Volume fraction and velocity fields of nearly uniform granular flows in a narrow channel geometry with smooth bed

The combined knowledge of the velocity and volume fraction fields is crucial for investigating the dynamics of granular flows, especially in the dense-collisional regime where both frictional and collisional dissipation mechanisms are significant. A laboratory investigation on steady dry granular flows in a straight channel is reported, where slip conditions are allowed at the basal surface and side walls. The stochastic-optical method (SOM), proposed by Sarno et al. (2016) for estimating the volume fraction in granular mixtures, is applied for the first time to granular flows. The velocity at the free surface and at the flume sidewall is measured by using a multi-pass particle image velocimetry (PIV) approach. The measurements of the velocity and volume fraction reveal a superimposition of different dynamic structures, which can be distinguished by means of a volume fraction threshold. Additionally, the profiles of measured volume fraction are exploited to estimate the pressure distribution, so as to numerically describe the velocity profiles by using the μ(I) rheology. It is found that the employment of the experimental volume fraction is superior in describing the flow dynamics, especially near the free surface.     

Control of the continuous rheocasting process

In the continuous rheocasting process, a semi-solid alloy is obtained from the exit port of the apparatus at a given rate and with a given fraction of solid. This solid fraction is dependent on the corresponding temperature within the solid-liquid range which should be controlled accurately by the process parameters for a given rheocaster stirring chamber. For this purpose a heat flow model has been established for the continuous rheocasting of Bi-17 wt% Sn alloy. The heat transfer calculations are based on the solution of the two-dimensional partial differential equations using a finite difference method. An excellent agreement between calculations and experimental results is found. Computations are carried out in order to find the influence of stirring chamber dimensions on the alloy exit temperature and therefore, the volume fraction of solid. The influence of input metal temperature and metal flow rate on the exit temperature and volume fraction of solid are also found.     

Experimental and CFD study on a cyclonic classifier with new flow pattern

Physical principle of conventional top-inlet classifier (CTC) with reverse-flow pattern leads to the heavily fine particles entrainment in coarse fraction. Present work concentrates on the flow-field design for less downward airflow at near-wall region of the classifier. A new middle-inlet classifier (NMC) is proposed and analyzed using computational fluid dynamics (CFD) method and powder classification experiments. The results showed that new flow pattern characterized by a pair of vortexes was created in the new classifier. The upper vortex with 80% of the total air volume moves upward and forms the washing effect at near-wall region, which effectively reduces the fine particles entrainment in coarse fraction. The downer vortex with reverse-flow pattern discharges the coarse particles timely. The radial centrifugal sedimentation combined with the axial counter-current washing effect dominates the particle classification in the NMC. Compared to the CTC, classification accuracy index of the NMC with double-vortex averagely increases by 27% with a pressure drop reduction of more than 38%. This work offers a new principle for high-efficiency particle classification and new strategy for improving the classification performance of turbo air classifiers and hydrocyclones.     

Molecular Dynamics Modeling of Latent Heat Enhancement in Nanofluids

A discrete computational approach based on molecular dynamics (MD) simulations is proposed for evaluating the latent heat of vaporization of nanofluids. The computational algorithm, which considers the interaction of the solid and the fluid molecules, is used for obtaining the enhancement of the latent heat of a base fluid due to the suspension of nanoparticles. The method is validated by comparing the computed latent heat values of water with standard values at different saturation temperatures. Simulation of a water–platinum nanofluid system is performed, treating the volume fraction and size of nanoparticles as parameters. The trends in the variation are found to match well with experimental results on nanofluids. Discussions are also presented on the limitations of the proposed model, and on methods to overcome them.     

