Molten Droplets Research Articles

The effect of ceramic surface structure modification method on the ignition and combustion behavior of non-metallized and metallized gel fuel particles exposed to conductive heating

The study explores the effect of roughness characteristics of ceramic surfaces processed by diamond abrasives (ground and polished) and laser radiation on the consistent patterns and characteristics of the ignition and combustion of non-metallized and metallized gel fuel (GF) particles based on oil-filled cryogels exposed to conductive heating. Ceramics were produced from SiC F1200 powder using spark plasma sintering. It was established that when combustible liquid droplets in a normal state, as well as metallized and non-metallized GF particles are ignited and burned on a ceramic surface, the ignition delay times can be controlled by the characteristics of the heating surface roughness. Laser surface treatment of ceramics was shown to be a high-potential technique to control the ignition delay times of combustible liquid droplets (±48 %), as well as metallized and non-metallized GF particles (±35 %). A reduction in the ignition delay times of liquid fuels, when using laser surface treatment of ceramics, calls for developed, multi-level roughness with Sdr more than 6.3 %. To decrease the ignition delay times of GF on a ceramic surface by using laser treatment, it is necessary to create a texture with roughness parameters Sdr, Sq, Sz close to the respective characteristics of a polished (molecularly-smooth) heating surface. Evidence was provided that laser surface treatment of ceramics is more promising than mechanical processing using diamond abrasives, which is the most common method currently used in mechanical engineering. This is explained by the fact that laser surface treatment of ceramics creates conditions for enhancing the dispersion of GF melt droplets when ignited, as well as reduces the burnout time by creating a layered multimodal texture of the heating surface, characterized by more developed roughness as compared with the texture resulting from the mechanical processing by means of diamond abrasives.

Flow and Heat Transfer Characteristics of Melt Droplet with Non-uniform Heating Temperature

Flow and Heat Transfer Characteristics of Melt Droplet with Non-uniform Heating Temperature

Effect of Internal Decay Power and Surface Cooling on Nuclear Fuel Droplet Solidification During MFCI in a SFR—A Numerical Study

Melting of the nuclear core is one of the severe accident scenarios in a Sodium-cooled Fast Reactor (SFR). During such an event, molten corium may come into contact with the coolant sodium. This interaction of the molten fuel and the coolant is commonly termed molten fuel–coolant interaction (MFCI) in the nuclear industry. In this study, a numerical analysis is carried out to study the solidification of a molten fuel droplet in the liquid sodium pool. In the first part of the study, the effect of constant internal heat generation on the solidification of the droplet is evaluated with convective heat dissipation prescribed at the droplet surface. The internal heat generation (decay power) and the heat transfer coefficient are varied as parameters, and the time required for complete solidification of the molten droplet is obtained. Based on the results, the freezing of the droplet is categorized into three regimes: conduction limited, transition, and internal heat generation dominated regimes. It is observed that the solidification process of nuclear fuel droplets generated during MFCI is not influenced by internal heat generation and lies in a conduction-limited regime for decay power level prevailing in a medium-sized SFR. Hence, in the next part of the study, the numerical analysis is carried out by incorporating the time-dependent decay power and the temperature-dependent heat transfer coefficient in the computational model by developing user-defined subroutines depicting a realistic scenario of an accident. The results of the analysis show that because of the high subcooling of sodium, film boiling is ruled out; nucleate boiling with a maximum heat transfer rate occurs briefly. The heat transfer coefficient then declines as the interface temperature between the droplet and the sodium decreases rapidly until the natural convective regime is reached. A parametric study on the droplet diameter is also carried out by varying the diameter from 0.5 to 10 mm, spanning the typical particle size spectrum expected during MFCI.

