Simultaneous imaging techniques for temperature and velocity fields in thermal fluid flows

In this paper, non-intrusive imaging techniques for simultaneously measuring temperature and velocity fields of thermal liquid flows are described. The experimental methods for temperature imaging considered here are the thermo-sensitive liquid crystal tracers and the laser-induced fluorescence, which are combined with particle image velocimetry (PIV) to measure simultaneously the corresponding 2D and 3D velocity fields. The features of these experimental methods and characteristics are examined, covering the measurable temperature range, uncertainty of measurement, time response and so on. The successful examples of simultaneous measurement of temperature and velocity fields are described for turbulent Rayleigh-Bérnard convection of a horizontal fluid layer and turbulent buoyant plume issuing from a circular nozzle, and their physical mechanisms of transport phenomena are explained.     

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分析了磁流体力学效应对液态金属自由表面射流稳定性的影响。从射流的感应电势、电流、速度等方面,解释射流在磁场中稳定的原因。数值计算结果验证了理论分析。     

Bubble Velocity and Size Measurement with a Four‐Point Optical Fiber Probe

The possibility to measure the velocity and size of individual bubbles in a high‐void fraction bubbly flow is investigated by using a four‐point optical fiber probe. The air bubbles have an initial spherical equivalent diameter ranging from 4 to 10 mm and the void fraction is up to 0.3. Firstly, single bubble experiments show that intrusiveness effects, i.e. bubble deformations due to the probe, are negligible provided that the bubble approaches the probe at the axis of the central fiber. A selection criterion is utilized for multiple bubble experiments. A good compromise can be found between the required accuracy, the duration of the measurements and the number of validated bubbles required for reliable statistical averaging. In an air‐water high‐void fraction vertical bubbly pipe flow, the void fraction obtained with the instrument is found to be in good agreement with both local single‐fiber probe measurements, and with the volume average void fraction obtained from pressure gradient measurements. The area average volumetric gas flow rate, based on the bubble velocity and void fraction as measured with the four‐point probe, agree with the measured gas flow rate. Also, the liquid velocity is measured by means of a laser‐Doppler anemometer, to investigate the slip velocity. The results show that reliable and interesting measurements can be obtained by using a four‐point optical fiber probe in high void fraction flows.     

Three measuring techniques for assessing the mean wall skin friction in wall-bounded flows

The present paper aims at evaluating the mean wall skin friction data in laminar and turbulent boundary layer flows obtained from two optical and one thermal measuring techniques, namely, laser-Doppler anemometry (LDA), oil-film interferometry (OFI), and surface hot-film anemometry (SHFA), respectively. A comparison among the three techniques is presented, indicating close agreement in the mean wall skin friction data obtained, directly, from both the OFI and the LDA near-wall mean velocity profiles. On the other hand, the SHFA, markedly, over estimates the mean wall skin friction by 3.5–11.7% when compared with both the LDA and the OFI data, depending on the thermal conductivity of the substrate and glue material, probe calibration, probe contamination, temperature drift and Reynolds number. Satisfactory agreement, however, is observed among all three measuring techniques at higher Reynolds numbers, Re _x >10⁶, and within ±5% with empirical relations extracted from the literature. In addition, accurate velocity data within the inertial sublayer obtained using the LDA supports the applicability of the Clauser method to evaluate the wall skin friction when appropriate values for the constants of the logarithmic line are utilized.     

Gas volume fraction and velocity profiles

Upward inclined gas-liquid flows are frequently encountered in the oil industry and data relating to the local gas volume fraction distribution and the local gas velocity distribution is important, for example, in pressure gradient prediction and in modeling oil well ‘blowouts’. In this paper measurements are presented of the local gas volume fraction distribution and the local axial gas velocity distribution which were taken in bubbly air-water flows in an 80 mm diameter pipe which was inclined at angles of 0°, 15° and 30° to the vertical. Qualitative arguments are presented to explain the influence of the liquid superficial velocity on the local gas volume fraction distribution in inclined flow and also to explain the very high axial gas velocities observed towards the upper side of the inclined pipe.     

