A study of flame propagation mechanisms in lycopodium dust clouds based ondust particles' behavior

Following our earlier study on the behavior of lycopodium dust flames, furt her experiments using a particle image velocimetry system with a high-resol ution video camera have been conducted to clarify the mechanisms of laminar dust flame propagation in a vertical duct. Lycopodium, a nearly equal-size d particle, has been recognized as being monodispersed, but it was found th at an actual lycopodium dust cloud consisted of individual and agglomerated particles. Corresponding to the particle forms, the reaction zone showed a double flame structure, consisting of enveloped diffusion flames (spot fla me) of individual particles and diffusion flames (independent flame) surrou nding some particles. Due to the convective flow caused by a flame, part of the gravitational settling particles was shifted to the surrounding sides and the rest of the particles changed their movements to upwards in front o f the flame. Such particle movement causes a dynamic variation in dust conc entration ahead of the flame, which propagates at lower dust concentration rather than the mean concentration. Although the flame moved discontinuousl y on a micro scale, an overall constant flame velocity was found, presumabl y due to the dynamic variation in dust concentration and induced flow ahead of the flame. Judging from the above-mentioned movement of single particle s in front of the flame, a residence time of the unburnt particle in the pr eheating zone is needed to form combustible gases close to the particle. Th is residence time depends on the preheating zone thickness, the particle ve locity and the flame propagation velocity. The observation of the movement of a single particle suggested a flame propagation mechanism where an envel oped and diffusion lycopodium dust flame discontinuously propagates from on e particle to those adjacent in a laminar suspension. (C) 2001 Elsevier Sci ence Ltd. All rights reserved.