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.     

A study on the relationship between upstream and downstream conditions in swirling two-phase flow

Inline fluid separation is a concept, which is used in the oil and gas industry. Inline fluid separators typically have a static design and hence changing inlet conditions lead to less efficient phase separation. For introducing flow control into such a device, additional information is needed about the relationship of upstream and downstream conditions. This paper introduces a study on this relationship for gas/liquid two-phase flow. The downstream gas core development was analyzed for horizontal device installation in dependence of the inlet gas and liquid flow rates. A wire-mesh sensor was used for determining two-phase flow parameters upstream and a high-speed video camera to obtain core parameters downstream the swirling device. For higher accuracy of the calculated void fraction, a novel method for wire-mesh sensor data analysis has been implemented. Experimental results have shown that void fraction data of the wire-mesh sensor can be used to predict the downstream behavior for a majority of the investigated cases. Additionally, the upstream flow pattern has an impact on the stability of the gas core downstream which was determined by means of experimental data analysis.     

Uncertainty and intrusiveness of three-layer wire-mesh sensor

A wire-mesh sensor (WMS) has been applied to estimate the bubble velocity of an air-water bubbly flow in a vertical channel with a square cross-section. The WMS provides instantaneous cross-sectional gas fraction distributions which are measured by detecting the local electrical conductivity between two electrode wires crossing each other at right angles. The applied WMS has three planes of wire grids separated by 1.5 mm in the axial direction. The wires of the central grid are used as transmitter electrodes, while the wires of the two external grids are connected to the receiver inputs of the electronic unit. In this way, the sensor has two measuring planes, located between the transmitter grid and both receiver planes. Individual bubble diameters are calculated from the measured gas fraction data by using a bubble identification algorithm, and the bubble velocity is evaluated by cross-correlating the instantaneous gas fraction profiles. In case of WMS measurements, the intrusive effects caused by the wires cannot be neglected. In this study, the effect of the intrusive WMS on the bubble velocity was studied by high speed camera (HSC) observation. Bubble parameters were extracted from both WMS and HSC data. A comparison of bubble size and velocity was carried out for each bubble individually. It was found that bubbles are strongly decelerated when they collide with the wire grids in case of low liquid velocities. The effect decreases with growing liquid velocity and finally turns into a slight acceleration which corresponds to the degree of the cross-section obstruction by the wires.     

Comparison between wire-mesh sensor and ultra-fast X-ray tomograph for an air–water flow in a vertical pipe

A comparison between ultra-fast X-ray CT and a wire-mesh sensor is presented. The measurements were carried out in a vertical pipe of 42 mm inner diameter, which was supplied with an air–water mixture. Both gas and liquid superficial velocities were varied. The X-ray CT delivered 263 frames per second, while the wire-mesh sensor was operated at a frequency four times higher. Two different gas injectors were used: four orifices of 5 mm diameter for creating large bubbles and gas plugs and a sintered plate with a pore size of 100 μm for generating a bubbly flow. It was found that the wire-mesh sensor has a significantly higher resolution than the X-ray CT. Small bubbles, which are clearly shown by the wire-mesh sensor, cannot be found in the CT images, because they cross the measuring plane before a complete scan can be performed. This causes artifacts in the reconstructed images, instead. Furthermore, there are large deviations between the quantitative information contained in the reconstructed tomographic 2D distributions and the gas fractions measured by the sensor, while the agreement is very good when the gas fraction is obtained by a direct evaluation of the X-ray attenuation along the available through-transmission chords of the tomography set-up. This shows that there is still potential for an improvement of the image reconstruction method. Concerning the wire-mesh sensor it was found that the gas fraction inside large bubbles is slightly underestimated. Furthermore, a significant distortion of large Taylor bubbles by the sensor was found for small liquid velocities up to 0.24 m/s. This effect vanished with growing superficial water velocity.     

Miniature conductivity wire-mesh sensor for gas-liquid two-phase flow measurement

A miniature conductivity wire-mesh sensor for gas-liquid two-phase flow measurement in small channels is presented. The sensor design is similar to the conventional wire-mesh sensor for larger flow cross sections with wire electrodes stretched across the flow channel in two adjacent planes and with perpendicular wire orientation between planes. Conductivity measurement is performed by special electronics which consecutively applies bipolar voltage pulse excitation to the sender wires and measures electrical current flow in the wire crossings at the receiver wires. The new design is based on printed circuit board technology. A prototypical sensor made of 2×16 stainless steel wires each of 50 μm diameter was manufactured and applied to two-phase flow measurement inside the mixing chamber of an effervescent atomizer. Accuracy of the sensor was studied for static liquid distributions using microphotography and for dynamic two-phase flow by comparison of wire-mesh sensor data with radial gas fraction profiles obtained from X-ray microtomography measurements.     

