Smoldering spot ignition of natural fuels by a hot metal particle

Abstract

The spotting ignition of combustible material by hot metal particles is an important pathway by which wildland and urban spot fires are started. Upon impact with a fuel, such as dry grass, duff, or saw dust, these particles can initiate spot fires by direct flaming or smoldering can transition to flaming. In spite of the relative frequency that fires are initiated by hot metal particles, there is little work published that addresses the ignition capabilities of hot metal particles landing on natural fuels, especially regarding smoldering ignition. This work is an experimental and analytical study of how the flaming and smoldering propensities of powdered natural fuel beds in contact with hot metal particles are affected by differences in the particle characteristics, particularly the effect of particle melting, which adds energy to the particle. In the experiments, stainless steel and aluminum particles ranging in size from 1.6 to 8 mm in diameter are heated to various temperatures between 500 and 1100°C and dropped onto a fuel bed composed of a powder grass blend. It is observed that the ignition boundary both for flaming and smoldering follows a hyperbolic relationship between particle size and temperature, with smaller particles requiring higher temperatures to ignite the fuel. For both metal particles smoldering ignition occurs at significantly lower temperatures than flaming ignition. A simplified numerical model is developed to help understand smoldering ignition by a metal particle and to examine how the melting influences the ignition process. Good qualitative agreement is obtained between the model predictions and the experiments suggesting that the model provides a first step toward the theoretical modeling of this complex problem.