Measurement and scaling of minimum ignition energy transition for spark ignition in intense isotropic turbulence from 1 to 5 atm

This paper presents high-pressure minimum ignition energies (MIE) and their scaling from measurements on spark discharges in lean methane/air mixtures at the equivalence ratio phi = 0.6 by cylindrical electrodes with flat ends in near-isotropic turbulence over a range of turbulent intensities (u'/S-L = 0-50), from 1 to 5 atm, using a large dual-chamber, constant-pressure, fan-stirred explosion facility, where S-L is the laminar burning velocity. Voltage and current waveforms of spark discharges with nearly square profiles are carefully generated for accurate determination of MIE, commonly defined as the 50% successful ignitability. Applying high-speed schlieren imaging, we observe a drastic change of kernel development from turbulent flamelet to distributed like with island formation and local quench even at 5 atm, when u'/S-L is greater than some critical values depending on p. It is found that the scaling slopes of MIET/MIEL versus u'/S-L change abruptly from a linear increase to an exponential increase when u'/S-L > (u'/S-L)(c), showing ignition transition. The subscripts T and L represent turbulent and laminar properties, MIEL approximate to 6.84 mJ (1 atm), 2.81 (3 atm), and 2.11 (5 atm), and the transition occurs at (u'/S-L)(c) approximate to 12 (1 atm), 24 (3 atm), and 34 (5 atm). It is also found that the above scattering MIE T/MIE L data at different u'/S-L and p can be merged together into a single curve when scaled with a pressure-corrected kernel (reaction zone, RZ) Peclet number, Pe* = Pe(RZ) (p/p(0))(-1/4), showing the first and fourth power dependence before and after MIE transition at a critical Pe* approximate to 3.6. Pe(RZ) = u'eta(k)/alpha(RZ), eta(k) is the Kolmogorov length scale of turbulence, alpha(RZ) is the thermal diffusivity estimated at the instant of kernel formation, and p(0) = 1 atm. These results reveal a self-similar spark ignition phenomenon. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.