Aluminum Droplets Research Articles

Dynamic behavior of metal droplets impacting on porous surfaces

During the operation of solid rocket motors, the behavior of condensed particles impacting the wall will have a remarkable influence on the structure and performance of the engine. Especially when the aircraft is under overload flight conditions, the condensed particles will form a local high concentration particle flow under the action of inertia force, continuously scouring the surface of the insulation layer, seriously affecting the thermal protection structure and the work safety of the engine. Therefore, it is an essential issue to master the behavior mode of the condensate particle impinging the wall and clarify its dynamic characteristics and evolutionary mechanism. In this paper, the dynamic behavior of aluminum droplets impacting on the porous surface is experimentally investigated by preparing the porous wall, the influence mechanism of the porous structure on the spreading process of aluminum droplets is clarified, and the effects of the droplet's initial parameters as well as surface environment are analyzed. Combined with the fluid of volume method, the flow process of droplets on the porous surface is simulated. With the variation of the dimensionless parameters M and N, the main behavior patterns of the droplets obtained so far are rebound, adhesion, partial rebound, partial adhesion, and porous seepage. The presence of pore structure enhances the hydrophobicity of the wall and makes the droplets more easily broken during spreading. When the droplet initial energy is certain and the wall structure conditions change, there is a strong competitive relationship between its spreading and penetration. When the droplet initial energy is increased, its spreading and penetration strengths are significantly increased. The research results can provide a reference for the erosion process of condensed-phase droplets impinging on the char layer and provide theoretical basis and data support for the design and optimization of SRMs.

Study on the wetting characteristics of liquid-Al/ SiO2 interface with Si content in liquid-Al

Aiming to address the issue of aluminum slag adhering to the interior walls of vacuum lifting ladles in the aluminum electrolysis industry, Molecular dynamics simulations are employed to investigate the wetting behavior and adhesion characteristics of aluminum droplets on silica surfaces. Our findings indicate that with an increase in silicon content within the droplets, wetting diminishes and complete wetting of the silica substrate by aluminum droplets is not achieved. The presence of silicon reduces both the melting point temperature and surface tension of aluminum droplets, leading to a significant increase in contact angle and a corresponding decrease in wettability. Moreover, an elevated silicon content within molten aluminum droplets substantially decreases their interaction energy with the substrate surface. The virtual wall method is applied to examine average separation force and work of separation for detaching aluminum droplets from the substrate, revealing that adhesion characteristics are predominantly governed by mutual adhesive forces. This paper presents an atomic-level analysis elucidating how silicon as an alloying element influences adhesion between liquid aluminum and solid surfaces.

Study on the collision characteristics between high-temperature alumina droplets and char layer

The collision characteristics between high-temperature alumina droplets and the char layer in solid rocket motors are of great significance for the accuracy of slag deposition and flow-field simulations, however, the current research on the collision characteristics of the alumina droplets and char layer is still in a blank state. This study is based on the high-temperature molding method to prepare the char layer and compare the porosity with that of the char layer in solid rocket motors, indicating that the two are relatively similar in structure and can be applied to droplet impact experiments. An experimental study on the collision of alumina droplet with the char layer was conducted using a high-temperature alumina droplet impact experimental system. The experimental results show that the adhesion behavior of alumina droplets is related to the rough structure of the char layer and the high viscosity dissipation of the process of droplets impacting the char layer, and the droplets adhere during the retraction stage with violent oscillation. The rebound behavior of the droplets on the wall was characterized by “tail dragging”, “spinning” and “asymmetric rebound” phenomena due to the combination of high surface tension and the pinning effect of wall roughness. A regime map of the rebound/adhesion results of droplets impact the char layers was constructed. At the same speed, droplets with smaller particle sizes are more likely to adhere to the char layer. We established a relationship between the rebound and adhesion behavior. Based on the experimental results, the relationship for the maximum spreading factor of the droplets was established, providing a theoretical basis for the in-depth understanding and study of the droplet collision process in solid rocket motors.

