Combustion Of Aluminum Particles Research Articles

Numerical study on multiphase combustion characteristics of aluminum-based powder-fueled water ramjet engine

Powder-Fueled Water Ramjet Engine (PFWRE) is of great attraction for high-speed and long-voyage underwater propulsion, as well as air-water trans-media navigation applications due to its high energy density and thrust adjustability. However, the complex multiphase combustion process in the combustor significantly affects engine performance. In this study, a detailed model for aluminum particle combustion in water vapor is developed and validated via literature data as well as the ground direct-connected test we conducted. Thereafter, the numerical study on the multiphase combustion process inside the aluminum-based PFWRE combustor is carried out within the Euler–Lagrange framework using the developed model. Results show that a reverse rotating vortex pair before the primary water injection causes particles to flow back towards the combustor head and leads to product deposition. Aluminum particles external to the powder jet have shorter preheating time than internal particles and burn out in advance. The analysis of the particle combustion process indicates that the flame structure inside the combustor consists of the particle preheating zone, the surface combustion heat release zone, the gas-phase combustion heat release zone, and the post-flame zone. In the present configuration, as the particle size increases from 10 μm to 20 μm, the preheating zone length increases from 35 mm to 85 mm. Meanwhile, heat release from gas-phase combustion decreases, and the average temperature of the combustor head first increases and then decreases. This study not only provides insight into the multiphase combustion characteristics of the aluminum-based PFWRE combustor but also offers guidance for the design of the combustion organization schemes and engine structure optimization.

Micron-sized aluminum particle combustion under elevated gas condition: Equivalence ratio effect

Micron-sized aluminum (Al) particle combustion under elevated gas condition is critical for improving energetic material performance, impacting propulsion and explosives technology. This study utilizes Eulerian-Lagrangian method to investigate Al particle combustion dynamics, encompassing both heterogeneous reaction (HTR) and homogeneous reaction (HMR). It focuses on the critical role of the equivalence ratio in single Al particle combustion, highlighting the interplay between HTR and HMR, aiming to optimize energy release and emission control. Our study identifies four stages in the combustion of a single Al particle, where the highest heat release is attributed to HTR, succeeded by HMR, and the minimal from unburned Al vapor. We observe a decline in the total heat release rate with an increasing equivalence ratio, primarily due to the differential impacts of heterogeneous and homogeneous reactions. The thermal-runaway stage in HTR is governed by the particle temperature, while the subsequent decaying stage is influenced by either the diminishing effective Al droplet diameter or the availability of oxygen, contingent upon the fuel conditions. Utilizing Cantera software to analyze HMR allows us to elucidate the thermal effects of elementary reactions and the key reaction pathways. These findings underscore the complex interactions between Al particles and the surrounding gas, providing insights into optimizing the conditions for Al-containing reaction systems.

Measurement of the temperature of burning aluminum particles using multi-spectral pyrometry

ABSTRACT Aluminum is used in the composition of many high explosives and contributes to their blast effect. This work focuses on the study of the combustion of a single isolated particle of size between 30 and 100 µm in an arbitrary atmosphere, whose pressure and composition are chosen to be close to those encountered in a high-explosive fireball. This gaseous atmosphere is still largely unreferenced, and this work aims to provide a better understanding of combustion in this environment. The use of an electrostatic levitator coupled with multi-spectral pyrometric diagnostics enables us to select a single particle and measure its temperature during combustion. Photomultipliers (PMs) are used to follow the temporal evolution of the particle’s light emission during combustion. In this work, we also used PMs as pyrometric devices to measure temperature. The diagnostic allows us to determine the temporal evolution of the integrated temperature of the condensed phase during combustion of the aluminum particle. By way of example, a temperature of 3300K was obtained for the combustion of Al in air at 1 bar. The data collected will also be used to verify the assumptions made in the combustion models (e.g. temperature stability during combustion).

