Influence of Gravity and Ambient Pressure on Combustion and Flammability of n-Heptane and 1-Propanol Droplets in Air-Diluent Environments

Abstract

Reduced gravity and normal gravity combustion experiments were performed with individual fiber-supported n-heptane and 1-propanol droplets with initial diameters in the 1-mm size range. Experiments were performed with air-inert mixtures at 0.1 MPa or 0.3 MPa and about 298 K, where helium, carbon dioxide, or xenon were separately used as inerts. The amount of inert gas required to suppress combustion was generally higher in reduced gravity than normal gravity, and higher mole fractions of xenon were required for suppression of combustion relative to helium or carbon dioxide. Helium was found to perform the best as a flame suppressant on a mass basis, i.e., mass fractions of helium required to suppress combustion were significantly smaller than for the other inert gases. Increasing the amount of inert gas generally decreased burning rates. Results indicate that ambient gas transport properties and the fuel type play an important role in determining flammability and combustion behaviors of the droplets. Comparison with cup burner results also show that gas transport properties influence flammabilities much more strongly for droplets than for cup burners.