计算混凝土中平均最邻近集料表面间距的正十二面体模型

带状模型是以解析解的方式研究混凝土中集料尺寸分布和集料体积分数对平均最邻近集料表面间距影响的模型。但是,由带状模型的计算公式计算浆体的体积无法得到正确的结果。鉴于上述缺点,提出采用单一粒径球形粒子体系的正十二面体模型。以符合Fuller分布集料为例,结合数量基平均粒径方法,采用正十二面体模型研究了集料尺寸分布和集料体积分数对平均最邻近集料表面间距的影响。研究结果表明:随着集料体积分数的增加和集料细度的提高,平均最邻近集料表面间距减小;对所研究的集料体积分数(60％～75％)和集料尺度分布(Fuller分布)而言,平均最邻近集料表面间距值在90～232μm之间变化;在给定的集料分布下,集料体积分数在55％～75％范围内,平均邻近集料表面间距与集料体积分数之间近似呈线性关系。     

水泥基复合材料浆体厚度分布的定量表达

依据统计物理学领域中的弦长密度函数,提出了水泥基复合材料邻近集料间浆体厚度分布的定量估计方法.借助计算机模拟方法生成的模型结构验证该理论方法的可靠性.通过浆体厚度的数量基概率密度曲线和累计概率曲线的比较发现,理论解和模拟结果之间能较好吻合.在常用集料体积分数下,用该解析解预测了集料符合Fuller分布的砂浆和混凝土中浆体厚度分布及其平均值随集料体积分数和集料细度的变化关系.结果发现:邻近集料间浆体厚度平均值的变化范围为190～860 μm.     

混凝土中邻近集料表面最近间距分布的计算机模拟

混凝土中邻近集料问界面过渡区的相互影响程度、混凝土中原生裂纹的尺度范围以及中心质假说中集料效应圈的范围等都涉及到混凝土中最邻近集料表面间距分布的问题。由于常规的实验方法无法给出混凝土中集料空间分布的信息,同时以往的计算机模拟方法由于采用随机分布方式分布粒子,导致无法得到较高集料体积分数的模型混凝土结构,而采用具有粒子动态混合密实功能的SPACE系统,模拟了高集料体积分数混凝土的结构。在假定模型混凝土中集料的最小粒径为1mm的前提下,以符合Fuller分布为例,研究了模型混凝土中集料粒径分布和集料体积分数对邻近集料表面最近间距分布的影响。     

纤维曲率对界面过渡区初始微观结构影响的计算机模拟

借助计算机模拟技术,以单粒径球形粒子体系在不同尺度的圆截面纤维周围堆积时固相体积分数分布为例,研究了纤维直径与粒子直径比值变化对纤维-浆体界面过渡区初始微观结构的影响。将模拟计算结果与单粒径球形粒子体系在平集料表面的固相体积分数分布曲线进行对比。研究了纤维曲率对多尺度球形粒子体系在纤维表面固相体积分数分布曲线的影响,并将该模拟计算结果与多尺度球形粒子体系在平集料表面的固相体积分数分布曲线进行对比。结果发现：无论是对单粒径球形粒子体系而言,还是对多尺度球形粒子体系而言,初始界面过渡区的微观结构与纤维的曲率无关。球形粒子沿不同曲率纤维周围的固相体积分数分布曲线的形式与在平集料表面周围的粒子固相体积分数曲线的分布形式相同,曲线第一个峰值点的位置(即初始界面过渡区厚度的位置)也都出现在模型水泥平均质量半径的位置附近,水胶比的降低,会使初始界面过渡区的厚度略有减小。因此,可以用平集料表面-浆体结构取代圆截面纤维-浆体结构来研究各种因素对初始界面过渡区微观结构的影响。     

集料形状和尺寸对混凝土边界效应的影响

为了探讨集料对混凝土边界效应的影响机理,利用计算机模拟技术,分析不同形状和尺寸的单尺度椭圆形集料随机堆积对混凝土边界效应的影响。结果显示：椭圆形集料的形状和尺寸的变化不影响集料固相体积分数曲线的形状,均可以分为先上升、后下降、再上升、最后趋于稳定的曲线,趋于稳定时的集料固相体积分数约等于集料的面积分数。通过统计和分析得...     

