Wire-mesh Sensor Research Articles

Assessment of a hybrid CFD model for simulation of complex vertical upward gas–liquid churn flow

Gas–liquid multiphase flow can be observed within different industrial processes, and Computational Fluid Dynamics (CFD) can be utilized as a tool for scrutiny of this kind of flows. Although the CFD simulations of multiphase are computationally-demanding, they can deliver a great deal of information. But, the larger point is whether the available CFD multiphase flow models are able to deliver a realistic solution for a complex flow pattern like churn flow? And if yes, to what extent are the results accurate?To shed light on these issues, the Eulerian–Eulerian MultiFluid VOF model offered by ANSYS FLUENT 15 (2015. 15.0 User's Guide, ANSYS Inc.) was used to simulate high flow rate air–water multiphase flow in a 76.2mm-diameter pipe upstream of an elbow in the vertical-horizontal configuration. In the simulations, superficial gas velocity ranged from 10.3m/s to 33.9m/s, and two superficial liquid velocities of 0.3, and 0.79m/s were employed.From the CFD simulations, data such as phase distributions, mean void fractions, and average void fraction time series were extracted. They were then compared to experimental Wire Mesh Sensor (WMS) data formerly obtained. Interestingly, evaluation of the model revealed that it was successful in terms of capturing different liquid structures present within the flow and delivering void fraction data which were in agreement with those of experiments.

Investigations of single-phase flow mixing characteristics in a wire-wrapped 37-pin bundle for a sodium-cooled fast reactor

Flow mixing characteristics inside a wire-wrapped 37-pin bundle were measured using a wire-mesh sensing system. The subchannel flow mixing within a sodium-cooled fast reactor (SFR) core subassembly is an important phenomenon to predict temperature distributions for the core thermal design and safety analysis. To identify the mixing characteristics experimentally, a dedicated test facility including the wire-mesh sensor system and tracing liquid injection system was developed. The conductivity fields at the end of a 37-pin bundle were visualized in several different flow conditions which are corresponding to 20–115% flow of the prototype reactor nominal flow based on the Reynolds number. A CFD preliminary analysis under pre-determined boundary conditions was also performed to verify the design parameters and range of various operating values of the current mixing test. The experimentally identified mixing characteristics were compared with the CFD results, which show reasonable agreements with each other. The current experimental work includes the uncertainty evaluation by performing the separated test for the elementary parameters.

Structures in gas–liquid churn flow in a large diameter vertical pipe

Gas–Liquid two phase co-current flow in a vertical riser with an internal diameter of 127 mm was investigated in the churn flow pattern. This paper presents detailed experimental data obtained using a Wire Mesh Sensor. It shows that the most obvious features of the flow are huge waves travelling on the liquid film. Wisps, large tendrils of liquid and the product of incomplete atomisation, which had previously detected in smaller diameter pipes, have also been found in the larger diameter pipe employed here. The output of the Wire Mesh Sensor has been used to determine the overall void fraction. When examined within a drift flux framework, it shows a distribution coefficient of ∼1, in contrast to data for lower gas flow rates. Film thickness time series extracted from the Wire Mesh Sensor output have been examined and the trends of mean film thickness, that of the base film and the wave peaks are presented and discussed. The occurrence of wisps and their frequencies have been quantified.

On the accuracy of gas flow rate measurements in gas–liquid pipe flows by cross-correlating dual wire-mesh sensor signals

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.

A New Statistical Parameter for Identifying the Main Transition Velocities in Bubble Columns.

The identification of the main flow regime boundaries in bubble columns is essential since the degrees of mixing and mass and heat transfer vary with the flow regime. In this work, a new statistical parameter was extracted from the time series of the cross-sectional averaged gas holdup. The measurements were performed in bubble columns by means of conductivity wire-mesh sensors at very high sampling frequency. The columns were operated with an air/deionized water system under ambient conditions. As a flow regime indicator, a new dimensionless statistical parameter called “relative maximum number of visits in a region” was introduced. This new parameter is a function of the difference between the maximum numbers of visits in a region, calculated from two different division schemes of the signal range.