Influence of Aluminum Content and Agglomerates Initial Velocity on Erosion in Solid Rocket Motor

Abstract Despite the aluminized propellants offering a high specific impulse, the challenge of nozzle erosion adversely impacts the rocket's performance and its potential for reusability. This study presents a numerical model aiming to predict the mechanical erosion of the propulsion chamber nozzle. The model employs an Eulerian/Lagrangian approach to simulate the complexity of the flow field within the rocket combustion chamber and the interactions between the continuous phase and particles. The model also includes a simplified representation of the aluminum particle combustion process, besides the consideration of secondary breakup phenomena in liquid droplets. Experimental and numerical data from the literature were used to validate the numerical model. Subsequently, the model was utilized to explore the impacts of increasing propellant aluminum content and varying particles' injection velocities on the nozzle mechanical erosion. The outcomes indicated that higher aluminum content leads to a 4-10% increase in nozzle erosion compared to the 15% content case. Furthermore, the aluminum particles tend not to fully burn within the combustion chamber and contribute to nozzle erosion. Lastly, particles with higher initial velocity at the inlet of the combustion chamber increase the nozzle mechanical erosion despite the observed decrease in incident mass flux.

Comprehensive modeling of ignition and combustion of multiscale aluminum particles under various pressure conditions

The ignition and combustion of aluminum particles are crucial to achieve optimal energy release in propulsion and power systems within a limited residence time. This study seeks to develop theoretical ignition and combustion models for aluminum particles ranging from 10 nm to 1000 μm under wide pressure ranges of normal to beyond 10 MPa. Firstly, a parametric analysis illustrates that the convective heat transfer and heterogeneous surface reaction are strongly influenced by pressure, which directly affects the ignition process. Accordingly, the ignition delay time can be correlated with pressure through the pb relationship, with b increasing from –1 to –0.1 as the system transitions from the free molecular regime to the continuum regime. Then, the circuit comparison analysis method was used to interpret an empirical formula capable of predicting the ignition delay time of aluminum particles over a wide range of pressures in N2, O2, H2O, and CO2 atmospheres. Secondly, an analysis of experimental data indicates that the exponents of pressure dependence in the combustion time of large micron-sized particles and nanoparticles are –0.15 and –0.65, respectively. Further, the dominant combustion mechanism of multiscale aluminum particles was quantitatively demonstrated through the Damköhler number (Da) concept. Results have shown that aluminum combustion is mainly controlled by diffusion as Da > 10, by chemical kinetics when Da ≤ 0.1, and codetermined by both diffusion and chemical kinetics when 0.1 < Da ≤ 10. Finally, an empirical formula was proposed to predict the combustion time of multiscale aluminum particles under high pressure, which showed good agreement with available experimental data.

Modeling of micron-sized aluminum particle combustion in hot gas flow

This paper presents a model for micron-sized aluminum (Al) particle combustion in hot oxidizing environments, forming hollow spheres. The model comprises four sub-models describing the physical and chemical processes during Al combustion: melting of the solid core, ejection of liquid Al droplets from the breaking solid shell, vaporization of liquid droplets, and ignition and establishment of vapor flame surrounding the solid particles. The model is of critical importance when the ambient gas temperature is higher than the melting point of the Al core, Tc,m, but lower than the alumina (Al2O3) shell’s melting point, Ts,m. In the model, the Al core is assumed to be surrounded by a thin, compact alumina shell that blocks the diffusion of oxidizer into the core and prevents surface reactions. The alumina shell’s cracking and liquid Al’s eruption are triggered by thermal expansion and pressure buildup in the liquid core. The splashed liquid Al droplets vaporize quickly and initiate gas-phase reactions, followed by the vaporization of the liquid Al core as the particle temperature Tp increases. Al vapor combustion heat is redistributed to simulate the gaseous flame near the particle. The model is implemented using the Lagrangian particle tracking method and is validated through simulations of micron-sized Al particle combustion in hot gas and comparison with experiments. The results can explain the formation of the sharp-edged holes on hollow aluminum oxide spheres and the ignition behavior observed in the experiments.

Combustion Enhancement of Gel Propellant Containing High Concentration Aluminum Particles Based on Carbon Synergistic Effect.

The addition of aluminum particles to gel propellants can improve combustion performance. However, the agglomeration of aluminum during the combustion process can result in a series of negative effects. In this paper, the aluminum agglomeration inhibition method of gel propellant based on carbon synergistic effect is proposed. Carbon particles exhibit excellent combustion properties, and the gaseous product CO2 generated during combustion can mitigate the agglomeration of aluminum. The research demonstrates that incorporating carbon particles into aluminum-containing gel effectively reduces the incomplete combustion of aluminum particles and increases the volumetric calorific value of the gel. When the mass fraction of carbon is 5 wt%, the volume calorific value of the gel reaches the highest. Meanwhile, the rheological experiments show that the addition of carbon particles can improve the shear-thinning properties of the gel, which is beneficial to the atomization and combustion processes of the gel.

