Combustion of Aluminum-Based Slurry Agglomerates

Abstract

The disperse, structural, and electrophysical characteristics of fine alumina produced by combustion of metal droplet agglomerates were studied experimentally. Data were obtained by transmission electron microscopy and video recording of aerosol particles moving in a homogeneous electric field. The aerosol particles are aggregates with sizes ranging from a fraction of a micrometer to a few micrometers and a fractal dimension of 1.60 ± 0.04 which consist of primary particles with sizes of a few to hundred nanometers. Most of the aggregates have electric charges, both positive and negative. The characteristic charge of the aggregates is equal to a few units of elementary charge. Some large aggregates rotate when the electric field polarity changes, i.e., they are dipoles. According to current concepts [1, 2], fine alumina is formed in the combustion of liquid aluminum drops by "chemical" condensation in the zone of diffusion mi-croflame around the particle. The microflame is at a distance of the order of the particle size from the particle surface. Further coagulation growth of oxide particles occurs in the "smoke tail" formed by particle motion in the carrying gas flow. The size distribution and morphology of the aerosol particle microaggregates formed by condensation-coagulation growth can depend appreciably on the magnitude of the electric charges gained by primary particles during the physicochemical stages of combustion [3]. In the literature, little experimental information can be found on the charge (electrophysical) and morphological properties of fine alumina particles formed during combustion of aluminum droplets [4]. The present work is an attempt to fill this gap. Alumina aerosol was produced by combustion of aluminized solid propellant samples. The sample in the form of a parallelepiped 20-25 mm in length with a section of 1 × 1.5 mm was burnt at atmospheric pressure in a twenty-liter container in air filtered from aerosols. The mass of aluminum in the samples was ≈6 mg. The combustion surface of the sample generated several tens of aluminum droplet agglomerates with sizes of 100-500 µm, which burnt up while falling in the container within tenths of a second. This resulted in fine alumina aerosol accumulated in the container. Aerosol was sampled 6 min after burning, and then, its disperse , morphological, and charge characteristics were analyzed. The grain-size composition and morphology of aerosol particles were studied by transmission electron microscopy (JEM-100SX). Samples for electron mi-croscopy were collected by a thermal precipitator. To observe the motion, coagulation, and behavior of the aerosol in an external electric field, we used a facility for video recording in real time (25 frames per second), which consisted of the following basic units. 734

Metal Slurry Droplet and Spray Combustion

“…This matches very well with the reported experimental observations. Turns et al [98] report final sizes of 60 to 80%. Salita [83] estimates the diameter of the residual aluminum oxide at about 70%.…”

“…From single particle tests, it is known that additional oxide is collected at the burning aluminum droplet [98,83]. A vapor phase diffusion flame results, and the surface oxidation of the aluminum gets inhibited due to the lack of oxidizer available at the droplet surface.…”

“…It is also noted that an effect of the oxide condensation on the particle surface is that, even as the aluminum is continuously depleted from the droplet, liquid oxide accumulates on the particle, and the particle radius does not go to zero upon the completion of aluminum vaporization. Turns et al [98) have experimentally obscrved final particle sizes of 60% to 80% of the initial aluminum particle size. At present, an empirical correlation (Hermsen [43]) continues to be widely employed to predict aluminum particle burning rate in solid rocket motors (Sabnis et al [82]).…”

“…Petela [69] constructs a simple mathematical model for combustion of coal-oil droplets. Turns and coworkers [98,111], experimentally and analytically studied the ignition of aluminum slurry droplets and the vapor phase combustion of aluminum. Individual droplets were supported on silicon carbide filaments, and exposed to oxygen mole fractions ranging from 0.10 to 0.25 and gas temperatures from 1250 to 1800 K, at one atmospheric pressure.…”

“…Combustion of metal-based slurries has been studied [5,6,7,68,95,98,111,112] for potential development of high energy density fuels for air breathing propulsion systems as ramjets. Additionally, coal-based slurries have been studied [32,84,93,94] for burning coal in liquid fueled combustors, and calcium-based slurries for reducing SO 2 emissions [63] from coal fueled power plants.…”