Impact mechanism of gas temperature in metal powder production via gas atomization

This paper aims at studying the influence mechanism of gas temperatures(300 K, 400 K, 500 K, and 600 K) on gas atomization by simulating the integral atomization process of the close-coupled nozzle in vacuum induction gas atomization(VIGA). The primary atomization is simulated by the volume of fluid(VOF) approach, and the second atomization is studied by the discrete phase model(DPM) combined with the instability breakage model. The results show that, at an increased gas temperature, the influences of gas–liquid contact angle and gas temperature in the recirculation zone on the primary atomization are virtually negligible. However, increasing the gas temperature will increase the gas–liquid relative velocity near the recirculation zone and decrease the melt film thickness, which are the main reasons for the reduced mass median diameter(MMD, d50) of primary atomized droplets. During the secondary atomization, increasing the gas temperature from 300 K to 600 K results in an increase in the droplet dispersion angle, which is beneficial to the formation of spherical metal powder. In addition, increasing the gas temperature, the positive effect of gas–liquid relative velocity increase on droplets refinement overweighs the negative influence of the GMR decrease, resulting in the reduced MMD and diameter distribution interval. From the analysis of the atomization mechanism, the increase in atomization efficiency caused by increasing the temperature of the atomizing gas, including primary atomization and secondary atomization, is mainly due to the increase in the gas drag force difference between the inner and outer sides of the annular liquid film.     

A mass-conservative average flow model based on finite element method for complex textured surfaces

A mass-conservative average flow model based on the finite element method(FEM) is introduced to predict the performances of textured surfaces applied in mechanical seals or thrust bearings.In this model,the Jakobsson-Floberg-Olsson(JFO) boundary conditions are applied to the average flow model for ensuring the mass-conservative law.Moreover,the non-uniform triangular grid is utilized,which can deal with the problem of complex geometric shapes.By adopting the modeling techniques,the model proposed here is capable of dealing with complex textured surfaces.The algorithm is proved correct by the numerical experiment.In addition,the model is employed to gain further insight into the influences of the dimples with different shapes and orientations on smooth and rough surfaces on the load-carrying capacity.     

A Model for the Contact Angle of Liquid Droplets on Rough Surfaces

Up to now, measured results of the contact angle on rough surfaces have been explained usually based on the Wenzel equation （1936） and the Cassie-Baxter equation （1944）. However, these equations do not take into account considerations of liquid wetting behaviors on rough surfaces, and this leads to poor understanding of the mechanisms of contact between liquid droplets and rough surfaces （e.g. contact angle hysteresis）. We propose a new model for the contact angle of liquid droplets. By means of the present model, we can well understand the evperimental data which could not be well explained by the Wenzel equation and the Cassie-Baxter equation.     

Capillary filling in closed-end nanotubes

Capillary filling in small length scale is an important process in nanotechnology and microfabrication. When one end of the tube or channel is sealed, it is important to consider the escape of the trapped gas. We develop a dynamic model on capillary filling in closed-end tubes, based on the diffusion–convection equation and Henry's law of gas dissolution. We systematically investigate the filling dynamics for various sets of parameters, and compare the results with a previous model which assumes a linear density profile of dissolved gas and neglect the convective term.     

Multi-phase field simulation of grain growth in multiple phase transformations of a binary alloy

This work establishes a temperature-controlled sequence function, and a new multi-phase-field model, for liquid–solid–solid multi-phase transformation by coupling the liquid–solid phase transformation model with the solid–solid phase transformation model. Taking an Fe–C alloy as an example, the continuous evolution of a multi-phase transformation is simulated by using this new model. In addition, the growth of grains affected by the grain orientation of the parent phase(generated in liquid–solid phase transformation) in the solid–solid phase transformation is studied. The results show that the morphology of ferrite grains which nucleate at the boundaries of the austenite grains is influenced by the orientation of the parent austenite grains. The growth rate of ferrite grains which nucleate at small-angle austenite grain boundaries is faster than those that nucleate at large-angle austenite grain boundaries. The difference of the growth rate of ferrites grains in different parent phase that nucleate at large-angle austenite grain boundaries, on both sides of the boundaries, is greater than that of ferrites nucleating at small-angle austenite grain boundaries.     

