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Wire-mesh sensors are used for a high-speed visualization of a gas–liquid flow as well as for the measurement of void fraction profiles, bubble size distributions and gas velocity distributions. Recent progress was made in designing and constructing such sensors for an application in a hot steam–water mixture. Two types are presented: (1) a sensor with an inner diameter of 52.3 mm with a measuring matrix of 16×16 and (2) a sensor of 195 mm inner diameter with 64×64 measuring points. Both devices can be operated at 7 MPa and a temperature of max. 286 ∘ C. The spatial and temporal resolutions are equal to earlier used sensors for air–water flow at ambient conditions (3 mm, 2500 fps). In the paper, the function of the sensors is illustrated by presenting flow visualizations obtained at two vertical test sections of the Rossendorf TOPFLOW facility. The pipes are approximately 9 m long and have inner diameters equal to the diameters of the measuring cross sections mentioned above. The results show how the flow structure depends on the thermodynamic parameters by comparing measurements performed at 1, 2, 4 and 6.5 MPa and 180, 212, 250 and 280 ∘ C, correspondingly, under adiabatic conditions with earlier air–water tests.

The structure of a steam-water flow in a vertical pipe of 195.3 mm inner diameter was studied using novel wire-mesh sensors for high-pressure/high-temperature operation (max 7 MPa/286 °C). Tests were carried out at pressures of 1 and 2 MPa under nearly adiabatic conditions as well as with slightly sub-cooled water (6 K at max). Steam was injected into sub-cooled water and condensed during the upwards flow. The evolution of radial gas fraction profiles and bubble size distributions along the pipe in a high-pressure steam-water flow was measured for the first time. The experimental data allow correlating the intensity of steam condensation in contact with sub-cooled water with the structure of the interfacial area and the bubble size distribution, which is very important for the model development. The data were used to test the complex interaction of local bubble distributions, bubble size distributions and local heat and mass transfer. The model considers a large number of bubble classes (50). This allows the investigation of the influence of the bubble size distribution. The results of the simulations show a good agreement with the experimental data. The condensation process is clearly slower, if the injection nozzle diameter is increased (from 1 to 4 mm orifices). Also bubble break-up has a strong influence on the condensation process because of the change of the interfacial area. Some modelling errors arise from the uncertainty of the interfacial area for large bubbles and the heat transfer coefficient.

The paper presents the results of a study on the motion of single (individual) bubbles rising in upward shear liquid flow in the vicinity of a vertical wall. Bubbles were found to slide along the wall when their diameter is small. Bubbles could also experience multiple collisions with the wall at certain experimental parameters (geometry of a channel, range of superficial liquid velocity, bubble size, etc). The latter was theoretically predicted by solving the equation of the bubble motion for the lateral direction in the boundary layer of the channel. For this, constitutive models available in the literature for the interfacial forces acting on a bubble in the vicinity of the wall were used. A simplified 1D model predicting bubble lateral displacement near the wall and taking into account the balance of drag and non-drag forces acting on a bubble was proposed. The numerical results were verified against the experimental ones obtained by non-intrusive high-speed video observations and subsequent image processing. The experiments on the bubble motion were conducted in a vertical acrylic duct having a square cross-section of 25 cm 2 and a height of approximately 1.3 m. Desalinated water and air both taken at room temperature were used in the experiments. All measurements on the bubble motion were performed at channel heights between 0.8 and 1 m above the gas injection point.

Bubbly flow is encountered in a wide variety of industrial applications ranging from flows in nuclear reactors to process flows in chemical reactors. The presence of a second phase, recirculating flow, instabilities of the gas plume and turbulence, complicate the hydrodynamics of bubble column reactors. This paper describes experimental and numerical results obtained in a rectangular bubble column 0.1 m wide and 0.02 m in depth. The bubble column was operated in the dispersed bubbly flow regime with gas superficial velocities up to 0.02 m/s. Images obtained from a high speed camera were used to observe the general flow pattern and have been processed to calculate bubble velocities, bubble turbulence parameters and bubble size distributions. Gas disengagement technique was used to obtain the volume averaged gas fraction over a range of superficial gas velocities. A wire mesh sensor was applied, to measure the local volume fraction at two different height positions. Numerical calculations were performed with an Eulerian-Eulerian two-fluid model approach using the commercial code CFX. The paper details the effect of various two-fluid model interfacial momentum transfer terms on the numerical results. The inclusion of a lift force was found to be necessary to obtain a global circulation pattern and local void distribution that was consistent with the experimental measurements. The nature of the drag force formulation was found to have significant effect on the quantitative volume averaged void fraction predictions.

The development of a technique for high spatial and temporal resolution film thickness measurement in oil-water flow is presented. A capacitance measurement system is used to measure thin water films near to the wall pipe. The system consists of seven parts: a signal generator, analog multiplexers, a planar sensor, auto-balancing bridges, logarithmic amplifiers, an analog-digital converter and a control unit. The measurement system is controlled by a NI FPGA RIO System. The FPGA has two state machines. The machines control the synchronization between the transmitter circuit board, the digitizer and a computer. FPGA technology has showed to be a flexible electronics, which let to change, easily, some important features as acquisition rate, number of channels and samples, among others. Experimental results of the technique indicated that the system can measure thickness between 400 µm and 2200 µm. Film thickness in oil-water flow was studied in a 12-m long vertical glass pipe, with 50.8 mm of internal diameter, using mineral oil (828 kg/m3 of density and 220 cP of viscosity) and tap water.

Very few experimental imaging studies using a compact neutron generator have been published, and to the knowledge of the authors none have included tomography results using multiple projection angles. Radiography results with a neutron generator, scintillator screen, and camera can be seen in Bogolubov et al. (2005), Cremer et al. (2012), and Li et al. (2014). Comparable results with a position-sensitive photomultiplier tube can be seen in Popov et al. (2011). One study using an array of individual fast neutron detectors in the context of cargo scanning for security purposes is detailed in Eberhardt et al. (2005). In that case, however, the emphasis was on very large objects with a resolution on the order of 1cm, whereas this study focuses on less massive objects and a finer spatial resolution. In Andersson et al. (2014) three fast neutron counters and a D-T generator were used to perform attenuation measurements of test phantoms. Based on the axisymmetry of the test phantoms, the single-projection information was used to calculate radial attenuation distributions of the object, which was compared with the known geometry. In this paper a fast-neutron tomography system based on an array of individual detectors and a purpose-designed compact D-D neutron generator is presented. Each of the 88 detectors consists of a plastic scintillator read out by two Silicon photomultipliers and a dedicated pulse-processing board. Data acquisition for all channels was handled by four single-board microcontrollers. Details of the individual detector design and testing are elaborated upon. Using the complete array, several fast-neutron images of test phantoms were reconstructed, one of which was compared with results using a Co-60 gamma source. The system was shown to be capable of 2mm resolution, with exposure times on the order of several hours per reconstructed tomogram. Details about these measurements and the analysis of the reconstructed images are given, along with a discussion of the capabilities of the system and its outlook.