Wetting and Spreading Characteristics of the Impact of Molten Aluminum Droplets on Surfaces

Wetting and Spreading Characteristics of the Impact of Molten Aluminum Droplets on Surfaces

Modulating Wetting States of Molten Aluminum Droplets on SiO2 Surfaces via Vertical Sinusoidal Vibrations.

Vibration affects the wetting behavior of droplets, and it is feasible to use vibration to modulate the adhesion characteristics of droplets. In this paper, the effect of vertical sinusoidal vibrations on the wettability of molten aluminum droplets on the substrate surfaces of smooth and with nanopillars is investigated. The increase in the frequency or amplitude of the vibration leads to a rise in the interfacial potential energy between the molten droplets and the substrate, which in turn leads to the occurrence of the Wenzel-Cassie transition. Once the vibration frequency reaches the threshold values, the molten droplets leave from the substrate, that is, dewetting occurs. The molten droplets in the Wenzel state undergo a Wenzel-Cassie transition before dewetting occurs. A phase diagram describing the frequency thresholds at which the molten aluminum droplets undergo dewetting and the Wenzel-Cassie transition at different amplitudes is plotted. For a specific amplitude, the frequency of vibration required for dewetting to occur in molten aluminum droplets in the Young state is lower than that in the Wenzel state. The needed vibrational frequency for dewetting or the Wenzel-Cassie transition decreases with increasing amplitude.

Unidirectional self-actuation transport behavior of metallic aluminum nanodroplets on SiO2/Fe surfaces

To address the issue of adhesion between liquid aluminum and the inner wall of the vacuum ladle during aluminum electrolysis production, molecular dynamics simulations are used to study the self-actuation behavior of aluminum droplets on the refractory materials SiO2 or Fe. Aluminum droplets are separated from the SiO2 surface by their self-actuation behavior in order to solve the issue of aluminum liquid adhesion to the inner wall. Furthermore, the impact of temperature, size, and wedge angle on the self-actuation behavior of droplets is investigated. The results show that droplet self-actuation is not possible when the droplet temperature is below 900 K. However, when the droplet temperature exceeds 900 K, the speed of droplet motion increases with the temperature rise. Within the radius range of 25 Å–32.5 Å, smaller droplet sizes are more advantageous for the self-actuation behavior of the droplets. The displacement of a droplet on a wedge-shaped wetting gradient substrate is significantly greater than on a rectangular wetting gradient. Additionally, the initial velocity of the droplet's motion increases with the wedge angle. However, a larger wedge angle leads to a decrease in the final displacement of the droplet. The results of the study will offer a new method to address the issue of adhesion between liquid aluminum and the inner wall of the vacuum ladle.

New experimental method for the simultaneous determination of concentration and size profiles of condensed combustion products around a burning aluminum droplet

When an aluminum particle burns in the diffusive regime, a cloud of nanometric particles of its primary oxide, alumina, is formed around the droplet. Characterizing this oxide smoke surrounding a burning aluminum droplet is essential. The original contribution of this paper is to introduce an experimental approach that allows for simultaneous size and concentration measurement of the particles composing the oxide smoke. This paper employs an electrodynamic levitator to facilitate the self-sustained combustion of an isolated aluminum particle, with a radius of 35μm, in atmospheric air. This levitation apparatus is paired with an optical setup that allows for a multi-spectral light extinction method. This technique is specifically designed to measure the size and concentration of alumina particles below the micrometric scale, which is scarcely documented in existing literature. Consequently, spatially resolved concentration and size profiles of the nanometric alumina particles are measured and compared to simulated data from the literature. The radii of alumina particles in the cloud are found to evolve closely around 80 nm. Volume fractions are evaluated from 2.8⋅10−4 near the particle surface to 3⋅10−5 further in the cloud. Therefore, significant concentrations of alumina particles are revealed during the short (15 to 20 ms) combustion process at distances of the order of magnitude of a few initial particle diameters. Such characterization could then lead to a better assessment of the interaction distance among aluminum particles as they burn collectively.Novelty and significance statementThis article introduces the first experimentally determined profiles of alumina concentration and size distribution surrounding a single burning aluminum droplet. The uniqueness of our experimental setup lies in its ability to study of the most fundamental case of the autonomous combustion of a single levitating particle, without any convective effects. A new non-intrusive method specifically designed provides unprecedented results in the field of metallic particle combustion. Data provided in the paper are crucial as they serve as the first reference case for future simulation and experiments. The results introduced in this paper highlight the inability of current simulation methods to accurately account for nucleation and condensation processes, and emphasize the need for a non-invasive characterization method.

