Cell Models for Viscous Sintering

The kinetics of viscous sintering are determined for models consisting of cylinders intersecting so as to form cells with cubic, tetrahedral, and octahedral shapes. These models are shown to be quite resistant to Rayleigh instabilities, and to densify at similar rates. The constitutive parameters (free strain rate, uniaxial viscosity, and Poisson's ratio) are presented for all of the models.     

Simulation of the Viscous Sintering of Coated Particles

Problems associated with the sintering of mixtures of hard and soft particles can be obviated by coating the hard particles (e.g., alumina) with a soft phase (e.g., glass). An analysis of the sintering kinetics of such materials based on finite-element simulations of the sintering of a pair of rigid spheres coated with a viscous material is presented. Until the contact radius is greater than the thickness of the coating, the sintering kinetics are very similar to those obtained if the rigid core is not present. The sintering rate decreases as the rigid cores approach each other and is limited by flow in the gap between the core particles. If the coating thickness is in excess of 20%, the simulations predict that a packing of such particles sinters to full density at a rate comparable to that of particles without a rigid core.     

Simulation of the Viscous Sintering of Two Particles

The viscous sintering of two initially spherical particles is modeled as creeping flow in response to surface tension. The governing equations are solved using the finite-element method. The condensed results of this simulation, the center-to-center approach velocity, and the rate of change of contact area were presented earlier. Some details of the flow field during sintering and the evolution of the neck geometry in the early stages of sintering are presented here. Important numerical issues are detailed.     

Viscous Sintering on a Rigid Substrate

Two analyses are presented for the sintering kinetics of a porous glass layer on a rigid substrate. The first treatment uses a continuum model, with constitutive equations and the free strain rate derived from an appropriate microstructural model. Predictions are obtained for the sintering kinetics and the magnitude of the tensile stress in the layer. During sintering, shrinkage is not permitted in the plane of the substrate, but the resulting microstructural anisotropy is ignored by the model. A second treatment represents the sintering layer by tubes whose axes are normal to the substrate. The densification kinetics of this model are in reasonable agreement with the results of the continuum model. Therefore, the effects of microstructural anisotropy (pore orientation) are likely to be small, and either model can be used     

用改进的单元胞模型数值模拟液体穿过球形颗粒群的流动

The cell model developed since 1950s is a useful tool for exploring the behavior of particle assemblages,but it demands further careful development of the outer cell boundary conditions so that interaction in a particle swarm is better repressented.In this paper,the cell model and its development were reviewed,and the modifications of outer cell boundary conditions were suggested.Athe cell outer boundary,the restriction of uniform liquid flow was removed in our simulation conducted in the reference frame fixed with the particle.Zero shear stress condition was used to evaluate the outer boundary value of the stream function.Boundary vorticity was allowed to evolve to values compatible to existing stream function at the free shear outer boundary.The fore-aft symmetry of vorticity distribution at the outer boundary is thought critical to ensure the continuity of inflow and outflow between touching neighbor cells,and is also tested in the modified cell model.Numerical simulation in terms of stream function and vorticity based on the modified cell models was carried out to shed light on the interaction between liquid and particles.Lower predicted drag coefficient by the modified cell models was interpreted with the feature of flow structure.The drag coefficient from the simulation was also compared with correlations of drag coefficient reported in literature.It is found that the modified cell model with the uniformity of external flow relaxed and the fore-aft symmetry of boundary vorticity enforced was the most satisfactory of the overall performance of prediction.     

Viscous Sintering under a Uniaxial Load

An analysis is presented for viscous sintering under a uniaxial load; numerical results are presented for compressive loads. The load increases the axial contraction rate, hut counteracts the radial contraction rate. When the load is similar in magnitude to the capillary stress driving sintering, the radius remains essentially constant as the body densifies. Following a suggestion by Bordia, the viscosity and sintering kinetics can be determined in one experiment by measuring the axial and radial shrinkages simultaneously.     

Viscous Sintering of a Bimodal Pore-Size Distribution

The cylinder model, used previously to analyze the viscous sintering of flame hydrolysis preforms and gels, is shown to be consistent with other models of early- and late-stage sintering. The model is then applied to a bimodal pore-size distribution in which the different shrinkage rates of small and large pores produce local stresses. The effect of these stresses on the sintering rate is determined and shown to be substantial. Initially, the small pores accelerate the densification of the large pores; later, shrinkage of the isolated large pores is resisted by the sintered remains of the small pores. Consequently, the time to reach full density is nearly independent of the initial volume fraction of small pores.     

