不同振动模式下颗粒分离行为的数值模拟

利用三维离散元法对垂直方向上的直线、圆和椭圆振动模式颗粒分离过程进行了数值模拟研究,对直线振动时上层大颗粒的波动及圆和椭圆振动时出现的聚集、循环等现象的形成机理进行了分析,并讨论了振动强度对各振动模式下颗粒分离形态的影响规律. 研究表明,综合运用空隙填充、侧面驱动的颗粒运动和能量非均匀分布三种机理,并结合颗粒群的速度矢量分布情况能够较好地解释各振动模式下的颗粒分离行为. 振动强度对圆和椭圆振动模式的分离形态具有显著的影响,并在振动强度约为3时,各种振动模式均具有良好的颗粒分离效果和稳定的颗粒运动状态. 关键词： 振动模式 颗粒分离 离散元法 数值模拟     

振动颗粒物质“巴西果”分离效应实验和理论研究

本文通过实验和理论研究了影响"巴西果"分离的因素及其物理机理. 分析了振动加速度、大小颗粒尺寸和密度对分离时间的影响, 并利用流体模型对分离时间作估算, 对实验结果进行定性解释. 结果表明在振动频率固定时, 调节振动加速度是控制"巴西果"分离的一个主要手段. 振动加速度存在一个临界值, 当高于此临界值时, "巴西果"分离的主要物理机理由对流机理转变为几何填空机理, 且振动加速度对分离影响变小, 大颗粒尺寸对分离的影响增大. 可通过调节大颗粒的尺寸来改变分离效果. 当大、 小颗粒密度比为1时, 仍会出现"巴西果"分离现象. 增大小颗粒尺寸或密度可以促进"巴西果"分离.     

竖直振动管中颗粒的上升运动

通过实验和数值模拟研究了竖直振动管中颗粒的受激运动.将一直管插入静止的颗粒料层中,并在管内预填充一定高度的颗粒,对直管施加竖直振动.振动强度较弱时,管内的颗粒在重力作用下向下运动;当振动达到一定的强度时,管内颗粒不下降反而克服重力的作用向上运动,随着振动强度的进一步提高,即使不在管内预填充高于颗粒床层的颗粒,颗粒也会沿着振动管逆重力向上运动.颗粒的上升高度与运动速度强烈依赖于振动强度.通过高速相机记录颗粒和直管在单个振动周期内的运动,并结合离散元(DEM)法模拟管内颗粒的受力变化规律,给出了颗粒上升的机理.此研究对实现散体物料的定向输运提供了一种新方法.     

云南松松塔中两种新木脂素圆二色谱的密度泛函研究

采用梯度校正的密度泛函理论方法,计算了云南松松塔中分离的2种新木脂素的绝对结构为7S,8R的手性结构、电子吸收光谱、电子圆二色谱、红外振动光谱和振动圆二色谱,计算值与实验值相符. 分子轨道和振动模式分析,两种新木质素具有相似的手性光谱. 电荷密度转移对电子圆二色谱具有主要贡献;而振动圆二色谱主要源于分子骨架运动.     

云南松松塔中两种新木脂素圆二色谱的密度泛函研究

采用梯度校正的密度泛函理论方法,计算了云南松松塔中分离的2种新木脂素的绝对结构为7S,8R的手性结构、电子吸收光谱、电子圆二色谱、红外振动光谱和振动圆二色谱,计算值与实验值相符. 分子轨道和振动模式分析,两种新木质素具有相似的手性光谱. 电荷密度转移对电子圆二色谱具有主要贡献;而振动圆二色谱主要源于分子骨架运动.     