An improved electrical-conductance sensor for void-fraction measurement in a horizontal pipe

The electrical-impedance method has been widely used for void-fraction measurement in two-phase flow due to its many favorable features. In the impedance method, the response characteristics of the electrical signal heavily depend upon flow pattern, as well as phasic volume. Thus, information on the flow pattern should be given for reliable void-fraction measurement. This study proposes an improved electrical-conductance sensor composed of a three-electrode set of adjacent and opposite electrodes. In the proposed sensor, conductance readings are directly converted into the flow pattern through a specified criterion and are consecutively used to estimate the corresponding void fraction. Since the flow pattern and the void fraction are evaluated by reading conductance measurements, complexity of data processing can be significantly reduced and real-time information provided. Before actual applications, several numerical calculations are performed to optimize electrode and insulator sizes, and optimal design is verified by static experiments. Finally, the proposed sensor is applied for air-water two-phase flow in a horizontal loop with a 40-mm inner diameter and a 5-m length, and its measurement results are compared with those of a wire-mesh sensor.

A methodology to quantify the uncertainty in liquid holdup measurements with wire mesh sensor

The uncertainty in the holdup measurements of a capacitance based WMS with 32×32 wires has been experimentally evaluated, and a methodology for its quantification is proposed for horizontal flow. Investigation is performed in laboratory and in-situ experiments under stagnant conditions with an emphasis on the effect of the mesh grid orientation on the measurements. Also, potential impact of misalignment in the pipe inclination is explored by slightly inclining the pipe for both tests. Finally, dynamic flow conditions are tested in a high pressure (1.37MPa) facility with horizontal stratified-wavy oil/air two-phase flow for gas and liquid superficial velocity ranges of 2.8m/s≤νSg≤6.9m/s and 0.01m/s≤vSL≤0.05m/s, respectively.The angle between the phase interface and the sensor wires is ineffective while the misalignment in the pipe inclination plays a major role in the deviations of the holdup measurements. Using the proposed methodology, the measurement uncertainty from laboratory tests is shown to follow a logarithmic increase as a function of the measured holdup for smaller holdup values (HL≤15%) and to be lower than 1.5% for HL>15%. This behavior is intrinsic to WMS and a representative of the measurement uncertainty in the actual flow loop installation.Under actual flow conditions, the holdup measurements of the trapped liquid by WMS show an offset compared to the measurements via flow imaging which can be corrected by using the uncertainty quantified in the laboratory tests. However, the dynamic measurements with WMS show a good agreement with the holdup of the trapped liquid volume within the quantified uncertainty bounds.

Dual-modality wire-mesh sensor for the visualization of three-phase flows

Three-phase gas–liquid–liquid flows are very common in petroleum extraction, production, and transport. In this work a dual-modality measuring technique is introduced which may be well applied for three-phase flow visualization. The measuring principle is based on simultaneous excitation with two distinct frequencies to interrogate each crossing point of a mesh sensor, which in turn are linked to conductive and capacitive parts of fluid impedance. The developed system can operate eight transmitter and eight receiver electrodes at a frame repetition frequency up to 781 Hz. The system has been evaluated by measuring reference components. The overall measurement uncertainty was 8.4%, which considering the fast repetition frequency of measurements is suitable for flow investigation. Furthermore, a model-based method to fuse the data from the dual-modality wire-mesh sensor and to obtain individual phase fraction of gas–oil–water flow is introduced. Here a parametrized model is fitted to the measured conductivity and permittivity distributions enabling one to obtain phase fraction from measured data. The method has been applied and tested to the acquired data from a mesh sensor in static and dynamic three-phase mixtures of gas, oil, and water. Fused images and quantitative values show good agreement with reference values. The newly developed dual-modality wire-mesh sensor has the potential to investigate three-phase flows to a good degree of detail, being a valuable tool to investigate such flows.

