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 CO₂ 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.     

A correlation for burn time of aluminum particles in the transition regime

A study of the combustion times for aluminum particles in the size range of 3–11 μm 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 μm, 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.     

Mathematical combustion model of nanoaluminum-air suspension

This paper presents a combustion model of a nano-aluminum-air (nAl-air) suspension. The special feature of the model is performing a local mathematical model of the oxidant diffusion through an aluminum oxide layer on the particle surface taking into account the aluminum-oxidant reaction to simulate the combustion of nano-size aluminum (nAl) particles. The oxidation rate of the aluminum particles and the associated with this process the rate of heat release are determined from the solution of the local combustion problems for the entire set of nAl particles in the suspension. To obtain the suspension state parameters we solve the equation system, which includes the energy conservation equations for the gas and particles, the mass-conservation equation for the gas-dispersed mixture and the motion equations for the gas and particles controlling for the particle velocity lag. The model considers gas expansion and thus gas and particle motion. The developed model does not require setting the ignition temperature of nAl particles. The study provides the calculated propagation rate of the combustion front in the nAl-air suspension depending on the nAl mass concentration and on the initial temperature of the suspension.     

Heterogeneous combustion of a pair of interacting particles with an arbitrarily shaped surface

The heterogeneous combustion of two immobile interacting particles with arbitrarily shaped surfaces is theoretically described under conditions of rapid chemical reaction, where the concentration of oxidizer molecules on the surface of the particles can be considered to be zero. The problem is solved for an arbitrary temperature dependence of molecular transport coefficients when molecular transport in the vicinity of the particles takes place by diffusion. Analytical formulas found allow for the direct characterization of the combustion of individual spheroidal particles and interacting spherical particles in several particular cases. At the same volumes, heavily elongated and oblate particles are shown to burn significantly faster. The approach of the particles significantly influences the time of burning the finer particle if its size is far less than that of the coarser one.     

Influence of particle size on the breaking of aluminum particle shells

Rupturing the alumina shell (shell-breaking) is a prerequisite for releasing energy from aluminum powder. Thermal stress overload in a high-temperature environment is an important factor in the rupture of the alumina shell. COMSOL Multiphysics was used to simulate and analyze the shell-breaking response of micron-scale aluminum particles with different particle sizes at 650 ℃ in vacuum. The simulation results show that the thermal stability time and shell-breaking response time of 10 μm-100 μm aluminum particles are 0.15 μs-11.44 μs and 0.08 μs-3.94 μs, respectively. They also reveal the direct causes of shell breaking for aluminum particles with different particle sizes. When the particle size is less than 80 μm, the shell-breaking response is a direct result of compressive stress overload. When the particle size is between 80 μm and 100 μm, the shell-breaking response is a direct result of tensile stress overload. This article provides useful guidance for research into the energy release of aluminum powder.     

Modes of particle combustion in iron dust flames

The so-called argon/helium test is proposed to identify the combustion mode of particles in iron dust flames. Iron powders of different particle sizes varying from 3 to 34 μm were dispersed in simulated air compositions where nitrogen was replaced by argon and helium. Due to the independence of the particle burning rate on the oxygen diffusivity in the kinetic mode, the ratio between the flame speeds in helium and argon mixtures is expected to be smaller if the particle burning rate is controlled by reaction kinetics rather than oxygen diffusion. Experiments were performed in a reduced-gravity environment on a parabolic flight aircraft to prevent particle settling and buoyancy-driven disruption of the flame. Uniform suspensions of the iron powders were produced inside glass tubes and a flame was initiated at the open end of the tube. Quenching plate assemblies of various channel widths were installed inside the tube and pass or quench events were used to measure the quenching distance. Flame propagation was recorded by a high-speed digital camera and spectral measurements were used to determine the temperature of the condensed emitters in the flame. The measured flame speeds and quenching distances were in good agreement with previously developed one-dimensional, dust flame model where the particles are assumed to burn in a diffusive mode and heat losses are described on a volumetric basis. However, a significant drop of the ratio of flame speeds in helium and argon mixtures was observed for finer 3 μm particles and was attributed to a transition from the combustion controlled by diffusion for larger particles to kinetically controlled burning of micron-size particles. In helium mixtures, the lower flame temperatures measured in suspensions of fine particles in comparison to larger particles reinforces this assumption.     

