Numerical investigation of sodium droplet ignition in the atmospheric air

Abstract

The ignition behaviour of sodium droplets in the atmospheric air has been studied numerically with two available models of pre-ignition stage combustion. Surface reaction is very important in the pre-ignition stage, and the different reaction rate-controlling processes involved in this stage are explained using the shrinking core model. The droplet ignition behaviour is studied by considering the energy balance at the droplet surface in terms of rate of heat generation from the surface oxidation reaction and rate of heat loss to the ambient air. Ignition delay times are evaluated numerically using the two pre-ignition models with different ranges of values to the main parameters that can affect the ignition behaviour of the sodium droplets. Based on these results, the relative capability of the models has been brought out in predicting the droplet ignition behaviour, so that the better model could be chosen for the sodium spray fire analysis code being developed. Analysis results show that the reaction kinetics limited pre-ignition model predicts the limit of ignitability for sodium droplets under different initial and convective conditions, whereas the mass transfer limited pre-ignition model predicts this only under very low oxygen concentrations.