Combustion Of Aluminum Particles Research Articles

Experimental investigation of the aluminum combustion in different O2 oxidizing mixtures: Effect of the diluent gases

The existence of different distinct phases during the aluminum combustion process was unambiguously proved and widely discussed in several studies on single burning aluminum particles. Indeed, in the case of a combustion in diffusion vapor phase, the formation of growing aluminum oxide caps on the droplet surface is suspected to drive the transition from a symmetric to an asymmetric regime. Especially based on diverse observations for aluminum particles burning in O2/N2, a hypothesis was proposed on the effect of N2 which probably contributes to the regime transition occurrence. To extend research efforts and get a further understanding of the asymmetric transition development, and, more generally on aluminum combustion, an analysis in O2/N2, O2/Ar and O2/He oxidizing mixtures is presented. This work is based on an advanced experimental technique allowing to levitate a single particle and visualize its self-sustained combustion with high temporal and spatial resolution. Combustion parameters of ignited micron-sized aluminum particles (30–100 µm) at atmospheric pressure were accurately determined using optical diagnostics. Results provide a new quantified dataset and allow to compare the relative effect of N2, Ar and He as diluent gases in the aluminum combustion process. Burning times are found to be similar, regardless of the considered diluent gas, as well as the aluminum consumption rate in symmetric regime. However, estimations on the relative duration of the symmetric phase demonstrate a potential influence of N2 which promotes the regime transition in comparison with Ar and He for low molar fraction of oxidizer. The ratio between flame and droplet diameters is also modified depending on the inert gas and is determined. Observed trends are discussed and compared to available data to improve the description and quantification of the relative influence of the inert gases.

Laser-Induced Ignition and Combustion of Individual Aluminum Particles Below 10 μm by Microscopic High-Speed Cinematography

Metal aluminum has been widely used as an ingredient in propellant, gunpowder and thermite, but there is less understanding of the combustion mechanism of aluminum particles from submicron to several microns in diameter. This paper proposes to experimentally investigate the ignition and combustion characteristics of individual aluminum particles below 10 μm. A specific in situ diagnostic experimental apparatus was first designed for directly observing the ignition and combustion behaviors of individual aluminum particles, with a submicrometer spatial resolution and a temporal resolution of tens of microseconds. Direct observation through microscopic high-speed cinematography demonstrated that, when heated by a continuous laser, individual aluminum particles thermally expanded, followed by shell rupture; the molten aluminum core overflowed and evaporated, leading to ignition and combustion. Further results showed that, when the laser power densities were gradually increased (5.88, 7.56 and 8.81 × 105 W/cm2), the durations of thermal expansion, melting and evaporation were shortened. The required time for the aluminum particles to expand to 150% of their initial diameter was shortened (34 s, 0.34 s and 0.0125 s, respectively). This study will be beneficial to further extend the investigation of other individual metal particles and reveal their combustion mechanism by direct observation.

Software for numerical simulation of the combustion of aluminum particles suspended in air

This paper presents a computer model for studying the processes of ignition and combustion of air suspended aluminum particles, based on a system of differential equations of the gas phase balance and the probability density function for temperature and particle size. The model is implemented as software for a two-dimensional non-steady state case. A new implicit difference scheme consisting in two - and three-point approximations for the first and second derivatives is proposed and implemented. A set of calculations is performed and the results of the reacting mixture characteristics distribution, the areas of the maximum heat release and their relationship with the most air suspension probable state are presented.

The Onset of Asymmetric Combustion of Aluminum Particles

ABSTRACT Combustion of an aluminum droplet in an oxidizer atmosphere is known to proceed in several stages. Ignition is followed by a first combustion stage in which the droplet is surrounded by a spherical diffusion flame, but this flame and the conditions at the droplet surface eventually cease to be uniform, giving way to a second combustion stage characterized by an asymmetric, spinning flame. This is followed by a final stage in which an oxide cap develops in the droplet. Oxygen reaches the surface of the burning droplet as aluminum suboxides and other oxygenated species produced at the flame, and dissolves in the liquid. The beginning of asymmetric combustion is associated with the appearance of an oxygen-rich liquid phase that is immiscible with aluminum. A simple model of the onset of asymmetric combustion is proposed that focuses on the adsorption of at the surface, the heterogeneous reaction generating , and the transfer of this species to the liquid bulk. Other aspects of the combustion are drastically simplified. The model shows that spherically symmetric combustion becomes unstable at a certain time, after which oscillatory perturbations develop that resemble the spinning flame observed experimentally. The instability depends on the interplay of adsorption of and production of at the surface, and disappears when the second of these processes is very fast.