Effect of heat treatment on microstructure evolution and magnetocaloric properties of droplet melted Mn–Fe–P–Si alloys

Practical implementation of Mn–Fe–P–Si alloys requires not only excellent magnetocaloric but also mechanical properties. Although these properties can be improved by droplet melting process, the effect of heat treatment on droplets has not been clarified. An investigation has been carried out on the microstructure and magnetocaloric properties of Mn1.20Fe0.75P0.45Si0.55 droplets annealed at different conditions. Experimental results show that the as-cast sample is composed of Fe2P and (Mn,Fe)3Si phases. Tuning the annealing condition leads to fluctuations in Fe2P phase fraction and compositions, thereby affecting the magnetocaloric properties. Increasing the annealing temperature from 1273 to 1373 K, Curie temperature (TC) increases from 204.0 to 277.0 K, thermal hysteresis (ΔThys) reduces from 2.5 to 0.5 K, and isothermal entropy change (ΔSm) doubles to 18.5 J/(kg·K) under a 5 T field. The increase in annealing time leads to the 34.0% increase in TC to 301.5 K, the 66.7% decrease in ΔThys to 1.0 K, but the slight decrease in ΔSm to 11.9 J/(kg·K) at an annealing time of 8 h. The larger relative cooling power (RCP) at higher temperatures and longer times are related to the increase in the Fe2P fraction. Moreover, optimizing the heat treatment condition to 1373 K and 4 h makes the alloy lay at the ideal border of the first-order and second-order magnetic transition, thus obtaining high ΔSm (18.5 J/(kg·K)), large RCP (508.8 J/kg), and extremely low ΔThys (0.5 K) around room temperature. Consequently, this optimal processing produces a competitive candidate for near room temperature magnetic refrigeration.

Development of technology for the production of granular materials with an ultrafine structure from high-strength aluminum alloys

The processes of obtaining granules of high-strength aluminum alloys В95 and В96ц with an increased content of Zr (up to 0,5 atomic %) under conditions of high crystallization rates of melt droplets are studied. A technology is developed for obtaining pressed semi-finished products from the obtained granules, which ensures the preservation of a finely dispersed structure, and, consequently, high mechanical properties of the obtained material. Keywords: granular material, ultrafine structure, high-strength aluminum alloy, crystallization, degassing, briquetting, pressing, strength. MaximZharov@mail.ru

From impact to solidification in drop-on-demand metal additive manufacturing using MetalJet

Drop-on-demand metal jetting is a promising additive manufacturing (AM) technology that is gaining interest due to its capability to directly print complex single and multi-material components at high resolutions. It also has key advantages over other metal AM techniques, such as avoiding powder handling and extensive post-processing. In this method, parts are built via spatially controlled deposition of individual molten droplets onto a substrate. Therefore, the success of the process entirely depends on the behaviour of these single droplets from deposition to solidification including their interactions with the substrate, which is scarcely investigated to date. To fill this research gap, the in-house MetalJet platform was used to investigate the spreading and solidification of metallic micro-droplets at low Weber numbers. This was undertaken onto various substrates using a range of jetting and substrate temperatures through an integrated experimental, analytical, and computational approach. This study reports that increasing the substrate temperature enhanced the diffusion between the droplet and substrate, hence improving the bonding. Moreover, ripples forming on a droplet’s periphery during solidification disappeared at elevated substrate temperatures, resulting in improved inter-droplet bonding. Furthermore, the significant role of the substrate wettability and thermal properties, which control the droplet’s dynamics and solidification behaviour, respectively, is elucidated. This highlights the importance of substrate material selection using this technology. The results presented in this article underpin the optimal process conditions under which the 3D structures produced with this technology can exhibit reliable integrity and consistency. This represents a step forward in the direct metal printing of high resolution functional multi-material components.

