Modeling of oxidation of molybdenum particles during plasma spray deposition

An oxidation model for molybdenum particles during the plasma spray deposition process is presented. Based on a well-verified model for plasma chemistry and the heating and phase change of particles in a plasma plume, this model accounts for the oxidant diffusion around the surface of particles or splats, oxidation on the surface, as well as oxygen diffusion in molten molybdenum. Calculations are performed for a single molybdenum particle sprayed under Metco-9MB spraying conditions. The oxidation features of particles during the light are compared with those during the deposition. The result shows the dominance of oxidation of a molybdenum particle during the flight, as well as during deposition when the substrate temperature is high (above 400 °C).     

Particle sedimentation during processing of liquid metal-matrix composites

A novel electrical resistance probe technique to measure thein situ volume fraction of ceramic particles in molten metals was applied to the measurement of sedimentation rates of 90-μm-diameter silicon carbide particles in molten aluminum. The results indicate that the rate strongly depends on volume fraction; the time to clarify a 0.15-m depth increased from approximately 60 to 500 seconds as the particle volume fraction increased from 0.05 to 0.30. Maps showing the changes in volume fraction throughout the melt were generated. A multiphase hydrodynamic model was developed to describe the sedimentation. Using volume fraction-dependent drag coefficients from work in aqueous systems, the model was able to simulate the experimental results remarkably well. The experimental and modeling results indicate that there was little agglomeration or network formation during sedimentation. The implications of the results for solidification and particle pushing are discussed. Formerly Graduate Student, McMaster University     

Experiments on Deposition Behavior of Fine Sediment in a Reservoir

The deposition behavior of fine sediment is an important phenomenon, and yet it is unclear to engineers concerned about reservoir sedimentation. Laboratory experiments were conducted to produce both quasi-homogeneous flow and a turbidity current region divided by a plunge section. Silica powder (a noncohesive sediment) and kaolin (a cohesive sediment) were used as the suspended material. Because the effective gravitational force is the primary driving force for velocity in turbidity currents, the velocity profile was closely related to the concentration profile. The deposition rate of noncohesive coarser particles exponentially decays along the flow path. Most of the coarser particles were deposited in the quasi-homogeneous flow region or within a small distance downstream of the plunge section. The plunge did not carry those coarser particles further downstream. Deposition in the region of the turbidity current was found mainly by cohesive particles. Hydraulic sorting exists in the quasi-homogeneous flow region for noncohesive coarser particles, but becomes less significant in the downstream portion with deposition rates becoming mildly decayed. For fine cohesive particles, hydraulic sorting for the deposited gradation is not significant.     

Mesoscale Measures of Nonaffine Deformation in Dense Granular Assemblies

We probe the origins of nonaffine deformation in discrete element simulations of densely packed granular systems under biaxial compression, using a new local measure of nonaffine micropolar deformation. This measure represents the deviation of particle motion from that dictated by a measure of micropolar strain and curvature, devised on the scale of a particle and its first ring of neighbors. Highly correlated mesoscopic structures emerge and contribute significantly to nonaffine deformation, an aspect of material behavior that is largely neglected in existing constitutive models. Distinct regimes of deformation were observed: a globally affine regime in which particles move in uncorrelated Brownian motion with some “rattlers” present followed by a globally nonaffine regime involving rattlers, microbands, vortices, confined buckling of force chains, and persistent shear band(s). Structural development leading to and during persistent shear banding is elucidated. The highest levels of nonaffine deformation were observed during drops in the macroscopic stress ratio: here, nonaffine deformation is essentially confined to the shear band, where confined buckling of force chains is the intrinsic mechanism. Degrees of correlation between the local measures of nonaffine strain and curvature confirm the key role of particle rotations in shear bands. Probability density functions of local nonaffine deformation exhibit power law behavior cut off by an exponential tail, consistent with experiments. Self-diffusion anisotropy was observed inside the shear band. The degree of diffusion is greater tangential rather than normal to the shear band; particles in buckling force chains exhibit the highest degree of diffusion along the band. Self-diffusion of particles after the peak stress ratio was found to be superdiffusive during each drop in stress ratio, and diffusive during each rise in stress ratio. The implications of these results for constitutive modeling are discussed, with particular attention paid to the significance of confined buckling of force chains.     

