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 φ = 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′/SL)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, MIE ∼ δ³ based on an order-of-magnitude criterion where δ 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.