Mechanical Properties of Arc Coatings Sprayed with Cored Wires with Different Charge Compositions

The mechanical properties (hardness, cohesion, and residual stresses) of arc coatings designed for operation under conditions of boundary friction and corrosive-abrasive wear are analyzed. The coatings were formed by arc spraying cored wires (CW) with different charge compositions (the content of carbon, aluminum, and boron in CW charge varied). It is shown that the hardness of the coatings increases with an increase in the carbon content in them up to 1 wt. %, and then decreased due to an increase in the content of residual austenite in their structure. The level of residual stresses of the first kind in such coatings increased by four times with an increase in the carbon content to 2 wt. %. The hardness of the coatings and the level of residual tensile stresses in them also increase with a decrease in the aluminum content in them. In this case, the cohesive strength of the coatings increased due to the implementation of aluminothermic reactions in the droplets of the CW melt during their flight and crystallization on the sprayed surfaces. However, then, with an increase in the aluminum content in the coatings of more than 2 wt. %, their cohesive strength decreased. The level of residual tensile stresses in coatings with a high content of retained austenite decreased after heat treatment (tempering) of the specimens. Sometimes, after tempering, these stresses even transformed into residual compressive stresses (in particular, under using CW C1.4Cr14Ni2). At the same time, the tempering of specimens with a predominance of ferrite in the coating structure increased the level of residual tensile stresses in them, which is due to the precipitation of finely dispersed carbides or borides. It has been shown that the addition of boron-containing components (ferrochromium-boron, chromium-boron) to the composition of the CW charge leads to a significant increase in the hardness of the coatings. Thus, an increase in the boron content in coatings from 0 to 4 wt. % leads to an increase in their hardness from 320 HV to 1060 HV. However, this is accompanied by an increase in tensile residual stresses in the coatings and a decrease in their cohesive strength.

Interfacial bonding flame retarded coating of nylon 6: Anti-dropping, self-extinguishment and water-proof

Melt-dripping is one of the key focuses for flame retardant modification research of nylon 6 (PA6). In this paper, amine-activated cyclotriphosphazene containing siloxane (N-APESP) was prepared via two-step synthesis of cyclotriphosphazene, KH550 and 1,3-propane diamine. Then, Si-O-Si crosslinking structure containing cyclotriphosphazene, called Si-CP structure, was formed on the PA6 surface by interfacial bonding and hydrolytic condensation of N-APESP. The interfacial bonding among Si-CP structure, melamine (MA) and PA6 via the reaction of aldehyde and amino, was characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). It was verified that there was 2–5 μm Si-CP coating containing on the PA6 surface by means of XPS depth profiling. The test results showed that the interfacial bonding Si-CP coating improved the anti-dropping, self-extinguishment and water-proof of PA6 plates. The water proof of the specimens were retained, especially for MA/PA6, with water contact angle of more than 90°. Compared with the UL-94 V-2 rating of the contrast specimens, the interfacial bonding made Si-CP@(10%MA/PA6) reach V-0 rating because of anti-dropping and after-flame time reduction. Meanwhile, the after-flame time of Si-CP@PA6 and Si-CP@(5%MA/PA6) was reduced by 76.34% and 77.56%, respectively. The flame-retardancy mechanism of the Si-CP coating was explored by thermogravimetry, cone calorimetry and char residue characterization. It was Si-CP coating that played an important role in the formation of strengthened expanded bubbly structure with rich P and Si during heating and combustion, so that the barrier effect was enhanced and the melting droplet problem was solved.

Transient Thermo-Fluid Analysis of Free Falling CuCl and AgCl Droplets with Liquid-to-Solid Phase Change

Hydrogen extraction from nature is a time-consuming and energy-intensive procedure. Most of the current methods of extracting H2 are not eco-friendly, and the thermochemical copper-chlorine (Cu-Cl) cycle is a promising alternative since the ingredients are continuously recycled within the cycle without discharging pollutants into the atmosphere. In this study, the heat recovered from molten cuprous chloride (CuCl) salt produced in one of the reactors and quenched in a water bath is analyzed numerically to determine the amount of thermal energy that can be recovered and improve the efficiency of the Cu-Cl cycle. The quenching cell is simulated in an inert atmosphere since CuCl is highly reactive in the presence of oxygen. The interactions of various diameters of CuCl droplets within nitrogen (N2) are numerically modeled in COMSOL Multiphysics. Silver chloride (AgCl) is also used in this study to validate the phase-change process. It was discovered in this study that during the free fall, the outer surface of the molten droplets solidifies, and the phase change of droplets slowly propagates radially inwards, which slows down the energy dissipation. It was also determined that the average internal temperature of the droplet does not change substantially with droplet diameter or quenching height. Based on this study, the net energy recovered after quenching was calculated to be around 23 kJ during 1 kg of H2 production.

