Local Void Fraction Measurements Research Articles

Measurement of Local Void Fraction of Air-Water Flow in an 8 × 8 Rod Bundle Using High-Resolution Gamma-Ray Tomography

High-resolution two-phase flow data in the rod bundle are important in the development and validation of high-fidelity models for computational fluid dynamics and subchannel codes, in particular, those pertaining to light water reactor cooling systems. The Michigan Adiabatic Rod Bundle Flow Experiment (MARBLE) has been constructed as a modular assembly of an 8 × 8 lattice rod bundle to simulate scaled pressurized water reactor and boiling water reactor subchannel assemblies. To establish a high-spatial resolution database of the void fraction in the reactor fuel assembly geometries, tomographic measurements were performed with the High-Resolution Gamma-ray Tomography System, which was designed and built in house; the detector system has a spatial resolution of less than 1.0 mm using 240 LYSO (Lu1.8Y0.2SiO5) scintillators with a fan-beam array. In the present study, the local void fraction was measured with the MARBLE facility under various air-water flow conditions (jg = 0.04 to 0.85 m/s and jl = 0.12 to 0.77 m/s) covering from bubbly to cap-turbulent flows. The local void fraction was also successfully measured under nonuniform and asymmetric air bubble distribution conditions with an investigation of the effect of spacer grids and mixing vanes on void drift across subchannels.

Novel Twofold Use of Photographic Technique for Simultaneous Flow Boiling Image Recording and Void Fraction Computation in a Mini-Channel Experiment

The paper presents a novel twofold use of the photographic technique for flow boiling heat transfer investigation in the horizontal mini-channel. The dedicated measurement system was constructed to record basic thermal and flow parameters, i.e., boiling liquid inlet and outlet temperatures and pressures, and also temperatures inside the heating block to calculate heat flux going into the liquid. A high-speed video camera equipped with synchronous movement system was applied to combine the recording of two-phase flow images with simultaneous local void fraction measurements both based on the same photographic data set. The data were collected, managed, and refined with the scripts developed in the MathWorks Matlab 2019b environment. The synchronous use of two intelligent techniques in the scripts, i.e., the background subtraction technique and the statistical analysis of individual pictures allowed obtaining reliable experimental results. The proposed method of the void fraction determination ensures high measurement accuracy.

Flow induced vibration of two-phase flow passing through orifices under slug pattern conditions

Flow restricting orifices are important components for flow and pressure control in the oil, gas, and power generation industries. In many situations, the piping systems are operating under two-phase flow conditions. These orifices affect the redistribution of the two phases and induce vibrations in the piping structures. These vibrations often occur during a slug flow pattern and can severely affect the integrity of a structure; therefore, in the present work, both the vibrations of the piping structure and the characteristics of the two-phase flow dynamics are experimentally investigated in the presence of an orifice. In this study, an air–water​ two-phase flow mixture was utilized at different superficial velocities ranging from 0.12 m/s to 1.7 m/s for air and 0.46 m/s to 1.3 m/s for water. Synchronized flow visualization images, along with local void fraction and vibration measurements, were used to evaluate the response of the structure as the slug flow passed through the orifices. Detailed instantaneous local void fraction measurements were carried out to understand the underlying physics of the phenomenon and their effect on the vibration response of the structure. The results showed that not all slug flow cases excited the piping at the slug frequency. The amplitude of vibration around the slug frequency was found to increase as the superficial velocity of the gas phase increased.

Forced convection steam-water experimental database in a vertical annulus with local measurements

A new database of steam water flow in a vertical annulus is developed in the Multiphase Thermo-fluid Dynamics Laboratory (MTDL) at the University of Illinois at Urbana-Champaign. The focus of the dataset is on the two-phase phenomena of condensing, flashing, and saturated flow in a vertical channel. The 2 m annular channel with inner and outer diameter of 19.1 mm and 38.1 mm respectively is located downstream of an internally heated annular section. Five measurement ports in this unheated section provide measurement of pressure and local measurements of void fraction, interfacial area concentration, gas velocity, Sauter mean diameter, and liquid temperature. The dataset was designed for validation of one-dimensional system codes and multi-dimensional computational fluid dynamics codes. Detailed local measurements of three representative cases are shown to highlight the complexity of local two-phase flow phenomena observed along the test section. A parametric analysis is also conducted to show the sensitivity of system parameters on the axially evolving two-phase flow. The focus of the adiabatic steam-water flow fills an important gap in validation data between the simplified hydro-dynamic data of air-water flow and the complex thermo-fluid dynamic data of boiling flows.

