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

Abstract

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′/SL = 0–50), from 1 to 5 atm, using a large dual-chamber, constant-pressure, fan-stirred explosion facility, where SL 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′/SL is greater than some critical values depending on p. It is found that the scaling slopes of MIET/MIEL versus u′/SL change abruptly from a linear increase to an exponential increase when u′/SL > (u′/SL)c, showing ignition transition. The subscripts T and L represent turbulent and laminar properties, MIEL ≈ 6.84 mJ (1 atm), 2.81 (3 atm), and 2.11 (5 atm), and the transition occurs at (u′/SL)c ≈ 12 (1 atm), 24 (3 atm), and 34 (5 atm). It is also found that the above scattering MIET/MIEL data at different u′/SL and p can be merged together into a single curve when scaled with a pressure-corrected kernel (reaction zone, RZ) Péclet number, Pe* = PeRZ(p/p0)−1/4, showing the first and fourth power dependence before and after MIE transition at a critical Pe* ≈ 3.6. PeRZ = u′ηk/αRZ, ηk is the Kolmogorov length scale of turbulence, αRZ is the thermal diffusivity estimated at the instant of kernel formation, and p0 = 1 atm. These results reveal a self-similar spark ignition phenomenon.