Effect of Chemistry and Transport Properties on Near-Limit Flames at Microgravity

Abstract

The role of chemical kinetics in the behavior of near-limit flames at rnicrogravity (μg) was investigated experimentally by comparison of the properties of these flames in mixtures having varying chemistry but similar diffusive transport properties. Conversely, the importance of unequal rates of diffusion of thermal energy and mass was evaluated by comparison of flames in mixtures having similar chemistry but varying Lewis numbers, It was found that the characteristics of near-limit flames at μg were affected much more by Lewis number effects than chemistry. It is concluded that there are at least three regimes of near-limit behavior at μg, depending upon the Lewis number (Le): (1) cellular flames, which are highly resistant to extinguishment, for mixtures with Le≪l, (2) self-extinguishing flames and well-defined flammability limits for mixtures with Le≃ 1, and (3) ignition energy limited flame propagation for Le ≪ 1, Gradual rather than abrupt transitions between regimes were observed.

Keywords:

• Flammability limits
• ignition limits
• microgravity
• chemical kinetics
• transport processes
• flame instabilities