Combustion of Bimodal Nano / Micro Aluminum Suspension with New Reaction Rate Model (original) (raw)
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FLAME SPEED IN A BINARY SUSPENSION OF SOLID FUEL PARTICLES
Most natural and industrial combustible dusts have a wide distribution of particle sizes. Yet, the majority of experimental data on flame propagation in dust clouds are given in relation to some average particle size, and all known theoretical models of dust combustion consider only monosize suspensions. Since the ignition temperature and combustion rate of an individual dust particle are functions of particle size, the flame in real dust suspensions has a complex, multistage structure. As a first step toward understanding multisize dust combustion, the combustion of a suspension of two monosize powders (that in general can be also of different chemical nature) is investigated in the present work theoretically and experimentally. A simple analytical model developed for the flame in a fuel-lean binary suspension permits flame speed and structure to be analyzed as a function of the dust composition and combustion properties of individual particles. The flame speeds predicted by the binary model were compared with flame speeds calculated from a model of monosize dust flame using various average particle size representations. It is shown that averaging of the particle size in general fails to correctly predict the flame speed over the wide range of the binary dust compositions. The flame propagation speed in a binary suspension of aluminum and manganese powders was investigated experimentally by observing the laminar stage of flame propagation in a semi-open vertical tube. The model correctly predicts dependence of the flame speed on mixture composition (mass ratio of manganese and aluminum dusts in suspension) and the mixture composition at the limit of flame propagation.
Experimental studies on the burning of coated and uncoated micro and nano-sized aluminium particles
Aerospace Science and Technology, 2007
Two different approaches are used in this work to reduce the burning times of aluminium particles with the ultimate goal to improve the performances of solid propellants. One method is to coat the micro-sized particles by nickel, and the second is to decrease the particle sizes to nano-metric scales.A thin coating of Ni on the surface of Al particles can prevent their agglomeration and at the same time facilitates their ignition, thus increasing the efficiency of aluminized propellants. In this work, ignition and burning of single Ni-coated Al particles are investigated using an electrodynamic levitation setup and laser heating of the particles. The levitation experiments are used to measure the particle ignition delay time and burning time at different Ni contents in the particles.Decreasing the size of Al particles increases their specific surface, and hence decreases the burning time of the same mass of particles. In this investigation, a cloud of Al nano-particles formed in a combustion tube is ignited by an electric spark. The cloud experiments are used to measure comparative flame front propagation velocities for different Al particle sizes with and without organic coating.The results and their analysis show that both methods reduce the Al burning time. Ni coating reduces significantly the ignition time of micro-sized Al particles and hence the total burning time compared to non-coated particles. Nano-sized particle clouds burn faster than micro-sized Al particle clouds.Deux approches sont étudiées pour diminuer le temps de combustion des particules d'aluminium dans le but d'améliorer les performances des propergols solides. Il s'agit d'une part d'enrober les particules de tailles micrométriques dans une fine couche de nickel, et d'autre part, d'utiliser des particules d'aluminium de taille nanométrique.Une mince couche de nickel couvrant les particules d'Al permet d'empêcher leur agglomération et facilite leur allumage. Dans ce travail, l'allumage et la combustion des particules isolées d'Al enrobées dans du Nickel sont étudiés à l'aide du lévitateur électrodynamique du LCSR équipé d'un dispositif d'allumage par laser. Ce dispositif permet de déterminer les temps d'allumage et de combustion des particules en fonction de la composition du milieu gazeux environnant, la pression et le contenu en Nickel de la particule. Les expériences sont conduites notamment dans de l'air et le CO2 jusqu'à 40 bars et des pourcentages en Nickel de la particule de 0 à 15% en masse.Diminuer la taille des particules à des échelles nanométriques augmente leur surface spécifique et par conséquent diminue le temps de combustion d'une même masse d'Al. Dans cette étude, un nuage de nano particules d'Al est formé dans un tube de combustion et allumé par des électrodes. Ces expériences permettent de déterminer les vitesses comparatives de propagation du front de flamme en fonction de la taille des particules et de la nature de leur enrobage (alumine ou des matériaux organiques).Les résultats préliminaires et leurs analyses montrent que les deux méthodes permetent de réduire d'une façon significative les temps de combustion des particules de'aluminium.