Studies of flow field characteristics during the impact of a gaseous jet on liquid–water column

Both experimental and numerical studies were presented on the flow field characteristics in the process of gaseous jet impinging on liquid–water column. The effects of the impinging process on the working performance of rocket engine were also analyzed. The experimental results showed that the liquid–water had better flame and smoke dissipation effect in the process of gaseous jet impinging on liquid–water column. However, the interaction between the gaseous jet and the liquid–water column resulted in two pressure oscillations with large amplitude appearing in the combustion chamber of the rocket engine with instantaneous pressure increased by 17.73% and 17.93%, respectively. To analyze the phenomena, a new computational method was proposed by coupling the governing equations of the MIXTURE model with the phase change equations of water and the combustion equation of propellant. Numerical simulations were carried out on the generation of gas, the accelerate gas flow, and the mutual interaction between gaseous jet and liquid–water column.Numerical simulations showed that a cavity would be formed in the liquid–water column when gaseous jet impinged on the liquid–water column. The development speed of the cavity increased obviously after each pressure oscillation. In the initial stage of impingement, the gaseous jet was blocked due to the inertia effect of high-density water, and a large amount of gas gathered in the area between the nozzle throat and the gas–liquid interface. The shock wave was formed in the nozzle expansion section. Under the dual action of the reverse pressure wave and the continuously ejected high-temperature gas upstream, the shock wave moved repeatedly in the nozzle expansion section, which led to the flow of gas in the combustion chamber being blocked, released, re-blocked, and re-released. This was also the main reason for the pressure oscillations in the combustion chamber.     

Surface tension effects on the behaviour of a rising bubble driven by buoyancy force

In the inviscid and incompressible fluid flow regime,surface tension effects on the behaviour of an initially spherical buoyancy-driven bubble rising in an infinite and initially stationary liquid are investigated numerically by a volume of fluid (VOF) method. The ratio of the gas density to the liquid density is 0.001, which is close to the case of an air bubble rising in water. It is found by numerical experiment that there exist four critical Weber numbers We₁,~We₂,~We₃ and We₄, which distinguish five different kinds of bubble behaviours. It is also found that when 1≤We2, the bubble will finally reach a steady shape, and in this case after it rises acceleratedly for a moment, it will rise with an almost constant speed, and the lower the Weber number is, the higher the speed is. When We >We₂, the bubble will not reach a steady shape, and in this case it will not rise with a constant speed. The mechanism of the above phenomena has been analysed theoretically and numerically.     

Studying Validity of Single-Fluid Model in Inertial Confinement Fusion

The validity of single-fluid model in inertial confinement fusion simulations is studied by comparing the results of the multi- and single-fluid models. The multi-fluid model includes the effects of collision and interpenetration between fluid species. By simulating the collision of fluid species, steady-state shock propagation into the thin DT gas and expansion of hohlraum Au wall heated by lasers, the results show that the validity of single-fluid model is strongly dependent on the ratio of the characteristic length of the simulated system to the particle mean free path. When the characteristic length L is one order larger than the mean free path A, the single-fluid model＇s results are found to be in good agreement with the multi-fluid model＇s simulations, and the modeling of single-fluid remains valid. If the value of L/A is lower than 10, the interpenetration between fluid species is significant, and the single-fluid simulations show some unphysical results; while the multi-fluid model can describe well the interpenetration and mix phenomena, and give more reasonable results.     

Mechanism from particle compaction to fluidization of liquid–solid two-phase flow

A new model of particle yield stress including cohesive strength is proposed, which considers the friction and cohesive strength between particles. A calculation method for the fluidization process of liquid–solid two-phase flow in compact packing state is given, and the simulation and experimental studies of fluidization process are carried out by taking the sand–water two-phase flow in the jet dredging system as an example, and the calculation method is verified.     

