No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Wire-mesh sensors: A review of methods and uncertainty in multiphase flows relative to other measurement techniques
Abstract
Void fraction has always been an important parameter in the study of multiphase flows and its measurement has proven difficult over the years. This paper is a state of the art review of the application of conductivity based wire-mesh sensors (WMS) for the measurement of void fraction, bubble size, and gas fraction velocity in mul-tiphase flows and their associated uncertainties. At this point in time there is no golden standard for void fraction measurement, so a large bulk of this work is on the uncertainty of the WMSs relative to other void fraction measurement methods, namely radiative methods. It is shown using the available data that the WMS have a void fraction measurement uncertainty of ± 10.5% over a variety of flow regimes relative to other measurement methods. However, the accuracy of the instrument is largely based on its applicability to a particular flow. For example, the WMS is an excellent choice when entrapment in the sensor due to surface tension is minimized resulting in best results at higher flow rates compared to radiative methods. An assessment into the uncertainty of velocity and bubble size measurements is also performed: analyzing the current algorithms available and studies on these measurements in comparison with high speed cameras and ultrafast X-ray tomography. The current functioning form of the wire-mesh sensors were developed by Prasser in 1998 as a tomographic technique for the measurement of void fraction using a conductivity approach, as performed by earlier researchers. Later developments with the senors resulted in various techniques that allow for the measurement of velocity and interfacial area concentration.
Discover the world's research
- 20+ million members
- 135+ million publications
- 700k+ research projects
... X-ray [28], [29] and gamma ray [30], or alternatively by high-speed camera [31]. Based on such works, it was found that WMS provides an average void fraction estimation within 10.5% deviation regardless of the flow regime [32]. In this direction, Vuong et al. [33] presented an experimental VOLUME 9, 2021 This work is licensed under a Creative Commons Attribution 4.0 License. ...
... Although the numerical models based on electrical field simulation have brought several insights about the physics of the sensor, none of them was able to reproduce deviations in the same range of those found in experiments, i.e. in the range of 10.5%, as reported in the literature [32]. All previous works [27], [36]- [43] have in common that they model only the electric field distributions (via FEM) but neglecting the correct description of the measuring electronics. ...
... In contrast, the experimental and the synthetic data from the proposed model (FEM+EC) show deviations of up to 17.7% and 14.7%, respectively, for patterns with large paraffin structures when the cut-off procedure is neglected. By applying the cut-off procedure, deviations below than 10.5% for all configurations were found, which are in agreement with the uncertainty reported in the literature (11% relative to other measurement techniques [32]). Those results suggest that the capacitance wire-mesh sensor has a performance similar as the conductivity one when used to measure conductive mixtures such as tap water (∼420 µS/cm) (at least for static experiments). ...
A new wire-mesh sensor model based on electric field and circuit simulations is presented. In our approach, the excitation and amplification stages of the capacitance WMS are created as macromodels and coupled to the electrodes of a 3D geometry of the sensor. Thus, the effects caused by nonideal characteristics of the amplifiers are considered (e.g. finite open-loop gain and bandwidth). In order to evaluate the performance of the model, a static validation based on phantom measurement was performed. The phantoms were created with paraffin to emulate typical flow regimes, i.e. annular, slug and bubble flow. A mapping containing the position and the electrical properties of the patterns was obtained by image processing and incorporated to the field simulation. Hence the numerical simulations could be directly compared to the experimental data. The results show that coupling the external circuits to the capacitance WMS model is crucial to provide reliable synthetic data.
... To understand and control the bubble-induced agitation in a gas-liquid two-phase flow, it is necessary to measure the distribution of gas-phase properties ( Prasser and Häfeli, 2018;Tompkins et al., 2018;Risso, 2018;Mathai et al., 2020;Shi et al., 2020 ). This has been mostly achieved by shadowgraphybased optical methods ( Lindken and Merzkirch, 2002;Bröder and Sommerfeld, 2007;Sathe et al., 2010;Kim et al., 2016;Kim and Park, 2019;. ...
... It was first used to measure a discrete planar distribution of water droplets in oil transportation ( Johnson, 1987 ). Heretofore, the technology has been improved towards increasing the sensing rate ( Prasser et al., 1998 ), measuring the bubble size ( Prasser et al., 1998;Prasser and Häfeli, 2018;Tompkins et al., 2018 ) and velocity ( Manera et al., 2006 ). It was also tested for harsh operational conditions such as high pressure and temperature ( Pietruske and Prasser, 2007 ) and miniaturization in a small channel ( Hampel et al., 2009 ). ...
... In this case, the sensor can be equipped with insulated wires with protective wire-coating to ensure the reliability of the system ( da Silva et al., 2007 ). Due to sophisticated calibration and optimizations required for the cases with a higher volume void fraction, on the other hand, the conductivity-based sensor has been utilized more widely to measure the size and rise velocity of the rising bubbles ( da Silva et al., 2007;Prasser and Häfeli, 2018;Tompkins et al., 2018;Shi et al., 2020 ). ...
In this work, we propose a conductivity-based single-layer wire-mesh sensor (WMS) system to simultaneously measure the planar bubble distribution, equivalent diameter, and rise velocity of micro-to-millimeter sized bubbles in a gas-liquid two-phase flow. Compared with the conventionally available WMS system, the proposed system is capable of simultaneously measuring the bubble size and velocity using a single-layer sensor by establishing the correlation between the effective time taken for the bubble to pass through the sensor wires and its rise velocity, which is validated for micro-to-millimeter sized bubbles. Using this correlation, we have improved the recursive bubble identification method to develop a nonlinear model for bubble size, making it possible to selectively measure and distinguish between the bubbles in micro-to-millimeter scales (previous sensors were validated for large bubbles only). To accomplish this, we also investigated the differences in the dynamics of bubble-wire contact in detail. In a vertical upward bubbly flow, the present sensor was employed for a wide range of bubble sizes (200μm to 3.5 mm on average) and liquid velocity of 0−0.5 m/s; the measured data were in good agreement with those measured with a high-speed shadowgraphy. Finally, we discuss some issues of the proposed system, which requires a further attention.
... The quantification of phase fractions is commonly based on scaling the measured signals with the aid of a single-phase reference measurement of water and an appropriate normalization approach, cf. e.g., [9]. Since the electrical conductivity of water is known to be temperature-dependent [10][11][12][13], the temperatures of the single-phase reference measurement and the two-phase measurement must be identical in order to calculate correct phase fractions. ...
... Although the so-called histogram calibration offers a possible alternative, it is not universally applicable, as it fails for e.g., annular or stratified flow patterns, cf. [9]. ...
... Especially in case of vapor-liquid flows operated at saturation conditions, it is difficult to eliminate vapor formation, cf. [9]. Also Hoffmann [3] reports on this problem with regard to the start-up procedure of a solar thermal facility with direct steam generation. ...
Wire-mesh sensors are well-established scientific instruments for measuring the spatio-temporal phase distribution of two-phase flows based on different electrical conductivities of the phases. Presently, these instruments are also applied in industrial processes and need to cope with dynamic operating conditions increasingly. However, since the quantification of phase fractions is achieved by normalizing signals with respect to a separately recorded reference measurement, the results are sensitive to temperature differences in any application. Therefore, the present study aims at proposing a method to compensate temperature effects in the data processing procedure. Firstly, a general approach is theoretically derived from the underlying measurement principle and compensation procedures for the electrical conductivity from literature models. Additionally, a novel semi-empirical model is developed on the basis of electrochemical fundamentals. Experimental investigations are performed using a single-phase water loop with adjustable fluid temperature in order to verify the theoretical approach for wire-mesh sensor applications and to compare the different compensation models by means of real data. Finally, the preferred model is used to demonstrate the effect of temperature compensation with selected sets of experimental two-phase data from a previous study. The results are discussed in detail and show that temperature effects need to be handled carefully—not merely in industrial applications, but particularly in laboratory experiments.
... It is a reliable tool which will be used as a reference to evaluate the ultrasonic void fraction measurement. The technique has a high spatial and temporal resolution allowing detailed visualization of the flow phases [25]. Figure 5 is a representation of a WMS with a 12 × 12-wire grid. ...
... where the voltage readings V are proportional to fluids' electrical permittivity, (i, j) are wire indices, and k is the discrete temporal index (for details see [8] and [25]). By appropriate processing of the matrix α, it is possible to visualize the data in different ways, for instance, averaged one-dimensional time series, time-averaged 2D images, as well as threedimensional contour images, as was described in our previous work [8] and was also applied here. ...
In this article, we propose a novel ultrasonic technique to measure the gas void fraction in a liquid-gas intermittent swirling flow. The method converts the liquid film thickness measurement to a gas void fraction value considering a symmetrical flow pattern. We propose the use of the first echo peak instead of the standard maximum echo peak to estimate the liquid film thickness. The results are compared with a wire-mesh sensor, a reference for gas void fraction measurement, in a 1-inch inner-diameter vertical flow loop. A Root-Mean-Square-Deviation (RMSD) of 2.87% was observed between the proposed ultrasonic method and the reference sensor. An improvement of 6.06% if compared to the standard film thickness method (8.93%). The studied cases show that our method improves the use of ultrasound for gas void fraction measurement in a gas-liquid swirling flow.
... Ключевые слова: сетчатый кондуктометрический датчик, двухслойный датчик, пузырьковое течение. DOI: 10.21122/2220-9506-2021- 12-3-183-193 Introduction Justification of the reliability, safety and efficiency of nuclear power plant circuits is an important task for developers. ...
