The Discrete Regime of Flame Propagation in Metal Particulate Clouds

Flame propagation in lean suspensions of nonvolatile fuels (aluminum and iron) in oxidizing atmospheres is experimentally investigated. Suspensions of micron-sized fuel particles were dispersed in glass tubes using a flow of oxygen/argon mixture, and flame propagation into a quiescent mixture was observed via a variety of techniques. An independence of flame speed on oxygen concentration (from 15% to 30% oxygen in argon) was observed for the suspensions of aluminum particulates. This result is found to be consistent with theoretical estimations of the relative magnitudes of the particle combustion time and the inter-particle heat diffusion time, suggesting that flame propagation through the aluminum suspensions occurs in a regime controlled by the spatial discreteness of the multiphase medium. Experiments with iron exhibited the expected square root dependence of flame speed on oxygen concentration (from 15% to 60% oxygen in argon), demonstrating that propagation in this mixture is in the classical, continuum regime of thermal flames. The aluminum mixtures also exhibited an array of flame instabilities, including a pulsating oscillation that is theoretically predicted for mixtures with a high Lewis number, although acoustic interactions with the flame tube could not be ruled out in influencing the pulsations. The result that lean aluminum flames propagate in a discrete regime suggests that the statistical nature of the particulate suspensions will have a significant influence on flame propagation, giving rise to percolation-like behavior seen in previous computational simulations of reactive waves in discrete media.