Flame initiation in lean, quiescent and turbulent mixtures with various igniters

High speed flame photographs have been taken, following ignition, in an explosion bomb, equipped with four high speed fans to create turbulence. The different igniters studied, under quiescent and turbulent conditions of the the lean methane-air mixture, comprised o (i) a conventional two electrode spark system (ii) a surface discarge spark (iii) a plasma jet (iv) a plasma jet with wetted cavity (v) a torch from a pre-chamber Hot kernel volume, flame front wrinkling and flame speed were measured during the early stages of flame development. Systems (ii) to (v) were potentially superior to (i) in their ability to create a larger igniting volume of hot, reactive gas. The flame speeds for systems (iii) to (v) were enhanced by jet-generated turbulence and, additionally, by active species in the case of the plasma jet. In turbulent mixtures, after a few milliseconds the flame speed is predominantly a function of the turbulence. Ignition limits have been measured over a range of r.m.s. turbulent velocities and different igniters compared. With limit mixtures, the ratio of chemical to eddy lifetime is a useful correlating parameter. Ignition limits for the different igniters fall within an upper bound for flame propagation defined by a fixed value of this ratio.