不同介质下涡街流量计计量性能分析

本文针对涡街流量计在不同介质下的计量性能,依据流体力学,通过理论分析得出对于可压缩流体介质须考虑其可压缩性对涡街流量计的影响,为此选用四台不同口径的涡街流量计,在雷诺数相似准则指导下进行了实流标定实验,得到水、空气和蒸汽介质下的标定仪表系数,结果表明：介质可压缩性和涡街流量计几何尺寸是影响涡街流量计计量性能的重要因素,不同介质下涡街流量计的标定仪表系数不同,不可压缩流体水介质下的标定系数不能直接应用到可压缩流体蒸汽介质上,须进行修正,介质可压缩性和涡街流量计几何尺寸可作为修正因子。     

利用涡街流量计测量油水两相流流量

为了考察涡街流量计在油水两相流中的测量特性,在内径为50 mm的垂直上升管道内,对不同混合流量、含油率下的涡街信号进行了实验测量,并对油水两相仪表系数和斯特劳哈尔数予以分析.结果表明,在含油率5%～40%内,仪表系数相对误差小于4%,斯特劳哈尔数相对误差随含油率增加有变大趋势、随混合流量增加有减小趋势,而且在两相雷诺数2×104～5×104内可视为常数,并随雷诺数降低而升高.实验说明油水两相流中存在稳定的两相涡街,利用涡街流量计测量油水两相混合流量具有可行性.     

涡轮流量计在不同粘度介质下标定曲线形态的实验研究

涡轮流量计的准确度受被测介质及其运动粘度变化的影响。使用体积流量和仪表系数无法从变粘度实验中取得形态一致且可预测的标定结果。应用量纲分析导出雷诺数和斯特劳哈尔数作为描述涡轮流量计性能的无量纲参数。通过改变丙二醇-水溶液的体积浓度得到5个不同运动粘度的介质,分别用于标定一台DN25涡轮流量计。对比结果表明,不同粘度下的标定曲线在雷诺数小于7 400区域出现分离,标定数据最大相差0.9%。随着雷诺数增加,仪表系数中轴承阻滞部分的影响相对减小,标定曲线簇由分散趋于聚拢,标定数据差异小于0.1%。叶片表面的流动边界层发生层湍转捩时阻力的突变导致标定曲线出现驼峰,运动粘度越低,驼峰趋于平缓。轴承阻滞中的静态阻力部分是造成相同雷诺数下仪表系数差异的主要原因,这种差异随雷诺数减小而增加,所以,当校准介质和工作介质的运动粘度有显著差异时,涡轮流量计要避免工作在低雷诺数区域。     

基于LBM方法的钝体绕流数值模拟

在生活中高雷诺数的钝体绕流现象普遍存在,但准确计算其流场特性却并不容易。针对这一问题采用多松弛时间格子Boltzmann方法(LBM)与壁面自适应局部涡黏(WALE)模型相结合的方法(MRT-LBM-WALE),对定常流下雷诺数90 000的二维圆柱绕流进行数值模拟,同时应用增强壁面函数对壁面附近湍流黏性进行修正,测算了其阻力系数、升力系数、涡脱落频率和表压。计算结果与已有实验比较表明,阻力系数、升力系数以及涡街的脱落频率均跟实验值吻合较好,同时在涡街捕捉上也表现出较高的稳定性和精度。     

智能涡街流量计的设计与实现

涡街流量计众多的优点使其跻身通用流量计之一,但是当流速较低时,会存在测量不稳定的缺点。针对涡街流量传感器的非线性特性,本系统采用16位微处理器M SP430F435为核心,将传感器特性曲线存储在F lash存储器中,实现了对涡街传感器的非线性校正,从而扩大了涡街流量计的使用范围。同时该仪表还具有频率输出、4—20m A电流输出以及R S485通信功能,便于和传统仪表兼容。     

涡街流量计在蒸汽计量中的特性研究

蒸汽流量量值体系的溯源是保证蒸汽流量测量准确的关键。本文基于流体力学、热力学以及涡街流量计旋涡的产生机理,分析不同介质对涡街流量计的计量特性的影响,介质粘度的不同导致了三种介质测试下雷诺数的不同,影响到斯特劳哈数差异。但对涡街流量计的仪表系数影响不大,可忽略其影响。介质粘度的不同会导致流量范围的不同。该分析将有利于提高涡街流量计测量蒸汽流量的计量准确度。     

