电磁流量计在氧化铝生产中的应用

本文介绍了氧化铝工艺流程及流程物料特点,电磁流量计工作原理结构特点,介绍了电磁流量计在氧化铝生产中的应用状况,指出应用过程中存在的问题及解决办法。     

Electromagnetic flowmeter evaluation in real facilities: Velocity profiles and error analysis

The reliability and accuracy of a flowmeter depends on its installation conditions, as an improper installation may distort the end results. The objective of this work was to determine the errors obtained in an electromagnetic flowmeter of 100 mm diameter, installed in common real conditions, that is, not obeying the straight pipelines length recommended in flowmeter standards. To achieve this objective of the work, the electromagnetic flowmeter was installed in four different conditions: installed downstream of a 90° curve and downstream of two curves of 90°, both of short radius; downstream of a gate valve, openings of 100% and 50%. During the research, flow velocity profiles were obtained, with the purpose of evaluating the influence of profile distortion on the performance of the meter. The tests were carried out in the Laboratory of Hydraulics and Fluid Mechanics of FEC-Unicamp, Brazil, and the results are consubstantiated in comparative graphs, which demonstrate the influence of the velocity profile on the flowmeter response and accuracy for each setup bench.     

Study on a new electromagnetic flowmeter based on three-value trapezoidal wave excitation

Excitation technology plays an important part in the field of electromagnetic flowmeter (EMF). The form of the magnetic field directly determines the characteristics of the flow signal. No breakthrough has been made on the excitation technology since the invention of sinusoidal wave excitation, rectangular wave excitation, and double-frequency excitation. In this paper, firstly a new three-value trapezoidal wave excitation with transient responses is proposed. Then the signal model is established, and the verification experiments are carried out. Finally, flow calibration experiments and comparative experiments on the trapezoidal wave excitation are conducted on the experimental platform. The experiment results show that the electromotive force output by the EMF based on three-value trapezoidal wave excitation is linearly related to the flow velocity. When the flow velocity is 0.257 m/s, the relative error is only 1.635%. When the flow velocity reaches 2.133 m/s, the relative error is reduced to 0.432%. The three-value trapezoidal wave excitation with the transient analysis satisfies the requirements of the EMF with high accuracy. The research also shows that the excitation frequency has a great influence on the measurement accuracy of the EMF based on three-value trapezoidal wave excitation.     

Relative errors in evaluating the electromagnetic flowmeter signal using the weight function method and the finite volume method

In the signal analysis of an electromagnetic flowmeter, the weight function method (WFM) involves a numerical integration of products between the magnetic flux density vector, the weight function and the velocity profile function in two-dimensional analysis. The electromagnetic flowmeter signal can also be predicted by solving the governing flowmeter equation by the finite volume method (FVM). The present study is aimed at comparing the calculation accuracies of WFM and FVM. The test flows are fully developed laminar and turbulent flows in a straight pipe under uniformly distributed magnetic field. The calculation accuracies are also compared between different O-type grid systems. It was found that the accuracy of both methods depended strongly on the grid system. In particular, for turbulent flow whose axial velocity component changes very rapidly near the wall, the accuracy of WFM was found to depend strongly on the cell configuration near the electrodes.     

Calculation of the sensitivity of a straight tube Coriolis mass flowmeter with free ends

The weight vector theory for Coriolis mass flowmeters is applied to a simple theoretical meter configuration consisting of a single unsupported straight tube unattached to adjacent piping. The tube has free ends and vibrates in the fundamental mode. It is shown how the sensitivity of this meter depends in part on the interaction of the flow velocity profile with fluid vibrations occurring near the tube ends. This end effect is negative i.e. the meter reads lower than would be expected if end effects were ignored. On account of the end effect there is a predicted variation in sensitivity with Reynolds number (of the order 1% in a tube 25 diameters long) and this can be minimised by a certain choice of the sensor positions.     

