Development of capacitance sensor for void fraction measurement in a packed bed of spheres

A capacitance void fraction sensor (CVS) is applied to measure the volumetric averaged void fraction in a packed bed of spheres. The void fraction in the packed bed is one of the most important parameters to evaluate cooling characteristics in a porous debris bed during a severe accident of nuclear reactors, and the quantitative void fraction measuring technique for such porous flow channels should be developed. The CVS is a very simple method, and the void fraction is estimated from the electrical capacitance measured between the electrodes installed on the pipe. Generally, the linear relationship or Maxwell equation could be applied to estimate the void fraction from the capacitance measured by the CVS. However, the electrical field in the packed bed becomes complex due to the existence of spheres. Therefore, they may not be applied to the void fraction estimation in the packed bed. In this study, the CVS with a ring-type electrode configuration is used for the sphere-packed beds, and the applicability of the CVS is investigated. At first, the particle size and the pipe diameter are varied in the packed test section, and X-ray transmission imaging is used to clarify the relation between the void fraction and the capacitance in the packed bed. Then, it is found that the void fraction can be obtained by the coefficient in Maxwell's equation, depending on the packed bed properties. Finally, the measurement accuracy of the CVS for the sphere-packed bed is estimated by comparing it with a volumetric method, and the availability of the proposed method is shown.     

基于LS-SVM的油气两相流空隙率测量方法的研究

利用12电极电容层析成像系统电容传感器获取的66个电容测量值,基于最小二乘支持向量机（LS-SVM）算法,提出了两相流空隙率在线测量的新方法。该方法用LS-SVM来建立空隙率测量模型。在实际测量时,首先归一化ECT获取的电容测量值,然后将归一化电容值输入已经建立的空隙率模型即可计算出空隙率。实验结果表明：该方法是有效的,避免了复杂耗时的图像重建过程,测量误差在6%以内,测量时间小于0.08 s。     

Quality measurement of refrigerant two-phase flow in refrigeration cycles

In refrigeration cycles, quality measurement of two-phase refrigerant flow is required to monitor the cycle operation. Although sectional void fraction of the two-phase flow can be detected in several ways, the quality of the two-phase flow is hardly obtained from the sectional void fraction since velocities of liquid- and gas-phase in the pipe are different from each other. In this study, a new quality measuring method was developed by installing multiple narrow tubes in a test section. By installing a gas bypass tube with the multiple narrow tubes, the quality measurement having an accuracy within 0.03 was achieved in the quality range from 0.05 to 0.8. In addition, the influence of oil contamination in the refrigerant flow on the flow pattern in the narrow tube was examined. It was found that the flow pattern in the narrow tube became bubble flow by the mixing of oil.     

Application of dual-plane ERT system and cross-correlation technique to measure gas–liquid flows in vertical upward pipe

The progress of process tomography provides a new method for two-phase flow measurement. The dual-plane electrical resistance tomography (ERT) is combined with the correlation measurement technique to carry out the two-phase flow measurement in which the continuous phase is conductive liquid. The method of the estimation of void fraction and the disperse phase velocity by extracting the eigenvalue of the dual-plane ERT boundary measured data is presented. This method is applied to the transient flow-rate measurement of the air–water two-phase flow in vertical pipe. The information of disperse phase void fraction and distribution variation with time change can be considered adequately, and the estimated value of disperse phase void fraction and velocity can be gained fairly accurately in this method, which provides the data for the calculation of the transient flow-rate. The experiment results indicate that this kind of measurement method is valid when the distance between the upstream and downstream measured cross section is short enough.     

