Effects of Damköhler Number on Methane/Oxygen Tubular Combustion Diluted by N₂ and CO₂

To fundamentally elucidate the mixing and its effects on the characteristics of methane/oxygen flame in a rapidly mixed tubular flame burner, experiments were conducted under various oxygen mole fractions and flow rates. Two inert gases of nitrogen and carbon dioxide were used, respectively. The inert gas was added to both the oxidizer and fuel slits to maintain the oxidizer/fuel injection velocity ratio near unity. Based on flow visualization, the mixing process around injection slits and that in the axial downstream were discussed. The Damköhler number (D_a1), defined as the ratio of molecular mixing time to reaction time, was selected as a parameter to quantitatively examine the criterion for the establishment of tubular flame from low to ultrahigh oxygen mole fractions (0.21-0.86). The mixing around slit exit determined the tubular flame establishment. Due to a flow time between two neighboring injection slits of fuel and oxidizer, part of the fuel was mixed in the downstream swirling flow, resulting in luminous helical structures. Hence, the Damköhler number (D_a2), defined as the flow to the reaction time ratio, was examined. Detailed observations indicated that when D_a2 was smaller than unity, the flame was uniform in luminosity, whereas the flame was nonuniform when D_a2 ≥ 1. The value of D_a2 was about 1.5 times as D_a1; however, they correspond to different mixing zones and D_a2 can be more easily calculated. The differences in flame stability between N₂ and CO₂ diluted combustion were also studied.