Three-Dimensional Solar Cell Finite-Element Sintering Simulation

The sintering process is ubiquitous in manufacturing, but the design-oriented modeling of sintering has presented considerable challenges. This type of modeling is necessary to be able to predict deformation and thus design appropriate powder compacts so that after they are sintered, the desired dimensions will be achieved. Currently this is done through a costly and time-consuming trial and error process. In our research, an application of the Skorohod–Olevsky viscous sintering constitutive equation in a finite-element (FE) model is developed and used to model solar cell manufacturing. Experimental measurements are used to determine the properties of the solar cell materials, and these are used to calculate the parameters for the FE model. Simulation results are compared with experimental data and analysis has been made to evaluate the adequacy and usefulness of this approach.     

抽油机变周期运动泵内流场数值模拟

目前,油田现场已采用的变周期采油方式,具有提高泵效,降低能耗等优点,但变周期采油井底形态及抽汲参数、抽油泵结构和抽油泵沉没压力等因素对变周期采油影响规律尚不明确。针对上述问题,根据抽油泵变周期运动规律,应用FLUENT软件,利用动网格技术与UDF程序控制抽油泵内柱塞与球阀的运动,实现井下抽油泵变周期运动抽汲过程的仿真模拟。取得成果如下:在变周期上下冲程速比固定条件下,得到井下抽油泵泵内流场分布,以及不同冲程、冲次对抽油泵泵效的影响规律。取得的研究成果,可为现场变周期采油提供理论支持。     

Numerical Simulation of Solid-State Sintering: I, Sintering of Three Particles

A kinetic, Monte Carlo model, capable of simulating microstructural evolution sintering in a two-dimensional system of three particles, has been presented. The model can simulate several mechanisms simultaneously. It can simulate curvature-driven grain growth, pore migration and coarsening by surface diffusion, and densification by diffusion of vacancies to grain boundaries and annihilation of these vacancies. Morphologic changes and densification kinetics are used to verify the model.     

铁矿石低碳烧结的数值模拟

建立了烧结过程传热模型,利用Fluent软件对常规烧结及以10%, 20%, 30%(等能量输出)竹炭和板栗壳炭替代常规燃料进行烧结的温度场进行数值求解,计算值与测定值对比验证模型的可靠性,以料层最高温度和料层冷却速率对各方案进行评价. 结果表明,竹炭和板栗壳炭20%替代方案最高温度高于1573 K时的料层厚度分别为0.49和0.47 m,大于常规烧结及10%和30%替代方案,能有效提高烧结矿成块固结量;竹炭和板栗壳炭30%替代方案料层冷却速率小于120 K/min时的料层厚度分别为0.458和0.480 m,大于常规烧结及10%和20%替代方案,有助于提高烧结矿机械强度;用燃点较高的生物质炭替代常规燃料有利于提高料层最高温度,用燃点较低的生物质炭替代常规燃料有利于加快烧结速率.     

Sintering Stress of Nonlinear Viscous Materials

An analytical model for the sintering stress of materials characterized by a nonlinear viscous behavior during densification is proposed. The growing applications in the field of nanosized powders processing (in particular, consolidation of high surface area components used in supercapacitors, rechargeable batteries, gas absorbers) have renewed the interest in this fundamental parameter of sintering science, because of the sintering stress’ characteristic inverse proportionality with respect to the powder particles radius. This increase in the magnitude of the sintering stress is also responsible for power‐law creep being the mechanism that underlies densification even without the application of any additional external load, and therefore for a nonlinear viscous behavior of the solid material. The analytical treatment of problems involving nonlinear viscous materials has traditionally involved complex self‐consistent methods and approximations, unless the local case of an isolated pore embedded in a fully dense skeleton was considered. The paper proposes a simple first‐order iterative method that allows the derivation of both bulk modulus and sintering stress of a material containing an arbitrary amount of pores, as functions of porosity and of the material's nonlinearity parameter, namely strain rate sensitivity. An expression for densification kinetics is also obtained and compared with experimental data.     

