Combustion Of Aluminum Particles Research Articles

Burning of aluminum particles in water

Experiments in a constant-volume reactor and in a model rocket combustor demonstrated the possibility of convective burning of mixtures of water with PAP-2 flaky aluminum powder with the formation of alumina and hydrogen. It was shown that the porosity of the mixture is an important factor determining the mode of its burning. The burning occurred at a pressure of several hundred atmospheres in the convective mode.

Design of the steam generator in an energy conversion system based on the aluminum combustion with water

The paper shows the preliminary design of the superheated steam generator to be used in a novel hydrogen production and energy conversion system based on the combustion of aluminum particles with water. The system is aimed at producing hydrogen and pressurized superheated steam, using the heat released by the Al–H2O reaction. The interest on this type of technology arises because of the possibility of obtaining hydrogen with very low pollutant and greenhouse gas emissions, compared to the traditional hydrogen production systems, such as the steam reforming from methane.The analysis of the combustion chamber and the heat recovery system is carried out by means of a lumped and distributed parameter numerical approach.The multi phase and gas mixture theoretical principles are used both to characterize the mass flow rate and the heat release in the combustion chamber and within the heat exchangers in order to relate the steam generator performance to the system operating parameters.Finally, the influence of the steam generator performance on the whole energy conversion system behavior is addressed, with particular care to the evaluation of the total power and efficiency variation with the combustion parameters.

IGNITION AND COMBUSTION OF NICKEL COATED AND UNCOATED ALUMINUM PARTICLES IN HOT POST-FLAME ENVIRONMENT

It is well known that aluminum particles require a high temperature to achieve ignition, often in excess of 1900 K. Lowering the ignition temperature of Al particles can increase the efficiency of solid propellant-based propulsion systems and thermobaric explosives. Current methods to reduce the ignition temperature include both reducing the particle size to the nano-scale and/or applying coatings that aid ignition. In this work, an experimental study of ignition and combustion of isolated 5 wt% Ni-coated and uncoated aluminum particles was conducted. Two particle sizes (nominally 32 μm and 9 μm in diameter) were examined. The ignition and combustion properties of these aluminum particles were observed in the postflame zone of a multi-diffusion flat-flame burner at atmospheric pressure. The results showed that the applied nickel coating decreased the ignition temperature of the Al particles by 750 K on average for 32 m particles and by 300 K for smaller 9 m particles. Measurements of particle burning times indicated that the application of the nickel coating does not affect the overall burning time of the particles. Thus, the application of nickel coatings on Al particle significantly decreases the ignition temperature while not affecting the overall combustion behavior. I. Introduction luminum is an energetic fuel additive for solid propellants and solid fuels in different propulsion systems, and also has effectiveness for particle damping in suppressing combustion instability in rocket motors. This has led aluminum particles to become widely employed in rocket propulsion applications. The importance of aluminum particle combustion in propulsion systems has led to a large number of combustion studies. Furthermore, aluminum particles have also been used in thermobaric explosive (TBX) applications where Al particles need to be ignited rapidly at low temperatures after the detonation of the high explosive charge. Beckstead 1 performed a comprehensive summary of aluminum particle combustion from a wide range of sources. He found a common ignition temperature of about 2,300 K, which is near the melting temperature of the aluminum oxide shell (2,327 K). Additionally, Beckstead's summary found that the ignition delay of Al particles is dependent on the thickness of the oxide shell. Burning time data of the Al particles was also compiled to determine if the D 2 law was applicable. It was found that the D n law was more applicable for larger particle sizes due to the oxide layer formation with the exponent being 1.5 or 1.8, depending upon the coefficient used in the correlation. Additionally, studies that compared the relative effectiveness of different oxidizer species including oxygen, water vapor, and carbon dioxide were compiled in Beckstead's work 1 . It was concluded that O2 was the most effective oxidizer, H2O was between 50 to 60 percent as effective, and CO2 was about 22 percent as effective. Beckstead's correlation is widely accepted for correlating aluminum combustion behavior for particle sizes above 20 m. Without consideration of the effective oxidizer mole fraction, pressure, and ambient gas temperature, Beckstead’s preliminary correlation indicated a D 1.99 law. When the diameter exponent is close to 2.0, the combustion can be regarded as a diffusion-limited process for relatively large particles beyond 20 m. Bazyn, et al. 2 conducted a study on the combustion of 10 μm-sized aluminum particles in the reflected region of a shock tube and found pressure exponents of −0.9, 0.2, and 0.3, for O2, H2O, and CO2 respectively, (i.e., tb  pi n where pi is the partial pressure of the i th