Application of X-ray radioscopic methods for characterization of two-phase phenomena and solidification processes in metallic melts

X-ray attenuation techniques are an important diagnostic tool for investigating liquid metal two-phase flows or solidification studies in metallic alloys. X-ray visualization enables a general, intuitive understanding of flow phenomena or pattern formation in opaque liquid metal systems. Real-time and in-situ observations of the density distribution within thin solidifying samples achieve a spatial resolution of a few microns and contribute significantly to an improved understanding of dendritic growth processes. Moreover, X-ray radioscopy is a useful tool for a non-invasive, in-situ visualization and characterization of gas bubbles in nontransparent melts or for observations of the formation of metal foams. In this paper we consider three different fields of application which are under intensive investigation at HZDR and TUD: the bottom-up solidification of Ga-In alloys under the influence of buoyancy-driven and electromagnetically driven convection, the injection of Ar gas into liquid GaInSn, the study of Al foams with respect to foam formation and the characterization of their internal structure.     

Experimental determination of space charge density associated with flow electrification phenomenon: Application to power transformers

At the solid/liquid interface, a charge zone called the Electrical Double Layer (EDL) appears. It is constituted of two zones of opposite sign, one in the solid and another one in the liquid. When a liquid flows through a pipe, an axial streaming current is generated. This current is due to the convection of the charges coming from the electrical layer in the liquid. The experimental methods for the determination of space charge density have been clearly identified in the case of fully-developed EDL. On the contrary, there is a need for further exploration in the case of a non-fully developed EDL. The purpose of this paper is to calculate the fully developed space charge density at the wall by streaming electrification experiments which are conducted in the case of non-fully developed EDL. Moreover, it introduces a simplified procedure for the experimental determination of space charge density as a function of liquid flow velocity. This procedure can be conducted for the investigation of flow electrification phenomena in transformers with oil/metal configuration.     

液态金属自由表面膜流MHD效应的数值模拟

对液态金属自由表面膜流在强磁场下的磁流体力学效应进行了数值模拟研究,获得了液态金属自由表面的形状、截面流速分布及截面上的电动势分布,从而能对膜流的一些磁流体动力学行为作出解释。数值计算结果与理论分析和实验结果符合较好。由实验和数值模拟结果可以得出,液态金属膜流通过强磁场时,磁场会阻碍膜流的运动。     

Pressure–velocity coupling allowing acoustic calculation in low Mach number flow

Low Mach number flow computation in co-located grid arrangement requires pressure–velocity coupling in order to prevent the checkerboard phenomenon. Two broad categories of pressure–velocity coupling methods for unsteady flows can be distinguished based on the time-step dependency of the coupling coefficient in the definition of the transporting velocity on a face of a control volume. As an example of the time-step independent category, the AUSM⁺-up scheme is studied. As an example of the second category, Rhie–Chow momentum interpolation methods are studied. Within the momentum interpolation techniques, again two broad categories can be distinguished based on the time-step dependency of the coupling coefficient used for unsteady flow computations, but when a steady state is reached. Variants of Rhie–Chow interpolation methods in each subcategory are studied on critical test cases. The result of the study is that for a good representation of unsteady flows containing acoustic information, the pressure–velocity coupling coefficient must explicitly depend on the time-step, but that the transporting velocity must become independent of the time-step when a steady state is reached.     

Simultaneous density and velocity measurements in a supersonic turbulent boundary layer

A novel technique for simultaneous measurements of instantaneous whole-field density and velocity fields of supersonic flows has been developed.The density measurement is performed based on the nano-tracer planar laser scattering(NPLS) technique,while the velocity measurement is carried out using particle image velocimetry(PIV).The present experimental technique has been applied to a flat-plate turbulent boundary layer at Mach 3,and the measurement accuracy of the density and velocity are discussed.Based on this new technique,the Reynolds stress distributions were also obtained,demonstrating that this is an effective means for measuring Reynolds stresses under compressible conditions.     

