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 φ = 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 MIE_T/MIE_L 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, MIE_L ≈ 6.84 mJ (1 atm), 2.81 (3 atm), and 2.11 (5 atm), and the transition occurs at (u′/S_L)_c ≈ 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) Péclet number, Pe^∗ = Pe_RZ(p/p₀)^-1/4, showing the first and fourth power dependence before and after MIE transition at a critical Pe^∗ ≈ 3.6. Pe_RZ = u′η_k/α_RZ, η_k is the Kolmogorov length scale of turbulence, α_RZ is the thermal diffusivity estimated at the instant of kernel formation, and p₀ = 1 atm. These results reveal a self-similar spark ignition phenomenon.