A new approach to detection of vortices using ultrasound

The measurement principle of vortex flowmeter is based on von Karman vortex shedding phenomenon. Frequency of vortices, behind the bluff body, is proportional to the mean flow velocity. There are different ways of detection of vortices, and different sensors are used (presser sensors, capacitive sensors, thermo-resistance sensors, ultrasonic sensors, etc.). Proposed method to vortex identification, presented in this paper is based on simultaneous detection of pair of vortices with opposite circulation, by means of two pairs of ultrasonic transducers. A beam of ultrasound, from ultrasonic transmitter to ultrasonic receiver is transmitted perpendicularly to the vortex street. The received ultrasonic signal is amplitude and phase modulated. Frequency of demodulated signal is equal to the frequency of vortices. This technique allows a number of advantages comparing to conventional solutions: reduction, or elimination of noises caused by installation vibration and disturbances in the flow, higher sensor sensitivity, which as a result leads to a possibility of a reduction of the bluff body size, i.e. reduction of the pressure drop on the flow meter, increase of the measurement range in the low flow region, the possibility of redundant operation of the flow meter, reduced measurement uncertainty, instrument technology improvements, improved reliability of the instrument, assured improved statement of complete uncertainty contributions, improved metrology of the equipment as such and calibration procedures that contribute to measuring capabilities etc. For experimental testing a prototype vortex flowmeter of a nominal inner diameter (ID) 50 mm is developed. A cylindrical bluff body for vortex shedding is used. Ultrasonic transducers based on piezo-crystal PZT-5A, inserted in the wall of the vortex meter casing are utilized. The testing of prototype ultrasonic vortex flowmeter is realized on the calibration station on the water. The results at the testing point to the possibility of measuring flow of liquid fluids at velocities less than 0.5 m/s, with an uncertainty better than ±1%.     

双钝体涡街流量计流体振动特性的试验研究

研究了双钝体旋涡脱离的流体动力学特性以及利用双钝体增强流体振动的最佳组合结构。给出了多种钝体组合结构的斯特劳哈尔数的试验结果,表明双钝体结构具有常定的斯特劳哈尔数。用50 mm口径涡街流量计进行的试验证明,双钝体组合在一定的条件下能获得理想的旋涡重叠,从而增强流体的振动。在钝体的轴对称点上,还观察到强度和频率相同,相位相差180°的流体振动点,利用优化的双钝体结构和差动传感技术,研制出小流量灵敏度和抗干扰性能很好的新型旋涡流量计。     

Experimental verification and numerical simulation of a vortex flowmeter at low Reynolds numbers

This study presents experimental verification and numerical simulations of a vortex flow meter in the Reynolds number range between 8300 and 50,000. A custom-designed bluff body with a wedge back shape was used in the flowmeter. A shedding frequency of the flowmeter was measured in an air duct using a hot-film probe. To evaluate the accuracy of the flowmeter, a measurement uncertainty analysis was performed. Numerical simulations of the vortex flowmeter were performed with the open source code OpenFOAM. Transient simulations of periodic vortex shedding behind the bluff body were performed using different simulation methods depending on the pipe Reynolds number, such as Direct Numerical Simulation (DNS), Large Eddy Simulation (LES) and Unsteady Reynolds Averaged Navier Stokes (URANS) method. The simulated vortex shedding frequencies matched the experimental data very well. Experiments and simulations demonstrated a clear linear dependence of the shedding frequency on the volumetric flow rate over the entire range of Reynolds numbers. In addition, numerical simulations were used to study the main mechanisms of vortex formation and shedding behind the considered bluff body.     

Computationally lean algorithm of novel optimal FIR adaptive filter for vortex signal extraction

In a vortex flowmeter vortices are generated by a bluff body, inserted in the path of flow, which has a piezoelectric sensor embedded in it. This piezosensor develops a signal having a fundamental frequency that is proportional to flow. The flow measurement relies completely on extraction of true vortex signal and estimation of the correct frequency. A novel adaptive FIR filter has been designed and implemented using low power computational resource (8.25 mW), which gives better results than an existing contemporary system when tested on an industrial flow rig. Further more a comparative study of autocorrelation, EMD Scales filter and proposed algorithm is carried on the good and bad vortex signals. From this comparative study it is seen that proposed algorithm is effective for bad vortex signals and low flowrates where vortex signals are weak.     