A Simple Model for Measuring Refractive Index of a Liquid Based upon Fresnel Equations

Due to many experimental data required and a lot of calculations involved, it is very complex and cumbersome to model prism-based liquid-refractive-index-measuring methods. We develop a new method of mathematical modelling for measuring refractive index of a liquid based upon the Fresnel formula and prism internal reflection at an incident angle less than the critical angle. With this method, only two different concentrations measurements for a kind of solution can lead to the determination of computational model. Measurements are performed to examine the validity of the theoretical model. Experimental results indicate the feasibility of the theoretical model with an error of 1%. The method is also capable of measuring even smaller changes in the optical refractive index of the material on a metal surface by the surface plasma resonance sensing techniques.     

Detection of moving objects and flows in liquids by ultrasonic phase conjugation

The possibility for the application of the method of parametric phase conjugation of ultrasonic waves in measuring the velocity of moving objects and flows is investigated. Results of experimental measurements of the Doppler frequency shift are presented for a low-frequency wave (1 MHz) generated by phase-conjugate waves (10 MHz and 11 MHz) propagating in opposite directions in the presence of a moving scatterer. The super high sensitivity of the phase of the low-frequency wave to variations in the spatial position of the scatterer is used to measure the velocity of the object. The presence of flows in the region of propagation of phase-conjugate waves returned leads to an uncompensated Doppler shift of the phase of the phase-conjugate wave at the primary radiation source. The implementation of this feature of ultrasonic phase conjugation for the detection and measurement of the flow velocities in a liquid is demonstrated experimentally.     

一种新的光子多普勒速度频谱分析方法

光子多普勒速度计可给出飞层表面某一速度带内颗粒群速度随时间演化的频谱数据, 在冲击动力学实验尤其是微喷射及其混合研究中得到广泛应用. 本文提出一种新的光子多普勒频谱数据分析方法, 可推断出混合区厚度变化和前端等效颗粒尺度. 利用该方法, 对一些典型状态下喷射混合速度频谱开展分析, 获得了不同冲击压力、气体条件下颗粒度数据, 证实了气体环境下喷射颗粒的气动破碎现象, 以及破碎后尺度与初始条件的依赖性, 为喷射混合物理规律研究提供了重要依据.     

FORCED AND SELF-EXCITED VIBRATIONS OF PIPES CONTAINING MOBILE BOILING FLUID CLOTS

Numerical modelling of the dynamic behaviour of a pipe containing inner non-homogeneous flows of a boiling fluid has been carried out. Inasmuch as the efforts to solve this problem analytically are confronted by considerable difficulties connected with varying system mass, geometry and discontinuity of equation coefficients, computational techniques for simulating pipe dynamics have been developed based on using of numerical time integration methods and transfer matrix methods together with orthogonalization procedures relating to the space variables. The system vibrations at different values of the parameters of the flow non-homogeneity and its velocity are observed. The possibility of forming stable and unstable flows depending on the character of the non-homogeneity and the velocity of fluid clots has been found.     

NMR velocimetry and spectroscopy at microscopic resolution in small rheometric devices

We describe several different rheometric devices for use within the nuclear-magnetic-resonance probe of a standard widebore microimaging system. These include both vertical and horizontal Couette cells and the cone- and -plate cell, which produce shearing flows, and the four-roll mill and the opposed-jet (cross-flow junction) cells which produce extensional flow. We demonstrate that velocity images can be obtained for each and that detailed information about local shear and extension rates can be extracted. These systems have considerable potential for use in the study of non-Newtonian viscosity, and of molecular ordering under shear or extension.     

