Design Of Test Section Research Articles

Flow-Induced Vibration Experiment in Fuel Pin Bundle with Heavy Liquid Metal Flow: Test Section Design and Measurement Methods

The present paper describes the experimental setup for flow-induced vibration in a fuel pin bundle test section with flowing lead. The test section is a 37-pin mock-up representative of the GEN-IV ALFRED fuel assembly and is manufactured to investigate the turbulence-induced vibrations in the pins. The pins are instrumented with longitudinal strain gauges (SGs), and with three SGs per monitoring point, the displacement signal can be recovered in terms of amplitude and frequencies. To achieve this goal, a complex algorithm is implemented in the data acquisition control system of the test section. The test section will be installed in the Heavy Liquid metal Experimental loop for Nuclear Applications (HELENA) facility at the ENEA Brasimone R.C., in which a mechanical pump for lead circulation is present. The test matrix is proposed with the mass flow rate varying from 10 to 50 kg/s and a constant temperature of 450°C for all the tests. Briefly, the experimental procedure is presented to carry out the experimental campaign.

Conceptual design of narrow rectangular channel test section for research reactor application

Conceptual design was carried out on test section for studying heat transfer characteristics of narrow rectangular channel for research reactor application. Thermal hydraulic design requirements were specified through literature study on mid- to high-powered research reactors currently under operation or design. Engineering design was then carried out in streamlined fashion with help of in-house design code and computational fluid dynamics analysis. Considering various constraints and uncertainties in terms of engineering hot channel factors, sensitivity studies were performed and realistic test section design was obtained. For final design, stochastic operable heater power map is generated by merging individual operable ranges from each constraint, with random sampling of hot channel factors.

Conceptual Design of Narrow Rectangular Channel Test Section for Research Reactor Application

Conceptual Design of Narrow Rectangular Channel Test Section for Research Reactor Application

Upgrading the PLINIUS platform toward smarter prototypic-corium experimental R&D

In the recent years, CEA Cadarache PLINIUS prototypic corium platform has been upgraded: more powerful induction generator for e.g. MCCI experiments, development of thermitic melting, e.g. for new corium spreading tests in VULCANO, new MERELAVA facility for corium flooding, new VITI setups for thermophysical property measurements, KROTOS FCI test with full depth X-ray visualization of water pool. Main features of these facilities are described. Designs for further improvements such as the new FUJISAN and VITAE setups for characterizations of the aerosol released during prototypic fuel debris cutting, as well as the design of STROMBOLI test section for studying Lower Head pool stratification inversion evolution are also presented.

Experimental investigation of the influence of boiling-induced ageing on high heat flux flow boiling in a copper microchannel

Copper, due to its ease of machining and high thermal conductivity, is the most commonly used substrate for heat sink applications. This paper presents an experimental investigation of the effect of boiling-induced copper ageing on the heat transfer and critical heat flux (CHF) characteristics of flow boiling of water in a mini/micro-channel. Ageing study was conducted for two cases; Case-1 consists of repeated boiling runs in a channel with freshly machined copper surface for a constant mass flux, and Case-2 consists of repeated boiling runs in the same channel with the copper surface obtained after cleaning the aged surface with 0.1 M HCl solution. A unique feature of the test section design is that the microchannel, which is 0.47 mm deep, 1.01 mm wide and 40 mm long, has been machined on the PEEK cover plate. This enables the in-situ measurement of contact angle on the channel copper surface, which is heated and undergoes ageing during the course of the experimental runs. The heat transfer coefficient reduces by up to 63% and 70% for Case-1 and Case-2, respectively. However, the results of both Case-1 and Case-2 indicate no significant change in the CHF due to repeated boiling runs. The EDS analysis of the channel copper surface indicates thermal oxidation during ageing, and the SEM and confocal microscopy analyses show the change in surface morphology due to ageing. The results of the contact angle measurement indicate that the copper surface becomes hydrophilic after ageing. The average static contact angle on the surface reduced from 91ο to 32ο during the Case-1 study and from 98ο to 28ο during the Case-2 study. The possible reasons for the reduction in the boiling heat transfer coefficient and insignificant change in the CHF are discussed. Experiments were also carried out to study the effect of mass flux on the CHF using the fully aged channel. The CHF was found to increase with the increase in mass flux. The study compares the obtained experimental CHF values with the predictions made by the correlations available in literature.