Field recovery from digital inline holographic images of composite propellant combustion base on denoising diffusion model.

Digital inline holography has gained extensive application in the optical diagnosis of solid propellant combustion. However, this method confronts several challenges. Firstly, the calculation time required for reconstruction and depth of field extension is excessively long. Secondly, the excessive smoke, airflow, and flame during combustion cause significant interference and poor reconstruction quality, which reduces the accuracy of particle identification. To address these issues, we have developed a holographic image reconstruction technique for aluminum particle combustion based on the Attention Mechanism, U-net, and Diffusion models. This approach enables end-to-end reconstruction of aluminum particle combustion holographic images, while effectively circumventing the interference of airflow combustion and flame.

Explosion characteristics and reaction mechanism of nano-aluminum/methanol fuel spray in a square closed vessel

Nano-aluminum/methanol fuel has broad application prospects in the field of ship and automobile energy, but disasters caused by its spray explosion tend to be ignored. In this study, a 16.2-L visualization sealed chamber was employed to assess the impact of varying aluminum powder blending ratios (from 2 wt% to 15 wt%) on the flame morphology, flame speed, and explosion overpressure of nano-aluminum/methanol fuel spray explosions. The results show that, compared with pure methanol, the spray explosion flame of nano-aluminum/methanol fuel is more continuous and brighter. Within the microscopic flame structure, the agglomeration and combustion of aluminum particles become more evident as the mass fraction of nano-aluminum powder increases. When the aluminum powder mass fraction surpasses 8 wt%, the flame's instantaneous velocity showcases oscillatory propagation patterns similar to those observed in dust explosions. The Pmax and (dP/dt)max of the nano-aluminum/methanol fuel spray explosion initially increase, then decrease with the rise in nano-aluminum powder mass fraction, peaking at 4 wt%. Compared to pure methanol, the enhancements in Pmax and (dP/dt)max are over 23.1 % and 73.3 %, respectively. By integrating the TG analysis of reactants with the SEM analysis of explosion residues, a combustion mechanism for the single droplet explosion of the nano-aluminum/methanol fuel was established. Numerical simulations suggest that nano-aluminum augments the generation rate and quantity of O free radicals via elemental reactions R2 (Al + O2 <=> AlO + O), R4 (AlO + O2 <=> AlO2 + O), and R8 (AlO2 <=> AlO + O). This amplifies combustion reactivity, expedites fuel consumption, and bolsters flame propagation. The research provides empirical evidence supporting the safe application of nano-aluminum/methanol fuels.

Aluminum particle combustion model in mixture of oxygen and helium

Physical and mathematical models of aluminum particles combustion in oxygen-containing media are developed. In terms of these models substantial regularities of combustion processes and corresponding experimental data are explained. The physical model is based on division of space around particle on preflame region, flame region and region of nonequilibrium processes. In the preflame region a zone of oxygen dissociation is set. In the region of nonequilibrium processes formation of condensed metal oxide does not proceed. In the regions, where equilibrium processes occur, mathematical description of combustion is implemented with taking into account results of thermodynamic calculations. Combustion of particle of initial diameter 200 μm is modeled in medium «79% Не + 21% O2» under pressure 0,1 MPa. Characteristic sizes of regions boundaries, distributions of temperature and oxidative components around particle are determined. Satisfactory agreement of modeling results with experimental data is obtained. Developed calculation methodology can be adapted to analysis of combustion parameters of particles of different sizes in mixtures of oxygen with other gases. Also the methodology may be used in design of technological and energy plants on fuels containing powdery aluminum.

Experimental and numerical investigation of aluminum particle combustion driven instability in solid rocket motors