混凝土界面渗流的集料体积率阈值算法及其影响因素评价

提出了混凝土界面渗流的集料体积率阈值算法并定量评价了影响该阈值的因素.通过引入周期性边界条件,在立方体单元内模拟混凝土中集料和界面的分布.基于模拟的混凝土细观结构,给出了集料体积率阈值算法并讨论了模拟单元边长对界面渗流概率的影响,在该算法的有效性获得实验验证后,定量评价了界面厚度、最大集料直径和集料级配对集料体积率阈值的影响.结果表明:集料体积率阈值随着界面厚度的增大而减小,但随着最大集料直径的增大而增大;集料级配对集料体积率阈值有很大的影响,其影响程度在20%左右,最大集料直径对集料体积率阈值的影响仅为7%.     

表面接枝二分散聚合物的自洽场理论

建立了表面接枝多分散聚合物的自洽场理论.通过对二分散聚合物进行自洽场模拟,研究了排除体积参数对分子链密度分布的影响,考察了分子链的摩尔分数与自由端分布的关系并将模拟结果与强伸展理论进行了比较.通过计算分子链的链段密度分布和伸展轨迹,讨论了二分散聚合物分子链的伸展行为.     

基于雷诺应力模型的脱油旋流器流场特性研究

采用雷诺应力模型(RSM)对油水分离用水力旋流器进行了数值模拟,模拟出了循环流和短路流现象,得到了内部流场的轴向速度、径向速度和切向速度的分布规律;模拟结果与实验结果吻合较好,说明该湍流模型和计算方法的选取是正确的;另外,针对油相体积分数为2%,油滴粒径为40μm的混合介质进行分析,得出了油水二相的体积分数分布。在旋流器的轴心处油相体积分数最大,最大处混合介质中含油体积分数高达98.9%;在壁面附近体积分数很小,说明该水力旋流器的分离效果较好。通过数值模拟为进一步研究水力旋流器内部流场的分布和结构优化设计奠定了基础。     

水平管道水合物浆液流动特性的数值模拟

基于欧拉双流体模型,结合颗粒动力学理论,研究不同水合物颗粒体积分数、不同流速下非均匀颗粒水合物浆液流动过程中水合物颗粒聚集行为、流动压降及浆液速度分布。结果表明:浆液流动压降主要受流速影响,速度越大压降越大。颗粒体积分数在20%—35%,压降变化很小,当颗粒体积分数达到55%时,压降明显变大。水合物颗粒聚集程度随流速增大而降低,低水合物颗粒体积分数下,颗粒聚集呈悬浮状。随颗粒体积分数增大,水合物颗粒呈堆积状聚集,易造成管道堵塞;水合物浆速度梯度随颗粒体积分数增大而增大。随流速增大,速度由非对称变成对称分布,出现较大的速度梯度。保障水合物浆稳定安全流动存在一个临界颗粒体积分数和经济流速。数值模拟为水合物浆液管道输送技术的研究提供理论参考。     

变截面流化床理论模型及数值模拟

基于双流体模型,在一定假设的条件下推导得到了用于描述变截面床流化过程的局部平衡模型。对于局部平衡模型,空间导数项的离散采用五阶精度的WENO有限差分格式,时间导数项的离散采用TVD Runge-Kutta型的离散格式,对流量突变后的瞬态过程进行了模拟,得到固相体积分数在整个变化过程中沿床高的分布以及床高变化规律和床层表面颗粒速度变化曲线。对于流量突增过程,在床内不同位置形成了一系列的连续波,当所有的连续波到达床面整个变化过程结束。而对于流量突然减小过程,将会有固相体积分数间断在分布板处形成,当所有间断到达床面时,塌落过程结束。     

Effect of Volume Fraction on Transient Structural Behavior of Aerosol Particles Using Off-Lattice Kinetic Monte Carlo Simulation