Ostwald ripening in lorentz-force stabilized CuPb dispersions at low volume fractions. I—Experimental observations

We present measurements and theoretical investigations on Ostwald-ripening of Cu particles in a liquid PbCu melt with low volume content of the phase dispersed. For this purpose we used a novel technique that is able to suppress sedimentation of particles in an electrically conducting matrix. The experiments on solid-liquid CuPb dispersions in a low volume fraction range, which is not accessible by conventional experimental techniques, reveal the existence of stationary particle size distributions. The particle growth differs significantly from the growth law of the LSW-theory for diffusion-controlled Ostwald ripening, indicating that in this system the particle coarsening is not only diffusion limited. We outline a new growth model based on the assumption of preferred places for the trapping of Cu-atoms at dislocation sites emerging at the particle surface.     

Critical Review of Aerosol Particle Transport Models for Building HVAC Ducts

Literature on aerosol behavior in conventional heating ventilating and air conditioning duct systems that is directly applicable to modeling and simulation of deposition and resuspension of particulate matter is reviewed. Open literature discussing particle resuspension from duct surfaces is rare and most studies investigating particle deposition consider only clean duct surfaces. There is a similar lack of studies on particle deposition and resuspension in mechanical components found in air distribution systems (e.g., heat exchangers, fans, dampers, etc.) and in duct flows where the turbulent velocity profile is not fully developed. An example calculation of penetration through a simple duct system illustrates the characteristics of results produced by the identified modeling methods.     

Analytical Analysis of Rainfall Infiltration Mechanism in Unsaturated Soils

To improve the understanding of the influence of hydraulic properties and rainfall conditions on rainfall infiltration mechanism and hence on the pore-water pressure distributions in single and two-layer unsaturated soil systems, an analytical parametric study has been carried out. Parameters considered in this study include saturated permeability (ks), desaturation coefficient (α), water storage capacity (θs?θr), and antecedent and subsequent rainfall infiltration rate (qA and qB). Moreover, the influence of soil profile heterogeneity is also investigated. The calculated results demonstrate that the infiltration process and pore-water pressure response are primarily controlled by both qα/ks and ks/α. Generally the larger the value of qα/ks?, the greater the reduction of negative pore-water pressure in shallow soil layer. The larger the ratio of ks/α, the faster is the advancement of wetting front. Among the three hydraulic parameters, the effects of α and ks on pore-water pressure response are much more significant than that of (θs?θr). However, the relative importance of ks and α depends on the initial negative pore-water pressure range in the ground. In addition, the influence of antecedent infiltration rate (qA) on pore-water pressure response appears to be much more significant than that of subsequent infiltration rate (qB).     

金纳米颗粒烧结的分子动力学模拟

采用分子动力学模拟方法研究了不同尺寸Au纳米颗粒在烧结过程中晶型转变及烧结颈长大机制.研究发现纳米颗粒的烧结颈生长主要分为两个阶段:初始烧结颈的快速形成阶段和烧结颈的稳定长大阶段.不同尺寸纳米颗粒烧结过程中烧结颈长大的主要机制不同:当颗粒尺寸为4 nm时,原子迁移主要受晶界（或位错）滑移、表面扩散和黏性流动控制;当尺寸在6nm左右时,原子迁移主要受晶界扩散、表面扩散和黏性流动控制;当颗粒尺寸为9 nm时,原子迁移主要受晶界扩散和表面扩散控制.烧结过程中Au颗粒的fcc结构会向无定形结构转变.此外,小尺寸的纳米颗粒在烧结过程中由于位错或晶界滑移、原子的黏性流动等因素会形成hcp结构.      