基于介质可压缩性的涡街流量计仪表系数修正

基于涡街流量计基本原理,结合流体力学分析了介质可压缩性对涡街流量计计量特性的影响。考虑到涡街流量计流动截面突然改变导致流体介质参数发生改变的现象与差压式流量计有相似之处,类比标准孔板的介质可膨胀性系数经验公式提出基于介质可压缩性的涡街流量计仪表系数修正数学模型。通过最小二乘拟合对实流标定实验数据进行数值分析得到仪表系数的可压缩性修正公式。最后分析了修正仪表系数的误差和不确定度,最大误差为-0.64%,相对扩展不确定度均在1%以内。研究结果对采用蒸汽介质的涡街流量计仪表系数跨介质标定具有指导意义。     

低流速涡街信号的数字滤波技术研究

通过分析涡街信号特点,对低流速涡街信号提出一种基于数字滤波理论的方法来进行信号处理。该算法采用短时傅里叶分析初步判断涡街信号所处的频段范围,然后从多组数字带通滤波器中选取最合适的一组进行滤波,从而统计涡街信号的频率。介绍了数字滤波主要参数的选择方法,并全面论述数字滤波的实现方式。实验结果表明,该方法有很强的去噪声能力与低流速测量能力。并且该算法严格控制了CPU的计算量,便于一般单片机应用系统的实现。     

基于小波变换的涡街流量计信号处理方法

涡街流量计有许多优点,应用比较广泛。但是,涡街流量计易于受到由管道振动和流场扰动引起的噪声干扰。涡待流量计中的处理电路不能保证仪表在工业现场的测量精度。本文研究基于小波变换的涡街流量计信号处理方法。本文介绍小波变换的基本原理和快速算法,分析小波滤波器的幅频特性,研究调整滤波器中心频率的方法,给出涡街信号的处理过程,进行仿真和实验测试。仿真和实验结果表明,小波变换能有效地减小了噪声影响,使频率测量的精度高,处理实时。小波变换是涡街流量计信号处理的一种新方法。     

基于CFD的超声波流量计最优声道位置研究

时差法超声波流量计圆形管道内不同声道位置上流体平均流速与管道截面平均流速的关系已被广泛研究,但是对于非圆形测量管体尚缺乏相应报道.本文针对某种方形管道,采用计算流体动力学(CFD)方法获得管道内的流场分布,进而通过数值计算得到时差法超声波流量计不同声道上K系数(即超声波传播路径上流体平均流速与管道截面流体平均流速之比)随雷诺数的变化关系,并选取K系数随雷诺数变化最小的声道作为最优声道,研究结果有利于简化流量补偿计算.     

Comparison of flow characteristics around an equilateral triangular cylinder via PIV and Large Eddy Simulation methods

The flow structures around an equilateral triangular cylinder, which is commonly used as a vortex shedder in the vortex flowmeter, were investigated experimentally and numerically. Flow characteristics such as vorticity contours, patterns of sectional streamlines, velocity vectors, velocity fields, Reynolds stress correlations, Strouhal numbers and drag coefficients were examined using the Particle Image Velocimetry (PIV) technique and the Large Eddy Simulation (LES) turbulence model. Experimental studies were performed in an open water channel for Re=2.9×10³, Re=5.8×10³ and Re=1.16×10⁴ based on the equilateral triangle edge. A sharp-tip corner of the cylinder with a triangle cross-section was exposed to the upstream side while the other two sharp-tip corners were placed on the downstream side. Numerical studies were also completed at Reynolds numbers in the range of 2.9×10³≤Re≤1.16×10⁵ to obtain the changes in the Strouhal numbers and drag coefficients. When the results of PIV and LES are considered in the same interval of Reynolds numbers, the maximum and minimum values of each flow pattern were nearly the same. The time-averaged patterns had considerable symmetry with respect to the axis line passing through the sharp-tip corner of the cross-section of the triangular cylinder. The Strouhal number was independent of the Reynolds number and was found to be approximately 0.22. The drag coefficient decreased with increasing Reynolds numbers while increasing the Power Spectral Density (PSD) and the vortex shedding frequency. For the same Reynolds numbers, the experimental and numerical results were in good agreement. Therefore, the LES turbulence model is recommended for applications of flow around this type of bluff body that is generally used in the design of vortex flowmeters to generate vortex shedding.     

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.     

Vortex wake transit time measurements for flow metering

A method of flow measurement is described based on using a vortex wake as a flow tracer shed from a low-blockage-ratio bluff body, the velocity of which is measured by cross-correlation. Preliminary comparisons are made between measurement iof the vortex shedding frequency and the vortex wake transit time between the two ultrasonic beams to determine the flow rates. Whilst preliminary results are confined to a 50 mm diameter sensing head, there is no upper limit to pipe size using the same suitably extended bluff body.     