Lattice-Boltzmann simulation of particle-laden flow over a backward-facing step

This paper deals with the numerical simulation of gas－solid two-phase flows in an Eulerian－Lagrangian scheme. The particle tracks are calculated using a recently developed exponential Lagrangian scheme, and the approach presently used for the computation of fluid phase is based on a modified Lattice-BGK model. Different from earlier publications, the present study employs a two-way coupling mechanism to handle the interactions between carrier phase and dispersed phase in the model. This new model is applicable to simulating gas－solid two-phase flows. For example, based on the scheme, we have recaptured some phenomena of planar laminar particle-laden flow over a backward-facing step in this research, and found a new interesting phenomenon.     

Critical assessment of effective interfacial potentials based on a density functional theory for wetting phenomena on curved substrates

In this Letter we examine an effective interfacial Hamiltonian approach for wetting phenomena based on two different density approximations in the framework of a density functional theory. The system under consideration is an attractive spherical wall subject to adsorption by a metastable liquid. We argue that, contrary to a planar geometry, in the spherical case the popular sharp-kink approximation leads to a considerable disagreement for the film thickness with numerical results obtained from density functional theory. We show that the deviation originates from the inaccuracy of the predicted liquid-gas surface tension. We further demonstrate that the prediction can be substantially improved when a soft-interface approximation is adopted, such that the liquid-gas interface is approximated by a smooth monotonic function.     

Pairwise correlations and wetting transitions at a solid-gas interface

We summarize the main results of our study of the density-density correlation function for Sullivan's model of a gas adsorbed on a solid substrate. In the approach to complete wetting, when a thick film of liquid density is adsorbed at the substrate, long-ranged transverse (parallel to the surface) correlations develop at the edge of the film where the density profile is similar to that of a liquid-gas interface. For a class I wetting situation the range of the transverse correlations increases and ultimately diverges as the bulk gas pressure approaches the saturated vapour pressure. We comment on other situations where long-ranged correlations arise and mention the possibility of observing these in diffraction experiments and in computer simulations. Sullivan's model always predicts a second-order phase transition between class II and class I wetting. By extending his model and allowing the attractive part of the solid-fluid potential to be longer-ranged than the attractive fluid-fluid potential we find that this wetting transition can become a first-order (Cahn) transition.     

The apparent state of droplets on a rough surface

The factors influencing the state and wetting transition of droplets on a rough surface are both complex and obscure. The change in wetting is directly reflected by changes under the contact condition of the droplets with the surface. The recent study about the wettability of the superhydrophobic surface under the condensing condition arouses the new understanding about the apparent state of droplets on a rough surface. In this work, to validate the existence of droplets in an intermediate state, a microscale pillar topological polydimethylsiloxane (PDMS) surface was manufactured and its wettability under various conditions was studied. According to the experimental data, it is proposed that the wetting state of a rough surface may be embodied using the contact area ratio of a solid/liquid/gas droplet with the projective plane. A general calculation model for the apparent contact angle of droplets is given and expressed diagrammatically. It is found that the measured apparent contact angles of droplets at different states on the surface falls within the range predicted by our proposed equation. Supported by the National Natural Science Foundation of China (Grant No. 50606025)     

正癸烷纳米液滴润湿特性的分子动力学研究

流体液滴在固体表面的浸润性对其润滑性能至关重要.本文利用分子动力学方法研究了正癸烷纳米液滴在铜表面上的润湿特性.结果表明：在平坦光滑表面上,壁面的厚度和分子数目对润湿效果影响不大.随着壁面能量势阱参数εs 增大,接触角线性减小.随着温度升高,液滴的接触角减小.在沟槽粗糙表面上,随着粗糙度因子增大,对于疏液表面,接触角增大到一定值后基本保持不变,符合Cassie理论;中性和亲液表面接触角则会减小,为Wenzel润湿模式.当表面分数增大时,疏液与亲液表面接触角整体呈减小的趋势,对中性表面影响不大.当温度升高时,粗糙疏液表面接触角会增大,润湿效果更差,而粗糙中性和亲液表面液滴润湿性会更好.     

