Occurrence probability of microexplosion in droplet combustion of miscible binary fuels

Abstract

Occurrences of microexplosion in droplet combustion of miscible fuel mixtures were studied. Experiments were performed using unsupported droplets of n-alkane/n-hexadecane mixtures injected into atmospheric air in normal gravity. It was found that the occurrence of microexplosion is stochastic and cannot be predicted by the classical criterion for microexplosion occurrence using the limit of superheat and the droplet temperature. An occurrence model for the microexplosion based on the homogeneous nucleation theory is presented and shows that the occurrence probability of microexplosion is, in general, controlled by the ratio of the liquid-phase lifetime to the nucleation time. For binary fuel mixtures whose constitutent fuels have very different volatilities, the occurrence probability of microexplosion is controlled by the ratio of the liquid-phase lifetime to the nucleation time during the quasi-steady vaporization period after the transition period. The nucleation time is inversely proportional to the nucleation rate and superheated liquid volume. The droplet temperature and the limit of superheat affect the occurrence probability through the nucleation rate. The model shows that the occurrence probability reaches a maximum at a certain initial concentration of the fuel mixture. The effects of the initial droplet diameter on the occurrence of microexplosion were also investigated. The occurrence probability was found to depend largely on the initial droplet diameter. It decreases with the decrease in the initial droplet diameter.