摩擦颗粒体系各向同性压缩过程中的堵塞行为

利用分子动力学方法,模拟了5000个表面轻微摩擦(μ=1×10^-4)的弹性颗粒组成的体系在各向同性压缩过程中的堵塞(jamming)现象,研究了颗粒体系对边壁压强P和对关联函数第一个峰的高度g₁随体积分数φ的变化规律. 结果表明,当体系的体积分数φ大于临界体积分数φ_c时,P(φ)曲线表现出明显的黏滑(stick-slip)行为,法向的力-力关联函数、切向的力-力关联函数和颗粒的位置-位置关联函数同时发生跳变,说明宏观黏滑现象源自内部的力位形和几何构形的调整. 体系在φ缓慢增大过程中得到了不同的堵塞态,随着颗粒粒径的缓慢减小,体系得以松弛(unjamming),实现松弛过程并得到相应的φ_c. 对于不同堵塞态,其边壁压强P与φ-φ_c遵从幂律标度P∝(φ-φ_c)^0.964. 关键词： 堵塞相图 对关联函数 力-力关联函数 标度规律     

竖直振动激励下颗粒毛细上升行为研究

竖直振动激励下颗粒毛细上升现象为颗粒物料的提升、输运和采集提供了一种全新的技术路径.然而,已有颗粒毛细上升行为研究仍存在明显不足,特别是缺乏对重力加速度、水平振动分量、颗粒粒径分布影响的深入探究.针对这些问题,采用离散元方法,对不同操作条件下颗粒毛细上升现象开展数值模拟研究,并对颗粒最终毛细上升高度和平均毛细上升速度进行计算分析.结果表明,在低重力条件下,颗粒毛细效应仍能发生,颗粒最终毛细上升高度和平均毛细上升速度随重力加速度均呈现先增加后减小的趋势;颗粒最终毛细上升高度对水平振动分量的变化不敏感,而平均毛细上升速度随水平振动分量的增大而增加;在平均粒径相等的情况下,粒径服从高斯分布的颗粒比单一粒径颗粒的最终毛细上升高度最大值对应的临界管径更大,并且同处于堵塞效应影响的管径区域时平均毛细上升速度也更快.     

各向同性压缩过程中圆盘体系的基本性质

结合理论分析与数值模拟研究仅有线性排斥力的二维无序圆盘体系在准静态各向同性压缩过程中的基本性质.引入平均约化重叠量、约化压强等概念,基于微观的力学分析,导出约化压强与体系的组成成分、几何统计量及形变统计量之间的关系.基于数值模拟结果,讨论平均接触数增量、各种接触类型的百分比、平均约化重叠量、约化压强等物理量在大范围压缩的过程中随体积分数增量的变化,给出一个形式简洁的近似约化压强公式.模拟结果表明圆盘体系在远离临界阻塞态及接近临界阻塞态时的压缩响应不同.     

溶液液滴蒸发变干的环状沉积

液体蒸发驱动的颗粒自组装现象在许多的工业技术中有重要应用. 本文利用显微镜观测含有颗粒物质的液滴变干后留在固体表面的颗粒形成的环状沉积图案. 采用微米粒径的SiO₂小球水溶液液滴蒸发变干模拟咖啡环的形成过程, 结果发现液滴蒸发过程中接触线的钉扎是环状沉积的必要条件. 在液滴蒸发过程中颗粒随着补偿流不断的向液滴边缘移动, 聚集在接触线处形成环. 液滴蒸发变干后残留在液滴内部的颗粒数随颗粒质量分数的增加而增加, 可以达到单层的颗粒排列. 而玻璃衬底上的颗粒环在颗粒质量分数很小时, 形成单层排列, 且一排一排地生长. 蒸发过程中颗粒环由于液滴边缘的尺寸限制向液滴中心缓慢移动. 这会导致液滴中不同大小颗粒的分离. 关键词： 液滴 接触线 蒸发 颗粒     

颗粒分层过程三维离散元法模拟研究

采用软球干接触模型对球形及非球形颗粒的分层过程进行了三维离散元法模拟研究,从颗粒间作用力、转动力矩和能量变化的角度分析了颗粒分层机理,讨论了颗粒的粒度比对分层速度的影响规律.结果表明,分层过程中,大颗粒比小颗粒活跃,非球颗粒由于具有较高的动能而比球颗粒活跃,在一定程度上弥补了颗粒形状对分层过程的影响.大颗粒间的平均法向、切向作用力、平均力矩及平均动能均大于小颗粒.颗粒分层速度随着粒度比的增加而显著增大,当粒度比大于临界粒度比3时,分层速度的增幅减缓.     

