Analysis of supersonic shear effects on flame characteristics of an evaporating flame-holder

Flame stabilization and its interaction with flow characteristics are crucial to the development of air-breathing propulsion engine. In this paper, experiments and numerical simulations have been conducted in both uniform and nonuniform inflowing streams to investigate supersonic shear effects on flame characteristics of an evaporating-flame holder. Specifically, supersonic shear is imposed by introducing a mixing layer between a subsonic stream of Mach number Ma=0.15 and a supersonic stream of Ma=1.55. The flames are imaged by high-speed photography with the flame boundary being identified. Three dimensional simulations with k-ε turbulence model, discrete phase model (DPM) for liquid droplets, and eddy-dissipation-concept (EDC) combustion model, are further performed to shed lights on the complex flow characteristics and interactions with flame shapes. Results show that the vortex structure and the size of recirculation zone after the flame holder are similar with and without shear implying that the presence of shear flow will not affect flame stabilization. The supersonic shear instead can modulate the flame shape and location within a combustor by shortening 30% of the overall flame length. Specifically, the hot products can be pulled away from the top wall and entrained towards the mixing layer. The flame shape is less sensitive to the increase of fuel supply due to the blockage of flame development from the recirculation zone in further downstream and the flame being entrained toward the mixing layer. Further flow and fuel/air mixing analysis shows that the outer shear layer of the recirculation zone after the flame holder plays an important role on flame stabilization. The flame front adjusts adaptively with the spatial distribution of the outer shear layer, which would be modulated by the presence of supersonic shear.