Joint utilization and harmless elimination of aluminum dross and refined magnesium slag to simultaneously recover metallic aluminum and fusing agent

Refined magnesium slag and aluminum dross are two typical hazardous solid wastes that contain significant amounts of leachable fusing agent and aluminum droplets encapsulated by dense oxidized films, respectively. This study creatively proposes a safe and green method for the joint utilization of these two wastes. The interfacial reaction behavior revealed that the dense oxidized films of the aluminum droplets were significantly broken by the erosive action of the fusing agent, providing the necessary conditions for the movement of aluminum droplets. Consequently, the aluminum droplets successfully broke free from the oxidized films and separated together with the fusing agent from the dross under the force of supergravity. The recovery ratios of metallic aluminum and fusing agent reached 98.95 % and 98.13 %, while the aluminum and fusing agent contents in the tailings were reduced to 0.82 wt% and 3.71 wt%. The study also discusses the leaching characteristic of the tailings and the scalability for industrial applications of this method in detail. This study not only achieves valuable resource recovery but also reduces the leaching risk and alleviates the land occupation and ecosystem pressure caused by industrial wastes. The tailings can be harmlessly utilized in related fields through subsequent scientific treatment.

VOF based model of numerical simulation for single aluminum droplet evaporation and combustion

ABSTRACT This study investigates the combustion of a single aluminum droplet in a high-temperature convective setting, focusing on the interplay between the environment and droplet evaporation combustion. Using the VOF numerical method, a two-dimensional multiphase flow combustion model for micron-sized aluminum droplets is developed. Experimental validation compares predicted droplet diameter with experimental data. The combustion behavior of aluminum droplets in high-temperature convective environments is analyzed, emphasizing gas-phase combustion during stable stages. Parametric variations in flow velocity and oxygen concentration reveal the formation of vortices around the droplet, influencing aluminum vapor accumulation and evaporation rates. Combustion primarily occurs at the droplet’s windward side, with reaction rates decreasing downstream. Increasing gas velocity from 1.5 m/s to 3 m/s thins the aluminum vapor concentration boundary layer, boosting combustion rates by 35%. Elevating oxygen concentration from 20% to 50% brings the flame closer to the droplet, accelerating combustion rates by 2.7 times, suggesting oxygen concentration’s role in reaction acceleration and combustion duration reduction. This research offers insights for simulating aluminum droplets in propellant applications.

Aluminum oxide droplet collisions: Molecular dynamics study

In this work, the induced coalescence mechanism and dynamic contact behavior of alumina (Al2O3) droplets at different impact velocities were investigated for the first time from a microscopic point of view by molecular dynamics (MD) methods through the analysis of axial speed, shrinkage, neck radius ratio, contact force, temperature, kinetic energy, surface energy, and the amount of change in the internal energy of the droplets. The results show that the minimum speed at which collisional coalescence of Al2O3 droplets occurs is 30 m/s. When the speed is lower than 30 m/s, the droplets undergo bounce phenomena due to the Coulomb force. Under the high-speed impact, the inertia force of Al2O3 droplets acts less than the surface tension and viscous resistance. The droplets don’t get squashed in the whole collision process. For the different initial velocities, the magnitude of the contact force on a unilateral droplet during the collision process does not always increase with speed. When the collision speed is not higher than 400[Formula: see text]m/s, the contact force on the droplets eventually stabilizes at about 0.28[Formula: see text]Kcal/(mol⋅Å), whereas this value is about 0.36[Formula: see text]Kcal/(mol⋅Å) and about 0.5[Formula: see text]Kcal/(mol⋅Å) for the intervals from 500[Formula: see text]m/s to 700[Formula: see text]m/s and from 800[Formula: see text]m/s to 1000[Formula: see text]m/s, respectively. The increase of the droplet’s initial speed has a limited contribution to the temperature of the system after the collision, and the amount of loss of the total energy (the sum of kinetic energy, surface energy, and internal energy changes) becomes more pronounced, even up to about 20% when the speed reaches 900[Formula: see text]m/s. At the same time, we predicted the Al2O3 melting point and compared it with the standard melting point with an error of 2%, proving the accuracy of the model. This work can strengthen our understanding of the industrial processes with applications in high-energy nanomaterials, rocket propellants, rocket structure design and performance optimization.

