QUANTITATIVE CAPACITIVE MEASUREMENTS OF VOIDAGE IN GAS-SOLID FLOWS

ABSTRACT

New capacitance probes were developed for quantitative, time-dependent measurements of voidage in gas-solid flows. Based on a unique guard circuit which nearly eliminates all stray and cable capacitances, these probes are fast (2 kHz) and they do not require in situ calibration. Two configurations were studied: small parallel plates for recording voidage profiles, and a non-intrusive design for local voidage measurements near a wall Static tests were performed using a fixed bed of known voidage and dielectric constant. The probes were also demonstrated in a two-dimensional fluidized bed of glass beads.     

QUANTITATIVE CAPACITIVE MEASUREMENTS OF VOIDAGE IN GAS-SOLID FLOWS

New capacitance probes were developed for quantitative, time-dependent measurements of voidage in gas-solid flows. Based on a unique guard circuit which nearly eliminates all stray and cable capacitances, these probes are fast (2 kHz) and they do not require in situ calibration. Two configurations were studied: small parallel plates for recording voidage profiles, and a non-intrusive design for local voidage measurements near a wall Static tests were performed using a fixed bed of known voidage and dielectric constant. The probes were also demonstrated in a two-dimensional fluidized bed of glass beads.     

LDV MEASUREMENTS OF SOLID-LIQUID SLURRY FLOW USING REFRACTIVE INDEX MATCHING TECHNIQUE

A refractive index matched facility for studying solid-liquid slurry flow was developed. The refractive index matching of the solid and liquid facilitates the use of non-intrusive—laser velocimetry to measure solid and liquid velocities for slurries with solid concentrations as high as 50 percent by volume. Silica gel and sodium iodide solution in water were used us the refractive index matched solid and liquid respectively. Tests were conducted on solid liquid slurry with 5 percent solid concentration by volume in the Reynolds number range of 700 to 8000. The Reynolds number ranges for the four flow regimes, stationary bed, saltation, heterogeneous and homogeneous flow were identified. It was shown that signal processing technique utilizing histograms and signal amplitude discrimination can be used to discriminate between liquid and solid velocities. The details of the facility and the results of the tests conducted on the slurry are discussed in this paper.     

PRESSURE CHANGES AT A SUDDEN EXPANSION IN GAS-SOLID FLOWS

There is a pressure recovery when a two-phase mixture passes through a pipe enlargement. This paper describes an experimental project for the determination of this pressure recovery in air-solids systems. The solids used are medium size wax and lucite particles. Two pipe diameter ratios were investigated, 0.63 and 0.76. The Reynolds numbers for the experiments varied between 45000 and 72000 and the loading ratios were low to intermediate (up to 6), it was observed that the major part of the pressure recovery in these mixtures was due to the air alone and that only a small part of it (up to 25%) was due to the deceleration of solid particles.     

PRESSURE MEASUREMENTS IN A FULL-SCALE FLY ASH SILO

In cooperation with the Danish electrical power supply organizations Elkraft and Elsam, test series have been carried out in a circular fly ash bin with a diameter of 16 m and a height of 28 m. The bin is emptied by an arrangement that causes sector-wise fluidizing inthe slightly inclined bottom. This silo has been equipped with 19 pressure cells to measure the total pressure, and the same number of pore pressure cells were made and mounted in accordance with theoretical and experimental results developed at the Department of Structural Engineering, Technical University of Denmark.

An interpretation of the test results shows that two types of flow field can exist, depending on certain conditions: one with mass flow and an almost rotationally symmetric pressure distribution, and one with partial funnel flow and a non-uniform pressure distribution. During flow it has been observed that the flow field can suddenly change between these two types. Such changes have been found to be connected with a considerable loss of potential energy, giving rise to dynamic phenomena in the structure. The measured pressure were generally within the range predicted by DIN 1055 except in the conical part of the silo.     

MODELING OF SALTATION AND TURBULENCE EFFECTS IN A HORIZONTAL GAS-SOLID BOUNDARY LAYER

This work is devoted to the study of the behavior of solid particles in a horizontal air boundary layer. A two-fluid model is selected to analyze the particle motion in the mixed regime where both saltation and turbulence effects are important. Boundary layer approximations are applied to the solid phase equations, using conventional space/time averaging. Resulting equations are shown to be in agreement with the phase averaged equations generally used in two-fluid models. Closure is achieved by means of a gradient law, however the particle diffusion and momentum transfer coefficients are distinguished using a variable particle Schmidt number. Turbulent particle diffusion is modeled using an available analytical model, which is modified to take the saltation effect into account in the particle r.m.s. velocity. Comparison between experimental and numerical results shows that the following analysis is acceptable within the experimental and numerical error. The model slightly underestimates the particle mean velocity in the outer region of the boundary layer, but yields satisfactory values of mass fluxes. The present simple approach adequately describes the particle behavior in a horizontal turbulent boundary layer, with the main originality being the realistic dependence of the particle Schmidt number upon the saltation phenomenon.     