Application of the MiniPanda test case ‘erosion of a stratified layer by a vertical jet’ for CFD validation

In order to allow development and validation of CFD models for hydrogen mixing and transport in the containment, a comprehensive experimental test campaign was performed at the small scale MiniPanda facility at ETH Zurich. The considered test series aimed at studying the turbulent erosion of a stratified light gas by means of a vertical air jet with different momenta. Due to its new and innovative measurements, e.g., with temperature wire mesh sensors, the global mixing and the local interaction of jet and stratification are characterized in a high resolution in space and time. Both are essential for a detailed model assessment, to identify possible error sources and rate their effect on the global scenario evolution. Consequently, the tests are well suited for CFD model development and validation and complement the data basis gained before, e.g., in the frame of the joint OECD/NEA-SETH-2 Project (2007–2010) (OECD/NEA, 2012).Based on a description of the MiniPanda facility and the ‘layer erosion’ test series, the application of a U-RANS CFD approach, capable to be applied also for large scale application, is discussed. Numerical model uncertainties are minimized according to the best practice guidelines before a systematic comparison against the experimental data is performed and the capability of the model to predict the turbulent mixing at the interface and the inter-compartment mass transfer is successfully validated.

Initial Study of a Wire Mesh Tomography Sensor for Liquid/Gas Component Investigation

Experimental studies have been carried out to study the principle operation of the conductive type wire-mesh tomography sensor and analyse the wire-mesh tomography sensor for the liquid/gas two-phase flow interface and void fraction distribution in a process column. The measurement of the two-phase flows in the process column is based on the cross-sectional local instantaneous conductivity. The sensor consists of two planes of parallel electrode wires with 16 electrodes each and was placed orthogonally with each plane. The sensor electrode wires were made of tinned copper wire with an outer diameter of 0.91 mm which stretched over the sensor fixture. Therefore, this result in the mesh grid size with 5.53×5.53mm². The wire-mesh sensor was tested in a horizontal liquid/gas two-phase flows process column with nominal diameter of 95.6 mm and the sampling frequency of 5882.3529 Hz. The tomogram results show that the wire-mesh tomography provides significant results to represent the void fraction distribution in the process column and estimation error was found in the liquid/gas interface level.

Capacitance wire mesh imaging of bubbly flows for offshore treatment applications

The impact of ship motion on bubbly flow was emulated using a swell simulator to expose flow structure changes emerging in bubble columns relevant to offshore floating applications. Roll, roll+pitch, yaw, heave and sway were implemented at various frequencies and changes in bubbly flow resulting from the imposed motions were monitored for the first time by means of a dual capacitance wire mesh sensor to measure local gas holdup and velocity. Visualizations of the two-phase flow revealed that roll, roll+pitch, and high-frequency sway were the most impactful in terms of bubble zigzag and swirl, and bubble-clustering and segregation due to vessel dynamic inclinations. As a consequence of these motions, lateral migration of bubbles and their clustering enhanced liquid recirculation and local streamwise gas velocity. Compared to static vertical bubble column, bubbly flow pattern was barely altered by yaw and low-frequency sway except the heave displacements which tended to slowdown the bubble rise.