We have developed an optical technique for the two-dimensional mapping of water film thickness. The technique is based on infrared light absorption. A near-infrared camera is used to capture the radiation returning from a surface illuminated by a halogen lamp. The attenuation of the back-scattered radiation is used as a measure for the thickness of the water film covering the surface. The method was calibrated using well-defined liquid films between a glass plate and the surface. Series of instantaneous, two-dimensional thickness profiles of wavy turbulent free-falling films along a vertical wall were measured at a frame rate of 200 Hz. The evolution of complex flow patterns with three-dimensional instabilities such as long waves and capillary waves was observed under isothermal conditions. For the validation of the method, simultaneous independent measurements were taken together with an electrical high-speed liquid film sensor.

The role of the lift force in the stability of the homogeneous flow regime in a bubble column is investigated. Instabilities caused by the lift force may be one important reason for the transition from a homogeneous to a heterogeneous bubble column. The lift force acts on rising bubbles in a lateral direction when gradients of the liquid velocity are present. Such gradients may result from circulation cells in the bubble column as well as from local disturbances. Depending on the sign of the lift force, such local disturbances in a homogeneous flow may be damped or enhanced. The corresponding feedback mechanism was analyzed by means of linear stability analyses. As a result, criteria for stability were obtained for monodispersed flow, for a flow with two bubble size groups, and, finally, with some additional approximations also for the case of N bubble size groups or a given bubble size distribution. In a next step, the effects were investigated by CFD calculations. The obtained criteria for stability fit well with the observed effects in CFD simulations. Recently, two different groups confirmed the stability criteria experimentally.

For the investigation of air/water slug flow, a horizontal channel with rectangular cross-section was built at Forschungszentrum Rossendorf. The channel allows the investigation of air/water co- and counter-current flows at atmospheric pressure, especially the slug behaviour. Optical measurements were performed with a high-speed video camera, and were complemented by simultaneous dynamic pressure measurements. Moreover velocity-fields were measured using Particle Image Velocimetry (PIV). A CFD simulation of the stratified co-current flow was performed using the code CFX-5, applying the Euler-Euler two fluid model with the free surface option. The grid contains 4105 control volumes. The turbulence was modelled separately for each phase using the k- based shear stress transport (SST) turbulence model. To achieve wave generation in such a short channel, the inlet water level had to be varied in time. For this purpose, the water level history was taken from a recorded image sequence and set as time-dependent boundary condition at the model inlet. The results show a wave formation up to slug development with closure of the whole channel cross-section and consequently an increase of the pressure level behind the slug. Despite unsteady conditions at the inlet of the test channel and simplified initial conditions in the model, the slug simulation with CFX is in good qualitative agreement with the experiment, while the slug length increases during its progression, witch was not observed in reality.

Eine in ruhendem Wasser aufsteigende Luftblase wird parallel zur Wasseroberfläche in zwei zueinander senkrechte horizontale Richtungen parallel projiziert. Diese beiden Projektionen werden zunächst als Ellipsen approximiert. Anhand der Ellipsen wird die Blase dann als Rotationsellipsoid modelliert. Dieses eignet sich zur Analyse der Orientierung der Symmetrieachse der Blase auf ihrem Weg durch das Wasser. Dies wird an einem Ensemble schraubenartig aufsteigender Blasen verdeutlicht.

Measurements of the cross-sectional distribution of the gas fraction and bubble size distributions were conducted in a vertical pipe with an inner diameter of 51.2 mm and a length of about 3 m for air/water bubbly and slug flow regimes. The use of a wire-mesh sensor obtained a high resolution of the gas fraction data in space as well as in time. From this data, time averaged values for the two-dimensional gas fraction profiles were decomposed into a large number of bubble size classes. This allowed the extraction of the radial gas fraction profiles for a given range of bubble sizes as well as data for local bubble size distributions. The structure of the flow can be characterized by such data. The measurements were performed for up to 10 different inlet lengths and for about 100 combinations of gas and liquid volume flow rates. The data is very useful for the development and validation of meso-scale models to account for the forces acting on a bubble in a shear liquid flow and models for bubble coalescence and break-up. Such models are necessary for the validation of CFD codes for the simulation of bubbly flows.

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 st...

A two-phase flow in a rectangular bubble column of 100×20 mm cross-section and 1.5 mm height was studied using a high-speed video system. Series of images were taken at different elevations at a frequency of 500 Hz. The images were processed using a bubble recognition algorithm. In such a way, an individual bubble in the gas swarm could be tracked. The time-averaged velocity profiles and the turbulent diffusion coefficients were derived as a function of the superficial gas velocity. The lateral displacement of bubbles travelling over a certain vertical distance was transformed into a probability density distribution in order to measure the turbulent diffusion coefficient of the gaseous phase. The shape of the distributions obtained was found to fit well to the Gaussian standard distribution. The dispersion coefficients were observed to grow proportionally to the square root of the vertical distance. The diffusion coefficients were calculated from the proportionality factor and were compared with some correlations published in the literature. The experiments were performed for superficial gas velocities ranging from 1 to 6 mm/s. The bubbles were generated either by a porous sparger or a set of capillaries placed at the bottom of the column. The measurements were taken at different heights between 1 and 1.2 m where the bubble cloud was occupying the entire cross-section.

