Effects of Damkoehler Number on Methane/Oxygen Tubular Combustion Diluted by N_2 and CO_2

摘要

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 oxidizerlfuel injection velocity ratio near unity. Based onflow visualization, the mixing process around injection slits and that in the axial downstream were discussed. The Damkoehler 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 Damkoehler 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 inflame stability between N_2 and CO_2 diluted combustion were also studied.