A Numerical Study On The Extinction Characteristics Of Droplet Combustion Under Microgravity Conditions

Abstract

Droplet combustion is of interest from both fundamental and application viewpoints. The latter is related to combustor performance and fire safety issues in space flights. Influences of diluent in the atmosphere on the combustion characteristics and on extinction provides insight into fire extinguishment phenomena and effectiveness of various diluents as fire suppressants in low gravity environments. Extinction of methanol droplets for varying levels of ambient carbon-dioxide, helium and oxygen concentration, burning in a quiescent microgravity environment was studied numerically to determine the relative effects of these diluents on extinction on, as well as on the limiting oxygen index. The results show distinct regimes of diffusive and radiative extinction. The transition from diffusive to radiative extinction is strongly influenced by ambient carbon dioxide concentration. Results for helium as the diluent show an increased burning rate and a dominance of extinction due to diffusive heat loss. Simulations were also conducted for different initial water concentrations solvated within the methanol. An extinction characteristics curve is proposed utilizing predicted burning rate, flame standoff ratio and ambient diffusivity, which are the governing variables for the combustion process. The dependence of observables (burning rate, flame stand-off, radiative and diffusive extinction) on physical, thermochemical, and kinetic parameters is discussed.