A numerical study of thermal shrinkage influence on the impact dynamics of alumina droplets thermally sprayed on flat, grit-blasted and laser treated surfaces

Thermal spray splat morphology and its microstructure are of significance to the coating resulting properties. Depending on the material, the changes in density as the droplet solidifies during splat development and solidification could have a substantial role in the particle deformation dynamics, formation of porosity and subsequent adhesion to the flat or rough target surface.In the current study, the effect of shrinkage on the final splat shape is numerically investigated for collisions occurring under several high-speed impact conditions using the Volume-of-Fluid (VOF) method on real surface topography. This influence is also studied for the interaction of two successive impacting particles. The developed models study the splat impact, development and dynamics of heat transfer on flat, laser-patterned, grit-blasted surfaces and numerically designed features. The results of droplet material variable density cases are compared with those assuming constant density. Experimentally obtained single splat depositions are also used to validate the developed models and obtained numerical results.Findings show that micro substrate surface features lead to splat stretching, porosity and limited finger creation subsequent to improved solidification through interfacial contact area increase. Macro surface characteristics, however, lead to splashing and separation from the substrate surface driven by fluid instabilities. Irrespective of the interface topography, shrinkage processes reduce droplet spreading and accentuate splat feature discontinuities such as porosity.

Microscale Surface Defects Influence on Thermally Sprayed Alumina Droplets Deformation and Splashing Dynamics

Abstract During plasma spraying, interaction between splats and surface microsized features can be critical to the splat dynamic progress and consequently to the coating microstructural development and interfacial bonding. The transient spreading of molten alumina impacting a flat substrate exhibiting micro-obstructions, commonly produced during surface machining, grinding and/or even polishing, is numerically investigated using a three-dimensional model comprising of splat solidification and shrinkage developments. Single isolated splats are also experimentally characterized using top surface scanning electron microscope analysis. Droplets impacting directly onto a microsized surface protuberance show no signs of premature splashing behavior. The microscopic features (<2.5 μm) are not able to generate flow instabilities to initially affect the splat inherent overall spreading. However, subsequent splat peripheral contact with target surface micro-obstructions, characterized by peak and valley features, induces peripheral lift, waviness, and instability. It follows that the ejected destabilized material shears/fractures during stretching triggering the formation of splash fingers. Solidification plays a major role in detracting the role of surface micro-obstructions, i.e., surface roughness, in splashing phenomena.

Experimental study of liquid aluminum droplet breakup characteristics based on a Drop-on-demand (DOD) magnetohydrodynamic actuation

In magnetohydrodynamic (MHD) drop-on-demand (DOD) actuation, Lorentz force generated in different excitation stages are exerted on the liquid metal jet to form a "push-pull" mechanism, which is crucial for generating accurate and repeatable metal droplets. In this study, the important parameters of the excitation current waveform in the magnetohydrodynamic (MHD) process, including the influence of the excitation voltage pulse width ratio and current amplitude on the velocity, energy, volume, and breakup length of aluminum droplets are studied. The conversion process of surface energy and kinetic energy during the formation and fall stage of liquid droplets is analyzed. The results show that the negative and positive excitation voltage pulse width ratio of the MHD pump has a significant effect on the droplet velocity, breakup time, size, and sphericity. The amplitude of the excitation current and Hartmann number has a significant impact on the generation of stable droplets. The results show that the single droplet ejection state can be achieved within the Ha number between 319.5 and 391.1. As the Ha number increases, the volume, length, kinetic energy and surface energy of the droplets at breakup time also increase. The size of droplets can be adjusted by changing the amplitude of excitation waveform.