Fractal-like aggregates exhibit interesting properties that determine their physicochemical advantages, and thus, the control and prediction of aggregation is critical for many applications. An off-lattice kinetic Monte Carlo (KMC) simulation was performed to investigate the aggregate evolution from primary particles to three-dimensional fractal aggregates, at three different volume fractions. We have found that at low volume fraction, aggregation kinetics is slow, and aggregate morphology is widely open and stringy, with fractal dimension of (D_f) 1.8, in which the system is constantly preserved in the dilute regime. In denser volume fractions, however, the aggregation kinetics appears to be accelerated and aggregate morphology is more compact and less stringy due to the transition from dilute to dense regimes. Moreover, the volume fractions determine what kind of coagulation mechanism may occur to produce aggregates with different morphologies. At low volume fraction, coagulation is predominated by coagulation between aggregates in which the maximum probability of interpenetration event is only 18%. This suggests that aggregates at low volume fraction can maintain their self-similarity behavior. While at high volume fraction, coagulation is predominated by two subsequent coagulation mechanisms, namely, primary particle–aggregate and aggregate–aggregate interaction. The probability of interpenetration event increases up to 40%. In addition, the interpenetration process as well as the primary particle–aggregate coagulation, particularly in the dense regime, could produce superaggregates with a hybrid structure with a high fractal dimension at large size scales and a low fractal dimension at small scales. A detail mechanism for the formation of superaggregates was discussed.

Copyright © 2015 American Association for Aerosol Research     

基于随机细观骨料的钢筋表面氯离子浓度概率分布研究

氯盐侵蚀诱发钢筋锈蚀是导致钢筋混凝土结构耐久性失效的主要因素之一。研究钢筋表面氯离子浓度概率分布对于预测海洋工程的服役性能具有十分重要的意义。通过引入随机厚度的界面过渡区,建立了包含骨料、界面过渡区以及砂浆的三相混凝土细观模型。在此基础上,结合氯离子扩散理论,建立了氯离子扩散细观数值模型。研究结果表明,骨料对氯离子的扩散具有阻碍作用,增加了氯离子的扩散路径。通过对6 000个钢筋表面氯离子浓度的模拟结果进行统计分析,发现:当骨料率为40%(体积分数)时,钢筋表面氯离子浓度呈双峰分布;当骨料率小于40%时,钢筋表面氯离子浓度近似呈正态分布;随着骨料率的增加,钢筋表面氯离子浓度不断减小,变异系数不断增大。     

Approximate migration coefficient of percolated interfacial transition zone by using the accelerated chloride migration test

In this study, the electrochemical technique is applied to accelerate chloride ion migration in cement-based material to estimate its migration coefficient. In order to investigate the chloride migration coefficient of percolated interfacial transition zone (ITZ) on the chloride migration coefficient of specimen, specimens with cylindrical aggregates of the same height as the specimen were cast and tested. In this study, the volume fraction of aggregate is constant and the varied lateral surface area of the aggregate cylinder was obtained by using different diameters and number of aggregate. The chloride migration coefficient of cement-based material was determined experimentally as a function of the lateral surface area of aggregate. A model obtained for the migration coefficient of cement-based material and the regression analysis are used to determine the approximate chloride migration coefficient of the percolated ITZ. Based on the experimental and regression analytical results, the approximate percolated ITZ migration coefficient is 40.6, 35.5, and 37.8 times of the altered migration coefficient of matrix mortar for the water/cement (w/c) ratio of 0.35, 0.45, and 0.55, respectively.     

On-line measurement of ultrafine aggregate surface area and volume distributions by electrical mobility analysis: I. Theoretical analysis