... However from the point of physics this procedure is not fully justified since it leads to a violation of the of conductivity balance at the WMS plane. In addition in [10] it was shown that the elimination of conductivity overshoots by the linear cut method leads to an overestimation of the value of the instantaneous local gas content when using mesh sensors. ...
One of the important tasks in carrying out a computational justification of the reliability and safety of equipment that is part of the projected nuclear power plants today is the modeling of the bubbly regime of the coolant flow. In this regard the aim of this work is the use of extended methods of using matrix conductometric systems which are widespread in research practice for study of gas-liquid flows.
The work uses a method of primary processing of experimental data aimed at eliminating of excess conductivity in the cells of the developed wire mesh sensor which makes it possible to obtain the values of the true volumetric gas content in the investigated area.
Subsequent analysis of the possibilities to estimate the volumes of registered gas bubbles by the gradient method as well as the size of the interface in the sensor cells which plays a key role in modeling the interfacial heat and mass transfer.
Comparison of readings values with the control instruments cues showed a good agreement. The presented work is an adaptation of the use of a conductometric measuring system for the study of multicomponent flows with the aim of further application for the study of two-component flows in the channels of the core simulator using wire mesh sensors.
... Recent years' the WMS developed by Prasser (Prasser et al., 2005;Prasser, 2007;Pietruske and Prasser, 2007;Beyer et al., 2010;Banowski et al., 2016) can measure the void fraction of the whole cross section instantaneously with a high spatial and temporal resolution. A comprehensive evaluation of WMS proposed by Tompkins et al. (2018) shows that the WMS is a good choice for this study. The authors of the present paper manufactured a WMS to measure the critical void fraction for bubbly-slug flow transition. ...
... In this paper, the multi bubbles measuring error is more concerned because the critical void fraction is a macro parameter for multiple bubbles two phase flow. The available data shows that the WMS has a uncertainty of ±10.5% for void fraction measurement, but the accuracy is largely based on the practical application (Tompkins et al., 2018). So a calibration experiment is needed for this study. ...
Two-phase flow is an important and common phenomenon in reactor systems, and there are significant differences in the heat and mass transfer characteristics of two-phase flow under different flow patterns. Researchers have proposed a variety of flow pattern transition mechanisms to predict the two-phase flow. But limited by the means of measurement, the critical void fraction for flow pattern transition can’t be determined. Two types of bubbly-slug flow transition criteria in vertical circular tubes are studied in this paper and a wire mesh sensor (WMS) was manufactured to measure the critical void fraction for bubbly-slug transition. One hundred forty-seven visualization tests were carried out for validation.
... The sensor accuracy and its intrusiveness has been investigated by several works (most recently by [57]), revealing the void fraction uncertainty to be less than 5% when compared against techniques such as ultra-fast X-ray CT [32,58] and gamma measurements [59,60] and less than 11% relative to secondary measurement parameters (e.g. bubble size measurements, flow rates or comparison via simulation, etc.) regardless of flow regimes [34,[61][62][63][64]. ...
... With no intention of being exhaustive, the next paragraphs summarize the main applications of conductivity wire-mesh sensor in recent years. Readers can also refer to previous reviews on wire-mesh sensor and its applications [57,65,66]. ...
Multiphase flow is a commonly seen transient and complex dynamic system in many industrial processes. The phase fraction and velocity are two of the most important parameters for flow monitoring and measurement. Due to the advantages of simplicity in sensor structure, low fabrication costs and fast response, conductance sensors have received broad attentions in horizontal, vertical and inclined multiphase pipe flow measurement. A conductance sensor measures the multiphase mixture conductivity between two electrodes in contact with the fluid to determine the phase fraction. Combined with cross-correlation technique, the velocity can also be acquired. This paper presents a review on the basic sensing principle, different types of conductance sensors and their applications in flow monitoring, flow pattern identification, phase fraction determination and velocity measurement of multiphase flow. Finally, based on the conclusion of the disadvantages, advantages and limitations of this technique, the insight into trends for future development are given.
... Despite using the same sensors in the outlets, we needed to consider the effects of the 4×4 WMS uncertainties. According to [21], WMS can measure the void fraction within 11%-relative to other methods-but the accuracy can be better, depending on the flow pattern. There was also a spatial resolution issue, as WMS pitch effects caused differences in the order of 1 to 4% for bubbly flows, but up to 20% peak at a low void fraction. ...
Wire-mesh sensors are used to determine the phase fraction of gas–liquid two-phase flow in many industrial applications. In this paper, we report the use of the sensor to study the flow behavior inside an offshore oil and gas industry device for subsea phase separation. The study focused on the behavior of gas–liquid slug flow inside a flow distribution device with four outlets, which is part of the subsea phase separator system. The void fraction profile and the flow symmetry across the outlets were investigated using tomographic wire-mesh sensors and a camera. Results showed an ascendant liquid film in the cyclonic chamber with the gas phase at the center of the pipe generating a symmetrical flow. Dispersed bubbles coalesced into a gas vortex due to the centrifugal force inside the cyclonic chamber. The behavior favored the separation of smaller bubbles from the liquid bulk, which was an important parameter for gas-liquid separator sizing. The void fraction analysis of the outlets showed an even flow distribution with less than 10% difference, which was a satisfactorily result that may contribute to a reduction on the subsea gas–liquid separators size. From the outcomes of this study, detailed information regarding this type of flow distribution system was extracted. Thereby, wire-mesh sensors were successfully applied to investigate a new type of equipment for the offshore oil and gas industry.
... Wire-mesh sensor (WMS) schematic and electrode circuit diagram[6]. ...
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.
... There are several measurement methods to determine the void fraction, among which the capacitance, conductance, optical, ultrasonic, or radiative methods are renowned (Tompkins et al., 2018). Radiative methods are non-intrusive techniques that employ radiations such as neutron, gamma, and X-rays. ...
Void fraction is an important parameter to design and simulate thermal systems involving two-phase flows. In this research, an experimental investigation of the void fraction in adiabatic two-phase flow of R1234yf in horizontal and smooth tubes with an internal diameter of 4.8 mm was carried out. For the experimental tests, the vapor quality ranges from 0.1 to 1 while two saturation temperatures (15 and 25°C) and two mass flow rates (180 and 280 kg.m-2s-1) are investigated. The quick-closing valve(s) method is used to measure the volumetric void fraction. Tests are also undertaken with R134a used as a reference in this study. The results highlight that the void fraction of R1234yf is 5% lower than that for R134a. In addition, the experimental data of R1234yf were compared against seven correlations from the literature: the Baroczy and the Hughmark correlations were shown to provide the best prediction, with a mean absolute error of 2% and 3.2%, respectively.
... Auf Grundlage von Ergebnissen einer Simulationsstudie zu den Messsignalen eines Leitfähigkeits-Gittersensors für eine idealisierte, feindisperse Blasenströmung empfiehlt Prasser et al. [13] die Anwendung des Maxwell-Ansatzes zur Umrechnung der primär gemessenen Leitwerte in lokale Gasgehalte. Tompkins et al. [14] gibt darüber hinaus den Ansatz nach Bruggeman zur möglichen Umwandlung der Leitwerte in Phasenanteile an. Eine Validierung der beiden Ansätze anhand experimenteller Daten liegt nach Wissen der Autoren bisher jedoch nicht vor. ...
de In experimentellen Versuchen wurde die Genauigkeit eines Leitfähigkeits‐Gittersensors mit einer zeitlichen und räumlichen Auflösung von 5 kHz und 4,8 mm in Bezug auf die Bestimmung von Blasengrößen untersucht. Als Referenzwert werden einzelne Luftblasen definierten Gasvolumens im Größenbereich von 2 – 12 mm in eine ruhende Flüssigkeit aufgegeben. Die Ergebnisse zeigen eine erhöhte Messunsicherheit für Blasen, deren Durchmesser kleiner als die Auflösung des Gitternetzes ist. Die ermittelte Blasengröße hängt in diesem Fall stark von der örtlichen Position der aufsteigenden Gasblasen im Gitternetz ab.
Abstract
en An experimental study to assess the accuracy of a wire‐mesh sensor with a temporal and spatial resolution of 5 kHz and 4.8 mm in dependence of bubble size has been carried out. As a reference, single air bubbles with a defined bubble size of 2 – 12 mm are injected in a stagnant liquid phase. The results show a higher uncertainty for bubble diameters below the grid resolution of the sensor. In this case, the bubble size depends strongly on the local bubble position within the mesh grid during its passage.
... Following Kline-McClintok's method to determine the experimental uncertainties, it was readily verified that the pressure time-series were measured with an uncertainty of ±10% with a confidence level of 95% ( [19,20]). It is worth mentioning that this uncertainty is well within the normally accepted range in most multiphase flow contexts (e.g., [21][22][23]). Accordingly, at least 3 experiments were conducted for each entry of the experimental matrix (i.e., for every (q g , q l ) combination). ...
We characterize the long-term development of high-viscosity gas–liquid intermittent flows by means of a detrended fluctuation analysis (DFA). To this end, the pressures measured at different locations along an ad hoc experimental flow line are compared. We then analyze the relevant time-series to determine the evolution of the various kinds of intermittent flow patterns associated with the mixtures under consideration. Although no pattern transitions are observed in the presence of high-viscosity mixtures, we show that the dynamical attributes of each kind of intermittence evolves from one point to another within the transport system. The analysis indicates that the loss of a long-range correlation between the pressure responses are due to the discharge processes.