Bending,Free Vibration and Buckling Analysis of Functionally Graded Plates via Wavelet Finite Element Method

Following previous work, a wavelet finite element method is developed for bending, free vibration and buckling analysis of functionally graded (FG) plates based on Mindlin plate theory. The functionally graded material (FGM) properties are assumed to vary smoothly and continuously throughout the thickness of plate according to power law distribution of volume fraction of constituents. This article adopts scaling functions of two-dimensional tensor product BSWI to form shape functions. Then two-dimensional FGM BSWI element is constructed based on Mindlin plate theory by means of two-dimensional tensor product BSWI. The proposed two-dimensional FGM BSWI element possesses the advantages of high convergence, high accuracy and reliability with fewer degrees of freedoms on account of the excellent approximation property of BSWI. Numerical examples concerning various length-to-thickness ratios, volume fraction indexes, aspect ratios and boundary conditions are carried out for bending, free vibration and buckling problems of FG plates. These comparison examples demonstrate the accuracy and reliability of the proposed WFEM method comparing with the exact and referential solutions available in literatures.     

Dynamics of Formation and Parameters of a Fog upon Abrupt Expansion of a Compressed Vapor–Gas Volume

This paper deals with an experimental investigation of structural changes in the compressed vapor–gas volume of a water vapor–hydrogen–air system, which are caused by an abrupt expansion of the volume due to the loss of integrity of the confinement shell. A homogeneous system transforms to a heterogeneous water vapor–microdroplets–hydrogen–air mixture behind the rarefaction wave. The microdroplet sizes and their bulk concentration are measured during the formation of a fog. A method is proposed for estimating the volume fraction of microdroplets from the measurements of light absorption on one wavelength of the IR region.     

玻璃纤维对磷酸镁水泥砂浆力学性能的增强作用及机理

添加不同体积比的玻璃纤维,按照一定比例配制玻璃纤维增强磷酸镁水泥。研究了玻璃纤维增强磷酸镁水泥的抗压强度、抗折强度以及耐水性,并采用电镜扫描的方法对其微观结构进行了观察。研究结果表明,玻璃纤维对磷酸镁水泥的抗压强度和抗折强度都有一定贡献,其中纤维的最佳体积掺量约为2.5%,但超过最佳掺量后,抗压和抗折强度都有所降低。另外,实验结果还表明,稍过量的玻璃纤维能够暂时"包裹"未反应基材,可能在浸水环境中发生又一轮反应,从而抵消因浸水造成的强度损失,这可能是一种改善磷酸镁水泥耐水性的新方法。此外,本工作提供了与实验结果一致的纤维增强机理的可能解释。     

颗粒团聚分布的复合材料数值模型

目的 提出颗粒团聚分布的数值模型参数化建模方法,以高效建立颗粒有序分布的复合材料微观构型。方法 以颗粒尺寸、颗粒团聚区域的半径以及团聚区域局部体积分数和整体体积分数的比值作为表征颗粒团聚结构的基本建模控制参数,首先建立了圆形/球形区域内的Voronoi多胞元结构,其次编写基于Python语言的接口程序,根据角点坐标和几何拓扑关系将多胞元结构导入有限元软件中,并对每个胞元按团聚区域局部体积分数进行缩放,形成了圆形/球形区域的颗粒团簇,最后使用随机顺序吸附算法将团聚颗粒在代表单元内周期性随机投放,生成了颗粒团聚分布的复合材料二维/三维微观构型。结果 使用不同控制参数进行数值建模测试,生成一个颗粒数目在1 000以内的二维/三维颗粒团聚微观构型仅需要数分钟,验证了本文方法的有效性。使用二维模型对单轴拉伸进行数值模拟,结果表明,材料宏观塑性随着颗粒团聚程度的增大而降低。结论 本文建模方法可以为复合材料结构设计提供有效的数值仿真支撑。     

Calculating the elastic moduli of steel-fiber reinforced concrete using a dedicated empirical formula

The equivalent inclusion method (EIM) is adopted to study the characteristics of the equivalent material properties of steel-fiber reinforced concrete as a function of the volume fraction and the length to diameter ratio of the fibers. It is found that the equivalent material moduli of concrete reinforce with randomly orientated and distributed fibers are insensitive to the length to diameter ratio of the steel fibers. A set of empirical formulae is then proposed for the purposes of engineering applications. The proposed empirical model can simplify the calculation of the equivalent material moduli. Verifications of the proposed empirical formulae with the EIM model and with experimental data are performed with two examples. The first is a compression test. The second is 4 point bending test. The empirical formulae, based on the equivalent inclusion method proposed in this study, represent an alternative means of quickly calculating the effective elastic modulus of steel-fiber reinforced concrete materials.     

