三维漏斗中颗粒物质堵塞问题的数值实验研究

对于工程和实验中使用漏斗颗粒流而言,连续稳定的流量是必要的.当漏斗口较小时,很容易发生堵塞行为.堵塞现象对于交通流、疏散问题等也具有重要的意义.前人主要使用扰动的方法破坏漏斗中已有的堵塞,以便引起下一次堵塞,加快实验进程.本文利用自主开发的基于GPU(graphics processing unit)的密集颗粒流模拟程序,主要研究当三维漏斗开口打开后的第一次堵塞行为,不再引入扰动.详细讨论了漏斗开口尺寸、漏斗锥角等几何参数对坍塌规模的影响.发现对于坍塌规模的概率分布符合前人的研究结果,可以分为两部分:峰的左边呈幂函数上升形式,峰的右边呈指数衰减趋势.对于漏斗开口尺寸和漏斗锥角而言,均存在一个临界值使得堵塞不再发生.     

格子Boltzmann方法分析加热尺寸和瑞利数对可变形开口腔内自然对流的影响

采用格子Boltzmann方法对可变形腔体内自然对流问题进行数值研究，给出平均努赛尔数的经验关系式.腔体左壁加热长度分为左壁面的整个区域（H）和左壁面的中间区域（0.5H）两种情况，右壁向外界环境开放，上下边界绝热且可以上下移动，以此调节右出口尺寸.主要研究瑞利数（10⁴ ≤ Ra ≤ 10⁶），右出口尺寸（1.0H ≤ L ≤ 2.0H），左壁加热尺寸（L_h=0.5H或L_h=H）对腔体内等温线、流线、局部努塞尔数和平均努赛尔数的影响.结果表明：腔体内换热随着瑞利数的增大越来越强烈，表现为椭圆形准静止区域更加靠近上绝热壁，且热分层厚度逐渐变小，平均努赛尔数增加.而右出口尺寸的增加，对于两种加热尺寸下腔内的换热效果有不同程度影响，其中与加热尺寸为左壁面的全部区域L_h=H相比，加热尺寸为左壁面的中间情况L_h=0.5H时，右侧开口尺寸的增加对换热效果的影响不显著.此外，左壁加热尺寸为0.5H时显示出比加热尺寸为H时更高的平均传热效率.最后，针对不同的加热尺寸，提出加热面平均努赛尔数与Ra数及右壁面开口尺寸L^*之间函数关系的经验预测，拟合效果满足工程实践与设计需要.     

旋转离心叶轮内部非定常流动实验研究

利用激光成像速度仪(PIV)测量了旋转离心叶轮内部的非定常流场,获得了旋转离心叶轮内部相对速度的非定常流场分布。详细分析了叶轮内部非定常流动现象和流动规律。通过实验研究发现旋转离心叶轮内部的流动是非定常,在叶轮出口处,叶片的吸力面与轮盖的夹角区存在一个低速区,并观察到了明显的射流/尾迹结构。射流区和尾流区的大小和范围在沿盘盖方向和跨叶片方向上是不同的。射流区和尾流区之间不存在明显的分界线。     

Dynamics of density waves in a two dimensional funnel on an inclined plane

We report here experiments on two-dimensional funnel flow of diameter glass beads on an inclined plane. We have investigated the properties of the flow according to the outlet size D of the funnel and the gravity. We have identified three different regimes. For small funnel outlet sizes, there is no significant change in flow density: the flow is rather steady and homogeneous. For intermediate outlet sizes (), the flow is intermittent, consisting of spatially ordered density waves propagating upwards. At bigger outlet sizes, density waves do not exhibit any ordering and the flow dynamics becomes chaotic. In addition, we find that the flow dynamics is independent of the funnel opening angle except close to the channel flow configuration. Finally, it is stressed that the interactions between the beads and the inclined plane play a crucial role in the mechanism of formation of density waves. Received: 9 July 1998 / Received in final form and Accepted: 18 September 1998     