Fabrication of stainless-steel microfibers with amorphous-nanosized microstructure with enhanced mechanical properties

Metallic glasses (MG) have attracted much attention due to their superior hardness and good corrosion resistance. However, designing new MG compositions is still a big challenge, and their integration into different systems is limited when they are in the shape of bulk materials. Here, we present a new method for the fabrication of MG in the form of microfibers which could greatly help them to be integrated within different systems. The newly proposed technique has the ability to form MG structure from commercially available alloy compositions thanks to its significantly improved quenching rate(~ 108 K.s−1). In this technique, individual melt droplets are ejected on a rotating wheel forming a thin film which are ruptured upon solidification leading to the formation of MG microfibers. In this regard, we have fabricated microfibers from a commercial DIN 1.4401 stainless-steel which could form a completely amorphous structure confirmed by DSC, XRD, and HRTEM. The fabricated MG microfibers show an increased hardness for more than two-fold from 3.5 ± 0.17 GPa for the as-received stainless-steel to 7.77 ± 0.60 GPa for the amorphous microfibers. Subsequent heat-treatment of the microfibers resulted in a nanocrystalline structure with the presence of amorphous regions when the hardness increases even further to 13.5 ± 2.0 GPa. We propose that confinement of both shear transformation zones and dislocations in the heat-treated MG microfibers plays a major role in enhancing strength.

Effect of Flux Ratio on Droplet Transfer Behavior in Metal-Cored Arc Welding

The effect of flux ratio on metal transfer behavior during metal-cored arc welding was elucidated through investigation using a standard solid wire and three metal-cored wires with flux mass ratios of (2-2) 10%, 15%, and 20%. Investigation was performed using a shadowgraph technique based on images recorded with a high-speed camera equipped with back-laser illumination. We observed that the droplet transfer frequency increased with both the welding current and flux ratio, with the effect of flux ratio being more dominant at low currents. We surmise that this is because the wire sheath area decreases as the flux ratio is increased. Hence, when the welding current is the same, a reduction in the sheath area (i.e., an increase in flux content) leads to an increase in the current density in the sheath, which enlarges the electromagnetic force at the tip of the wire and aids droplet detachment. Conversely, Joule heating is higher at high welding currents than at low currents. This increased temperature shortens the flux column inside the wire, such that the current flow into the molten droplets is more uniform. Hence, the droplet transfer frequency does not increase significantly if the flux ratio is increased in the high current range.

Multi‐Stage Enhanced Removal of Inclusions During Electroslag Remelting Process by a Static Magnetic Field

An axial static magnetic field (ASMF) is superimposed during the electroslag remelting (ESR) process to regulate the inclusion removal (GCr15 bearing steel) in the current study. After applying a 50 mT ASMF, the metal pool of the ESR ingot becomes shallower and the solidified dendritic structure tends to grow axially, together with a higher inclusion removal efficiency. The number of inclusions in ESR ingot decreases, and the inclusion with larger size (>10 μm) is almost eliminated. By tailoring the morphology of the liquid melt film (LMF), droplet, and metal pool, the ASMF applied in this study strengthens the kinetic conditions (inclusion migrated to slag/metal interface) for the inclusion removal, thus realizing the multi‐stage enhanced removal of inclusion during ESR process.

