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.     

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.     

Flow measurement based on the combination of swirler and differential pressure under slug flow

The alternating appearance of elongated bubbles and liquid slugs of slug flow in the pipe causes severe pressure fluctuation. As a result, measuring the flow rate of the slug flow with the throttling unit based differential pressure method is difficult. This paper investigates a new swirler-based flow measurement method in slug flow. The swirler converts the slug flow into a swirling annular flow, and the differential pressure method is used to measure the flow rate. The influences of gas and liquid flow rates on the differential pressure ΔP_X across the swirler as well as its downstream axial differential pressure ΔP_Z are investigated. ΔP_X^0.5 increases linearly as the liquid mass flow rate increases, and the slope of the curve increases as the gas mass flow rate increases. The influence of gas mass flow rate on ΔP_X^0.5 is comparable to that of liquid mass flow rate on ΔP_X^0.5. ΔP_Z^0.5 increases linearly with increasing gas/liquid mass flow rate, and the slope of the curve of ΔP_Z^0.5 with m_l differs slightly from the slope of the curve in single-phase water conditions. Based on the research presented above, new empirical correlations of mass flow rate based on ΔP_X and ΔP_Z are established respectively. The superficial liquid velocity ranges from 0.6 to 2 m per second, while the superficial gas velocity ranges from 2 to 6 m per second. If the gas mass flow rate and ΔP_X are known, the relative error of liquid mass flow is less than 3%. The relative error of the gas mass flow rate is less than 10% if the liquid mass flow rate and ΔP_X are given. The calculation accuracy of the flow measurement model using ΔP_X is better than the calculation accuracy of the flow measurement model using ΔP_Z.     

Research on wet gas separation method based on swirl and ejection cycle technology

Gas-liquid co-production often occurs in the middle and late stages of natural gas production. Efficient wet gas separation is very important in natural gas transportation and measurement. The traditional separation device has a relatively low separation efficiency due to the flow patterns, a new type of pipe separator was designed based on the swirl and ejection cycle technology. A new systematical separation procedure with three main steps was proposed simultaneously. The wet gas was forced to form an annular flow by a swirler. Then, the liquid film flows into the annular gap. The wave layer was introduced into the swirl separation again with the self-circulating ejection system for fine separation finally. Laboratory experiments and Computational Fluid Dynamics (CFD) simulations show that the additional swirl process can decrease the flow pattern influence effectively, and the separation efficiency can increase to more than 90%. The separation efficiency is mainly determined by the gas superficial velocity, and while the velocity of the inlet gas is less than 21 m/s, the separation efficiency is up to 93%. The separation efficiency prediction model was established based on liquid film porosity, and the prediction relative error is less than 10%. The new device, the separation procedure, and the test results can provide constructive technical reference for the real-world pipe separator application.     

The parameters measurement of air–water two phase flow using the electrical resistance tomography (ERT) technique in a bubble column

The air–water two-phase flow is investigated in a bubble column with a height of 2 m and a diameter of 0.282 m by using the Electrical Resistance Tomography (ERT) technique. The flow characterization are measured by applying ERT sensors of three vertical sections with superficial gas velocities in the range 0.027–0.156 m/s. Based on the cross-correlation technique and dynamic gas disengagement (DGD) theory, the bubble Saunter diameters are obtained and the local axial velocity about two phases flow can be calculated. The results show that with increased gas superficial velocity the distribution of bubble size is gradually widespread. Moreover, the local velocity of gas bubble swarm has a center peak distribution with increased gas superficial velocity.     

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.     

Refined reconstruction of liquid–gas interface structures for stratified two-phase flow using wire-mesh sensor

Wire-mesh sensors (WMS), developed at HZDR [4], [13], are widely used to visualize two-phase flows and measure flow parameters, such as phase fraction distributions or gas phase velocities quantitatively and with a very high temporal resolution. They have been extensively applied to a wide range of two-phase gas–liquid flow problems with conducting and non-conducting liquids. However, for very low liquid loadings, the state of the art data analysis algorithms for WMS data suffer from the comparably low spatial resolution of measurements and from boundary effects, caused by e.g. flange rings – especially in the case of capacitance type WMS. In the recent past, diverse studies have been performed on two-phase liquid–gas stratified flow with low liquid loading conditions in horizontal pipes at the University of Tulsa. These tests cover oil–air flow in a 6-inch ID pipe and water–air flow in a 3-inch ID pipe employing dual WMS with 32×32 and 16×16 wires, respectively. For oil–air flow experiments, the superficial liquid and gas velocities vary between 9.2 m/s≤ν_SG≤15 m/s and 0.01 m/s≤ν_SL≤0.02 m/s, respectively [2]. In water–air experiments, the superficial liquid and gas velocities vary between 9.1 m/s≤ν_SG≤33.5 m/s and 0.03 m/s≤ν_SL≤0.2 m/s, respectively [17], [18]. In order to understand the stratified wavy structure of the flow, the reconstruction of the liquid–gas interface is essential. Due to the relatively low spatial resolution in the WMS measurements of approximately 5 mm, the liquid–gas interface recognition has always an unknown uncertainty level. In this work, a novel algorithm for refined liquid–gas interface reconstruction is introduced for flow conditions where entrainment is negligible.     