Drag force and heat transfer characteristics of deformable alumina droplets in compressible flows

This paper investigates the force and heat transfer characteristics of deformable alumina droplets in compressible flow. The numerical scheme couples the Navier–Stokes equations with the volume-of-fluid method, fuzzy theory, and a proportional–derivative controller. The effects of the Reynolds number, Weber number, and relative Mach number on the droplet deformation and the drag and heat transfer characteristics are studied. The results show that the fuzzy theory coupled with the proportional–derivative controller allow the droplet to reach the quasi-steady state more efficiently and robustly. The drag coefficient and Nusselt number of the droplet increase with the degree of deformation and the relative Mach number between the flow field and the droplet. The relative Mach number and the Weber number are weakly coupled with the drag coefficient and the Nusselt number. Finally, the inner two-phase flow fields of a solid rocket motor are calculated. The mechanisms whereby particle deformation influence the inner flow field of the solid rocket motor are analyzed.

The effect of crystallographic orientation of α-Al2O3 on the wetting behavior and adhesion characteristics of aluminum droplets

To solve the problem of adhesion of aluminum fluid to the inner wall of the vacuum ladle in the aluminum electrolysis industry, molecular dynamics simulation is performed to research the wetting behavior of Al droplets on the surfaces of the α-Al2O3 substrates C (0001), M ( 11ˉ00 ), and R ( 11ˉ02 ) at 1073 K. Meanwhile, the adhesion characteristics of the Al droplet are evaluated by the potential of the mean force (PMF) for the separation of the Al droplets from different surfaces of the α-Al2O3 substrate. The results show that the wetting behavior of Al droplets on the α-Al2O3 substrate is influenced by the different crystallographic orientations. The diffusion of Al droplets in the x-o-y plane of the substrate exhibits isotropic. The PMF and the interfacial potential energy reveal that the magnitude of the adhesion work in the solid-liquid separation of Al droplets from α-Al2O3 substrates follows the order C (0001) > R ( 11ˉ02 ) > M ( 11ˉ00 ). These findings characterize the wetting properties and adhesion behavior of Al droplets on an atomic scale and provide a theoretical basis for the selection of materials for the inner wall of the vacuum ladle.

Aluminum droplet, oxide cap and flame segmentation in burning Al/AP propellants by combining YOLOv7 and two-stage cluster

Aluminum additives have complex effects on propellant combustion, and high-speed microscopic imaging is a valuable tool to investigate these effects. However, challenges arise from issues like out-of-focus images and grayscale variations, hindering structural information extraction. This study introduces a segmentation method to segment the oxide cap, aluminum droplet, and enveloping flame, combining YOLOv7 detection and two-stage cluster segmentation, integrating geometrical data into the primary cluster. The method is rigorously evaluated with metrics, yielding impressive results: 84.4% Mean Intersection over Union (MIoU), 91.1% Precision (Pr), 92.4% Recall (Re), and 89.3% F1 score. These metrics affirm its effectiveness. Accurate segmentation facilitates the extraction of essential information, including position, shape, and motion data. This information is vital for understanding combustion mechanisms, such as reaction nonuniformity, combustion rate, and motion impetus and the further enlightenment of the investigation of propellents.

