Interactions between molten metal droplets impinging on a solid surface

We photographed molten tin droplets (2.2 mm diameter) landing off-center on a circular splat formed by the impact and solidification of another, identical drop. Final splat shapes were sensitive to the spacing between droplet centers, which was varied from 1.0 to 5.0 mm. We used a three-dimensional model of spreading and solidification to simulate interactions between droplets. The model applied a fixed-grid Eulerian control volume method to solve the fluid dynamics and energy conservation equations. A volume-of-fluid algorithm was used to track free surface deformation. Predictions of droplet shapes during impact from the model agreed well with photographs. By following temperature variations at different points on the surface of the first splat we could identify locations where remelting occurred and the splats fused together. Splat shapes observed in experiments with large tin droplets qualitatively resembled those obtained by plasma-spraying nickel powders on a steel surface.     

Non-equilibrium solidification of the molten metal droplets impacting on a solid surface

A measuring method for the kinetic coefficient and activation energy of molten metals has been developed. This method is based on a splat thickness measurement of a molten metal droplet deposited on a polished metal substrate. An analytical solution of a non-equilibrium crystallization of a molten metal droplet impacting on a solid substrate relates the thickness of the splat to the kinetic coefficient. The dimensionless number showing the departure of the equilibrium phase transition from the non-equilibrium transition follows from the theory. The predicted values of the kinetic coefficient and activation energy agree well with the existing literature data.     

Thermal interactions of a molten tin drop with water triggered by a low-pressure shock

Thermal interactions of a molten tin drop with water were studied with a dropping contact mode in a shock tube geometry. The interaction was triggered by collapsing the initial vapor/gas bubble with a low-pressure (s< 0.8 MPa) shock. The hollow, porous, shell-like debris indicates that violent boiling, or homogeneous nucleation, of penetrated water, followed by turbulent mixing, might be a dominant mechanism for fragmentation of tin drops. An empirical correlation was obtained for the fragmentation time scale. The average heat transfer rate during the interaction was found to be in the range 1–10kW. The conversion efficiency of thermal energy to mechanical energy of the water column above the tin drop was found to be in the range 0.1%–1.0%.     

Scattering of radiation by a fiber with a rough surface

We present a new theory on scattering of radiation by real fibers in engineering systems. The theory covers the scattering by fibers consistently over an entire range of fiber sizes from smaller to larger than the wavelength of radiation. A geometrical technique is presented for modeling a fiber with a rough surface of a three-dimensional homogeneous microstructure. Scattering characteristics of the fiber are described based on the electromagnetic wave theory of diffraction to investigate the relationship among three main factors of scattering: wavelength of radiation, fiber diameter, and fiber microstructure. With an increase in fiber diameter and a corresponding magnification of a rough fiber surface microstructure, the scattering changes from particle scattering to surface scattering. The transition occurs in a size parameter range of P = 100–300. © 1999 Scripta Technica, Heat Trans Asian Res, 28(4): 322–335, 1999     

Heat transfer in a forced wall jet on a heated rough surface

Many stUdies of wall Jets on smooth, flat and curveds~es haVe been examined over the past fifty years.Glauertll] realized the first comprehensive study of thewall jet. A survey by Launder and Rodiln summarizedmost of the known results on tulbulent jet flows on flatsauce. The stUdy of the effect of significant rouglmesscaused by the deposition of pericles on a heated wallsubmited tO acoustic vibrations, can be interesting foraPPlications in problems of cooling in engine brineblades, for exa…     

Heat transfer augmentation for air jet impinged on a rough surface

An experimental investigation of heat transfer from a round air jet impinging normally from below onto a heated square plate was performed. The objective of the investigation was to study the effect of roughness on both the heat transfer and the fluid flow characteristics. Smooth and rough plates were, therefore, used in the course of the experiments. The heat transfer data were collected for four jet Reynolds numbers, ranging from 6500 to 19 000. The Reynolds numbers are based on the jet-exit velocity (U_e) and the nozzle-exit diameter (D), Re_e=U_eD/ν. The nozzle-to-plate distance ranged from 0.05 to 15 nozzle-exit diameter to cover both the potential core and the far regions of the jet flow. The roughness was composed of cubes of 1 mm dimension distributed uniformly along the plate. The local and average Nusselt number values for the rough plate showed an increase ranging from 8.9% to 28% over those for the smooth plate. Roughness was found to have a strong effect on the flow characteristics; it affected the mean velocity as well as the turbulence intensity of the flow. The mean velocity profiles for the smooth case at radial distances of r/D=1 and r/D=2.5 showed steeper near-wall velocity gradients compared with the profiles of the rough case, where r is the radial distance measured from the plate center along the plate centerline. In addition, roughness caused an increase in the turbulence intensity of the flow.     