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.

Modeling of Cellular Detonation in Gas Suspensions of Submicron and Nano-Sized Aluminum Particles

Cellular detonation in monodisperse suspensions of submicron and nano-sized aluminum particles is numerically simulated. Approaches of mechanics of heterogeneous media are applied. The transition from the continuum to free-molecular regime of the flow around the particles is taken into account in the processes of interphase interaction. Particle combustion is described within the framework of the semi-empirical model developed previously. Results calculated for two-dimensional flows in a plane channel for suspensions of aluminum particles with the particle size ranging from 1 µm to 100 nm are presented. The regular structure of cellular detonation is found to transform to an irregular structure as the particle size decreases. An increase in the peak pressure and enlargement of the detonation cell are also noted, which is attributed to enhancement of the activation energy of reduced kinetics caused by the transition to the kinetic regime of combustion of aluminum particles.

Structural response of aluminum core–shell particles in detonation environment**Project supported by the National Natural Science Foundation of China (Grant No. 11772058).

Natural aluminum particles have the core–shell structure. The structure response refers to the mechanical behavior of the aluminum particle structure caused by external influences. The dynamic behavior of the structural response of aluminum core–shell particles before combustion is of great importance for the aluminum powder burning mechanism and its applications. In this paper, an aluminum particle combustion experiment in a detonation environment is conducted and analyzed; the breakage factors of aluminum particles shell in detonation environment are analyzed. The experiment results show that the aluminum particle burns in a gaseous state and condenses into a sub-micron particle cluster. The calculation and simulation demonstrate that the rupture of aluminum particle shell in the detonation environment is mainly caused by the impact of the detonation wave. The detonation wave impacts the aluminum particles, resulting in shell cracking, and due to the shrinkage-expansion of the aluminum core and stripping of the detonation product, the cracked shell is fractured and peeled with the aluminum reacting with the detonation product.

Combustion of frozen bimodal aluminum particles and water mixtures

This paper presents a combustion model of nano-and microsized aluminum mixture frozen in water. The model takes into account combustion of aluminum particles in water vapor, the motion of combustion products, the temperature and velocity differences between particles and gas. The obtained results of the combustion rate depending on pressure and mass ratio between dispersed Al powders are in good agreement with the experimental data described in scientific literature.

Combustion of aluminum particles in a high-temperature furnace under various O2/CO2/H2O atmospheres

The study focusing on the combustion of flowing aluminum particles and the properties of condensed phase products has important guiding significance for the practical application of aluminum-based propellants. Based upon an in-house built dynamic combustion experimental system, the dynamic combustion process of aluminum particles and the properties of condensed phase products under different atmospheres were studied in detail. The microstructure, size distribution and active aluminum content of samples were analyzed by field emission scanning electron microscopy, laser particle analyzer and inductively coupled plasma atomic emission spectroscopy. By monitoring the temperature distribution at different points in the furnace, the heat release of the samples at different positions is approximated, and the combustion efficiency is calculated. In the atmosphere containing CO2, the maximum combustion efficiency can reach the value of 94.41%, followed by that in H2O atmosphere, which had the value of 81.19%. Finally, under the N2 containing atmosphere, the combustion is the weakest, and has the value of only 53.91%, confirming that the combustion followed the following descending order: CO2 > H2O > N2. The condensed phase products were mainly composed of agglomerates formed by the aggregation of particles and alumina smoke. It is well known that the reaction of the sample in the furnace not only follows the melt-dispersion mechanism, but also the diffusion mechanism. The high-speed camera captured four typical combustion forms of aluminum particles during flow, which are stable combustion, release of alumina smoke, crushing and extinction. The average burning time of four stages were studied. The two reaction mechanisms occurring under the same reaction conditions are determined by the nature of aluminum particles themselves.

Ignition and combustion of nano-sized aluminum particles: A reactive molecular dynamics study

Combustion mechanism of nano Al particles remains controversies for years. In this paper, we use ReaxFF molecular dynamics simulations to study thermodynamic and structure properties of Al particles from ignition to combustion. Two typical core–shell nano Al models with different oxide shell thickness are studied. Effect of radiative heat transfer is also considered in the combustion stage. Our results show that no shell cracks are observed during both heating and burning periods. The oxide shell hinders the thermal diffusion of core Al atoms and causes the abnormal melting rule. The stress variations at the core–shell interface are the most dramatic during the heating period. The combustion processes are controlled by diffusion behaviors of components in particles and external oxygen molecules. Detailed structural evolution processes are studied by displacement magnitude analysis. Particle with thinner shell has a shorter ignition delay and a higher combustion temperature which directly affect the radiative heat transfer rate during the combustion period.

