Aluminum Droplets Research Articles

A PHYSICAL MODEL FOR THE PREDICTION OF ALUMINUM OXIDE RESIDUES

This work proposes a new model to estimate the size of oxide residues produced during the combustion of aluminum. Such large oxide residues contribute to the two-phase loss performance in solid rocket motors. The model includes two physical mechanisms: deposition of oxide smoke from the surrounding flame and oxide creation on aluminum surface by heterogeneous reactions. Parametric numerical simulations at the aluminum droplet scale are performed to estimate the rate of deposition and surface production, in order to feed a zero-dimensional model of oxide lobe growth and final oxide residue. Results are in line with available measurements with an oxide residue-to-aluminum size ratio in the range 0.5 ~ 0.8, depending on conditions. The model suggests that surface reactions are important for low pressures and small particles, while deposition prevails otherwise.

Wetting Characteristics of Aluminum Droplets Before and After Embedding Carbon Atoms on the Surface of Heterologous Silicon Carbide

Wetting Characteristics of Aluminum Droplets Before and After Embedding Carbon Atoms on the Surface of Heterologous Silicon Carbide

Heat transfer process in the collision of high-temperature aluminum droplets with an inclined wall

The heat transfer process occurring when high-temperature aluminum droplets collide with a solid surface is investigated via numerical simulation and experimentation. The wall temperature at the impact point rapidly increases, with some oscillations, reaching a maximum value of 1054.5 K. The maximum wall heat flux density during the droplet–wall collision is 63,135 kW/m2. As the droplet spreads on the wall, the temperature at the impact point undergoes a two-stage decrease: a rapid decrease followed by a slow decrease. During the initial stage of the droplet–wall collision, the wall temperature is asymmetrically distributed, with the increase in temperature at the advancing edge spreading more slowly than that at the receding edge. Similarly, the corresponding heat flux density distribution at that time is also asymmetrically distributed, with higher heat flux density at the trailing edge of the spread than at the leading edge.

Study on the dynamic numerical simulation of flow and combustion in hybrid rocket motors based on a discrete phase model

Numerical simulation based on the discrete phase model and dynamic mesh model can more accurately simulate the working process for hybrid rockets, including the motion and evaporation process of particles and regression process of the fuel grain. The dynamic numerical simulation of combustion and flow in hybrid rocket motors based on a discrete phase model is carried out. Aluminum and aluminum hydride are added to the internal flow field as discrete droplets. Then, a hybrid rocket is designed with 95 % hydrogen peroxide as oxidizer and 38 % aluminum hydride+20 % aluminum+42 % hydroxyl-terminated polybutadiene as the fuel and used to verify the model by firing test. A high degree of agreement between experiment and simulation findings is found. The average deviations for thrust and pressure are 3.6 % and 5.9 %, respectively. It offers the required backing for using the simulation method to investigate the flow and combustion process for the hybrid rockets. The results also demonstrate the transient characteristics of the inner flow field, including the separation and merging of vortexes in the pre-chamber. It is found that a peak of the regression rate appears at the front of the grain and moves backward. This is mainly because the vortex appears here and enhances its heat transfer. Through the discrete phase model, it is worth noting that the aluminum droplet content gradually decreases at the nozzle over time, which is consistent with the firing test. Further research on the impact of aluminum hydride content on the performance of the hybrid rockets has been carried out. The results show that the higher aluminum hydride content can lead to better performance for the hybrid rockets. The thrust, pressure, specific impulse, and characteristic velocity enhance with the increase of aluminum hydride content. An interesting discovery is that the average regression rate is maximum at 48 % aluminum hydride content. This research innovatively couples dynamic mesh and discrete phase models, which can help better understand the working process for hybrid rockets and provide important guidance for the selection of fuel formula.