Экспериментальное исследование теплогидравлических процессов в модели тепловыделяющих сборок с микротвэлами

The test section design of the TVS-MEI experimental setup intended for studying the hydrodynamics and heat transfer in a fuel assembly with micro fuel elements is developed, and the setup hydraulic circuit is modernized. The setup process characteristics correspond to the operational parameters of VVER-1000 reactor plants (a pressure up to 16 MPa and coolant temperature up to 350°C). The internal heat release in the bed of metal pebbles is obtained by high-frequency induction heating. A technology for compacting the test section made of high-strength alundum ceramics and a special clamping device for holding the bed were developed. The fuel assemblies with micro fuel elements have the outer geometrical parameters fully identical with those of the conventional assemblies with fuel rods. A technology for installing, wiring, and sealing thermocouples in the test section has been developed. Experimental studies aimed at determining the pressure loss and flow friction coefficient for a cylindrical pebble bed were carried with the following coolant operating parameters: P = (2--7) MPa and G = (0.05--0.5) kg/s. In processing the obtained experimental results, the dependences of pressure loss on the coolant mass velocity and the pebble bed flow friction on the Reynolds number were identified and plotted. The first experimental data on the temperature distribution in the pebble bed are obtained. The main objective of the experiments was to determine the possibility of heating the considered test section by using the chosen method.

High-fidelity PIV measurement of cross flow in 5 × 5 rod bundle with mixing vane grids

In order to achieve high-fidelity PIV measurement for cross flow in rod bundles, the measurement approach with the two-dimensional (2D) PIV has been optimized systematically and the systematic and random errors are analyzed. The optimizations, covering the design of test section, the setting of optical system, the PIV parameter configuration and the PIV correlation algorithm, were established based on massive sensitivity analysis. Utilizing the optimized PIV configuration, cross flow measurements were conducted in a 5 × 5 rod bundle with the rods 9.5 mm in diameter and the rod pitch of 12.6 mm, which is the typical configuration of fuel assembly in pressurized water reactors (PWRs). The FEP tubes filled with deionized water are employed as the matched index of refraction (MIR) rods to realize uniform illumination across the rod bundle. On each investigated cross section the cross flow was measured in the 16 inner subchannels. In order to maximize the magnification factor, four subchannels are measured in a single test run. Several substantial error sources were identified and accounted for in the error and uncertainty analysis. The systematic error consists mainly of the perspective error and displacement variation error, which were quantified and used to correct the measured results. The uncertainty of RMS velocity is the combination of random error and error due to limited sampling number, while the uncertainty of mean velocity equals to the error resulted from limited sampling number assuming that the systematic error is perfectly compensated. In the experiment the relative standard uncertainty of RMS velocity (to the bulk velocity) is within 0.6%, and the relative standard uncertainty of mean velocity (to the bulk velocity) is less than 0.4%. The measurements were carried out with the subchannel Reynolds number (Re) of 39600 at five cross sections, i.e. 3Dh, 5Dh, 10Dh, 15Dh and 20Dh downstream of a spacer grid. The experimental results showed the detailed cross flow features, including the evolution of vortices, in the rod bundle. The measured lateral flow was substantially dominated by the mixing vanes. Drastic turbulence generation by mixing vanes was observed downstream of a spacer grid. The generated turbulence decayed rapidly within 10Dh downstream of the spacer grid and then damped gradually.

