Effect of particle-size distribution on sintering

A sintering model, taking into account the effect of particle-size distribution and the effect of grain growth, has been derived. The model predicts a dependence of densification on the width of the particle-size distribution. This dependence is strongly affected by the occurrence of grain growth. Prior to the occurrence of grain growth, the model predicts that the densification rate increases and then decreases as the particle-size distribution width of the original powder increases. After grain growth occurs, the densification rate decreases as the particle-size distribution width of the starting powder increases.     

A transient distribution of particle assemblies at the concluding stage of phase transformations

There are a number of experimental and theoretical papers testifying that the large-time behavior of the particle-size distribution function heavily depends on the initial conditions at the final stages of phase transformation processes. However, still now there is no theoretically derived distribution confirming these conclusions. The present paper is concerned with a new theoretical approach verifying the fact that the large-time distribution can actually be dependent on the details of the initial data. The concluding stage of evolution of a particulate ensemble as a result of coalescence and coagulation processes is considered. The transient kinetic and balance equations for the particle-size distribution function are modified into a single nonlinear equation for arbitrary collision frequency factors. This integro-differential equation is solved analytically in the limit of large times. The obtained particle-size distribution function depends on the initial condition and describes the concluding stages of transient phase transformation processes. The area of applicability of the present asymptotic solution for the large-time particle-size distribution is discussed. The obtained distribution function is in agreement with experiments.     

应力对Ag颗粒增强SnCu基复合钎料蠕变性能的影响

使用搭接面积为1mm2的单搭接钎焊接头,研究了恒定温度下应力对Ag颗粒增强SnCu基复合钎料钎焊接头蠕变寿命的影响,结果表明:Ag颗粒增强SnCu基复合钎料的蠕变抗力优于99.3Sn0.7Cu基体钎料;随着应力的增大,复合钎料及其基体钎料钎焊接头的蠕变寿命均呈下降趋势,且应力对复合钎料钎焊接头蠕变寿命的影响比基体钎料明显.     

The influence of particle geometry and the intermediate stress ratio on the shear behavior of granular materials

The behavior of granular materials is very complex in nature and depends on particle shape, stress path, fabric, density, particle size distribution, amongst others. This paper presents a study of the effect of particle geometry (aspect ratio) on the mechanical behaviour of granular materials using the discrete element method (DEM). This study discusses 3D DEM simulations of conventional triaxial and true triaxial tests. The numerical experiments employ samples with different particle aspect ratios and a unique particle size distribution (PSD). Test results show that both particle aspect ratio (AR) and intermediate stress ratio \((b=({\upsigma }_{2}'-{\upsigma }_{3}')/({\upsigma }_{1}'-{\upsigma }_{3}'))\) affect the macro- and micro-scale responses. At the macro-scale, the shear strength decreases with an increase in both aspect ratio and intermediate stress ratio b values. At the micro-scale level, the fabric evolution is also affected by both AR and b. The results from DEM analyses qualitatively agree with available experimental data. The critical state behaviour and failure states are also discussed. It is observed that the position of the critical state loci in the compression \((e-p')\) space is only slightly affected by aspect ratio (AR) while the critical stress ratio is dependent on both AR and b. It is also demonstrated that the influence of the aspect ratio and the intermediate stress can be captured by micro-scale fabric evolutions that can be well understood within the framework of existing critical state theories. It is also found that for a given stress path, a unique critical state fabric norm is dependent on the particle shape but is independent of critical state void ratio.     

Investigation of spherical and non-spherical binary particles flow characteristics in a discharge hopper

The particulate flow characteristics of granular particles including mung beans, wooden spheres, wooden cylinders, wooden cubic, plastic spheres, plastic cylinders, and their binary mixtures in a flat bottom hopper were studied experimentally using high-speed high-resolution camera recordings. An image processing method based on the color threshold was developed to calculate the area ratio for binary mixtures. The mass discharge rates, the residue inclination angles, and the changes in edge height and center height with time were also investigated. In addition, the effects of different initial packing patterns on the particulate flow, mixing and segregation behaviors were analyzed and compared. It is interesting to find that the shape of particles and the initial binary particle packing patterns have significant effects on the discharging characteristics. The results of this study provide helpful guidance for industrial applications and provide experiment data for computational model validation.     

Drag,lift and torque acting on a two-dimensional non-spherical particle near a wall

Gas-solid granular flows with non-spherical particles occur in many engineering applications such as fluidized beds. Such flows are usually contained by solid walls and always some particles move close to a wall. The proximity of a wall considerably affects the flow fields and changes the hydrodynamic forces and torque acting on particles moving near the wall. In this paper, we numerically investigate the drag, lift and torque acting on a non-spherical particle in the vicinity of a planar wall by means of lattice Boltzmann simulations. To gain an exhaustive understanding of the complex hydrodynamics and study the influence of various geometrical and flow parameters, a single 2D elliptical particle is selected as our case study. In the simulations, the effect of particle Reynolds number, distance to the wall, orientation angle and aspect ratio on drag, lift and torque is studied. Our study shows that the presence of a wall causes significant changes in hydrodynamic forces, with increasing or decreasing drag and lift forces, depending on the distance from the wall. Even the direction of lift and torque may change, depending on both the distance from the wall and particle orientation angle. Also, an ellipse with higher AR experiences larger hydrodynamic forces and torque whatever the gap size and orientation angle.     