This work investigates the role of the lift force for the stability of a homogeneous bubble column. Instabilities caused by the lift force may be one important reason for the transition from homogeneous to heterogeneous bubble column. On rising bubbles the lift force acts in a lateral direction, when gradients of the liquid velocity are present. Non-uniform liquid velocity fields may be induced if the gas fraction is not equally distributed, e.g. caused by local disturbances. This feedback mechanism is studied in the paper. It was found, that a positive lift coefficient (small bubbles) stabilizes the flow, while a negative coefficient (large bubbles) leads to unstable gas fraction distributions, and thus it favours the appearance of a heterogeneous bubble column regime. The turbulent dispersion force has always a stabilizing action, i.e., it partially compensates the destabilization induced by a negative lift coefficient. A stability analysis for a mono-dispersed system nevertheless showed, that influence of the lift force is much larger, compared to the influence of the turbulent dispersion force, if only bubble induced turbulence is considered. Thus, the stability condition is practically the positive sign of the lift force coefficient. The extension of the analysis to two bubbles classes, from which one being small enough to have a positive lift coefficient, results in a minimum fraction of small bubbles needed for stability. Finally a generalized criterion for N bubble classes and for a continuous bubble size distribution is given.

This paper reports on the modeling and simulation of flashing-induced instabilities in natural-circulation systems, with special emphasis on natural-circulation boiling water reactors (BWRs). For the modeling the 4-equation two-phase model FLOCAL [Rohde, U., 1986. Ein teoretisches Modell fur Zweiphasen-stromungen in wassergekulthen Kernreaktoren und seine Anwendung zur Analyse des Naturumlaufs im Heizreaktor AST-500. Ph.D. dissertation, Akademie der Wissenschaften der DDR, Dresden], developed at the Forschungszentrum Rossendorf (FZR, Germany), has been used. The model allows for the liquid and vapor to be in thermal non-equilibrium and, via drift-flux models, to have different velocities. The phenomenology of the instability has been studied and the dominating physical effects have been determined. The results of the simulations have been compared qualitatively and quantitatively with experiments [Manera, A., van der Hagen, T.H.J.J., 2003. Stability of natural-circulation-cooled boiling water reactor during start up: experimental results. Nuc. Technol., 143] that have been carried out within the framework of a European project (NACUSP) on the CIRCUS facility. The facility, built at the Delft University of Technology in The Netherlands, is a water/steam 1:1 height-scaled loop of a typical natural-circulation-cooled BWR.

Gamma tomography has been used to investigate the fluid distribution within multiple axial planes of a hydrodynamic test coupling under operating conditions. The tomographic data was obtained by phase correlated averaging of projections from the homogeneously rotating parts of the coupling. By application of the filtered backprojection reconstruction technique to the data we reconstructed average fluid fraction distributions in the transversal planes at different operating points of the coupling.

We devised and tested a computed tomography approach that utilises a scanned electron beam X-ray source to produce fast tomographic image sequences of transient density distributions. Potential application areas for this technique are the visualisation and measurement of two-phase and particle flows in thermofluid dynamics research, chemical processes, or transport systems for fluids and bulk solids. In our setup we used a linear deflection pattern for the electron beam and a non-annular detector arc to record transmission data of an object from different projection angles. This approach gives the highest achievable axial resolution and is comparatively moderate in effort and costs. For the inverse problem we applied iterative image reconstruction techniques to reconstruct the density distribution from a limited data set. The method has been experimentally tested on static and dynamic phantoms with a frame rate of 1000 images per second and a spatial resolution of approximately 1 mm in plane and axial.

This study investigates the characteristics of upward air–water flow for non-symmetric initial condition in the 50 mm vertical tube. Wire mesh sensor is utilized to obtain the local properties of the gas phase i.e. void fraction, gas velocity and bubble size. To generate the non-symmetric initial flow condition, air is supplied from a single injecting hole and mixed into the liquid flow inside the pipe through a narrow-ring shaped exit. In this case, large bubble can be generated at the inlet. In this experiment, the characteristics of bubble formation at the inlet and flow evolution along the pipe are emphasized. The results show the capability of the measurement technique to analyze the non-symmetric distribution of the gas phase. The effects of superficial gas and liquid velocity on the bubble distributions and on the local void fraction spreading at the inlet are clarified. The flow evolution dominated by bubble coalescence and break up is discussed in term of the development of Sauter mean diameter over the cross-section of the pipe. The results also show an inconsistency with uniform flow conditions at low gas flux. Hence, the effect of phase distribution spreading is proposed. In addition, the development of rising gas velocity in term of mean bubble size during coalescence and break up is revealed.

