Proceedings of the World Congress on Momentum, Heat and Mass Transfer (MHMT’16) Prague, Czech Republic – April 4 – 5, 2016 Paper No. CSP 125 DOI: 10.11159/csp16.125 CSP 125-1 Effect of Solid Fuel Particle Size on Burning Rate Using Computational Analysis Thomas Marino 1 , Vahid Motevalli 2 1 NSWC West Bethesda, MD, USA 2 Tennessee Technological University, College of Engineering 1010 N. Peachtree Ave., Clement Hall, Cookeville, TN USA thomas.marino@navy.mil; vmotevalli@tntech.edu Abstract - Aluminium as fuel using steam oxidant is an attractive energy source in combustion systems where high energy densities are desired. Most experimental and computational work has been done for combustion chamber operating at or near atmospheric pressures using oxidants of air or pure oxygen. Experimental studies conducted for particles ranging in size of 10-700 microns suggest that smaller particles may experience shorter combustion residence time. This computational analysis provides a detailed examination of metal-fuel combustion at pressures above atmospheric and shows that combustion performance of aluminium particles directly relate to the particle size and distribution. The computational model is applied to a linear-type dump combustor. The effects of a range of particle sizes are investigated using monodispersed and polydispersed particle distributions. Characterization of the combustion process is addressed by studying particle ignition delay, burn time, and particle emission as a function of particle diameter and mass fraction. The computational models include non-isotropic turbulence, empirically derived ignition and reaction rate criteria as well as convective and radiant heat transfer. The commercial computational fluid dynamics solver Fluent is used to perform these computations. Sub-models are added to the standard Fluent capability for reaction rate equations, oxide nucleation in the discrete and continuous phases, dissociation, particle emissivity, and particle drag. The numerical results indicate that polydispersed fuel can lead to better overall performance than monodispersed fuel with the same mean diameter at 20 microns and below. Ignition and burn times can become accelerated in distributions relative to the monodispersed sizes for the same mean diameters for the full range of sizes investigated. The optimum ignition time was determined to be at the 10-micron polydispersed distribution. Burn time can be optimized with a distribution mean of 10 microns, with diminishing increase and variability above a mean distribution of 20 microns, as well as very little increase in maximum burn time. Keywords: Metal combustion, particle size, numerical analysis, polydispersed 1. Introduction Combustion using aluminium particles as fuel is an attractive energy source where high energy densities are desired. The Office of Naval Research (ONR) has been investigating the potential of an engine that burns aluminium with oxygen from seawater [1]. A sea-breathing propulsion system does not need to carry its oxidizer with it, saving vehicle space and weight, and allowing it to more easily maintain the speeds and range needed to be an efficient platform. This seawater- breathing engine can be used in a high-speed torpedo and small, unmanned submarines. Applications can be extended for an engine that can breathe oxygen from air or CO2. Ideally the same engine could be developed to operate across platforms and operating environments, including terrestrial air breathing combat vehicles like the M1 Abrams Tank, subsonic stealth aircraft, and Mach 6 to 15 intercept missiles. An advanced technology demonstration (ONR Advanced Technology Demonstration 1999) proposed replacing the battery and diesel configuration on submarines. This technology is also being presented as a replacement for batteries, fuel cells, and Sterling Engines on small submarines and unmanned undersea vehicles [2 and 3]. An aluminium combustor could also be used to power a steam generator, which could be used in fuel cells for mining methane from a hydrate lattice and for cracking of the methane molecule. High-pressure combustion chambers are also desired to drive small turbines that can be used for power sources. Still in the conceptual stages are projects that would use aluminium-fuelled propulsion for a combination tank and submarine, as well as a combination airplane and submarine.