Three-Dimensional Finite-Element Analysis of Viscous Sintering

A finite-element model is developed and implemented on a massively parallel supercomputer to study the viscous sintering of a three-dimensional (3-D) configuration of three particles. Model development is described, implementation is discussed, and calculations are compared with benchmark results for a two-dimensional (2-D) axisymmetric case. For the first time, detailed simulation results are shown for the viscous sintering of a 3-D, three-particle system. The calculations predict center approach of particle pairs concurrent with asymmetric neck growth, effects which ultimately lead to an inward rotation of the outer particles. Anisotropic shrinkage arises due to this 3-D rearrangement of the particles during sintering.     

Viscous Sintering with a Pore-Size Distribution and Rigid Inclusions

The model developed for sintering of a bimodal pore-size distribution is generalized to describe an arbitrary distribution. The model is further extended to allow for the presence of nonsintering (i.e., rigid) inclusions. This analysis uses the self-consistent approach that takes account of the local stresses created by differential sintering rates.     

成品油罐自动脱水装置数值模拟

成品油外输时对其含水率有严格要求,因此,成品油在储运过程中,尤其是在油罐储存过程中必须要进行脱水。成品油罐的脱水设备在运行过程中应尽量满足低能耗、低污染的要求。本文以成品油罐自动脱水装置为研究对象,建立了水力数学模型,对流经自动脱水装置的油水混合物进行了流动状态的数值模拟,进一步验证了自动脱水装置设计的合理l生,为成品油罐自动脱水装置的安装使用提供了经验。     

Numerical Simulation of Drying of a Deformable Anisotropic Porous Medium Using the Lattice Boltzmann Method

The convective drying of an anisotropic saturated deformable porous media has been carried out numerically in the present paper based on the Darcy-Brinkmann extended model. The considered sample is a porous medium that is assumed to be both hydro-dynamically and thermally anisotropic. All walls of the sample are subjected to a convective heat flux. Furthermore, the thermal equilibrium condition is assumed to be applicable to the current investigation. The principal axis of the anisotropic porous medium ranges between 0 and 90°. The Lattice Boltzmann method (LBM) is used to solve the differential equation system. Special attention is given throughout this work to understand the effect of dynamic and thermal anisotropic parameters and the mass boundary conditions on macroscopic fields.     

用智能型单回路调节器实现对间歇式窑炉烧成曲线的控制

简要介绍了用智能型单回路调节器实现对间歇式窑炉烧成曲线的控制及调节方法。     

Fabrication of Mullite and Mullite-Matrix Composites by Transient Viscous Sintering of Composite Powders

Mullite was fabricated by a process referred to as transient viscous sintering (TVS). Composite particles which consisted of inner cores of α-alumina and outer coatings of amorphous silica were used. Powder compacts prepared with these particles were viscously sintered to almost full density at relatively low temperatures (∼1300°C). Compacts were subsequently converted to dense, fine-grained mullite at higher temperatures (∼1500°C) by reaction between the alumina and silica. The TVS process was also used to fabricate mullite/zirconia/alumina, mullite/silicon carbide particle, and mullite/silicon carbide whisker composites. Densification was enhanced compared with other recent studies of sintering of mullite-based composites. This was attributed to three factors: viscous flow of the amorphous silica coating on the particles, avoidance of mullite formation until higher temperatures, and increased threshold concentration for the development of percolation networks.     

Viscous Sintering of Spherical Particles via Finite Element Analysis

A robust finite element model is employed to study the viscous sintering of three-dimensional, axisymmetric particles from several initial configurations. Particle shape evolution and flow fields are presented which yield new insight into the behavior of these systems. The effects of the initial particle size distribution and configuration are considered.Model predictions are validated by direct comparisons with experimental measurements.     

基于格子Boltzmann方法的封闭三角腔自然对流的数值模拟

建立了二维不可压缩D2G9格子Boltzmann模型,耦合二维TD2Q5热格子Boltzmann模型,在非平衡态外推的边界条件下,首先对不同Eckert数(Ec)和Prandtl数(Pr)时Couette流的温度场进行数值模拟,计算结果与解析解吻合良好,且在Ec变化很大的条件下,计算结果仍与解析解相符,验证了模型的准确性和稳定性. 然后对封闭三角空腔内不同Rayleigh数(Ra)下的自然对流流场和温度场进行了数值模拟,结果与文献计算值吻合良好,说明格子Boltzmann方法的TD2Q5热模型可用于高Ra时的空腔热流动模拟.