From the beginning of BWR technology it was realized that a BWR can become unstable under particular circumstances caused by a feedback between the thermal-hydraulics and the neutronics. This instability can result in oscillations of the power and the flow rate, which is an unwanted phenomenon. The NACUSP project addresses the stability issues in current and future BWRs by expanding the basic understanding through well structured testing and analyses of experimental data, by analyses of existing operational stability data from three different European reactors (Forsmark, Leibstadt, Cofrentes), by applying this knowledge via efficient models and validated computer codes to operating reactors and reactor designs, and by developing general guidelines for reactor operation and design on how to avoid BWR instabilities. In order to cover the parameter range as efficiently as possible, four existing, sophisticated thermohydraulic test facilities (CLOTAIRE [Gouirand, J.M., 1988. CLOTAIRE Program, description and manufacturing of the mock-up, CEA Cadarache, DRE/STRE/LGV 88–876.] DESIRE [van de Graaf, R., van der Hagen, T.H.J.J., Mudde, R.F., 1994. Two-phase flow scaling laws for a simulated BWR assembly. Nucl. Eng. Des. 148, 455–462.] CIRCUS [de Kruijf, W.J.M., van der Hagen, T.H.J.J., Mudde, R.F., 2000. CIRCUS; a natural circulation two-phase flow facility, Eurotherm Seminar No. 63, 6–8 September 1999 Genoa, Italy, 391–395] and PANDA [Dreier, J., Huggenberger, M., Aubert, C., Bandurski, T., Fischer, O., Healzer, J., Lomperski, S., Strassberger, H.-J., Varadi, G., Yadigaroglu, G., 1996. The PANDA facility and first test results, Kerntechnik 61, 214–222]) have been selected. To extrapolate from small-scale separate-effect testing conditions to full-scale integral reactor conditions one needs to rely on the performance of computer codes (MONA [Hoyer, N., 1994. MONA, a 7-Equation Transient two-phase flow model for LWR dynamics, Proceedings of the International Conference on New Trends in Nuclear System Thermohydraulics, pp. 271–280], ATHLET [Krepper, E., Prasser, H.-M., 1999. Natural circulation experiments at the ISB-VVER integral test facility and calculations using the thermal-hydraulic code ATHLET. Nucl. Technol. 128, 75–86], RAMONA-3(-5) [Grandi, G., Hoyer, N., Belblidia, L., 1998. Two-fluid thermal hydraulics capabilities of RAMONA-5, Proceedings of the International Conference on the Physics of Nuclear Science and Technology, October 5–8, 1998, Long Island, NY, USA, 1621–1625], LAPUR-V [Otaduy, P.J., March-Leuba, J., 1990. LAPUR User's Guide, NUREG/CR-542, ORNL/TM-11285], DWOS, FLICA [Toumi, I., Bergeron, A., Gallo, D., Royer, E., Caruge, D., 2000. FLICA-4: a three-dimensional two-phase flow computer code with advanced numerical methods for nuclear applications, Nucl. Eng. Des. 200, 139–155], SAPHYR, RELAP5/MOD3 [Idaho National Engineering Laboratory, 1995. RELAP5/MOD3 Code Manual, Vols. 1–6, INEL-95/0174, NUREG/CR-5535], TRAC-BF1 [Idaho National Engineering Laboratory, 1992. TRAC-BF1/MOD1; an advanced best-estimate computer program for BWR accident analysis, Vols. 1–3, EGG-2626, NUREG/CR-4356]. For specific items CFD codes are applied as well. Within NACUSP a linear stability analysis tool is developed. Four of the three experimental facilities within the project have yielded a large, unique database. Natural-circulation and stability characteristics at nominal pressure were collected from extensive experiments at the DESIRE facility. Low-pressure characteristics were measured at the PANDA (large scale) and the CIRCUS (small scale) facility. The phenomena encountered (flow rate and stability trends, the occurrence of flashing induced oscillations) can be explained from basic physical models and are now well understood. Several thermal-hydraulic codes have been benchmarked—some successfully, others with limited success—against these data. The CLOTAIRE (nominal pressure, large scale) facility has been modified and is now ready for experiments. Nuclear power plant data from three BWRs (Cofrentes, Forsmark and Leibstadt) have been collected. A very complete set of reactor data was obtained from measurements at the Leibstadt BWR during cycle-19. During this test, which was performed within the NACUSP project, safe reactor operation was demonstrated at extreme conditions, covering the entire power-flow map. State-of-the-art, coupled thermal-hydraulic—neutronics codes are being benchmarked on these data. The development of an easy to use, rapid and efficient analytical tool for parametric studies on BWR stability is in progress. The last year of NACUSP will be devoted to finalising the before mentioned activities and to use the tools developed and the experience obtained for developing general guidelines for designing and operating BWRs.

Zwei asynchron rotierende Objekte sollen gemeinsam tomografisch rekonstruiert werden. Dafür werden tomografische Projektionen immer nur dann vorgenommen, wenn beide Objekte eine bestimmte Relativposition zueinander aufweisen. Die Objekte erscheinen dadurch wie fest miteinander verbunden. Vorhandene radiale Symmetrien der Objekte können ausgenutzt werden. Als praktisches Anwendungsbeispiel dient eine hydrodynamische Kupplung.

During two-phase flow blowdown from pressure vessels considerable pulsations of the discharged mass flow rate were found. Regions of mass flow rate instability were predicted by a linear stability analysis. The pulsations are caused by the following closed feedback circuit: boil up – level swell – void fraction of the discharged mixture – critical discharge rate – velocity of pressure decrease – boil up. They were also observed in transient simulations of the depressurization. Finally, the regions of instability were confirmed by experiments. The possibility of the occurrence of pulsations increases with the volume of the ventline and the volume void fraction of the discharged mixture. They may influence the rate of pressure relief from pressure vessels as well as from chemical reactors.

Within the pressurized water reactor (PWR) safety analyses, attention has increasingly focused in recent years on boron dilution events that could potentially lead to reactivity transients. Mixing of the low-boron water with the ambient coolant of higher boron content provides an important mitigation mechanism before the low-boron water enters the core.Experimental support is needed to validate the computational tools to be applied to analyze the mixing of the low-boron water. Experiments were performed in the three test facilities - the Upper Plenum Test Facility (UPTF), the Primaerkreislauf (PKL), and the Rossendorf coolant mixing model (ROCOM) - in Germany.The relevant PKL and UPTF tests were focused on small-break loss-of-coolant accident (SBLOCA) scenarios with reflux-condenser mode and restart of natural circulation. The two test facilities represent a typical western-type PWR and are/were operated by Siemens/KWU now Framatome ANP in Germany. While the restart of natural circulation was investigated in the PKL system test facility (volume 1:145, height 1:1), the UPTF experiments dealt with the mixing of water flows with different boron concentration in the cold legs, reactor pressure vessel (RPV) downcomer, and the lower plenum (all these components were full-scale models).The results from the PKL test facility demonstrate that in case ofmore » a postulated SBLOCA with reflux condensation phase, natural circulation does not start up simultaneously in all loops. This means that slugs of condensate, which might have accumulated in the pump seal during reflux-condenser mode of operation, would reach the RPV at different points in time. The UPTF tests showed an almost ideal mixing of water flows with different boron concentration in the RPV downcomer.The ROCOM test facility has been built in a linear scale of 1:5 for the investigation of coolant mixing phenomena in a wide range of flow conditions in the RPV of the German KONVOI-type PWR. The test results presented are focused on the mixing of a slug of deborated water during the startup of the first reactor coolant pump. Based on experimentally determined pulse responses, a semianalytical model for the description of coolant mixing inside the KONVOI RPV has been developed. Calculations for a presumed boron dilution event during the startup of the first reactor coolant pump have been carried out by means of the semianalytical model and independently by means of the computational fluid dynamics code CFX-4. The semianalytical model is able to describe the time dependent behavior of the deboration front at each fuel element position in a good agreement with the experiment. All main mixing effects, observed in the experiment, are also reproduced by the CFX calculation.« less

The wire-mesh sensor developed by the Forschungszentrum Rossendorf produces sequences of instantaneous gas fraction distributions in a cross section with a time resolution of 1200 frames per second and a spatial resolution of about 2–3 mm. At moderate flow velocities (up to 1–2 m·s−1), bubble size distributions can be obtained, since each individual bubble is mapped in several successive distributions. The method was used to study the evolution of the bubble size distribution in a vertical two-phase flow. For this purpose, the sensor was placed downstream of an air injector, the distance between air injection and sensor was varied. The bubble identification algorithm allows to select bubbles of a given range of the effective diameter and to calculate partial gas fraction profiles for this diameter range. In this way, the different behaviour of small and large bubbles in respect to the action of the lift force was observed in a mixture of small and large bubbles.