Optical measurement method of non-spherical particle size and concentration based on high-temperature melting technique

Optical particle counter (OPC) is always used to measure particles in high-temperature gas pipelines through gas cooling, leading to the original molten droplets condensing into irregular solid particles. To solve the problem, an optical measurement method of non-spherical particle size and concentration based on high-temperature melting technique is proposed. By establishing the theoretical dynamic model of optical measuring volume (OMV) of commercial OPC (Welas 3000, PALAS GMBH, Germany) with changing gas temperature and studying the thermal expansion characteristics of aerosol conduit, the influence of gas temperature on OPC optical sensor is analyzed, and OPC is applied at 650 °C. On this foundation, irregular glass particles are melted into spherical shape by high-temperature melting technique to realize the optical measurement of particle size and mass concentration. By comparing the online and off-line results of mass concentration, the deviation between them is less than 5%, verifying the feasibility of the proposed method.

The Surface Tension and Density of Molten Sc_{2}O_{3}, La_{2}O_{3}, Y_{2}O_{3}, Al_{2}O_{3}, and MgO Measured via a Pendant Droplet Method

A thermal imaging furnace (TIF) was used to study the physical properties of molten Sc_{2}O_{3} , La_{2}O_{3}, Y_{2}O_{3}, Al_{2}O_{3} and MgO via a containerless method. The density and surface tension of molten droplets of Sc_{2}O_{3} , La_{2}O_{3}, Y_{2}O_{3}, Al_{2}O_{3}, and MgO suspended from their own solid were measured in situ using live video under a range of oxygen partial pressures (1rightarrow 10^{-6} atm O_{2}). To the best knowledge of the authors, this is the first time the surface tension and density of molten MgO and Sc_{2}O_{3} have been reported. A relationship between molten oxide surface tension and the enthalpy of vaporization is proposed, and a comparison is drawn between the oxide properties and similar relationships previously established for liquid metals.

SEINA: A two-dimensional steam explosion integrated analysis code

In the event of a severe accident, the reactor core may melt due to insufficient cooling. the high-temperature core melt will have a strong interaction (FCI) with the coolant, which may lead to steam explosion. Steam explosion would pose a serious threat to the safety of the reactors. Therefore, the study of steam explosion is of great significance to the assessment of severe accidents in nuclear reactors.This research focuses on the development of a two-dimensional steam explosion integrated analysis code called SEINA. Based on the semi-implicit Euler scheme, the three-phase field was considered in this code. Besides, the influence of evaporation drag of melt and the influence of solidified shell during the process of melt droplet fragmentation were also considered. The code was simulated and validated by FARO L-14 and KROTOS KS-2 experiments. The calculation results of SEINA code are in good agreement with the experimental results, and the results show that if the effects of evaporation drag and melt solidification shell are considered, the FCI process can be described more accurately. Therefore, it is proved that SEINA has the potential to be a powerful and effective tool for the analysis of steam explosions in nuclear reactors.

Size distribution evolution and viscosity effect on spherical submicrometer particle generation process by pulsed laser melting in liquid

Spherical submicrometer particles (SMPs) can be fabricated by pulsed laser melting in liquid (PLML). In this process, raw nanoparticles dispersed in liquid are irradiated by a pulsed laser, heated over the melting point to generate molten droplets, and quenched to form SMPs. The particle size increment is caused by aggregating and merging raw particles in liquid by transient heating. In this study, the viscosity effect on the formation process of SMPs by PLML process is studied by changing aqueous glycerin solution concentration. In highly viscous liquid, smaller SMPs are generated due to the ineffective aggregation process. The temporal evolution of cumulative volume frequency of particles can be simulated, considering the constraints of the non-melting at larger size due to high heat capacity and evaporation for smaller particles by overheating. The SMP size can be controlled by viscosity of the liquid, prior disaggregation time, and liquid temperature.