染料敏化太阳电池中TiO2颗粒界面接触对电子输运影响的研究

采用溶胶-凝胶法制备TiO₂浆料,通过丝网印刷技术印刷和不同温度曲线烧结TiO₂薄膜,并应用于染料敏化太阳电池(DSC).高分辨透射电子显微镜发现,低温下多孔薄膜中TiO₂颗粒之间呈现点接触,510 ℃烧结后TiO₂颗粒间由点接触变为面接触,近邻颗粒数增多,接触面积增大.同时采用强度调制光电流谱(IMPS)和强度调制光电压谱(IMVS)技术,研究了不同颗粒接触方式和接触面积对电子传输与复合的影响.结果表明:在420- 510 ℃之间,随着烧结温度提高,颗粒接触面积增大,电子传输时间(τ _d)缩短,电子有效扩散长度(L _n)增大,暗电流减小;当烧结温度达到550 ℃时,薄膜比表面积减小,多孔结构坍塌,表面态密度增大,电子传输时间(τ _d)增大.电池光伏特性研究表明:在480-510 ℃范围内烧结得到的TiO₂薄膜,电池短路电流密度(J_sc)最佳,电池效率(η)最好. 关键词： 界面接触 电子输运 暗电流 染料敏化太阳电池     

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。     

颗粒堆的体积分数与制备流量关系的实验研究

通过实验研究发现,颗粒堆的体积分数随制备颗粒堆的颗粒流流量指数衰减,减小或增加流量到一定程度时,体积分数都达到饱和;出料口直径与颗粒粒径的比值小到一临界值时,随着流量的减小体积分数增加急剧变缓;颗粒粒径小到一临界值时,随着流量的增加,体积分数的减小急剧变缓.结合颗粒物质的强耗散性、空气作用、瓶颈效应和碰撞理论解释了实验现象,从连续性原理出发推出的颗粒堆体积分数随制备流量变化的函数与实验数据的拟合公式相同.     

平面射流场中纳米颗粒的成核与凝并

采用大涡模拟和直接积分矩方法,数值模拟了在Reynolds数为8300的平面射流中,水蒸气（相对湿度φ=70%）和硫酸蒸气（质量分数为5×10^-6）二元体系中纳米颗粒的成核与凝并,详细分析了颗粒数密度、体积密度和平均粒径的分布.计算结果表明.射流场混合动量厚度的增长和实验结果一致;射流场的拟序结构导致了涡核中心处硫酸蒸气浓度的明显减小,而纳米颗粒数密度则明显增加;拟序结构的出现导致颗粒碰撞概率增大,提高了颗粒凝并效率;在颗粒数密度较大的涡核中心,颗粒成核作用增强,从而加 关键词： 纳米颗粒 成核 凝并 平面射流     

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。     

Jamming as a critical phenomenon: a field theory of zero-temperature grain packings

A field theory of frictionless grain packings in two dimensions is shown to exhibit a zero-temperature critical point at a nonzero value of the packing fraction. The zero-temperature constraint of force balance plays a crucial role in determining the nature of the transition. Two order parameters, , the deviation of the average number of contacts from the isostatic value, and , the average magnitude of the force per contact, characterize the transition from the jammed (high packing fraction) to the unjammed (low packing fraction state). The critical point has a mixed character with the order parameters showing a jump discontinuity but with fluctuations of the contact force diverging. At the critical point, the distribution of phi shows the characteristic plateau observed in static granular piles. The theory makes falsifiable predictions about the spatial fluctuations of the contact forces.     

基于超二次曲面的颗粒材料缓冲性能离散元分析

自然界或工业中普遍是由非球形颗粒组成的复杂体系,与球形颗粒相比,非球形颗粒间的高离散和咬合互锁可使冲击载荷引起的能量有效衰减实现缓冲作用.基于连续函数包络的超二次曲面单元能准确地描述非球形颗粒的几何形态,并可精确地计算单元间的接触碰撞作用.本文采用离散元方法对冲击载荷作用下非球形颗粒物质的缓冲性能进行数值分析,并与圆柱体冲击的理论结果和球体冲击的实验结果进行对比验证.在此基础之上,进一步研究了筒底作用力在不同颗粒层厚度和形状等因素影响下的变化规律.计算结果表明:不同颗粒形状都存在一个临界厚度H_c.当HH_c时,缓冲率随H的增加而增加;当HH_c时,缓冲率的变化不再显著并趋于稳定值.此外,减小颗粒表面尖锐度和增加或减小圆柱形和长方形颗粒的长宽比都会提高颗粒材料的缓冲效果.     