Dynamic characteristics of molten droplets and hot particles falling in liquid pool

The dynamic characteristics of molten droplets and hot particles at the very beginning of their fall into coolant pools are presented. The falling course of a single droplet or a single hot particle was recorded by a high-speed camera and a curve of velocity vs. time was obtained. Emphasis was placed on the effects of the droplet’s size and temperature, the coolant’s temperature and properties, and the droplet’s physical properties on the moving behavior. The results for the all cases showed that the velocity of a falling droplet/particle decreased rapidly but rebounded shortly, at the beginning of droplet/particle falling in the coolant. Following such a V-shaped evolution in velocity, the droplet/particle slows down gradually to a comparatively steady velocity. An increase in either coolant temperature or droplet temperature results in a larger velocity variation in the “J-region”, but a smaller deceleration when it moves out of the “J-region”. The elevated volatility of a coolant leads to a steeper deceleration in the “J-region” and beyond. The bigger size of a particle leads to a greater velocity variation in the “J-region” and terminal velocity. A high melting point and thermal conductivity as well as lower heat capacity contribute to dramatic variation in the “J-region” and low terminal velocity.     

The behavior of water droplets on the heated surface

The effect of the volume of distillate droplets and temperature of the heated wall on evaporation process was investigated. The strong influence of thermal-physical and geometrical parameters of the wall on droplet evaporation is shown. A change in the ratio of droplet diameter to the wall thickness can lead to a change in evaporation regimes. Distribution of interface temperature along the droplet length is measured. At evaporation the wall temperature under the droplet changes significantly. A new method for measurement of the droplet mass on the heated wall is presented. The regimes of droplet boiling differ significantly from pool boiling. The droplet mass is directly changed in time; evaporation is separated into several stages.     

固体表面振荡液滴接触角演化

固体表面液滴振荡存在于很多实际生产当中,了解和掌握其中的界面特性对于实际的生产具有指导意义。利用可视化实验台对吹风条件下固体表面液滴的振荡现象进行了观察。实验在不同的条件下进行,着重风速、液滴尺寸和表面粗糙度等因素对实验现象的影响。实验观察到液滴与固体壁面接触角在振荡过程中有一定的变化规律,而接触线在振荡过程中始终保持不动。通过对实验结果的分类整理和对比,发现固体表面振荡液滴接触角变化和风速、液滴尺寸及表面粗糙度存在一定的关系。最后,通过接触线力平衡条件初步分析解释了接触角的演化规律。     

Dynamic characteristics of molten droplets and hot particles falling in liquid pool

The dynamic characteristics of molten droplets and hot particles at the very beginning of their fall into coolant pools are presented. The falling course of a single droplet or a single hot particle was recorded by a high-speed camera and a curve of velocity vs. time was obtained. Emphasis was placed on the effects of the droplet’s size and temperature, the coolant’s temperature and properties, and the droplet’s physical properties on the moving behavior. The results for the all cases showed that the velocity of a falling droplet/particle decreased rapidly but rebounded shortly, at the beginning of droplet/particle falling in the coolant. Following such a V-shaped evolution in velocity, the droplet/particle slows down gradually to a comparatively steady velocity. An increase in either coolant temperature or droplet temperature results in a larger velocity variation in the “J-region”, but a smaller deceleration when it moves out of the “J-region”. The elevated volatility of a coolant leads to a steeper deceleration in the “J-region” and beyond. The bigger size of a particle leads to a greater velocity variation in the “J-region” and terminal velocity. A high melting point and thermal conductivity as well as lower heat capacity contribute to dramatic variation in the “J-region” and low terminal velocity.     