Al2O3夹杂物在钢-渣界面处的运动特性及去除率

通过理论计算和分析,研究了夹杂物颗粒在钢-渣界面处夹杂物去除层内的运动特性及去除率。结果表明:在夹杂物去除层内,Al2O3夹杂物颗粒的布朗扩散上浮临界尺寸为1.33μm,直径小于临界尺寸的夹杂物颗粒很难上浮去除;布朗碰撞的优势区域主要是直径为2.5μm以下的夹杂物颗粒之间以及直径为2.5～5μm夹杂物颗粒与0.5μm以下的微小颗粒之间的碰撞;直径为20～150μm的夹杂物颗粒在钢-渣界面去除层中9min内很容易完全上浮去除,而直径小于10μm的夹杂物颗粒去除率很低且升高缓慢,是提高钢液洁净度的主要控制对象。     

In situ stress-strain response of small metal particles embedded in a ceramic matrix

The in situ stress-strain response of metal particles embedded in a ceramic matrix was obtained by combining the measurement and the modeling of the crack opening displacement field of a crack in a brittle material bridged by metal particles. The experiments were done on a composite made from platinum particles with a volume fraction of 10% in a magnesium aluminate spinel matrix. The size of the platinum particles was varied from 1 to 12 μm to study the influence of scale on the deformation behavior. Large strain to failure (85%) and ultimate tensile strength of 550 MPa were obtained for the 1 μm particles. But the larger particles failed at a strain of less than 25%; the ultimate tensile strength was also lower. This difference in ductility is explained in terms of debonding at the metal ceramic interface. It is argued that the debonding depends on the length of the dislocation pile up at the interface, and, therefore, on the particle size. The results and the metallographic observations are consistent with a model presented here; in this model the failure condition is given by a combination of the intrinsic yield stress of platinum, and the hydrostatic constraining stress in the metal particle.     

Analysis of Dimensional Changes During Sintering of Fe—Cu

Abstract

The dimensional changes of Fe–Cu compacts during sintering are of considerable interest for the production of sintered parts. Previous investigations of the swelling and shrinkage behaviour were mostly based on dilatometric measurements and led to partially contradictory results. In the present investigation Fe and Cu powders of ideal sphericity were compacted to densities of up to 93%, sintered for different times, and then rapidly cooled. The simultaneous measurement of density and microstructural parameters permitted the quantitative distinction of the swelling mechanism of Fe–Cu compacts into four main contributions; I penetration of the melt between Fe particles and II along the grain boundaries; III diffusion of Cu into the Fe particles from the particle surface; and IV diffusion of Cu into the Fe grains from the grain boundaries. Whereas the penetration between the Fe particles is the result of pure capillary forces the penetration along grain boundaries is thought to be a special case of solution-reprecipitation.

Particle rearrangement during sintering of Fe–Cu compacts could be separated into two stages. Primary particle rearrangement, just after the melt phase occurs, leads to rapid densification of loose packed and slightly compacted specimens. In highly compacted specimens it can be neglected. The penetration of the melt along the grain boundaries leads to disintegration of the Fe particles, which enables densification by rearrangement of the individual grains of the Fe particles.     

深井充填两相流管道沉积机理研究

为解决某深井矿山因管道沉积造成的安全生产问题,通过对深井充填管道沉积机理进行研究,基于固液两相流相关理论,分别从细颗粒结垢和粗颗粒沉积2个方面来阐释两相流管道沉积机理。运用FLUENT流体动力学软件进行建模和分析,将浆体中固体细颗粒与清水混合的均质悬液作为连续相,较粗颗粒作为离散相,根据浆体中粗颗粒的浓度分布和管道速度分布特点来确定管道内颗粒物沉积规律。结果发现,充填两相流管道沉积的主要原因是细颗粒结垢和粗颗粒沉积相结合。数值模拟的两相流充填管道沉积过程与理论分析提出的粗细颗粒沉积物理模型相吻合。     