Air–water two-phase flow measurement using a Venturi meter and an electrical resistance tomography sensor

A method for air–water two-phase flow measurement is proposed using a Venturi meter combined with an Electrical Resistance Tomography (ERT) sensor. Firstly, the real-time flow pattern of the two-phase flow is identified using the ERT sensor. Secondly, the void fraction of the two-phase flow is calculated from the conductance values through a void fraction measurement model, developed using the LS-SVM regression method. Thirdly, the mass quality is determined from the void fraction through void fraction-quality correlation. And finally, the mass flowrate of the two-phase flow is calculated from the mass quality and the differential pressure across the Venturi meter. Experimental results demonstrate that the proposed method is effective for the measurement of the mass flowrate of air–water flow. The proposed method introduces the flow pattern information in the measurement process, which minimizes the influence of flow pattern on the conventional differential pressure based methods. In addition, the mass quality is calculated from the void fraction, so the difficulty to obtain the mass quality in conventional methods is also overcome. Meanwhile, the new method is capable for providing concurrent measurements of multiple parameters of the two-phase flow including void fraction, mass quality and mass flowrate as well as an indication of the flow pattern.     

Flow pattern identification for gas-oil two-phase flow based on a virtual capacitance tomography sensor and numerical simulation

Gas-oil two-phase flow is widely encountered in oil exploitation and transportation pipelines. It's complex and transient changes of flow regimes present a great challenge for accurate and real-time measurement. As a non-invasion and real-time measuring method, electrical capacitance tomography (ECT) is suitable for the transient measurement of non-conductive gas-oil flow. However, the highly random and nonlinear nature of multiphase flow make it difficult and limited to investigate the flow parameters based on either static or dynamic measurement. In this research, the whole process of dynamic measurement of ECT applying in gas-oil two-phase flow is thoroughly studied, including simulation calculation, experimental validation and comprehensive data analysis. A simulation approach by coupling the flow and electrostatic field is proposed based on a virtual ECT sensor, in order to monitor the gas-oil two-phase flow characteristics. Based on FLUENT and COMSOL platform, the numerical simulation under six typical flow patterns in a horizontal pipe is carried out. Combining the visualized image generated by ECT measurement and the theory of flow pattern transition, the formation mechanism and structural characteristics of different gas-oil flow patterns are analyzed in detail. Furthermore, this research attempts to analyze the signal fluctuation characteristics caused by flow pattern change, in order to access more in-depth flow information implied in the original capacitance data, via time-series analysis as well as frequency domain analysis based on Flourier Transform. At last, a series of dynamic experiment is conducted to verify the feasibility of the simulation and data analysis approach. The experiment focuses on the flow pattern transition, gas-liquid dynamic characteristics and noise influence in the actual process. It can be concluded from the results of simulation and experiment tests, combining the visualized images and the dynamic characteristics of capacitance signals can make it more effective and intuitive for flow pattern identification, which might be used for the online measurement in real-industry process.     

High-resolution gas–oil two-phase flow visualization with a capacitance wire-mesh sensor

The application of a novel wire-mesh sensor based on electrical capacitance (permittivity) measurements for the investigation of gas–oil two-phase flow in a vertical pipe of 67 mm diameter under industrial operating conditions is reported in this article. The wire-mesh sensor employed can be operated at up to 5000 frames per second acquisition speed and at a spatial resolution of 2.8 mm. By varying the gas and liquid flow rates, different flow patterns, such as bubbly, slug and churn flow, were produced and investigated. From the images of gas void fraction distribution, quantitative flow structure information, such as time series of cross-sectional void fraction, radial void fraction profiles and bubble size distributions, was extracted by special image-processing algorithms.     

Gas–oil two-phase flow measurement using an electrical capacitance tomography system and a Venturi meter

A new flowrate measurement method for gas–oil two-phase flow using an electrical capacitance tomography technique and a Venturi meter is reported in this paper. A hybrid image reconstruction algorithm combining the Tikhonov regularization and the Algebraic Reconstruction algorithm is used to obtain images. The cross-sectional void fraction is determined from the images. A Venturi meter is used as a velocity measurement device whilst five measurement models are introduced to predict the total mass flowrate of gas–oil two-phase flow. Experimental results obtained show that the performance of the proposed flowrate measurement system is effective.     