纳米结构超疏水表面冷凝现象的三维格子玻尔兹曼方法模拟

采用三维多相流格子玻尔兹曼方法 (lattice Boltzmann method, LBM),对纳米结构超疏水表面液滴的冷凝行为进行模拟研究.通过Laplace定律和光滑表面的本征接触角理论对三维LBM模型进行定量验证.模拟分析了超疏水表面纳米阵列的几何尺寸和润湿性的局部不均匀性对冷凝液滴形核位置和最终润湿状态的影响规律.结果表明,较高的纳米阵列使液滴在纳米结构间隙的上部侧面和底部优先形核长大,通过采用上下不均匀的间隙可避免液滴在底部形核长大,而在上部侧面形核的冷凝液滴在生长过程中向上运动,其润湿状态由Wenzel态转变为Cassie态;较低的纳米阵列使液滴在纳米结构底部优先形核长大,液滴的最终润湿状态为Wenzel态;润湿性不均匀的纳米结构表面使液滴在阵列顶端亲水位置处优先形核长大,成为Cassie态.冷凝液滴在不同几何尺寸的纳米结构表面上的最终润湿状态的模拟结果与文献报道的实验结果符合良好.通过模拟还发现,冷凝液滴在生长过程中的运动行为与液滴统计平均作用力的变化有关.本文的LBM模拟再现了三维空间中液滴的形核、长大和润湿状态转变等物理现象.     

基于MRT-LB伪势模型的液滴撞击液膜数值研究

运用改进的格子玻尔兹曼（LB）伪势多松弛多相模型,研究单/双液滴撞击液膜的流动特性.考察单液滴在不同气液相密度比时撞击液膜的发展.随着密度比的减小,冠状水花顶端开始向内弯曲,且底部半径显著减小.在大密度比情况下研究双液滴撞击液膜.结果表明：双液滴撞击液膜有中心射流的形成;液滴水平间距的增大,延缓中心射流的出现,并降低初始中心射流的高度;随Re数的增加,中心射流的高度明显增大.     

Some aspects of the wetting transition1

The wetting of an attractive planar wall by liquid at liquid-gas coexistence is investigated from a microscopic point of view. A model for interface detachment due to thermal fluctuations is first worked out. An empirical framework for distinguishing between first-order and continuous wetting transitions is then set up, and quantified by the introduction of the van der Waals mean field model. This is solved for strong, short-range and weak, long-range forces. The role of fluctuations is reconsidered, and the nature of the required corrections examined, both for wall-fluid and fluid-fluid interfaces.This paper was presented at the conference on the Statistical Mechanics of Phase Transitions-Mathematical and Physical Aspects, Tebo, Czechoslovakia, September 1–6, 1986.     

Hydrogen and helium films as model systems of wetting

Optical experiments on the wetting properties of liquid ⁴He and molecular hydrogen are reviewed. Hydrogen films on noble metal surfaces serve as model systems for studying triple point wetting, a continuous transition between wetting and non-wetting. By means of optically excited surface plasmons, the adsorbed film thickness for temperatures around, and far below, the bulk melting temperature is measured, and the physical mechanisms responsible for the transition are elucidated. Possible applications for other experiments in pure and applied research are discussed. Thin films and droplets of liquid helium are studied on cesium surfaces, on which there is a first order wetting transition. Our studies concentrate on dynamical observations via surface plasmon microscopy, which provide insight into the morphology of liquid helium droplets spreading at different temperatures. Features corresponding to pinning forces, the prewetting line, and the Kosterlitz-Thouless transition are clearly observed.     

Capillary Forces between Submillimeter Spheres and Flat Surfaces at Constant Liquid Volumes

We investigate the capillary forces between submillimeter spheres and flat surfaces at constant liquid volumes theoretically and experimentally. An iterative method is used to estimate the capillary force with contact angles as the boundary conditions and the constant volume as a constraint. The theoretical analysis shows that the maximum capillary force between them decreases with the increase of the liquid bridge volume at small contact angles. The experimental results show that the force is smaller than the theoretical values at the initial separation distances. It is also observed that the force first increases and then decreases with an increasing separation distance in some cases. These phenomena of capillary forces hysteresis are explained according to the wetting hysteresis.