Influence of cone angle on the performance of a wafer cone flowmeter

The present study explores the effect of upstream disturbances like a single 90°bend, double 90° bends (in plane and out of plane) on the performance of wafer cone flowmeters with same beta ratio (β) of 0.77 but different half cone angles (α) of 30° and 45°. The influence of these disturbances on the upstream and downstream axial velocity (u) profiles are studied experimentally. The orientation effects, if any, are also studied experimentally. The minimum upstream distances required to get a fully developed flow for these disturbances vary with type of upstream disturbance, beta ratio (β) and half cone angle (α) of the wafer cone flowmeter. The study is carried out for a single phase flow with air as working medium at high Reynolds number (Re_D = 144000). From the results obtained from this study, it may be concluded that the wafer cone flowmeter with a beta ratio (β) of 0.77 and a cone angle of 30° requires less upstream distance compared to the wafer cone flowmeter with a beta ratio (β) of 0.77 with a cone angle of 45° for all the disturbances under consideration.     

Three-dimensional MHD simulations of the electromagnetic flowmeter for laminar and turbulent flows

The interaction between an electrically conducting fluid and an external magnetic field in an ideal cylindrical electromagnetic flowmeter is numerically investigated for both laminar and turbulent flows. Induced electric potential in the fluid, and the difference in potential at the measuring electrodes are directly obtained by including MHD effects in the CFD simulations. Fully developed laminar and turbulent flows are simulated. The computed electric potential difference on the electrodes agrees with analytical values for small Hartmann number cases, where the induced Lorentz force is small. Turbulent flow produces a more uniform electric potential distribution in the flow meter cross-section than laminar flow. These integrated MHD/CFD simulations couple the MHD effect with flow dynamics without deriving a weighting function with an assumed velocity profile, which will be necessary for electromagnetic flow meters when the Hartmann number is not small.     

Effects of flow disturbance on an ultrasonic gas flowmeter

A series of tests are carried out to assess the effects of flow disturbance on a small dimension ultrasonic gas flowmeter. Flow disturbances generated by cone couplings, and single and double elbows are investigated. Measurements with a 100 D straight pipe upstream with a smooth connection to the meter body are used as a reference. Our measurements show that the symmetrical disturbance produced by a cone coupling at a 12 D distance from the transducer path does not impair the performance of the flowmeter. An asymmetrical disturbance, such as a single or a double elbow at the same distance, seems generally to give an underestimation of the flow velocity, resulting in reading errors of −1% or worse. Measurements with straight pipes of 10 D, 20 D, 40 D and 80 D between the disturbance and the flowmeter have also been made showing that 10 D can cause an overestimation of flow velocity. Increasing the length of the straight pipe generally decreases the error. More than 80 D straight pipe between the disturbance and the flowmeter is required to give a result within ±1% of reference conditions. The angle between the elbow plane and the transducer plane is changed from 0 to 315° in 45° steps. The meter error is plotted as a function of inlet angle, showing a clear relationship between these values.     

Flow measurement with an electromagnetic flowmeter in two-phase bubbly and slug flow regimes

In order to investigate the characteristics of an electromagnetic flowmeter in two-phase flow, an alternating-current electromagnetic flowmeter was designed and manufactured. The signals and noise from the flowmeter under various flow conditions were obtained, and analyzed in comparison with the flow patterns observed with a high-speed charge-coupled device camera.

An experiment with void simulators, in which a rod-shaped non-conducting material was used, was carried out to investigate the effect of bubble position and void fraction on the flowmeter. Two-phase flow experiments, encompassing bubbly to slug flow regimes, were conducted with a water–air mixture.

The simple relation ΔU_TP=ΔU_SP/(1−), relating the flowmeter signal between single-phase flow and two-phase flow, was verified with measurements of the potential difference and the void fraction for a bubbly flow regime. Due to the lack of homogeneity in a real two-phase flow, the discrepancy between the relation and the present measurement increased slightly with increasing void fraction and superficial liquid velocity j_f.