Combustion and Flame, 2010
The Liang/Beckstead aluminum-particle combustion model has been successfully joined with a detailed chemical-kinetic mechanism. The model has been used to investigate the effect of oxidizer concentration, pressure, and particle diameter on the combustion of CO 2 /Ar and O 2 /Ar with micrometer-sized aluminum particles. The simulation results when varying the oxidizer compare well with experimental data. With CO 2 as the oxidizer, the trend of each simulated diameter follows that of the experimental data, especially the simulated 7 lm particles compared to experimental data for a mass average diameter of 11 lm. For oxygen, the simulated burn times with a particle size of 11 lm has excellent agreement compared to experimental data with a mass average diameter of 11 lm. The simulation results for both CO 2 /Ar and O 2 /Ar show a transition from kinetically-controlled combustion to diffusion-controlled combustion as the pressure increases. The burn time of the particles decreases as the pressure increases, until the diffusion-controlled combustion regime is reached and then the pressure has no effect on burn time. The opposite is true for the CO 2 experimental data, in that the observed burn time increases with increasing pressure. The simulations indicate that the observed experimental trend could be the result of using a distribution of particle diameters. As the pressure decreases, larger particles may not ignite and the apparent burn time does not increase. The effect of particle diameter was also investigated. The effects of particle size, oxidizer, and oxidizer concentration on the calculated surface temperatures are also shown. This is the first model to show the beginning of the transition from diffusion-limited to kinetic-limited combustion control for aluminum particles.
Studies on the burning of micro-and nano-aluminum particle clouds
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, 2014
Flame propagation studies for Al nanoparticles (80 nm) and micron particles (3-4.5 lm) mixed with MoO 3 in both an open and confined burn setup were examined. A scanning electron microscopy (SEM) analysis of the reactants and products reveals quantitative size data that contributes toward an understanding of the governing reaction mechanisms. For the confined burn tube experiments, nanoscaled reactants exhibited a flame speed of 960 m/s, the same as has been reported in previous experiments. Micron scale particles exhibited a flame speed of 402 m/s, much higher than the 244 m/s obtained previously for 1-3 lm particles. These flame speeds are in quantitative agreement with predictions based on the recently developed melt-dispersion mechanism (MDM) describing the reaction of Al particles. It also demonstrates that some micron particles can reach flame speeds just 58% lower than the fastest nanoparticles, while micron scale particles are less expensive and do not have the pre-combustion safety and environmental issues typical of nanoparticles. The SEM analysis reveals a significant (at least by factor of 3.7 for nanoparticles) reduction in Al particle size post combustion, which is in agreement with the MDM and in contrast to the predictions based on diffusion mechanisms. Open burn experiments with nanoscale reactants have flame speeds of 12 m/s and product particle sizes almost as small as those in the burn tube experiments. However, the presence of some large particles, which may grow based on the diffusion mechanism, exclude evaporation and the homogenous nucleation mechanism. For open burn experiments with micron reactants, with flames speeds of 9 m/s, SEM analysis shows a molten-resolidified product with no distinguishable particles and cavities containing numerous nanoparticles with a measured diameter of 36 nm.
Aluminum particle combustion in turbulent flames
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 and numerical studies on the burning of aluminum micro and nanoparticle clouds in air
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.
On possibility of vapor-phase combustion for fine aluminum particles
Combustion and Flame, 2009
A simplified heat transfer model applicable for vapor-phase combustion of individual fine metal particles predicts existence of a critical particle diameter, below which the vapor-phase flame alone cannot be self-sustaining. Other heat generation mechanisms (i.e. surface oxidation) should complement the vapor-phase flame. The predicted critical particle diameter is a function of the flame temperature and pressure. For single aluminum particles burning in atmospheric pressure air, CO 2 and H 2 O, the predicted critical particle diameters are close to 6, 7, and 15 lm, respectively.
A correlation for burn time of aluminum particles in the transition regime
Proceedings of the Combustion Institute, 2009
A study of the combustion times for aluminum particles in the size range of 3-11 lm with oxygen, carbon dioxide, and water vapor oxidizers at high temperatures (>2400 K), high pressures (4-25 atm), and oxidizer composition (15-70% by volume in inert diluent) in a heterogeneous shock tube has generated a correlation valid in the transition regime. The deviation from diffusion limited behavior and burn times that could otherwise be accurately predicted by the widely accepted Beckstead correlation is seen, for example, in particles below 20 lm, and is evidenced by the lowering of the diameter dependence on the burn time, a dependence on pressure, and a reversal of the relative oxidizer strengths of carbon dioxide and water vapor. The strong dependence on temperature of burn time that is seen in nano-Al is not observed in these micron-sized particles. The burning rates of aluminum in these oxidizers can be added to predict an overall mixture burnout time adequately. This correlation should extend the ability of modelers to predict combustion rates of particles in solid rocket motor environments down to particle diameters of a few microns.