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

Abstract

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 << MIET. Moreover, when Le ≈ 0.3, MIE transition is shown to appear at dgap = 0.3 mm, but is clearly suppressed at dgap = 0.58 mm beyond which successful ignition is very easy to achieve. These findings are important for spark ignition in premixed turbulent combustion.