Determination of binary mixture vapor-liquid critical densities from coexisting density data

Two-phase vapor-liquid equilibrium (VLE) isochores for binary mixtures are defined as the thermodynamic paths along which the overall density and composition are fixed. Data along such isochores are generated from a modified Leung-Griffiths model fit to experimental data for the binary system nitrogen-methane. The behavior of the liquid volume fraction along these isochores is found to be similar to that for pure fluids. Rectilinear diameters for varying overall densities (fixed composition) are seen to be nearly coincident. Straight-line diameters and the critical liquid volume fraction method are utilized to predict critical densities using data near and removed from the critical point. Both methods give acceptable results but the critical liquid volume fraction method is more accurate. A critical literature review of the need for binary mixture critical densities is presented and a proposed experimental procedure is given for the determination of mixture critical densities.     

Figure of merit for the thermal performance of cementitious composites containing phase change materials

This paper presents a novel method to quantitatively characterize the thermal performance of composite materials containing phase change materials (PCM) based on a figure of merit we termed the energy indicator. The method features (i) commonly used specimen geometry, (ii) straightforward experimental implementation, and (iii) sensitivity to relevant design parameters including PCM volume fraction, enthalpy of phase change, composite effective thermal conductivity, and specimen dimensions. The experimental method and the concept of energy indicator were demonstrated on PCM-mortar composites using various volume fractions of two commercial microencapsulated PCMs. This was supported by transient two-dimensional heat transfer simulations. The energy indicator was shown to increase linearly with increasing microencapsulated PCM volume fraction and latent heat of fusion and quadratically with the specimen radius. This figure of merit can be used to rapidly screen and select microencapsulated PCM composite materials for energy efficient buildings or crack-resistant concretes.     

Free vibration analysis of functionally graded conical shell panels by a meshless method

A free vibration analysis of metal and ceramic functionally graded conical shell panels is presented using the element-free kp-Ritz method. The first-order shear deformation shell theory is used to account for the transverse shear strains and rotary inertia, and mesh-free kernel particle functions are employed to approximate the two-dimensional displacement fields. The material properties of the conical shell panels are assumed to vary continuously through their thickness in accordance with a power-law distribution of the volume fractions of their constituents. Convergence studies are performed in terms of the number of nodes, and comparisons of the current solutions and those reported in literature are provided to verify the accuracy of the proposed method. Two types of functionally graded conical shell panels, including Al/ZrO₂ and Ti–6Al–4V/aluminum oxide, are chosen in the study, and the effects of the volume fraction, boundary condition, semi-vertex angle, and length-to-thickness ratio on their frequency characteristics are discussed in detail.     

Comparison of methods for wall boundary conditions as applied to the calculation of turbulent heat exchange characteristics

Methods for setting and realizing wall boundary conditions numerically in calculating turbulent flows is considered. A method for realizing weak boundary conditions on the wall with discretization of Reynolds-averaged Navier–Stokes equations by the control volume approach is discussed. The results of calculations for a number of model problems obtained within the framework of different approaches to the near-wall modeling are compared to the data of the physical experiment and the available correlation dependences. The grid dependence of the solution in using the method of near-wall functions is compared to that in using weak boundary conditions.     

Numerical Investigation of Nanofluid Thermocapillary Convection Based on Two-Phase Mixture Model

Numerical investigation of nanofluid thermocapillary convection in a two-dimensional rectangular cavity was carried out, in which the two-phase mixture model was used to simulate the nanoparticles-fluid mixture flow, and the influences of volume fraction of nanoparticles on the flow characteristics and heat transfer performance were discussed. The results show that, with the increase of nanoparticle volume fraction, thermocapillary convection intensity weakens gradually, and the heat conduction effect strengthens; meanwhile, the temperature gradient at free surface increases but the free surface velocity decreases gradually. The average Nusselt number of hot wall and the total entropy generation decrease with nanoparticle volume fraction increasing.     