Aluminum particles are extensively employed to enhance the energetic properties of propellants. However, the heat release rate (HRR) fluctuations resulting from aluminum combustion can potentially affect the stability of solid rocket motors (SRMs). Using a self-made high-pressure combustion diagnostics system and laser ignition system, this work investigates the combustion behavior of single aluminum particle under propellant atmosphere and acoustic excitation. An unsteady aluminum combustion model using Ranz-Marshall correlation and involving the heat and mass transfer equation was developed. The D2 law of aluminum particle combustion in a real propellant under 0.5 MPa was fitted as t = D2/0.31. The measured combustion rate of laser-ignited aluminum particles was found to increase from 0.381mm2/s to 0.465 mm2/s when an acoustic excitation of sine wave(200 Hz, 20Pa) was applied to the propellant, which agreed well with the prediction of the proposed unsteady combustion model whose error was 4.7%. Then, the verified correlation was included in the SRM to calculate the HRR fluctuation of the aluminum on acoustic boundary near the propellant burning surface. The effects of particle diameter, gas velocity, oscillation amplitude and frequency on the HRR fluctuation were obtained. It was found that the HRR fluctuation increases with the increasing particle diameter, gas velocity, and oscillation amplitude, and that due to the acoustic boundary effect, the HRR fluctuation is more sensitive under low-frequency oscillation. Based on the HRR fluctuation of aluminum combustion and particle damping, the growth rate of instability resulting from aluminum combustion in an SRM was investigated. The results showed that the growth rate of instability resulting from aluminum combustion were 9.38s-1 and 24.06s-1, while the particle damping (−25.8s-1) caused by combustion residuals, which suggests the aluminum particles play a critical role in SRMs combustion instability.

Laser ignition of solid propellants using energetic nAl-PVDF optical sensitizers

Laser ignition of propellants could provide better control of the ignition process and improve safety. However, sustained laser ignition of solid propellants is often challenging in practice due to a lack of optical absorptivity at convenient wavelengths. Common optical additives, i.e., opacifiers, improve the absorption characteristics but are typically inert and fail to contribute to the reactivity of the propellant. In this work, ammonium perchlorate (AP)/hydroxyl-terminated polybutadiene (HTPB) composite propellants are optically sensitized by adding reactive nano-aluminum (nAl)/polyvinylidene fluoride (PVDF) particles (ranging from 600 to 1000 µm). These propellants are compared with neat (no additive) AP/HTPB, and other AP/HTPB propellants with carbon black or nano-aluminum (nAl) additives. The nAl/PVDF additives enable sustained ignition with relatively low laser energy levels (< 5 J/cm2) at common wavelengths (1064 nm). Only the propellant with added nAl/PVDF exhibited sustained ignition. The ignition delays of the nAl/PVDF propellant ranged from 2 to 5 ms, depending on the energy density of the laser. We propose that the sustained ignition in nAl/PVDF propellants is due to the lower critical energy needed for nAl/PVDF composites (estimated 0.5 J/cm2 compared to 15 J/cm2 needed for AP/HTPB based propellant). The propellant's calculated critical energy is similar to the reported values of nAl/PVDF composites. The nAl/PVDF based propellant has a faster burning rate and increased pressure dependence due to the PVDF's gas generation and rapid burning of aluminum particles near the surface. Potential applications for this propellant, such as flash ignition and subsurface ignition, are also explored.

Study on the Transformation of Combustion Mechanism and Ejection Phenomenon of Aluminum Particles in Methane Flame

In solid propellants, the combustion of aluminum particles often occurs in a hydrocarbon combustion atmosphere. In order to study the combustion energy release process of aluminum particles during propellant combustion, we carried out a study of the combustion behavior of aluminum particles in the combustion atmosphere of hydrocarbon fuels and conducted experiments using a plane flame burner to observe the combustion process of aluminum particles in a methane plane flame combustion atmosphere. High-speed microscopy revealed a new special combustion phenomenon: ejection combustion with the release of internal components from a point on the particle at high speed, in addition to the already observed particle microexplosions. Both phenomena show faster-than-normal combustion with short combustion energy release times. The experiments also showed that the combustion behavior of aluminum particles changes with the combustion environment. As the ambient effective oxidizer mole fraction increases from 13% to 29%, the basic combustion behavior of aluminum particles changes from vapor evaporation combustion to multiphase surface combustion. In addition, the percentage of aluminum particles burned by ejection increases from 18.2% to 49.2%, which becomes the dominant mechanism in the special combustion phenomenon of aluminum particles. This paper argues that the multiphase surface combustion provides higher heating rates due to the heat production collected on the particles and the diffusion combustion in the air around the aluminum particles, compared with the evaporation combustion. Therefore, the rate of temperature rise within the particle is affected by the ambient oxidant concentration, leading to a transformation from microexplosion to ejection combustion. The effect of the temperature of the combustion environment on this phenomenon has also been investigated through experiments conducted under different conditions.