一种新型电容式空隙率传感器仿真研究

该文提出了一种侵入式多环套状结构电极的电容传感器,并研究它对气液两相流空隙率测量的可行性。通过建立多环套状电容传感器装置的有限元仿真数学模型,运用ANSYS软件对电容传感器有限元模型进行了二维静电场仿真。其分析了极板间的电位分布,计算不同相分布下传感器的电容及变化量,分析空隙率的设定值与仿真值对比结果,并以层流和环流为例比较并分析传感器的灵敏度以及受流型的影响。仿真结果表明,多环套状电极结构电容传感器可运用于两相流的空隙率测量技术上,为多环套状电极结构电容传感器的设计提供了部分理论依据。     

12电极电容层析成像系统的设计

电容层析成像技术(ECT)是基于电容敏感原理的过程层析成像技术(PT).它可进行多相流的相浓度、流型、流量等参数的在线测量,是目前最具发展前景的多相流参数检测方法.本文以12电极电容层析成像系统为研究对象.主要介绍一种基于交流激励型的电容层析成像的C/V转换电路,并分析了该电路的工作原理.该电路能抑制杂散电容的干扰,能...     

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.     

Intermittent flow parameters from void fraction analysis

Two-phase horizontal intermittent flow in straight pipes is experimentally investigated. A new procedure is proposed to characterize the flow through the statistical analysis of the instantaneous cross-sectional averaged void fraction obtained by means of ring impedance probes. The algorithm, based on the statistical analysis of the void fraction records, allows the main intermittent flow parameters, such as slug frequency and length, time average void fraction, minimum and average liquid film height to be evaluated. The procedure is validated through flow visualizations, as obtained from a fast digital video camera.Experiments on air-water horizontal flows in 40 and 60 mm inner diameter pipes are performed. The operating conditions cover the 0.3–4.0 and 0.6-3.0 m/s gas and liquid superficial velocity ranges, respectively.An extensive comparison with literature data shows a general agreement with present measurement. The reliability of both the instrumentation and the signal analysis procedures allows new correlations for minimum and average liquid film height in stratified regions to be proposed. Finally proper dimensionless numbers were applied to correlate frequency data in a wide range of superficial velocity values.     

Identification of oil–gas two-phase flow pattern based on SVM and electrical capacitance tomography technique

The correct identification of two-phase flow patterns is the basis for the accurate measurement of other flow parameters in two-phase flow measurement. Electrical capacitance tomography (ECT) is a new visualization measurement technique for two-phase/multi-phase flows. The capacitance measurements obtained from the ECT system contain flow pattern information, and then six feature parameters are extracted. The support vector machine (SVM) has a desirable classification ability with fewer training samples. The inputs of the SVM are extracted feature parameters of different flow patterns. Simulation and static experiments were carried out for typical flow patterns. Results showed that this method is fast in speed and can identify these flow patterns correctly.     

双层电容层析成像传感器及应用

电容层析成像在气力输送过程中具有十分重要的应用地位，可进行固相浓度分布测量，流型判断。利用双层电容层析成像传感器不仅可以进行浓度测量还可以进行相关测速。基于这种思想，设计了一种双层电容传感器，与电容测量系统结合，用于气力输送过程固相浓度和速度的测量。     

基于ECT传感器和模式识别的气液两相流空隙率测量新方法研究

基于电容层析成像(ECT)传感器和模式识别理论,提出了一种新型的气液两相流空隙率测量方法。根据流型几何特征分别建立了空隙率的三个模板库。测量时先对流型进行分类,再调用对应的模板库利用距离测度进行模板匹配,从而得到空隙率。实验结果表明该方法是可行的,有助于克服测量结果易受流型影响的问题,同时速度优于传统ECT技术方法,获得一个管截面空隙率值所花费的测量时间小于50ms,最大测量误差可小于5%,满足工业在线运行需要。     