Whereas there is no difference in the shape of the raw signal between single-phase flow and bubbly flow, the signal amplitude for bubbly flow is higher than that for single-phase flow at the same water flow rate, since the passage area of the water flow is reduced. In the case of slug flow, the phase and the amplitude of the flowmeter output show dramatically the flow characteristics around each slug bubble and the position of the slug bubble itself. Therefore, the electromagnetic flowmeter shows a good possibility of being useful for identifying the flow regimes.     

On the effect of the electrode shape and contraction section on the right-angled electromagnetic flowmeter

Flow measurement is of great importance in many industries, such as the chemical industry, energy and urban drainage. Electromagnetic flowmeter is widely used due to its wide range, high response and suitability for multiphase flow detection. To meet the needs of small-caliber and low-flow applications and wider installation scenarios, this paper proposes a new insertion-type right-angled bent electromagnetic flowmeter. The measurement signal of the electromagnetic flowmeter is closely related to its internal fluid flow state, thus computational fluid dynamics (CFD) method is used to optimize the electrode shape and contraction section shape of the electromagnetic flowmeter. Through comparison experiments of flat electrodes and cone electrodes, a better model is obtained. Furthermore, three new models of the contraction section are proposed: the sharp contraction type, the gradual contraction type, and the Widosinski curve contraction section. The Widosinski curve contraction section is easier to stabilize the flow field inside the flowmeter. The experimental data are in good agreement with the results of the large eddy simulation, and the results show that the measurement performance of the cone electrode is better than that of the flat electrode. When measuring small flow rates, the error of the cone electrode is only 5%, much smaller than the error of the flat electrode. The findings of this study provide a new idea for the development and optimization of the measurement performance of the right-angled electromagnetic flowmeters.     

Installation effects on ultrasonic and electromagnetic flowmeters: a model-based approach

The paper describes the basic ideas of a model-based installation-effects analysis method and presents results from a collaborative research programme being conducted at the National Institute of Standards and Technology (NIST) in Gaithersburg, MD, USA, and at Tampere University of Technology in Finland. The analysis method is based on a combination of flow modelling and flowmeter modelling. In this paper, installation effects on electromagnetic and travel-time-difference ultrasonic flowmeters are discussed. The installation cases are single elbow and double elbow out-of-plane piping configurations. The results show that significant shifts from ‘ideal’ meter performance can occur in such disturbed flow conditions. The flowmeter modelling results also show that these significant shifts can be reduced by altering the operational arrangements and performance characteristics of the meters. It is concluded that such flowmeter modelling can be very useful in: (1) redesigning flowmeters to make them less susceptible to installation effects, (2) constructing new meters, or (3) arranging special meter configurations to deal with specific disturbed flows.     

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.     

插入式电磁流量计的研制

文中介绍了插入式电磁流量计的特点以及它的设计难点,给出了设计方案和样机的初步试验结果.     

Measurement of high-water-content oil-water two-phase flow by electromagnetic flowmeter and differential pressure based on phase-isolation

When the oil field has been exploited by long-term water-flooding, it will be in high water-content stage of production. However, it is a great challenge for high-water-content measurement due to oil droplets extremely dispersed in the water. In this paper, we developed a phase-isolation based method for high-water-content oil-water two-phase flow measurement. Phase-isolation was realized by axial-flow swirler to concentrate scattered and random oil droplets into the pipe center and change the inlet flow pattern into a particular annular flow before measuring. Owing to the axisymmetric velocity and phase distribution, the electromagnetic flow meter avoided the effect of random distribution of insulating phase, and then had a good measurement performance for total volume flow rate. Furthermore, we respectively studied using axial pressure drop, radial pressure drop and the ratio of the two pressure drops to measure water content. The results showed that the ratio of the two pressure drops not only improves the resolution of oil and water, but also effectively reduces the impact of error transfer. In the dual-parameter measurement experiment, the relative errors of total volume flow rate and water content were almost within ±5%.     