Micro wire-mesh sensor for two-phase flow measurement in a rectangular narrow channel

A micro wire-mesh sensor (μWMS) based on an electrical conductivity measurement between electrodes installed on the walls has been developed for gas–liquid two-phase flow measurements in a narrow rectangular channel. This measuring method applies a principle of conventional wire-mesh tomography, which can measure the instantaneous void fraction distributions in the cross-section of the relatively large flow channel. In two-phase flow measurement using μWMS the void fraction distributions in the narrow channel were obtained by the measured conductivities between electrodes arranged on each wall. Therefore, the gas phase structures and the bubble behaviors can be investigated in the flow channel with narrow gap. In the present paper, a μWMS for the air–water flow between parallel flat plates with a gap of 3 mm was developed and simultaneous measurements with a high speed video camera were conducted to compare the measured results in bubbly flow.     

Applications of wire-mesh sensors in multiphase flows

This article offers an overview of the applications of the wire-mesh sensor (WMS) in different environments. It presents a critical review of the literature, with relevant and recent implementations, remarkably in gas–liquid and liquid–liquid flow, comparing it with other techniques. In addition, it is shown how the sensor is adapted to each application and its different geometries, showing its flexibility. The advantages and disadvantages of the use of the WMS are analyzed. This technique can provide information about local, chordal, cross-section or in-situ volume profiles/distributions of phase fraction; velocities, size and distributions of droplets/bubbles; frequency of periodic structures; interfacial area; film thickness; flow regimes and thermal distribution.     

Two-phase flow pattern classification based on void fraction time series and machine learning

Two-phase flow is a complex phenomenon present in several industrial applications such as chemical reactors, power generation, and in the exploration, production, and transport of oil and natural gas. The classification of the flow pattern is a fundamental step in such applications, as it influences several derived parameters and sub-processes such as flow rate, void fraction, and pressure drop estimation. In this paper, we propose an objective approach for classifying flow patterns using time series of void fraction (from a wire-mesh sensor), signal processing and machine learning. As novel approach, the time series is modeled as a stochastic process of independent and identically distributed samples with probability density function described by a Gaussian mixture model. The estimated parameters of the mixture are then fed into a Support Vector Machine (SVM), yielding the flow pattern classification. Tests were performed with a vertical liquid–gas flow database from a 52.3-mm-diameter pipe and the results indicate a great potential for application in real systems. The average accuracy and F-score obtained was higher than 0.94 for different test sets, with standard deviation lower than 0.08 for accuracy a lower than 0.11 for F-Score, demonstrating the efficiency and generalization of the proposed method.     

Determination of velocity and angular displacement of bubbly flows by means of wire-mesh sensors and correlation analysis

For the analysis of air–water bubbly flows in vertical pipes a method is presented which determines the radial profiles of velocity and angular displacement of the gaseous phase. Measurements rely on two so-called wire-mesh sensors, put into the flow behind each other and capturing the electrical conductivity distribution in the pipe cross-section at two different axial positions. The time-resolved signal is directly translated into transient two-dimensional distributions of the gaseous phase at both sensor positions. Sensor data analysis is based on two-dimensional cross-correlation within concentric cylindrical planes. The axial and angular displacement of the gaseous phase manifest themselves as the maximum position of the cross-correlation function.     

空气质量流量传感器的流场分析及安装位置优化

对管道气体流场模型进行了说明.空气质量传感器安装在滤清器出口的进气管道中,用Fluent仿真软件对模型做流场分析,改变传感器水平方向和垂直方向的安装位置,得出多组速度数据进行比较分析,确定其最优安装位置.仿真结果表明,随着传感器的垂直方向安装位置从管道底部向管道顶部的上移,且水平方向安装位置从滤清器出口向外移,其热电阻表面流场的同一水平方向的速度一致性变好,但垂直方向速度梯度变大,兼顾速度一致性和速度梯度,得到传感器的热电阻处于管道中心垂直往上h,且距离滤清器出口1.3H的优化位置.     