Research on signal amplitude of the Kármán vortex street in gas–liquid two-phase flow with high void fraction

Based on Biot–Savart law and single-phase flow Kármán vortex characteristics, flow field has been analyzed when gas–liquid flow past a fixed bluff body with high void fraction. Vortex signal characteristics have been studied for stratified two-phase flow on atmospheric conditions in a horizontal pipe. To discuss the relation between void fraction and vortex signal amplitude spectrum, this paper sets up the vortex-induced pressure field model for gas–liquid two-phase flow and gives the relationship between void fraction and relative amplitude spectrum of two-phase flow to single-phase flow. An algorithm is proposed for predicting the two-phase flow parameters. Experiments were performed using air–water as working fluid along with a test tube diameter of 50 mm, at gas volume flow rate of 20–68 m³/h, and void fraction of 0.9–1. The results indicate that calculations by the vortex-induced pressure field model on the amplitude spectrum of vortex signal are in good agreement with the experimental data, and relative errors of the algorithm predictions on gas volume flow rate and liquid volume flow rate are 0.08 and 0.56, respectively.     

Influence of blockage and shape of a bluff body on the performance of vortex flowmeter with wall pressure measurement

Experimental and numerical investigations are carried out with various bluff body shapes to identify an appropriate shape which can be used for vortex flowmeter application. In both the cases vortex shedding frequency is inferred from the fluctuation of wall pressure. The numerical simulations are carried out with cylindrical and triangular bluff bodies to understand the vortex shedding phenomenon and to identify an appropriate turbulence model for this class of flows with wall pressure measurement. The simulations reveal that the k-ε RNG model predicts the Strouhal number closer to the experimental results than other models. The experimental investigations are carried out with several bluff body shapes, such as triangular, trapezoidal, conical, cylindrical and ring shapes, with water as the working medium. In this context, the effects of sampling rate, tap location and blockage effects are explored. The results suggest that the axisymmetric tapping is better than differential pressure tapping in terms of signal amplitude. The non-dimensional location of the static pressure tap is found to be 0.714 times diameter of pipe times blockage. The trapezoidal bluff body is found to be the best among all the bluff bodies investigated in terms of signal amplitude and constancy of Strouhal number. The vortex flowmeter performance is also measured under disturbed flow conditions created by using gate valve and bends. These results are significant because they provide an optimum bluff body shape and blockage, and also present the performance of vortex flow meter under disturbed flow conditions which is rather seldom reported in the literature.     

差压式涡街质量流量计取压位置的研究

在涡街流量计中，流体通过涡街发生体后会产生压力损失及由旋涡引起的压力波动，根据这一特点，本文提出利用差压检测技术，通过单路差压传感器同时感受由涡街发生体引起的流体双重变化特性，测量流体质量流量的新方法。本文重点对差压检测取压位置进行研究，利用空气和水两种流体介质进行了一系列实验，得到不同取压位置的差压信号与流量关系，确定了能正确测量质量流量的差压取压位置。结果表明，该测量方法结构简单，是测量质量流量的有效方法。     

双钝体涡

涡街流量计测量应用广泛,但小流量信噪比低、抗干扰性能差.本论文提出以双钝体组合强化流体振动,从而降低涡街流量计的计量下限,提高其抗干扰性能.通过实验研究获得了优化的双钝体组合,表明双钝体组合能达到强化流体振动和提高流量计信噪比的目的.     

低含气率气液两相流涡街特性研究

气液两相涡街现象广泛存在于生产实际中,由于相间作用,研究变得较为复杂。通过数值模拟和实流试验,对水平管低含气率情况下气相对两相涡街的影响进行研究。建立三维计算流体力学（Computational fluid dynamics, CFD）仿真模型,采用VOF多相流模型和RNG k-e 湍流模型,模拟气液两相钝体绕流。仿真与试验结果在定量频率上具有良好的一致性。结合仿真与试验结果,从近尾迹流场与涡拓扑、斯特劳哈尔数两方面,对不同含气率两相涡街特性进行对比研究。结果表明,在稳定涡街范围内,随着气相含率增大,对涡街起重要作用的滞止区长度增大,导致斯特劳哈尔数线性减小;同时,涡街周期性和信号质量变差,涡街能量降低,这是由于涡中心吸入密度小、速度大的气泡,造成涡的旋转能量降低,进而影响涡街的稳定性。     

Vortex shedding mechanisms in single and dual bluff bodies

This paper presents studies of vortex shedding mechanisms in a single bluff body and two dual bluff body combination. These studies are based on video imaging of vortex shedding in open channel flow. Three thousand frames were analysed for each geometry under specified flow conditions and used to derive statistical distributions for the periods of individual cycle. For each of the combinations, the vortex mechanism for a typical half cycle is presented in six stages. The differences in mechanisms between the three geometries are highlighted and used to explain the differences in the statistical distribution of cycle periods.     

