高超声速火星进入环境中颗粒运动特性研究

火星大气中会发生不同规模的沙尘暴,大气中蕴含的尘埃颗粒会对高速进入的火星探测器表面造成侵蚀并导致壁面热流增加,给探测器的热防护系统设计带来巨大挑战.文章针对高超声速火星进入环境两相流动问题,基于Euler-Lagrange框架建立非平衡流场与颗粒的单向耦合计算方法,采用模态半径为0.35μm的火星大气颗粒分布模型,研究不同尺寸颗粒在流场中的运动轨迹,获得高温相变模型对颗粒运动的影响以及不同粒径颗粒的撞击能量分布.结果表明,颗粒在高温流场中运动会吸热融化甚至蒸发,高温相变模型导致的颗粒直径减小对小尺寸颗粒运动轨迹有较大影响;当前计算状态下,直径3μm以上的颗粒具有较大的Stokes数且颗粒半径在运动过程中基本保持不变,其运动轨迹受流场影响较小,该尺寸颗粒的撞击分数均达95%以上,是造成壁面撞击的主要颗粒尺寸;撞击能量分数结果表明,直径3～10μm之间的颗粒是撞击能量的主要来源,约占总撞击能量的80%.     

电致失效力学

电致失效力学研究单调或交变电场载荷下由应力引起的失效行为,它包含了电致断裂、电致疲劳、电致迁移与电致损伤等新研究课题．本文概述了电致失效力学的领域与课题,并深入讨论了电致应变诱导断裂疲劳的机理及电迁移损伤的力电耦合过程．研究结果表明,电致失效力学可提供铁电陶瓷致动器和集成电路的若干关键设计参数．对铁电陶瓷多层共烧致动器,该分析提供其层厚、外加电场强度和交变电场循环周数．对集成电路内导线,该分析提供其允许电流密度和临界线长．      

基于格子Boltzmann方法的电流变液动力学建模及数值模拟

电流变悬浮液内部结构对外电场的快速响应发生在指定的控制空间中,在这一特指的时间和空间尺度上电流变悬浮液的物理行为特征主要为剪切速率低和流动阻尼大,即Mach和Reynolds数一般不大,可以视为微尺度流动来加以研究。针对这一流动特征,基于介观动理论的格子Boltzmann方法,建立了电流变悬浮液两相流动的离散颗粒运动模型,通过该模型进行了动力学模拟,结果表明,该模型解决了分子动力学模型难以描述的因颗粒运动造成局部流场流变特性改变的难题,以及该流场双向耦合过程中对颗粒运动的影响。     

流动聚焦实验

介绍了一种制备微纳米量级颗粒的流动聚焦技术,它的最基本的特点是从毛细管流出的液体由高速运动气体驱动经小孔聚焦形成稳定的锥,锥顶端射出的微射流因不稳定性破碎成小颗粒.实验在自行设计的装置上完成.分析了流动聚焦技术中影响锥-射流以及颗粒形貌的因素,总结了过程中装置的结构参数以及气体压力降、液体流量和物性等参数对射流的直径的影响.结果显示该技术制备的颗粒单分散性好,直径达到微米和亚微米量级,在工业方面有重大应用前景.     

颗粒流体气液相分离空间实验研究

正颗粒物质团簇形成机制是包括研究星际尘埃行为及深空探测等微重力科学研究的重要问题。微重力环境是实验观察颗粒气体形成及研究其本征行为的必要条件,项目利用卫星的较长时间的稳定微重力环境,有系统地研究颗粒物质团簇形成条件、弛豫冷却过程,建立完善理论模型,并建立可     

泥石流反粒径分布的一种定性解释

假设泥石流中的粗颗粒（如石块等）以跳跃的方式前进,通过分析单个颗粒在定常充分发展的流场中的运动,给出了单一起跳速度下以及具有一定分布的起跳速度下,颗粒数密度随高度的分布,并给出了起跳粒径有分布时颗粒的平均粒径随高度的分布,定性地解释了泥石流中的反粒径分布现象。     

固液两相流体中的空泡溃灭计算

本文导出了固液两相流体中球空泡溃灭的运动方程,计算并讨论了空泡溃灭过程中的颗粒运动和颗粒对空泡溃灭的影响,得到了固相浓度、颗粒尺寸等因素与空泡溃灭之间的定性关系。在分析过程中,考虑了液体与固体颗粒之间的阻力耦合作用。     

