Effects of leading edge entrainment on the double flame structure in lifted ethanol spray flames

Abstract

OH planar laser-induced fluorescence and smoke visualization have been performed in the near field of a turbulent ethanol spray flame to investigate reaction zone structure and the effects of air entrainment on combustion. Annular air co-flow surrounds an axisymmetric spray injector utilizing a pressure-swirl atomizer to supply a hollow cone fuel spray. OH fluorescence demarcates reaction zone contours while smoke visualization indicates ambient air entrainment and turbulent mixing within the spray. A double flame structure is observed, appearing as two diverging flame fronts originating at the stabilization point, and consists of an outer diffusion flame with an inner structure that transitions from mixing controlled to partially premixed combustion downstream of the leading edge. Without co-flow, the inner branch of the double structure burns intermittently with large regions of local extinction often observed, resulting from a high droplet flux and possibly high strain/scalar dissipation rates. Addition of 0.29 m/s co-flow lifts the flame base enough to increase air entrainment and enhance inner zone combustion. The inner zone burns continuously, with no apparent local extinction, due to turbulent mixing between entrained oxidizer and fuel vapor generated by easily vaporized droplets present in the recirculations along the shear layer. The polydisperse spray distribution yields larger droplets which are able to cross the inner reaction zone and then vaporize in the hot region bounded by the double flame structure. This region serves as a fuel source to feed both the stable outer diffusion flame and the diffusive structures of the inner zone. In both cases, the flame leading edge stabilizes in the low-speed flow just outside the periphery of the spray cone, where flame propagation against the incoming flow is possible.