一种测量液膜厚度的超声相控阵实验装置

气液两相流存在于核反应堆蒸发、飞行器冷却、化工生产降膜蒸发等过程,界面波的动态测量对工业过程监控和生产优 化具有重要意义。 界面波的准确识别与特性参数测量是开展科学研究与工程实践的重要前提。 基于超声相控阵测量系统,设 计了扇扫的测量方式,可以用于气液界面清晰的流型中液膜厚度和界面波形态三维测量。 通过静态标定和圆管验证,确定了像 素点和液膜厚度之间的关系,在气相表观流速为 0. 071 9~ 0. 431 6 m/ s,液相表观流速为 0. 056 7~ 1. 416 1 m/ s 的工况下进行实 时动态实验,获得了实时流动过程中较高精度的截面气液相界面信息,并构建了管道内部界面波三维分布形态,为界面波特性 研究提供了一种实验参考方法。     

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.     

A new method of entrainment fraction measurement in annular gas–liquid flow in a small diameter vertical tube

A new method was introduced to measure liquid entrainment fraction in gas–liquid two-phase upward annular flow in a vertical tube (i.d.=9.525 mm). In this method, a new liquid–gas separator was designed and the chemically-based titration method was used to effectively measure the entrainment fraction in real time. Experiments were conducted at low system pressure (∼1 atm), and relatively low gas and liquid superficial velocities ($V_{\mathrm{sg}}=25.8$–45.5 m/s, and $V_{\mathrm{sl}}=0.15$–0.30 m/s). Data analysis shows that the results are repeatable and occupy the range commonly seen in annular flow. The entrainment fraction was found to be under 7% for all the experimental set points. The repeatability of the test results and comparisons with previous entrainment data indicate that the new technique can perform as well as the film removal technique.     

Electrical resistance tomography coupled with differential pressure measurement to determine phase hold-ups in gas–liquid–solid outer loop bubble column

Radial variation of the gas hold-ups and mean hold-ups are investigated in a 90 mm outer loop bubble column using electrical resistance tomography (ERT) with two axial locations (Plane 1 and Plane 2). In all the experiments, air is used as the gas phase, tap water as liquid phase, and polypropylene particles as solid phase where the superficial gas velocity is varied from 0.02 to 0.25 m/s. The effect of operating conditions, solid concentration on mean hold-ups and radial gas hold-ups distribution is discussed. Gas hold-ups and solid hold-ups results using ERT are in very good agreement with conventional estimation and correlations obtained using pressure transmitter methods. Meanwhile, the results show that the gas hold-ups in the centre region increase constantly with an increase in the superficial gas velocity, namely there is a maximum hold-up at the centre of cross-section. But, solid hold-ups distribution is very homogeneous for high gas velocity. According to the visible image, the gas–liquid flow behaviours are obtained for gas–liquid–solid outer loop bubble column. Furthermore, the results also indicate that ERT is a very powerful tool for diagnosing the ‘inside’ flow behaviour of gas–liquid–solid three phase bubble column.     

Characterization of slug flows in horizontal piping by signal analysis from a capacitive probe

The slug flow is a common occurrence in gas–liquid piping flows. Usually it is an undesirable flow regime since the existence of long lumps of liquid slug moving at high speed is unfavorable to gas–liquid transportation, so that considerable effort has been devoted to study its hydrodynamic characteristics. In this work, a capacitive probe was used for dynamic measurements in the horizontal air–water slug flows, for several flow rates. The acquired signals were representative of the effective liquid layer thickness near every cross sectional area of the flow, instead of merely the holdup or void fraction in a finite volume of the flow. This was possible because probe had a thin sensing electrode that minimizes the axial length effect on the measurements. Tests were performed in a 34 mm i.d. acrylic pipe, 5 m long; in which slug flows as well as stratified-smooth and stratified-wavy flows were generated. Signal analysis techniques were applied for flow regime identification and toward characterization of these two-phase flows: Power Spectrum Density (PSD) from Fourier Transform and Probability Density Function (PDF) from Statistical Analysis. Therefore, PSD and PDF graphs were taken as signatures of each flow under test and a correlation was calculated for each PSD and PDF set of data, which showed to be a robust parameter for correct flow regime identification.     

Quantitative multiphase flow characterisation using an Earth's field NMR flow meter

We present a novel nuclear magnetic resonance (NMR) multiphase flow metering system with the ability to interpret the flow regime and quantify both the liquid volumetric flowrate and holdup for gas-liquid flows. The flow measurement apparatus consists of a pre-polarising permanent magnet upstream of an Earth's field radio frequency NMR detection coil. In this work, the system is applied to measure the free induction decay (FID) NMR signal of gas-liquid flows at a range of flow rates in both the stratified and slug flow regimes. Tikhonov regularisation is applied to fit a model equation to the acquired FID signal in order to determine the relevant liquid velocity probability distribution. Signal interpretation applied to the individual NMR scans allows monitoring of both the liquid velocity and holdup with time. The NMR estimate of the liquid holdup is comparable to video analysis of the flowing stream through a transparent pipe section. The accuracy of the NMR metering system is successfully validated against an independent in-line rotameter measurement of the liquid volumetric flowrate during multiphase flow. Finally, analysis of the temporal variation in measured liquid flowrate is shown to clearly distinguish the stratified and slug flow regimes.     