Electrical mobility analyzers are usually calibrated for spherical particles, and provide number, area and volume distributions for spherical particles. However, these instruments cannot be directly used to obtain the surface area and volume distributions for aggregates. Aggregates are important in technological applications, such as the manufacture of fine powdered materials, and in air pollution and atmospheric sciences. Thus, nanoparticle chain aggregates of low fractal dimension are another important limiting case, in addition to spheres; a method is described which makes it possible to relate aggregate surface area and volume distributions to the electrical mobility diameter. This is accomplished by equating the migration velocity of an aggregate to that of a sphere. Particles of equal migration velocities will trace similar paths in the mobility analyzer and have the same mobility diameter (neglecting the Brownian diffusive spread). By equating the migration velocities of a sphere and aggregate, the number and size of the primary particles composing the aggregate can be related to the diameter of a sphere with the same migration velocity.The calculation of aggregate surface areas and volumes requires two theoretical “modules”, one for the drag on the aggregates and the other for aggregate charging efficiency. Two modules selected from the literature were used. The results indicate that the surface area distributions of aggregates with random orientation are somewhat over-predicted when calculated directly from the mobility diameter. However, the volume distributions are greatly over-predicted, up to a factor of ten compared with values based on the mobility diameter. The affect of aggregate orientation on surface area estimates was also examined.     

DEM simulation of aggregation of suspended nanoparticles

A DEM-based model was developed and examined for simulation of aggregation in suspensions of α-alumina nanoparticles. In the model, the random Brownian diffusion and the externally induced dielectrophoresis (DEP) motion were considered as the driving mechanisms for the transport of particles in colloidal suspension. To simulate particle interactions, the non-contact surface force and the contact force were taken into account using the well-known Derjaguin-Landau-Verway-Overbeek (DLVO) theory and the soft-sphere model, respectively.Specifically, the model was used to study the effects of pH, solid volume fraction and external AC electric field on α-alumina aggregate growth which was expressed in terms of coordination number, longest dimension, and fractal dimension. The simulations were carried out over a pH range of 4-10, solid volume fraction of 0.02-0.4, and a variety of AC electric fields. In relatively dilute suspensions, the aggregates predominantly exhibited chainlike structures, whereas at high solid volume fraction, aggregates with complex netlike structures were formed. It was also evident that, in concentrated colloidal suspensions, DEP had a negligible influence on aggregate growth over the examined conditions. The effect of DEP however, was found to be more noticeable on aggregate structure leading to the formation of more compact aggregates with a greater particle number density. The break-up and reattachment of sub-aggregates as well as the rearrangement of nanoparticles in the particle assemblies and subsequent curling of the loose network promoted by a strong AC electric field was deemed to be responsible for this structural transformation. Finally, the DEM-based model was used to predict the size of α-alumina aggregates over a range of pH. The predictions were found to be in good agreement with the published experimental data, particularly around the isoelectric point.     

Influence of aggregate size and volume fraction on shrinkage induced micro-cracking of concrete and mortar

In this paper, the influence of aggregate size and volume fraction on shrinkage induced micro-cracking and permeability of concrete and mortar was investigated. Nonlinear finite element analyses of model concrete and mortar specimens with regular and random aggregate arrangements were performed. The aggregate diameter was varied between 2 and 16 mm. Furthermore, a range of volume fractions between 0.1 and 0.5 was studied. The nonlinear analyses were based on a 2D lattice approach in which aggregates were simplified as monosized cylindrical inclusions. The analysis results were interpreted by means of crack length, crack width and change of permeability. The results show that increasing aggregate diameter (at equal volume fraction) and decreasing volume fraction (at equal aggregate diameter) increase crack width and consequently greatly increases permeability.     

聚乙二醇与十二烷基羟丙基磺基甜菜碱相互作用的介观动力学模拟

采用介观动力学(MesoDyn)模拟方法研究了聚乙二醇(PEG)与十二烷基羟丙基磺基甜菜碱(DSB)在水溶液中的聚集行为,从介观层次上探讨了PEG以及与DSB混合体系的聚集过程和聚集形态,并对混合体系中DSB的体积分数、温度和剪切速率等影响因素进行了分析。实验结果表明,PEG在水溶液中随体积分数增加会形成球形、棒状、蠕虫状聚集体和网状结构;PEG与表面活性剂DSB之间存在着较强的协同作用,在较低的DSB体积分数下PEG即可产生聚集现象;随着温度的增加,PEG/DSB混合体系聚集变得较为困难,而且聚集的速度也逐渐变慢,说明整个聚集过程是放热的;并且剪切作用对体系聚集行为影响较大。