Multimodal partial cavity shedding on a two-dimensional hydrofoil and its relation to the presence of bubbly shocks

The shedding dynamics of partial cavitation forming on a NACA0015 hydrofoil is known from the previous studies to be multimodal, exhibiting abrupt changes in Strouhal number as the cavitation number is reduced (Arndt et al. in Instability of partial cavitation: a numerical/experimental approach. National Academies Press, Washington, D.C., Retrieved from the University of Minnesota Digital Conservancy. http://hdl.handle.net/11299/49781 , 2000). The present study aims to understand the underlying shedding mechanisms responsible for this abrupt change in dynamics. As was observed in Ganesh et al. (J Fluid Mech 802:37–78, 2016), the shedding process can result from both re-entrant liquid flow and the formation of propagating bubbly shock waves within the cavity. Time-resolved X-ray densitometry and high-speed videography are combined with time synchronous measurements of acoustic noise produced by the cavity to examine the formation and shedding of the partial cavitation on a NACA0015 hydrofoil. From the experiments on the hydrofoil, it was observed that the mean cavity length increased with decreasing cavitation number, as expected. At higher pressures, stable partial cavities formed that shed smaller vapor clouds mainly due to the re-entrant liquid flow in the cavity closure. With a reduction in cavitation number, the partial cavity grew in length, and shedding often occurred when the cavity was pinched-off from its leading edge. Once a large region of vapor was shed, the pressure pulse caused by its subsequent collapse could suppress the growth of the newly forming partial cavity. This feedback led to complex, multistep cavity shedding dynamics. At still lower cavitation numbers, bubbly shocks were found to be responsible for the strongest periodic shedding of vapor clouds. Spectral analysis of the acoustic signal revealed a multimodal nature, which was more pronounced at lower cavitation numbers, and similar dynamic behavior was also found for the time varying local void fraction measurements in the vicinity of cavity closure. The occurrence of strongest multimodal behavior was also characterized by the presence of bubbly shock waves as the dominant mechanism of shedding. From the measurements and analysis, it is concluded that at least four different types of shedding modes occurred over a range of cavitation numbers at a fixed attack angle (10°).

Numerical investigations of the accuracy of conductivity wire-mesh sensors

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.

A study on steam-water two phase flow distribution in a rectangular channel with different channel orientations

Experimental study on steam-water two phase vertical and inclined upward flow (15–90°) was performed in a rectangular channel with cross section of 17 mm × 10 mm under atmospheric pressure to investigate the phase distribution and the average void fraction in the cross section which were obtained from the local void fraction measurement by a conductivity probe. The inlet superficial velocities of the steam and water varied from 0.72 to 3.85 m/s and from 0.11 to 0.3 m/s respectively. A high speed camera was used to identify the flow patterns. Experimental results show that the phase distribution curves are significantly affected by channel orientation and the average void fraction first decreases and then increases with the increase of orientation. Based on the drift-flux model, two parameters, namely, the distribution parameter (C0) and the drift velocity (Ugm) have been studied in detail. Both the distribution parameter and the drift velocity are found to be functions of orientation. The distribution parameter decreases with the increase of orientation while the drift velocity first increases and then decreases with the increase of orientation., Based on the experimental data, an improved drift-flux model is proposed especially for the slug and churn flow, which predicts the void fraction in an inclined channel with good accuracy.

Development of procedures for local void fraction measurements

AREVA operates a world-wide unique thermal hydraulic platform to ensure high safety standards in the nuclear industry. This platform is operated as an accredited test and inspection body according to ISO 17025 and 17020 to grant high and independently confirmed quality standards. The aim of the tests is to demonstrate the reliability of components and systems, mainly under operational or accidental conditions. In addition to that it is also the aim of the tests to increase the understanding of the fluid dynamic processes. Especially under operational conditions it is very difficult to gain local measurement data. As an example for AREVA’s strategy in the field of advanced two-phase flow measurement techniques, this paper gives an overview on the current activities regarding the development of these measurement procedures focused on the local void fraction measurements under two-phase flow conditions in an annulus. With increasing requirements with respect to the local resolution, the efforts for measurement techniques increase as well. For that purpose AREVA has built up a specific test loop to develop adequate measurement techniques and corresponding validation processes. This loop allows the adjustment of water/steam properties representative of operational and accident conditions of LWRs and has been used to develop a procedure to measure the void fraction distribution in an annulus representing the hydraulic diameter of a typical LWR core sub-channel by means of a gamma tomography system. This paper describes the process, which has been established to reach a reliable high special resolution measurement. Further steps planned to increase the validity of the measurement procedure will also be discussed.

