Ignition of Aluminum Particles and Clouds

Proceedings of the Combustion Institute, 2011

A hybrid two-phase numerical methodology is used to investigate the flow-field subsequent to the detonation of a spherical charge of TNT with an ambient distribution of a dilute cloud of aluminum particles. The interaction of the particle cloud with the contact surface results in Rayleigh-Taylor instability, which grows in time and gives rise to a mixing layer where the detonation products mix with the air and afterburn. At early times, the ambient particles get engulfed into the detonation products and ignite. Subsequently, they catch up with the Rayleigh-Taylor structures, and the vortex rings around the hydrodynamic structures cause transverse dispersion that results in the clustering of particles. Then, the particles leave the mixing layer and quench, yet preserve their hydrodynamic foot print. Preferential heating and combustion of particles occurs due to clustering. A higher initial mass loading in the ambient cloud results in larger clusters due to stronger/larger vortex rings around the hydrodynamic structures. A larger particle size results in the formation of fewer and degenerate clusters when the initial width of the cloud is larger. A theoretical model is used to predict the bubble amplitudes, and are in good accordance with the simulation results. Overall, this study has provided some useful insights on the explosive dispersal of dilute aluminum particle clouds and the gas dynamics of the flow field in the mixing layer.

Understanding the shock and detonation response of high explosives at the continuum and meso scales Applied Physics Reviews 5, 011303 (2018);

International Journal of Thermophysics, 2018

HAL (Le Centre pour la Communication Scientifique Directe), 2007

The aim of the present work is to reduce the burning time of aluminum particles with the ultimate goal to improve the performances of solid propellants. Aluminium nanoparticles have gained importance because of their increased reactivity as compared with traditional micro-sized particle. Decreasing the size of Al particles increases their specific surface area, and hence decreases the burning time of the same mass of particles. Nevertheless another consequence of decreasing the particle size is an increase of alumina mass fraction in the reactant powders passivated in air. An experimental program is initiated to determine flame propagation velocities of micro-sized (around 6 µm) and nano-sized (around 250 nm) aluminum particle clouds. Another goal of this study is to estimate the gas phase temperature from AlO molecular spectra and the temperature of condensed phase emitters in the flame using emission spectroscopy. To this end, an experimental setup is developed to investigate the flame characteristics of particle clouds ignited by an electric spark in a glass tube. The present results show that nano-sized Al particle clouds burn faster than micro-sized particle clouds for the same global particle mass concentration in air. The cloud flame propagation velocity depends also on the particle concentration. The temperature measurements indicate a consistent value around 2900 K for all nano-Al particle burning clouds and 3300 K for micro-Al particle clouds. The results of the condensed phase temperature show, first a stable temperature and then a decreasing trend along the axis of the flame.

Combustion and Flame, 2013

a b s t r a c t Predictive mechanisms for particle ignition and combustion rates are required in order to develop optimized propellant and energetic formulations using micron-sized metal powders, such as aluminum. Most current descriptions are based on laboratory experiments performed in stationary or laminar combustion configurations. However, turbulent environments exist in most applications and validity of the present descriptions for such environments has not been established. This experimental study is aimed to measure burn times for aluminum particles burning in environments with different levels of turbulence. A laminar air-acetylene flame is produced, and auxiliary tangential jets of air with adjustable flow rates are used to achieve different controlled levels of turbulence. Fine spherical aluminum powder is injected in the flame axially using a flow of nitrogen. The streaks of burning particles are photographed using a camera placed behind a mechanical chopper interrupting the photo-exposure with a pre-set frequency. The obtained dashed streaks are used to measure the particle burn times for different flow conditions. The particle burn times are correlated with the particle size distribution to obtain the burn time as a function of the particle size. The results are processed to obtain a correction for the Al particle burn rate as a function of the turbulence intensity, I. The measured burn times are longer than predicted for the micronsized Al particles using a correlation based on survey of earlier experiments, mostly with coarser Al powders. Increased turbulence intensity results in substantial reduction of the particle burn time. Present data suggest that the burn rate for particle combustion in a laminar environment should be multiplied by 1 + 18.2I, to estimate the acceleration of aluminum combustion in turbulent environments.