Influence of blockage and upstream disturbances on the performance of a vortex flowmeter with a trapezoidal bluff body

Experimental investigations are conducted on vortex flowmeter with the differential wall pressure measurement method. The bluff body employed is trapezoidal in shape and water is used as the working fluid. Three different blockages (0.14, 0.24 and 0.3) are considered in this study. The performance of the vortex flowmeter is studied both under fully developed condition and in the presence of flow disturbances. The flow disturbance is created using 45° swirl generator and gate valve placed at different upstream distances. The performance of the flowmeter is also evaluated in the presence of a Laws Vanes flow conditioner placed downstream of the swirl generator. The blockage ratio of 0.3 is found to be the best among all the blockages studied under both disturbed and undisturbed conditions.     

Design conditions for optimal dual bluff body vortex flowmeters

Examination of the performance of a large number of dual bluff body combinations leads to the conclusion that optimum repeatability of vortex shedding may be obtained with combinations satisfying certain basic conditions. One possible condition is the coincidence of the positions of maximum vortex strength for the individual bluff bodies which make up the combination. The aim of this paper is to describe the results of a test of the above principle using rectangular bluff bodies. The first stage is to find the mathematical relationship between the position of maximum strength and the depth of a single rectangular bluff body. The next stage is to use this relationship to find which combinations satisfy the above coincidence condition. These ‘optimal’ combinations were then tested against a number of ‘non-optimal’ combinations as controls. The overall conclusion is that the above condition is just one of five conditions underlying the design of optimum dual bluff body vortex flowmeters.     

Development of side wall type permanent magnet flowmeter for sodium flow measurement in large pipes of SFRs

Sodium cooled Fast Reactors (SFR) require measurement of liquid sodium flow in its primary and secondary circuits. For the primary system of the pool type concept of SFR design, flowmeters have to be immersed in sodium pool and require flow sensors which can withstand high temperatures up to 550 °C, nuclear radiation and chemically reactive sodium environment. Secondary circuits and safety grade decay heat removal (SGDHR) circuits of SFR need flow measurement in stainless steel (SS) pipes of diameter varying from 15 mm to 800 mm. For small pipes, flowmeters with permanent magnet flowmeter with ALNICO-V magnet assembly is the unanimous choice. Conventional permanent magnet flowmeters (PMFM) for large pipelines become bulky, heavy and have installation problems. For sodium flow measurement in large pipelines a few other alternate methods are considered. In the case of Prototype Fast Breeder Reactor (PFBR), which is at an advanced stage of construction at Kalpakkam, flow in the 800 mm diameter secondary main circuit is measured by means of a bypass flowmeter. Other sensors that could be deployed include eddy current flowmeters (ECFM), which are introduced into the pipe to measure flow velocity in the pipe, ultrasonic flowmeters and permanent magnet based side wall flowmeters. In permanent magnet based side wall flowmeter (SWFM), a permanent magnet block is mounted on one side of the large pipe and the magnetic field produced by the magnet penetrates through the pipe and interacts with the flowing sodium and induces an electro motive force (emf) proportional to the flow. This is a compact, cost effective and fairly accurate method for flow measurement in large pipelines of SFR circuits. SWFM is suitable for pipelines of 100 mm and above. In the present work a side wall flowmeter for 100 mm pipe is designed, manufactured, calibrated and tested in an existing sodium facility. Voltage signal developed in SWFM for different flowrates was simulated with three dimensional Finite Element Model (FEM) and validated with experimental results. Effect of asymmetric magnetic field on flowmeter voltage signal and dependence of flowmeter voltage signal on position of electrodes was also analyzed with model. The feasibility of use of this type of flowmeter for large pipelines of SFRs is demonstrated.     

低雷诺数下涡街流量计线性度的试验

在DN25水流量标准装置上对装有梯形漩涡发生体的涡街流量计在低雷诺数下进行试验研究,利用数字信号处理方法(频谱分析方法)得到涡街脱落频率,较传统模拟信号处理方法在保证较好线性度的基础上,有效地扩展了测量下限。绘制雷诺数—仪表系数曲线,发现梯形发生体在低雷诺数下仪表系数K呈递减趋势,当Re>2 000时K趋于常数,这与圆柱发生体在低雷诺数时曲线递增的趋势截然相反。通过分析得知,管道内流速分布的影响、发生体形状的影响和流速与频率关系的影响是导致上述变化趋势的3个主要影响因素。     

The mapping of vortex fields around single and dual bluff bodies

The development and use of a system to measure important parameters of the vortex field in a coordinate grid around several single bluff bodies and dual bluff body combinations are described. Two hot wire sensors were used: a reference sensor at a fixed position and a moving sensor with position coordinates adjusted by using computer control. The strength, frequency and regularity of the vortex shedding are found from the auto-power spectral density of the moving sensor signal. The phase difference between moving and reference sensor signals is found from the cross-power spectral density function between the signals. The results are presented as maps of vortex parameters plotted as a function of sensor position coordinates. The main features of these maps, including the enhancement of vortex shedding from certain dual bluff body combinations, is then discussed.     