... However, in the context of a typical industrial process environment there are several challenges associated with its measurement. Current technologies for the determination of BSD are often intrusive or disruptive to the process flow, for example, using wire-meshes to measure the fluid conductivity to infer the BSD [16] [17]. On the other hand, non-invasive techniques, such as photographic analysis, require a clear optical path to exist between the measurement sensor and the process [18]. ...
In this paper, ultrasonic phased arrays are deployed as an imaging tool for industrial process analysis. Such arrays are typically used for sonar, medical diagnosis and non-destructive testing, however, they have not yet been applied to industrial process analysis. The precise positioning of array elements and high frequencies possible with this technology mean that highly focused images can be generated that cannot currently be achieved using ultrasound tomography. This paper aims to highlight the potential of this technology for measurement of bubble size distribution (BSD) and to demonstrate its application to both intrusive and non-invasive process measurement. Ultrasound images of bubble reflectors are generated using the total focusing method deployed using a 32 element, 5 MHz linear phased array and an image processing algorithm for BSD determination is presented and evaluated under stationary and dynamic acquisition conditions. It is found that the sizing accuracy is within 10% for stationary reflectors larger than 4λ in diameter and that the algorithm is stable across the expected spatial variation of reflectors. The phased array is coupled to a six-axis robotic arm to scan a solid sample containing bubble reflectors at velocities up to 500 mms–1. The sizing accuracy is within 45% for bubbles larger than 4λ in diameter and at velocities up to 300 mms–1. However, above this velocity the algorithm breaks down for reflectors smaller than 9λ in diameter. The ultrasound system is applied to a stream of air bubbles rising through water that is verified via photographic analysis. Images were generated both intrusive and non-invasive, via a 10 mm Perspex barrier, to the process stream. The high bubble density in the process stream introduced scattering, limiting the measurement repeatability and the sample size in the measured distribution. Notwithstanding, this result demonstrates the potential of this technology to size bubbles for intrusive and non-invasive process analysis.
... The interfacial area concentration represented by the small bubbles was between 1 and 6% of the total interfacial area concentration measured by the wire-mesh sensor, and maybe surprisingly, the fraction of the small bubbles seemed to decrease with increasing flow rate, Table 2. The influence of the small bubbles can therefore be considered to be almost insignificant in view of the other uncertainties of the wire-mesh sensor measurement (Tompkins et al., 2018). ...
The international EU-SGTR and ARTIST projects investigated the transport of fission products in the form of aerosols during SGTR severe accidents. The major finding of the two projects was that there was significant retention of aerosols in the steam generator secondary side, and that the retention was increased by more than an order of magnitude if the secondary side of the steam generator was flooded with water. Furthermore, the experiments with the flooded secondary side showed that the aerosol particle retention was significantly increased due to the presence of submerged structures, i.e., the tube bundle, as compared to an empty pool. The increased aerosol retention was attributed to the interactions of the high velocity gas jet discharged from the tube break with the dense bundle of the steam generator tubes. Under these conditions, the two-phase flow is very complex due to the high gas velocities and complicated geometry of the steam generator secondary side.
To determine the effect of the tube bundle on aerosol retention, hydrodynamic characteristics of an empty pool and a flooded steam generator secondary side were measured in a facility equipped with wire-mesh sensors. The facility was equipped with either a tube bundle consisting of 221 steam generator tubes, or with a single tube in the center of the facility. The flow development could be followed by making the measurements at different distances between the gas injection and the measurement point, and using different flow rates. The facility was operated at close to ambient conditions.
Void fraction, bubble size distributions, gas phase velocity as well as interfacial area concentration were determined based on the wire-mesh sensor data. In addition, the penetration depth of the initial large gas bubble into the channel was studied for the closest break-sensor distances. The investigations show distinct differences between the flow characteristics in the single tube geometry and in the tube bundle. As compared to the single tube, the flow was more confined in the tube bundle due to the interactions of the flow with the tubes. The interfacial area between the liquid and gas phase is larger with the tube bundle than with the single tube and also the bubble size distributions show distinct differences between the two geometries.
... As EIT takes measurements only in the contour of the object of interest, it is an advantageous approach for the characterization of multiphase flows, since it does not distort the flow geometry [1]. Although wire-mesh sensors [12] are currently a successful technique for imaging multiphase flows, their intrusiveness exposes them to erosion by solid particles, for instance, in an oil-industry flow typically composed of oil, gas, water and sand, aside from hydrates and paraffin. As a step towards typifying these complex flows, experimental analyses in laboratories are commonly performed with air-water two-phase flows. ...
Electrical impedance tomography (EIT) is a severely ill-posed nonlinear inverse problem. In order to obtain solutions with physical meaning, the inverse of the model of measurements requires the combination of information from various sources. This paper proposes a new approach through Kalman filtering for adaptive integration of EIT measurements, Tikhonov regularization and evolution models for the characterization of a two-phase air–water fluid flow. The Tikhonov regularization factor is embedded into the observation error covariance matrix, thus allowing for individual adjustment for each of the regularization equations. The filter outputs for different evolution models—random walk, advective and advective–diffusive—are compared in terms of estimate convergence and physical meaning. With the random walk evolution model the analysis of experimental data shows that the proposed information fusion strategy provides fewer artifacts, enabling a more effective identification of the phase interfaces. When the other two evolution models are incorporated into the Kalman filter and compared with the random walk model, faster and more accurate estimates of the flow are obtained even away from the electrodes, as well as sharper phase interfaces are identified. The results suggest that the reason for this improved performance is the fused information from the upstream–downstream dynamics of the advective and advective–diffusive models with the outer-inner structure influence of measurements.
... Fig. 4d displays a photo of the WMS. It was successfully employed for gas-liquid flow by da Silva et al. [26],Azzopardi et al., [13,9,5] , Vuong et al. [76],Shaban et al. [66], Zhao et al. [81], Tompkins et al. [72], Prasser and Hafeli [64,3,2,8] . ...
Reliable and accurate prediction of the transition from spherical cap bubble to slug flow is crucial not only to the operation of industrial facilities such as the crude oil pipelines, bubble column, and nuclear reactors but also for model development for computational fluid dynamics (CFD) studies. The present paper presents a review of the transition mechanics from spherical cap bubble flow to slug flow in vertical pipes. The bubble flow was split into sub-regions, bubbles and spherical cap bubbles and the mechanisms to classify them (i.e., bubble terminal velocity and cap bubble velocity) was analysed. For now, the literature review does not present some important previous works. This paper presents an original data set of gas–silicone oil in vertical pipes to support the new findings. The experimental two-phase data classifies the flow patterns using the probability density function (PDF) and shows the important flow variables such as average void fraction, pressure gradient, slug body void fraction, liquid slug, Taylor bubble and slug unit lengths, structural velocity and frequency obtained by electrical capacitance tomography (ECT) and a wire mesh sensor (WMS).
... (1)) assumes a linear relationship between local gas fraction and measured conductance which is suitable in case of void fractions close to zero or one, when the overall bubble surface areas are essentially larger than the WMS grid resolution [16]. Thus, this assumption is questionable in case of dispersed bubbles, which is why other calculation approaches such as Maxwell [16] or Bruggemann [17], have been suggested. Further, the calculation of gas velocities based on an empirical correlation causes uncertainties that might have a major impact on the reconstructed bubble diameter, especially in case of small bubbles. ...
Two Machine Learning algorithms – LASSO and Random Forest – are applied to derive regression models for the prediction of gas bubble diameters using supervised learning techniques. Experimental data obtained from wire-mesh sensor (WMS) measurements in a deionized water/air system serve as the data base. Python libraries are used to extract features characterizing WMS measurement signals of single passing bubbles. Prediction accuracy is largely increased with the obtained regression models, compared to well-established methods to predict bubble sizes based on WMS measurements.
The wire-mesh sensor (WMS) is a kind of advanced instrument for gas-liquid two-phase flow measurements. In previous research, the sensitive volume was assumed to be uniformly distributed in the cross-section. The received current was assumed to represent the conductivity averaged over a uniform sensitive volume (USV), which is defined by symmetry considerations within the geometry of the electrode grids of the sensor. The void fraction within USV can be further determined based on the conductivity of USV. However, it is unphysical to determine the USV’s conductivity by the received current directly, since the true sensitive volume has an irregular shape due to the complexity of the potential field in the WMS. This study proposes an iterative method to determine the normalized conductivity averaged over each USV from the received current of WMS. The iterative method is based on potential field simulations within the electrode geometry of the WMS and an iterative process. To evaluate the performance of the iterative method, different normalized conductivity distributions were pre-defined. The maximum deviation between the pre-defined normalized conductivity and the normalized conductivity value determined by the new method was within 3%, instead of 30% for the conventional method. The performance of the new method for WMSs with different axial distance between transmitter and receiver grids La was also evaluated. For a larger La, more iterations need to be carried out to reach this accuracy. The limitation of this iterative method has been discussed. To reconstruct the sub-USV non-conductive phase distribution, the resolution of iterative method should be enhanced to describe the distribution of sub-USV non-conductive phase in future studies.