Convective burning of fine ammonium nitrate–aluminum mixtures in a closed volume bomb

It is commonly assumed that the burning of ammonium nitrate–aluminum mixtures is much less prone to undergo a transition to explosion and detonation than similar mixtures based on ammonium perchlorate. However, this conclusion has been made for mixtures based on commercial-grade ammonium nitrate with large particles. In this study, the combustion of fine loose-packed mixtures of ammonium nitrate and aluminum in a closed-volume bomb has been examined. It has been shown that fine mixtures (ammonium nitrate with a particle size of less than 40 µm and an ASD-4 aluminum powder with spherical particles with a size of about 4 µm) undergo high-intensity combustion; in experiments with a stoichiometric mixture, explosions are observed. The explosions occur in the initial phase of convective combustion and lead to abrupt pressure pulsations with an amplitude of a few kilobars and to the destruction of the cup in which the sample is placed. The dynamics of development of the explosion has been analyzed in detail using numerical simulation. According to the results of experiments with varied parameters—the degree of dispersion of the ammonium nitrate powders, the aluminum content in the mixture, the length and diameter of the charge, and the level of pressure generated by the combustion of the igniter,—threshold conditions have been determined to separate the following modes: the absence of ignition, layer-by-layer combustion, or convective combustion with a transition into an explosion in experiments with a stoichiometric mixture.     

Development and characterization of high performance solid propellants containing nano-sized energetic ingredients

This paper addresses the development of a pair of layered solid propellants suitable for use in a fast-core gun-propellant charge application. A baseline propellant combination was formulated using RDX particles and thermoplastic-elastomer binder as the major ingredients and CL-20 and nitroguanadine as separate additives for high- and low-energy propellants. The propellant’s burning rate was characterized and insufficient burning-rate ratio between the fast and slow baseline propellants was found. Impetus obtained from the combustion of the combined baseline propellants was also found to be far from the demanded value of 1300 J/g. Several modifications were made by introducing nano-sized aluminum particles and ultra-fine boron particles as well as high-energy oxidizer HNF into the propellant formulation. It was found that the addition of nano-sized aluminum particles can enhance the propellant burning rate only when the propellant contains oxidizers with a positive oxygen balance. Without the presence of positive oxygen balance oxidizer, the exothermic reaction of aluminum and boron particles occurs at a large distance from the burning surface introducing an energy-sink effect. The results obtained from the combustion of the advanced propellants show that an average impetus of 1299 J/g, a flame temperature of 3380 K with a burn rate ratio around 3 between the fast- and the slow-burning layers can be achieved. These conditions are desired for fast-core layered propellant applications. The impact sensitivities of the baseline, intermediate and advanced propellants were measured. The results show that addition of HNF and nano-sized aluminum exhibited improved impact sensitivity at levels that can be considered acceptable for deployment.     

Ignition and combustion of a dense powder jet of micron-sized aluminum particles in hot gas

Aiming at the potential implementation of aluminum as a primary fuel in powder-fueled ramjets or engines, this work seeks to investigate the ignition and combustion characteristics of a dense gas-suspended jet of micron-sized aluminum particles in a hot flow with controlled temperature and compositions. Aluminum particles with a mean diameter of 40 µm are aerosolized using a custom-made feeder and carried into the burner by a nitrogen stream. The powder jet with a particle density of up to 1–3 kg/m³ can be ignited and burned violently at a surrounding gas temperature as low as 1500 K. The lowered ignition temperature of the powder jet can be attributed to a cooperative mechanism resulting in fast reactions. Meanwhile, the ignition delay time decreases from ∼25 to ∼5 ms when the surrounding temperature increases from 1500 to 2200 K. The burning powder jet generates strong luminance and AlO emission signals detected by a spectrometer. Particle image velocimetry (PIV) and camera pyrometry are used to derive the two-dimensional velocity and average projected temperature distribution, respectively. Furthermore, a high-speed camera with a microscopic lens captures the transition from dispersed combustion to group combustion that forms a large-scale flame column wrapping the entire powder jet. The aluminum oxide produced in the columnar flame forms a large number of nanosized smoke particles in the condensation region. Finally, a numerical model considering the collective effect of the powder jet is developed to predict the particle temperature history during the ignition stage, which shows good agreement with the temperature profiles derived from camera pyrometry and PIV techniques.     