Freezing-induced splashing during impact of molten metal droplets with high Weber numbers

The impact of molten tin droplets (0.6 mm diameter) on solid surfaces was observed for a range of impact velocities (10–30 m/s), substrate temperatures (25–200 °C) and substrate materials (stainless steel, aluminum and glass). The substrate was mounted on the rim of a rotating flywheel and the collision of single droplets with the moving substrate was photographed. Droplet impact Reynolds number ranged from 2.2 × 10⁴ to 6.5 × 10⁴ and Weber number from 8.0 × 10² to 7.2 × 10³. On a hot surface there was no splashing and droplets spread to form disk-like splats with smooth edges. Solidification around the edges of droplets spreading on cold surfaces created a solid rim that obstructed flow and triggered splashing. An analytical model was developed to predict the transition temperature at which splashing disappeared by assuming that the thickness of the solid layer had to equal that of the splat in the time the droplet spread to its maximum extent in order to obstruct liquid flow. The model predicted the transition temperature for aluminum and stainless steel surfaces, assuming that thermal contact resistance between the droplet and substrate varied between 10⁻⁶ and 10⁻⁷ m² K/W. The model also predicted that tin droplets would not splash on glass surfaces maintained at or above room temperature, and this was confirmed by experiments.     

Modelling of impingement phenomena for molten metallic droplets with low to high velocities

Thermal spray coatings are formed by accelerating a stream of powder particles towards a targeted substrate surface where they impact, deform, and adhere. A fundamental understanding of the splat formation can pave the way for future developments in thermal spray technology through better understanding. Numerical modelling is applied in this investigation which simulates the detailed transient flow of a molten metal droplet impacting, deforming, and solidifying on a flat, solid substrate. The computations are carried out on a fixed Eularian structured mesh using a volume of fluid method to simulate the boundary between the metallic and atmospheric-gas phases. The results shed light on the break-up phenomena on impact and describe in detail how the solidification process varies with an increasing impact velocity.     

固体表面液滴在吹风作用下的振荡特性

利用可视化实验观察了水平表面液滴在吹风条件下的振荡现象。针对水液滴在不同型号砂纸打磨的铜表面进行实验,观察到液滴在吹风条件下发生上下和左右两种振荡模式,在一定的条件下两种模式可以相互转化。通过细致的观察分析,总结归纳了振荡及转化发生的规律。实验结果表明：液滴的振荡特性同表面租糙度、液滴尺寸、风速等有一定的关系。振荡周期随着液滴尺寸增大而增大,随着风速的增大阶跃式增大;随粗糙度增大,液滴振荡周期有先增大再减小的趋势。     

Convective nanofluid flow over a vertical cone with a rough surface

In recent literature, the analysis of a combined convective flow over a cone has received a lot of attention. To explore the convection effects of flow over a cone in greater detail, in this investigation, we have considered a cone with a rough surface, which is entirely a new flow problem. Recent studies have shown the influence of roughness on fluid flow over several geometries, but flow over a rough conical surface has not been studied so far. In addition, we have analyzed the effects of nanoparticles, magnetohydrodynamic (MHD), and suction/blowing, which could have significant impacts on characteristics of fluid flow over the cone with a rough surface. Initially, the governing equations, which are partial differential equations with a high degree of nonlinearity, are nondimensionalized through Mangler's transformations. Later, linear equations are obtained via the method of quasilinearization, which is then solved numerically through finite difference approximations. The roughness of the cone's surface has notable effects on fluid flow, that too away from the origin. In fact, the roughness increases the friction at the cone's surface. Furthermore, the magnetic field applied at the wall increases the surface friction. Thus, the combination of roughness and MHD helps delay the boundary layer separation. On the other hand, the suction reduces the temperature of the fluid and increases the energy transport strength, while the thermophoresis parameter exhibits the contrary nature. Therefore, the combined consideration of these two could enhance energy transport strength in several industrial applications.     

Effect of surface tension on the temperature of burning metal droplets

An effect of Laplace pressure resulting in elevated boiling temperatures for fine burning metal droplets is discussed. The effect is evaluated for both Al and Mg. An increase in the boiling temperature suggests that the temperature of fine Al particles can become equal to or even higher than the temperature of the vapor phase Al flame. The high particle surface temperature may be associated with accelerated surface reaction rates. For Al, such surface reactions may generate vapor-phase aluminum suboxides, resulting in a reduced heat effect and extended burn times for fine particles and nanoparticles. For Mg, the particle temperature remains below the vapor phase flame temperature despite an elevated boiling point. For fine particles, surface reaction producing solid MgO growing directly on the particle surface is possible.     