Effect of the particle-size distribution on the backscattering ratio in seawater

Mie theory is used to model the backscattering ratio (the ratio of the backscattering coefficient to the total scattering coefficient) of marine particles with the assumption that they follow a Junge-type size distribution. Results show that the backscattering ratio is very sensitive to the presence of submicrometer particles and depends strongly on the shape of the size distribution. However, it is not affected significantly by absorption and does not vary with wavelength over the visible range. The implications for modeling of backscattering and ocean color in terms of phytoplankton pigment concentration are discussed.     

An approach to simulate the motion of spherical and non-spherical fuel particles in combustion chambers

The objective of this paper is to identify a numerical method to simulate motion of a packed or fluidized bed of fuel particles in combustion chambers, such as a grate furnace and a rotary kiln. Therefore, the various numerical methods applied in the areas of granular matter and molecular dynamics were reviewed extensively. As a result, a time driven approach was found to be suited for the numerical simulation of particle motion in combustion chambers. Furthermore, this method can also be employed to moving boundaries which are required for the present application e.g. travelling grate. The method works in a Lagrangian frame of reference, which uses the position and orientation of particles as independent variables. These are obtained by time integration of the three-dimensional dynamics equations derived from the classical Newtonian approach for each particle. This includes the keeping track of all forces and momentums acting on each particle at every time step. Viscoelastic contact forces include normal and tangential components with viscoelastic models for energy dissipation and friction. The particle shapes are approximated by spheres and ellipsoids with a varying size and ratio of the semi-axis accounting for the variety of particle geometries in a combustion chamber. For these shapes the overlap of particles during contact is expressed by a polynomial of 4th order in the two-dimensional case and a polynomial of 6th order in the three-dimensional case. A new algorithm to detect two-dimensional elliptical particle contact with sufficient accuracy was developed. It is based on a sequence of coordinate transformations and has demonstrated its reliability in numerous applications. Finally, the method was applied to simulate the motion of spherical and elliptical particles in a rectangular enclosure, on a travelling grate, and in a rotary kiln. Received: 16 November 2001     

Modeling influence of hysteretic moisture behavior on distribution of chlorides in concrete

Aggressive environmental conditions, such as exposure to the sea climate or use of de-icing salts, can have a considerable influence on the durability of reinforced concrete structures due to corrosion-induced damage of reinforcement. Recently, the coupled 3D chemo-hygro-thermo-mechanical (CHTM) model for simulation of processes related to the chloride induced corrosion of steel reinforcement in concrete was developed. In the model, it is assumed that for wetting and drying of concrete, the transport of water is controlled by a single sorption curve. However, it is well known that concrete exhibits a hysteretic moisture behaviour, which significantly influences the distribution of moisture and chlorides. To account for the hysteretic moisture behaviour of concrete and for simulating a more realistic time and space distribution of moisture, the CHTM model was further improved. The proposed hysteretic model is implemented into a 3D finite element code and it is validated using a numerical example, which shows reasonably good agreement with the available test results. Similar to what is observed in the experimental tests, it is shown that due to the wetting and drying of the concrete surface, the peak concentration of chloride moves progressively deeper into the concrete specimen.     

The effect of particle size distribution on barite reduction

The present study is a follow-up of investigation on barite reduction to barium sulfide as a function of starting particle size distribution and temperature of reaction. In this work we study the high temperature reduction process of barite from the view point of particle size distribution. Conversion-time data have been obtained using iodometry method in each isothermal condition. A modified kinetic model used to express the carbothermic reduction process. To obtain the values of activation energy and frequency factor, the same expression was selected for each sample at all temperatures. The rate of reaction is found to be related to the particle size distribution and the gasification reaction of coke which has influence on reduction process. The kinetic parameters calculated from standard analysis of isothermal kinetic data indicated that the particle size of barite controlled the reaction when it was coarser than 400 mesh both in presence and absence of catalyst.     

Effect of rubber particle size on the impact tensile fracture behavior of MBS resin with a bimodal particle size distribution

We performed impact tensile fracture experiments on methylmethacrylate–butadiene–styrene (MBS) resin with small and large particles in a bimodal size distribution, and examined the effects of particle size on fracture behavior by fixing the total rubber content (28 wt%) and the small particle size (about 140 nm), and varying the size of large particles (about 490 nm or 670 nm). Dynamic load P′ and displacement δ′ of single-edge-cracked specimens were measured using a Piezo sensor and a high-speed extensometer, respectively. A P′−δ′ diagram was used to determine external work U _ex applied to the specimen, elastic energy E _e stored in the specimen, and fracture energy E _f for creating a new fracture surface A _s. Energy release rate was then estimated using G _f = E _f/A _s. Values of G _f were correlated with fracture loads and mean crack velocity v _m determined from load and time relationships. We then examined the effect of particle size on G _f and v _m, and results indicated that particle size plays an important role in changing the values of G _f and v _m.     

The motion of a neutral spherical particle between coaxial cylindrical surfaces

The tangential and normal components of a dipole force acting upon a neutral spherical particle moving parallel to the axis of symmetry of a coaxial cylindrical channel with different permittivities of the internal and external walls are calculated in a nonrelativistic approximation of the fluctuational electromagnetic theory. Upon a limiting transition, the obtained formulas coincide with the previous results for a particle moving inside a cylindrical channel or parallel to the generatrix of a convex cylindrical surface. The problem is considered in the general case of different temperatures of the particle and the surface.