbulence re-declares its dominance leading to the increase of MIET. There is no TFI when dgap > 1 mm regardless of Le. The scenario changes to MIE transition when dgap = 2 mm at Le ≈ 2.3, where MIEL
ISSN:0010-2180
1556-2921
DOI:10.1016/j.combustflame.2019.04.029