阻塞态颗粒介质的慢速阻力

报导了不同尺寸的大球缓慢压入颗粒床过程中所受阻力随深度变化的实验测量结果,发现阻力曲线在不同的深度区域有不同的增长规律,存在凹—凸转变.阻力在较浅的区域满足1.3次方的幂率增长,而在较深的区域趋向0.3次方的幂率增长.通过研究凹凸转变中拐点的性质,发现这种阻力增长速度趋缓的转变不是来源于前人认为的器壁支撑导致的介质压强饱和,而是来源于侵入物自身的体积效应.此外,适用于颗粒介质快速阻力的静水压力描述并不适用于阻塞态颗粒介质的慢速阻力行为,实验表明慢速阻力对深度的依赖关系不是线性,且测得的静水压力系数也远大于理论估算值. 关键词： 颗粒介质 阻力 力链重组     

运动物体在颗粒物质中的动力学过程及最大穿透深度仿真研究

利用离散元法仿真了运动物体在颗粒物质中的三维动力学过程, 仿真采用周期边界条件, 并考虑了重力、接触力、阻尼力、摩擦力的影响. 将仿真结果和相关的三维实验结果进行了对比, 两者符合较好. 仿真结果表明穿透深度与运动物体的冲击速度、运动物体质量、颗粒介质床的密度均有关系. 运动物体质量越大, 速度越快, 则穿透越深, 而且穿透深度和质量呈线性关系. 仿真过程较为真实地再现了小颗粒的飞溅现象. 关键词： 颗粒物质 动力学过程 仿真 离散元法     

重力驱动下运动物体在颗粒介质中的最大穿透深度

通过高速摄像的跟踪，研究了重力场中球形下落物体对松散颗粒床进行撞击并进入颗粒床内的运动过程.运用已得到的颗粒体系中运动物体的阻力模型，分析了物体质量对穿透深度的影响.研究表明，当物体与颗粒床撞击的初始速度较小时，物体在颗粒床中的最大穿透深度与物体质量呈近似线性关系. 实验得到的结果与模型符合很好. 关键词： 颗粒体系 阻力 动力学过程     

固态氚增殖包层中球床堆积性能研究

基于球床堆积实验和离散元数值模拟对包层中球床的堆积性能做了初步研究。圆柱形一元(单尺寸颗粒)球床的堆积性能的结果显示随着球床直径与颗粒直径比的增大,球床的平均堆积因子逐渐增高,实验与模拟结果一致;采用二元颗粒(双尺寸颗粒)、提高颗粒粒度比可以显著提高球床的堆积因子,二元球床的堆积因子随着大颗粒体积分数的增加先增加后减小,在大颗粒体积分数约为60%~80%时达到最大。优化了二元球床的填充工艺,最终二元球床的堆积因子基本达到0.8,但球床的均匀性欠佳。     

玻璃-橡胶混合颗粒的力学响应研究

通过直剪实验和离散元模拟, 研究掺杂了橡胶软球的玻璃体系的力学响应. 改变颗粒固体中橡胶颗粒的含量, 研究体系剪切强度以及剪胀变化等特性, 发现随着橡胶颗粒的增加, 会出现剪胀到剪缩的相转变现象, 且混合颗粒固体的弹性有了很大的提高. 实验研究发现, 随着体系中橡胶颗粒含量的增加, 剪切屈服强度值逐渐减小, 体积发生从剪胀到剪缩的相转变现象, 但临界剪切强度在一定橡胶颗粒含量范围内保持一致; 实验所采取的剪切速率下, 应力应变曲线能较好重合, 即实验处于率无关区域; 混合样品的屈服强度值随正压力的增大而增大. 离散元模拟也得到了上述结果, 另外模拟还发现, 随着橡胶颗粒含量的增加, 颗粒的平均配位数增大; 橡胶颗粒含量和正压力对剪胀-剪缩相转变的位置有影响, 橡胶颗粒含量较小时, 在较大的正压力下易发生相转变现象, 且剪胀-剪缩相转变点对应的平均配位数在5.6-5.9之间; 在橡胶颗粒含量小于30%时, 混合颗粒样品的残剪强度与不掺杂的颗粒体系相近; 大于30%时, 残剪强度随橡胶颗粒含量的增加而减小; 残剪强度随正压力加大而增加.     

