Experimental investigation on the void fraction characteristics of a sudden expansion tube using wire-mesh sensors

Void fraction is an essential parameter of gas-liquid two-phase flow and experiments were executed to investigate the void fraction fluctuation characteristics of gas-liquid two phase flow through a sudden expansion tube. Two 16 × 16 wires mesh sensors were applied to measure the phase distribution of upstream pipe(pipe-32) and downstream pipe(pipe-50). The superficial gas velocity is in the range of 3.46 m/s - 22.46 m/s and the superficial liquid velocity ranges from 0.034 m/s to 0.414 m/s. Flow pattern evolution of upstream and downstream pipes was reconstructed and compared. The experiment results show that, in contrast to pipe-32, the void fraction of pipe-50 shows different trends with the increase of liquid and gas velocity. Liquid-carrying capacity is essential in the relationship between the void fraction of pipe-32 and pipe-50. The critical superficial liquid and gas velocities are proposed to characterize the liquid-carrying capacity. The maximum critical superficial gas and liquid velocity is 15.56 m/s and 0.207 m/s, respectively. Besides, a model is proposed to describe the relationship of void fraction between pipe-32 and pipe-50. It is found that the prediction error is less than ±10% in the case of annular flow.     

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.     

Two-dimensional two-fluid model for air-oil wavy flow in horizontal tube

This study concerns the development of a two-dimensional two-fluid model for wavy flows in horizontal tubes. To deal with the curved walls of the liquid and gas phases and the gas-liquid interface simultaneously, the bipolar coordinate system was used. Experiments on air-oil mixture flow in horizontal tubes with diameters of 20 and 40 mm were conducted to observe wavy flow patterns accompanying the two-dimensional (2D) and Kelvin-Helmholtz (KH) waves and to measure the pressure gradient under different flow conditions. Two different previous correlations for the interfacial friction factor were employed in the model for predicting the wavy flows with 2D and KH waves. Predictions of the model of the liquid film height, the average values of wall shear stresses of each phase, and the average interfacial shear stress were compared for different diameters and different superficial gas and liquid Reynolds numbers. Also presented are detailed predictions of the model for four different flow conditions, including the local values of interfacial shear stress, wall shear stress of the liquid phase, interfacial friction factor, liquid film height, and two-dimensional velocity distribution in the liquid phase at the cross-section of the tube.

     

Parametric study of two-phase flow by integral analysis based on power law distribution

To understand the fluid dynamic forces acting on a structure subjected to two-phase flow, it is essential to obtain detail information on the characteristics of that flow. The distributions of flow parameters across a pipe, such as gas velocity, liquid velocity and void fraction, may be assumed to follow a power law (Cheng 1998; Serizawa et al. 1975). The void fraction profile is, for example, uniform for bubbly flow, whereas for slug flow it is more or less parabolic. In the present work, the average values of momentum flux, slip ratio and other parameters were derived by integral analysis, based on approximate power law distributions. A parametric study with various distributions was performed. The existing empirical formulations for average void fraction, proposed by Wallis (1969), Zuber et al. (1967) and Ishii (1976), were considered in the derivation of the present results. Notably, the unsteady momentum flux for slug flow was approximated.     

Void fraction measurement in steam–water two-phase flow using the gamma ray attenuation under high pressure and high temperature evaporating conditions

