A transition on minimum ignition energy for lean turbulent methane combustion in flamelet and distributed regimes

Minimum ignition energy (MIE) of lean methane-air mixtures is quantitatively measured using a high-power pulse generator which can vary ignition energies of a spark-electrode in the central position of a large fan-stirred cruciform burner. The burner equipped with a pair of counter-rotating fans and perforated plates can be used to generate isotropic turbulence having a very wide range of turbulent intensities (u') up to 8 m/s with negligible mean velocities. Observations of ignition, flame kernel development, and subsequent flame propagation in the central uniform region of the burner are recorded by a CMOS high-speed camera (5000 frames/s), showing distributed-like flames of very dispersive and fragmental structures with filiform edges for the first time. A complete MIE data set of lean methane-air mixtures at the equivalence ratio phi = 0.6 as a function of u'/S-L is obtained, where S-L is the laminar burning velocity. It is found that there is a transition on values of MIE due to different modes of combustion. Before the transition, MIE only increases gradually with u'/S-L. Across the transition when u'/S-L > 24 corresponding to the commonly defined turbulent Karlovitz number Ka = (u'/S-L)(2) (Re-T)(-0.5) > 8, MIE increases abruptly, where Re-T is the turbulent Reynolds number based on the integral length scale of turbulence. This transitional value of Ka is much greater than the Klimov-Williams criterion (Ka = 1). Since values of MIE under different levels of turbulence should be relevant to the size of the reaction zone at least in the beginning of turbulent combustion, M I E similar to delta(3) based on an order-of-magnitude criterion where delta is the reaction zone thickness. It is thus concluded that this new experimental finding proves the existence of both thin and broken reaction zones regimes proposed by Peters for a new regime diagram of premixed turbulent combustion. (C) 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.