The wire-mesh sensor (WMS) is a promising instrumentation for two-phase flows investigation. Since the WMS is a kind of intrusive sensor, some researchers have focused on its intrusive effects on the measurement results. However, apart from the intrusive effects, the uniform sensitive volume assumption also brings a systematic error. For a WMS, it is assumed that there is a linear relationship between the current received by the receiver wire and the void fraction within a square sensitive volume, of which the side length equals to the lateral distance between two adjacent wires. Since the complexity of the potential field, the uniform sensitive volume assumption brings a systematic error. In order to evaluate this systematic error, a cuboid with zero conductivity was introduced in 9 × 9 WMS potential field simulations. The results indicate that the systematic error caused by the uniform sensitive volume assumption increases in the wire plane axial distance. The wire diameter has little effects on the measurements. However, in view of eliminating the intrusive effects, the wire diameter should be as small as possible. The phenomena of over-shoots and cross-talk, which can be observed in experiments, were explained by the potential field simulation results. With the wire plane axial distance increase, the over-shoots and cross-talk will be exacerbated. Even though the over-shoots and cross-talk can cause a deviation for local void fraction measurements, they would not cause any systematic error for sectional average void fraction measurements in a square cross-section.
The slip velocity between phases is an important parameter for evaluating the volumetric flow rate of each phase in vertical pipes. This paper presents an improved empirical correction for calculating the slip velocity of two-phase, oil-water bubble flows in large-diameter vertical pipes, based on a series of experimental tests. Water holdup data were obtained through experiments on an oil-water mixture flowing through a transparent test section in a 16-m-long vertical pipe with an inner diameter of 124 mm. The water holdup ranged from 0.58 to 1, and the observed flow pattern was bubble flow. Based on these experimental data, a modified correction was developed to improve the accuracy of the prediction of slip velocity for oil-water bubble flow systems. In addition, the proposed correction was validated against published experimental data for pipes with diameters of 2–6 in. The performance of the proposed correction against an experimental dataset (from our tests and published data) was also compared with that of six other, widely accepted corrections. The proposed correction predicted the slip velocity of two-phase, oil-water bubble flow in large-diameter vertical pipes more accurately than previous corrections did.
Measurements of the composition of multiphase flows, especially water-cut in oil-water flow, is a frequently encountered problem in the petroleum industry. New techniques that can offer improvements towards that are continuously sought and the use of microwave sensors is considered very promising. The current paper reports on the performance of a microwave cylindrical resonator, integrated into a multiphase flow experimental facility, used to determine water-cut in an upward flowing oil-water mixture. The performance and suitability of two microwave resonant modes (TM010 and TM110) was studied. The TM110 mode was found to be less dependent on the spatial phase distribution of the oil-water flowing mixture and provided more consistent results across a broader flow regime. The relative errors between the predicted and actual water-cut were from −6.53% to 9.16% and relative errors of 78% of the total data points were inside −5% to 5%. The results suggest that appropriate sensor design and careful selection of the operating frequency/resonant pattern can offer a powerful technique for real time, on-line and non-destructive determination of water-cut in multiphase flow systems.
Transient gas–liquid two-phase flows are often encountered in nuclear industries. Investigating the dynamics of the transient gas–liquid flows is of significance for optimizing the reaction processes and predicting the thermal–hydraulic phenomena relevant to the performance of nuclear reactors. In this study, a multi-nozzle gas inlet was designed to mimic the transient gas–liquid flows in a horizontal pipe. A conductance wire-mesh sensor (WMS) was used to visualize the phase distribution at the pipe cross-section. Based on the WMS data, a cost-based recurrence analysis (CBRP) was developed to reveal the dynamics of transient gas–liquid flows. Different transition processes from plug flow to slug flow were observed in the experiment and explored by the determinism (DET) of the CBRP. Note that special flow structures, i.e., wave bridge and pseudo slug, were encountered during the flow pattern transition, and their generation mechanisms were uncovered. The processes of the flow pattern transition were also characterized by the slug frequency and the probability density function of the water holdup, which presented a good agreement with the CBRP results.
Mixtures of oil and water are common in oil-producing wells. When the oilfield has a mid-late exploitation status, the production of oil and water gradually decreases and the comprehensive water content sharply increases with time. An accurate evaluation of the volumetric flow rate of each phase in the pipe is of importance for diagnosing production problems, developing an engineering design, and guiding reservoir management activities. In this study, the slip behaviours we investigated based on the high-quality experimental data obtained from a multiphase flow loop with a large-diameter (inner diameter: 5.5 inch) vertical pipe with low flow rates (<30 m³/d). Furthermore, the interpretation accuracies of five interpretative models of vertical oil/water flow ware comprehensively evaluated. The results show that the slip trends to increase with decreasing flow rates. However, it was found that, when the flow rate is less than 7 m³/d, the flow rate minimally affects the slip. Additionally, the variation tendency of the slip velocity as a function of the water holdup is not stable, hence, a reasonable interpretative model should be selected according to the water holdup. The slip velocity gradually increased, and was found to be nearly linear, and the slip model proposed by Hasan and Kabir (1999) demonstrated the best adaptability under the conditions of the tested data and a water holdup of less than 0.9. The slip velocity exponentially increased with increasing water holdup and the modified drift-flux model proposed by Wu et al. (1992) was found to be more appropriate under the condition that the water holdup exceeds 0.9. Overall (i.e. for all of the test data), the Hasan model (1999) yielded better processing results for the vertical oil/water flow system under the condition of low flow rates. The analysis in this study provides better insight into the slip between the oil and water phases in a two-phase flow system with a low flow rate. In addition, the interpretative model with the best predictive capability is highlighted in this paper.
A wire-mesh sensor (WMS) is a widely used instrument to visualize and estimate derived parameters of multiphase flows, e.g., gas void fraction or liquid hold-up. The spatial resolution of obtained flow images is associated with the number of crossing points formed by the transmitter and receiver wires of a given sensor. This may be a limitation for applications that require high spatial resolution since WMS is an intrusive device and the increase of electrodes may increase pressure drop and deform/fragment bubbles. In order to minimize such undesirable effects and maximize the sensor resolution, we employed a reconstruction algorithm based on the minimum mean-square error (MMSE) estimator to increase image resolution of WMS with fewer wires than commonly reported in the literature, i.e., here, we apply
$8\times 8$
,
$6\times 6$
,
$4\times 4$
, and
$2\times 2$
sensors for 1-in pipe. Since standard regularization approaches may provide incorrect solutions for such configurations, a new methodology to obtain the prior model is presented. In our approach, the prior is assumed as a multivariate Gaussian model, which is extracted from experimental flow data of a
$16\times 16$
WMS (the most common resolution for 1-in pipe). Finally, the sensitivity matrix obtained by electric field simulation and the experimental prior model is incorporated into the MMSE algorithm to restore experimental flow data of the low-resolution sensors. The experiments were performed in a flow loop operating at slug flow. The experimental results suggest that the MMSE estimator combined with the experimental prior model has a high potential not only to improve image resolution but also to correct the average void fraction estimation.
The impedance flow sensors have usually been considered as the competitive components in the multi-flow measurement, primarily due to their advantages of fast response, high dynamic range and sensitivity, non-invasive and/or non-intrusive measurements. In this review, the impedance sensors have been categorized into two categories, i.e., the probes for local/average parameter measurement and sensor arrays for distributed parameter measurement. Maxwell’s equations and boundary conditions have been referred as a starting point to understand the measuring principle of impedance sensors, from which the sensitivity distribution can be attained. The sensitivity distribution can be utilized to characterize the sensor and employed as an objective function in optimizing the sensor geometry. The EMA models usually offer a way to relate the impedance measurements to individual phase fraction, which have been critically reviewed. In addition, the application range, spatial and temporal resolutions, advantages and disadvantages of the sensor arrays, i.e., the wire mesh sensor, field focusing sensor, electrical resistance and capacitance tomography sensors have also been provided. Finally, the future trends of impedance sensor development have been briefly discussed.
Pseudo-slug flow occurs between the slug and segregated flows and occupies a non-negligible area in the flow pattern map especially for gas condensate pipeline/wellbore where the liquid flow rate is low. An experimental study was conducted in a 3-in. flow loop with a valley configuration to investigate pseudo-slug flow characteristics using wire-mesh sensors, which include total average liquid holdup, liquid holdups in pseudo-slug body, and film region, respectively, pseudo-slug frequency, body length, and structure velocities.
Comparison with conventional slug characteristics shows that pseudo-slugs have lesser liquid holdup in slug body, shorter body lengths, and smaller structure velocities. The pseudo-slug characteristics were also compared with predictions from existing correlations for conventional slug flows, as no correlation is available for pseudo-slug flow. The comparison shows poor agreement as expected, which necessitates the development of correlations for pseudo-slug flow specifically.
The spatial evolution of pseudo-slugs is also discussed. Two initiation and dissipation mechanisms are described based on the location where pseudo-slugs are initiated and dissipated. In general, pseudo-slugs are shorter and slower near the inlet of uphill section compared the farther downstream locations in the uphill section.
The existing evidence clearly shows that the physical phenomena governing gas/liquid two-phase flows are quite complicated even in the case of smooth conduits and simple geometries. Needless to say, the development of experimental, analytical and computational methods for predicting such flows in complex geometries is an even more complicated and challenging task. A configuration of interest to a broad range of industrial applications, including nuclear reactors, deals with the flow of two-phase mixture along the channels formed between narrow arrays of multiple parallel cylindrical elements (tubes or rods). The alignment of such elements is normally accomplished by installing spacer grids placed at regular distances along the flow. The presence of spacers actually affect flow conditions, including the velocity field, pressure drop, heat transfer and, in the case of two-phase flows, phase distribution.