振动颗粒物质中倍周期运动对尺寸分离的影响

实验研究了竖直振动颗粒床中,倍周期运动对尺寸分离的影响.实验中,当振动加速度足够大时,系统中出现稳定的对称对流,进一步增大振动加速度到某个临界值时,还会出现倍周期运动.观察表明,背景颗粒的对流运动对分离过程起主导作用,对流速度决定着分离过程的快慢,而在2倍周期和4倍周期分岔之后,分离时间有所减慢.对引起对流运动的起因进行了分析,以此为基础分析了倍周期运动产生影响的物理机理,并对分离时间进行了定量计算,结果与实验值符合很好. 关键词： 颗粒物质 “巴西果”效应 倍周期分岔 对流     

巴西果效应分离过程的计算颗粒力学模拟研究

采用离散单元模型对巴西果问题进行了模拟研究,采用本数学模型可以准确地预测出垂直机械振动导致的不同大小颗粒的分离现象.分析了影响振动床内颗粒分层的因素,讨论了振动频率、振动幅度等参数对分层的影响,发现振动频率和振动幅度会对分离效果造成较大的影响.当振动频率或振动幅度较小时,颗粒整体分离效果都较差,但分离的稳定性较好,当振动频率或振动幅度较大时,整体分离效果仍然较差,同时分离的稳定性也较差,振动分离过程中存在一个频率和振幅适中的最优操作点. 关键词： 分层 振动 模式形成 离散单元方法     

基于Hough变换的金刚石颗粒测量方法研究

针对金刚石颗粒图像分析时的颗粒边缘非闭合性问题,本文提出了一种基于Hough变换的金刚石颗粒测量新方法。该方法首先对图像进行形态学去噪、平滑滤波和Canny边缘检测等预处理,得到金刚石颗粒图像的初始边缘曲线,然后采用Hough变换提取边缘曲线的直线特征,并通过极径和极角对多个直线特征进行判断和图形识别,得到确定的金刚石颗粒边缘轮廓,最后对得到的边缘轮廓进行拟合,测量出金刚石颗粒的粒径、椭圆度和圆度等参数大小。该方法在VC+ 环境下,用OpenCV编程技术对其进行了实验验证,结果表明：该方法可以准确快速地测量出金刚石颗粒特征参数,为金刚石颗粒的等级评定提供了技术支持。     

可激发气体分子声弛豫过程振动模式能量转移速率计算

研究可激发气体中振动模式能量转移速率和声弛豫过程形成的关系,将单一气体Tanczos弛豫方程理论[J.Chem.Phys.25,439(1956)]扩展应用于混合气体中振动模式的振动-振动(V-V)和振动-平动(V-T)能量转移速率的计算。在室温下CO₂,CH₄,CL₂,N₂和O₂组成的多种混合气体中,振动模式能量转移速率的计算结果表明:对于多个振动模式所形成的声复合弛豫过程,各振动模式的声激发能可由V-V能量转移相互耦合后传递给具有最快V-T转移速率的最低振动频率振动模式,再通过该振动模式的V-T转移退激发形成主弛豫过程。这种选择最快转移路径的声激发量弛豫方式,造成了大多数可激发气体中声弛豫吸收谱的实测数据只存在一个吸收峰的现象。从而提供了一个可通过计算微观振动能量转移速率分析混合气体声弛豫过程形成机理的理论模型。      

振动颗粒混合物中的三明治式分离

在竖直振动两种颗粒的混合物的实验中，观察到了一种新的分离现象——“三明治”式分离，即大而重的颗粒被夹在两层小的轻颗粒之间.这不同于“巴西果”效应导致的大而重的颗粒在上的两层有序结构.实验表明当振动加速度大于某个临界值时这种三层有序将取代两层结构，而且是稳定的.实验中观察到导致“三明治”式分离的两种不同的分离过程.对这两种过程的物理机理及其与振动加速度、振动频率及颗粒尺寸等因素的关系做了研究，并给出了相图. 关键词： 颗粒物质 振动 有序结构 三明治式分离 巴西果效应     

