Johnston, Howard Gregory (2015-12). Experimental Effects of Coal-Limestone Mixtures on Dust Dispersion Behind a Moving Shock Wave. Master's Thesis.
Secondary dust explosions in coal mines or industrial settings are known to cause greater catastrophic hazards than the coupled primary explosions themselves. The shock waves produced during a primary explosion, which are initiated by inadvertent stimuli in an explosive atmosphere such as methane, lift surrounding coal particles from neighboring areas, and if added in an effort to create an inert mixture, limestone as well. Dust dispersion can influence the severity of a secondary explosion as the particles can ignite from passing shock waves. A shock tube modified to evaluate dust dispersion provides the optical access to characterize the shock-wave/dust-layer interaction. This experimental study characterized the dust dispersion of coal-limestone mixtures and moisture-varied limestone dust as it is likely present in the hazardous environment. The dust rise height was measured with respect to time after the shock passage, where regardless of the sample, initial laminar dust growth rates increased with Mach number. Laminar and unstable regimes were also identified in the data samples. The moisture-varied limestone samples were tested at three shock Mach numbers, namely Ms = 1.1, 1.23, and 1.4, and the trending data show an average increase of 10% in overall lifting heights and 20% in initial linear growth rates for the moisture-reduced, dried samples as compared to undried samples stored in standard temperature and pressure (STP) conditions. Conceivably, the effective moisture reduction in the samples led to fewer agglomerations and/or reduced densities, influencing the ability of lift forces to act on the particles. In addition, limestone may bond or agglomerate more readily to coal particles when undried, reducing the likelihood of ignition. The coal-limestone mixture samples were tested at two shock Mach numbers, namely Ms = 1.24 and 1.57, with the 75% coal sample having the largest and 25% coal sample having the smallest combined dust dispersion parameters. Dust dispersion parameters affect how quickly the dust transitions to an unstable interface layer, readily increasing the chances of ignition through increased mixing and dispersion. As the limestone content is increased, dust grows faster, larger, and tend to transition into larger instabilities on the dust-gas boundaries. The highest dust growth rate, shortest transition time, and largest average dust height will affect the ability of the dust sample to ignite. Increasing undried limestone content while maintaining the lowest possible dust dispersion would both help the mixture remain inert and expose less coal particle surface area to the reactive atmosphere. These parameters are fluid-particle dispersion dynamics.