An analytical method for plain weave fabric composites

A two-dimensional closed-form analytical method has been developed for the thermoelastic analysis of two-dimensional orthogonal plain weave fabric laminae. Strand undulation and continuity in both the warp and fill directions, actual strand cross-section and weave geometry, strand fibre volume fraction and possible gap between the two adjacent strands have been considered in the analysis. Good correlation is observed between the predicted thermoelastic properties and experimental results.     

Measurements of γ/γ' Lattice Misfit and γ' Volume Fraction for a Ru-containing Nickel-based Single Crystal Superalloy

A conventional X-ray diffractometer has been used to determine the γ/γ lattice misfit and γ volume fraction for a Ru-containing nickel-based single crystal superalloy at room temperature.The rocking curve was used to characterize the distribution of subgrains.The diffraction peaks obtained by ω-2θ scan were used to determine the γ/γ lattice misfit and γ volume fraction.A three peaks fitting model was proposed.The peak fitting results are in good agreement with the model.The X-ray diffraction results indicate that the nickel-based single crystal superalloy was not a perfect monocrystalline material,which is comprised of many subgrains;and each subgrain also consists of large numbers of mosaic structures.In addition,two anomalous reflection phenomena were found during the experiment and discussed with respect to their occurrence and impact on the measurement.The experimental results show that the γ/γ lattice misfit and γ volume fraction will be various at the different regions of its dendritic microstructure.The average γ/γ lattice misfit and γ volume fraction of the experimental alloy are approximately-0.2% and 70%,respectively.Furthermore,the γ volume fraction calculated by atom microprobe(AP) data is also basically consistent with the experimental results.     

Mixture law including particle-size effect on fracture toughness of nano- and micro-spherical particle-filled composites

The effects of particle diameter and volume fraction on fracture toughness of nano- and micro-spherical particle-filled composites were investigated. The purpose was to create a mixture law of fracture toughness based on experimental results of spherical silica particle–filled epoxy composites and a theoretical approach. The fracture toughness of composites was found to be tailored independently by exchanging different particle sizes, and elastic and viscoelastic properties were found to be governed by the volume fraction of the particles. In a theoretical analysis, a mixture law of fracture toughness, composed of the elastic moduli, diameter, and volume fraction of particles and the elastic moduli of matrix resins was proposed. Its validity was demonstrated in a comparison with the experimental results.     

Effect of temperature on rheological properties of copper oxide nanoparticles dispersed in propylene glycol and water mixture

This paper reports on experimental investigation of the rheological behavior of copper oxide nanoparticles dispersed in a 60:40 propylene glycol and water mixture. Nanofluids of a particle volume concentration from 0 to 6% have been tested in this study. The experiments were conducted over a temperature range of -35 degrees C to 50 degrees C to establish their behavior for use as a heat transfer fluid in cold climates. The experiments reveal that this nanofluid in the range of particle volume percentage tested exhibits a Newtonian behavior. A new exponential correlation has been developed from the experimental data, which expresses the viscosity as a function of particle volume percent and the temperature of the nanofluid. The slope of relative viscosity curve was found to be higher at lower temperatures.     

Application of meshless EFG method in fluid flow problems

This paper deals with the solution of two-dimensional fluid flow problems using the meshless element-free Galerkin method. The unknown function of velocity u(x) is approximated by moving least square approximants u^h(x). These approximants are constructed by using a weight function, a monomial basis function and a set of non-constant coefficients. The variational method is used for the development of discrete equations. The Lagrange multiplier technique has been used to enforce the essential boundary conditions. A new exponential weight function has been proposed. The results are obtained for a two-dimensional model problem using different EFG weight functions and compared with the results of finite element and exact methods. The results obtained using proposed weight functions (exponential) are more promising as compared to those obtained using existing weight functions (quartic spline and Gaussian)