Experimental Thermal and Fluid Science, 2010

An experimental study has been conducted to determine flame propagation velocities in clouds of micro-(4.8 lm) and nano-(187 nm) aluminum particles in air at various concentrations. The experimental results show faster flame propagation in nanoparticle cloud with respect to the case of microparticles. Maximum flame temperature has been measured using a high-resolution spectrometer operating in the visible range. Analysis of combustion residual shows that nanoparticles combustion is realized via the gas-phase mechanism. A three-stage particle combustion model has been proposed based on these observations. Model parameters have been fitted to match the experimental results on the flame velocity and maximum temperature. Particle burning time is estimated from the flame simulations.

Journal of Loss Prevention in The Process Industries, 2017

In this paper, the combustion mechanisms of aluminum particles suspended in air and dust flame propagation mechanisms in dust-air mixtures, more specifically, in aluminum dust-air mixture are reviewed. Experimental data about individual aluminum particles combustion are collected and explained with details. It is shown from previous studies that despite the well understanding of the individual particles combustion process, the ignition and the inter-particles interaction need more investigation seen that potential differences exist between the combustion regimes of isolated particles and a group of particles. The combustion regime may affect the ignition temperature resulting in the difference in the burning velocity. Additional experimental data and theoretical/numerical studies are needed. It is afterwards shown that, at these temperatures level, the thermal radiation contribution could be significant. Therefore, optical properties of aluminum burning particles and alumina particles are determined based on Mie scattering solutions showing that both aluminum and alumina are poor emitters but excellent scatterers. Then, several flame propagation models which take into account the thermal radiation exchanges are reviewed. The review of existing models reveals that the knowledge of the nature of radiative exchanges remains piecewise and basics need to be founded in order to be able to test modeling routes and plan experiments. The interaction of the different heat exchange modes and combustion regimes within the flame propagation process should be studied with a minimum of simplifying assumptions and the nature and characteristics of thermal radiation exchanges should be the key.

Combustion and Flame, 2010

This paper presents experimental results on ignition of micron-sized spherical Al particles by a CO 2 laser in H 2 O/N 2 , CO 2 , air, H 2 O/air and CO 2 /O 2 gaseous environments. Al powder with nominal particle sizes in the range of 4.5-7 lm is aerosolized using a parallel plate capacitor by charging particles contacting the electrodes. A thin, laminar aerosol jet is formed using a gas purged through the capacitor and issuing together with aerosolized particles through a small opening in the top electrode. The jet is fed into a focused CO 2 laser beam. A part of the gas mixture is fed as a shroud flow around the central aerosol jet to stabilize and shield the aerosol jet from surrounding air. The velocities of particles in the jet are varied in the range of 0.1-3 m/s. For the H 2 O-containing gas mixtures, the gas lines are heated to $150°C. A numerical simulation using Fluent CFD code is used to determine the gas composition at the laser focal spot. In experiments, for a given environment and selected particle velocity, the laser power is increased until the particles ignite. The ignition is detected optically using a photomultiplier. The laser power thresholds required for ignition of spherical aluminum particles are measured at varied particle velocities for each environment. The lowest thresholds are found for CO 2 /O 2 mixture and the highest for the H 2 O/N 2 mixture. Addition of O 2 to H 2 O or CO 2 reduces the ignition thresholds. The experimental data are processed to determine the kinetic parameters of a simplified Arrhenius description of the exothermic reaction leading to the particle ignition in different oxidizing environments.

2006

For many explosives, only a fraction of the chemical energy is released in the detonation. Calorimetry data for TNT from Ornellas (1984) shows that when the ambient gas is inert, there is substantially less total energy released than when the ambient gas is air. This data indicates that burning of the explosion byproducts plays a key role in the overall energetics of the system. In this paper, we briefly discuss the models and the numerical methods used for the simulations and present the computational results. We also discuss future directions our work on the development of SDF explosives