Extinction Characteristics of Cup-Burner Flames in Microgravity

Abstract

Carbon dioxide extinguishes flames through dilution process. The extinction characteristics of CO 2 were previously studied using a cup -burner flame under normal-gravity conditions. As the diffusion flames behave differently in microgravity compared to those on earth, it is important to understand the structure of cupburner flame and the extinction characteristics of CO 2 for 0g conditions. A numerical study was performed in the present paper using a time-dependent, axisymmetric mathematical model and by incorporating detailed chemical kinetics of CH 4 and O2. Calcula tions were performed for the cup-burner flame under different gravitational forces. It was observed that the cup-burner flame ceases to flicker under gravitational forces less than 0.5g. As the buoyancy force was reduced, the flame diameter increased, the tip of the flame opened, and the flame at the base became vertical. Through numerical experiments it was found that radiative heat loss was solely responsible for the extinction of flame in the tip region under 0g conditions. In contrast, 1g flames were not affected much by the radiative heat losses. Calculations were made by adding CO2 to air stream to obtain the limiting volume fraction of CO2 for extinguishing the 0g flame. Similar to that observed in 1g flames, addition of CO2 destabilized the flame base, which then moved downstream in search of a new stabilization location. For CO2 volume fractions greater than 19.1 %, the flame base moved out of the computational area, as it could not find a stabilization point within the domain. This limiting concentration for 0g flame is ~ 32% higher than that obtained for the same flame under normal-gravity conditions. Calculations made by ignoring radiation for the limiting flame under 0g conditions yielded a stable flame. This study suggested that it is important to consider radiation while