Micro-alumina particle volatilization temperature measurements in a heterogeneous shock tube

Peak flame temperatures in aluminum particle combustion should approach the volatilization temperature of the product alumina. References are divided in assigning this temperature anywhere between 3200 and 4000 K, which can provide significant uncertainty not only in numerical models for combustion but also in the interpretation of flame structure from temperature measurements. We present results in the controlled conditions of the UIUC heterogeneous shock tube of volatilization temperature, made by measuring the extinction of light by nano- and micro-alumina particles at non-resonant wavelengths at different ambient temperatures. At 10 atm, there is a sharp cutoff at 3860 K beyond which nano-particles volatilize and stop extinguishing within the shock tube test time. Numerical modeling of the evaporation rate of these particles is used to assign a volatilization temperature of 4340 K at 10 atm. Similarly, a volatilization temperature of 4260 K at 3 atm is measured. From our analysis, the best estimate for the volatilization temperature at 1 atm was 4189 ± 200 K, which is consistent with the high range of volatilization temperature reported in the literature.

Combustion characteristics of micron-sized aluminum particles in oxygenated environments

Experimental data describing combustion of micron-sized aluminum particles as a function of their size are limited. Often combustion characteristics are derived indirectly, from experiments with aerosolized powder clouds. In a recently developed experiment, micron-sized particles cross two laser beams. When each particle crosses the first, low-power laser, it produces a scattered light pulse proportional to the particle diameter. The second, powerful CO 2 laser beam ignites the particle. The optical emission pulse of the burning particle is correlated with its scattered light pulse, so that the combustion characteristics are directly correlated with the size for each particle. In this work, emission signatures of the ignited Al particles are recorded using an array of filtered photomultipliers to enable optical pyrometry and evaluate the molecular AlO emission. Processing of the generated data for multiple particles is streamlined. Experiments are performed with spherical aluminum powder burning in atmospheric pressure O 2/N 2 gas mixtures with the oxygen concentrations of 10%, 15%, and 21% (air). In air, the AlO emission peaks prior to the maximum in the overall emission intensity, and the latter occur before the maximum of the particle temperature. The temperatures at which particles burn steadily increase with particle size for particles less than 7.4 μm. For coarser particles, the flame temperature remains constant at about 3040 K. In the gas mixture with 15% O 2, the flame temperatures are observed to increase with particle size for the entire range of particle sizes considered, 2–20 μm. At 10% O 2, the flame temperatures are significantly lower, close to 2000 K for all particles. The intensity of AlO emission decays at lower oxygen concentrations; however, it remains discernible for all environments. The results of this study are expected to be useful for constructing the Al combustion models relaxing the assumption of the steady state burning.

Characteristics of the underwater explosion of a nonideally detonating aluminum-rich energetic material

An experimental study of the characteristics of the explosion of mixtures of ammonium perchlorate, aluminum, and nitromethane with a large excess of aluminum (1.45 to 1.66 g/cm3 in density) confined in plastic enclosures and immersed in small elastic-wall reservoirs with water is conducted. It is shown that composite charges, 20 mm in diameter, surrounded by a water layer of thickness 20–30 cm and detonate in a nonideal detonation mode. High-speed cinematography records show the possibility of the intense mixing of the detonation products with the surrounding water and of the burning of excess aluminum particles in a heterogeneous cloud. The time scales of the development of secondary energy release by burning of aluminum particles in water are estimated. The possibility of controlling the characteristics of the pressure waves generated by the explosion, for example, by means of a preliminary bubbling of the water with air near the charge, is demonstrated.

コンポジット推進薬の燃焼表面近傍でのアルミニウム粒子の燃焼

The combustion behavior of the aluminum particles near the burning surface of the aluminized composite propellant was studied. The combustion of the strand in the strand burner and the computational fluid dynamics (CFD) around the burning aluminum particle were performed. The temperature histories from combustion experiments and the temperature distributions from CFD were obtained. The temperature fluctuations of the burning aluminum particles in the temperature histories were observed. The aluminum particles ignite and burn near the burning surface. When the aluminum particles ignite much nearer the burning surface, the temperature gradient is increased in the reaction zone. The burning aluminum particle made a streamline shaped high temperature area around it. Therefore the burning aluminum particle could increase the temperature of the combustion gas widely, the temperature gradient near the burning surface was increased and the burning rate was increased.