The void fraction is one of the most important parameters used to characterize gas–liquid two-phase flow, and a myriad of researchers have investigated it under the adiabatic flow conditions. The gamma ray attenuation is a frequently used non-intrusive method for measuring component volume fraction in gas–liquid two-phase flow system. In this paper, firstly, the influence of the various parameters and test conditions on the gamma ray attenuation have been completely examined, such as the calibration of Count Rate for pure gas and liquid phases, the influences of fluid temperature, phase changing point and fluid mass velocity, distance between gamma ray attenuation measuring instrument and experimental section etc. Secondly, the measurement of void fraction was taken in the vertically upward pipes under high pressure and high temperature evaporating conditions. The experimental results of void fraction were compared with the data in reference literature for measurement, the results from the gamma ray attenuation show good agreement with the literature for air–water two-phase flows, but for the evaporating conditions, a small number of compared data beyond the statistical approach for 90% of confidence interval due to some reasons, such as heat flux, the diameter of Taylor-bubbles, longitude of slugs etc. Finally, six predicted correlations from four groups were selected for comparing with the experimental data. The most of compared data were within the statistical approach for 85% of confidence interval. In general, the void fraction was rarely investigated and the available data was limited under high temperature and high pressure evaporating conditions. The investigations of present study are helpful to resolve the difficulties of measuring for gas–liquid two-phase flows concerning to the heated evaporating condition.     

Flow regime identification of steam-water two-phase flow using optical probes,based on local parameters in vertical tube bundles

Flow regime identification based on local parameters of axial upward two-phase flow in vertical tube bundles, at high-temperature and high-pressure, was performed using optical probes. A staggered arrangement of the tube bundles was simulated inside a non-circular test channel, the tube size and pitch are same as that in a real steam generator of a PWR under design. Optical probes were utilized to acquire the void fraction, interface frequency, and fluctuation characteristics of the local void fraction at two typical locations (centroid of the three tubes, named op-1, and centre of the minimum gap between two tubes, named op-2). The system pressure ranged from 5 to 9 MPa, mass flux from 100 to 350 kg m⁻² s⁻¹, thermodynamic steam quality from 0 to 1, and inlet fluid temperature from 263.9 to 303.3 °C, depending on the saturation pressure. This study investigated local parameters and flow pattern characteristics of high-pressure steam-water two-phase flow in vertical tube bundles using optical probes, with the measurement error of less than 2%. Results showed that local void fraction at op-1 was much larger than that at op-2, and the local void fraction difference between op-1 and op-2 increased first and then gradually decreased, which was primarily affected by the transition in flow regimes. The flow pattern characteristics of steam-water two-phase flow were described based on three aspects, namely, variation in interface frequency with local void fraction, fluctuation characteristics of local void fraction, and statistical analysis of local void fraction deviating from the average. Additionally, the flow regime identification criteria, applicable to the steam-water two-phase flow in vertical tube bundles, were proposed based on local parameters.     

An experimental study on real–time analysis of two–phase peristaltic slug flows in dialysis machines

Two–phase flows appear in many industrial and biomedical applications. One of the most vital biomedical applications of two–phase flows is in hemodialysis machines due to air embolism and heparin injection. Since these flows have a very complex and intermittent nature, studying their dynamics is a very challenging and fundamental problem. The purpose of this article is to present an experimental study on the dynamics of two–phase peristaltic slug flows. The measurement strategy is based on the image processing technology. The characteristic parameters of the two–phase pulsatile slug flows, including the slug length, as well as the translational velocity and frequency of the slug motion, are measured, and the effect of the liquid flow rate and liquid superficial velocity is investigated. The results show that the average and maximum slug velocities, and also the dominant amplitude of the slug velocity increase with the flow rate and liquid superficial velocity, while it is not possible to clearly predict a correlation between the liquid superficial velocity and the slug length. The measurement strategy presented in this article can be used in the control and alarm systems of smart dialysis machines.     

Model analysis for differential pressure two-phase flow rate meter in intermittent flow

Multiphase flow rate metering is a challenging problem, specially for flow patterns other than wet-gas. This paper brings forward a new comparative analysis of three differential pressure calibration models suited for liquid dominated two-phase flows, in a total of seven model configurations. First, the models are compared theoretically and classified in terms of the type of input data required. Then, experimental data of over 300 horizontal air–water experiments, for $1$” and $2$” pipe diameters, supports quantitative analyses of the prediction accuracies and sensitivity of the superficial velocities of gas and liquid to measurement errors in the model input variables. Finally, a method for assessing the decoupled measurement errors for the void fraction and gas velocity is shown, as these variables are typically subject to higher uncertainties. It results that, though the void fraction is shown to be systematically under evaluated in more than 10%, the total mass flow rate is estimated through the Paz et al. (2010) model with an overall root mean squared deviation (RMSD) of 5.75% for the $2$” data. Also, the use of gas velocity measurements, even if subject to considerable errors, decreased the RMSD for the gas superficial velocity by more than half for the $1$” data.     