Robust computational framework for wire-mesh sensor potential field calculations

With the development of the next generation of nuclear reactor safety system codes fast underway, increased importance has been placed on enhancing physical closure correlations and amassing representative benchmark-quality experimental data for validation purposes. Wire-mesh sensors, a reputable experimental measurement technique with sufficient spatial and temporal resolution to serve such goals, and related data reconstruction algorithms have been the subject of renewed interest as researchers attempt to characterize their measurement uncertainty. To assist in such investigations, the present work establishes a comprehensive numerical framework with which to quantify the electric potential field around wire-mesh sensors. Using the finite-volume foundations of OpenFOAM, a numerical solution algorithm is developed to predict the transmitted electric current between transmitter and receiver electrodes for both homogeneous and heterogeneous electrical conductivity fields. A detailed verification against seminal numerical calculations and robust validation procedure is included to ensure the accuracy of the proposed methodology. Parametric studies of spherical bubble diameter, lateral crossing position, and spheroidal shape influence are conducted to provide preliminary insights into wire-mesh sensor operation and the suitability of various calibration approaches. Observed trends in the transmitted currents reveal overshoots relative to calibration conditions, which are fundamentally linked to the maldistributed electric potential field in heterogeneous bubbly flows. The present investigation offers a vital first step towards a comprehensive multi-physics model of multiphase flow around a wire-mesh sensor.     

一种新型的电容式油气两相流相份额传感器

本文介绍了一个高精度的电容式两相流相份额传感器。该传感器是非插入式,传感部件与测量电路集成一体,排除了寄生电容和杂散电容的影响,使测量精度和稳定性大大提高;输出是直流电压信号,便于计算机自动采集数据;动态反应快,适于实时在线测量。可用于管内油气两相流相份额测量、流型识别、两相流量测量和流动状态监测     

Measurement of film fluctuations and aerated characteristics during gas-liquid slug-churn flow transition by wire-mesh sensor

Vertical upward gas-liquid slug flows are frequently encountered in chemical processes and petroleum industries. The measurement of the film fluctuations and the aerated characteristics is of great significance for uncovering the mechanism of slug-churn flow pattern transitions. In this study, a conductance wire-mesh sensor (WMS) measurement system is designed based on a Field Programmable Gate Array (FPGA) to visualize the structures of vertical gas-liquid flows. Liquid film flooding is a significant factor prompting the transition from slug to churn flow. Based on the WMS data, the 3D film structures are derived to indicate film instability during the flow pattern transition. Three types of film fluctuations in stable slug flow, unstable slug flow, and churn flow are presented. Liquid slug aeration is another important factor contributing to the slug-churn flow transition. The spatial distribution and the diameters of the gas bubbles in the liquid slug are detected by the WMS. The coalescence behavior of the bubbles is uncovered. Finally, mechanistic models based on the film flooding and slug aeration are constructed to predict the boundary of the flow pattern transition. The performance of the film flooding model and slug aeration model in predicting the onset of churn flow is evaluated.     

An experimental study on characteristics of two-phase flows in vertical pipe

The characteristics of two-phase flow in a vertical pipe are investigated to provide information for understanding the excitation mechanisms of flow-induced vibration. An analytical model for two-phase flow in a pipe was developed by Sim et al. (2005), based on a power law for the distributions of flow parameters across the pipe diameter, such as gas velocity, liquid velocity and void fraction. An experimental study was undertaken to verify the model. The unsteady momentum flux impinging on a ‘turning tee’ (or a ‘circular plate’) has been measured at the exit of the pipe, using a force sensor. From the measured data, especially for slug flow, the predominant frequency and the RMS value of the unsteady momentum flux have been evaluated. It is found that the analytical method, given by Sim et al. for slug flow, can be used to predict the momentum flux.     

Velocity distribution of liquid phase at gas-liquid two-phase stratified flow based on particle image velocimetry

Horizontal gas-liquid flows are commonly encountered in the production section of the oil and gas industry. To further understand all parameters of the pipe cross-section, this paper use particle image velocimetry to study the circular pipe cross-section liquid velocity distribution rule. Firstly the focus is on the software and hardware combination of image correction system, to solve the influence of different refractive indexes of medium and pipeline curvature caused by image distortion. Secondly, the velocity distribution law of the corrected stratified flow (the range of liquid flow of 0.09-0.18 m³/h, and gas flow range of 0.3-0.7 m³/h) cross-section at different flow points of the pipeline cross-section at x=0 and in the Y direction at the maximum liquid velocity is studied. It is found that these distribution laws are caused by the influence of the interphase force of the gas-liquid interface and the resistance of the pipe wall. The current measurements also produce a valuable data set that can be used to further improve the stratified flow model for gas-liquid flow.     