Thermal hydraulic performance analysis of a post-CHF heat transfer test facility

Post critical heat flux (post-CHF) heat transfer may occur in loss of coolant accidents (LOCAs) in water-cooled nuclear reactors when makeup water quenches the uncovered fuel rods in the reactor core. The post-CHF regimes are characterized by low heat transfer coefficient and high wall temperature due to the vapor-wall contact. A good understanding of the hydrodynamics and heat transfer process in the post-CHF regimes, especially low-quality film boiling, such as inverted annular film boiling (IAFB) and inverted slug film boiling (ISFB), is imperative for accurately modeling the post-CHF phenomena and predicting the fuel rod cladding temperature and void fraction during accidents. This paper first discusses limitations of existing post-CHF hydrodynamics and heat transfer models/correlations and then identifies needs of post-CHF experimental data under high-flow and high-pressure conditions for model improvement. To narrow down the gap in the available experimental data, a Post-CHF Heat Transfer (PCHT) test facility is designed to perform quasi-steady-state low-quality film boiling experiments under high-pressure (ranging from 0.14 to 3.45 MPa), high-flow (ranging from 150 to 2000 kg/m2-s), and high inlet subcooling (up to 50 °C) conditions. In addition to the experimental study, a numerical model using COMSOL Multiphysics tool is developed to inform the test section design and to investigate wall heat flux and wall temperature distributions in the post-CHF regimes. The COMSOL Multiphysics simulation results show that quasi-steady-state post-CHF phenomena can be achieved in the PCHT test section. Considering thermal non-equilibrium conditions in the post-CHF regimes, a one-dimensional two-fluid model is implemented into a computer code to predict the void fraction using the wall temperature information obtained from the COMSOL simulation results. A correlation for the liquid-side Nusselt number is obtained for the IAFB regime based on previous experimental data and is implemented into the code, which results in an improvement in the agreement between the calculated void fraction and corresponding experimental data. With the new Nusselt number correlation, a parametric study of the effects of the heat flux, pressure, mass flux, and inlet subcooling on the liquid-side Nusselt number is studied. In addition, flow regime transition in the post-CHF regimes is studied by comparing the existing correlation and experimental data. Furthermore, the Weber number and liquid-side Nusselt number are calculated to examine whether there exists any correlation between these two non-dimensional numbers and the flow regime transition. The simulation results provide insights for design improvement of the PCHT test section and assist the development of a test plan for future IAFB and ISFB tests under high-pressure, high-flow, and high-subcooling conditions in the PCHT test facility.

Pre-test analysis of an LBE solidification experiment in TALL-3D

Abstract Coolant solidification is a phenomenon of potential safety importance for Liquid Metal Cooled Fast Reactors (LMFRs). Coolant solidification can affect local flow, heat transfer and lead to partial or complete blockage of the coolant flow paths jeopardizing decay heat removal function. It is also possible that reduced flow circulation may increase coolant temperature, counteract solidification and prevent complete blockage of the flow. Complex interactions between local physical phenomena of solidification and system scale natural circulation make modelling of solidification uncertain. Development and validation of adequate models requires validation grade experimental data. In this work we discuss results of analysis carried out in support of experiment development, specifically, design of a solidification test section and a test matrix for TALL-3D experimental facility (lead-bismuth eutectic (LBE) thermal-hydraulic loop). The aim of the analysis is experimental design that satisfies requirements stemming from the process of model qualification. We focus on two aspects: (i) design of solidification test section (STS) for investigation of solidification phenomena in lead-bismuth eutectic (LBE), and (ii) effect of the STS pool on the system scale behavior of the TALL-3D facility. Selection of the STS characteristics and experimental test matrix is supported using computational fluid dynamic (CFD) and system thermal-hydraulic (STH) codes.

Flow condensation in a mini channel with serrated fins with jet impingement cooling: Experimental study and development of new correlation

The present study experimentally investigated the heat transfer characteristics of flow condensation in a mini channel with serrated fins. The hydraulic diameter of the aluminum channel was 1.28 mm with 11 offset rectangular channels. The vapor of refrigerant R134a was cooled by cold water with jet impingement. After the calibration of jet impingement cooling, the influences of different parameters on condensation heat transfer were identified with the experimental conditions of vapor quality from 0 to 1, heat flux from 5 kW/m2 to 52 kW/m2, mass flux from 72 kg/(m2·s) to 145 kg/(m2·s) and saturation pressure from 0.38 MPa to 0.50 MPa. Flow patterns during condensation were also observed through the visualization design of test section. The experimental results reveal that the vapor quality and mass flux have insignificant influence on the local heat transfer coefficient. The heat transfer coefficient increases with the heat flux and decreases with the saturation pressure. Annular flow is dominant during the whole condensation process. As the existing empirical correlations fail to give reasonable prediction of the present experimental results, a new one is developed with the mean relative deviation and mean absolute relative deviation of 2.3% and 12.0%, respectively. The present work can provide valuable guidance for the design of mini channel condenser for engineering application.

