Can the phenomenon of turbulence facilitated ignition through differential diffusion occur in laser-induced spark ignition?

This study investigates experimentally whether turbulence facilitated ignition (TFI) through differential diffusion, previously observed by conventional electrode spark ignition (ESI) under two restrictions at sufficiently large Le >> 1 and at sufficiently small d_gap (typically less than 1 mm), could also exist in laser-induced spark ignition (LSI). Le is the effective Lewis number and d_gap is the electrode gap distance. The same n-butane/air mixture at the equivalence ratio ϕ = 0.7 with Le ≈ 2.2 >> 1 is applied for both ESI and LSI experiments in the same fan-stirred cruciform burner capable of generating near-isotropic turbulence with negligible mean velocities. Values of laminar and turbulent minimum ignition energies (MIE_L and MIE_T) at 50% ignitability are statistically measured as a function of the r.m.s. turbulence fluctuation velocity (u'). For ESI at small d_gap = 0.8 mm having TFI, MIE_T first decreases with increasing u', of which a minimum MIE_T ≈ 19.2 mJ occurs at u' = 0.92 m/s < MIE_L ≈ 23 mJ; then MIE_T increases drastically to be greater than MIE_L when u' ≥ 1.4 m/s (a non-montonic MIE transition). By using three different focal lengths of 70/150/200 mm for laser breakdown where corresponding values of MIE_L are respectively 8.51/12.74/15.77 mJ, we discover no TFI for LSI because not only MIE_T > MIE_L at any u' but also MIE_T >> MIE_L when u' is greater than some critical value of u'_c ≈ 2 m/s, showing the monotonic MIE transition. This is attributed to the deformation of laser kernel by a third lobe generated by the toroidal rings in the direction opposite to the laser having both negative and positive curvature segments, and negative curvature segments can enhance reaction rate through differential diffusion that prevents the occurrence of TFI in LSI.