An experimental study on combustion characteristics of flames in a coaxial flow with a stagnation point

By using a stagnation-point coaxial flow generated by a lower coaxial burner and an upper quartz plate, an inner (or outer) premixed flame influenced by outer (or inner) oxygen content is experimentally developed to simulate and study double-flame burning structures modified by interactions of flamelets in turbulent combustible flows. In the experiments, fuel–air and oxygen–nitrogen mixtures are therefore introduced into outer (or inner) and inner (or outer) flows, respectively. This experimental arrangement allows either the inner flame or the outer flame to be located at different planes by separately adjusting the compositions and injection velocities of the inner and outer flows. An inner (or outer) planar premixed flame with a small outer (or inner) lifted tail or an inner (or outer) nonplanar premixed flame and an outer (or inner) trumpet-shaped diffusion flame can be developed in the flow field. The lifted tail and the trumpet-shaped diffusion flame are stabilized along the interface between the inner and outer jets in the coaxial flow. The inner (or outer) premixed flame influenced by the outer (or inner) oxygen content may experience transports of mass and thermal diffusion parallel to the flame surface. It endures the flow stretch tangent to the flame surface. Furthermore, in the flow field, the directions of flow convection for both inner and outer flows are the same (both divergent). The combustion characteristics, including extinction, blow off, flashback, the transition from the flat flame to the hat-shaped flame, and the ignition and development of diffusion flame are reported and discussed. Finally, the measurements of flame shape and temperature distribution are involved.