Soldering a gas diffusion layer to a stainless steel bipolar plate using metallic tin

A novel investigation to decrease the interfacial contact resistance of stainless steel bipolar plates was performed. A thin layer of Sn was electrodeposited onto a bipolar plate and subsequently joined with a gas diffusion layer through hot-pressing at a temperature around the melting point of tin. This procedure was optimised, depositing 30 μm of Sn onto the stainless steel bipolar plate before hot-pressing at 230 °C and 0.5 bar for 20 min. A contact resistance of 5.45 mΩ cm² at 140 N cm^?2 was obtained, with low values maintained after exposure to both in-situ and ex-situ conditions. The in-situ testing in a fuel cell produced excellent results, with minor increases in contact resistance from 8.8 to 9.2 mΩ cm² and decreases in cell voltage from 0.714 to 0.667 V after 200 h of operation. These values are comparable to gold plated stainless steel, showing that combining a gas diffusion layer with electrodeposited Sn through hot-pressing is a promising low-cost coating for bipolar plates in PEM fuel cells.     

A theoretical and experimental study on hydrodynamics,heat exchange and diffusion during methane pyrolysis in a layer of molten tin

A theoretical and experimental study was performed on the heat exchange, hydrodynamics and diffusion during the transition of methane through a layer of tin melt with the aim of intensifying the pyrolysis process in a reactor filled with metal melt. Analysis of the experiment data shows that, as the methane supply rate increases from 25 to 250 ml/min, the duration of the contact between the gas bubbles and tin melt shortens, which leads to a decrease in the mole fraction of hydrogen in the resulting gas from 12% to 4.4% (for a reactor 10 cm in height). Using a cascade of mesh diaphragms is suggested to intensify the pyrolysis process for high methane supply rates. A unique float-type structure may be placed inside the reactor to solve the problem of continuous removal of the hard carbon deposits generated during methane pyrolysis and to control the metal melt level.     

Antidazzle effect of switchable mirrors prepared on substrates with rough surface

To examine the antidazzle effect of switchable mirrors, mirrors based on a magnesium-nickel alloy were prepared on three kinds of transparent substrates with different surface roughness. The mirrors prepared using a direct-current magnetron sputtering method have nearly the same surface morphology and roughness as the underlying substrates. The diffuse fraction of the total reflectance of the mirrors increases with surface roughness of the mirrors. Thus, by utilizing substrates with rough surfaces, the specular reflectance of the mirrors decreases and consequently it is possible to prepare switchable mirrors with an antidazzle effect.     

In situ synthesis of ultra-fine, porous, tin oxide-carbon nanocomposites via a molten salt method for lithium-ion batteries

Ultra-fine, porous, tin oxide-carbon (SnO₂/C) nanocomposites are fabricated by a molten salt method at 300 °C, and malic acid is decomposed as the carbon source. In situ synthesis is favourable for the combination of carbon and SnO₂. The structure and morphology are confirmed by X-ray diffraction analysis, specific surface-area measurements, and transmission electron microscopy (TEM). Examination of TEM images reveals that the SnO₂ nanoparticles are embedded in the carbon matrix, with sizes between 2 and 5 nm. The electrochemical measurements show that the nanocomposite delivers a high capacity with good capacity retention as an anode material for lithium-ion batteries, due to the combination of the ultra-fine porous structure and the carbon component.     

Treatment of antireflection on tin oxide coated anodized aluminum selective absorber surface

Nickel-pigmented anodic aluminum oxide, Ni–Al₂O₃ was pyrolytically coated with tin oxide (SnO₂). The undesirable increase of reflectance in the solar spectrum due to the high refractive index of the SnO₂ film was compensated by an antireflection layer. The layer was applied by a simple dipping technique in a bath containing a commercial colloidal silica sol which forms a silica (SiO₂) layer. The infrared reflectance is nearly unaffected by the silica sol treatment process. Preliminary test results indicate that treated samples are resistant to temperatures as high as 300°C as well as to corrosion in an 8% sulfuric acid solution. In addition, the optical properties were unaffected by outdoor exposure for two months.