Poststall Airfoil Performance and Vertical-Axis Wind Turbines

The sensitivity of vertical-axis wind turbine startup to airfoil aerodynamic performance characteristics is analyzed. Existing poststall airfoil data and the corrections used in acquiring them are critiqued. Uncertainties in the existing data that are large enough to impact on vertical-axis wind turbine startup modeling are established, with wind-tunnel blockage identified as a potential cause of the discrepancies. Using a conventional closed-section wind tunnel with blockage corrections applied to data and a passive, tolerant test section design, tests are conducted on five NACA 0015 airfoils with a range of chord lengths. The passive tunnel is found to be better at minimizing the effects of blockage, and it makes no use of correction formulas that are not strictly applicable when flow is separated because they are based on potential flow theory. Preliminary results are presented from a study varying Reynolds numbers and showing little variation in poststall performance for the NACA 0015 in the range of Reynolds numbers from to .

The new airfoil model NACA0015, modal analysis and flutter properties

The experimental airfoil model NACA0015 was used to study aeroelastic phenomena during self-excited profile vibration. It provides data for control of aeroelastic calculation programs at subsonic speeds of the stream. The model movability is two-dimensional with two-degree of freedom dynamic system, one in pitch and the second in plunge, and is proposed to be a dynamic system having two near corresponding eigenfrequencies. To quantitatively evaluate flow field using interferometry, a special test section design and profile was constructed. It utilized a large visual field for the optical system together with the option of changing support stiffness for both degrees of freedom. In this paper experimental results from the range of Reynolds numbers Re=(2.63–2.83)105 are published. The identified eigenvalues and eigenmodes for zero flow velocity are compared with measured flutter properties (frequency, modes and time evolutions) of the airfoil.

Numerical investigations of shock wave interaction with laminar boundary layer on compressor profile

The investigation of shockwave boundary layer interaction on suction side of transonic compressor blade is one of main objectives of TFAST project (Transition Location Effect on Shock Wave Boundary Layer Interaction). In order to look more closely into the flow structure on suction side of a profile, a design of generic test section in linear transonic wind tunnel was proposed. The experimental and numerical results of flow structure on a suction side of the compressor profile investigations are presented. The numerical simulations are carried out for EARSM (Explicit Algebraic Reynolds Stress Model) turbulence model with transition model. The result are compared with oil flow visualisation, schlieren pictures, Pressure Sensitive Paint (PSP) and static pressure.

A heat transport benchmark problem for predicting the impact of measurements on experimental facility design

