Flame speed in a binary suspension of solid fuel particles

Abstract

Most natural and industrial combustible dusts have a wide distribution of particle sizes. Yet, the majority of experimental data on flame propagation in dust clouds are given in relation to some average particle size, and all known theoretical models of dust combustion consider only monosize suspensions. Since the ignition temperature and combustion rate of an individual dust particle are functions of particle size, the flame in real dust suspensions has a complex, multistage structure. As a first step toward understanding multisize dust combustion, the combustion of a suspension of two monosize powders (that in general can be also of different chemical nature) is investigated in the present work theoretically and experimentally. A simple analytical model developed for the flame in a fuel-lean binary suspension permits flame speed and structure to be analyzed as a function of the dust composition and combustion properties of individual particles. The flame speeds predicted by the binary model were compared with flame speeds calculated from a model of monosize dust flame using various average particle size representations. It is shown that averaging of the particle size in general fails to correctly predict the flame speed over the wide range of the binary dust compositions. The flame propagation speed in a binary suspension of aluminum and manganese powders was investigated experimentally by observing the laminar stage of flame propagation in a semi-open vertical tube. The model correctly predicts dependence of the flame speed on mixture composition (mass ratio of manganese and aluminum dusts in suspension) and the mixture composition at the limit of flame propagation.