Comparison of void fraction measurements using different techniques in two-phase flow bubble column reactors

Local, area and volume average void fraction measurements have been conducted in gas-liquid two-phase flow through vertical pipes of different diameters under different modes of operation covering up-flow, down-flow and batch bubble column. The measurement techniques used include gamma ray densitometry, electrical resistance tomography (ERT), wire mesh sensor (WMS), optical void probe, and pressure transducers. The consistency among different measurement techniques has been examined by comparing the local, area and volume-average void fraction measurements made using the above methods in the same test section under the same flow conditions. The accuracy of the measurement techniques has been found to depend significantly on the mode of two-phase flow operation. The results show that the gamma densitometry and the pressure transducers can produce highly reliable measurements independent of the mode of operation. However, the optical void probe was found to underestimate the local values of void fractions, particularly, in the case of a co-current down-flow with sub-millimeter sized bubbles. Also, the electrical resistance tomography technique was found to be the least reliable method, as it underestimated the void fraction in the co-current up-flow as well as co-current down-flow operation. The accuracy of the wire mesh sensor was strongly dependent on the size of the bubbles relative to the wire mesh spacing.

Experimental study on two-phase flow natural circulation in a core catcher cooling channel for EU-APR1400 using air-water system

Ex-vessel core catcher cooling system driven by natural circulation is designed using a full scaled air-water system. A transparent half symmetric section of a core catcher coolant channel of a pressurized water reactor was designed with instrumentations for local void fraction measurement and flow visualization. Two designs of air-water top separator water tanks are studied including one with modified ‘super-step’ design which prevents gas entrainment into down-comer. In the experiment air flow rates are set corresponding to steam generation rate for given corium decay power. Measurements of natural circulation flow rate, spatial local void fraction distribution and re-wetting time near the top wall are carried out for various air flow rates which simulate boiling-induced vapor generation. Since heat transfer and critical heat flux are strongly dependent on the water mass flow rate and development of two-phase flow on the heated wall, knowledge of two-phase flow characteristics in the coolant channel is essential. Results on flow visualization showing two phase flow structure specifically near the high void accumulation regions, local void profiles, rewetting time, and natural circulation flow rate are presented for various air flow rates that simulate corium power levels. The data are useful in assessing the cooling capability of and safety of the core catcher system.

Experimental Study on Interfacial Area Transport of Two-Phase Flow under Vibration Conditions

An experimental study on air-water two-phase flow under vibration condition has been conducted using double-sensor conductivity probe. The test section is an annular geometry with hydraulic diameter of 19.1 mm. The vibration frequency ranges from 0.47 Hz to 2.47 Hz. Local measurements of void fraction, interfacial area concentration (IAC), and Sauter mean diameter have been performed along one radius in the vibration direction. The result shows that local parameters fluctuate continuously around the base values in the vibration cycle. Additional bubble force due to inertia is used to explain lateral bubble motions. The fluctuation amplitudes of local void fraction and IAC increase significantly with vibration frequency. The radial distribution of local parameters at the maximum vibration displacement is specifically analyzed. In the void fraction and IAC profiles, the peak near the inner wall is weakened or even disappearing and a strong peak skewed to outer wall is gradually observed with the increase of vibration frequency. The nondimensional peak void fraction can reach a maximum of 49% and the mean relative variation of local void fraction can increase to more than 29% as the vibration frequency increases to 2.47 Hz. But the increase of vibration frequency does not bring significant change to bubble diameter.

Two-phase flow phenomena along an adiabatic riser – An experimental study at the test-facility GENEVA

Long adiabatic riser geometries with low system pressures are present in a lot of energy and petrochemical processes. Natural-circulation systems are an appropriate solution to save operating and maintenance costs. Under certain circumstances natural-circulation systems tend to unstable mass flow, especially in the riser section. The pressure gradients can stress the construction materials and affect the heat transfer. This paper focuses on GENEVA test-facility and its natural-circulation circuit with heat input by steam condensation. The GENEVA test-facility is explained in detail with focus on the local void fraction measurement system in the adiabatic riser section. The differences to former natural circulation test-facilities are particular emphasized. Therefore a transient experiment is presented and analysed. Moreover the influence of flow restrictions at the downcomer outlet is explained and an experimental method is presented, to determine the maximum natural circulation mass flow of the natural-circulation circuit. Besides, a comparison between the two different riser inner diameters, which were used during the experiments, is presented. The convective heat transfer is analysed by taking into account different dimensionless numbers. A variety of experiments were performed up to 100kWel input power from the evaporators. Flashing and geysering as two types of occurred instabilities are stated and discussed in comparison to former test-facilities. Further phenomena like water hammer and counter current liquid flow are investigated. Based on these analyses constructive solutions can be derived, to stabilize the presented natural-circulation two-phase flow system.