基于激光诱导荧光成像技术的截面含气率检测研究

截面含气率作为气液两相流动过程中的基本参数之一,对石油管道的开采、输运,核反应堆冷却塔的设计等过程具有重要意义。本文提出了基于激光诱导成像技术和高速摄录系统的截面含气率直接检测方法,有效的避免管道曲率和介质折射率导致的光学畸变。在河北大学多相流循环装置进行实验,测量了18个流量点,液相流量测量范围10～35 L/min,气相流量测量范围2.0～3.0 L/min。运用计量比对的思想,对两种检测技术获得的截面含气率值求取偏差并进行修正,最大偏差仅为0.014 59。结果表明两种方法得到的截面含气率值具有较好的一致性,证明本文提出的荧光成像技术对气液两相分层流截面含气率的检测是有效的。     

A reference conductivity fitting method for ERT system to achieve void fraction of gas/liquid flow

The void fraction is one of the key parameters in the measurement of gas/liquid two-phase flow. It can be derived from the absolute conductivity distribution based on Maxwell׳s theory. With Electrical Resistance Tomography (ERT) technology, the absolute conductivity distribution is obtained by multiplying the relative conductivity image with the reference conductivity which is conventionally the liquid conductivity of a gas/liquid flow. Unfortunately the liquid conductivity is not always available. Therefore, a conductivity fitting method is proposed in this paper, to find an optimal reference conductivity, which will be used in substituting the liquid conductivity to reconstruct the quasi-absolute conductivity image. The optimal reference conductivity fitting method is proposed and validated by simulation and experiments under certain flow regimes, e.g. slug flow, annular flow and bubbly flow. The simulation and experimental results show that, independent from prior-knowledge, the fitted quasi-homogenous conductivity is close to the average conductivity of the sensing field. It also leads to a much more accurate estimation of void fraction than the conventional method using liquid conductivity as the reference. With the proposed method, the ERT technique can play a more significant role in the measurement of multiphase flow (MPF).     

On-line measurement of oil/gas/water mixtures,using a capacitance sensor

This paper describes the use of a non-intrusive capacitance transducer for the simultaneous on-line measurement of water and undissolved gas in crude oil. The water concentration of the flow is determined from the mean capacitance of the flowing mixture. The same transducer can be used simultaneously to determine the void fraction of the flow by measuring the instantaneous variation in the permittivity of the mixture created by fluctuations of the gas component. There is an interaction between these two measurements but this can be decoupled. This measurement principle was investigated experimentally, using process oil/water/air mixtures. Results are reported for mixtures with water contents up to 40% v/v and void fractions up to 15% v/v.A three-component volumetric flowrate measurement system, based on the capacitance transducer discussed in this paper was proposed. The work reported and the conclusions drawn relate to tests using mixtures of lubricating oil/fresh clean water/air. No work has so far been carried out in mixtures of crude oil/saline waters/hydrocarbon gases typical of the intended applications, nor have estimates been made of the effects that the variable nature of these actual components might have on the performance of the technique. Also, the tests were made on a Perspex section of pipe at nominal pressure whereas the real-life application would require measurements on a high-pressure/high-integrity stell pipeline; the practical problems of adapting the technique to the latter situation have not been examined though they are considered to be resolvable.     

Determination of void fraction in wet-gas vertical flows via differential pressure measurement

The measurement of void fraction in multiphase flow is important for a wide range of industrial processes. Existing methods for void fraction measurement require intrusive, expensive and potentially hazardous equipments which constrict the flow, adding both capital and operational costs. Two phase flow experiments were carried out at the National Engineering Laboratory (NEL) to measure void fraction via pressure drop in a vertical pipe. Additional experiments are carried out at Spirax Sarco Inc. to validate the efficacy of the method on steam/water flow mixtures at high temperature and pressure, in gas mass fraction range between 0.17 and 0.95 and void fraction range between 0.75 and 1.0. The void fraction calculated by the presented differential pressure (dP) method is confirmed via established correlations. The work demonstrates the efficacy of a low cost, non-intrusive method to determine void fraction in two phase flow over a wide range of flow conditions.