Hydrodynamics of gas–liquid cocurrent downflow in floating packed beds

An innovative experimental setup combining a robot with six-degree-of-freedom motions (translational: surge, sway, heave, and rotational: roll, pitch, yaw) and an embarked column instrumented with a capacitance wire mesh sensor was built to investigate experimentally the two-phase flow hydrodynamics in porous media within inclined stationary and oscillating packed beds under gas–liquid cocurrent descending flows. The hydrodynamic deviations with respect to the stationary straight column were thoroughly analyzed in terms of bed pressure drop, liquid saturation, regime changeover, and pulse flow properties. Non-tilting oscillations had marginal effects on the packed bed hydrodynamics which remained practically similar to the one prevailing in stationary vertical trickle beds. Conversely, tilting oscillations profoundly influenced the flow regime inside the packed bed as a result of the time-dependent nature of the gravitational and inertial (acceleration/deceleration) forces brought about by the hexapod rotational oscillations which caused important secondary transverse displacements of the fluids in the bed. Using moment analysis of the probability density functions of the registered liquid saturation time series, it was found that due to the periodic gravity-driven stratification, the transition to pulse flow shifted towards higher fluid velocities as the oscillation periods of roll or roll+pitch motions were increased.

Applications of wire-mesh sensors in multiphase flows

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

Study on void fraction distribution of plunging jet experiment with wire mesh sensor

The plunging jet is a phenomenon of two phase gas-liquid flow, which could be an effective method to sufficiently mix boric acid with de-ionized water, and is suitable for studying the Pressurized Thermal Shock (PTS) accident scenario of Pressurized Water Reactors(PWR). The air is entrained when the liquid jet hits the water surface, which generates bubbles during the air entrainment. This bubble entrainment has a crucial influence on the mixing process. The distribution of void fraction was studied in dependence of the jet length and flow rate. A movable three-layer Wire Mesh Sensor (WMS) is designed to measure sequences of instantaneous two-dimensional gas fractions with a high-resolution in space and time, which, from the data, derives the void fraction distributions. Comparisons of average void fraction are carried out with different jet lengths and flow rates.

Advanced image processing of wire-mesh sensor data for two-phase flow investigation

Two-phase gas-liquid flows, the simultaneous flow of a gas and liquid in a pipe, are commonly found in several industrial activities, among them during the extraction and processing of crude oil. Wire-mesh sensors are flow imaging devices which are able to generate images of phase distribution of two-phase flows within a pipe cross section at high spatial and temporal resolutions. Appropriated image processing algorithms are however necessary in order to extract important flow parameter from raw data. In this paper, we describe a bubble identification algorithm based on three-dimensional image segmentation. Results of bubbles velocity and flow rate measurement based on the development algorithm are validated against reference models showing good agreement.

Comparison of experimental and Computational Fluid Dynamics (CFD) studies of slug flow in a vertical riser

This paper presents a comparison of the results obtained from experiments and CFD studies of slug flow in a vertical riser. A series of two experimental investigations were carried out on a 6m vertical pipe with a 0.067m internal diameter charged with an air–silicone oil mixture. For the first set of experiments, the riser was initially full of air, and then liquid and gas flows set to liquid and gas superficial velocities=0.05 and 0.344m/s, respectively, electrical capacitance tomography (ECT) and wire mesh sensor (WMS) transducers were employed. In the second one, the riser was initially full of (static) liquid, and then liquid and gas flows set to liquid and gas superficial velocities=0.05 and 0.344m/s, respectively, only ECT was used. A characterisation of the observed slug flow regimes was carried out. This includes the evaluation of the instantaneous distribution of the phases over the pipe cross-section, the Probability Density Function (PDF) of void fraction, time series of cross-sectional void fraction, Power Spectral Density (PSD), structure velocity of the Taylor bubble, lengths of the liquid slug and Taylor bubble and void fractions in the liquid slug and Taylor bubble. The simulation results were validated both qualitatively and quantitatively against experimental data. A reasonably good agreement was observed between the results of the experiment and CFD.