Impact characteristics of high-temperature alumina droplets on a cold wall: Rebound and adhesion

The adhesion and rebound characteristics of alumina droplets in solid rocket motors are important for accurate prediction of slag deposition. This paper develops an experimental system for high-temperature alumina droplets impacting the wall. The system uses induction furnace to melt the alumina and use argon to squeeze the liquid alumina to obtain high-temperature alumina droplets. The particle diameter of the alumina droplets is 0.2 mm ∼ 6 mm, which can be used to explore the impact characteristics of the alumina drop on the cold wall. The experimental system is used to carry out the droplet impact experiments on the tungsten wall at different temperatures, and the droplet temperature is obtained by the two-color pyrometer. The studies show that the contact line stops moving during spreading when the small micron droplets adhere; while the larger droplets adhere, the contact line stops moving during retracting. When alumina droplets impact with the wall vertically then rebound, two sharp corners will be formed near the contact line. When they impact with the wall obliquely then rebound, the primary sharp corner and the secondary sharp corner will be formed gradually. The regime map of adhesion and rebound is obtained, and it is found that the condensation effect will dominate the adhesion stage when the impact energy of the droplet is small.

Chromium–Aluminum Coatings for Oxidation Protection of Titanium–Aluminum Intermetallic Alloys

This article explores the utilization of cathodic-arc deposition Cr-Al overlay coatings as oxidation protection for Ti-Al-Nb intermetallic alloys. The primary objective is to investigate PVD Al-Cr coatings deposited via cathodic-arc deposition without subsequent vacuum annealing. The microstructure, phase, and chemical composition of the coatings were characterized using scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction analysis. Isothermal exposure of samples in a laboratory air furnace was conducted, revealing the efficacy of Cr-Al coatings in protecting the Ti49-11Al-40Nb-1.5Zr-0.75V-0.75Mo-0.2Si (mass%) intermetallic alloy VTI-4 against oxidation. The findings highlight that the as-deposited coatings possess a layered structure and contain Al-Cr intermetallics. Post-exposure to the furnace without prior vacuum annealing results in coatings exhibiting a porous microstructure, raising concerns regarding oxidation protection. This investigation of Cr-Al coatings on a VTI-4 alloy substrate yields valuable insights into their nanolaminate structure and challenges associated with aluminum droplet fractions. The proposed additional vacuum heat treatment at 650 °C for 500 h effectively homogenizes the coating, leading to predominant Cr2Al and Ti-Al phases. Additionally, the formation of diffusion layers at the “coating–substrate” interface and the presence of oxide barriers contribute to the coatings’ heat resistance. Our research introduces possibilities for tailoring coating properties for specific high-temperature applications in aerospace, energy, or industrial contexts. Further refinement of the heat treatment process offers the potential for developing advanced coatings with enhanced performance characteristics.

A new strategy for eliminating bottom hole defects during aluminum droplet printing within a broad temperature range

A new strategy for eliminating bottom hole defects during aluminum droplet printing within a broad temperature range

Effect of Nanopillars on the Wetting State and Adhesion Characteristics of Molten Aluminum Droplets.

To solve the adhesion problem between molten aluminum and vacuum ladle liner during the electrolytic aluminum production process, the wetting state and adhesion properties of molten aluminum droplets on substrate surfaces with different nanopillars are investigated based on molecular dynamics. The results show that the adhesion strength of molten aluminum droplets in different wetting states has the pattern Young state > Wenzel state > Cassie state. Effects of increasing nanopillar height or interval are poles apart in the wetting state and adhesion characteristics of aluminum molten droplets. The critical height and critical interval of the nanopillar where the wetting state transition occurs are obtained. The increase of the nanopillar width can induce the wetting state transition from the Cassie state to the Wenzel state. In addition, the phantom wall method is applied to study the variation of the separation force. It is found that a peak in the separation force curve occurs when the molten droplet separates from the bottom of the nanopillar interval or the top of the nanopillar. The separation force curves of the droplets in the Young state and the Cassie state have single peaks, while the droplets in the Wenzel state have double peaks.