Low Cost True Monofiber Optical Probe for Local Void Fraction Measurements in Minichannels

Two phase flow inside minichannels is one of the most investigated research topic at present. The measurement of the flow rate parameters is fundamental to characterize the flow pattern and its evolution over time. This paper shows that an optical technique, well-known for large diameter pipes, can be applied to mini channels with a laminar mass flow rate. In particular, a Y-junction mono-fiber optic system with a chamfered tip probe has been built and tested. This method is applied to the local void fraction measurement in a copper capillary pipe with internal diameter of 2 mm and external diameter of 3.00 mm. Different probes have been developed and tested. The accuracy of the method depends on the size, the shape of the tip and on the tip distance from the pipe centre. Different distances and liquid flow rate have been tested. The two-phase flow pattern is also visualized and recorded by a high speed camera (FASTEC Troubleshooter 16000 fps) and post processed with an image analysis technique. A good agreement between the optical and the video signal has been observed.

Experimental investigation of counter current air-water flow in a large diameter vertical pipe with inners

Counter current two phase flow is encountered in a wide variety of industrial applications. This paper describes the experimental results obtained in a circular column of 240 mm diameter with two inner pipes. The counter current flow studied concerns an upward flow of air and a downward flow of water at ambient temperature and pressure. The following range of operating conditions was analysed: superficial air velocities up to 23 cm/s and superficial water velocities up to – 11 cm/s, corresponding to a global air volume fractions (gas holdup) up to 29%. The experimental investigation concerned (i) flow visualization, (ii) local data from a double fibre optical probe and (iii) gas holdup measurements. The images obtained from an optical camera were used to observe the general flow patterns. The data obtained from the double fibre optical probe were used to study the local flow characteristics. In particular, the local void fractions, the bubble velocities, the bubble mean diameters and the bubble diameter distributions and are presented and discussed. The bed expansion technique was used to obtain the gas holdup measurements for every operating condition. The gas holdup measurements are discussed, compared with existing correlations and used for investigating the flow regime transitions. Finally, the gas holdup and the local void fraction measurements data are compared and used for understanding the local hydrodynamics.

Model of bubble velocity vector measurement in upward and downward bubbly two-phase flows using a four-sensor optical probe

The knowledge of bubble behaviors is of considerable significance for a proper understanding and modeling of two-phase flows. To obtain the information on the bubble motion, a novel model was developed, by which the bubble velocity vector can be directly calculated from six time intervals measured with a four-sensor probe. The measurements of local bubble velocity vector and void fraction were performed in both upward and downward bubbly flows by using a four-sensor optical probe. The area-averaged void fraction and bubble velocity obtained from the probe agree well with those measured by other cross-calibration methods, and the measurement errors are within 15% under various flow conditions. Experimental results of the bubble velocity vector reveal that the bubble lateral migration may be suppressed in upward flows, but be strengthened in downward flows as the liquid flow rate increases. Also, with an increase in gas flow rate, the bubble velocity distribution varies into the power–law profile in upward flows, but into an off-center peak profile in downward flows. In addition, the void fraction shows a core peak distribution at low void fraction for downward flows, but a wall peak distribution for upward flows. However, when the void fraction is relatively high, it displays an off-center peak distribution for downward flows but a core peak distribution for upward flows.

Design of a non-intrusive electrical impedance-based void fraction sensor for microchannel two-phase flows

A non-intrusive electrical impedance-based sensor is developed for measurement of local void fraction in air–water adiabatic flow through rectangular microchannels. Measurement of the void fraction in microchannels is essential for the formulation of two-phase flow heat transfer and pressure drop correlations, and may enable real-time flow regime control and performance prediction in the thermal regulation of high-heat-flux devices. The impedance response of the sensor to a range of flow regimes is investigated for a configuration with two aligned electrodes flush-mounted on opposing microchannel walls. Numerical simulations performed on a multi-phase domain constructed from three-dimensional reconstruction of experimentally observed phase boundaries along with the corresponding experimental results serve to establish the relationship between void fraction and dimensionless impedance for this geometric configuration. A reduced-order analytical model developed based on an assumption of stratified gas–liquid flow allows ready extension of these calibration results to different working fluids of interest.