Mechanical Erosion Investigation in Solid Rocket Motor Nozzle Through Droplet Breakup and Surface Tension Influence

Abstract Erosion prediction of the solid propellent nozzle is vital for its design process. This erosion is caused by the impingement of agglomerated aluminum/aluminum oxide particles on the nozzle walls. Thus, a multi-phase numerical model is established based on the Eulerian–Lagrangian approach to model the aluminum particles burning inside the combustion chamber and simulate the mechanical erosion of the nozzle. The numerical model is validated against numerical and experimental results from the literature. Then it is simplified by eliminating the aluminum particles burning process as they do not reach the nozzle. The simplified model will be further used in modeling the agglomerates’ breakup and predicting the mechanical erosion for aluminum particles with lower surface tension. The results showed that applying the Reitz–Diwakar breakup model reduces the erosion rate by 6.2–24% depending on the injected droplets. In addition, it was found that a decrease in the erosion rate by 1–4.5% can be achieved by reducing the aluminum additive’s surface tension by 15%.

Emission thermometry of microwave-assisted alkali-doped propellant combustion

Recently, the application of high-power microwave fields to sodium-doped aluminized solid propellants has been shown to increase the atmospheric burning rate by up to 60%. Two main mechanisms have been proposed to increase the burning rate: (1) plasma enhancement by the presence of low-ionization threshold sodium and (2) the dielectric thermal runaway heating of metallic-oxide products. To examine the mechanisms of microwave energy deposition in the solid propellant burning strands, optical emission measurements were made during the application of a 2.46 GHz, 870 W microwave field in a single-mode microwave resonator. Flame temperature measurements are made via gas-phase AlO B2Σ+→X2Σ+(Δv=−1) electronic emission spectroscopy, spatially-resolved two-color imaging pyrometry of the condensed phase, and spatially-resolved two-line sodium emission thermometry. These experiments indicate an increase in the gas phase AlO temperature of ∼130 K, and a ∼1200 K increase in the sodium gas-phase temperature during the application of the microwave field. Gray body condensed-phase temperatures of burning aluminum particles indicated little temperature change, but the product plume temperature increased by ∼700 K. These temperature measurements of the vapor and condensed phase products indicate several important pathways for the thermalization of the field energy throughout the propellant flame structure, offering opportunities for further optimization of electromagnetically tunable energetic material combustion.

Aluminum Particle Ignition Studies with Focus on Effect of Oxide Barrier

Aluminum particle ignition behavior in open atmosphere rocket propellants fires is of particular interest for preventing accidents for rockets carrying high-value payloads. For nominal motor pressures, aluminum particles oxidize to aluminum oxide in the gas phase and release significant combustion energy while minimizing motor instability. During rocket abort or launch pad malfunction which occur under atmospheric or low pressure, behavior of aluminum particle combustion becomes complex and aluminum appears to melt, agglomerate or form a skeletal structure. Furthermore, an oxide shell of alumina instantly forms on any fresh aluminum surface which is exposed to an oxidizing environment. Aluminum combustion then strongly depends on the oxide layer growth, which is influenced by causative factors, including particle size, environmental gas composition, and heating rate. This work focuses on the effect of the oxide barrier which forms on the surface of aluminum that is recognized to impede combustion of aluminum in solid rocket propellants. Understanding the mechanism for breach of this barrier is deemed to be an important consideration in the overall process. In this discussion, results of various experiments will be discussed which have a bearing on this process. Basically, a recognized criterion is the melting of the oxide layer at 2350 K is sufficient. However, in other situations, depending on the mechanism of oxide formation, there will occur defects in the oxide shell which provide for aluminum ignition at lower temperatures. For slow heating in an oxidizing environment, where the oxide layer can grow thick, then ignition is more difficult. Because there is no uniform model to establish an ignition criterion due to the unknown history of an aluminum particle, this paper reports experimental findings involving oxyacetylene torch, thermogravimetric analysis with differential scanning calorimeter, aluminum particle heating, electric ignition and aluminum powder heating, to address the influence of the oxide layer on the aluminum particle ignition.

Горение частиц при взаимодействии излучения гиротрона со смесью порошков металл/диэлектрик

As a result of interaction under normal conditions in an air atmosphere of a microwave pulse of a gyrotron with a mixture of Al/Al2O3 powders, ceramic particles of morphology microprocesses are synthesized. A variant of a multi-stage process for the development of physical and chemical processes in a reactor, taking into account ignition. After the end of the microwave pulse, a cloud is observed consisting of hot aluminum oxide particles and burning aluminum particles. A certain influence of the combustion process of aluminum particles on the products of plasma-chemical synthesis. The paper also determines the average velocity of particles from the powder mixture, the burning time of aluminum particles, the surface temperature of the particles of the powder mixture, and the temperature of the gaseous medium in the upper part of the plasma-chemical reactor.