Deposition from Particle-Laden, Round, Turbulent, Horizontal, Buoyant Jets in Stationary and Coflowing Receiving Fluids

Results are presented from a series of laboratory experiments investigating the characteristic features of particle-laden, round, turbulent, buoyant jets discharged horizontally into stationary and coflowing receiving fluids. For the volumetric source concentrations of particles tested ( ～ 0.1%), the presence of the particle load was found to have no significant influence on mean buoyant jet trajectories. Deposition patterns on the bed of the receiving water container indicated the existence of two separate sedimentation processes for discharges into stationary or coflowing ambients, namely (1) a relatively concentrated, narrow band of particle accumulation associated with near-source fallout from the buoyant jet margins; and (2) a broader and more disperse downstream depositional fan associated with particle fallout from the radially-expanding surface gravity current formed by the impingement of the buoyant jet with the free surface of the receiving fluid. Scaling arguments have been developed and applied successfully to deposition length scales associated with these sedimentation patterns, allowing the quantitative characteristics and parametric dependences of the deposition distributions to be established.     

The Penetration Behavior of an Annular Gas–Solid Jet Impinging on a Liquid Bath: The Effects of the Density and Size of Solid Particles

Top-blow injection of a gas?Csolid jet through a circular lance is used in the Mitsubishi Continuous Smelting Process. One problem associated with this injection is the severe erosion of the hearth refractory below the lances. A new configuration of the lance to form an annular gas?Csolid jet rather than the circular jet was designed in this laboratory. With this new configuration, the solid particles fed through the center tube leave the lance at a much lower velocity than the gas, and the penetration behavior of the jet is significantly different from that with a circular lance where the solid particles leave the lance at the same high velocity as the gas. In previous cold-model investigations in this laboratory, the effects of the gas velocity, particle feed rate, lance height of the annular lance, and the cross-sectional area of the gas jet were studied and compared with the circular lance. This study examined the effect of the density and size of the solid particles on the penetration behavior of the annular gas?Csolid jet, which yielded some unexpected results. The variation in the penetration depth with the density of the solid particles at the same mass feed rate was opposite for the circular lance and the annular lance. In the case of the circular lance, the penetration depth became shallower as the density of the solid particles increased; on the contrary, for the annular lance, the penetration depth became deeper with the increasing density of particles. However, at the same volumetric feed rate of the particles, the density effect was small for the circular lance, but for the annular lance, the jets with higher density particles penetrated more deeply. The variation in the penetration depth with the particle diameter was also different for the circular and the annular lances. With the circular lance, the penetration depth became deeper as the particle size decreased for all the feed rates, but with the annular lance, the effect of the particle size was small. The overall results including the previous work indicated that the penetration behavior of an annular jet is much less sensitive to the variations in operating variables than that of a circular jet. Correlation equations for the penetration depth that show good agreements with the measured values have been developed.     

Influence of Stratification and Shoreline Erosion on Reservoir Sedimentation Patterns

Sedimentation in the main pool of a deep (maximum depth: 50?m), 227?km2 hydropower reservoir was modeled using a three-dimensional numerical model of hydrodynamics and sedimentation for different wind, inflow, and outflow conditions. Short-term velocity measurements made in the reservoir were used to validate some aspects of the hydrodynamic model. The effects of thermal stratification on sedimentation patterns were investigated, since the reservoir is periodically strongly stratified. Stratification alters velocity profiles and thus affects sedimentation in the reservoir. Sedimentation of reservoirs is often modeled considering only the deposition of sediments delivered by tributaries. However, the sediments eroding from the shorelines can contribute significantly to sedimentation if the shorelines of the reservoir erode at sufficiently high rates or if sediment delivery via tributary inflow is small. Thus, shoreline erosion rates for a reservoir were quantified based on measured fetch, parameterized beach profile shape, and measured wind vectors, and the eroded sediments treated as a source within the sedimentation modeling scheme. The methodology for the prediction of shoreline erosion was calibrated and validated using digital aerial photos of the reservoir taken in different years and indicated approximately 1?m/year of shoreline retreat for several locations. This study revealed likely zones of sediment deposition in a thermally stratified reservoir and presented a methodology for integration of shoreline erosion into sedimentation studies that can be used in any reservoir.     