The objective of this paper is to present a comparative analysis of the results of a combined experimental, theoretical, and computational study of phase distribution around and downstream from complex-geometry spacer grids with split-vane type mixing devices. The main emphasis has been given to the analysis of the effect of proper interpretation of the experimental data on the modeling consistency. The importance of the understanding of uncertainties and limitations associated with the results of multidimensional computer simulations performed using mechanistic modeling principles based on an average bubble size is also discussed.
Condensation of vapor bubbles in a subcooled liquid is known to influence heat transfer and pressure oscillation in subcooled boiling and direct contact condensation. This study reviews the published literature concerning interfacial heat transfer and bubble dynamics in the process of bubble condensation. The correlations for bubble condensation are analyzed and evaluated with a database covering a wide range of Reynolds, Jacob, and Prandtl numbers. Then, the investigations addressing bubble dynamics are reviewed, which focus on the bubble condensation patterns, motion, collapse, and the pressure oscillations induced by bubble condensation, as well as the effect of noncondensable gas and field. Despite the extensive experiments of bubble condensation available in the literature, it is shown that there is still a shortage of investigation focused on the variation of thermal boundary layer and turbulence formed near the bubble at the micro-scale, which could help to develop the prediction method of bubble condensation in the future. The transportation of noncondensable gas inside the mixture bubble and effect of capillary waves formed on the bubble surface on the actual vapor-liquid contact area and thermal boundary are also suggested to be further investigated to gain the thorough understanding of the bubble condensation process.
The analysis of the flow behavior within a cooling panel of a scaled water-cooled reactor cavity cooling system has been performed supported by high-resolution measurements of the void fraction at the risers’ outlet. The results have shown a stable, symmetric distribution of the flow through the risers during subcooled boiling conditions, characterized by small, slow bubbles with average void fraction lower than 0.3 for all risers. As the temperature increased to saturation, flow distribution appeared strongly asymmetrical and unstable, with peaks of void fraction as high as 0.9. Large, faster bubbles were observed during these peaks, immediately followed by flow of subcooled liquid water. Flow stagnation and inversion has also been observed. The high resolution void fraction results, complemented by the measurements of the coolant flow rate and temperatures, not only have further described the behavior of the system under hypothetical accident conditions, but represent a unique set of data to support the validation of system-level codes, and more advanced computational tools.
In this study, two machine learning based regression models are developed to predict diameters of single bubbles in a bubble column reactor based on wire-mesh sensor (WMS) measurement. Both Least Absolute Shrinkage and Selection Operator (LASSO) regression and a regression tree algorithm are used to predict bubble diameter with supervised learning techniques. Measurements are carried out in a DN150 column filled with deionized water and air as the continuous phase while WMS passage of single bubbles is investigated. A novel method for definition of different labels characterizing the passing bubble is introduced. Based on the defined labels, Machine Learning regression models are developed to predict bubble sizes. Methods for dimensionality reduction are applied, allowing for an investigation of each labels influence on model prediction quality. Both regression models perform similar or better than well-established approaches to calculate bubble diameter based on WMS measurement. As a highlight, it is shown that bubble diameters even below the sensor’s spatial resolution can be predicted with an accuracy of ±13% using the regression tree model, which is about 1/3 of the conventionally assumed measurement uncertainty at bubble diameters below the sensor’s spatial resolution.
Flow pattern identification for simultaneous gas-liquid flow in pipes is a central problem in the petroleum industry. However, there are many conventional methods used for flow patterns identification that come with some form of restrictions limited to specific operating conditions. Although previous studies made efforts to make flow patterns identification free from biases (subjectivity), objective predictions are yet to be fully explored. For the first time, this study employs a 4-layer convolutional neural network (CNN) architecture and threshold segmentation algorithm to classify industrial working fluids of air/silicone oil flow patterns in a vertical pipe. Also, an advanced wire mesh sensor (WMS) instrumentation was used to obtain the cross-sectional frame images from a series of 52 experimental runs. This study made an attempt to use the sequential cross-sectional frames original data obtained from the experimental WMS because it is the crossing wires of the WMS that interact with the flow field and also capture the main flow pattern transitional zones. The obtained experimental testing results indicate that the supervised CNN model has an accuracy of 99.90% better than the seven-benchmarked supervised machine learning classification models used in recognizing bubbly, slug and churn flow patterns. The CNN model experimental test accuracy also outwits other related flow patterns identified in the literature when compared, thereby affirming the model's robustness. Since the WMS provide high spatial and temporal resolutions data, the average pixel intensity values from the segmented images were used as the flow indicator in identifying the transition zones within the main flow patterns. Furthermore, the Locally Interpretable Model-agnostic Explanation (LIME) algorithm for the first time was used to explain and interpret features in the WMS flow images that were contributing to the overall CNN classification scores. Finally, the CNN trained model was able to classify the main flow patterns and segmented transitional zones with certainty and confidence without subjectivity which is inherent in the direct conventional methods.
Inverse Uncertainty Quantification (UQ) is a process to quantify the uncertainties in random input parameters while achieving consistency between code simulations and physical observations. In this paper, we performed inverse UQ using an improved modular Bayesian approach based on Gaussian Process (GP) for TRACE physical model parameters using the BWR Full-size Fine-Mesh Bundle Tests (BFBT) benchmark steady-state void fraction data. The model discrepancy is described with a GP emulator. Numerical tests have demonstrated that such treatment of model discrepancy can avoid over-fitting. Furthermore, we constructed a fast-running and accurate GP emulator to replace TRACE full model during Markov Chain Monte Carlo (MCMC) sampling. The computational cost was demonstrated to be reduced by several orders of magnitude. A sequential approach was also developed for efficient test source allocation (TSA) for inverse UQ and validation. This sequential TSA methodology first selects experimental tests for validation that has a full coverage of the test domain to avoid extrapolation of model discrepancy term when evaluated at input setting of tests for inverse UQ. Then it selects tests that tend to reside in the unfilled zones of the test domain for inverse UQ, so that one can extract the most information for posterior probability distributions of calibration parameters using only a relatively small number of tests. This research addresses the "lack of input uncertainty information" issue for TRACE physical input parameters, which was usually ignored or described using expert opinion or user self-assessment in previous work. The resulting posterior probability distributions of TRACE parameters can be used in future uncertainty, sensitivity and validation studies of TRACE code for nuclear reactor system design and safety analysis.
In nuclear reactor system design and safety analysis, the Best Estimate plus Uncertainty (BEPU) methodology requires that computer model output uncertainties must be quantified in order to prove that the investigated design stays within acceptance criteria. "Expert opinion" and "user self-evaluation" have been widely used to specify computer model input uncertainties in previous uncertainty, sensitivity and validation studies. Inverse Uncertainty Quantification (UQ) is the process to inversely quantify input uncertainties based on experimental data in order to more precisely quantify such ad-hoc specifications of the input uncertainty information. In this paper, we used Bayesian analysis to establish the inverse UQ formulation, with systematic and rigorously derived metamodels constructed by Gaussian Process (GP). Due to incomplete or inaccurate underlying physics, as well as numerical approximation errors, computer models always have discrepancy/bias in representing the realities, which can cause over-fitting if neglected in the inverse UQ process. The model discrepancy term is accounted for in our formulation through the "model updating equation". We provided a detailed introduction and comparison of the full and modular Bayesian approaches for inverse UQ, as well as pointed out their limitations when extrapolated to the validation/prediction domain. Finally, we proposed an improved modular Bayesian approach that can avoid extrapolating the model discrepancy that is learnt from the inverse UQ domain to the validation/prediction domain.
In this study, a new and improved electrical conductance sensor is proposed for application not only to a horizontal pipe, but also an inclined one. The conductance sensor was designed to have a dual layer, each consisting of a three-electrode set to obtain two instantaneous conductance signals in turns, so that the area-averaged void fraction and structure velocity could be measured simultaneously. The optimum configuration of the electrodes was determined through numerical analysis, and the calibration curves for stratified and annular flow were obtained through a series of static experiments. The fabricated conductance sensor was applied to a 45 mm inner diameter U-shaped downward inclined pipe with an inclination angle of 3◦ under adiabatic air-water flow conditions. In the tests, the superficial velocities ranged from 0.1 to 3.0 m/s for water and from 0.1 to 18 m/s for air. The obtained mean void fraction and the structure velocity from the conductance sensor were validated against the measurement by the wire-mesh sensor and the cross-correlation technique for the visualized images, respectively. The results of the flow regime classification and the corresponding time series of the void fraction at a variety of flow velocities were also discussed.
This article presents an assessment of the accuracy of gas flow rate measurement in gas-liquid pipe flows by cross-correlating dual wire-mesh sensor signals. The differences between the estimated and the actual gas superficial velocities in different flow regimes were analyzed. It was demonstrated that this gas flow rate measurement method is susceptible to significant systematic errors, some of which are inherent to the use of cross-correlation and others which are specific to wire-mesh sensors. It was concluded that this method would be accurate only for flow conditions within narrow ranges.