Particle temperature and potassium release during combustion of single pulverized biomass char particles

This work investigated the combustion characteristics of single pulverized biomass-derived char particles. The char particles, in the size range 224–250 µm, were prepared in a drop tube furnace at pyrolysis temperatures of 1273 or 1473 K from four types of biomass particles – wheat straw, grape pomace, kiwi branches and rice husk. Subsequently, the char particles were injected upward into a confined region of hot combustion products produced by flat flames stabilized on a McKenna burner, with mean temperatures of 1460, 1580 and 1670 K and mean O₂ concentrations of 4.5, 6.5 and 8.5 vol%. The data reported include particle temperature, obtained using a two-color pyrometry technique, and potassium release rate, measured using a laser-induced photofragmentation fluorescence imaging technique. In addition, particle ignition delay time and burning time, obtained from the temporal evolution of the thermal radiation intensity of the burning char particles, are also reported. The results indicated that ignition of the char particles occurs simultaneously with the starting of the potassium release, then the particle burning intensity increases rapidly until it reaches a maximum, after which both the particle temperature and the potassium release rate remain approximately constant until the end of the char oxidation process. The char ignition process is temperature controlled, and the char oxidation process is oxygen diffusion controlled, with the total potassium release being independent of the oxygen concentration and the temperature of the combustion products. The combustion behavior of the chars studied is more affected by the char type than by the conditions used to prepare them.     

真实比热模型中铝粉尘两相爆轰波的数值研究

采用多流体模型对铝粉尘两相爆轰波进行数值模拟,研究颗粒能量计算方法对起爆和传播过程的影响.以前的固相颗粒能量的计算一般采用固定比热方法,本文采用随温度变化的真实比热.由于铝颗粒及其产物氧化铝的比热变化很大,模拟得到的爆轰波的速度、压力和波后参数变化和采用固定比热存在较大的差异.变比热计算得到的爆轰波压力、传播速度和实验结果更加接近,而固定比热的计算方法会对这些参数造成高估.对爆轰波的形成进行研究,发现起爆距离主要受起爆能量影响,但是相对于固定比热模型,采用变比热模型得到的起爆距离较短.     

Flame propagation of nano/micron-sized aluminum particles and ice (ALICE) mixtures

Flame propagation of aluminum–ice (ALICE) mixtures is studied theoretically and experimentally. Both a mono distribution of nano aluminum particles and a bimodal distribution of nano- and micron-sized aluminum particles are considered over a pressure range of 1–10 MPa. A multi-zone theoretical framework is established to predict the burning rate and temperature distribution by solving the energy equation in each zone and matching the temperature and heat flux at the interfacial boundaries. The burning rates are measured experimentally by burning aluminum–ice strands in a constant-volume vessel. For stoichiometric ALICE mixtures with 80 nm particles, the burning rate shows a pressure dependence of r_b = aPⁿ, with an exponent of 0.33. If a portion of 80 nm particles is replaced with 5 and 20 μm particles, the burning rate is not significantly affected for a loading density up to 15–25% and decreases significantly beyond this value. The flame thickness of a bimodal-particle mixture is greater than its counterpart of a mono-dispersed particle mixture. The theoretical and experimental results support the hypothesis that the combustion of aluminum–ice mixtures is controlled by diffusion processes across the oxide layers of particles.     

