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

At the 31st Combustion Symposium, Shy et al. found a transition on minimum ignition energy (MIE) of methane-air mixtures at the equivalence ratio phi=0.6 in intense isotropic turbulence, where ignition energies of a spark-electrode was quantitatively measured by an energy-adjustable high-power pulse ignition system. Using the same methodology, this paper presents for the first time two new MIE data sets at phi=0.7 and 0.8 over a wide range of turbulent intensities. It is found that MIE transition due to different modes of turbulent combustion depends on a turbulent Karlovitz number (Ka) indicating the time ratio between chemical reaction and turbulence, for which MIE first increases gradually with Ka and then increases drastically when Ka>Ka(c)approximate to 4 similar to 9 depending also on phi. The effect of the electrode gap on ignition energies and turbulence influence to centrally-ignited, outwardly propagating flames are also discussed.