Analysis of viscosity effect on turbine flowmeter performance based on experiments and CFD simulations

Viscosity effect is one important factor that affects the performance of turbine flowmeter. The fluid dynamics mechanism of the viscosity effect on turbine flowmeter performance is still not fully understood. In this study, the curves of meter factor and linearity error of the turbine flowmeter changing with fluid viscosity variations were obtained from multi-viscosity experiments (the viscosity range covered is 1.0×10^–6 m²/s–112×10^–6 m²/s). The results indicate that the average meter factor of turbine flowmeter decreases with viscosity increases, while the linearity error increases. Furthermore, Computational Fluid Dynamics (CFD) simulation was carried out to analyze three-dimensional internal flow fields of turbine flowmeter. It was demonstrated that viscosity changes lead to changes of the wake flow behind the upstream flow conditioner blade and the flow velocity profile before fluid entering turbine rotor blade, which affect the distribution of pressure on the rotor blades, so impact the turbine flowmeter performance.     

Upstream installation and misalignment effects on the performance of a modified electromagnetic flowmeter

The performance of a modified Danfoss 50 mm diameter electromagnetic flowmeter has been investigated when installed downstream from three different pipe diameters—50 mm, 55 mm and 45 mm. The effects of a 3 mm misalignment of the flowmeter, in both the vertical and horizontal planes, with respect to each of the three upstream pipe diameters has also been identified. The largest percentage errors are reported for the 45 mm upstream diameter pipe, with the flowmeter misaligned by 3 mm in the horizontal plane. The vertical and horizontal mean velocity and root-mean-square velocity profiles, measured within the flowmeter using laser Doppler anemometry, show significant variations in comparison with the ideal, fully developed profiles.     

基于EEMD-Hilbert谱的气液两相流钝体绕流流型识别

为了有效识别气液两相流的流型,以水和空气为实验介质,以涡街流量计为元件诱发钝体绕流,通过管壁差压法获取气液两相流钝体绕流的尾迹波动信号,采用集总经验模态分解对信号进行分解,通过Hilbert变换得到Hilbert边际谱,利用最大互相关系数法对固有模态函数进行筛选,选取特征固有模态函数能量比分别与体积含气率、两相雷诺数构建流型图。结果表明,构建的两类新流型图对单相水、泡状流、塞状流、弹状流等典型流型的识别率分别可达91.67%和88.89%,能较好地满足工程实际应用的需求。     

Development of a gas micro-flow transfer standard

LNE has ability to calibrate gas micro-flow rates using the dilution method for nitrogen flow rates in the range from 2 µg/s to 200 µg/s or helium ones in the range 0.75–30 µg/s. In addition, a primary constant pressure flowmeter for leak rate measurements from 0.05 µg/s to 35 µg/s is also available. This equipment will be used to validate the dilution method below 30 µg/s. In order to compare these reference facilities, LNE is developing a micro-flow transfer standard (µFTS) in collaboration with ATEQ France, a manufacturer of control equipment for leak testing. The flowmeter consists mainly of an array of three stainless steel capillaries designed to cover the ranges from 0.035 µg/s to 0.35 µg/s, 0.35 µg/s to 3.5 µg/s and 3.5 µg/s to 35 µg/s for nitrogen (0.1–100 ml/h). A dynamic model of the µFTS determines the mass flow rate from the input pressure, the differential pressure of the capillary, the gas temperature, viscosity and density and the length and radius of the capillary. A comparison of both reference methods is carried out with the µFTS from 0.35 µg/s to 35 µg/s.     

An investigation of friction and wear performances of bonded molybdenum disulfide solid film lubricants in fretting conditions

The friction and wear performances of bonded MoS₂ solid film lubricants with the counterpart steel ball rubbing were investigated in fretting wear conditions in order to inquire into the load-carrying capacity and wear mechanisms of bonded MoS₂ solid film lubricants under dry friction conditions. Experimental results show that the bonded MoS₂ solid film lubricants have excellent anti-friction and wear-resistance performances within a wide load range between 20 N and 800 N and within a wide oscillatory frequency range between 5 Hz and 30 Hz. It is found through analyses of the transfer films formed in the surface of the counterpart steel ball investigated by SEM, XPS and AES, that the thickness of the transfer film formed is about 38 nm and the oxidation of MoS₂ in the transfer films does not occur during dry friction process. The high load and frequency promote the formation of a compact transfer films. The compact transfer films are believed to be the predominant mechanism giving rise to high load-carrying capacity, and excellent wear-resistance performances of the bonded MoS₂ solid film lubricants.