Combustion of 10–100 μm aluminum droplets in detonation products gases

AbstractWe describe a two‐phase model of combustion effects in aluminized high explosive (HE) charges. It is based on: (i) a Gas Dynamic Model of the expansion of the detonation product gases and their turbulent combustion with air; and (ii) a Heterogeneous Continuum Model of aluminum (Al) droplets and their combustion with the detonation product gases. Initial conditions are based on an analytical similarity solution for a cylindrical Chapman‐Jouguet (CJ) detonation propagating at the CJ detonation velocity. The CJ jump conditions are computed at the thermodynamic equilibrium state by the Cheetah code, assuming the Al droplets are inert. We assume that the Al is 10 % of the charge mass and occurs as droplets at the CJ state. Different initial droplet diameters, ranging from 10 to 100 microns, are studied. A hydrodynamic combustion model based on large Damköhler numbers is employed in this study, . It has a square‐root dependence on the Reynolds number (Re) and inversed‐squared dependence on the droplet diameter ( ). The burnout time ( ) of the Al droplets has a three‐halves dependence on the droplet diameter, . After burnout, the detonation products act as detonation products of the HE charge with active Al. They turbulently mix with air and form a combustion layer on the outer edge of the fireball. Details of the two‐phase model, initial conditions and evolution of the flow field will be described.

Study on heat transfer model of solid rocket motor combustor under high-temperature two-phase flow scour state

To address the issues with the multi-phase flow and heat transfer characteristics of the gas inside solid rocket motor combustor, this paper first establishes a mathematical model form that can express the heat transfer characteristics between gas and wall, and obtains the mathematical model of heat transfer by means of experiment and numerical simulation. The results show that heat transfer in the wall surface has obvious hysteresis during the actual working process of solid rocket motor, and that temperature in the wall surface will gradually reach the peak value after the motor works. The heat transfer trend between the numerical simulation results and the experimental data is relatively consistent, and the errors of heat transfer calculation in the stable working section of the motor can be controlled by 15%. It shows that the calculation model with the Sommerfeld number as the criterion selected in this paper can better reflect the heat transfer characteristics between the alumina droplet and the wall in combustor. The maximum error between the fitted value and calculated value is 9.43%, which indicates that the numerical model of heat transfer established under the state of two-phase gas and wall scouring has certain reliability.

An environmental-friendly method for recovery of aluminum droplets from aluminum dross: Mechanical activation and super-gravity separation

Aluminum dross produced from the smelting process of aluminum is a toxic solid waste that easily hydrolyzes and releases poisonous gases in contact with water, which contains large numbers of fine oxidized aluminum droplets. In this study, an environmental-friendly method for efficient recovery of aluminum droplets from aluminum dross was proposed through mechanical activation and super-gravity separation. Firstly, the aluminum dross with different particle sizes of 8.0–50.0 mm, 4.0–8.0 mm, and < 4.0 mm was mechanically activated, and the oxidized films of metallic aluminum droplets were greatly destructed via milling. However, the driving force of gravity was not sufficient to drive the molten aluminum droplets to detach from the oxidized films at high temperatures. With the enhancement of the super-gravity field, almost all of the aluminum droplets efficiently escaped from the oxidized films and fully recovered from the aluminum dross within 5 min with a high recovery ratio of 97.14% and a high purity of 99.17 wt%. Compared to the conventional process of separating aluminum, this study provides an environmental-friendly method to efficiently recover aluminum droplets from aluminum dross without the problems of secondary aluminum dross production, dust pollution, and gas pollution.

