Observations on burning and flame-spread of black powder

Abstract

Results are reported concerning some experimental observations on the combustion of particles of black powder and some theoretical considerations on the process of flame spread through porous matrices of the material. The experimental work involved high-speed cinemicrography and scanning electron micrography of particles ignitied in air by a pulsed laser, by a spark, and by a hot wire. The theoretical work involved definition of two flame-spread models, a random walk of hot particles through the matrix producing ignition and multiplying in number at each stop, and a gas-like motion of the reacting region through the bed. The experiments demonstrate emission of hot high-velocity particles from a liquid layer on the surface of the burning propellant. They also identify extinctions subsequent to the short ignition stimuli and slow spread of flame along continuous surfaces. The theory produces formulas for the order of magnitude of the spread time and for its functional dependence on the void fraction of the matrix, the surface to volume ratio of the particles, and other design parameters. It is emphasized that many uncertainties remain, and that further experimental and theoretical work is needed.