X波段高功率微波TE11模圆极化器

 基于圆波导TE₁₁模的模式简并特性和微波在椭圆波导中传输两个正交TE11模式相速不同的性质,研制了一种带有椭圆波导结构的圆波导TE₁₁模圆极化器。该圆极化器通过圆波导到椭圆波导的过渡段,将输入的线极化TE₁₁模式分成两个等幅、正交的TE₁₁模,然后调整椭圆波导长度,使得两个正交的TE₁₁模式的相位差为90°,实现了TE₁₁模式微波线极化到圆极化的转换。利用时域有限差分软件优化设计了该圆极化器,并按照优化的结构尺寸加工了一套实验装置进行了实验测试,测试结果表明：在工作频率9～10 GHz范围内,该圆极化器轴比小于1 dB,驻波比小于1.1,且功率容量大于1.6 GW。     

Filtration of aerosol particles by elliptical fibers: a numerical study

Filtration is an important technology for nanoparticle control and reduction of human exposure to nanoparticles. Filtration by non-circular fibers is gaining more attention in search for better filter quality; elliptical fibers have extra significance because they may be used in simple models of dust loading. We carry out numerical simulation to investigate filtration by fibers with elliptical cross-sections. The drag force on individual fibers is in good agreement with analytical solutions for elliptical fibers obtained from the cell model. Our simulation covers mechanisms for particle capture due to interception, inertial impaction, and diffusion. Single-fiber efficiencies of different elliptical fibers are computed. The figure of merit, which is an indicator of the ratio of the collection efficiency to the pressure drop, is used to evaluate the performance of different elliptical fibers. It is found that blunt and close to circular fibers perform better for particles dominated by the effects of interception and inertial impaction, whereas long and slim fibers perform better for particles dominated by the diffusion effect. For very small nanoparticles, the diffusion effect is important and long and slim elliptical fibers may improve the filter performance.     

涂覆石墨烯的椭圆形电介质纳米线光波导的模式特性分析

设计了一种涂覆石墨烯的椭圆形电介质纳米线光波导.采用分离变量法,在椭圆柱坐标系中,借助Mathieu函数,得到了色散方程.通过数值求解色散方程,可以得到模式的有效折射率和场分布,从而得到模式的传播长度.研究了工作波长、结构参数以及石墨烯的费米能对模式特性的影响,并给出了前五个模式的品质因数.计算表明,当波长从4.3μm增加到8.8μm,这5个模式的有效折射率的实部减小,基模和一阶模的传播长度增大,二阶模的传播长度先增大后减小.当改变纳米线结构参数半长轴和半短轴时,对基模和一阶模的模式特性影响较小,对二阶模的模式特性影响较大.当石墨烯的费米能从0.45 eV增加到0.72 eV时,有效折射率的实部减小,传播长度可以达到2μm左右.分离变量法得到的结果与有限元方法得到的结果完全一致.本文工作可以为基于涂覆石墨烯的电介质纳米线的光波导的设计、制作和应用提供理论基础.     

High-power TE11 and TM11 circular polarizers in oversized waveguides at 70 GHz

Elliptical deformation of oversized, smooth-wall circular waveguides can produce choosable elliptical or circular polarization from a linearly polarized TE11 or TM11 mode used as intermediate linearly polarized modes in TEO1 to HE11 mode conversion sequences in electron cyclotron resonance heating (ECRH) of magnetically confined thermonuclear fusion plasmas with high-power gyrotrons. Mode coupling in elliptically distorted overmoded circular waveguides has been studied theoretically and experimentally in order to optimize TE11 (and TM11) polarizers (I.D.=27.79 mm) for the 1 MW/70 GHz long-pulse (3s) ECRH system on the Garching Stellarator W VII-AS. Coupling coefficients for ellipticity coupling of non-degenerate modes are given (coupled-mode differential equations formalism). The polarization converters essentially consist of smooth-wall circular waveguides which are gradually squeezed. A sine-squared function of the length coordinate is used to get an almost elliptical crosssection in the middle and circular cross sections at both ends. Arbitrary elliptical polarization states can be generated introducing an extremely low level (<<1%) of undesired spurious modes. Well defined differential phase characteristics have been achieved.     