Sensitivity analysis and uncertainty quantification on aluminum particle combustion for an upward burning solid rocket propellant

A sensitivity analysis was conducted on an aluminum particle combustion simulation in an upward burning solid rocket propellant, via a computational fluid dynamics code, with comparison to experimental data. Correlations from Beckstead were used to formulate the evaporation rates for aluminum particles. Using one of the correlations for the sensitivity testing, it was found that as the propellant surface velocity increases, the maximum temperature would decrease, but it would increase the gas temperature closer to the propellant surface. Conversely, reducing the propellant surface velocity would increase the maximum temperature, but overshoot the experimental results. The baseline simulation data demonstrates reasonable agreement with the experimental data.

Analysis of aluminum particle combustion in a downward burning solid rocket propellant

In this study, aluminum particle burn in a gap created between a downward facing combusting propellant surface and an upward facing inert surface was modeled with sensitivity testing on propellant surface velocity, propellant surface temperature, propellant surface radiation temperature, parcel count, particle diameter, turbulent Prandtl number and turbulent Schmidt number. Decreasing the turbulent Prandtl and turbulent Schmidt number appeared to have little effect on the maximum temperature. Increasing/decreasing the velocity of the propellant surface and propellant surface temperature showed greater sensitivity on the plume temperature. Perturbation of the propellant surface temperature had the greatest influence on the maximum gas temperature. By increasing and decreasing the surface temperature by ten percent of its original value, a temperature difference of 265.4 K and 248.7 K were observed, respectively.

Impact ignition and combustion of micron-scale aluminum particles pre-stressed with different quenching rates

Pre-stressing aluminum (Al) particles by annealing and quenching alters dilatational strain and is linked to increased particle reactivity. The quenching rate associated with pre-stressing is a key parameter affecting the final stress state within the Al particle, with faster quenching rates theoretically favoring a higher, more desirable stress state. Micron scale Al particles are annealed to 573 K, then quenched at different rates (i.e., 200 and 900 K/min), mixed with bismuth oxide (Bi2O3), and the Al + Bi2O3 mixtures are examined under low-velocity, drop-weight impact conditions. Both quenching rates showed increased impact ignition sensitivity (i.e., between 83% and 89% decrease in ignition energy). However, the slower quenching rate showed a 100% increase in pressurization rate compared to untreated particles, while the faster quenching rate showed a 97% increase in peak pressure, indicating that these two quenching rates affect Al particles differently. Surprisingly, synchrotron X-ray diffraction data show that the 200 K/min quenched particles have a higher dilatational strain than the untreated particles or the 900 K/min quenched particles. Results are rationalized with the help of a simple mechanical model that takes into account elastic stresses, creep in the alumina shell, and delamination of shell from the core. The model predicts that Al powder quenched at 200 K/min did not experience delamination. In contrast, Al quenched at 900 K/min did not have creep but does have delamination, and under impact, delamination led to major fracture, greater oxygen access to the core, and significant promotion of reaction. Thus, the increase in quenching rate and shell-core delamination are more important for the increase in Al reactivity than pre-stressing alone.

Numerical Simulation of Combustion of a Metallized Composite Solid Propellant with Additives of Nanosized Aluminum Particles

The paper presents a numerical simulation of combustion of a metallized composite solid propellant with additives of micron-and nanosized aluminum particles. The model takes into account the thermal effect of decomposition of the condensed phase, convection, diffusion, the exothermic chemical reaction in the gas phase, heating and combustion of aluminum particles in the gas flow, the flow of combustion products, the particle velocity lag relative to the gas. The effect of the Al particle size and mass fraction, emitted from the burning surface, on the burning rate is also taken into consideration.

Exit of a Heterogeneous Detonation Wave into a Channel with Linear Expansion. II. Critical Propagation Condition

Propagation of a detonation wave in monodisperse suspensions of reacting particles (based on the model of the suspension of aluminum particles in oxygen) in channels with linear expansion is studied within the framework of mechanics of heterogeneous reacting media. Reduced kinetics is described with allowance for the transitional (from diffusion to kinetic) regime of combustion of micron-sized and submicron-sized spherical aluminum particles. The effects of the channel width, particle diameter, and expansion angle on propagation conditions and detonation regimes are determined. The critical channel width is found to be a nonmonotonic function of the expansion angle, which is associated with qualitatively different wave patterns behind an oblique step. Flow charts are constructed, and the results are compared with solutions of problems of heterogeneous detonation wave propagation in channels with a backward-facing step and with sudden expansion.