Jamming of packings of frictionless particles with and without shear

By minimizing the enthalpy of packings of frictionless particles, we obtain jammed solids at desired pressures and hence investigate the jamming transition with and without shear. Typical scaling relations of the jamming transition are recovered in both cases. In contrast to systems without shear, shear-driven jamming transition occurs at a higher packing fraction and the jammed solids are more rigid with an anisotropic force network. Furthermore, by introducing the macrofriction coefficient, we propose an explanation of the packing fraction gap between sheared and non-sheared systems at fixed pressure.     

Jamming III: Characterizing randomness via the entropy of jammed matter

The nature of randomness in disordered packings of frictional and frictionless spheres is investigated using theory and simulations of identical spherical grains. The entropy of the packings is defined through the force and volume ensemble of jammed matter and this is shown to be difficult to calculate analytically. A mesoscopic ensemble of isostatic states is then utilized in an effort to predict the entropy through the definition of a volume function that is dependent on the coordination number. Equations of state are obtained relating entropy, volume fraction and compactivity characterizing the different states of jammed matter, and elucidating the phase diagram for jammed granular matter. Analytical calculations are compared to numerical simulations using volume fluctuation analysis and graph theoretical methods, with reasonable agreement. The entropy of the jammed system reveals that random loose packings are more disordered than random close packings, allowing for an unambiguous interpretation of both limits. Ensemble calculations show that the entropy vanishes at random close packing (RCP), while numerical simulations show that a finite entropy remains in the microscopic states at RCP. The notion of a negative compactivity, which explores states with volume fractions below those achievable by existing simulation protocols, is also explored, expanding the equations of state. The mesoscopic theory reproduces the simulations results in shape well, though a difference in magnitude implies that the entire entropy of the packing may not be captured by the methods presented herein. We discuss possible extensions to the present mesoscopic approach describing packings from random loose packing (RLP) to RCP to the ordered branch of the equation of state in an effort to understand the entropy of jammed matter in the full range of densities from RLP to face-centered cubic (FCC) packing.     

多光谱可见光通信信道串扰分析

在可见光通信领域, 通过波分复用技术可以增加信道个数, 从而提高系统通信容量. 然而发光二极管(LED)的辐射光谱具有一定线宽, 当信道个数增加, 信道间隔将变小, 尽管有滤光片的通道选择, 但LED的辐射光谱会出现重叠从而产生信道串扰. 本文基于LED光谱重叠现象分析了多光谱波分复用可见光通信系统的信道串扰问题. 首先结合LED的物理机制和实际LED的光谱形状对其光谱进行建模; 然后根据光谱重叠现象和可见光通信信道推导出信道串扰公式; 最后利用不同中心波长的LED在两通道可见光通信系统中验证了信道串扰公式的正确性. 仿真和实验结果表明, 当两信道的信道间隔大于28 nm时, 两信道之间的信道串扰不超过-13.6 dB. 对多光谱波分复用可见光通信系统的信道串扰分析对未来可见光通信增加信道数量有一定指导作用.     

Effects of short-range attraction on Jamming transition

Enormous progresses to understand the jamming transition have been driven via simulating purely repulsive particles which were somehow idealized in the past two decades. While the attractive systems are both theoretical and practical compared with repulsive systems. By studying the statistics of rigid clusters, we find that the critical packing fraction φ_c varies linearly with attraction μ for different system sizes when the range of attraction is short. While for systems with long-range attractions, however, the slope of φ_c appears significantly different, which means that there are two distinct jamming scenarios. In this paper, we focus our main attention on short-range attractions scenario and define a new quantity named "short-range attraction susceptibility" χ_p, which describes the degree of response of the probability of finding jammed states pjto short-range attraction strength μ. Our central results are that χ_p diverges in the thermodynamic limit as χ_p ∝|φ-φ_c~∞|~(-γ_p), where φ_c~∞ is the packing fraction at the jamming transition for the infinite system in the absence of attraction. χ_p obeys scaling collapse with a scaling function in both two and three dimensions, illuminating that the jamming transition can be considered as a phase transition as proposed in previous work.     

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The packing behaviors of pebble bed in blanket were investigated by packing experiment and discrete element modeling. The packing behaviors of cylinder mono-sized pebble beds show that the average packing factor is gradually increased with increase of the diameter ratio of the bed to pebbles. The experimental and simulated results of cylinder mono-sized pebble beds are in good agreement. Using binary-sized pebble packing and increasing the size ratio of binary pebbles can increase the packing factor of pebble bed, significantly. For binary-sized pebble bed, the average packing factor increases first and then decreases with the increase of the large pebble volume fraction. The maximum value is achieved when the larger pebble volume fraction is about 60%~80%. By optimizing the filling process of the pebble bed, the packing factor of the binary-sized pebble bed has reached 0.8, but the uniformity of the binary-sized bed is poor.