Numerical simulation,validation, and analysis of two-phase slug flow in large horizontal pipes

Multiphase flow, especially two-phase gas-liquid flow, is of great importance for a variety of applications and industrial processes, for example in the nuclear, chemical, or oil and gas industries. In this contribution, we present simulation results for gas-liquid slug flow in large horizontal pipes. Six test cases with different oil, water, and gas flow rates are considered, which cover a wide range of different slug flows. The numerical predictions are validated by comparison with experimental data obtained from video observations. The relative error of the mean liquid level between experiment and simulation is less than 12.3% for all but one test cases. Furthermore, a frequency analysis is performed. The single-sided amplitude spectrum as well as the smoothed power spectral density are calculated. For both, experimental and simulation data, one observes an increase of the dominant frequencies if the ratio of liquid and gas superficial velocity is increased.     

Image processing techniques for high-speed videometry in horizontal two-phase slug flows

Two-phase flow measurements are very common in industrial applications especially in oil and gas areas. Although some works in image segmentation have analyzed gas–liquid slug flow along vertical pipes, few approaches have focused on horizontal experiments. In such conditions, the detection of the Taylor bubble is challenging due the great amount of small bubbles in the slug area and, thus, requires a special treatment in order to separate gas from liquid phases. This article describes a new technique that automatically estimates bubble parameters (e.g. frequency, dimension and velocity) through video analysis of high-speed camera measurements in horizontal pipes. Experimental data were obtained from a flow test section where slug flows were generated under controlled conditions. Image processing techniques such as watershed segmentation, top-hat filtering and H-minima transform were applied to detect and estimate bubble contour and velocities from the observed images. Finally, the estimated parameters were compared to theoretical predictions, showing good agreement and indicating that the proposed technique is a powerful tool in the investigation of two-phase flow.     

Experimental investigation of gas-liquid flow in a vertical venturi installed downstream of a horizontal blind tee flow conditioner and the flow regime transition

A venturi device is commonly used as an integral part of a multiphase flowmeter (MPFM) in real-time oil-gas production monitoring. Partial flow mixing is required by installing the venturi device vertically downstream of a blind tee pipework that conditions the incoming horizontal gas-liquid flow (for an accurate determination of individual phase fraction and flow rate). To study the flow-mixing effect of the blind tee, high-speed video flow visualization of gas-liquid flows has been performed at blind tee and venturi sections by using a purpose-built transparent test rig over a wide range of superficial liquid velocities (0.3–2.4 m/s) and gas volume fractions (10–95%). There is little ‘homogenization’ effect of the blind tee on the incoming intermittent horizontal flow regimes across the tested flow conditions, with the flow remaining intermittent but becoming more axis-symmetric and predictable in the venturi measurement section. A horizontal (blind tee) to vertical (venturi) flow-pattern transition map is proposed based on gas and liquid mass fluxes (weighted by the Baker parameters). Flow patterns can be identified from the mean and variance of a fast electrical capacitance holdup measured at the venturi throat.     

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.     