Heterogeneous Continuum Model of Aluminum Particle Combustion in Explosions

A heterogeneous continuum model is proposed to describe the dispersion and combustion of an aluminum particle cloud in an explosion. It combines gasdynamic conservation laws for the gas phase with a continuum model for the dispersed phase, as formulated by Nigmatulin. Interphase mass, momentum, and energy exchange are prescribed by the phenomenological model of Khasainov. It incorporates a combustion model based on mass conservation laws for fuel, air, and products. The source/sink terms are treated in the fast-chemistry limit appropriate for such gasdynamic fields, along with a model for mass transfer from the particle phase to the gas. The model takes into account both the afterburning of the detonation products of the booster with air and the combustion of the Al particles with air. The model equations are integrated by high-order Godunov schemes for both the gas and particle phases. Numerical simulations of the explosion fields from 1.5-g shock-dispersed-fuel charges in 3 different chambers are performed. Computed pressure histories are similar to measured waveforms when the ignition temperature model is employed. The predicted product production is 10–14% greater than that measured in the experiments. This fact can be ascribed to unsteady ignition effects not included in the modeling.

Modeling of aluminum particle combustion with emphasis on the oxide effects and variable transport properties

A simplified analytical modeling of single aluminum particle combustion was conducted. Ignition and quasi-steady combustion (QSC) were separately formulated and integrated. Both the heat transfer from the hot ambient gas and the enthalpy of heterogeneous surface reaction (HSR) served to cause the particle ignition. Conservation equations were solved for QSC parameters in conjunction with conserved scalar formulation and Shvab-Zeldovich function. Limit temperature postulate was formulated by a sink term pertinent to the dissociation of the aluminum oxide near the flame zone. Effective latent heat of vaporization was modified for the thermal radiation. Ignition and QSC of the aluminum particle were predicted and discussed with emphasis on the effect of the aluminum oxide and variable properties. The model was validated with the experiments regarding ignition delay time, burning rate, residue particle size, flame temperature, QSC duration, and stand-off distance of the envelop flame. Agreement was satisfactory and the prediction errors were limited within 10%.

Emissivity of Aluminum-Oxide Particle Clouds: Application to Pyrometry of Explosive Fireballs

Pyrometry measurements of clouds of high-temperature particles require an estimate of the spectral dependence of the particle emissivity. Common assumptions for this dependence range from e λ ~ λ -2 to e λ ~ constant. Depending upon the assumption used, there is uncertainty in the temperature of 100 s to a 1000 K in high-temperature clouds. Such errors are not apparent in goodness of fit of spectral data. A heterogeneous shock tube was used to measure the emissivity of aluminum oxide in an inert environment as a function of temperature (2000-3500 K), wavelength (0.55-0.95 μm), and particle diameter (50 nm-10 μm). In micro-sized alumina particles, the spectral dependence upon temperature transitions from decreasing with wavelength to increasing with wavelength with the dependence being roughly gray at about 3000 K. Because of local minima in the e λ vs λ curve, a power-law (λ n ) dependence is insufficient to describe the emissivity. However, if such a dependence is assumed, n transitions from -1.4 to 0.5 as temperature increases from 2500-3500 K. Nano-sized alumina particles exhibit an even stronger spectral dependence. At 2678 K, n is approximately -1.2 but reaches as high as 2.1 at 3052 K. Considering optical depth issues, there is merit in gray emissivity approximations for high-temperature (~3000-3300 K) particles typical of aluminum particle combustion.

Combustion times and emission profiles of micron-sized aluminum particles burning in different environments

Optical signatures and combustion times for micron-sized aluminum particles are measured in oxidizing environments including nitrogen mixtures with oxygen, carbon dioxide, and water. Particles in a room temperature oxidizing gas stream are fed into a CO 2 laser beam where they are ignited. Prior to entering the CO 2 laser beam, each particle crosses a second, low energy laser beam and produces a scattered light signal used to determine the particle size in real time. The correlation between the measured particle sizes and their burn times produces an experimental trend that is compared to various correlations reported in the literature. In addition to the burn time measurements, detailed optical signatures are recorded for micron-sized aluminum particles burning in different environments. For aluminum burning in water vapor, the optical signature of the particle is substantially weaker than in other environments, possibly indicating a primarily surface oxidation. It is shown that semi-empirical and widely used τ b – D n expressions for the particle burn time, τ b , as a function of its diameter, D, are inaccurate for the conditions that are different from those used to establish respective trends initially. It is observed that the effect of oxygen concentration on combustion time of micron-sized aluminum particles is weak when oxygen concentrations exceed 21%. Combustion times increase substantially for lower oxygen concentrations. Aluminum particle combustion times are substantially longer than predicted for experiments with water and carbon dioxide oxidizers. For all environments, the observed effect of particle size is relatively weak and the exponents in the descriptive D n relations appropriate for the current experiments vary approximately from 0.3 to 1. The exponent close to 0.3 adequately describes current results for those oxidizing environments for which heterogeneous reactions appear to dominate.

