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

Abstract

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 dgap (typically less than 1 mm), could also exist in laser-induced spark ignition (LSI). Le is the effective Lewis number and dgap 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 (MIEL and MIET) at 50% ignitability are statistically measured as a function of the r.m.s. turbulence fluctuation velocity (u'). For ESI at small dgap = 0.8 mm having TFI, MIET first decreases with increasing u', of which a minimum MIET ≈ 19.2 mJ occurs at u' = 0.92 m/s < MIEL ≈ 23 mJ; then MIET increases drastically to be greater than MIEL 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 MIEL are respectively 8.51/12.74/15.77 mJ, we discover no TFI for LSI because not only MIET > MIEL at any u' but also MIET >> MIEL 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.