Exploring impact, spreading, and bonding dynamics in molten metal deposition for novel drop-on-demand printing

This study delves into the dynamics of molten metal deposition (MMD)-based drop-on-demand (DoD) printing, focusing on the interaction between aluminum droplets and substrates. Nozzle-to-substrate distances below 20 mm prevent droplet pinch-off, while distances exceeding 40 mm result in no droplet-substrate adhesion. Operating within the substrate temperature range of 350–550 °C is crucial, avoiding delamination, and shrinkage pits at lower temperatures, and substrate deformation and heightened oxidation at higher temperatures. Droplet adhesion is impeded at nozzle temperatures below 700 °C. Impact-driven and inviscid, DoD-MMD exhibits spreading outpacing overall solidification, particularly at higher temperatures. Surface tension forces dominate, influencing droplet spreading and leading to underdamped interfacial oscillations. Weak droplet-substrate adherence, facilitated by Van der Waals forces, allows easy droplet detachment, beneficial for successive drop-on-drop deposition. Unique to DoD-MMD, ridges along the droplet periphery act as solidification paths, influenced by thermal contraction and surface tension. The spherical pancake shape of the droplet, characterized by a solidification angle >90°, is elucidated through Weber and Freezing numbers. The final deposited droplet width to initial diameter ratio increases with the droplet temperature and deposition height. In contrast to other metal DoD studies, the spreading factor decreases with a rise in substrate temperature, attributed to intensified oxidation at higher substrate temperatures.

Computation of aluminum droplet ejection and flight in microgravity

Persistent and frequent space explorations are limited by real-time parts supply and it is significant to achieve space manufacture, such as the droplet-based printing. In order to study the microgravity environment effect on droplet-based printing, this paper proposed an effective method to research the effect of space station environmental g-jitter (according to NASA experimental signal) on droplet ejection and flight process. The heat transfer, transient gravity acceleration and Marangoni effect are studied based on the improved model. The results show that the g-jitter is in the order of 10−3 magnitude and the effect on aluminum droplet ejection could be ignored when nozzle diameter (dn) is smaller than 0.1 mm and so do ground case. Combined with the ground microgravity simulation of SnPb alloy lateral ejection experiments, it can be obtained that the ejected single droplet is characterized by capillary number (Ca≤0.002), the droplet flight trajectory is dependent on the Froude number (Fr) and the laminar-turbulence transition exists during the droplet flight process. The relationship formulation between the deviation along gravity direction and the deposition distance is introduced to control and adjust the droplet flight trajectory. Moreover, the temperature effect on material property, such as surface tension and so on, has effect on the droplet breakup length. The g-jitter field enhances the Marangoni effect resulting in the deviation of droplet trajectory. In microgravity field, the computational droplet oscillation period is well coincidence with theoretical equation. Improved KIC-SST based model is available for simulating droplet-based printing in microgravity.

Detailed numerical simulation and experiments of a steadily burning micron-sized aluminum droplet in hot steam-dominated flows

Detailed numerical simulations are conducted in comparison with experimental results to study the flame structure and burning rate of a steadily burning aluminum droplet in hot steam-dominated environments. The droplet surface temperature, flame temperature, and flame stabilization position are measured along with the droplet burning rate estimated from the droplet size evolution. A numerical model accounting for detailed transport properties and chemical kinetics is presented and applied to unveil the flame structure, species and temperature distributions, and heat/mass transfer between the droplet and the surrounding gas. The numerical results of the temperature, velocity, and species distribution profiles demonstrate that the aluminum vapor flame is of classical diffusion flame structure, where near the droplet, there is a non-negligible amount of AlOAl apart from the main product Al2O3(l). This supports the deposition and formation of an alumina cap on the surface proposed in the literature. The simulation correctly captured the flame temperature and flame stabilization distance for a range of droplet sizes. Net heat flux analysis shows that conduction heat from the flame front accounts for less than 30% of the heat needed in aluminum evaporation, which warrants further quantification on other heat sources. The experimental and numerical results enrich the knowledge of the heat/mass transfer and chemical reactions near the droplet, which helps deepen the understanding of aluminum droplet burning.

Development of the Free Drift (FD) Method to Measure the Laser-thrust Generated on an Aluminum Droplet

Development of the Free Drift (FD) Method to Measure the Laser-thrust Generated on an Aluminum Droplet