Characterizing char particle fragmentation during pulverized coal combustion

A particle population balance model was developed to predict the oxidation characteristics of an ensemble of char particles exposed to an environment in which their overall burning rates are controlled by the combined effects of oxygen diffusion through particle pores and chemical reactions (the zone II burning regime). The model allows for changes in particle size due to burning at the external surface, changes in particle apparent density due to internal burning at pore walls, and changes in the sizes and apparent densities of particles due to percolation type fragmentation. In percolation type fragmentation, fragments of all sizes less than that of the fragmenting particle are produced. The model follows the conversion of particles burning in a gaseous environment of specified temperature and oxygen content. The extent of conversion and particle size, apparent density, and temperature distributions are predicted in time.Experiments were performed in an entrained flow reactor to obtain the size and apparent density data needed to adjust model parameters. Pulverized Wyodak coal particles were injected into the reactor and char samples were extracted at selected residence times. The particle size distributions and apparent densities were measured for each sample extracted. The intrinsic chemical reactivity of the char to oxygen was also measured in experiments performed in a thermogravimetric analyzer. Data were used to adjust rate coefficients in a six-step reaction mechanism used to describe the oxidation process.Calculations made allowing for fragmentation with variations in the apparent densities of fragments yield the type of size, apparent density, and temperature distributions observed experimentally. These distributions broaden with increased char conversion in a manner that can only be predicted when fragmentation is accounted for with variations in fragment apparent density as well as size. The model also yields the type of ash size distributions observed experimentally.     

爆轰和燃烧法合成SrAl2O4：Eu2+，Dy3+纳米发光粉

 在低温条件下分别用爆轰法和燃烧法制备出了SrAl₂O₄：Eu²⁺,Dy³⁺ 纳米发光粉。从合成条件、热处理温度等方面详细对比了爆轰法和燃烧法对所制备的SrAl₂O₄：Eu²⁺,Dy³⁺纳米发光粉的晶体生长行为、粒子形貌和光学性质等的影响。研究表明,随着热处理温度的升高,爆轰法制备的纳米发光粉的平均粒径逐渐增大,而燃烧法制备的纳米发光粉的平均粒径先减小后增大,在600 ℃时平均粒径存在一个极小值。在同样热处理温度下,爆轰法制备的纳米发光粉的平均粒径增长明显高于燃烧法合成的纳米发光粉的平均粒径。最后讨论了长余辉的发光机理,并给出了如何改进合成方法的建议。     

On turbulent chemical explosions into dilute aluminum particle clouds

We use a hybrid two-phase numerical methodology 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. Rayleigh–Taylor instability ensues on the contact surface that separates the inner detonation products and the outer shock-compressed air due to interphase interaction, which grows in time and results in a mixing layer where the detonation products afterburn with the air. At early times, the ambient particles are completely engulfed into the detonation products, where they pick up heat and ignite, pick up momentum and disperse. Subsequently, as they disperse radially outwards, they interact with the temporally growing Rayleigh–Taylor structures, and the vortex rings around the hydrodynamic structures results in the clustering of the particles by also introducing local transverse dispersion. Then the particles leave the mixing layer and quench, yet preserve their hydrodynamic ‘footprint’ even until much later; due to this clustering, preferential heating and combustion of particles is observed. With a higher initial mass loading in the ambient cloud, larger clusters are observed due to stronger/larger hydrodynamic structures in the mixing layer – a direct consequence of more particles available to perturb the contact surface initially. With a larger particle size in the initial cloud, clustering is not observed, but when the initial cloud is wider, fewer and degenerate clusters are observed. We identify five different phases in the dispersion of the particles: (1) engulfment phase; (2) hydrodynamic instability-interaction phase; (3) first vortex-free dispersion phase; (4) reshock phase; and (5) second vortex-free dispersion phase. Finally, a theoretical Buoyancy-Drag model is used to predict the growth pattern of the ‘bubbles’ and is in agreement with the simulation results. Overall, this study has provided some useful insights on the post-detonation explosive dispersal of dilute aluminum particle clouds.     

Detonation models of fast combustion waves in nanoscale Al-MoO3 bulk powder media

The combustion of nanometric aluminum (Al) powder with an oxidiser such as molybdenum trioxide (MoO₃) is studied analytically. This study focuses on detonation wave models and a Chapman-Jouget detonation model provides reasonable agreement with experimentally-observed wave speeds provided that multiphase equilibrium sound speeds are applied at the downstream edge of the detonation wave. The results indicate that equilibrium sound speeds of multiphase mixtures can play a critical role in determining speeds of fast combustion waves in nanoscale Al-MoO₃ powder mixtures.     