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.     

A novel tomographic sensing system for high electrically conductive multiphase flow measurement

Electrical resistance tomography (ERT) has been widely applied in order to extract flow information from various multiphase flows, e.g. the concentration and velocity distributions of the gas phase in gas–water two phase flows. However, the quality of measurement may become very poor from a multiphase flow whose continuous phase has a considerably high electrical conductivity, e.g. seawater (5.0 S/m), using a conventional current-injected ERT system. It is known that a large current excitation is necessary in order to enhance the measurement sensitivity. In practice, it will be very challenging to build a current source with a large amplitude (more than 75 mA) and a high output impedance at a high excitation frequency. This paper presents an implementation of an ERT system with a voltage source and current sensing to overcome the limits of the current source. The amplitude of the current output can reach more than 300 mA. A logarithmic amplifier is used to compress the signal’s dynamic ranges from 18.32 dB to 1.66 dB. The structure and features of this system are presented in this paper and the performances of key circuits are reported. Finally the experimental results from a highly conductive flow (1.06 S/m) are analysed and compared with the measurements obtained from a low conductive flow.     

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.     

Characterization of the hydrodynamic flow regime in bubble columns via computed tomography

The current work evaluates the potential of Computed Tomography (CT) measurements for flow regime characterization. Experiments were carried out in a pilot scale (0.162 m diameter) bubble column using an air–Therminol LT system at ambient as well high operating pressures (0.4 and 1 MPa). The superficial gas velocities were varied from 1 to 20 cm/s at intervals of 1 cm/s. The steepness of the gas holdup radial profile was analyzed to demarcate the hydrodynamic flow regime. The regime transition velocities obtained from CT measurements are compared with the drift flux model. An increase in operating pressure was found to delay the regime transition and at higher pressures, a transition occurred over a range of superficial gas velocities. The current state of correlation prediction is evaluated against the experimental transition velocities.     

Effect of IR Transceiver orientation on gas/liquid two-phase flow regimes

Multiphase flows play a vital role in many industrial and naturally occurring processes. Recent trend of miniaturization in mini/micro fluid reactors, compact heat exchangers and micro thrusters requires a thorough knowledge on multiphase flow phenomena in mini/micro channels. The present work is focused on the effect irradiation behavior of infrared rays (IR) during gas liquid two phase flow consisting of thin liquid films inside a mini channel. The influence of size and shape of the slug regime and liquid film thickness on IR rays is analyzed with COMSOL Multi physics package. Experiments are carried out in a 2.5 mm diameter borosilicate glass tube with wall thickness of 0.3 mm. The refraction and transmittance behavior of IR rays on slug and bubbly flow is studied by analyzing the Current-time output of an IR photodiode kept at different angles with the test section. The results are found to be in good agreement with experimental image processing technique and COMSOL results. The results obtained will be useful for designing of IR sensor arrays sensitive to multiphase flows. It can also be used for measurement of liquid film thickness with proper calibration.     

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.     

Flow regime recognition in a long pipeline-riser system based on signals at the top of the riser

To identify conveniently multiphase flow regimes in subsea pipeline-risers, we study in this paper experimentally two-phase flows in a 1657 m long pipeline with an S-shaped riser to simulate field experiment, within a wide range of gas and liquid velocities. Three flow regimes, namely severe slugging, transitional flows, and stable flows, are analyzed based on three differential pressure and one pressure signals at the top of the riser; comparatively speaking, the positions of these signals in the experimental system are similar to those of the sea level signals in industrial fields, which are easy and less expensive to obtain. The obtained signals are decomposed into six scales via a multi-scale wavelet analysis, and further four statistical parameters on each scale are extracted, including mean values, standard deviations, ranges, and mean values of absolute. We compared the effects of six SVM classifiers with different kernel functions on the recognition rate of flow regimes, and it is found the recognition rates of SVM classifier with quadratic and cubic kernel functions are the highest. Further, the principal component analysis is employed to reduce the dimension of statistical parameters and it indicates that the recognition rate tends to increase with the rising number of principal components from 1 to 6, and it remains constant if the principal component number is further increased. Moreover, The results suggest that the recognition rate obtained from the pressure difference between the top of the riser and the separator peaks, and then it comes that from the pressure signal at the top of the riser, and that for the pressure difference signal at the top of the riser is the least satisfying one. As for the optimal differential pressure signals between the top of the riser and the separator, the results show that the recognition rate increases rapidly from 70.2% to 90.4% when the sample duration rising from 2.3 s to 18.6 s, and when the sample duration exceeds 74.4 s, the recognition rate exceeds 92.9% and remains unchanged.