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 (MIE_L and MIE_T) 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 (d_gap = 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, d_gap, and u′, we explain what causes two distinct phenomena: Turbulent Facilitated Ignition (TFI) meaning MIE_L >> MIE_T and MIE Transition meaning a change from MIE_T ≥ MIE_L to MIE_T >> MIE_L 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 d_gap < 1 mm (d_gap > 1 mm), demonstrating large (very small or negligible) positive curvature. This explains why TFI, an unusual phenomenon, only occurs at sufficiently small d_gap < 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 d_gap = 0.58 mm and Le ≈ 2.3, a non-monotonic decrease and increase of MIE_T with increasing u′ is observed, because the dissipation of ignition kernel by sufficiently intense turbulence re-declares its dominance leading to the increase of MIE_T. There is no TFI when d_gap > 1 mm regardless of Le. The scenario changes to MIE transition when d_gap = 2 mm at Le ≈ 2.3, where MIE_L << MIE_T. Moreover, when Le ≈ 0.3, MIE transition is shown to appear at d_gap = 0.3 mm, but is clearly suppressed at d_gap = 0.58 mm beyond which successful ignition is very easy to achieve. These findings are important for spark ignition in premixed turbulent combustion.