Динамічні процеси у твердопаливних ракетних двигунах та їх взаємодія з вібраціями конструкції ракети: стан питання та актуальні проблеми

The most critical operating conditions of solid rocket motors (SRMs) are often due to the development of dynamic processes characterized by excess values of operating parameters. Pressure surges and a sharp increase in the combustion product temperature may impair the strength of the combustion chamber structure, cause its failure, and lead to critical conditions of the motor operation, up to extinguishing the propellant combustion in the motor. It is shown that both in steady and in unsteady operating conditions of an SRM, dynamic processes in its combustion chamber feature a complex interrelation of a large number of processes in the gas-dynamic space of the combustion chamber: physical, chemical, and thermodynamic (heat and mass exchange) processes. It is found that current studies of SRM operation instability are aimed at identifying mechanisms of combustion chamber pressure oscillations, which are usually due to combustion product vortex formation in the chamber space and acoustic feedback resulting from collisions of vortices with the SRM’s combustion chamber components or nozzle. Other lines of investigation are the analysis of SRM resonant damping and the establishment of a relationship between aluminum droplet combustion and SRM internal instability. It is noted that accelerations and vibrations of mixed-propellant combustion surfaces may greatly affect the combustion rate and the agglomeration, on-surface confinement, and burn-up of metal additives, which, in its turn, governs the combustion chamber acoustics. It is pointed out that the interaction of SRM combustion chamber pressure oscillations and the response of the SRM structure observed in flight tests of some rockets should be taken into account in predicting the stability of SRM dynamic processes. This interaction may call into question the sufficiency of SRM static tests and subsequent conclusions on the magnitude of its dynamic effect on the rocket structure.

Measurement of alumina film induced ablation of internal insulator in solid rocket environment

This study investigates the ablation of internal insulation induced by the deposition of an alumina film with different lateral film speeds. A sub-scale test solid rocket motor (SRM) was designed in an impinging jet configuration to form an alumina film on the sample and to encourage the lateral movement of the film by a high-speed wall jet. Fifteen static fire tests of the test SRM were conducted with six different jet velocities (Vjet = 100 m/s, 150 m/s, 200 m/s, 268 m/s, 330 m/s, and 450 m/s) that indirectly affected the velocity of the wall jet and the deposition rate of alumina droplets. The ablation velocity was deduced from the difference in the sample thickness after a test using a coordinate measuring machine. The droplet deposition mass flux and wall jet velocity were obtained via two-phase flow simulation with the same jet velocity and effective pressure. As a result, the characteristics of alumina-induced ablation and the changes in ablation with jet velocities were obtained. The area within 0.8 × jet diameter was focused upon, where the ratio of ablation velocity to incoming alumina mass was constant for each jet velocity, and showed a similarity in jet structure. When the ablation velocity was increased from 2.05 to 9.98 mm/s with increasing jet velocity, the ratio of the ablation velocity and alumina mass flux decreased from 1.07×10−4 to 0.49×10−4 m3/kg as Al2O3–C reactions became less efficient with a reduced residence time of the film. Because the decrease in residence time by the wall jet is more pronounced for slow reactions involved in Al2O3–C reactions, fast reactions in Al2O3–C reactions are less affected and result in a convergence of the volumetric rate of ablation per unit mass of alumina.

Numerical study of the impact and aggregation characteristics of alumina droplets on a wall in the solid rocket motor

The impact and aggregation of multiple alumina droplets on the inner wall of the solid rocket motor may generate a larger rebound aggregated droplet or adhere to the wall to form a liquid film. The rebound and adhesion characteristics of the aggregated droplet are of great significance for slag deposition and two-phase flow prediction in the motor. In this paper, the VOF (Volume of fluid) method is used to study the impact and aggregation process of multiple alumina droplets on the wall. Detailed numerical verifications are carried out, including the impact and aggregation of moving droplets, the aggregation and rebound of stationary droplets and the impact process of a single alumina droplet on the wall. It is found that the aggregated droplet will rotate when two droplets aggregate under small droplet diameter ratio, which is caused by the asymmetry aggregation process. The oscillation process intensifies when three droplets impact and aggregate on the wall, as a result, the aggregated droplet is easier to adhere to the wall. The viscous dissipation in the aggregation of droplets is huge, and the total viscous dissipation energy accounts for about 50% of the total energy when the aggregated droplets rebound. The influence of droplet diameter on the V⁎ (rebound recovery coefficient) of the aggregated droplet and the rebound/adhesion results are dominated by the viscosity effect. While the influence of the parameters involving the initial spatial arrangement of droplets is dominated by the oscillation process, which will reduce the effective rebound kinetic energy, making a lower V⁎ and the aggregated droplet easier to adhere.