Measurements of liquid-phase turbulence in gas–liquid two-phase flows using particle image velocimetry

Liquid-phase turbulence measurements were performed in an air–water two-phase flow loop with a circular test section of 50 mm inner diameter using a particle image velocimetry (PIV) system. An optical phase separation method-–planar laser-induced fluorescence (PLIF) technique—which uses fluorescent particles and an optical filtration technique, was employed to separate the signals of the fluorescent seeding particles from those due to bubbles and other noises. An image pre-processing scheme was applied to the raw PIV images to remove the noise residuals that are not removed by the PLIF technique. In addition, four-sensor conductivity probes were adopted to measure the radial distribution of the void fraction. Two benchmark tests were performed: the first was a comparison of the PIV measurement results with those of similar flow conditions using thermal anemometry from previous studies; the second quantitatively compared the superficial liquid velocities calculated from the local liquid velocity and void fraction measurements with the global liquid flow rate measurements. The differences of the superficial liquid velocity obtained from the two measurements were bounded within ±7% for single-phase flows and two-phase bubbly flows with the area-average void fraction up to 18%. Furthermore, a preliminary uncertainty analysis was conducted to investigate the accuracy of the two-phase PIV measurements. The systematic uncertainties due to the circular pipe curvature effects, bubble surface reflection effects and other potential uncertainty sources of the PIV measurements were discussed. The purpose of this work is to facilitate the development of a measurement technique (PIV-PLIF) combined with image pre-processing for the liquid-phase turbulence in gas–liquid two-phase flows of relatively high void fractions. The high-resolution data set can be used to more thoroughly understand two-phase flow behavior, develop liquid-phase turbulence models, and assess high-fidelity codes for multi-phase flows.

Development of impedance void meter for evaluation of flow symmetry

Often it is useful or even necessary to know whether the void fraction is evenly distributed in two-phase flows. In industrial systems, unevenly distributed flows can be a factor in vibrations and other undesirable occurrences. In bubble column chemical reactors, uneven distribution of the void can result in reduced reaction efficiency. In experiments, it is often desirable to have symmetrically distributed void for local measurements since this allows for more rapid data collection. Current systems for determining the symmetry of the void distribution in distributed flows either suffer from very long measurement time requirements or cause significant disturbances to the flow. To address the need for approximate evaluation of flow symmetry without causing excessive disturbance to the flow, a new impedance void meter design has been developed and tested under a variety of conditions. The ability of the new design to evaluate the void distribution symmetry has been tested through comparison with conductivity probe measurements of local void fraction, and the accuracy of the impedance meter has been verified through comparison with the conductivity probe measurements and with typical impedance void meter measurements.

Measurements of two-phase flow patterns and local void fraction in vertical rectangular minichannel

Abstract Technology advancements entail a necessity to remove huge amounts of heat produced by today’s electronic devices based on highly integrated circuits, major generators of heat. Heat transfer to boiling liquid flowing through narrow minichannels is a modern solution to the problem of heat transfer enhancement. The study was conducted for FC-72 boiling in a rectangular, vertical and asymmetrically heated minichannel that had depths of 0.5-1.5 mm, a width of 20 mm and a length of 360 mm. The heat flux increased and decreased within the range of 58.3-132.0 kWm−2, the absolute pressure ranged from 0.116 to 0.184 MPa and the mass flux was 185-1139.2 kgm−2s−1. The boiling process took place on a flat vertical heating surface made of Haynes-230 0.1 mm thick acid-proof rolled plate with the surface roughness of 121 μm.

Two-Phase Flow-Induced Forces on Piping in Vertical Upward Flow: Excitation Mechanisms and Correlation Models

Momentum variation in two-phase flow generates significant low frequency forces, capable of producing unwanted and destructive vibrations in nuclear or petroleum industries. Two-phase flow-induced forces in piping were previously studied over a range of diameters from 6 mm to 70 mm in different piping element geometries, such as elbows, U-bends, and tees. Dimensionless models were then developed to estimate the rms forces and generate vibration excitation force spectra. It was found that slug flow generates the largest forces due to the large momentum variation between Taylor bubbles and slugs. The present study was conducted with a 52 mm diameter U-bend tube carrying a vertical upward flow. Two-phase flow-induced forces were measured. In addition, two-phase flow parameters, such as the local void fraction, bubble size and velocity, and slug frequency were studied to understand the relationship between the force spectra and the two-phase flow patterns. A new two-phase flow pattern map, based on existing transition models and validated using our own local void fraction measurements and force spectra, is proposed. This paper also presents a comparison of the present dimensionless forces with those of previous studies, thus covers a wide range of geometries and Weber numbers. Finally, a dimensionless spectrum is proposed to correlate forces with large momentum variations observed for certain flow patterns.