Tip Opening of Burner-Stabilized Flames

The tip-opening mechanism of burner-stabilized flames is investigated computationally using premixed propane + air mixtures. The temperature, net production rate, and reaction rates are investigated for rich mixtures. The flame tip structure was analyzed on the basis of reaction rates to understand the conditions of the equivalence ratio at which the tip-opening phenomenon occurs. Numerical predictions of tip opening are in good agreement with experimental observations. The study revealed that the tip-opening phenomenon starts at phi = 1.4. As the mixture becomes rich, the tip opening was found to increase. When the flame tip opens, the volumetric heat release rate at the tip was found to be less than 50% of the heat release rate at the flame shoulder. An increase in the flame tip thickness was observed around 30% from equivalence ratios of 1.3-1.4. The effect of the temperature on the propane burner flame structure is studied by performing simulations at three different mixture inlet temperatures of 300, 350, and 400 K. When the temperature of the unburnt gas mixture increases, the propane-air tube burner flame tip opening begins at more fuel-rich conditions compared to that of the mixture at ambient temperature. A detailed sensitivity analysis was carried out to identify the reactions having large sensitivities.