Simulation of Cup-Burner Flames in Microgravity

The extinction process of cup-burner flames under normal-gravity conditions were previously studied. As the low-speed diffusion flames behave differently in microgravity compared to those on earth, it is important to understand the structure of cup-burner flame and its extinction characteristics under 0g conditions. A numerical study was performed in the present paper using a time-dependent, axisymmetric model and by incorporating detailed chemical kinetics of CH4 and O2. Calculations 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 predominantly 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 loss. Introduction A fire, whether within a spacecraft or in an occupied space on extraterrestrial bases can lead to mission termination or loss of life. The advent of longer duration missions to the moon, Mars, or aboard the space stations increases the likelihood of fire mishaps. Therefore, development of efficient fire safety systems and procedures for the space-oriented living represents a mission-critical task. This requires an understanding of the inhibition mechanisms of fire-suppressing and inert agents in microgravity flames. Experimental or numerical studies for the investigation of the inhibitory effects of halogenated hydrocarbons on flames have been performed in either premixed or diffusion systems. Premixed flames are selected mainly because the overall reaction rate, heat release, and heat and mass transport can be described with a fundamental parameter—the laminar burning velocity; on the other hand, most common fires are of the diffusion type and often become dynamic in nature with large vortical structures entraining additional surrounding air. The predominant experimental techniques for studying fire suppression in diffusion flames are the cup-burner and opposing-jet configurations. In both these experiments, agents are quasi-statically added to either the fuel or air stream. The opposed-jet configuration offers very simple flames that can be modeled using onedimensional analysis and, hence, is often used for the development of chemical kinetics models for different agents. From a fire safety point of view, however, the most hazardous situation is a low-strain-rate diffusion flame such as the one established over a cup burner where flames are more stable and larger concentrations of agent are required to achieve extinction. Studies on cup-burner flames are also important since the amount of agent required for extinguishing these flames is believed to scale to the requirements in common fires. Under normal gravitational conditions, a laminar jet diffusion flame formed over a cup burner with negligibly small fuel flow rate and a low-speed annular air flow develops large-scale, low-frequency (1-40 Hz), organized buoyancy-induced vortices on the air side of the flame. _____________________________ * Corresponding author: [email protected] Proceedings of the Third Joint Meeting of the U.S. Sections of The Combustion Institute