This work presents the application of the “Predictive Modeling of Coupled Multi-Physics Systems” (PM_CMPS) methodology conceived by Cacuci (2014) to a “test-section benchmark” problem in order to quantify the impact of measurements for reducing the uncertainties in the conceptual design of a proposed experimental facility aimed at investigating the thermal-hydraulics characteristics expected in the conceptual design of the G4M reactor (GEN4ENERGY, 2012). This “test-section benchmark” simulates the conditions experienced by the hottest rod within the conceptual design of the facility's test section, modeling the steady-state conduction in a rod heated internally by a cosinus-like heat source, as typically encountered in nuclear reactors, and cooled by forced convection to a surrounding coolant flowing along the rod. The PM_CMPS methodology constructs a prior distribution using all of the available computational and experimental information, by relying on the maximum entropy principle to maximize the impact of all available information and minimize the impact of ignorance. The PM_CMPS methodology then constructs the posterior distribution using Bayes’ theorem, and subsequently evaluates it via saddle-point methods to obtain explicit formulas for the predicted optimal temperature distributions and predicted optimal values for the thermal-hydraulics model parameters that characterized the test-section benchmark. In addition, the PM_CMPS methodology also yields reduced uncertainties for both the model parameters and responses.As a general rule, it is important to measure a quantity consistently with, and more accurately than, the information extant prior to the measurement. For the test-section benchmark, it is shown that the maximum coolant and rod surface temperatures responses are the most important measurable quantities. Using all of the available information, the PM_CMPS formulas yield optimally predicted best-estimate nominal model parameter values and reduced predicted standard deviations for the predicted parameters, such that: (i) the model parameters displaying the largest relative sensitivities and largest relative standard deviations are affected the most; and (ii) the model parameters with zero sensitivities remain unaffected by applying the PM_CMPS methodology. The PM_CMPS formulas also yield optimally predicted best-estimate nominal values for the model responses, along with correspondingly reduced predicted standard deviations.Assimilating within the PM_CMPS methodology an accurate measurement of the benchmark's maximum coolant temperature has the following impacts on the quantity that has been measured (i.e., on the maximum coolant temperature): (i) the nominal value of the predicted maximum coolant temperature is closer to the more “accurate” experimental value; (ii) the originally computed standard deviation is decreased significantly. In addition, the measurement of the maximum coolant temperature impacts not only the quantity being measured, but also impacts the other responses of interest, e.g., the maximum temperature in the rod and the maximum rod surface temperature. In all cases, the predicted standard deviations were decreased substantially, indicating that the predicted values are more accurate than the originally computed ones.The PM_CMPS formulas were also used to predict the impact of the simultaneous assimilation of the maximum coolant and the maximum rod's surface temperature measurements. As expected in view of the fact that the PM_CMPS methodology uses Bayes’ theorem to combine information, the assimilation of two accurate experiments reduces the predicted standard deviations of the maximum rod, rod surface and coolant temperatures even further. The test-section benchmark and methodology presented in this work can serve as a paradigm for quantifying the impact of experimental measurements in the analysis, design and/or operation of other thermal-hydraulics experimental facilities.

An accurate wall temperature measurement using infrared thermometry with enhanced two-phase flow visualization in a convective boiling system

This paper presents an experimental strategy to achieve the accurate wall temperature measurement using infrared (IR) thermometry with the enhanced flow visualization. Our particular interest is focused on the measurement of two-phase flow parameters in a convective boiling system which involves a large heated area. For the present application, the important issues such as the design of test section and material selection were discussed along with our decision-making process. Then, the IR-based temperature tracking algorithm was established based on the multi-layer heater wall design proposed. To apply this algorithm, however, the optical properties of materials must be identified first. Thus, the optical features of selected materials, e.g., soda-lime glass and indium-tin-oxide (ITO) were investigated and the measured values were validated through experiments.The wall temperature tracking algorithm on the proposed heater wall design was validated both for steady-state and transient conditions. Also, such algorithm was proved to be applicable for the heat flux measurement. Finally, the feasibility of the present approach was demonstrated through a subcooled flow boiling experiment. The results showed that both the hydrodynamic motions of bubble and the corresponding wall temperature can be captured with high fidelity using the measurement strategy presented, from which the interrelation between the sliding vapor bubbles and the wall heat transfer were discussed.

An experimental study of bubble departure diameter in subcooled flow boiling including the effects of orientation angle, subcooling, mass flux, heat flux, and pressure

The effects of orientation angle, subcooling, heat flux, mass flux, and pressure on bubble departure diameter in the isolated bubble regime of subcooled flow boiling were studied by high-speed video in a two-phase flow loop that can accommodate a wide range of flow conditions. Specifically, the following ranges were explored: orientation angles of 0° (downward-facing horizontal), 30°, 45°, 60°, 90° (vertical), and 180° (upward-facing horizontal); mass flux values of 250, 300, 350, and 400kg/m2s, corresponding to Froude numbers between 0.42 and 1.06; pressures of 101 (atmospheric), 202, and 505kPa; two values of the subcooling degrees (10 and 20°C); and two heat fluxes (0.05 and 0.10MW/m2). The combination of the test section design, high-speed video camera and LED lighting results in high accuracy (order of 20μm) in the determination of the bubble departure diameter. The data indicate that the bubble departure diameter increases with increasing heat flux, decreasing mass flux, decreasing subcooling, and decreasing pressure. Also, the bubble departure diameter increases with decreasing orientation angle, i.e. the largest bubbles are found to detach from a downward-facing horizontal surface. The mechanistic bubble departure diameter model of Klausner et al. and its recent modification by Yun et al. were found to correctly predict all the observed parametric trends, but with large average errors and standard deviation: 65.5±75.8% for Klausner's and 37.9±51.2% for Yun's. Since the cube of the bubble departure diameter is used in subcooled flow boiling heat transfer models, such large errors are clearly unacceptable, and underscore the need for more accurate bubble departure diameter models.