Longitudinal structures in the near field of a plane wall jet

In the present study, the flow field near the orifice of a plane wall jet is in focus. Two main flow regions may be distinguished in the wall jet, i. e., a free shear layer away from the wall and a boundary layer close to the surface. In both of these layers, streamwise coherent structures are detected by means of smoke visualization and hot-wire measurements. The structures, which occur naturally, have different spanwise scales and emerge at different distance from the nozzle. Effects of the flow velocity, upstream perturbations, and acoustic excitation on the generation and characteristics of the streamwise disturbances are investigated, and especially the interaction between the two layers is studied. In order to resolve the complex 3D flow by means of hotwires a system for accurate automated traversing and data acquisition has been developed. In each flow case time-dependent measurements were taken in (X, Y, Z) space of about 3000 to 25,000 points, and it was found that the value of outlet velocity and the frequency of Kelvin — Helmholtz rolls have a clear influence on the size of the structures. Higher outlet velocities and higher frequencies of triggered two-dimensional roll-ups lead to a decrease in the size of longitudinal structures.     

传像光纤束单丝芯径大小对光纤像面全息图的影响

光纤像面全息可以获得较高衍射效率的全息图和再现较大的物像。文中分析了相干光激励下传像光纤束单丝芯径的大小对其输出光场的影响，以及输出光场中低阶模与高阶模对全息照相的不同作用，在此基础上，提出缩小传像束单丝芯径，增多单丝根数，可以提高光纤像面全息图衍射效率和分辨率的观点，并通过实验进行了验证。     

Atom funnel running with evanescent light generated by a thick hollow light beam

The use of a hollow light beam reduces the loss of atoms inside and outside an evanescent-light funnel. Therefore, we superimpose an LG₀₁ beam with a small hole onto a doughnut beam with a large outer diameter and couple the resultant hollow light beam to the 240-μm-wide outlet of an inverted trigonal-pyramidal funnel. Compared to the case where a Gaussian light beam is employed for generation of evanescent light, the flux intensity of the output cold ⁸⁷Rb atoms rises 13-fold when the inner diameter of the 4.5-mm wide hollow light beam is 1.7 times as large as the size of the outlet under optimal blue-detuning conditions.     

Ultrasonic particle size fractionation in a moving air stream

Identification of bio-aerosol particles may be enhanced by size sorting before applying analytical techniques. In this paper, the use of ultrasonic acoustic radiation pressure to continuously size fractionate particles in a moving air stream is described. Separate particle-laden and clean air streams are introduced into a channel and merged under laminar flow conditions. An ultrasonic transducer, mounted flush to one wall of the channel, excites a standing ultrasonic wave perpendicular to the flow of the combined air stream. Acoustic radiation forces on the particles cause them to move transverse to the flow direction. Since the radiation force is dependent upon the particle size, larger particles move a greater transverse distance as they pass through the standing wave. The outlet flow is then separated into streams, each containing a range of particle sizes. Experiments were performed with air streams containing glass microspheres with a size distribution from 2-22 μm, using a centerline air stream velocity of approximately 20 cm/s. An electrostatic transducer operating at a nominal frequency of 50 kHz was used to drive an ultrasonic standing wave of 150 dB in pressure amplitude. The microsphere size distributions measured at the outlet were compared with the predictions of a theoretical model. Experiments and theory show reasonable correspondence. The theoretical model also indicates an optimal partitioning of the particle-laden and clean air inlet streams.     

Droplet breakup in an asymmetric microfluidic T junction

Breakup of non-uniform droplets in an asymmetric T junction consisting of an inlet channel and two different-size outlet channels has been investigated numerically. Also, an analytical approach in the limit of the lubrication approximation has been extended to provide some analytical relations to study the system and verify the numerical results. Parameters that are important in the performance of the system have been determined and discussed. Our results indicate that smaller droplets can be produced by increasing the capillary number. As the geometry becomes symmetric the pressure drop decreases. Our results also reveal that the breakup time and the pressure drop for this system are smaller than the previous suggested method for producing non-uniform droplets, i.e., a uniform size T junction with different-length outlet channels.     

