Extinguishment of methane diffusion flames by carbon dioxide in coflow air and oxygen-enriched microgravity environments

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• 作者：Fumiaki Takahashi ; Gregory T. Linteris ; Viswanath R. Katta
• 关键词：Diffusion flame stabilization ; Spacecraft fire suppression ; Carbon dioxide ; Reaction kernel ; Microgravity
• 刊名：Combustion and Flame
• 出版年：2008
• 出版时间：October 2008
• 年：2008
• 卷：155
• 期：1-2
• 页码：37-53
• 全文大小：3099 K

文摘

Microgravity experiments and computations have been conducted to elucidate stabilization and extinguishment mechanisms of methane diffusion flames, in the cup-burner configuration, with CO₂ added gradually to a coflowing air or oxygen-enriched stream. The minimum extinguishing concentration of CO₂ under low oxidizer velocities (<20 cm/s) was measured in microgravity achieved by parabolic flights of the NASA Reduced Gravity Aircraft. Transient computations with full chemistry and a gray-gas radiation model were performed to reveal the detailed flame structure and extinguishment processes. To compensate for the overestimation of radiative heat losses at high concentrations of radiating CO₂, the Planck mean absorption coefficient was multiplied by a correction factor (0C1). The fuel-lean peak reactivity spot (the so-called reaction kernel) at the flame base stabilized the trailing diffusion flame. The calculated temperature along the trailing flame decreased downstream due to radiative cooling, leading to local extinction at <1300 K and flame tip opening. As CO₂ was added to the oxidizer: (1) the calculated maximum flame temperature decreased toward a threshold (≈1600 K); (2) the reaction kernel weakened (i.e., lower heat release rate) but nonetheless remained at a nearly constant temperature (≈1450 K); (3) the flame base stabilized increasingly higher above the burner rim, parallel to the axis; until finally, (4) blowoff-type extinguishment occurred. In the lifted flame, the broadened reaction kernel supported a super-lean reaction branch on the oxidizer side as well as the trailing diffusion flame on the fuel-rich side (no triple flame structure was formed).