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

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

Experimental characterization of vertical downward two-phase annular flows using Wire-Mesh Sensor

Annular two-phase flow has been vastly investigated because of its large and deep involvement in industrial processes, particularly in nuclear engineering and petroleum production facilities. Much effort has been devoted to investigating upward flows involving the flow patterns, void fraction, as well as local interfacial characteristics. However, research for vertical downward two-phase flow, especially of the interfacial characteristics, are comparatively scarce. In order to gain insight on void fraction, interfacial structures and characteristics frequencies, experimental work was performed in downward annular two-phase flow with water and air as process fluids at low pressure conditions. A flow loop, including a 76mm ID, 16.5m long vertical pipe, has been instrumented. A state-of-the-art instrument for two-phase flow measurements based on the fluid conductivity, namely dual Wire-Mesh Sensor (WMS) has been utilized to acquire the experimental data. A total of 43 data points have been acquired at superficial liquid velocities that ranged from 0.005m/s to 0.10m/s and superficial gas velocities that varied from 10m/s to 31m/s. The effects of liquid viscosity on the measured parameters are also investigated using two different viscosities of 1 and 10cP. Analysis of time series void fraction data from the dual Wire-Mesh Sensors allows the determination of cross-sectional averaged void fraction, local time averaged void fraction distribution, liquid phase distribution around the tube periphery, interfacial structure frequencies, pseudo 3D reconstruction as well as Probability Density Function (PDF) and Power Spectral Density (PSD). The experimental results indicate that the interfacial shape and frequencies are significantly altered by the superficial gas velocity. Comparisons between mechanistic model predictions and the acquired experimental data show a maximum absolute average relative error of approximately 7% for the cross-section void fraction.

Measurement of Bubbly Two-phase Flow in Vertical Pipe Using Multiwave Ultrasonic Pulsed Dopller Method and Wire Mesh Tomography

Abstract Two measurement methods which are multiwave ultrasonic pulsed Doppler (multiwave UVP) method and wire mesh tomography (WMT) have been applied to the measurement of bubbly two-phase flow. Velocity profiles of bubbles and liquid have been measured using multiwave UVP. Simultaneously, cross-sectional void fraction distribution has been measured using WMT. As the result, a combination method has been set up. The measured parameters are indispensable in order to obtain detailed structures of two-phase flow. Multiwave UVP method exploits two basic ultrasonic frequencies which are 2 MHz and 8 MHz. Simultaneous measurement of velocity profiles of bubbles (using 2 MHz frequency) and those of liquid (using 8 MHz frequency), at the same position, is enabled. A multiwave ultrasonic transducer (multiwave TDX) which is able to emit and receive the two ultrasonic frequencies along the measurement line at the same time has been applied. The signal processing is based on the pulsed Doppler method. Using the combination method, first, instantaneous velocity profiles and void fraction distribution have been measured simultaneously for air-water counter-current bubbly flow in a vertical pipe. Flow structure has been clarified. Effect of initial condition on void fraction distribution has been confirmed. Next, measurements have been carried out for subcooled boiling bubbly flow in a vertical pipe. For measurement of subcooled boiling flow, a high temperature wire mesh sensor (WMS) has been developed. A method for separation of velocity profiles of bubbles with different sizes and velocities has been suggested.

Experimental investigation of interfacial structures within churn flow using a dual wire-mesh sensor

A challenging area in the field of multiphase flow is the study of churn flow. According to the multiphase flow community, churn flow has not been widely investigated in intermediate and large diameter pipes at high gas and liquid flow rates. The present work deals with an experimental study of upward vertical air–water flow in a 76.2mm I.D. pipe. Superficial gas velocities ranging from 10 to 38m/s and four superficial liquid velocities (0.30, 0.46, 0.61 and 0.76m/s) were employed. The experimental data points are mostly located in churn flow and at the transition between churn and annular flow. A dual 16×16 Wire Mesh Sensor (WMS) was used to obtain the temporal/spatial variations of phase distributions over the pipe cross-section at one specific axial location (L/D=236).Sequences of phase distributions, axially sliced images, virtual 3-D images as well as void fraction time-series were used to distinguish between different interfacial structures such as slugs and huge waves. Results showed that huge waves occur with either a continuous gas core with a distinct boundary between two phases or a core with a gas–liquid mixture. Furthermore, velocities and frequencies of interfacial structures were obtained. Results are qualitatively and quantitatively consistent with the previous findings available in literature.