A procedure for correcting for the effect of fluid flow temperature variation on the response of capacitive void fraction meters

In gas–liquid flows through test loops, the fluid temperature increases, causing a change in the dielectric properties, mainly of the liquid component, which interferes with the response of capacitive void fraction meters. This phenomenon causes, for example, deviations of about 4% in the meter response for each 10 ^∘C variation of temperature in the air–water flow. In this work, a simple procedure is proposed for correcting for the response of capacitive void fraction meters being affected due to the flow temperature being different from the calibration temperature. Static and dynamic tests were performed on a capacitive void fraction measuring system with double-helix electrodes. The results showed the effectiveness of the proposed compensation technique, which allowed a significant reduction of the measurement deviations.     

Precise gamma based two-phase flow meter using frequency feature extraction and only one detector

Flow regime information can be used to enhance measurement accuracy of flowmeters. Void fraction measurement and regime identification of two-phase flows including, liquid and gas phases are crucial issues in oil and gas industries. In this study, three different regimes including annular, stratified and homogeneous in the range of 5%–90% void fractions, were simulated by Monte Carlo N-Particle (MCNP) Code. In simulated structure, a Cesium 137 source and only one NaI detector were used to record received transmitted photons. Fast Fourier Transform (FFT) was applied to the registered signals of the detector in order to analyze in the frequency domain. Several features of signals in the frequency domain were extracted. These features were the average value of fast Fourier transform, the amplitude of dominant frequency, variance, Kurtosis and RMS (root mean square). Different combinations of these features were investigated in order to find the best features with the best separation ability for using as the inputs of Artificial Neural Network (ANNs). Two different Multi-Layer Perceptron (MLP) neural networks were used to recognize flow regimes and predict the void fraction. In regime identification procedure, all of the three mentioned regimes were recognized correctly and in the volume fractions prediction procedure, the void fraction was also estimated with a Mean Relative Error (MRE) percentage of less than 0.5%. In all of the previous studies, at least two detectors were used. Using the proposed method in this paper, number of detectors was reduced to one.     

High-speed liquid film sensor for two-phase flows with high spatial resolution based on electrical conductance

A sensor is presented, that allows high-speed measurements of two-phase film flows. The sensor consists of electrodes flush to the wall, that measure the electrical conductance in a liquid film. This sensor has a time resolution of 10 kHz and 64×16 measuring points, with a spatial resolution of 3.12 mm² and a maximum film thickness range of 0.8 mm. The shape of the sensor electrodes is optimized by calculating the potential field of the liquid film. The calculated sensor characteristic is compared with experimental data received by imposing static liquid films of known thickness on top of the sensor. Further dynamic measurements of co-current air–water flow in a horizontal channel have been conducted. The statistical analysis concerning film thickness distribution and flow patterns is generally in agreement with literature. The high time and spatial resolution allows a wave analysis over a wide range of wavelengths and wave velocities. By using a continuous wavelet transform, a significant amount of additional information has been retrieved, particularly in regard to ripple waves. Preliminary experiments show the ability of the sensor to detect droplet deposition onto the liquid film. An autoregressive filter has been applied to separate droplet impacts from periodic wavy structures.     

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.     

Quality check of wire-mesh sensor measurements in a vertical air/water flow

Extensive measurements were executed for a vertical upward air/water flow to generate a high-quality database for the development and validation of CFD-Codes for two-phase flows (e.g. for models on bubble forces or on coalescence and break-up). Thereto, in a pipe with a nominal diameter of 200 mm, the wire-mesh sensor technology was used. The present paper aims on the assessment of uncertainty caused by the experimental procedure and especially global deviations arising from the use of the wire-mesh sensor technology. Special attention was paid to the plausibility and accuracy of the data regarding the evolution of the vertical multiphase flow. In the result, a clear and consistent trend regarding their evolution with increasing distance from the position of the gas injection was found. Comparisons of the trend of time and cross-section averaged gas volume fraction along the pipe height with the theoretically expected values were carried out. From the measured radial profiles of the void fraction and the velocity of the gas phase, the superficial gas velocity at the wire-mesh sensor is integrated over the cross-section and compared with the set value from the test matrix. Thus, a general uncertainty analysis of the sensor data is possible.     

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.     

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).