As CFD (Computational Fluid Dynamics) continues to grow in the nuclear industry the need for validation is essential to this growth. The focus of this paper is to highlight the validation of AREVA’s 2-Phase CFD models in a rod bundle with spacer. The experimental work is based on void distribution measurements using a 64×64 wire mesh sensor at the Paul Scherrer Institute adiabatic test loop fuel rod bundle (SUBFLOW). The 2-Phase models are based on the Eulerian multiphase framework where the interaction models are further developed using user routines in a commercial CFD tool to achieve stability and accuracy in a rod bundle configuration with a spacer. The 2-Phase CFD models coincide with the SUBFLOW experiments in showing the effect of void collecting in the center of the sub-channels slightly downstream of the spacer and further downstream the smaller bubbles migrate toward the rod surfaces whereas the larger bubbles stay in the sub-channels. Good agreement is achieved between CFD and void distribution experiments. The multi-phase CFD method was used by AREVA to improve the performance of the new products GAIA and ATRIUM™ 11.Copyright © 2014 by ASME

This work presents a study of a new technique for non-intrusive measurement of liquid-liquid flow. The evaluated film thickness sensor has 64 × 16 measuring points and a measuring area of 128 × 32 mm. This sensor measures the electrical conductance and, through a calibration procedure, converts those signals in water film thickness. The experimental work was carried out in the Swiss Federal Institute of Technology Zurich (ETHZ) in Zurich, Switzerland.

A method for the prediction of the radial gas profile for a given bubble size distribution is presented. It is based on the assumption of the equilibrium of the forces acting on a bubble perpendicularly to the flow direction. These forces strongly depend on the bubble size [14, 18]. For the simulation of transient flow regime effects, the modelling of several bubble classes in a 1D model and consideration of their radial profiles seems to be more promising than a detailed 3D modelling. The radial profile of the liquid velocity is calculated by the model of Sato [21, 22]. On the basis of this velocity profile, radial distributions are calculated separately for all bubble classes according to the given bubble size distribution. The sum of these distributions is the radial profile of the gas fraction. It is used in an iteration process to calculate a new velocity profile. There is a strong interaction between the profiles of liquid velocity and gas volume fraction. The model is the basis of a fast running one-dimensional steady state computer code. The results are compared with experimental data obtained for a number of gas and liquid volume flow rates. There is a good agreement between experimental and calculated data. In particular, the change from wall peaking to centre peaking gas fraction distribution is well predicted.

For the analysis of the core behaviour during boron dilution transients and main steam line breaks, coupled neutron kinetic/ thermal hydraulic codes are necessary. These codes must contain models of the coolant mixing on its way from the inlet nozzles to the core entrance, because the reactivity insertion strongly depends on the distribution of temperature and boron concentration at the core inlet. To model the coolant mixing the ROCOM test facility was built, a fluid dynamic 1:5 scaled model of the Konvoi reactor. The experimental results at steady state flow conditions are presented in a survey form.

In 1995 at the integral test facility ISB-VVER in Elektrogorsk near Moscow, natural circulation experiments were performed that were scientifically supported by the Forschungszentrum Rossendorf. These experiments were the first of this kind at a test facility that models VVER-1000 thermal hydraulics. Using the code ATHLET, which is being developed by Gesellschaft fur Anlagen-und Reaktorsicherheit, pre- and posttest calculations were done to determine the thermal-hydraulic events to be expected and to define and tune the boundary conditions of the test. The conditions found for natural circulation instabilities and cold-leg loop-seal clearing could be confirmed by the tests. The main thermal-hydraulic phenomena were identified and compared with the results gained during similar experiments on VVER-440 test facilities. Besides the thermal-hydraulic standard measuring system, the facility was equipped with needle-shaped conductivity probes for measuring the local void fractions.

The Siemens AG is developing the new innovative boiling water reactor concept SWR1000. New features are the passive safety systems (e.g. emergency condensers, building condensers, passive pressure pulse transmitters, gravity-driven core flooding lines). For the experimental investigation of the emergency condenser effectiveness, the NOKO test facility has been constructed at the Forschungszentrum Jülich in cooperation with Siemens. This test facility has an operating pressure of 10 MPa and a maximum power of 4 MW for steam production. The emergency condenser bundle consists of eight tubes and is fabricated with planned geometry and material of the SWR1000. In more than 200 experiments, the emergency condenser power was determined as a function of pressure, water level and concentration of noncondensables in the pressure vessel as well of pressure, water level and temperature in the condenser. Post test calculations of NOKO experiments were performed with an improved version of ATHLET. To calculate the heat transfer coefficients during condensation in horizontal tubes it was necessary to develop the module KONWAR and to implement it in ATHLET. KONWAR is based on the flow regime map of Tandon and includes several semiempirical correlations for the determination of the heat transfer coefficients. The comparison bet-ween calculations and experiments shows good agreement.

Siemens AG is developing the innovative boiling water reactor concept SWR1000. New features are the passive safety systems (e.g. emergency condenser, building condenser, passive pressure pulse transmitter). For the experimental investigation of the emergency condenser effectiveness, the NOKO test facility has been constructed at the Forschungszentrum Julich. The facility has an operating pressure of 10 MPa and a maximum power of 4 MW. The emergency condenser test bundle consists of eight tubes and is fabricated with planned geometry and material of the SWR1000. In more than 200 experiments, the emergency condenser capacity was determined as a function of pressure, water level and concentration of non-condensables in the pressure vessel, as well as of pressure, water level and temperature in the condenser. For the evaluation of the NOKO experiments, the program system CASH-Graphics (Computergestutzte Auswertung und Unsicherheitsanalyse) was developed. This evaluation is the basis for the determination of the operation conditions of the emergency condenser. Post-test calculations of NOKO experiments were performed with an improved version of ATHLET. To calculate the heat transfer coefficients during condensation in horizontal tubes, the module KONWAR has been developed and implemented in ATHLET. KONWAR is based on the flow regime map of Tandon and includes several semi-empirical correlations for the determination of the heat transfer coefficients. The comparison between calculations and experiments shows a good agreement.