The Influence of Powder Porosity on the Bonding Mechanism at the Impact of Thermally Sprayed Solid Particles

High-velocity oxy-fuel (HVOF) thermal spraying can generate dense depositions without melting the powders during spraying. Our previous study showed that most HVOF-sprayed particles are in solid state prior to impact on the substrate. The deposition of solid particles requires sufficient deformation of the particles as a result of a high impact. This report is a continuation of our previous work to study the bonding mechanism for thermally sprayed solid particles. The same hard material, WC-Co powder, is studied by considering the porosity inside the particles. The detailed deposition mechanism is examined by dynamically tracking the particle impingement using finite element analysis (FEA) models. The results confirm that the deposition of high-speed solid particles is caused mainly by the particle deformation and further implies that deformation is enhanced with increase in porosity alone. Therefore, a possible way to increase the deposition efficiency of hard cermet coating could be to use a properly designed porous powder.     

Direct Numerical Simulation of Microparticle Motion in Channel Flow with Thermophoresis

The motion of microparticles in a vertical flow channel driven by drag and thermophoretic forces was simulated using the direct numerical simulation method with the particles tracked using the Lagrangian method. The particle motions were analyzed for five sizes of particles (dp = 1, 2.5, 10, 20, and 100?μm) at three temperature differences (Δt = 0, 130, and 180°C). The results showed that the effect of thermophoresis on the deposition near the wall decreases with increasing particle diameter and can be neglected for particles with dp = 100?μm. Since the deposition rates of inhalable particles (PM10) increase dramatically as the thermophoresis force increases, especially for particles with dp = 1?μm, thermophoresis is an effective method for collecting inhalable particles.     

The effect of the carbide phases on the crack grow of 0.5 pct Mo-B steels under impact,cyclic, and monotonically increasing loading

A study of the influence of carbide phases on the cracking resistance of as-quenched and of quenched and tempered 0.5 pct Mo-B steels was made using notched or notched and precracked specimens that were subjected to impact, cyclic, and monotonically increasing loading. The carbide influence on fracture, while limited in extent, was found to increase as load, loading rate, volume fraction, and particle size increase. The results for the asquenched condition showed that the susceptibility of these steels to crack initiation under impact loading at temperatures below - 100°F is greater when even a small amount of titanium carbide (less than 0.2 vol pet of 1 to 5 μm particles) is present than when none is present. At room temperature, this same carbide concentration has no influence on impact properties, fatigue-crack initiation (in the presence of a notch), fatigue-crack growth rate, or the ductile fracture resistance under monotonically increasing loading at slow strain rate. In the case of the quenched and tempered materials, the alloy containing a large amount of M₂₃C₆ (2 vol pct of 1 to 10 μm particles) exhibited behavior similar to that observed in the as-quenched materials containing titanium carbide. That is, the presence of M₂₃C₆ was associated with increased susceptibility to crack initiation for impact loading at low temperature. In addition, at room temperature this alloy had a reduced impact energy for crack propagation. For monotonically increasing loading at slow strain rate, this same carbide distribution had no influence on the net section stresses required to initiate stable or unstable crack growth. These stresses fall closely in line with, respectively, the yield stress and tensile strength of the material. The alloy containing M₂₃C₆ required less crack opening for a given crack extension—an effect most pronounced after maximum load. Finally, some attention is directed to the use of Charpy test data to assess fracture resistance for modes of loading other than impact.