PREDICTION OF HEAT TRANSFER IN A TURBULENT GAS-SOLID PIPE FLOW USING A TWO-FLUID MODEL

The dynamic and thermal behaviour of a turbulent gas-solid flow in a vertical pipe has been numerically studied by means of an original two-fluid approach which includes the effects of particle-particle and particle-wall collisions as well as particle-turbulence interactions. A k-ε-k_p model for the flow dynamics has been combined with the energy balances of each phase. Comparisons have been carried out with available experimental data, first for the dynamic validation, and then for the thermal portion of the problem, in the case of rather large panicles (200 to 500 µm). Heat transfer from wall to suspension is seen to be strongly affected by particles. As a consequence, a suspension Nusselt number decrease of 20% may be observed at loading ratio smaller than two. Further increase is possible if more panicles are injected. Predictions had excellent agreement with experimental data for 500 µm particles. Slight discrepancies appear for smaller particles, probably due to more complex, partially understood panicle-turbulence interactions.     

MAGNETOSTRICTION EFFECTS IN CRACK LENGTH MEASUREMENTS

Abstract— Fatigue crack lengths were measured in a ferromagnetic material (a low alloy steel) using the d.c. potential drop technique. Significant cyclic fluctuations in potential were observed. Using discriminatory tests it has been established that the fluctuations may be attributed to inverse magnetostriction known as the Villari effect. The strain rate dependence of the fluctuations was investigated using sinusoidal, square and sawtooth waveforms, and the amplitude of the fluctuations was shown to be proportional to strain rate. In some cases the amplitude of the fluctuations was of similar magnitude to the crack length measurement potential.     

MONITORING CONTAMINANT PARTICLES IN GASES: A REVIEW

Especially in the microelectronics and pharmaceutical industries, particles that are microns in size and even smaller lessen the quality and quantity of the products produced. Reducing losses due to contamination requires detecting particles in air, in other gases, and in liquids, as well as on surfaces. This review covers the monitoring of particles in gases, from the perspective of contamination control primarily in the microelectronics industry.     

结构振动模态局部化评述

近年来，力学系统的振动模态局部化现象受到了人们极大的关注。研究表明，在一定条件下，小量失调对某些结构系统的振动特性有很大影响。为了促进模态局部化方面的研究工作，本文在总结国内外大量文献的基础上，根据我们自己的研究，较系统地介绍了结构振动模态局部化研究的历史、现状及前景等若干重要问题。详细讨论了结构失调的影响、与阻尼的类比特性；物理学局部化与结构动力学局部化的联系和区别；局部化现象的产生条件、产生机理、利与弊、与频率曲线转向的关系；针对不同结构系统的不同研究方法。同时，还列举了大量国内外参考文献，以备研究者参阅。     

形状自由的高性能有限元方法研究的一些进展

作为工程和科学计算的主要工具,有限元方法已经得到了广泛的应用,但是仍然受到网格畸变敏感等固有难题的困扰,并且一直没有能够彻底根治。该文系统介绍了新型有限元方法--形状自由的高性能有限元方法研究的最新进展,包括平面问题和二维断裂问题的杂交应力函数有限元方法,中厚板问题的杂交位移函数有限元法,平面和三维问题的新型非对称有限元方法。这些方法在已有的杂交应力元法和非对称有限元法基础上,综合利用了解析试函数法、新型自然坐标方法、广义协调方法等先进技术,获得重要进展：所发展的单元模型精度高且稳定,在网格极端畸变的情况下仍可保持原有精度,具有形状自由的优异特性;同时破解了MacNeal局限定理,解决了中厚板边缘效应计算等难题。论文的最后对上述方法的特点以及后续的研究工作进行了讨论。     