Power law approximations to gas volume fraction and velocity profiles in low void fraction vertical gas–liquid flows

A dual sensor conductance probe was used to measure the distributions of the local gas volume fraction and the local gas axial velocity in vertical upward, bubby air–water flows in which the mean gas volume fraction was less than 0.1. Very limited data are available in the literature for such low volume fraction flows. The measured local gas volume fraction and velocity distributions were approximated by power law functions. The power law exponents associated with the measured local gas volume fraction profiles were found to be up to 30% higher than values predicted in the literature. The power law exponents associated with the measured local gas velocity profiles were also found to be somewhat higher than values predicted in the literature. The power law exponents for the measured local gas volume fraction and local axial gas velocity distributions at a given flow condition were combined to obtain an estimate of the ‘Zuber–Findlay’ distribution parameter C₀ at that flow condition. The mean value of C₀ for all of the flow conditions investigated was 1.09. This value of C₀ was found to give good agreement with the gradient of a plot of the mean gas velocity versus the homogeneous velocity u_h, where and u_h were obtained from reference measurements. This agreement is evidence for the good accuracy of the measured volume fraction and velocity profiles. Finally, the paper casts doubt upon previously published criteria regarding the optimum axial sensor separation in dual sensor probes.     

Capacitance wire-mesh sensor applied for the visualization of three-phase gas–liquid–liquid flows

This short communication describes the application of a capacitance wire-mesh sensor for the investigation of a gas–liquid–liquid three-phase flow in a laboratory setup. Experiments with air, silicone oil and water are performed first in static and second in dynamic flow conditions. The capacitance mesh sensor is capable of generating images of the cross-sectional distribution of relative permittivity values, which in turn is an indication to the phases present in the multiphase mixture. Initial tests show that the sensor is a valuable tool to investigate three-phase flows, which are very common in the oil industry.     

Bubble size measurement using wire-mesh sensors

A wire-mesh sensor with a time resolution of 1.2 kHz was used to measure bubble size distributions in a gas-liquid flow. It is designed for a pipe of 51.2 mm diameter and consists of two electrode grids with 16 electrodes each, put in the flow direction behind each other. The local instantaneous electrical conductivity is directly measured between all pairs of crossing wires, a tomographic image reconstruction is not necessary. The resulting 16 × 16 sensitive points are equally distributed over the cross section. This resolution is sufficient to detect individual bubbles, which are imaged in several successive frames during their transition through the measuring plane. To investigate the influence on bubbles, a model of the sensor was tested in a transparent channel with a rectangular cross section of 50 × 50 mm at liquid velocities between 0 and 0.8 m/s. A comparison with high-speed video observations has shown that the sensor causes a significant fragmentation of the bubbles. Nevertheless, the measured signals still represent the structure of the two-phase flow before it is disturbed by the sensor. Bubble sizes can therefore be determined by integrating local instantaneous gas fractions over an area of the measuring points occupied by the bubble. Bubble size distributions are obtained by analysing large assemblies of bubbles. The method was applied to study the formation of slug flow along a vertical tube. The bubble size distributions obtained show the effect of coalescence as well as bubble fragmentation.     

科氏流量计应用于气液两相流测量的实验研究

以空气-水为介质,对科氏流量计应用于气液两相流双参数测量进行了实验研究.实验过程中保持液相流量一定,通过加入不同体积分数的空气来分析含气率对科氏流量计测量精度的影响,采用Weisman垂直上升管气液两相流流型图与实验数据进行了比较.结合实验结果,初步归纳出含气量、流型和科氏流量计测量精度之间的关系,总结出液相中含气影响科氏流量计测量精度的主要因素及其影响规律,为进一步研究科氏流量计气液两相流测量误差修正提供了一种技术方法.     

Experimental study of bubbly swirling flow in a vertical tube using ultrasonic velocity profiler (UVP) and wire mesh sensor (WMS)

Two-phase air-water bubbly swirling flow through a pipe is a complex turbulent flow and its prediction is still challenging. The present paper describes the experimental investigation of the air-water bubbly swirling flow in vertical co-current flow. Swirling flow is induced by a twisted tape in a 20 mm inner diameter pipe. The flow is investigated using Ultrasonic velocity profiler (UVP), which allows the measurement of liquid and gas velocities simultaneously. Furthermore, simultaneous measurement of void fraction is performed using Wire mesh sensor (WMS). The experimental results reveal that swirling flow has significant impact on bubbles’ distribution. In low liquid flow rate, the average bubble velocity is fairly uniform along the radial position and void fraction increases in the near wall region. However, increasing liquid flow rate at constant gas flow rate leads to increase in void fraction in the core region, this is mainly due to drift velocity which is affected by centrifugal force. Experimental findings and parametric trends based on the effects of swirling flow are summarized and discussed.