Experimental methodology and heat transfer model for identification of ignition kinetics of powdered fuels Trent S. Ward, Mikhaylo A. Trunov, Mirko Schoenitz, Edward L. Dreizin * Department of Mechanical Engineering, New Jersey Institute of Technology, University Heights, Newark, NJ 07102-1982, United States Received 9 November 2005 Available online 5 September 2006 Abstract A methodology for investigating and quantifying the thermal processes leading to ignition of rapidly heated metal powders was devel- oped. The simple experiment involves observing ignition of a powder coated on the surface of an electrically heated filament and is well suited for a variety of powdered fuels. In an experimental case study, the ignition temperature of spherical Mg powder was detected opti- cally at different heating rates. To interpret the results, a heat transfer model was developed for a multilayer powder coating on the heated cylindrical filament. The thermal contact resistance between particles was determined from the measured bulk thermal diffusivity of the powder considering the experimental particle size distribution. An Arrhenius type expression was used to describe the exothermic chemical processes leading to ignition with the pre-exponent as an adjustable parameter. For Mg, a pre-exponent value identified by matching the calculations with the experimental data was found to be 10 10 kg/m 2 s. The match between the experimental and predicted temperatures and times of ignition was good for different heating rates, which validated the proposed heat transfer model and indicated that the developed methodology is practically useful. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Ignition temperature; Ignition kinetics; Metal powder; Reactive materials 1. Introduction Metal based fuels are widely used in propellants, explo- sives, and pyrotechnics because of their high combustion enthalpies. The most significant limitation of metal fuels is associated with the relatively low overall reaction rates. Specifically, reducing the ignition delay time is important, which can be defined as the period when a metal particle is introduced in the combustion system and represents a heat sink rather than a heat source. A number of advanced reactive materials are being developed to address this chal- lenge. Advanced models are also under development to describe combustion of such materials, often powdered fuels, in various experimental configurations. Analysis of reaction kinetics is often associated with a concept of igni- tion temperature. This temperature is most commonly defined as the lowest temperature of the environment at which the particle would self-heat and start burning [1,2]. This definition is adequate for slow particle heating, e.g., in experiments where the minimum temperature at which ignition occurs is determined by introducing a particle in an environment with gradually increasing temperature. In propellants and explosives, however, an initially cold fuel particle is introduced into a high-temperature combustion chamber or a fireball so that the particle is heated very rap- idly. The particle temperature can exceed the classically defined ignition temperature well before the particle can be considered a heat source, suggesting that a different description of the ignition process would be more useful for such applications. Following the rationale for introduc- ing the ignition delay, the ignition can be assumed to occur when the particle becomes a heat source rather than a heat sink. Typically, a particle will become a heat source at increasing temperatures with increasing heating rates. This 0017-9310/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2006.05.025 * Corresponding author. Tel.: +1 973 596 5751; fax: +1 973 642 4282. E-mail address: dreizin@njit.edu (E.L. Dreizin). www.elsevier.com/locate/ijhmt International Journal of Heat and Mass Transfer 49 (2006) 4943–4954