Combustion of a frozen bi-dispersed aluminum-water suspension

This paper presents a combustion model of nano- and microsized aluminum mixture frozen in water. The model takes into account combustion of aluminum particles in water vapor, the motion of combustion products, the temperature and velocity differences between particles and gas. The obtained results of the combustion rate depending on pressure and mass ratio between dispersed Al powders are in good agreement with the experimental data described in scientific literature.

Effect of particle size and oxygen content on ignition and combustion of aluminum particles

Particle size and oxygen content are two of the key factors that affect the ignition and combustion properties of aluminum particles. In this study, a laser ignition experimental system and flame test system were built to analyze the ignition and combustion characteristics and the flame morphology of aluminum particles. A thermobalance system was used to analyze the thermal oxidation characteristics. In addition, the microstructure of aluminum was analyzed by scanning electron microscopy. It was found that the oxidized products were some of the gas phase products agglomerated. Smaller particle size samples showed better combustion characteristics. The combustion intensity, self-sustaining combustion time and the burn-off rate showed a rising trend with the decrease in the particle size. Increasing the oxygen content in the atmosphere could improve the ignition and combustion characteristics of the samples. Four distinct stages were observed in the process of ignition and combustion. Small particle size samples had a larger flame height and luminance, and the self-sustaining combustion time was much longer. Three distinct stages were observed during the thermal oxidation process. The degree of oxidation for small-sized samples was significantly higher than that for the larger particle size samples. Moreover, it was observed that the higher the oxygen content, the higher the degree of oxidation was.

Cellular detonations in nano-sized aluminum particle gas suspensions

Formation of cellular detonation structures in monodisperse nano-sized aluminum particle – oxygen suspensions is studied by methods of numerical simulations of two-dimensional detonation flows. The detonation combustion are described within the semi-empirical model developed earlier which takes into account transition of the regime of aluminum particle combustion from diffusion to kinetic for micro-sized and nano-sized particles. The free-molecular effects are considered in the processes of heat and velocity relaxation of the phases. The specific features of the cellular detonation of nanoparticle suspensions comparing with micron-sized suspensions are irregular cellular structures, much higher pick pressure values, and relatively larger detonation cells. This is due to high value of activation energy of reduced chemical reaction of aluminum particle combustion in kinetic regime.

Modeling and numerical simulation of afterburning of thermobaric explosives in a closed chamber

An Eulerian-Lagrangian simulation of the thermobaric explosion and the propagating detonation waves with aluminum particle combustion in a closed chamber is carried out and validated very favorably with the pressure histories measured experimentally. A novel numerical framework based on the space-time conservation-element and solution-element method (CE/SE) was employed to simulate the complex detonating flow with detailed chemistry. A discrete particle technique was adopted with either a mono-dispersed or poly-dispersed particle distribution. The results between the experiment and simulation on pressure histories and corresponding impulse forces are compared and showed good agreement. In addition, the effects of metal combustion on afterburning were discussed in detail in terms of the reaction mechanisms and particle dynamics.

Study of aluminum particle combustion in solid propellant plumes using digital in-line holography and imaging pyrometry

The combustion of molten metals is an important area of study with applications ranging from solid aluminized rocket propellants to fireworks displays. This work uses digital in-line holography (DIH) to experimentally quantify the three-dimensional position, size, and velocity of aluminum particles during combustion of ammonium perchlorate (AP) based solid-rocket propellants. In addition, spatially resolved particle temperatures are simultaneously measured using two-color imaging pyrometry. To allow for fast characterization of the properties of tens of thousands of particles, automated data processing routines are proposed. Using these methods, statistics from aluminum particles with diameters ranging from 15 to 900 µm are collected at an ambient pressure of 83 kPa. In the first set of DIH experiments, increasing initial propellant temperature is shown to enhance the agglomeration of nascent aluminum at the burning surface, resulting in ejection of large molten aluminum particles into the exhaust plume. The resulting particle number and volume distributions are quantified. In the second set of simultaneous DIH and pyrometry experiments, particle size and velocity relationships as well as temperature statistics are explored. The average measured temperatures are found to be 2640 ± 282 K, which compares well with previous estimates of the range of particle and gas-phase temperatures. The novel methods proposed here represent new capabilities for simultaneous quantification of the joint size, velocity, and temperature statistics during the combustion of molten metal particles. The proposed techniques are expected to be useful for detailed performance assessment of metalized solid-rocket propellants.