收缩-扩张喷管内气固两相流运动规律研究

航空飞行器在简易跑道起飞或降落时,砂尘颗粒不可避免被发动机吸入,并与高速旋转的发动机叶片发生冲撞,导致叶片冲蚀损伤,显著降低发动机使用寿命,严重威胁飞行器的安全。试验中一般采用气动喷砂的方法将砂尘粒子加速至一定速度,并冲击试验件,以模拟发动机叶片的实际冲蚀过程。喷管是实现粒子加速的关键部件,该文通过试验和数值模拟对不同入口总压条件下收缩-扩张喷管加速砂尘颗粒的情况进行研究。实验中,通过压力传感器和双盘测速分别测量了喷管内壁静压和喷管出口处的颗粒速度;数值模拟中,采用实验入口总压条件,模拟了稀疏砂尘颗粒在喷管中的气固两相流运动,并详细分析了气流运动和颗粒的受力及加速情况。研究表明：数值模拟的气流压力分布及颗粒速度均与实验结果吻合;喷嘴出口处粒子速度随入口总压的增大而增大,且总压为0.23 MPa~0.4 MPa范围时的增长速率较大,而总压为0.4 MPa~0.56 MPa范围时的增长速率较小;粒子的加速主要发生在喷管的扩张段。     

DEFORMATION IN THE SHEAR ZONE IN METAL CUTTING,A COMPARATIVE REVIEW

The assumptions basic to a number of shear zone theories are discussed, the final shear strain values predicted by the theories being compared with the results of published experimental data. Theories and experimental data concerning chip grain direction are also discussed.     

CRACK CLOSURE IN SMALL FATIGUE CRACKS—A COMPARISON OF FINITE ELEMENT PREDICTIONS WITH IN-SITU SCANNING ELECTRON MICROSCOPE MEASUREMENTS

Abstract— A three dimensional, elastic-plastic, finite element analysis of fatigue crack growth and plasticity-induced crack closure has been performed for a range of small, semi-circular cracks. Predicted crack opening displacements have been compared with data obtained from in-situ SEM measurements for a coarse-grained aluminium alloy 2024-T351. The magnitude of fatigue crack closure measured from in-situ SEM measurements was consistently higher than that predicted from the finite element analysis. It is deduced that the higher closure stresses obtained from in-situ SEM measurements are due to the contact of asperities on the fatigue crack surfaces. A simple mathematical model is suggested to describe the fatigue crack closure stress caused by the combination of both a plastic wake and asperities on the fatigue crack surfaces. The predicted fatigue crack closure stresses and their dependence on crack size are consistent with experimental measurement.     

HIGH RESOLUTION CRACK GROWTH MEASUREMENTS IN VIBRATING BEAMS

Abstract— Fatigue crack propagation characteristics are measured by continuously determining the resonant frequency in vibrating systems, which is stabilized through a feedback control loop. The precisely controlled resonant frequency is related to the crack length by a nonlinear model based on fracture mechanics, hence crack growth can be monitored with respect to time with very high accuracy. The nonlinearity due to the opening and closing of the crack needs to be taken into account. In contrast to conventional fatigue tests, which require a long duration of time due to the high numbers of load cycles at low frequencies, the proposed technique operates at much higher frequencies, i.e. in the range of 100 Hz to 100 kHz. Thus the required time for measurements in the high cycle fatigue range is considerably reduced. The experimental setup is simple and inexpensive and does not require high energy inputs.     

FATIGUE OF STEEL REINFORCEMENT BARS IN CONCRETE: A REVIEW

Abstract— During the last few years there has been an intensification of interest in the fatigue performance of steel reinforcement bars in concrete structures. Although fatigue has not proved to be a problem to date, loading cycles are becoming increasingly severe so that the margin of reserve strength is progressively being reduced. In this paper the main parameters associated with fatigue of reinforcement are reviewed with particular attention given to conditions related to highway bridges. Fatigue testing can be either axial in air or by bending of reinforced concrete beams. The latter is experimentally less convenient but simulates more closely the service environment. Endurances can be influenced by type of steel, geometry and size of the bars, nature of the loading cycle, welding and presence of corrosion. The relative behaviour of butt welded joints is considered and it is shown that the reduction in fatigue strength commonly attributed to flaws and to the stress concentrations associated with welds does not always occur for bending fatigue of reinforced concrete beams.     

Organic Magnets—A History

This article summarizes the history associated with the research targeting the design and synthesis of new classes of magnets, namely, those prepared via conventional synthetic organic chemistry methodologies and where the building blocks of the magnetic materials are comprised of molecules as opposed to atoms and as a consequence frequently possess electron spins in p‐orbitals. Aspects of new chemistry and new materials evolved from these worldwide studies are highlighted.     

复合材料迭层板壳有限单元法现状分析

本文从剖析单元构成因素的角度出发,综合评述了目前用于复合材料板壳结构分析的各种有限单元,分析了这些单元的特点及其适用范围,并说明Mindlin类等参板壳单元广泛应用迭层板壳的原因以及存在问题、解决办法。还深入地讨论了杂交单元应用至迭层板壳计算的优点。