Salil Mohan

Sizing and burn time measurements of micron-sized metal powders

Detailed ignition and combustion mechanisms are needed to develop optimized propellant and energetic formulations using micron-sized metal powders, such as aluminum. Combustion researchers have traditionally used relatively coarse metal particles to characterize the burn time dependence on particle size. However, measurements of burn times for particles below 10 microm in diameter are still needed for aluminum powders and other metal fuels. The apparatus described here sizes the particles just before the ignition event, providing a direct correlation between individual particle size and its burn time. Two lasers were utilized: a 785 nm laser diode for sizing the particles and a 125 W CO2 laser for particle ignition. The particles crossed the 785 nm laser beam just before crossing the CO2 laser beam. The particle size was determined from the amplitude of the scattered 785 nm light pulse. The burn time was determined from the duration of the visible light emission produced from the ignited particle. The in situ measured particle size distributions compared well with the size distributions measured for the same powders by a commercial instrument using low angle laser light scattering. Our measurements with two nominally spherical aluminum powders, suggest that the burn times increase from 0.5 to approximately 2.5 ms as the particle diameters increase from 3 to 8 microm.

Aluminum Particle Ignition in Mixed Environments

An experimental study of aluminum particle ignition in different oxidizing environments is reported. Aluminum particles in the µm size range are ignited by a CO 2 laser in air, water vapor, carbon dioxide and mixtures of thereof. The particles are passing through the laser beam of fixed diameter at different velocities enabling experiments at varied heating rates and thus enabling one to determine the ignition kinetics. Ignition and combustion events are distinguished optically. Experiments are interpreted considering heat balance for a metal particle in a cold oxidizing environment. Experiments and estimates both show that achievement of a vapor phase flame around single Al particle is difficult in water vapor, when the adiabatic flame temperature is substantially lower than that in oxygenated environments. However, if the flame is established, the rate of reaction of aluminum in water is higher than that in other environments. Approximate Arrhenius-type descriptions for aluminum ignition in different oxidizing environments are proposed suitable for processes involving high heating rates of the order of 10 6 -10 7 K/s. Aluminum powder is widely used as a fuel additive in solid propellants, explosives, and pyrotechnics. Ignition and combustion of aluminum particles have been extensively studied in the past but many of the related processes are not understood sufficiently well to enable their quantitative modeling. Currently, research of aluminum ignition and combustion in various configurations is very active involving both experimental 1 - 4 and modeling 5 – 7 efforts. Quantitative description of particle ignition processes is of specific importance for the practical applications, in which such processes determine ignition delays and bulk burn rates for aluminum. An ignition model for aluminum particle in oxygen was suggested based on detailed thermo gravimetric (TG) studies of aluminum powders oxidation. 6 Oxidation was established to occur in several steps, including growth of the initial amorphous oxide layer, a phase change from the amorphous to γ-Al 2 O 3 polymorph accompanied with an increase in the oxide density and formation of discontinuities in a thin alumina scale, growth of γ-Al 2 O 3 and its transformations into θ- and later α-Al 2 O 3 polymorphs. Each alumina polymorph presents a specific diffusion resistance and thus is oxidized at a specific rate. The polymorphic phase transition result in stepwise changes in the oxidation rate. The rates of mass transfer processes accompanying oxidation of different alumina polymorphs and occurring polymorphic phase changes in alumina were quantified based on the TG measurements. 2 Combining the quantitative description of heterogeneous oxidation processes with the heat transfer analysis for aluminum particles introduced in a hot gas environment or heated by another source (e.g., laser beam) enables one to predict the ignition delay as a function of the particle size and external conditions. The model was validated experimentally for the aluminum particles rapidly heated and ignited in the CO 2 laser beam. 8 However, in many practical applications oxygen is not the primary oxidizer available for the ignition of aluminum powders. Instead, ignition occurs in CO 2 and H 2 O environments. 9, 10 This paper deals with experimental study of ignition of aluminum particles heated rapidly in well-controlled environments with H 2 O and CO 2 being the primary oxidizers. The laser ignition experimental methodology is similar to that for ignition experiments in air. 8 The experimental setup is modified to enable studies of aluminum ignition in water vapor, carbon dioxide, and mixed oxidizers.