Wire mesh sensors (WMS) are fast imaging instruments that are used for gas–liquid and liquid–liquid two-phase flow measurements and experimental investigations. Experimental tests were conducted at Helmholtz-Zentrum Dresden-Rossendorf to test both the capacitance and conductance WMS against a gamma densitometer (GD). A small gas–liquid test facility was utilized. This consisted of a vertical round pipe approximately 1 m in length, and 50 mm internal diameter. A 16 × 16 WMS was used with high spatial and temporal resolutions. Air–deionized water was the two-phase mixture. The gas superficial velocity was varied between 0.05 m s−1 and 1.4 m s−1 at two liquid velocities of 0.2 and 0.7 m s−1. The GD consisted of a collimated source and a collimated detector. The GD was placed on a moving platform close to the plane of wires of the sensor, in order to align it accurately using a counter mechanism, with each of the wires of the WMS, and the platform could scan the full section of the pipe. The WMS was operated as a conductivity WMS for a half-plane with eight wires and as a capacitance WMS for the other half. For the cross-sectional void (time and space averaged), along each wire, there was good agreement between WMS and the GD chordal void fraction near the centre of the pipe.
This volume presents state-of-the-art of reviews in the field of multiphase flow. In focusses on nonlinear aspects of multiphase flow networks as well as visualization experiments. The first chapter presents nonlinear aspects or deterministic chaos issues in the systems of multi-phase reactors. The second chapter reviews two-phase flow dynamics in combination with complex network theory. The third chapter discusses evaporation mechanism in the wick of copper heat pipes. The last chapter investigates numerically the flow dynamics and heat and mass transfer in the laminar and turbulent boundary layer on the flat vertical plate.
Intermittent two-phase flows are commonly encountered in the petroleum industry. Much attention has been focused by several researchers on intermittent flows existing at low superficial gas velocities (<10 m/s). There is limited work performed on intermittent structures persisting at higher superficial gas velocities (pseudo-slug flows). In the present experimental study, a conductivity-based Wire-Mesh Sensor (WMS) was used to visualize and characterize pseudo-slug flow. Experiments were performed in a 76.2 mm horizontal pipe with air and water as the working fluids at atmospheric conditions. The superficial gas and liquid velocities ranged from 9 m/s to 35 m/s and 0.45 m/s to 0.76 m/s, respectively. A 16 × 16 WMS was placed 17 m away from the pipe inlet to measure spatio-temporal void-fraction distribution. The WMS data acquisition frequency was set to 10 kHz. From the void-fraction time series data, the periodic pseudo-slug structures were visualized. The visualization suggested that unlike slug flow where the liquid structures fill the pipe cross-section, the pseudo-slugs were extremely aerated structures (high gas-liquid mixing) formed due to the gas penetration into the liquid slug body. This paper also presents the measurements of important hydrodynamic characteristics such as cross-sectional averaged void-fraction time series and mean void fraction. The effect of liquid viscosity on the visualized structures is also presented.
Wire-mesh sensors are widely used to characterize gas-liquid two-phase flows and single-phase mixing processes. The geometry of the electrode grids and the way of the signal readout generates a three-dimensional electrical field in the vicinity of the electrode wires. Resulting electrical currents at the receiver electrodes, representing the primary measuring information, are calculated by a three-dimensional potential field simulation within the sensitive volume formed by the electrode wires, whereas bubbles are taken into account as simplified, spherical or elliptical objects placed at different locations in the calculation domain. The response of the sensor to the passage of such synthetic bubbles is studied. A significant deviation from the linear dependency between the received current and the local instantaneous gas fraction is found. Overshoots of the current above the reference value obtained by calibration in plain liquid occur. Furthermore, the response of the sensor depends on the axial distance between the transmitter and the receiver electrode grids. Swarms of bubbles of small size passing through the grids of the wire-mesh sensor lead to an average decrease of the current which can be described by the average conductivity of an emulsion according to Maxwell.
Wire mesh sensors (WMS) are state of the art devices that allow high resolution (in space and time) measurement of 2D void fraction distribution over a wide range of two-phase flow regimes, from bubbly to annular. Data using WMS have been recorded at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) (Lucas et al., 2010; Beyer et al., 2008; Prasser et al., 2003) for a wide combination of superficial gas and liquid velocities, providing an excellent database for advances in two-phase flow modeling.
In two-phase flow, the interfacial area plays an integral role in coupling the mass, momentum and energy transport equations of the liquid and gas phase. While current models used in best-estimate thermal-hydraulic codes (e.g. RELAP5, TRACE, TRACG, etc.) are still based on algebraic correlations for the estimation of the interfacial area in different flow regimes, interfacial area transport equations (IATE) have been proposed to predict the dynamic propagation in space and time of interfacial area (Ishii and Hibiki, 2010). IATE models are still under development and the HZDR WMS experimental data provide an excellent basis for the validation and further advance of these models. The current paper is focused on the uncertainty analysis of algorithms used to reconstruct interfacial area densities from the void-fraction voxel data measured using WMS and their application towards validation efforts of two-group IATE models.
In previous research efforts, a surface triangularization algorithm has been developed in order to estimate the surface area of individual bubbles recorded with the WMS, and estimate the interfacial area in the given flow condition. In the present paper, synthetically generated bubbles are used to assess the algorithm’s accuracy. As the interfacial area of the synthetic bubbles are defined by user inputs, the error introduced by the algorithm can be quantitatively obtained. The accuracy of interfacial area measurements is characterized for different bubbles sizes and shapes, and for different WMS acquisition frequencies. It is found that while convex shapes are successfully analyzed by the reconstruction algorithm, some difficulties are faced with bubbles presenting internal cavities.
Utilizing the experimental HZDR database, the performance of the current two-group IATE model is evaluated. While the qualitative propagation of interfacial area is predicted sufficiently well, there is a discrepancy in magnitude between the model’s prediction and the experimental results. Overall, the study suggests that differences exist in the incidence of interaction mechanisms between small and large diameter pipes and further efforts are needed in order to extend the range of validity of current IATE models.
Transmission radiometry is frequently used in industrial measurement processes as a mean to assess the thickness or composition of a material. A common problem encountered in such applications is the so-called dynamic bias error, which results from averaging beam intensities over time while the material distribution changes. We recently reported on a method to overcome the associated measurement error by solving an inverse problem, which in principle restores the exact average attenuation by considering the Poisson statistics of the underlying particle or photon emission process. In this paper we present a detailed analysis of the inverse problem and its optimal regularized numerical solution. As a result we derive an optimal parameter configuration for the inverse problem.
Wire-mesh sensors and ultrafast X-ray tomography were developed to investigate two-phase flows with high spatial and temporal resolution. In this study we present results of a comparative study for both imaging techniques, where disperse gas-phase velocity was available from both techniques for the first time. For the experiments we designed a special test section which was operated at HZDR’s two-phase flow facility TOPFLOW. A very small vertical distance between X-ray planes and wire-mesh sensor guarantees that lateral flow changes during passage of the respective planes are almost negligible. Experiments were conducted with varying water and air superficial velocities, giving flow regimes from bubbly flow via slug flow to annular flow. Typical experimental results are presented and discussed in detail in this paper. Eventually, the application ranges for both measurement techniques are briefly discussed.
The main purpose of this study is experimental and numerical void fraction measurement in two-phase flow inside a vertical pipe by using gamma-ray. Three types of flow regimes including homogenous, stratified and annular were modeled in a vertical pipe by using polyethylene phantoms. These three flow regimes are basis regimes in two-phase flow and the other flow regimes are incorporated of these patterns. For all three modeled flow regimes all transmitted and scattered gamma rays in all directions were measured by setting a gamma ray source and detector around the pipe. Numerical modeling was done by using MCNP code to improve the accuracy and validation of experimental results. Finally, innovative correlations to predict the void fraction in two-phase flow in a vertical pipe was presented.
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.
A gas-liquid two-phase flow in a large diameter pipe exhibits a three-dimensional flow structure. The wire-mesh sensor (WMS) can acquire a quasi-three-dimensional void fraction distribution. Furthermore, the WMS can acquire a phasic-velocity distribution on the basis of the time lag of void signals between both sets of WMS. Previously, the acquired phasic velocity was one-dimensional distributions. The authors propose a method to estimate the three-dimensional phasic-velocity distribution from the same WMS data. A three dimensional velocity vector was determined on the basis of cross-correlation analysis. The flow direction is determined by the WMS measuring-point combination, whereby the cross-correlation coefficient between both sets of WMS measuring points reveals the peak. In addition, the flow structure can be extracted by size on the basis of a wavelet analysis. The proposed method was applied for two sets of 64 x 64 mesh sensors in an air-water flow in a vertical pipe with inner diameter of 224 mm. The proposed method can successfully visualize a swirl flow structure where large and small bubbles tend to move respectively in inward and outward directions in turn.
The main purpose of this study is experimental and numerical void fraction measurement for modeled two-phase flow inside a vertical pipe by using gamma-ray. Three types of flow regimes including homogenous, stratified and annular were modeled in a vertical pipe by using polyethylene phantoms. These three flow regimes are basis regimes in two-phase flow and the other flow regimes are incorporated of these patterns. For all three modeled flow regimes all transmitted and scattered gamma rays in all directions were measured by setting a gamma ray source and detector around the pipe. Numerical modeling was done by using MCNP code to provide appropriate correction coefficient for the void measuring and improve the accuracy and validation of experimental results.
