双嵌段共聚物薄膜介观结构的耗散粒子动力学模拟

采用耗散粒子动力学(Dissipative Particle Dynamics, DPD)方法模拟两平行平板间的双嵌段共聚物体系的介观结构. 模拟结果表明, 随板间距的增大, 体系分层数量的增加是不连续的, 在分层数量的增加过程中, 出现不规则结构的过渡区；聚合物链末端距随板间距的增大呈周期性振荡, 振荡幅度逐渐减小. 对模拟结果的分析表明：层状结构转变点与分层数量之间存在近似线性关系；层状结构转变点近似与共聚物链长的2/3次方成正比.     

在介观尺度研究多相复杂流体——四辊流变仪及其应用

研究多相复杂流体的流变学需要介观尺度下基本力学过程的信息.四辊流变仪可以产生旋转、剪切、拉伸等多种流场,且在流场中心形成一个停滞点,适于实时观察和测量介观尺度范围内与基质不相容的液滴的形变、破裂和聚并,或纤维、固体粒子、液晶等与液体组成的多相复杂流体的流变性质,还可以和多种仪器联用进一步扩展应用范围.本文综述了四辊流变仪的发展、主要部件、理论基础、用途以及应用四辊流变仪所做的主要研究,最后与其它流变仪器加以比较并总结了它的优缺点.     

Analysis of local behaviour in granular materials

A local scale, called the meso-scale, has recently been introduced to the multi-scale approach for 2D granular materials. This local scale is defined at the level of meso-domains enclosed by particles in contact. Stress and strain have been defined at this local scale, and their relation with the local structure has been studied. The purpose of this paper is to analyse the behaviour of granular materials at the meso-scale, i.e. the stress–strain–structure relationship at this scale. Analyses are performed on a 2D numerical granular sample subjected to a biaxial compression test and simulated with the Discrete Element Method (DEM). The sample is quite dense and it is loaded at a relatively low strain rate so that the state of the sample can be considered as being quasi-static. The size of sub-domains in the sample varies largely from 3 to 12 particles. It is shown that the evolution of the internal state of the sample corresponds, at the meso-scale, to a clear evolution of the quantity of meso-domains oriented in different directions. In addition, the behaviour of meso-domains is highly governed by their orientation rather than their density, especially for the strongly elongated meso-domains: the meso-domains oriented in the compression (resp. extension) direction behave like a dense (resp. loose) granular material.     

Experimental determination of Representative Volume Element (RVE) size in woven composites

A systematic approach is proposed to estimate the length scales of the representative volume element (RVE) in orthogonal plain woven composites. The approach is based on experimental full-field deformation measurements at mesoscopic scales. Stereovision digital image correlation (DIC) is conducted to determine the full-field strain distribution in on- and off-axis specimens loaded axially in tension. A sensitivity analysis is carried out to optimize the image correlation parameters. Using the optimized set of image correlation parameters, full-field strains are measured and used in conjunction with a simple strain averaging algorithm to identify the length scales at which globally applied and spatially-averaged local strains converge in values. The size of a virtual window containing local strain data, the average of which has the same value as the global strain, is identified as the RVE dimensions for the examined material. The smallest RVE sizes found in this work are shown to be both strain and angle dependent. The largest RVE dimension obtained is reported as a unique, strain and orientation insensitive RVE size for the woven composite examined.     

A review of multiscale CFD for gas–solid CFB modeling

Meso-scale structure is of critical importance to circulating fluidized bed (CFB) applications. Computational fluid dynamics (CFD) with consideration of meso-scale structures can help understand the structure-oriented coupling between flow, heat/mass transfer and reactions. This article is to review our recent progress on the so-called multiscale CFD (MSCFD), which characterizes the sub-grid meso-scale structure with stability criteria in addition to conservation equations. It is found that the mesh-independent solution of fine-grid two-fluid model (TFM) without sub-grid structures is inexact, in the sense that it overestimates the drag coefficient and fails to capture the characteristic S-shaped axial profile of voidage in a CFB riser. By comparison, MSCFD approach in terms of EMMS/matrix seems to reach a mesh-independent solution of the sub-grid structure, and succeeds in predicting the axial profile and flow regime transitions. Further application of MSCFD finds that neglect of geometric factors is one of the major reasons that cause disputes in understanding the flow regime transitions in a CFB. The operating diagram should, accordingly, include geometric factors besides commonly believed operating parameters for the intrinsic flow regime diagram. Recent extension of MSCFD to mass transfer finds that Reynolds number is insufficient for correlating the overall Sherwood number in a CFB. This is believed the main reason why the conventional correlations of Sherwood number scatter by several orders of magnitude. Certain jump change of state of motion around Reynolds number of 50–100 can be expected to clarify the abrupt decay of Sherwood number in both classical- and circulating-fluidized beds. Finally, we expect that the real-size, 3-D, full-loop, time-dependent multiscale simulation of CFB is an emerging paradigm that will realize virtual experiment of CFBs.     