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.

Results of a small-break loss-of-coolant accident experiment, conducted on the PMK-2 integral-type test facility are presented. The experiment simulated a 1% break in the cold leg of a VVER-440-type reactor. The main phenomena of the experiment are discussed, and in the case of selected events, a more detailed interpretation with the help of measured void fraction, obtained by a special measurement device, is given. Two thermohydraulic computer codes, RELAP5 and ATHLET, are used for posttest calculations. The aim of these calculations is to investigate the code capability for modeling natural circulation phenomena in VVER-440-type reactors. Therefore, the results of the experiment and both calculations are compared. Both codes predict most of the transient events well, with the exception that RELAP5 fails to predict the dryout period in the core. In the experiment, the hot- and cold-leg loop-seal clearing is accompanied by natural circulation instabilities, which can be explained by means of the ATHLET calculation.

An overview of a measurement method for liquid film thickness in annular flows based on cold neutron imaging is given here. Neutron imaging being a non-intrusive, contactless method is attractive option for two-phase flow investigations offering an excellent contrast. It can provide with information at a high spatial resolution on the flow structure, like the thickness of the liquid film in annular flows. The method has been optimized, and its performance, regarding bias, statistical accuracy, upper and lower detection limits, has been thoroughly quantified using computational tools and measurement results. The technique has been developed based on nuclear fuel bundle models; however, it is applicable practically to annular flows in any arbitrary flow channel geometry of interest., Experiments in Fluids, 54 (9), ISSN:0723-4864, ISSN:1432-1114

A conceptual design optimization of a fast neutron tomography system was performed. The system is based on a compact deuterium–deuterium fast neutron generator and an arc-shaped array of individual neutron detectors. The array functions as a position sensitive one-dimensional detector allowing tomographic reconstruction of a two-dimensional cross section of an object up to 10 cm across. Each individual detector is to be optically isolated and consists of a plastic scintillator and a Silicon Photomultiplier for measuring light produced by recoil protons. A deterministic geometry-based model and a series of Monte Carlo simulations were used to optimize the design geometry parameters affecting the reconstructed image resolution. From this, it is expected that with an array of 100 detectors a reconstructed image resolution of ~1.5 mm can be obtained. Other simulations were performed in order to optimize the scintillator depth (length along the neutron path) such that the best ratio of direct to scattered neutron counts is achieved. This resulted in a depth of 6–8 cm and an expected detection efficiency of 33–37%. Based on current operational capabilities of a prototype neutron generator being developed at the Paul Scherrer Institute, planned implementation of this detector array design should allow reconstructed tomograms to be obtained with exposure times on the order of a few hours.

We present the results of our recent studies on a cold-neutron imaging detector prototype based on THick Gaseous Electron Multiplier (THGEM). The detector consists of a thin Boron layer, for neutron-to-charged particle conversion, coupled to two THGEM electrodes in cascade for charge amplification and a position-sensitive charge-readout anode. The detector operates in Ne∕(5%)CF4, at atmospheric pressure, in a stable condition at a gain of around 10(4). Due to the geometrical structure of the detector elements (THGEM geometry and charge read-out anode), the image of detector active area shows a large inhomogeneity, corrected using a dedicated flat-filed correction algorithm. The prototype provides a detection efficiency of 5% and an effective spatial resolution of the order of 1.3 mm.

Chemical Engineering and Processing, 69, ISSN:0255-2701, ISSN:1873-3204

Novel high efficiency fast-neutron detectors were suggested for fan-beam tomography applications. They combine multi-layer polymer converters in gas medium, coupled to thick gaseous electron multipliers (THGEM). In this work we discuss the results of a systematic study of the electron transport inside a narrow gap between successive converter foils, which affects the performance of the detector, both in terms of detection efficiency and localization properties. The efficiency of transporting ionization electrons was measured along a 0.6 mm wide gas gap in 6 and 10 mm wide polymer converters Computer simulations provided conceptual understanding of the observations. For a drift lengths of 6 mm electrons were efficiently transported along the narrow gas gap, with minimal diffusion-induced losses; an average collection efficiency of 95% was achieved for the ionization electrons induced by a primary electron of a few keV initial energy. The 10 mm height converter yielded considerably lower efficiency due to electrical and mechanical flaws of the converter foils. The results indicate that detection efficiencies of around 7% can be expected for 2.5 MeV neutrons with 300 foils converters, of 6 mm height, 0.4 mm thick foils and 0.6 mm gas gap., 19 pages, 18 figures, Journal of Instrumentation (JINST)

The flow channel of the fluids in the nuclear reactor core and the heat exchanger is relatively small. In order to understand the two-phase flow phenomena in the narrow channel, many interesting studies have been carried out, for example, mini capillaries (Wong, et al., 1995, Han & Shikazono, 2009), rectangular channels with small gap (Mishima, et al., 1993, Xu, et al., 1999, Hibiki & Mishima, 2001) and rod bundles with tight-lattice geometry (Tamai et al., 2006, Sadatomi et al., 2007, Kawahara et al., 2008). Especially, a triangle tight-lattice rod bundle has been adopted as a fuel rod configuration in high conversion boiling water reactor (Iwamura et al., 2006, Uchikawa et al., 2007, Fukaya et al., 2009). This has a narrow gap of about 1mm in the coolant channel. Therefore, two-phase flow in the tight-lattice bundle or narrow channel should be clarified for thermal-hydraulic analysis of high conversion reactor. In the past several years, the acquisition of experimental data and the modelling of the flow in tight-lattice rod bundle have been done. Tamai et al. (2006) evaluated the effect of the gap width and the power profile from the pressure drop measured in tight-lattice 37 rod bundles. Sadatomi et al. (2007) studied the void fraction characteristics in double subchannels with tight-lattice array and the data was compared with some correlations and subchannel codes. Furthermore, their group estimated the wall and the interfacial friction forces from the measured void fraction and pressure drop using the same subchannel (Kawahara et al., 2008). However, the advanced measurement techniques of the spatio-temporal phase distribution and velocity field are required for the high accurate analysis of the flow.