Optimization of Synchropulsed MIG Welding Process Parameters for Welding of AW 5083 Sheets

Metal inert gas (MIG) welding is one of the processes most commonly used for joining metals, especially for joining aluminum and its alloys. The application of a pulsed current in an electric arc allows better controllability of the molten droplets and the arc transition, which subsequently leads to welds with characteristic flaky joints of better quality. In this paper, the optimization of parameters for welding aluminum alloys using the synchropulse welding process is investigated. By observing the input variables that have the greatest influence on the change in appearance of the welding current characteristics (delta wire feed from 0.1 to 6.0 m/min, frequency F from 0.5 to 3 Hz, duty cycle from 10% to 90%), it is possible to perform an optimization to achieve the desired output values. The output variables of the experiments are defined as insufficient/excessive throat thickness (mm), depth of penetration (mm), and weld width (mm); and for the desired quality of the welded joint the most acceptable range of its values is selected, the numerical optimization implementation. The experiment has shown that the delta wire feed has the greatest effect on the observed properties, while the influence of frequency F and duty cycle is somewhat smaller, but the factors responsible for the observed output properties are still significant. From all this, it is possible to select specific values of these input variables to define the best possible observed properties and to determine the characteristics of the defined mathematical models.

Li3PO4 electrolyte of high conductivity for all-solid-state lithium battery prepared by plasma spray

Li3PO4 (LPO) deposits were prepared for the electrolytes of all-solid-state lithium batteries (ASS-LBs) by atmospheric plasma-spraying (APS) with sintered powders of three different sizes. The effect of LPO particle size on the microstructure, chemical composition, lattice structure, and ion conductivity of LPO deposits was systematically investigated. Results showed that instead of the conventional lamellar-structured splats with limited inter-splat bonding, columnar-structured grains with chemical inter-splat bonding were formed in APS LPO due to its low melting point. The porosity of the deposits decreases with the decrease in feedstock particle size and deposits with porosity less than 1% can be deposited by using powders smaller than 50 µm. The XRD presented that all deposits have the same phase compositions as that of the sintered powders. Due to more intensively vaporization of P than Li from the molten droplets during APS, the Li/P atomic ratio in the deposits was higher than the stoichiometric ratio of 3 and ranges from 3.02 to 3.17. Owning to the high-quality inter- splat bonding and the slightly deviated Li/P atomic ratio, a high room-temperature ionic conductivity of 4.65 × 10−8 S.cm−1 was achieved for APS LPO deposit by using the LPO particles with sizes of less than 30 µm.

Critical conditions for melting of metallic wire in induction heating system through numerical simulation and experiments

Induction heating is a novel energy source that can be utilized in the melting of metallic wires for the development of green manufacturing processes such as drop-casting and additive manufacturing. The present work aims to find out the critical radius of the feedstock wire to be melted through an induction heating system. An analytical model is developed to find an initial assessment of the feasible parameters and assists the 3D coupled electromagnetic-thermal model to study the melting behaviour of the mild steel wire. Through these models, the time required to reach the melting point of the wire for various process conditions has been investigated. The coil current and the number of turns of the coil primarily influence the melting duration of the wire. The experiments on a high-frequency induction heating system suggests that the critical wire size of 1.6 mm transfer the metal droplets at 550 A coil current, 15 mm inner coil radius, 3-turn induction coil and 353 kHz frequency. At this point, the finite element model acts as an aid to develop the experimental system and provides satisfactory results when compared with experimental data. This work effectively presents a mathematical framework where critical size of wire is identified to create molten droplets in a specific induction heating system.

Electro-spark deposition of amorphous Fe-based coatings in vacuum and in argon controlled by surface wettability

Amorphous Fe-based coatings deposited on AISI 420S steel by electro-spark deposition in argon at atmospheric pressure and in vacuum using a Fe41Co7Cr15Mo14C15B6Y2 electrode were studied. At the same pulse energy, melt droplets formed during processing in vacuum wet the substrate surface better. This is due to the in situ formation of a thin layer during arc evaporation of the electrode in vacuum prior to the melt transfer. Vacuum-deposited coatings exhibit significantly lower corrosion current and wear rates in seawater.