Spark ignition probability and minimum ignition energy transition of the lean iso-octane/air mixture in premixed turbulent combustion

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 [tbnd] 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 Péclet 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 Г₁ = 1 + 0.4Pe_RZ in the pre-transition and Г₂ ∼ Pe_RZ⁴ 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.