LAUSR

Abstract This paper measures turbulent spark ignition probability and minimum ignition energy (MIE) of the pre-vaporized iso-octane/air mixture at an equivalence ratio ϕ = 0.8 at 373 K with Le ≈ 2.98 over a wide range of turbulent intensities ( u′ / S L ), where Le is the mixture's effective Lewis number and S L is the laminar burning velocity. Ignition experiments using a fixed 2-mm electrode gap are conducted in a large dual-chamber, constant-temperature/pressure, fan-stirred 3D cruciform burner capable of generating near-isotropic turbulence. Spark discharges having nearly square voltage and current waveforms are created for accurate determination of the ignition energy ( E ig ) across the electrodes. MIE E ig(50%) that is determined statistically from many repeated experiments at a given condition using a range of E ig to identify an overlapping energy band within which ignition and non-ignition coexist even at the “same discharge E ig ”, where the subscript “ig(50%)” indicates 50% ignitability. Results show that the increasing slopes of MIE T /MIE L = Г versus u′ / S L change drastically from linearly to exponentially when u′ / S L is greater than a critical value of 4.8, which is much smaller than previous rich methane data ( Le > 1) at ϕ = 1.2 with ( u′ / S L ) c ≈ 16 and at ϕ = 1.3 with ( u′ / S L ) c ≈ 24, revealing MIE transition. The subscripts T and L represent turbulent and laminar properties. When a reaction zone Peclet number Pe RZ = u ′ η k / α RZ is used for scaling, it is found that both present lean iso-octane and previous methane data can be collapsed onto a general correlation of Г 1 = 1 + 0.4 Pe RZ in the pre-transition and Г 2 ∼ Pe RZ 4 in the post-transition with the transition occurring at ( Pe RZ ) c ≈ 4.2, showing similarity on MIE transition. η k is the Kolmogorov length scale of turbulence and α RZ is the reaction zone thermal diffusivity estimated at the instant of kernel formation.