Study of the 3C-SiC surface crystalline orientation effect on the wettability of molten aluminium droplets by molecular dynamics simulation

Study of the 3C-SiC surface crystalline orientation effect on the wettability of molten aluminium droplets by molecular dynamics simulation

Combining a population balance approach with detailed chemistry to model the condensation of oxide smoke during aluminum combustion in spatially homogeneous reactors

At high temperatures, aluminum particles can be burned exothermally in oxidizing atmospheres in a similar way to carbon-based particulate fuels. The main reaction product, aluminum oxide (Al2O3, alumina), is solid under ambient conditions and may form through two distinct chemical pathways. On the one hand, vapourized aluminum burns in the gas phase, leading to the condensation of aluminum oxide into very fine smoke droplets that are thought to be nano-sized. On the other hand, heterogeneous reactions at the particle surface cause a direct conversion of aluminum into aluminum oxide and result in the transformation into a bi-phasic aluminum/alumina particle. As aluminum powder is being investigated as a potential carbon-free, recyclable energy carrier, we face unanswered questions about the droplet sizes within the oxide smoke and the techniques required for extraction from flue gases. This article constitutes a first step towards a detailed modelling framework for predicting alumina smoke size distributions.Physically, the oxide droplet size distribution is influenced by the ambient gas phase composition and shaped by the mutual competition of nucleation, condensational surface growth, evaporation, dissociation and coagulation. The heat release and dispersion temperature, on the other hand, are affected by chemical reactions, the gas-to-liquid phase change and radiation. In this article, we present a complete set of droplet formation and interaction kinetics for alumina droplets and analyze the interaction, competition and mutual reinforcement of the relevant physical processes in a perfectly stirred reactor and a partially stirred reactor. These simplified model formulations are representative of the dynamics that characterize the changes in gas phase composition and droplet size distribution in a single grid cell during the reaction and droplet formation fractional steps of a spatially inhomogeneous laminar reactive flow solver or a one-point, one-time probability density function (PDF) description. The partially stirred reactor model is extended to account for the presence of a reactive surface with small-scale variability in terms of surface composition. Detailed chemical and heterogeneous surface kinetics, also including NOx formation, are taken into account. Finally, we point out open challenges and incomplete kinetic or constitutive information.

Study on flow and heat transfer characteristics of 3D molten aluminum droplet printing process

The molten metal droplet printing technology can effectively realize a precise preparation of complex electronic devices at the micro scale by relying on the metallurgical bonding among the tiny molten metal droplets, which can improve the metallographic structure and mechanical properties of the fabricated parts. In this paper, the interaction process between metal droplets and movable substrates during the preparation of electronic devices is studied based on the coupled level set volume of fluid method (CLSVOF) and equivalent heat capacity method. The influence of the micro-bubbles and wall infiltration characteristics on the impingement spreading flow of metal droplets and their heat transfer cooling process is also investigated. The evolution mechanisms of droplet geometry and the distribution characteristics of droplet internal temperature and heat flow density during the impingement spreading process are discussed. The mechanism of high-temperature region formation under superhydrophobic wall conditions is analyzed. Results show that the microbubbles inhibit the spread of molten metal droplet. Under the same conditions, the spreading radius of hollow metal droplet is smaller than that of solid droplet, and their spreading height is higher than that of solid droplet. Different wall infiltration characteristics have similar degrees of influence on the spread of solid and hollow droplets. A larger wall contact angle for the same impingement mode at the same moment corresponds to a smaller droplet spreading radius and the higher the spreading height. With the participation of microbubbles, the heat transfer cooling process inside metal droplet becomes slow and demonstrates a less uniform temperature and heat flow density distribution. A high-temperature region is easily formed inside droplet under superlyophobic wall conditions, which triggers an uneven heat transfer cooling process at the bottom of the droplet. The results benefit to reveal the law of metal microdroplet impingement molding on different working surfaces and provide a theoretical reference for the preparation of electronics-related devices.