Two phase flows exist as a part of many industrial processes, including chemical processes, nuclear reactor systems, and heat exchangers. In all of these applications the interfacial area concentration is an important parameter for evaluating the interactions between the phases, including drag forces, heat transfer or chemical reaction rates. Many models for interfacial area concentration exist for dispersed bubbly flows; however this type of flow only exists at relatively low void fractions. Very few correlations exist for the prediction of cap-turbulent, slug, or churn-turbulent flows. In this paper a new correlation for predicting the interfacial area concentration beyond bubbly flows in large diameter pipes is derived using a two-bubble-group method (spherical and distorted bubbles as Group-1 bubbles and cap and churn-turbulent bubbles as Group-2 bubbles) and the two-group interfacial area transport equation. The derivation assumes steady state and fully developed flow, and is based on interfacial area transport source and sink terms for large diameter pipes developed by Smith et al., 2012a. Int. J. Heat Fluid Flow 33, 156–167. The resulting equations can be used to predict the void fraction for each group of bubbles and the Sauter mean diameter for each group of bubbles in addition to the total interfacial area concentration. The model is then benchmarked based on the data collected by Schlegel et al., 2012. Exp. Therm. Fluid Sci. 41, 12–22; Schlegel et al., 2014. Int. J. Heat Fluid Flow 47, 42–56. It is found that the correlation predicts the data for Sauter mean diameter of Group 1 bubbles with RMS error of 23.3% and bias of +1.83%. For Group 2 bubbles the RMS error is 24.0% and the bias is +5.35%. This indicates that the correlation somewhat over-predicts the bubble sizes. In spite of this the prediction error remains reasonable compared to the accuracy of previous correlations, and given that the experimental uncertainty can be as high as 15% for some flow conditions. The RMS error and bias in the total interfacial area concentration are 22.6% and −4.29%, respectively. This is consistent with the over-prediction of the Sauter mean diameters, but again is reasonable considering the experimental uncertainty and the prediction error of previous correlations. The model is also able to predict the trends found in the experimental data with varied liquid and gas velocities, representing a large improvement over previous modeling efforts. An expanded database of accurate interfacial area concentration measurements at higher pressures would allow further improvement of the model benchmark and expansion of the range of applicability of the model.
Air-water two-phase flow tests in a large vertical pipe of 194.1-mm inner diameter (i.d.) are reported. Close to the outlet of a 9-m-tall test section, two wire-mesh sensors are installed that deliver instantaneous void fraction distributions over the entire cross section with a resolution of 3 mm and 2500 Hz used for fast-flow visualization. Void fraction profiles, gas velocity profiles, and bubble-size distributions were obtained. A comparison to a small pipe of 52.3-mm i.d. (DN50) revealed significant scaling effects. Here, the increase of the airflow rate leads to a transition from bubbly via slug to churn-turbulent flow. This is accompanied by an appearance of a second peak in the bubble-size distribution. A similar behavior was found in the large pipe; though the large bubbles have a significantly larger diameter at identical superficial velocities, the peak is less high but wider. These bubbles move more freely in the large pipe and show more deformations. The shapes of such large bubbles were characterized in three dimensions. They can be rather complicated and far from ideal Taylor bubbles. Also, the small bubble fraction tends to bigger sizes in the large pipe.
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.
An application of wire–mesh sensors to obtain the interfacial area concentration in vertical pipes is presented as an alternative to the widely used multiple-tip electrical or optical fibre probes. The measuring data of a mesh sensor consists of a three-dimensional matrix of local instantaneous gas fractions measured at each crossing point of the wires and recorded as a time sequence. Bubbles are clearly distinguishable in this data matrix, since they represent regions of interconnected elements containing the gaseous phase. The method to deduce the interfacial area concentration from this data is based on a full reconstruction of the gas–liquid interface, where the interfacial area of each bubble is recovered as the sum of the surface area of all surface elements belonging to the given bubble. The new method can be applied to large bubbles with an arbitrary shape. To study the change of the interfacial area concentration along the pipe the distance between sensor and gas injection was varied. The axial development of the interfacial area density measured in the test pipe of 195.3mm inner diameter was compared to the measurements carried out by Sun et al. [Sun, X., Smith, T., Kim, S., Ishii, M., Uhle, J., 2002. Interfacial area of bubbly flow in a relatively large diameter pipe. Exp. Thermal Fluid Sci. 27, 97–109] in a pipe of 101.6mm diameter, which is the largest pipe for which interfacial area densities are presented in literature. An acceptable agreement was found, whereas deviations are consistent with the differences in the boundary conditions of both experiments.
For the first time, an experimental three-dimensional reconstruction and visualization of stationary and transient flashing flow in a vertical pipe (47mm diameter) is presented. The measurements have been performed by means of wire-mesh sensors. This type of sensor delivers two-dimensional void-fraction distributions in the pipe cross-section where it is mounted with a maximum sampling rate of 10,000 frames per second. A sampling rate of 1200 frames per second has been used in this work. Steam bubbles have been identified from the wire-mesh data and their complete three-dimensional reconstruction has been performed by taking into account the steam bubble velocity. For the estimation of the bubble velocity, two wire-mesh sensors positioned at a small axial distance from each other have been used. The velocity has been determined by cross-correlation of the two wire-mesh signals, by direct identification of the traveling time of the steam bubbles between the two sensors and by means of a drift-flux model. A comparison between the three methods of bubbles velocity measurement is reported. Stationary and time-dependent bubble size distributions have been derived. The stationary radial void-fraction profiles have been decomposed according to bubble size classes and compared with the results obtained with an equilibrium model.
An analysis is presented concerning the dynamic-bias which appears in radiation interrogation of fluctuating two-phase flow. It is shown that in certain cases it is possible to obtain closed form expressions for the dynamic-bias. Based on these analytical and calculational results, conditions are enumerated which must be emphasized if the dynamic-bias is to be minimized.RésuméOn présente une analyse de la déviation qui apparaît dans l'exploration par rayonnement d'un écoulement biphasique fluctuant. On montre qu'il est possible dans certains cas d'obtenir des expressions analytiques de la déviation. Basées sur ces résultats théoriques, on donne des conditions exploitables si la déviation est minimisée.ZusammenfassungEine Analyse der dynamischen Verzerrung bei Strahlungsrechnungen in fluktuierender Zwei-Phasenströmung wird angegeben. Es wird gezeigt, dass es in bestimmten Fällen möglich ist, geschlossen darstellbare Ausdrücke für die dynamische Verzerrung zu erhalten. Auf Grund dieser analytischen und rechnerischen Ergebnisse werden die Bedingungen aufgezählt, unter denen die dynamische Verzerrung minimalisiert werden kann.РефератДaeтcя aнaлиз динaмичecкoгo cмeщeния, пoявляющeгocя пpи вoздeйcтвии излyчeния нa пyльcиpyющee двyчфaзнoe тeчeниe. Пoкaзaнo, чтo в oяpeдeлeнныч cлyчaяч для динaмичecкoгo cмeщeния мoжнo пoлyчить выpaжeния в зaмкнyтoм видe. Ha ocнoвe peзyльтaтoв aнaлизa и pacчeтa пepeчиcляютcя ycлoвия, нa кoтopыe cлeдyeт oбpaтить внимaниe, чтoбы cвecти дo минимyмa динaмичecкoe cмeщeниe.
Experiments on suspensions of glass beads in electrolytes indicate that Bruggemann's approximation represents the dependence of effective conductance on volume fraction very satisfactorily when the dispersed phase contains a broad range of particle sizes. Data on narrow size ranges fall in between values predicted by the Maxwell and Bruggemann equations. These findings are consistent with the physical assumptions implicit in both theoretical developments.
Gamma-ray densitometry is a frequently used method for non-intrusive measurement of the void fraction in two- and multi-phase gas liquid pipe flows. Here it is demonstrated how a multi-beam configuration using a low-energy gamma-ray source and several detectors, enables the void fraction to be determined almost independent of the flow regime. An experimentally verified EGS4 Monte Carlo simulation has been developed of the multi-beam gamma-ray densitometer. This is an efficient tool in developing the densitometer as detector responses to different void fractions and flow regimes easily are generated. The simulations cover the full range of void fractions with homogeneous, annular and stratified flows. The model has also been applied to generate training data for a neural network which then was tested with experimental data. This has been done for a variety of detector positions in order to optimise the geometry of the multi-beam densitometer. Polypropylene phantoms (density = 0.91 g/cm 3 ) were used to represent oil in these experiments in order to have reliable and accurate references. By using experimental data as input to the neural networks, the void fraction was determined with an error of 3% regardless of the flow regime. The flow regimes were successfully recognised in all cases studied here, meaning that the system also provides tomographic information.
The factors contributing to the bias introduced in the experimental determination of void fractions in liquid flow systems by methods of radiation diagnosis are being examined. It is shown that error bounds for the radiation transmittance and for the void fraction can be established for certain well-defined void variations. The general validity of the analytical results has been experimentally tested and confirmed. In addition, analytical criteria are derived which relate radiation attenuation and certain parameters of the radiation diagnostic facility to the dynamic voiding effects.