Response of a dual triangulate bluff body vortex flowmeter to oscillatory flow

In this paper, on an experimental facility, the measurement characteristics of a diameter 50 mm dual triangulate bluff body vortex flowmeter in steady flow and oscillatory flow were investigated. Then, the Hilbert Huang Transformation (HHT) method was used to assess the anti-interference performances and the vortex street stability in oscillatory flow for the dual triangulate bluff body vortex flowmeter and a single bluff body vortex flowmeter. Offline simulation was carried out on the anti-interference performances of the dual triangulate bluff body vortex flowmeter signal noise in oscillatory flow by the method of the EMD-scales filter. The major findings are: (a) in most case, the EMD-scales filter may be as good at de-noising effect for the dual bluff body vortex flowmeter in oscillatory flow than that for the single bluff body vortex flowmeter in oscillatory flow. The vortex street stability in oscillatory flow for the dual bluff body is similar to that for the single bluff body. (b) In some special case, the EMD-scales filter is unable to play a better de-nosing role for the dual bluff body vortex flowmeter in oscillatory flow. The invalid condition of the EMD-scales filter for the dual bluff body vortex flowmeter in oscillatory flow is different to that of the single bluff body vortex flowmeter and it was advanced in this paper. (C) The vortex street stability for the dual bluff body vortex flowmeter is better than that for the single bluff body vortex flowmeter.     

Numerical simulation of flow past rectangular cylinders with different aspect ratios using the incompressible lattice Boltzmann method

This paper presents a numerical study of a uniform flow past a rectangular cylinder using the incompressible lattice Boltzmann method (ILBM). Firstly, we use the ILBM to simulate the flow past a square cylinder symmetrically placed in a two-dimensional channel and results are validated against the well-resolved results obtained using finite-difference method and finite-volume method. Secondly, the effects of the aspect ratio defined as R = width/height on the fluid forces, vortex shedding frequency and the flow structures in the wake are investigated. Aspect ratios ranging from 0.15 to 4.00 and four Reynolds numbers Re = 100, 150, 200 and 250 are selected for the investigation. The results show that the effects of aspect ratio on physical quantities such as drag and lift coefficients, Strouhal number and the vortex shedding mechanism are very notable in the range between 0 and 2. In general, the drag coefficient decreases with the aspect ratio and the decreasing rate is more distinct in the range of 0.15 ≤ R ≤ 2.0. There is no local maximum found at around R = 0.6 in the drag coefficient as reported for higher Reynolds numbers in the literature. However the root-mean-square value of the lift coefficient shows a maximum value at R ≈ 0.5 for all Reynolds numbers selected. The variation of Strouhal number with R appears to be different for four selected Reynolds numbers. Especially for Re = 250, a discontinuity in St, as has been observed for higher Reynolds numbers, is observed at around R = 1.45 where multiple peaks are found in the result of Fourier spectrum analysis of the lift force and irregular vortex shedding behavior with no fixed shedding frequency is observed from the instantaneous vorticity contours. Such discontinuity is not observed for Re = 100, 150 and 200. The present results using the LBM are compared with some existing experimental data and numerical studies. The comparison shows that the LBM can capture the characteristics of the bluff body flow well and is a useful tool for bluff body flow studies.     

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.     

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.     

Control of laminar vortex shedding behind a circular cylinder using tabs

Small, thin flat plates (called tabs hereafter) are attached to the upper and lower surfaces of a circular cylinder to control vortex shedding and reduce the mean drag and lift fluctuations at the Reynolds number of 100. We vary the location and size of the tabs and the distance between the adjacent tabs. The maximum amount of drag reduction by the tabs is 17%. It is found that the tabs perturb twodimensional vortex shedding and introduce spanwise mismatch of vortex shedding, which weakens the strength of vortex shedding or even annihilates vortex shedding. The present result suggests that these tabs are an effective passive device for the control of vortex shedding behind two-dimensional bluff bodies.     

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%.     

Numerical investigation of wake flow control by a splitter plate

Unsteady separated flow around a square cylinder is simulated by using vortex tracing method to investigate the wake flow control by a splitter plate attached to the base of a bluff body. The numerical method is evaluated with selected numerical parameters for the case without the splitter plate. Then the method is applied to computations for different splitter plate lengths. Instantaneous flow patterns are scrutinized to see how the splitter plate affects the vortex formation behind the body and the downstream shedding. It is confirmed that the drag and the frequency are significantly reduced by the splitter plate, suppressing vortex shedding in the wake.