The effect of a ledge in outlet electrode on operation efficiency of the two-chamber plasmatorch

Electric and thermal characteristics of the two-chamber plasmatorch with a stepped outlet electrode were studied experimentally. Formulas for determination of arc voltage, plasmatorch efficiency, and length of a narrow part of the outlet electrode were derived. It is shown that such a plasmatorch is more efficient than the plasmatorch with a smooth outlet electrode.     

FC-72在浸泡于液池中的微小圆管内的沸腾传热

本文对浸泡于FC-72液池中的两个微小圆管进行了沸腾实验研究,得到了沸腾曲线和传热系数,并用DV摄像机拍摄到了圆管出口处的沸腾状况,研究了管道尺寸对沸腾传热特性的影响。实验结果显示,管道尺寸对沸腾传热特性有显著的影响。传热系数和CHF随着管道尺寸的缩小而减小.直径为1.10mm的圆管出口处在低热负荷加热时发生了汽泡阻塞,并导致了剧烈的沸腾滞后现象。     

LNG船用BOG再液化装置工艺流程模拟

文中运用工程模拟计算软件ASPEN PLUS对LNG船用蒸发气体(BOG)再液化装置工艺流程进行了较全面和深入的模拟计算.通过对模拟计算结果进行分析,得到用丙烯预冷的再液化工艺流程中的主要工艺设备运行参数:海水冷凝器冷凝温度、BOG压缩机出口压力、丙烯压缩机出口压力以及混合制冷剂压缩机出口压力对再液化效率和能耗有着不同...     

Yerevan synchrotron injector electron beam transversal scan with vibrating wire scanner

We have developed extended-aperture (20 mm) vibrating wire scanner (VWS) for transversal scanning of electron beams with large transversal size. Test experiments were performed in open atmosphere on the 40 MeV electron beam of the Yerevan synchrotron injector with the 4–10 μA at outlet. A construction of VWS is elaborated for scanning beams with larger transversal size. This elaboration is a new precision tool for diagnostics of beams in accelerators, which may be successfully employed, because of universality of the used operation principle, for profiling of as well large-aperture proton and ion beams.     

Effect of wall thickness on the end corrections of the extended inlet and outlet of a double-tuned expansion chamber

Simple expansion chambers, the simplest of the muffler configurations, have very limited practical application due to the presence of periodic troughs in the transmission loss spectrum which drastically lower the overall transmission loss of the muffler. Tuned extended inlet and outlet can be designed to nullify three-fourths of these troughs, making use of the plane wave theory. These cancellations would not occur unless one altered the geometric lengths for the extended tube in order to incorporate the effect of evanescent higher-order modes (multidimensional effect) through end corrections or lumped inertance approximation at the area discontinuities or junctions. End corrections of the extended inlet and outlet have been studied by several researchers. However the effect of wall thickness of the inlet/outlet duct on end correction has not been studied explicitly. This has significant effect on the tuning of an extended inlet/outlet expansion chamber. It is investigated here experimentally as well as numerically (through use of 3-D FEM software) for stationary medium.     

供水温度对CO2空气源热泵系统性能的影响

为探究热泵供水温度对CO2空气源热泵系统性能的影响,保持室外环境温度15.5℃不变,调节热泵供水温度,测试冷却水流量、气冷器出水温度、压缩机排气温度、气冷器CO2进出口温差、压缩机排气压力、压缩机耗功量、系统制热量、气冷器热交换完善度、系统COP的变化情况。结果表明:供水温度由45℃升至85℃,气冷器出水温度、压缩机排气温度、气冷器CO2进出口温差、压缩机排气压力随之增加,冷却水流量随之减小。系统制热量增加了7.3%、气冷器热交换完善度下降了20.0%、系统COP下降了35%、压缩机功耗增加了65.1%。     

