Combustion and Flame, Vol.205, 371-377, 2019
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 (phi= 0.18 and 5.1) where Lewis numbers Le approximate to 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 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 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 approximate to 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 approximate to 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 MIET. 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 approximate to 2.3, where MIEL << MIET. Moreover, when Le approximate to 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. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.