几种液体金属自由表面的MHD不稳定性实验研究

介绍了在核工业西南物理研究院的液态金属实验回路(LMEL)上获得的几种可供液态偏滤器-限制器系统选用的液体自由表面的磁流体动力学(MHD)效应不稳定性的实验结果。实验发现：自由表面射流在穿越梯度横磁场时射程变短、截面变扁平,但MHD效应稳定,调节射流与磁场的夹角可以控制射流的流动特性;自由表面膜流MHD效应存在三种现象,即层流、溪状流和湍流。层流是由多束射流打到固体表面产生的(简称“射-膜流”),从MHD效应角度考虑,“射-膜流”将是四种可选液态偏滤器-限制器系统的液体自由表面形式中最佳的选择。同时,探讨了从物理的角度来理解四种自由表面形式的MHD效应的现象。     

Coherent light scattering spectroscopy for supersonic flow measurements

A unified discussion on coherent light scattering spectroscopy and its spectral dependence on molecular gas velocity, temperature and density is presented. The feasibility of using coherent Raman spectroscopy techniques for nonintrusive measurements of supersonic flow parameters is demonstrated and recent laboratory as well as wind tunnel experiments are reviewed. In addition, the advantages of using coherent Rayleigh-Brillouin spectroscopy for measuring high pressure flows are discussed.     

Absorption and Scattering of Light by Highly Concentrated Two‐phase Flows

The spray cone emerging during an extended metal atomization process (called spray forming) has been investigated in order to quantify the influence of highly concentrated multiphase flows on phase‐Doppler‐anemometry (PDA) measurements. Using this non‐intrusive, optical measurement technique not only the local particle size and velocity distributions of the spray can be obtained but also additional information about the mass flux in the multiphase flow. Since standard phase‐Doppler systems can be easily applied to low concentrated particle systems (spherical particles with smooth surfaces and an optical transparent continuous phase taken for granted) the application of this measurement technique to highly concentrated multiphase flows is more complex. Both the laser light propagating from the PDA device to the probe volume and the scattered one going backward to the PDA receiving system are disturbed by passing the highly concentrated multiphase flow. The resulting significant loss in signal quality especially concerns the measurement of the smaller particles of the spray because of their reduced silhouette (in comparison with the bigger ones). Thus, the detection of the smallest particles becomes partially impossible leading to measurement of a distorted diameter distribution of the entire particle collective. In this study the distortions of the measured distributions dependent on the particle number concentration as well as on the path length of the laser light are discussed.     

Velocity fluctuations in fluidized suspensions probed by ultrasonic correlation spectroscopy

Velocity fluctuations in a fluidized suspension of particles are investigated using two new ultrasonic correlation spectroscopies: diffusing acoustic wave spectroscopy and dynamic sound scattering. These techniques probe both the local strain rate and rms velocity of the particles, providing important information about the spatial extent of velocity correlations. Our results demonstrate the power of these techniques to probe particle dynamics of fluidized suspensions, and suggest that the velocity correlations are essentially independent of Reynolds numbers for Re(p)<1.     

表/界面水的扫描探针技术研究进展

表面和界面水在自然界、人们的日常生活以及现代科技中无处不在.它在物理、化学、环境学、材料学、生物学、地质学等诸多基础学科和应用领域起到至关重要的作用.因此,表面和界面水的功能与特性的研究,是水基础科学的一项核心任务.然而,由于水分子之间氢键相互作用的复杂性,及其与水-固界面相互作用的竞争,使得表(界)面水对于局域环境的影响非常敏感,往往需要深入到分子层次研究其微观结构和动力学过程.近年来,新型扫描探针技术的发展使得人们可以在单分子甚至亚分子尺度上对表(界)面水展开细致的实空间研究.本文着重介绍几种代表性的扫描探针技术及其在表(界)面水体系中的应用,包括:超高真空扫描隧道显微术、单分子振动谱技术、电化学扫描隧道显微术和非接触式原子力显微术.此外,本文还将对表(界)面水扫描探针技术研究面临的挑战和未来发展方向进行了展望.