基于双重介质模型的正仲氢转化器流动规律研究

为了研究正仲氢转化器中流体的流动规律，建立了催化剂堆积床的颗粒孔隙-堆床空隙的双重介质模型。基于双重介质模型分别建立颗粒孔隙和堆床空隙中正氢和仲氢的质量守恒方程，两种介质之间通过流量交换源项相互耦合。采用控制容法离散控制方程，并数值求解离散方程组。数值算例表明，由于颗粒孔隙与堆床空隙的差异，两种介质中的流动规律存在明显差异。     

燃尽风率对新型W火焰锅炉主燃区气固流动特性的影响

针对采用偏心旋流二次风燃烧技术的300 MWe旋流燃烧器W火焰炉,借助1/10冷模试验台,通过三维激光颗粒动态分析仪测量研究了不同燃尽风率下其主燃区内气固流动特性。随着燃尽风率减小,拱下回流区内回流速度不断增加,且回流区尺寸不断增大。随着燃尽风率由25%减小到10%,在分级分区域,颗粒的最大竖直速度由2 m/s增大到4 m/s.燃尽风率由20.3%减小到10%,拱下回流区内气固两相竖直脉动速度明显增大,气固两相湍流强度将不断增大.在乏气和分级风区域,燃尽风率15%下最大颗粒体积流率是燃尽风率20.3%的2至2.7倍,拱上气流下冲深度明显增加.随着燃尽风率减小,下冲颗粒开始折转向上的位置被推迟,下炉膛空间利用率将不断增加.     

颗粒数方程的求解及流化床数值模拟

流化床内颗粒聚并和破碎将影响颗粒相的流动特性.本文运用基于颗粒动理学理论的欧拉一欧拉气固多相流模型,利用直接矩积分方法求解颗粒数平衡方程,建立颗粒数密度与连续性方程、动量方程之间的关系,数值模拟流化床内两种不同直径颗粒发生聚并时气固两相流动特性。计算结果表明,颗粒聚并伴随着床内颗粒直径逐渐增大,床内颗粒流化状态逐渐变为固定床状态,两种颗粒直径均增加,且小颗粒的体积分数逐渐减小、大颗粒的体积分数增加。当仅考虑聚并过程时增加流化速度将导致床内颗粒体积平均直径变大。随着颗粒密度减小,床内体积平均直径增加。     

温度变化环境中Au纳米球光学性质的研究

理论分析了贵金属纳米颗粒介电函数的尺寸及温度修正,考虑环境介质折射率的色散及随温度变化关系,计算了温度变化环境中Au纳米球的光学性质.结果表明,体相和纳米Au金属介电函数实部均随着温度升高而增大,而体相材料和纳米Au介电函数虚部表现出不同的温度特性;对于纳米球的光吸收效率,温度升高,纳米球的吸收峰峰值增大;对10~100nm的Au纳米球吸收效率的温度灵敏度分析表明,Au纳米球的光吸收效率温度灵敏度随粒径增大而减小.     

TiO2颗粒尺寸对染料敏化太阳电池内电子输运特性影响研究

采用强度调制光电流谱(IMPS)和强度调制光电压谱(IMVS)研究了染料敏化太阳电池(DSC)内部电子传输和背反应动力学特性.在纳米TiO2薄膜厚度相同的情况下,借助于IMPS/IMVS测量了由3种不同TiO2颗粒尺寸大小薄膜制备出DSC的电荷传输特征参数值.IMPS/IMVS理论模型拟合实验测量数据的结果表明:在不同入射光强下,随着颗粒尺寸的增大,电子扩散系数(Dn)增大,而电子寿命(τn)减小,电子传输时间(τd)也减小.Dn随颗粒尺寸增大而增加归因于薄膜表面积的减小,而τn减小可以通过缺陷之间的跃迁频率来解释,τd减小是由于TiO2薄膜内缺陷浓度减小而导致的.     

Intruders in the dust: air-driven granular size separation

Using MRI and high-speed video we investigate the motion of a large intruder particle inside a vertically shaken bed of smaller particles. We find a pronounced, nonmonotonic density dependence, with both light and heavy intruders moving faster than those whose density is approximately that of the granular bed. For light intruders, we furthermore observe either rising or sinking behavior, depending on intruder starting height, boundary condition, and interstitial gas pressure. We map out the phase boundary delineating the rising and sinking regimes. A simple model can account for much of the observed behavior and shows how the two regimes are connected by considering pressure gradients across the granular bed during a shaking cycle.     