Theoretical study of vertical slug flow measurement by data fusion from electromagnetic flowmeter and electrical resistance tomography

Electrical resistance tomography (ERT) can be used to obtain the conductivity distribution or the phase distribution of gas/liquid flows (e.g. slug flow). Using proper parameter models and flow regime identification models, the measurement of phase size, void fraction, and pattern recognition can be realized. Electromagnetic flowmeters have been used to measure conductive single-phase liquid flows. However, neither ERT nor electromagnetic flowmeters (EMF) can provide accurate measurement of gas/liquid two-phase flows. This paper presents an approach to fuse the information from ERT and an electromagnetic flowmeter. A model for the measurement signal from the electromagnetic flowmeter has been developed based on the flow pattern and the phase distributions, which are obtained from the reconstructed images of ERT, aiming to reduce the measurement error of the electromagnetic flowmeter and enhance the measurement accuracy. Through the simulation research of virtual current density distribution, the feasibility of fusion of electromagnetic flowmeter and ERT to measure gas/liquid two-phase vertical slug flow is verified. By theoretical analysis, the relationship between the output of electromagnetic flowmeter and flow parameters is established. The electrical potential difference of the electromagnetic flowmeter, average velocity, volume flow rate and gas void fraction between the bubble size and location are also investigated. The fusion approach can be used to measure vertical slug flows.     

Experimental and numerical study on a novel thermal mass flowmeter that is insensitive to radial flow velocity distribution

A novel thermal mass flowmeter (TMF) is proposed by improving the composition and structure of the probe in this study. An experimental setup was developed to compare the effects of installation angles on the measurement characteristics of the novel and traditional TMF (flow velocity range of 1.0–8.0 m/s), and three-dimensional numerical models were established to compare the effects of axial positions and insertion depths on the measurement characteristics of novel and traditional TMF (flow velocity range of 0.05–8.0 m/s). The experimental results show that when the installation angle changes from 0° to 90°, the maximum power variation of traditional TMF is 16.5%, while that of the novel TMF is only 0.6%. The simulation results show that when the axial position changes from 9 to 1 m, the maximum power variation of traditional TMF is 11.5%, while that of the novel TMF is only 3.8%. When the insertion depth of the velocity sensor translates from the pipe center to 0.10 m upward, the maximum power variation of traditional TMF is 91.6%. The novel TMF is installed by thread or flange compression, with a fixed and unique insertion depth of D/2, there is no change in the insertion depth during measurement. In conclusion, the effect of the flow velocity distribution on the measurement characteristics is significantly reduced in the novel TMF compared to the traditional TMF, the measurement results are more accurate.     

便携式低功耗电磁流量计测量电路的设计

本文对低功耗电磁流量计的测量电路进行了研究,给出了测量电路的基本组成。根据被测信号的特点,即微弱信号在强干扰下的测量,阐述了测量电路的工作原理。本测量电路主要用于电池供电的电磁流量计中,可以实现便携式低功耗的工作要求。     

Reduction of electromagnetic interferences in measurements of fast-changing air velocity fluctuations by means of hot-wire anemometer

Air flow velocity measurements are the key issues in examining the condition and progress of mine ventilation process. In addition to standard measurements carried out in the stock exploitation process, it is necessary to conduct cognitive scientific research in this area. One of the important issues here is the measurement of fast-changing fluctuations of air velocity, enabling the knowledge of the spectral structure of the tested flow. Hot-wire anemometers are used for such measurements both in laboratories and in industrial conditions. A frequent metrological problem is the presence in the measuring area of a high level electromagnetic interference (EMI) from electrical networks and devices. Due to the metrological properties of hot-wire anemometers, such as high gain and wide frequency bandwidth, measuring signal in the presence of electromagnetic interferences may be subject to strong disturbances. Therefore, it is necessary to develop methods for reducing or eliminating these disturbances. The article presents such a new method based on the use of an additional induction loop in the anemometric sensor for compensation or reduction of interferences.     

A review of the theory of Coriolis flowmeter measurement errors due to entrained particles

Coriolis flowmeters operate with high accuracy when the medium metered is a single-phase incompressible fluid. Multi-phase fluids lead to measurement errors because of center-of-mass motion. In this paper we review the “bubble theory” which describes errors due to phase decoupling of two-phase fluids. Examples are provided with combined phase decoupling and compressibility errors.