考虑等效曲率的超二次曲面单元非线性接触模型

基于连续函数包络的超二次曲面单元可有效地描述自然界和工业生产中的非球体颗粒形态, 并通过非线性迭代方法精确计算单元间的接触力. 对于具有复杂几何形态的超二次曲面单元, 线性接触模型不能准确地计算不同接触模式下的作用力. 考虑超二次曲面单元相互作用时不同颗粒形状及表面曲率的影响, 本文发展了相应的非线性黏弹性接触模型. 该模型将不同接触模式下的法向刚度和黏滞力统一表述为单元间局部接触点处等效曲率半径的函数; 切向接触作用则借鉴基于Mohr-Coulomb摩擦定律的球体单元非线性接触模型的计算方法. 为检验超二次曲面单元接触模型的可靠性, 对球形颗粒间的法向碰撞、椭球体颗粒间的斜冲击过程、圆柱体的静态堆积和椭球体的动态卸料过程进行离散元模拟, 并与有限元数值结果及试验结果进行对比验证. 计算表明, 考虑接触点处等效曲率半径的超二次曲面非线性接触模型可准确地计算单元间的接触碰撞作用, 并合理地反映非球形颗粒体系的运动规律. 在此基础上进一步分析了不同长宽比和表面尖锐度对卸料过程中颗粒流动特性的影响, 为非球形颗粒材料的流动特性分析提供了一种有效的离散元方法.      

机器人单足系统沙土跳跃刚?散耦合动力学分析

在行星探索过程中涉及探测器在星壤上着陆、运动以及收集、存储某些样本材料等问题, 因此需要建立探测机器人在沙土上运动的动力学模型, 从而优化其系统构型. 近年来, 对跳跃型探测机器人研究得到了越多越多的关注. 本文采用离散元法对颗粒场进行建模, 以及采用多体动力学方法对机械系统进行建模, 对机器人单足系统在沙土上的跳跃问题进行耦合动力学仿真分析. 基于经典土力学Prandtl-Reissne理论, 从颗粒场受压分层的形式和动量传递出发, 对描述颗粒侵入阻力的惯性力动阻力项进行了修正, 提出了一种修正的Poncelet公式. 通过与离散元仿真结果对比, 说明所提出修正公式比原始的Poncelet公式更准确地计算了机械足受到的沙土侵入阻力, 尤其在达到一定侵入深度表现出更好的收敛性. 最后分析了机械腿足部的不同尺寸和形状对沙土中跳跃效果的影响, 给出了锥形足部和柱形足部的体积对跳跃效果影响的近似计算公式. 本研究将拓展刚?散耦合动力学理论, 并且为新型探测器在行星土壤上运动的系统设计提供技术支撑.      

滚筒干燥器中颗粒混合运动的三维数值模拟

滚筒干燥器中的颗粒混合运动是关系干燥效率的重要因素.为了研究滚筒干燥器中颗粒的混合特性,基于力学原理,采用离散元方法直接跟踪滚筒中的每一个颗粒.建立了三维数值实验平台,对滚筒中颗粒碰撞、摩擦的混合运动全过程进行了数值模拟,并着重探讨了滚筒转速、抄板半径及抄板数目等关键参数对滚筒中颗粒混合特性的影响.结果表明:随着转速的增加,颗粒混合均匀的速度加快;滚筒内壁抄板半径和数目对颗粒的混合特性也有重要影响;并发现了近壁区域颗粒的混合均匀程度比内部区域好.     

Three dimensional simulation of lunar dust levitation under the effect of simulated sphere body

Four typical models of irregular lunar dust are built based on the particle shape. Parameters of the simulated sample are determined by dimensional analysis and simulated triaxial test. The simulation was performed by a simulated sphere body acting as the lunar explorer experiencing free fall landing on moon surface from a certain height. Tracking the movement of lunar dust, the motion characteristics and levitation distribution were statistically analyzed. The influences of the landing speed of the simulated sphere body, friction coefficient and stiffness of the particles on the levitation of the dust were also discussed. Results show that the landing speed and the friction coefficient have greater influence on the number and height of levitating particles than other factors. The number and height of levitating particles increase with landing speed. While the friction coefficient increases, the number of levitating particles increases but the levitation height decreases. The stiffness of particle also has some influence on levitation height. The larger the stiffness of particles is, the smaller the levitation height. But it has little effect on the number of levitating particles.     

Surrogate atmospheric dust particles generated from dune soils in laboratory: Comparison with field measurement

Desert dust strongly influences the climate and the environment by altering the radiation budget, participating in atmospheric chemical reactions, and engaging the biogeochemical cycle. Studying its impacts requires knowledge of the physical and chemical properties of the original particles from the source origin. Unfortunately, the field collection of atmospheric samples in the desert is impractical. Fine surface soil is thus considered an alternative material from which to obtain the information applicable to dust particles from the desert soil. In this study, a laboratory system was set up to generate dust particles with simulated natural wind erosion processes from surface soils or directly from desert surfaces. Surface soils and field dust were simultaneously collected during a local blowing dust event. The comparison between the laboratory-generated dust and the field dust showed a consistent trend in particle size distribution and chemical composition. The chemical compositions were found to have explainable differences with the Certified Reference Materials for Asian dust (CJ-2). The laboratory-generated dust particles with the system could be applied as surrogates for soil-emitted dust in desert areas.     