Drag induced lift in granular media

Laboratory experiments and numerical simulation reveal that a submerged intruder dragged horizontally at a constant velocity within a granular medium experiences a lift force whose sign and magnitude depend on the intruder shape. Comparing the stress on a flat plate at varied inclination angle with the local surface stress on the intruders at regions with the same orientation demonstrates that intruder lift forces are well approximated as the sum of contributions from flat-plate elements. The plate stress is deduced from the force balance on the flowing media near the plate.     

Effects of air on the segregation of particles in a shaken granular bed

Effects of interstitial air on the motions of a large intruder in a shaken granular bed are studied experimentally as a function of ambient air pressure, particle size of the bed, and the density of the intruder. It is found that the intruder always rises from the granular bed in the absence of air. However, the intruder can acquire both positive and negative buoyancy in the presence of air. Negative buoyancy can be observed only when both the density of the intruder and the particle size of the bed are small enough. This negative buoyancy can be explained by the unusual air pressure distribution found in the bed.     

气压对颗粒物质振动分离的影响

文章作者系统地研究了垂直振动颗粒床中，在不同气压、颗粒尺寸以及密度情况下大球的运动规律。实验发现，系统处于真空状态或低气压时，大球总是向上运动。然而，当在通常大气压下，大球则会出现上升和下降两种运动状态。大球下降这种运动状态，只在大球的密度和颗粒床中颗粒尺寸足够小时才会出现，颗粒床中的负气压梯度导致大球下降。     

Reverse buoyancy in a vibrated granular bed: Computer simulations

We have performed molecular dynamics simulations of an intruder in a vibrated granular bed including interstitial fluid effects to account for the phenomenon of reverse buoyancy. We show that our model is able to reproduce the overall behaviour observed by previous experimental works and is the first finite-elements simulation to show the sinking of intruders lighter than the granular bed. To further advance our comprehension of this phenomenon, we studied the motion of the intruders in a single vibration cycle with respect to the bottom of the granular column, finding a substantial qualitative difference for heavy and light intruders and we compare these results with experiments using fine-sized glass beads. We show that, though heavy intruders seem unaffected by the force due to the fluid, the effect on light intruders is remarkable.      

Gas-mediated impact dynamics in fine-grained granular materials

Noncohesive granular media exhibit complex responses to sudden impact that often differ from those of ordinary solids and liquids. We investigate how this response is mediated by the presence of interstitial gas between the grains. Using high-speed x-ray radiography we track the motion of a steel sphere through the interior of a bed of fine, loose granular material. We find a crossover from nearly incompressible, fluidlike behavior at atmospheric pressure to a highly compressible, dissipative response once most of the gas is evacuated. We discuss these results in light of recent proposals for the drag force in granular media.     

Are Brazil nuts attractive?

We present event-driven simulation results for single and multiple intruders in a vertically vibrated granular bed. Under our vibratory conditions, the mean vertical position of a single intruder is governed primarily by a buoyancylike effect. Multiple intruders also exhibit buoyancy governed behavior; however, multiple neutrally buoyant intruders cluster spontaneously and undergo horizontal segregation. These effects can be understood by considering the dynamics of two neutrally buoyant intruders. We have measured an attractive force between such intruders which has a range of five intruder diameters, and we provide a mechanistic explanation for the origins of this force.     

Characterization of soliton damping in the granular chain under gravity

A soliton created in the horizontal granular chain damps due to gravity in the vertical chain. We show that there are two types of propagating modes, quasisolitary and oscillatory, in the vertical chain, depending on the strength of impulse. We find that the type of damping is a power law in depth or time. We also find that the absolute value of the exponent of the power law decreases as the strength of the initial impulse increases in the quasisolitary regime. In the oscillatory regime, however, in which the initial impulse is weak, the power-law exponent is independent of the strength of the initial impulse. We show that the power-law damping is caused by the gravitation which results in the change of the force constant at each contact.     

Fluctuation-induced casimir forces in granular fluids

We numerically investigate the behavior of driven noncohesive granular media and find that two fixed large intruder particles, immersed in a sea of small particles, experience, in addition to a short-range depletion force, a long-range repulsive force. The observed long-range interaction is fluctuation-induced and we propose a mechanism similar to the Casimir effect that generates it: The hydrodynamic fluctuations are geometrically confined between the intruders, producing an unbalanced renormalized pressure. An estimation based on computing the possible Fourier modes explains the repulsive force and is in qualitative agreement with the simulations.