Modular interference characteristic of two-mode fiber

The modular interference characteristics of circular-core and elliptical-core two-mode fibers are investigated in theory. The intensity distribution and figure of two-lobe mode patterns are evaluated and simulated quantitatively for different phase difference change between LP01 and LPeven11 mode. The interference mode patters of elliptical-core and circular-core two-mode fibers are compared, the result shows that the two-lobe interference patters of the two-mode fibers generate energy exchange and oscillation, and thedifference is that the interference mode patterns of circular-core two-mode fiber are almost elliptical, while the interference mode pattern of elliptical-core two-mode fiber is approximately circular on condition that proper selection of the ellipticity. Their two-dimensional (2D) profile determines the choice of the core shape of the information pick-up fiber.     

Chaotic mixing of granular materials in two-dimensional tumbling mixers

We consider the mixing of similar, cohesionless granular materials in quasi-two-dimensional rotating containers by means of theory and experiment. A mathematical model is presented for the flow in containers of arbitrary shape but which are symmetric with respect to rotation by 180 degrees and half-filled with solids. The flow comprises a thin cascading layer at the flat free surface, and a fixed bed which rotates as a solid body. The layer thickness and length change slowly with mixer rotation, but the layer geometry remains similar at all orientations. Flow visualization experiments using glass beads in an elliptical mixer show good agreement with model predictions. Studies of mixing are presented for circular, elliptical, and square containers. The flow in circular containers is steady, and computations involving advection alone (no particle diffusion generated by interparticle collisions) show poor mixing. In contrast, the flow in elliptical and square mixers is time periodic and results in chaotic advection and rapid mixing. Computational evidence for chaos in noncircular mixers is presented in terms of Poincare sections and blob deformation. Poincare sections show regions of regular and chaotic motion, and blobs deform into homoclinic tendrils with an exponential growth of the perimeter length with time. In contrast, in circular mixers, the motion is regular everywhere and the perimeter length increases linearly with time. Including particle diffusion obliterates the typical chaotic structures formed on mixing; predictions of the mixing model including diffusion are in good qualitative and quantitative (in terms of the intensity of segregation variation with time) agreement with experimental results for mixing of an initially circular blob in elliptical and square mixers. Scaling analysis and computations show that mixing in noncircular mixers is faster than that in circular mixers, and the difference in mixing times increases with mixer size. (c) 1999 American Institute of Physics.     

环形ZnO薄膜谐振器的横模抑制与测试分析

采用Tiersten方程研究了环形ZnO薄膜谐振器中横模寄生问题, 获得了环(圆)形薄膜谐振器的横模振动方程, 求得横模位移场解和频率色散方程; 然后采用电磁学模式合成理论进行分析, 发现环形薄膜谐振器横模频率与环形电极的内外径之比a/b有关, 振动模式可由圆形薄膜谐振器横模模式合成得到, 通过控制a/b能够抑制横模模式数和调控基膜频率. 采用外差激光干涉仪和网络矢量分析仪测量并比较了同批次的圆形和环形薄膜谐振器的上电极横模振动图样和电阻抗曲线. 振动图样显示环形薄膜谐振器振动模式可由半径为a和半径为b的圆形薄膜谐振器振动模式合成, 仅存在节圆数大于0的横模振动, 等于0的横模模式被抑制; 电阻抗曲线显示当a/b为0.436时, 环形薄膜谐振器的基频(约1217 MHz)和圆形的(0, 1)模式频率相等. 测量数据验证了模式合成理论的分析结果正确性, 为薄膜谐振器的横模抑制研究提供了理论基础和新方法.     

Raman scattering from gas-evaporated silicon small particles

Raman scattering measurements are reported on silicon small particles prepared by gas-evaporation technique. The crystalline structure is also observed for the sample having 70 A particles in average size. Four resolved component modes with Gaussian distribution function are identified with the three usual modes (LA, TO and allowed-TO) and a new surface mode. The surface mode of silicon particle, whose relative integrated intensity decrease with an increase of the particle size, is presented for the first time.