Clustering and combustion of dilute aluminum particle clouds in a post-detonation flow field

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.     

Combustion of bimodal nano/micron-sized aluminum particle dust in air

The combustion of bimodal nano/micron-sized aluminum particles with air is studied both analytically and experimentally in a well-characterized laminar particle-laden flow. Experimentally, an apparatus capable of producing Bunsen-type premixed flames was constructed to investigate the flame characteristics of bimodal-particle/air mixtures. The flame speed is positively affected by increasing the mass fraction of nano particles in the fuel formulation despite the lower flame luminosity and thicker flame zone. Theoretically, the flames are assumed to consist of several different regimes for fuel-lean mixture, including the preheat, flame, and post flame zones. The flame speed and temperature distribution are derived by solving the energy equation in each regime and matching the temperature and heat flux at the interfacial boundaries. The analysis allows for the investigation of the effects of particle composition and equivalence ratio on the burning characteristics of aluminum-particle/air mixtures. Reasonable agreement between theoretical results and experimental data was obtained in terms of flame speed. The flame structure of a bimodal particle dust cloud may display either an overlapping or a separated configuration, depending on the combustion properties of aluminum particles at different scales. At low percentages of nano particles in the fuel formulation, the flame exhibits a separated spatial structure with a wider flame regime. At higher nano-particle loadings, overlapping flame configurations are observed.     

An examination of the shrinking-core model of sub-micron aluminum combustion

We revisit the shrinking-core model of sub-micron aluminum combustion with particular attention to the mass flux balance at the reaction front which necessarily leads to a displacement velocity of the alumina shell surrounding the liquid aluminum. For the planar problem this displacement simply leads to an equal displacement of the entire alumina layer, and therefore a straightforward mathematical framework can be constructed. In this way we are able to construct a single curve which defines the burn time for arbitrary values of the diffusion coefficient of O atoms, the reaction rate, the characteristic length of the combustion field, and the O atom mass concentration within the alumina provided that it is much smaller than the aluminum density. This demonstrates a transition between a ‘d  ²–t’ law for fast chemistry and a ‘d–t’ law for slow chemistry. For the spherical geometry, the one of physical interest, the outward displacement velocity creates not a simple displacement, but a stress field which, when examined within the framework of linear elasticity, strongly suggests the creation of internal cracking. We note that if the molten aluminum is pushed into these cracks by the high internal pressure characteristic of the stress field, its surface, where reaction occurs, could be fractal in nature and affect the fundamental nature of the burning law. Indeed, if this ingredient is added to the planar model, a single curve for the burn time can again be derived, and this describes a transition from a ‘d  ²–t’ law to a ‘d  ^ν–t’ law, where 0<ν<1.     

Deflagration-to-detonation transition in mixtures of finely divided ammonium perchlorate with submicron aluminum powder

Deflagration-to-detonation transition in binary mixtures of fine ammonium perchlorate (20-μm grains) with submicron ALEX-L aluminum powder (0.2-μm particles) is studied using high-speed photography and pressure recording with quartz crystal sensors. The test mixtures were loaded in thin-walled quartz tubes of inner diameter 10 mm. The charges had a porosity of ~50%. It has been shown that, even under very mild conditions (low-strength shell and a weak source of initiation), the deflagration mode of mixture combustion easily transforms into the detonation mode. The shortest length of the region of transition from deflagration to normal detonation (not more than 30 mm) was observed for a lean mixture, with an aluminum content of ~5%. The mechanism of transition to detonation involves the stage of convective combustion, resulting in the formation of a brightly luminescent crescent-shaped area behind the primary flame front, which, in turn, generates a forward (in the direction of propagation) and a backward wave. The forward wave gives rise to low-speed detonation, which later transforms into normal detonation. The pressure profile within the region of low speed detonation is measured. A comparison with similar experiments in which ALEX-L alu- minum powder was replaced by ASD-4 aluminum (4 μm particles) shows that ALEX-L sensitizes the mixture, resulting in a dramatic reduction of the length of the transition region, making it possible to produce normal detonation in low-strength shells.