The conceptual design and operational principle of a novel high-efficiency, fast neutron imaging detector based on THGEM, intended for future fan-beam transmission tomography applications, is described. We report on a feasibility study based on theoretical modeling and computer simulations of a possible detector configuration prototype. In particular we discuss results regarding the optimization of detector geometry, estimation of its general performance, and expected imaging quality: it has been estimated that detection efficiency of around 5-8% can be achieved for 2.5MeV neutrons; spatial resolution is around one millimeter with no substantial degradation due to scattering effects. The foreseen applications of the imaging system are neutron tomography in non-destructive testing for the nuclear energy industry, including examination of spent nuclear fuel bundles, detection of explosives or drugs, as well as investigation of thermal hydraulics phenomena (e.g., two-phase flow, heat transfer, phase change, coolant dynamics, and liquid metal flow)., Comment: 11 Pages; 6 Figures; Proceeding of the International Workshop on Fast Neutron Detectors and Application FNDA2011, Ein Gedi, Israel, November 2011. Published on the Journal of Instrumentation; 2012 JINST 7 C02056

Experiments in Fluids, 51 (3), ISSN:0723-4864, ISSN:1432-1114

A network of custom developed anemometers is described, employed to measure the flow profile in a 0.9 m diameter pipe. The sensor is suitable to the harsh environment inside the large scale containment test facility PANDA. It is designed to work at velocities from 0.04–0.3 m/s in gas mixtures of varying composition, where other methods fail.

Thermal fatigue is a relevant problem in the context of life-time extension of nuclear power plants (NPP). In many piping systems in NPPs hot and cold water is mixed, which leads to high temperature fluctuations in the region close to the solid wall and resulting thermal loads on the pipe walls that can cause fatigue. One of the relevant geometric test cases for thermal fatigue is the mixing in T-junctions. In this study we apply large–eddy simulations (LES) to the mixing of hot and cold water in a T-junction. We perform a set of simulations by using different formulations of the LES subgrid scale model, i.e. standard Smagorinsky and dynamic procedure, to identify the influence of the modelled subgrid scales on the simulation results. The results exhibit a large difference between the models, which is caused by the use of turbulent viscosity wall–damping functions when applying the standard model.Copyright © 2009 by ASME

"The water hammer and inertia-driven cavitation hammer phenomena caused by the activation of fast-acting valves were studied in a pipeline test facility at Fraunhofer UMSICHT in the context of the EURATOM project WAHALoads. The main goal of the project is the prediction of the loads on equipment and support structures. The presented experiments tackle some scenarios typical for power plants and supply material for code validation with regard to the modeling of both thermohydraulic effects and fluid-structure interaction. The test facility Pilot Plant Pipework, representing an approximately 230 m long experimental pipeline, was upgraded in order to allow experiments at system pressures of up to 30 bar at maximum temperatures of about 180° C. The test rig was further equipped with a test segment that simulates a piping system and the associated supports typical for a (nuclear) power plant. For a better understanding of thermohydraulic processes during cavitation behind the fast-acting valve, novel instrumentation was applied. Wire-mesh sensors as well as local void probes were equipped with integrated microthermocouples and used for the local instantaneous measurement of both void fractions and fluid temperature. The fast temperature measurement combined with the instantaneous detection of the passage of the gas-liquid interface measurement reveals insights into the condensation heat transfer controlling the speed of the void collapse in the case of a condensational water hammer. Two-phase flows commonly occur in nature and in a multitude of other settings. They are not only of academic interest but are found in a wide range of engineering applications, continuing to pose a challenge to many research scientists and industrial practitioners alike. Although many important advances have been made in the past, the efforts to understand fundamental behavior and mechanisms of two-phase flow are necessarily a continuing process. Volume 8 of Multiphase Science and Technology contains the text of the invited lectures given at the Third International Workshop on Two-Phase Flow Fundamentals sponsored by the Electric Power Research Institute (EPRI) and the U. S. Department of Energy (DOE). Many of the world's foremost researchers, representing diverse cross sections of multiphase flow community, attended the workshop. The diversity of the participants enriched the discussions and conclusions that were reached. The purpose of the workshop was to define the state-of-the-art in two-phase flow, advance the science, and reach a consensus on future research directions. In particular, the specific emphasis was to evaluate the current understanding of the physics of multiphase flows as based on measurement and visualization, to evaluate the current state of the art in modeling of multiphase physics, to develop an agreed-upon basis for the balance equations for multiphase systems and their associated boundary conditions and closure relations, to evaluate and discuss the mathematical structure of the equations to describe multiphase flow, and to discuss and identify the methodologies needed to apply proper numerical procedures for the solution of the governing equations; that is, to fill existing gaps and bring together materials from diverse fields of multiphase flow. Multiphase Science and Technology is now being published as a quarterly journal, a mode of publication that will allow the material to be published more rapidly and regularly. Please contact Begell House for information on the previous eight hardbound volumes."

Continuing safety assessments are important for the present and future generations of nuclear reactors to ensure safe and efficient use of nuclear energy. For safety analysis purpose, lumped parameter codes as well as codes with 3D capabilities are used to predict and simulate the containment conditions following a hypothetical accident. The assessment of code capability to predict consequences of hypothetical accidents can be done by comparing code simulations with experimental data obtained in large scale facilities under prototypical thermal hydraulic conditions. The large scale PANDA test facility built at the Paul Scherrer Institute (PSI) in Switzerland is well suited to perform thermal hydraulic experiments for investigating integral containment system response during accidents and also to study multicompartmental 3D effects related to a Light Water Reactor (LWR). This paper presents the experimental results and the analysis of two tests performed in a containment that is represented by two large vessels (volume of each vessel is 90 m3 ) interconnected by a bended pipe of 1 m diameter. The tests address the possibility of the permanence of hydrogen stratification just beneath the containment dome under conditions determined by low velocity and a plume or jet of higher density which could be, for instance, due to steam produced by evaporation of the sump. Since practically no data are available on these specific phenomena in large scale test facilities, the capabilities of the codes could not be yet verified. During the experiments, a stratified helium-rich layer is created in upper part of one of the two vessels and thereafter, interaction of an ascending buoyant jet with the stratified helium layer is investigated. Helium is used to simulate hydrogen released by cladding metal water reaction. Two tests with initial densimetric Froude number equal to 22.6 (TestAir/He ) and 104 (TestSteam/He ) are investigated. The distance between the pipe exit for vertical injection of the fluid and the bottom front of the stratified helium rich layer is about 2m for both the investigated test cases. Results depicting the mixing of lower and upper vessel regions are presented in terms of helium concentration profiles and temperature contour maps. The results show that the maximum penetration height is controlled by the initial exit conditions (momentum flux and the buoyancy flux) and density stratification inside the test vessel. Additionally, for the given test conditions and geometry, most of the injected fluid remains trapped in upper part of one of the two test vessels in which helium layer is created.Copyright © 2008 by ASME

