Smoldering combustion in horizontal dust layers

Abstract

It has been shown by experimental studies and accidental occurrences in industrial facilities that a layer of organic dust can, under certain conditions, undergo sustained smoldering combustion. The purpose of this investigation was to examine the combustion, propagation, and ignition properties of horizontal layers of grain, grain dust, and wood dust. Combustion wave velocities have been measured and are presented as a function of average combustion wave depth, ignition source depth, and type of material tested. Tests have been made with and without a forced horizontal airflow over the layer surface. The effect of the airflow has been shown to be an increase in the combustion wave velocity and, under certain conditions, the possibility of transition from smoldering to flaming combustion. Minimum ignition energies as a function of power per unit area dissipated by the ignition source have been measured for three materials when ignited at a depth of 1.27 cm below the layer surface. Ignition in dust layers has been found to exhibit a statistical nature within certain ranges of power per unit area and time of dissipation, where only a fraction of the tests conducted under the same conditions were found to yield a positive ignition. A numerical solution of the conservation equations which describe a one-dimensional smoldering combustion wave has been obtained. This analysis gave good predictions of the combustion wave velocity and also yielded temperature and density profiles within the combustion wave having features which agree with experimental values. Kinetic constants of grain dust have also been estimated by using the model in conjunction with the experimental results.