Conditions for a split diffusion flame

Abstract

An unusual phenomenon has been observed in a methane jet diffusion flame subjected to axial acoustic forcing. At specific excitation frequencies and amplitudes, the driven flame splits into a central jet and one or two side jets. The splitting is accompanied by a partial detachment of the flame from the nozzle exit, a shortening of the flame by a factor of 2, and a change from the common yellow color of soot radiation to a clear blue flame. Such a phenomenon may be useful for the control of soot production or product species. The splitting is intermittent in time, bifurcating between the split flame and an ordinary single jet diffusion flame. The experiment consists of an unconfined axisymmetric methane jet formed by a short length of 0.4 cm diameter pipe. The pipe is connected to a large plenum surrounding a bass reflex loudspeaker enclosure that provides the excitation. Conditions producing split and bifurcated flames are presented. The drive frequencies required to cause bifurcation correspond to the first two peaks in the system's frequency response curve. Bifurcating behavior was observed at a wide range of flow rates, ranging from very small flames of Reynolds number 240 up to turbulent lift-off, at Re = 1000, based on the inner pipe diameter. It was not sensitive to nozzle length, but the details of the nozzle tip, such as orifice or pipe geometry, can affect the frequency range.