The simulation of the combustion of micrometer-sized aluminum particles with oxygen and carbon dioxide

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 μm particles compared to experimental data for a mass average diameter of 11 μm. For oxygen, the simulated burn times with a particle size of 11 μm has excellent agreement compared to experimental data with a mass average diameter of 11 μm. 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.

On possibility of vapor-phase combustion for fine aluminum particles

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 2O, the predicted critical particle diameters are close to 6, 7, and 15 μm, respectively.

Aluminum Particle Combustion in High-Speed Detonation Products

Aluminum particles ranging from 2 to 100 μm were subjected to the flow of detonation products of a stoichiometric mixture of hydrogen and oxygen at atmospheric pressure. Luminosity emitted from the reacting particles was used to determine the reaction delay and duration. The reaction duration was found to increase as d n with n ≈ 0.5, which is more consistent with kinetically controlled reaction rather than the classical diffusion-controlled regime. Emission spectroscopy was used to estimate the combustion temperature, which was found to be well below the flow temperature. This fact also suggests combustion in the kinetic regime. Finally, the flow field was modeled with a CFD code, and the results were used to model analytically the behavior of the aluminum particles.

Aluminum Combustion Driven Instabilities in Solid Rocket Motors

This work describes an unreported instability found out by numerical simulations on a solid rocket motor. A simple motor with a cylindrical port is considered and predicted to be stable by single-phase computational fluid dynamics computations. When aluminum combustion is modeled, the motor experiences a strong instability on the first longitudinal acoustic mode. However, no vortex shedding is observed, meaning that it is a genuine combustion instability. A detailed numerical study leads to the conclusion that the instability is thermoacoustic and results from a coupling between chamber acoustics and aluminum combustion heat release. A parametric study points out the importance of aluminum distributed combustion, particularly the thickness of the combustion zone and the aluminum heat of reaction. Theoretical assessment of this instability is also obtained by revisiting the acoustic balance theory. An additional thermoacoustic stability integral is derived and appears to be a driving term. This term also helps to shed light on this instability and explains some of the computational fluid dynamics results. In particular, the underlying mechanism is found to be primarily caused by the acoustic boundary layer which creates high acoustic velocities that enhance heat release from burning aluminum particles.

The Simulation of the Combustion of Micrometer-Sized Aluminum Particles with Steam

The Liang and 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, initial temperature, and pressure on the combustion of steam and micrometer-sized aluminum particles. The results compare well with experimental data investigating the effects of initial temperature and oxidizer concentration on burn time. The simulations and experimental data have opposite trends for the change in burn time as the pressure increased. The calculated flame temperature increases with increasing pressure, initial temperature, and oxidizer concentration. The effects of particle diameter, initial temperature, and pressure on the calculated flame temperature, flame structure, and species profiles were also investigated.

Characterization of a gas burner to simulate a propellant flame and evaluate aluminum particle combustion

This study details the characterization and implementation of a burner designed to simulate solid propellant fires. The burner was designed with the ability to introduce particles (particularly aluminum) into a gas flame. The aluminized flame conditions produced by this burner are characterized based on temperature and heat flux measurements. Using these results, flame conditions are quantified in comparison to other well-characterized reactions including hydrocarbon and propellant fires. The aluminized flame is also used to measure the burning rate of the particles. This work describes the application of this burner for re-creating small-scale propellant flame conditions and as a test platform for experiments that contribute to the development of a particle combustion model, particularly in propellant fires.

Size and morphology of the nanooxide aerosol generated by combustion of an aluminum droplet