Experiments with air-water flows have been carried out in a vertical pipe of ∼194-mm diameter and 9-m length, and a wide range of superficial liquid and gas velocities has been covered. At a distance of 7.6 m from the air injection, two wire-mesh sensors are installed, located at a distance of 63.3 mm from each other. The wire-mesh sensors measure sequences of instantaneous two-dimensional gas-fraction distributions in the cross section in which they are mounted, with a spatial resolution of 3 mm and a frequency of 2500 Hz. The spatial cross-correlations of the gas-fraction signals have been evaluated, and on their basis turbulent diffusion coefficients have been estimated. It is found that for a given liquid superficial velocity, a sudden increase of the diffusion coefficient takes place when the superficial gas velocity is increased above a certain value. The abrupt increase of the diffusion coefficient occurs in correspondence of the transition from mono- to bimodal bubble size distributions. The experimental diffusion coefficients are compared with the prediction of the Sato model (experimental gas-fraction profiles and bubble size distributions are given as input). Even though this model has been developed for bubbly flow, the general trends are well captured also in the churn-turbulent regime.

Closure laws are needed for the qualification of CFD codes for two-phase flows. In case of bubbly and slug flow, forces acting on the bubbles usually model the momentum transfer between the phases. Several models for such forces can be found in Literature. They show, that these forces depend on the liquid flow field as well as on the size and the shape of the bubbles. A validation of consistent sets of bubble force models for poly-disperse flows is given, basing on a detailed experimental database for vertical pipe flows, which contains data on the radial distribution of bubbles of different size as well as local bubble size distributions. A one-dimensional (1D) solver provides velocity profiles and bubble distributions in radial direction. It considers a large number of bubble size classes and is used for the comparison with the experiments. The simplified model was checked against the results of full 3D simulations done by the commercial code CFX-5.7 for simplified monodisperse cases. The effects of the number of bubbles classes as well as the effect of the lateral extension of the bubbles were analyzed. For the validation of bubble force models measured bubble size distributions were taken as an input for the calculation. On basis of the assumption of an equilibrium of the lateral bubble forces, radial volume fraction profiles were calculated separately for each bubble class. In the result of the validation of different models for the bubble forces, a set of Tomiyama lift and wall force, deformation force and Favre averaged turbulent dispersion force was found to provide the best agreement with the experimental data. Some discrepancies remain at high liquid superficial velocities.

Die CFD Modellierung hat Einzug gehalten in die nukleare Sicherheit. CFD-Codes werden erfolgreich eingesetzt, um komplexe Strömungsfelder in ganzen Druckwasserreaktoren zu berechnen. Dabei geht es vornehmlich um die Vermischung von Borsäure bei mit unboriertem Wasser und von kalten Notkühlwasser mit heißem Reaktorkühlmittel. Die Validierung anhand von Versuchen an verhätnismässig grossskaligen Testständen, die mit hochauflösender Leitfähigkeitsmesstechnik ausgestattet sind, zeigt die bereits erreichte hohe Zuverlässigkeit der Vorhersagen im Einphasenbereich. Diese Erkenntnisse können auf chemisch-verfahrenstechnische Anlagen übertragen werden. Beim Übergang zur Zweiphasenströmung treten zusätzliche Schwierigkeiten auf, die mit der Beschreibung der Prozesse an der Phasengrenze und der Modellierung der Topologie und Ausdehnung derselben zusammenhängen. Hierzu werden neuere Entwicklungen auf dem Gebiet der Zweifluidmodelle für Gas-Flüssigkeitsströmungen vorgestellt. Speziell handelt es sich um das Inhomogeneous MUSIG Model, das Bestandteil des Codes ANSYS CFX ist. Dieser Ansatz erlaubt die Beschreibung der Wechselwirkung mehrerer Blasenklassen unter Berücksichtigung der auf die Blasen wirkenden Kräfte und von Koaleszenz und Zerfall. Das Modell wurde ausgehend von Experimenten an vertikalen Rohrleitungen vorgeschlagen und dann anhand von Messergebnissen validiert. Auch hier kommen Leitfähigkeitssensoren zum Einsatz, die die Struktur der Phasengrenzfläche im gesamten Strömungsquerschnitt hochaufgelöst erfassen. Dabei wurden u.a. Strömungen mit Phasenübergang infolge Kondensation von Dampf an einer unterkühlten Flüssigphase untersucht. Mit solchen Gittersensoren wurden auch komplexe dreidimensionale Gas-Flüssigkeitsströmungen, z.T. bei Drücken bis zu 70 bar und Temperaturen bis 286 °C (Wasser-Dampf) vermessen, die charakteristische Merkmale von Strömungen in technisch relevanten Anlagenteilen aufweisen, wie Strömungsabriss, Staupunkte, Strahlausbildung, Rezirkulationsgebiete u.ä.. Der Vergleich zwischen Rechnungen und Experiment dokumentiert den erreichten Stand bei der dreidimensionalen Modellierung. Durch die Diskretisierung der zu untersuchenden Geometrie ist der CFD-Code in der Lage, die Einzelmodelle auch Geometrien zu übertragen, die bei der Validierung nicht betrachtetet wurden. Dies wird anhand von Rechenergebnissen für Sicherheitsventile demonstriert.

Measurements of two-phase flow parameters such as void-fraction, bubble velocities, and interfacial area densities have been performed in an upwards air-water flow at atmospheric pressure by means of a four-tip needle-probe and a wire-mesh sensor. For the first time, a direct comparison between the two measuring techniques has been carried out. Both techniques are based on the measurement of the fluid conductivity. For void fraction and velocity measurements, similarity exists between the two methodologies for signal analysis. A significantly different approach is followed, instead, for the estimation of the interfacial area concentration: while the evaluation based on the needle-probe signal is carried out by using projections of the gas-liquid interface velocity, the evaluation based on the wire-mesh signals consist in a full reconstruction of the bubbles interfaces. The comparison between the two techniques shows a good agreement.