Combustion of hydrogen peroxide gel and micron-aluminum mixtures

A newly reported oxidizer, hydrogen peroxide encapsulated in silica xerogel (hydrogen peroxide gel), is prepared and applied to the combustion of aluminum particles in this study. Linear and mass burning rates of the highly viscous mixture of hydrogen peroxide gel and micron-aluminum (μAl) were measured as a function of mixture composition, pressure, and particle size. Steady-state burning rates were obtained at room temperature using an optical vessel for a pressure range of 3.6–12.0 MPa in argon, an oxidizer-to-fuel weight ratio (O/F) from 0.8 to 2.0, and particle diameters between 1 and 12 μm. The homogeneous mixture of hydrogen peroxide gel and μAl was able to be ignited over a wide O/F range of 0.8–2.0, even at low pressures (e.g., 3.6 MPa). At a nominal pressure of 3.6 MPa, mass burning rates ranged between 4.45 g·cm−2·s−1 (O/F = 0.8) and 1.26 g·cm−2·s−1 (O/F = 2.0), which corresponded to linear burning rates of 35.1 and 8.5 mm·s−1, respectively. In the pressure range of 3.6–10.0 MPa, linear burning rates of the oxidizer-rich mixture (O/F = 1.7) were linearly increased with pressure increasing and the pressure exponent (n) was 1.05. The combustion reaction was kinetically controlled within this pressure range. However, when the pressure beyond ∼10 MPa, there was a transition in pressure dependence and the burning rate tended to be constant. In addition, burning rates of the mixture were inversely proportional to Al particle diameter, increasing by 3.06 times when the particle diameter was decreased from 11.02 to 1.84 μm at stoichiometric ratio.

Numerical study on combustion efficiency of aluminum particles in solid rocket motor

The combustion of aluminum particles in solid rocket motor plays an important role in energy release of propellants. However, due to the limited residence time, aluminum particles may not be burned completely, thus hindering the improvement of specific impulse. This study aims to explore the characteristics of aluminum combustion efficiency and its influencing factors by experiments and numerical simulations, providing a guideline for engine performance improvement. As an input of simulation, the initial agglomerate size was measured by a high pressure system. Meanwhile, the size distribution of the particles in plume was measured by ground firing test to validate the numerical model. Then, a two-phase flow model coupling combustion of micro aluminum particle was developed, by which the detailed effects of particle size, detaching position and nozzle convergent section structure on aluminum combustion efficiency were explored. The results suggest that the average combustion temperature in the chamber drops with increasing initial particle size, while the maximum temperature increases slightly. In the tested motors, the aluminum particle burns completely as its diameter is smaller than 50 μm, and beyond 50 μm the combustion efficiency decreases obviously with the increase of initial size. As the diameter approaches to 75 μm, the combustion efficiency becomes more sensitive to particle size. The combustion efficiency of aluminum particle escaping from end-burning surfaces is significantly higher than that from internal burning surface, where the particle combustion efficiency decreases during approaching the convergent section. Furthermore, the combustion efficiency decreases slightly with increasing nozzle convergent section angle. And theoretically it is feasible to improve combustion efficiency of aluminum particles by designing the convergent profile of nozzle.

Peculiarities of aluminum particle combustion in steam

This work experimentally addresses aluminum combustion in steam, pure or mixed with diluents, for aluminum particles in size range 40∼80 µm, using an electrodynamic levitator. High-speed videos unveil an unreported and complex mechanism in steam, not observed in other oxidizers. The detached flame is quite faint and very close to the surface. Alumina smoke around the droplet rapidly condenses and coalesces into a large, single orbiting alumina satellite. It eventually collides the main aluminum droplet while generating secondary alumina droplets. A unique feature is the presence of several distinct oxide lobes on the droplet, which merge only at the end of burning and encapsulate the remaining aluminum, possibly promoting an incomplete combustion. The measured burning times in pure water vapor are longer than expected and the efficiency of steam is found to be 30% that of oxygen, lower than the usually accepted value of 60%. A general correlation on burning time, including the major oxidizers, is proposed. Direct numerical simulations are conducted and are in line with experiments, in terms of burning rate or flame stand off ratio. Combustion in steam seems mostly supported by surface reactions, giving a faint flame with low gas temperatures and high hydrogen content. It is speculated that those two specific features could help explain the peculiarity of steam.