Yuriy L. Shoshin

Preparation and characterization of energetic Al-Mg mechanical alloy powders

Abstract Metals such as Al and Mg have high combustion enthalpies and they are widely used as additives in energetic materials for propellants, explosives, and pyrotechnics. However, long ignition delays and slow combustion kinetics limit their current applications. An approach suggested in this work is to design new metal-based materials in which pre-determined phase changes will occur and trigger ignition at a desired temperature and also accelerate the rate of heat release during combustion. As a first step, metastable solid solutions of Mg in Al (10–50% of Mg) have been produced by mechanical alloying. The ignition temperatures of the produced alloys in air were determined using digital imaging and three-color pyrometry of the electrically heated filaments coated with different alloy powders. Combustion of mechanical alloys in air was studied using a laminar, premixed flame aerosol burner. The ignition temperatures were around 1,000 K, much lower than the pure aluminum ignition temperature of about 2,300 K. The steady flames of mechanical alloy powders were produced at lower equivalence ratios and had higher propagation velocities than similar pure aluminum powder flames. Phase compositions of the combustion products were determined using X-ray diffraction. In addition to Al 2 O 3 and MgO, significant amounts of Al 2 MgO 4 were found in experiments.

Particle combustion rates in premixed flames of polydisperse metal—air aerosols

Abstract A new approach and experimental technique are proposed to determine times of metal particle combustion in flames of polydisperse aerosols. Laminar flames are produced in air at 1 atm, using aerosol jets formed by an electrostatic particulate method. The flame radiation intensities as a function of vertical coordinate are measured and compared with the flame radiation profiles reconstructed using experimental data and simplified models. The experimental data used include particle size distributions, flame velocities, and temperatures of metal ignition and combustion. The simplified models describe the particle ignition delay, combustion time, and particle flame radiation intensity as a function of particle diameter, D . Variable parameters of the models describing particle radiation intensities and combustion times are adjusted to achieve the best fit between the reconstructed and measured flame radiation profiles. A set of parameters providing the best agreement between the reconstructed and measured profiles is selected for several aerosol flames produced by powders of different sizes of the same material. These parameters are assumed to adequately describe particle combustion times and radiation intensities for the chosen material. The experimental radiation profiles for both aluminum and magnesium aerosol flames with particles of different sizes were found to be in very good agreement with the respective reconstructed profiles. For both metals, particle radiation intensities were well described by a D 3 -type expression. The combustion times for magnesium aerosol particles were well described by the traditional D 2 -law with the evaporation constant close to those reported earlier for single particles. Aluminum aerosol particle combustion was better described by a D 1 -law and combustion times of fine (

Laminar Lifted Flame Speed Measurements for Aerosols of Metals and Mechanical Alloys

This research develops and validates a novel experimental methodology for measurements of laminar flame speed of metal-air aerosols. The methodology is based on a recently developed laminar, lifted flame aerosol burner (LLFAB) using electrostatic fluidization to produce metal aerosol between the electrodes of a plate capacitor. An aerosol jet directed vertically up, and decelerating in a stagnant gas environment is produced. The jet is ignited, and the position of the propagating downward flame is stabilized at a location where the flame speed becomes equal to the jet velocity with the opposite sign. Therefore, the flame speed determines the vertical location of the lifted flame. Aerosol flame speed measurements using LLFAB are compared vs earlier measurements using Bunsen burner and flame tube in microgravity. The developed technique was used to compare the flame speed for pure aluminum and magnesium powders vs flame speed for a set of aluminum-based mechanical alloys using Mg, Ti, Zr, Li, MgH 2 , or C as alloying elements. It was observed that the flame speeds for all of the tested alloys, except the one with carbon, are higher than that of the pure aluminum aerosol.

Production of Well-Controlled Laminar Aerosol Jets and Their Application for Studying Aerosol Combustion Processes

Generation of steady-state solid aerosol jets with controllable parameters is often necessary in experimental studies and industrial processes. Most of the current approaches use a fluidized bed to produce an aerosol flow and always introduce initial turbulence into the jet. Toproduce a laminar aerosol jet, flow straighteners and long tubes are used that make the design cumbersome and inflexible. In addition, in a fluidized bed-type system, the aerosol number density and gas flow rate are inherently interdependent. In a new apparatus described in this paper, metal aerosol is produced using an electrostatic recharging of particles in a DC electric field of a parallel plate capacitor, a so-called electrostatic particulate method. The powder is aerosolized within the capacitor without using any gas flows and only a small velocity, a laminar gas jet is used to carry the aerosol away from the chamber through a small nozzle made in the top plate of the capacitor. It is shown that the aerosol number density is controlled by an electric field, independently of the gas flow rate. The usefulness and flexibility of the new technique for the aerosol combustion studies is demonstrated. Preliminary results on characterization of the produced small-scale, laminar, premixed, lifted aluminum-air flames are reported. The flame propagation velocities are measured and compared to the earlier results; overall flame dimensions and radiation profiles are determined. Individual particle flame zones are visualized in the aluminum-air aerosol flame for the first time.

Constant pressure flames of aluminum and aluminum-magnesium mechanical alloy aerosols in microgravity

In an effort to develop new aluminum-basedenergetic materials for advanced metallizedpropellants, explosives, pyrotechnics, and incen-diaries, metastable Al-Mg mechanical alloyshave been recently prepared and characterized[1]. In this work, the experimental setup de-signed for the constant pressure experiments onaerosol flame propagation in microgravity [2–4]was exploited to compare the flame propagationin the aerosols of pure Al versus Al-Mg me-chanical alloys. In addition, combustion prod-ucts produced with different metallic fuels werecollected and compared with one another.

Integral Radiation Energy Loss During Single Mg Particle Combustion

This work was an experimental study of radiation losses during combustion of a single Mg particle in air. Magnesium particles of 1.0-2.5-mm diameter were used in the experiments. For the first time the total radiation energy was measured directly. It is shown that the energy radiated was nearly a constant fraction (∼40%) of the total heat release during the combustion for all particles studied.