Research on the collision model of high-temperature alumina droplets with cold wall for solid rocket motors

The study on the adhesion and rebound behavior of high-temperature alumina droplets impacting on the cold wall is very meaningful for the accurate prediction of slag deposition and internal flow field in solid rocket motors. In this paper, based on high-temperature alumina droplet impact experimental system, the experiment of alumina droplet impacting on the cold wall was carried out, and the temperature response of alumina droplet during the impact is obtained by the colorimetric thermometry. The study shows that the high-temperature alumina droplets impacting on the cold wall in a certain range of Oh number and Re number exhibit two kinds of behaviors: rebound and adhesion; based on experiments, the distribution laws of rebound and adhesion behaviors of alumina droplets impacting the cold wall are obtained; based on the energy conservation model, and introducing the heat transfer model between the high-temperature droplet and the cold wall, the maximum spreading coefficient model of alumina droplet impact on the cold wall and the prediction model of rebound and adhesion results were established, and the maximum spreading coefficient model prediction results were in good agreement with the experimental data.

Rebound characteristics of burning aluminum droplets impacting surfaces

An experimental system is established to study the behavior of burning aluminum droplets impacting a wall in a solid-rocket-motor environment. First, the rebound process of droplets impinging on the wall is observed using shadow shooting; the size range of the burning aluminum droplets is 150–500 μm, and their range of impact speed is 1–5 m/s. Second, a formula is developed for predicting the droplet–wall contact time, and combined with energy conservation, a model for predicting the behavior of aluminum droplets impacting the wall is established. Furthermore, analyzing the droplet velocity reveals that the total restitution coefficient decreases gradually with increasing Weber number, whereupon a model for calculating the restitution coefficient is proposed. Compared with experimental results, the errors of the prediction model are within 15%. Finally, a new phenomenon involving the oscillatory motion of burning aluminum droplets is discovered; analyzing this oscillatory behavior gives an equation for the attenuation of the oscillation of droplets moving in a gas flow, and the results of this equation agree well with experimental data. The present experimental system and measurement method are suitable for fine measurements of the rebound process of burning aluminum droplets, and the present results lay a foundation for (i) studying the law governing the motion of aluminum droplets in a solid rocket motor and (ii) revealing the mechanism for interaction between them and the inner surface of the combustion chamber.

Dynamical behaviors of aluminum droplets impinging on horizontal and inclined surfaces

Aluminum (Al) droplets collide with the wall of the combustion chamber in a solid rocket motor at different angles of incidence and velocities, which exhibit rebounding, splashing, and other behaviors. It increases the loss of two-phase flow to degrade the performance of the engine and poses a challenge to the thermal protection system. In this study, an experimental system in which molten Al droplets impact the walls of the chamber is designed. The size of the Al droplets produced by the generator ranged from 120 μm to 2000 μm while their speeds of impact ranged from 0.85 m/s to 9 m/s. This was used to examine the process of impact of Al droplets on flat and inclined walls. The residence time and spreading diameter of the droplets were quantified to reveal their mechanism of collision. Their empirical correlations were obtained by regressing the experimental data, and the predictive error of the model was within 20% of the experimental values. Furthermore, a theoretical model was developed to analyze energy distribution during the collision between the droplets and the wall. It showed that the effect of viscous dissipation energy on droplet flow was negligibly small, but the adhesion energy of the droplet increased significantly with its initial energy. In addition, the dimensionless sliding distance was S≈8 when the tangential Weber number was greater than 90, where this was related to the high surface tension of the Al droplets. The results here provide a theoretical basis for the design of thermal protection system of solid rocket motors.