An assessment of void-fraction correlations and drift-flux models applied to stationary and transient flashing flows in a vertical pipe has been performed. Experiments have been carried out on a steam/water loop that can be operated both in forced- and natural-circulation conditions to provide data for the assessment. The GE-Ramp and Dix models are found to give very good predictions both for forced- and natural-circulation flow conditions, in the whole range of measured void fractions.Advanced instrumentation, namely, wire-mesh sensors, has been used to obtain a detailed picture of the void-fraction development in the system. On the basis of experimental data, a three-dimensional visualization of the transient flow pattern during flashing was achieved. A transition of the flow pattern between bubbly and slug/churn regimes was found.
The wire-mesh sensor technique has been successfully introduced into a fuel rod bundle geometry for the first time. In this context, a dedicated test facility (SUBFLOW) has been designed and constructed at Paul Scherrer Institut (PSI) in a co-operation with the Swiss Federal Institute of Technology (ETH Zürich). Two wire-mesh sensors designed and built in-house were installed in the upper part of the vertical test section of SUBFLOW, and single-phase experiments on the turbulent mass exchange between neighboring sub-channels were performed. For this purpose, salt tracer was injected locally in one of the sub-channels and conductivity distributions in the bundle measured by the wire-mesh sensor. Both flow rate and distance from the injection point were varied. The latter was achieved by using injection nozzles at different heights. In this way, the sensor located in the upper part of the channel could be used to characterize the progress of the mixing along the flow direction, and the degree of cross-mixing assessed using the quantity of tracer arriving in the neighboring sub-channels. Fluctuations of the tracer concentration in time were used for statistical evaluations, such as the calculation of standard deviations and two-point correlations.
Direct quantitative comparisons of four different probe methods were performed for determination of local gas holdup, vertical bubble length, bubble rising velocity, and bubble frequency. The methods include vertically projecting electroresistivity, horizontally projecting electroresistivity, U-shape light reflection, and light transmission. The measured bubble properties strongly depended on the size and configuration of the probe tips. The light transmission probe developed in this work can be used to determine the bubble properties effectively. In order to minimize interferences with bubble flow, the size of probe tip should be small as possible and its configuration must be vertically projecting.
Es werden verschiedene physikalische Konstanten heterogener Körper aus den Konstanten ihrer homogenen Bestandteile nach einer einheitlichen Methode berechnet. In dieser ersten Arbeit wird die Berechnung der Dielektrizitätskonstanten und der Leitfähigkeiten für Elektrizität und Wärme der Mischkörper aus isotropen Bestandteilen behandelt. Die Genauigkeit der älteren Formeln wird untersucht und die bis jetzt unbekannten Konstanten dieser Formeln werden berechnet. Sodann wird die Theorie geprüft an Messungen der Leitfähigkeit bei heterogenen Metallegierungen und an den DK. von gepreßten Pulvern und Emulsionen; die verschiedenen Formeln werden bestätigt. Bei dieser Anwendung werden einige Widersprüche zwischen früheren Untersuchungen aufgehoben und es wird versucht, einige ungenau bekannte DK. genauer zu bestimmen.
The use of an x-ray tube that emits a spectrum of energy was shown to be an effective source of monoenergetic radiaton for the measurement of the void fraction in two-phase, steam-water flow with the proper selection of the tube wall thickness, x-ray tube operating conditions, collimation, and traverse time. Assuming monoenergetic radiation, equations were derived for the local and average void fraction and the statistical error associated with the measurement, and were confirmed to be applicable by using the mock-up technique. This consisted of simulating the two-phase, steam-water flow patterns of the experimental system with mock-ups of Lucite and air. Exprimental data are presented to show that confidence could be placed in the measurements to within the probable statistical error, as all deviations of the measured values of the void fraction from the actual values were less than the probable error. Local and average void fractions for steam-water, two-phase flow are given. Application to other systems can be inferred from these results.
Gamma-ray densitometry is a frequently used non-intrusive method for determining void fraction in two- and multi-phase gas liquid pipe flows. The traditional gamma-ray densitometer using a 137Cs source and a scintillation PMT detector has proved itself reliable and robust. This paper presents a method using a low energy source (241Am), which offers the advantages of reduced size due to reduced shielding requirements, compact detectors, and lesser dependence on flow regime, due to its multibeam measurement configuration. These are important aspects with regard to future subsea and down-hole fluid flow measurement applications. The performance of single-beam and the compact multi-beam low-energy gamma-ray measurement principles was compared. Consideration of the measurement volume, defined by the detector area and the radiation beam, demonstrated the flow regime dependency of single-beam gamma-ray measurement principles. With the multi-beam low-energy gamma-ray measurement principle, the dependence on flow regime is negligible when several detector responses are combined. Use of phantoms and one movable detector verified the multi-beam gamma-ray measurement principle. The detector responses at several positions around the pipe were obtained for different flow regimes and void fractions.
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.
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.Highlights► The intrusive effect of three-layer wire-mesh sensor is studied by image processing. ► Bubble acceleration and deceleration due to the sensor are observed. ► Individual bubble parameters are compared with image processing.
A simple conductivity method for the measurement of void fractions in gas-electrolyte dispersions is described. The experimental correlation between the effective conductivity of the dispersion and the gas voidage is in very good agreement with the well-known Maxwell equation. The simplicity and versatility of the method are demonstrated in the experimental study of an electrolytic cell with simulated gas evolution. The measured gas void fraction has been correlated with the superficial gas and electrolyte velocities in accordance with a theoretical correlation given by Nicklin.
This article offers an overview of the instrumentation techniques developed for multiphase flow analysis either in gas/liquid or in gas/liquid/solid reactors. To characterise properly such reactors, experimental data have to be acquired at different space scale or time frequency. The existing multiphase flow metering techniques described give information concerning reactor hydrodynamics such as pressure, phases holdups, phases velocities, flow regime, size and shape of dispersed inclusions, axial diffusion coefficients. The measuring techniques are presented in two groups: the non-intrusive techniques that deliver global, cross-section-averaged or local data, and the intrusive probes that are dedicated to local measurements. Eventually some examples of multiphase instrumentation development are reported (trickle-bed and slurry bubble column at semi-industrial scale) in the refinery or petrochemical area.
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.
A wire-mesh sensor, which is based on local conductivity measurement, has been applied to studies on the characteristics of bubble flow in a rectangular channel (). Special design of the sensor allowed the measurement of the local instantaneous true gas velocity besides the measurement of the local instantaneous void fraction. A review of an already published method for true gas velocity measurement under consideration of the uncertainty caused by limitations in the sampling frequency is presented. A cluster-algorithm is proposed for the evaluation of bubble size distribution and volume flow reconstruction. The validity of this algorithm for spatial field reconstruction was benchmarked by theoretical considerations as well as comparison of the calculated with alternatively measured data. Good agreement was stated. The achieved information was used to obtain plots showing the bubble/slug velocity (up to the second statistical momentum) depending on the spherical-equivalent bubble diameter. This information was measured inside a transient bubble flow with void fraction of up to 20%. Occurring phenomena are explained by presented Fourier spectra of the cross-sectional averaged void fraction and the gas volume flow.
A computed tomographic scanner using γ-rays has been implemented for the measurement of void fraction and its distribution in two-phase flow systems such as fluidized beds and bubble columns. The automated scanner is capable of imaging flows in test sections between 2.5 cm and 45.0 cm in diameter. The developed system hardware, the adopted algorithm for image reconstruction and possible sources of error in measurement are discussed. Typical results for void fraction distribution in bubble columns are presented.
The paper presents an electrode-mesh tomograph for the high-speed visualisation of transient gas fraction distributions in two-phase flows in pipes. It is based on the measurement of the local instantaneous conductivity of the two-phase mixture. The time resolution of the device is 1024 frames per second. The sensor consists of two electrode grids with 16 electrodes each. This results in 16×16 sensitive points, which are equally distributed over the cross-section. The sensor is available in two designs: (1) wire-mesh sensor for lab applications and (2) sensor with enforced electrode rods for high mechanical loads. The device was recently tested in a vertical and a horizontal air–water flow in a pipe of 51.2 mm diameter.
This paper details the development of non-invasive ultrasonic tomography for imaging liquid and gas flow. Transmission-mode approach has been used for sensing the liquid/gas two-phase flow, which is a kind of strongly inhomogeneous medium. A 16-pair of ultrasonic sensors have been used. By using low excitation voltage of 20 V, fan-shape beam ultrasonic transmitters will emit ultrasonic pulses to the receivers. The investigations were based on the transmission and the reception of ultrasonic sensors that were mounted circularly on the surface of experimental vessel. The algorithms used to reconstruct the concentration profile for two-phase flow using fan-shape beam scanning geometry were presented. By using Hybrid-Binary Reconstruction algorithm (HBR), a real-time of ultrasonic transmission-mode tomography had been developed. Experiments showed that the performance is acceptable with the image reconstruction speed of ten frames per second. The results of the experiments and possible future improvements were also discussed.
Fluid dynamics of bubble columns at elevated temperature modelling and investigation with refractive fiber optic sensors
- Chabot
Chabot, J., Farag, H., de Lasa, H., 1998. Fluid dynamics of bubble columns at elevated
temperature modelling and investigation with refractive fiber optic sensors. Chem.
Eng. J. 70, 105-113. https://doi.org/10.1016/S0923-0467(98)00075-X.. URL:
http://www.sciencedirect.com/science/article/pii/S092304679800075X.
Liquid Films and Droplet Deposition in a BWR Fuel Element
- M Damsohn
Damsohn, M., 2011. Liquid Films and Droplet Deposition in a BWR Fuel Element (Ph.D.
thesis). ETH Zurich.