Progress in miniature liquid film combustors: Double chamber and central porous fuel inlet designs

One of the key limits to miniaturizing the size of liquid fueled combustors is the atomization process applied in meso-scale systems. A single-wall fuel-film combustor was introduced recently as one of the successful liquid fuel combustors at the meso-scale. Instead of atomizing the fuel, film combustors spread out a liquid film along the wall and absorb the heat transferred from the flame for vaporization. With a single-wall film design, however, there are some unexpected and disadvantageous combustion phenomena. This paper attempts to improve the single-wall film combustor by exploring separately a double chamber concept and a central porous fuel delivery concept. These two configurations help describe the limits and the potential of liquid fuel-film miniature combustors. The double chamber design demonstrates how heat transfer issues can be overcome by injecting the fuel-film on the outside of the primary combustor wall rather than on the inside, and the second design demonstrates a flame-holding mechanism using a porous material set on the bottom of the chamber. The combustion behavior in these two configurations is compared with that in the original single-wall miniature fuel-film combustor, revealing new aspects that are relevant to portable power generation with high specific energy and power.     

Simulation of polycrystal deformation with grain and grain boundary effects

Modeling the strengthening effect of grain boundaries (Hall-Petch effect) in metallic polycrystals in a physically consistent way, and without invoking arbitrary length scales, is a long-standing, unsolved problem. A two-scale method to treat predictively the interactions of large numbers of dislocations with grain boundaries has been developed, implemented, and tested. At the first scale, a standard grain-scale simulation (GSS) based on a finite element (FE) formulation makes use of recently proposed dislocation-density-based single-crystal constitutive equations (“SCCE-D”) to determine local stresses, strains, and slip magnitudes. At the second scale, a novel meso-scale simulation (MSS) redistributes the mobile part of the dislocation density within grains consistent with the plastic strain, computes the associated inter-dislocation back stress, and enforces local slip transmission criteria at grain boundaries.Compared with a standard crystal plasticity finite element (FE) model (CP-FEM), the two-scale model required only 5% more CPU time, making it suitable for practical material design. The model confers new capabilities as follows:

(1)

The two-scale method reproduced the dislocation densities predicted by analytical solutions of single pile-ups.

(2)

Two-scale simulations of 2D and 3D arrays of regular grains predicted Hall-Petch slopes for iron of 1.2 ± 0.3 MN/m^3/2 and 1.5 ± 0.3 MN/m^3/2, in agreement with a measured slope of 0.9 ± 0.1 MN/m^3/2.

(3)

The tensile stress-strain response of coarse-grained Fe multi-crystals (9-39 grains) was predicted 2-4 times more accurately by the two-scale model as compared with CP-FEM or Taylor-type texture models.

(4)

The lattice curvature of a deformed Fe-3% Si columnar multi-crystal was predicted and measured. The measured maximum lattice curvature near grain boundaries agreed with model predictions within the experimental scatter.

     

Focusing on the meso-scales of multi-scale phenomena—In search for a new paradigm in chemical engineering

To celebrate the 90th birthday of Professor Mooson Kwauk, who supervised the multi-scale research at this Institute in the last three decades, we dedicate this paper outlining our thoughts on this subject accumulated from our previous studies. In the process of developing, improving and extending the energy- minimization multi-scale （EMMS） method, we have gradually recognized that meso-scales are critical to the understanding of the different kinds of multi-scale structures and systems. It is a common challenge not only for chemical engineering but also for almost all disciplines of science and engineering, due to its importance in bridging micro- and macro-behaviors and in displaying complexity and diversity. It is believed that there may exist a common law behind meso-scales of different problems, possibly even in different fields. Therefore, a breakthrough in the understanding of meso-scales will help materialize a revolutionary progress, with respect to modeling, computation and application.     

Equivalent mechanical properties of textile monolayers from discrete asymptotic homogenization

The determination of the effective mechanical moduli of textiles from mechanical measurements is usually difficult due to their discrete architecture, which makes micromechanical analyses a relevant alternative to access those properties. Micropolar continuum models describing the effective mechanical behavior of woven fabric monolayers are constructed from the homogenization of an identified repetitive pattern of the textile within a representative unit cell. The interwoven yarns within the textile are represented as a network of trusses connected by nodes at their crossover points. These trusses have extensional and bending rigidities to allow for yarn stretching and flexion, and a transverse shear deformation is additionally considered. Interactions between yarns at the crossover points are captured by beam segments connecting the nodes. The woven fabric is modeled after homogenization as an anisotropic planar continuum with two preferred material directions in the mean plane of the textile. Based on the developed methodology, the effective mechanical properties of plain weave and twill are evaluated, including their bending moduli and characteristic flexural lengths. A satisfactory agreement is obtained between the effective moduli obtained by homogenization and numerical values obtained by finite element simulations performed over periodic unit cells.     

Modeling development on the meso-scale reacting transport phenomena in proton exchange membrane fuel cells

The catalyst layer (CL) of proton exchange mem-brane fuel cell (PEMFC) involves various particles and pores in meso-scale, which has an important effect on the mass, charge (proton and electron) and heat transport coupled with the electrochemical reactions. The coarse-grained molecular dynamics (CG-MD) method is employed as a meso-scale structure reconstruction technique to mimic the self-organization phenomena in the fabrication steps of a CL. The meso-scale structure obtained at the equilibrium state is further analyzed by molecular dynamic (MD) software to provide the necessary microscopic parameters for understanding of multi-scale and-physics processes in CLs. The primary pore size distribution (PSD) and active platinum (Pt) surface areas are also calculated and then compared with the experiments. In addition, we also highlight the implementation method to combine microscopic elementary kinetic reaction schemes with the CG-MD approaches to provide insight into the reactions in CLs. The concepts from CG modeling with particle hydrodynamics (SPH) and the problems on coupling of SPH with finite element modeling (FEM) methods are further outlined and discussed to understand the effects of the meso-scale transport phenomena in fuel cells.