Structure of Lean Turbulent Partially-Premixed Flames Stabilized in a Co-Axial Jet Flame Burner

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Lean premixed flames at very low equivalence ratios and high mean flow velocities can besustained in a co-axial jet flame burner. The flame shape involves a thin, highly turbulent flame region where most of the lean combustion occurs, and a base flame region which burns at the initial equivalence ratio. Due to the entrainment velocity induced by the high-speed inner jet flow, the flame also has a thin "pinched" region near the flame base. The entrainment of the hot product gas and mixing with the reactant, leading to preheating, along with the fact that the turbulence intensity increases gradually in the presence of the inner jet, are believed to be contributing factors in the stabilization of the flames in the current burner geometry. Addition of the inner jet of always lowers the overall equivalence ratio and enhances the stability limits. The minimum overall equivalence ratio achieved is 0.56 with a mean velocity of 60 m/s for the inner jet. This capability to generate lean premixed flames at high turbulence intensities and mean flow velocities is expected to have useful practical applications. Temperature measurements have been obtained using a point Rayleigh scattering technique, and the temperature profiles show a significant reduction in the flame temperature as the overall equivalence ratio is reduced using the inner jet air. The temperature pdf's show a shift of the cold reactant temperature to a higher value, indicating preheating of the reactant due to the entrainment of hot product gas that surrounds the lean flame. Also, there can be significant departure from flamelet behavior, associated with a bimodal temperature pdf, when the inner jet velocity becomes high. The temperature pdf's are no longer bi-modal at high turbulence intensities caused by the strong inner jet, with temperature pdf's showing a broadened temperature range in the flame zone.