Ignition of Aerosolized Reactive Particles at High Heating Rates
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Ignition of Aerosolized Reactive Particles at High Heating Rates Salil Mohan, Yuriy L. Shoshin, and Edward L. Dreizin Department of Mechanical Engineering New Jersey Institute of Technology Newark, NJ 07102, U.S.A. ABSTRACT This paper presents an experimental methodology, respective heat transfer model, and initial results describing ignition of rapidly heated, aerosolized powders of different materials. The experimental setup uses a CO2 laser as a heat source. The interaction of the laser beam with particles is particle size-dependent and only a narrow range of particle sizes is heated effectively. Therefore, the heat transfer model needs to be only analyzed for the particles with this specific size, which greatly simplifies the interpretation of experiments. The powder is aerosolized inside a plate capacitor by charging particles contacting the capacitor’s electrodes. A thin, laminar aerosol jet is carried out by an oxidizing gas through a small opening in the top electrode and is fed into a laser beam. The velocities of particles in the jet are about 1 m/s. The laser power is increased until the particles are observed to ignite. The ignition is detected optically. The ignition thresholds for spherical magnesium and aluminum powders were measured. The experimental data for magnesium, for which ignition kinetics is well known, were used to calibrate the detailed heat transfer model. The model was used to evaluate the ignition kinetics for aluminum powder. INTRODUCTION Ignition temperature of reactive materials has been classically understood in terms of Semenov’s thermal theory as the minimum environment temperature which leads to selfsustaining combustion of an inserted particle [1, 2]. This approach has been successfully used for applications dealing with fire safety or ignition of solid fuels in large furnaces where the heating rates are characteristically low [3, 4]. However, this approach becomes inadequate for applications in which the particles are heated rapidly, and the particle temperature can exceed the classically defined ignition temperature before the self-sustaining combustion is established. Related applications include ignition and combustion of reactive materials in explosives, propellants and pyrotechnics. To describe ignition for such application, one needs to analyze the specific transient heat transfer problem in which one or more of exothermic processes leading to the particle ignition are considered. Therefore, quantitative description of such, typically thermally controlled processes, is necessary for various energetic materials. The low-rate kinetics of exothermic reactions in related energetic materials are commonly characterized by thermal analysis, where the heating rates are in the range of 1 – 50 K/min. The extrapolation of the identified kinetics to the high heating rates is difficult and requires direct experimental verification. This difficulty led to development of new experimental approaches to directly characterize ignition kinetics for the heating rates in the ra
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