Study on particle removal efficiency of an impinging jet by an image-processing method

 An image-processing method is proposed to obtain the distribution of the removal efficiency of particles on a plate by an air jet. This method can be used to measure particle removal from a flat surface by processing the image of the reflected light from the surface. Factors affecting the particle removal efficiency such as air pressure, distance between the nozzle and the impinging surface and the impinging angle are discussed. Optimal conditions are determined to obtain the most effective particle removal by the air jet. Received: 10 April 2001 / Accepted: 2 August 2001     

RESEARCH ON THE COLLISION EFFICIENCY OF NANO-PARTICLE-TO-WALL WITH EXTERNAL ELECTRIC FIELD

着重考虑了电场作用下,壁面排斥力对颗粒-壁面碰撞效率的影响. 采用Matlab 中的龙格库塔方法,对颗粒-壁面碰撞的压缩阶段及回弹阶段的动力学方程组进行求解,并对Maxwell 速度分布进行积分获得不同速度方向下的碰撞效率. 研究表明,颗粒的碰撞效率随着碰撞角度的增加而增大,并最终达到一个临界值,即完全凝并时的碰撞效率;且颗粒的碰撞效率将因壁面排斥力的影响而减小,即壁面排斥力在一定程度上阻碍了颗粒的凝并.     

电场作用下纳米颗粒与竖直壁面的碰撞效率

着重考虑了电场作用下,壁面排斥力对颗粒-壁面碰撞效率的影响. 采用Matlab 中的龙格库塔方法,对颗粒-壁面碰撞的压缩阶段及回弹阶段的动力学方程组进行求解,并对Maxwell 速度分布进行积分获得不同速度方向下的碰撞效率. 研究表明,颗粒的碰撞效率随着碰撞角度的增加而增大,并最终达到一个临界值,即完全凝并时的碰撞效率;且颗粒的碰撞效率将因壁面排斥力的影响而减小,即壁面排斥力在一定程度上阻碍了颗粒的凝并.      

铝粉爆炸特性的实验研究

本文研究了扬尘湍流、铝粉浓度、颗粒度和气相中氧浓度等因素对铝粉爆炸特性的影响。研究结果表明,铝粉颗粒度对铝粉爆炸有十分明显的影响。颗粒度越小,其它因素对铝粉爆炸的影响也越明显。在粉尘爆炸中,较强的扬尘湍流能够使更多粉尘悬浮,有利于粉尘的燃烧并且加快了其燃烧速率。     

Removal of dust from flue gas in magnetically stabilized fluidized bed

A magnetically stabilized fluidized bed (MSFB, φ 500mm x 2100mm) was designed to study dust removal from flue gas. Based on the mechanism of dust removal in a fixed bed, the effects on collection efficiency of magnetic field intensity, ratio of flue gas velocity to minimum fluidization velocity, bed height, and particle average diameter, were investigated. Then feasible methods for MSFB to better remove dust were proposed. Over 95% of dust removal with MSFB can be achieved, when stable fluidization is maintained and when magnetic particles are frequently renewed.     

POWER LAW DISTRIBUTION AND SELF-ORGANIZED CRITICALITY OF DISPERSED PARTICLES

Research on particulate characteristics has been an important frontier in physics and chemistry during the past decades. It has however been mostly focused on granular materials with short-range interactions. In this work, it was found that the power law of particle size distribution applied to the long-range interacting system of floating dust in air, from which we deduced that self-organized criticality might hold for floating dust just as granular materials with short-range interactions. This feature may reveal underlying kinetic mechanisms, important in dispersed particle systems. In industry, power law of size distribution of dispersed particles can be used to investigate the change of dust size, and the power law parameter could be taken as an important index for dust separation.     

Removal of dust from flue gas in magnetically stabilized fluidized bed

A magnetically stabilized fluidized bed （MSFB,Ф 500mm×2100mm） was designed to study dust removal from flue gas. Based on the mechanism of dust removal in a fixed bed, the effects on collection efficiency of magnetic field intensity, ratio of flue gas velocity to minimum fluidization velocity, bed height, and particle average diameter, were investigated. Then feasible methods for MSFB to better remove dust were proposed. Over 95 % of dust removal with MSFB can be achieved, when stable fluidization is maintained and when magnetic particles are frequently renewed.     

Filtration characteristics of a binary multi-layer granular bed filter based on CFD-DEM coupling simulation

The coupled CFD-DEM method with the JKR (Johnson-Kendall-Roberts) model for describing the contact adhesion of dust to filter particles (FPs) is used to simulate the distribution pattern of dust particle deposition in the granular bed filter (GBF) with multi-layer media. The minimum inlet flow velocity must meet the requirement that the contact probability between dust and FPs is in the high contact probability region. The air flow forms vortices on the leeward side of the FPs and changes abruptly at the intersection of different particle size FPs layers. Dust particles form large deposits at the intersection of the first and second layers and the different particle size filter layers. Dual element multilayer GBF can further optimize the bed structure by interlacing filter layers with different particle sizes. Compared with single particle size multi-layer GBF, the bed pressure drop is reduced by 40.24%–50.65% and the dust removal efficiency is increased by 21.93%–55.09%.