动态调整出水温度的高效制冷方法研究

通过数学方法较为精确地建立了吸收式制冷机需满足负荷要求的数学模型,确立了蒸发器冷冻水出口温度与室外温度之间的关系式。进而建立了吸收式出水温度控制系统的动态特性模型,并采用并联程序法对水温控制系统进行仿真。     

后置蜗壳斜流叶轮进出口流场的实验研究

后置蜗壳斜流叶轮进出口流场的实验研究吴克启，于文华，张勇，胡晓，刘海波，赵联州（华中理工大学武汉４３００７４）关键词：后置蜗壳，斜流叶轮，进出口流场，实验一、前言后置导叶翼形和圆弧形斜流风机的设计及应用【川＼日益受到人们的关注并形成了性能良好的斜流风...     

Experimental Determination of the Transfer Function of a Differential Mobility Analyzer (DMA) in the nanometer size range

This paper describes the evaluation of the transfer function for the TSI-short differential mobility analyzer (DMA) in the nanometer particle size rang. The TSI-short DMA is identical with the TSI-long DMA (Model 3071) but has shortened inner and outer electrodes and the insulator material Teflon was replaced by black Delin. The DMA transfer function was determined by operating two identical DMAs in series. The DMA was investigated at a sheath to aerosol flow rate ratio of between 10 and 1. Usually for particles above 50 nm the parameters of the assumed triangular transfer function, i.e. height and half-width, approach the ideal values, which depend only on the flow ratio. For particle size below 50 nm the height decreases from 0.9 at 50 nm to 0.18 at 5 nm. The half-width increase from the ideal value 0.1 to about 0.17. The observed changes of the transfer function are due to diffusional effects resulting in losses mainly in the inlet and outlet and also broadening of the aerosol stream inside the DMA. The determined transfer functions were also compared with transfer functions calculated using a theoretical model. The agreement proved to be satisfactory. The half-widths and the transmission efficiencies of the conventional TSI-DMA (Model 3071), TSI-short with Teflon and also black Delin insulator as a function of particle size are compared to demonstrate the improvements in performance.     

A molecular dynamics simulation of nanoscale convective heat transfer with the effect of axial heat conduction

Effective heat dissipation from nano-fluidic devices is sometimes necessary to ensure their performance and lifespan. In the molecular dynamics simulation of nanoscale convective heat transfer, thermostats cannot be directly applied to the fluid because of the non-uniform temperature distribution. Periodic boundary is typically utilised, but unrealistic axial heat conduction exists when there is a temperature difference between the outlet and images of inlet atoms. In this paper, the effect of axial conduction caused by periodic boundary is investigated through the Péclet number (Pe). Taking viscous dissipation into consideration, the magnitude of outlet thermal diffusion is observed to decrease with increasing Pe. The local average temperature of fluid changes in an exponential form except in the region close to the outlet. Results show that the contribution of outlet axial conduction to the local average temperature is less than 2.0% when Pe > 10. The main reason is that the magnitude of fluid velocity and viscous heat dissipation in nanochannels is much larger than that in macro-channels at the same Péclet number.     

Numerical investigation of supersonic flow breakdown at the inlet duct throttling

The work presents the results of investigating the process of supersonic flow deceleration in a duct of the two-dimensional inlet throttled by variation of the outlet cross-sectional area. An inlet with three external compression shock waves designed for the freestream Mach number M_d = 7 was considered as an example for the investigation. A one-dimensional analysis of the conditions for realization of the supersonic flow deceleration regimes in the inlet duct with two throats — in the inlet entrance and at the inlet duct outlet, has been carried out. The parametric numerical computations of two-dimensional inviscid or turbulent flows in the inlet were performed with the use of the Euler and Navier—Stokes codes of the program package FLUENT. The critical conditions for the nonuniform flow in the outlet throat bringing to choking the inlet duct were determined.