The increasing recent interest in characteristics ofsubmicron oxide smoke generated by combustion ofaluminum droplets stems from environmental problemsassociated with application and utilization of alumi-nized propellant rocket motors and from the develop-ment of technologies of production of metal nanoox-ides in aerodisperse flames [1, 2]. Oxide particles rang-ing from nanometer to submicrometer size form andgrow in the flame zone surrounding a burning particle,this process being the starting point for their furtherevolution. Rational design of particle combustion pro-cesses in technical devices is based on an understand-ing of physicochemical processes that occur duringcombustion of a particle and on knowledge of how theircharacteristics depend on combustion conditions. Oneof the challenges in studying the aluminum particlecombustion mechanism is to study the formation andproperties of oxide nanoparticles. In this work, we stud-ied how the size distribution and morphology of theoxide aerosol formed by combustion of aluminum par-ticles in atmospheric air depends on the size of a burn-ing droplet. We intend to extend the pressure range inthe future.Burning droplets were produced by combustion of asmall sample (from 2 × 2 × 20 to 2 × 4 × 40 mm in size)of aluminized solid propellant, which was burnt in a20-L container in air filtered from aerosols. Duringcombustion, the sample expelled burning aluminumdroplets (agglomerates), which fell freely under grav-ity. The combustion lasted a few seconds. During thistime, the container was filled with “oxide smoke,” a fineaerosol. After completion of combustion, aerosol parti-cles coagulated, sedimented, and were partially depos-ited on the walls of the container. To study the evolutionof particles after burning, the resulting aerosol wasperiodically sampled. The aerosol sample was fedeither into a thermophoretic precipitator, to collect par-ticles for subsequent dispersion and morphologicalanalysis based on their electron microscopic images, orinto a Millikan cell, to observe aerosol particles andrecord their motion by a video camera with a micro-scope lens and a laser illuminator. The cell constructionimplies that a homogeneous electric field can beapplied. This, in combination with video recording ofparticle motion, makes it possible to determine the elec-tric charge of the particles. The sampling, video record-ing, and image processing techniques were describedelsewhere [1, 3].We carried out four series of experiments with pro-pellants differing in the size of generated burning drop-lets.In series 1, a model solid propellant containing25 wt % ammonium perchlorate (AP), 35 wt %cyclotetramethylenetetranitramine (HMX), 20 wt %binder, and 20 wt % aluminum was used. Inasmuch asaluminum droplets are agglomerated as the propellantburns, the distribution function of burning droplets gen-erated by the propellant was estimated in the followingmanner. The size distribution of agglomerates, as wellas the fraction of the metal involved in agglomeration(≈0.6), was determined using the technique in [4]. If weassume that the metal not involved in agglomerationpasses into the gas phase in the same form as it has inthe propellant, the set of particles generated by combus-tion consists of agglomerates and particles of initialaluminum. The distribution function of initial alumi-num was preliminarily determined on a Malvern 3600Eparticle size analyzer. Table 1 summarizes the charac-teristics of the set of polydisperse particles calculatedtaking into account the weight fractions. Hereinafter,D

Evidence for the transition from the diffusion-limit in aluminum particle combustion

This work presents experimental evidence that the transition from gas-phase diffusion-limited combustion for aluminum particles begins to occur at a particle size of 10 μm at a pressure of 8.5 atm. Measurements of the particle temperature by AlO spectroscopy and three-color pyrometry indicate that the peak temperature surrounding a burning particle approaches the aluminum boiling temperature as particle size is decreased to 10 μm when oxygen is the oxidizer. This reduction indicates that reactions are occurring at or near the particle surface rather than in a detached diffusion flame. When CO 2 is the oxidizer, the combustion temperatures remain near the aluminum boiling temperature for particles as large as 40 μm, indicating that the flame is consistently near the surface throughout this size range. Burn time measurements of 10 and 2.8 μm powders indicate that burn time is roughly proportional to particle diameter to the first power. The burn rates of micron- and nano-particles also show strong pressure dependence. These measurements all indicate that the combustion has deviated from the vapor-phase diffusion limit, and that surface or near-surface processes are beginning to affect the rate of burning. Such processes would have to be included in combustion models in order to accurately predict burning characteristics for aluminum with diameter less than 10 μm.

Numerical Simulation of Single Aluminum Particle Combustion (Review)

A two-dimensional, unsteady-state, kinetic-diffusion-vaporization-controlled numerical model for aluminum particle combustion is presented. The model solves the conservation equations, while accounting for species generation and destruction with a 15-reaction kinetic mechanism. Two of the major phenomena that differentiate aluminum combustion from hydrocarbon-droplet combustion, namely, condensation of the aluminum-oxide product and subsequent deposition of part of the condensed oxide onto the particle, are accounted for in detail with a submodel for each phenomenon. The effect of the oxide cap in the distortion of the species and temperature profiles around the particle is included into the model. The results obtained from the model, which include two-dimensional species and temperature profiles, are analyzed and compared with experimental data. The combustion process is found to approach a diffusion-controlled process for the oxidizers (O2, CO2, and H2O) and conditions treated. The flame-zone location and thickness are found to vary with the oxidizer. The result shows that the exponent of the particle-diameter dependence of the burning time is not a constant and changes from ≈1.2 for smaller-diameter particles to ≈1.9 for larger-diameter particles. Owing to deposition of the aluminum oxide onto the particle surface, the particle velocity oscillates. The effect of pressure is analyzed for a few oxidizers.