Linear Cascade Wind Tunnel Testing of Supersonic Inflow and Outflow Turbine Blades (Test Section Design and Flow Visualization Using Schlieren Techniques)

Linear Cascade Wind Tunnel Testing of Supersonic Inflow and Outflow Turbine Blades (Test Section Design and Flow Visualization Using Schlieren Techniques)

Counter-current flow limitation in a model of the hot leg of a PWR—Comparison between air/water and steam/water experiments

Abstract In order to investigate the two-phase flow behaviour in a complex reactor-typical geometry and to supply suitable data for computational fluid dynamics (CFD) code validation, a model of the hot leg of a pressurised water reactor was built at Helmholtz-Zentrum Dresden Rossendorf (HZDR). The hot leg model is devoted to optical measurement techniques, therefore, a flat test section design was chosen and equipped with large windows. In order to enable the operation at high pressures, the test section is installed in the pressure chamber of the TOPFLOW ( T ransient tw O P hase FLOW ) test facility of HZDR, which is used to perform the experiments under pressure equilibrium with the inside atmosphere. Counter-current flow limitation (CCFL) experiments were performed, simulating the reflux-condenser cooling mode appearing in small break loss-of-coolant-accident (LOCA) scenarios. The fluids used were air and water at room temperature and pressures of up to 3.0 bar, as well as steam and water at pressures of up to 50 bar and the corresponding saturation temperature of 264 °C. One selected 50 bar experiment is presented in detail: the observed behaviour is analysed and illustrated by typical high-speed camera images of the flow. Furthermore, the flooding characteristics obtained from the different experimental runs are presented in terms of the Wallis parameter and Kutateladze number, which are commonly used in the literature. However, a discrepancy was first observed between the air/water and steam/water series. Further investigations show that the steam was probably wet due to heat losses and liquid entrainment from the heater circuit. Consequently, a correction of the steam measurements was required. The amount of parasitic water was evaluated indirectly over the shift of the zero liquid penetration noticed in the CCFL diagram. Finally, the experimental results confirm that the Wallis similarity is appropriate to scale flooding in the hot leg of a pressurised water reactor over a wide range of pressure and temperature conditions.

Comparison of Countercurrent Flow Limitation Experiments Performed in Two Different Models of the Hot Leg of a Pressurized Water Reactor With Rectangular Cross Section

Abstract In order to investigate the two-phase flow behavior during countercurrent flow limitation in the hot leg of a pressurized water reactor, two test models were built: one at the Kobe University and the other at the TOPFLOW test facility of Forschungszentrum Dresden-Rossendorf (FZD). Both test facilities are devoted to optical measurement techniques; therefore, a flat hot leg test section design was chosen. Countercurrent flow limitation (CCFL) experiments were performed, simulating the reflux condenser cooling mode appearing in some accident scenarios. The fluids used were air and water, both at room temperature. The pressure conditions were varied from atmospheric at Kobe to 3.0 bars absolute at TOPFLOW. According to the presented review of literature, very few data are available on flooding in channels with a rectangular cross section, and no experiments were performed in the past in such flat models of a hot leg. Commonly, the macroscopic effects of CCFL are represented in a flooding diagram, where the gas flow rate is plotted versus the discharge water flow rate, using the nondimensional superficial velocity (also known as Wallis parameter) as coordinates. However, the classical definition of the Wallis parameter contains the pipe diameter as characteristic length. In order to be able to perform comparisons with pipe experiments and to extrapolate to the power plant scale, the appropriate characteristic length should be determined. A detailed comparison of the test facilities operated at the Kobe University and at FZD is presented. With respect to the CCFL behavior, it is shown that the essential parts of the two hot leg test sections are very similar. This geometrical analogy allows us to perform meaningful comparisons. However, clear differences in the dimensions of the cross section (H×W=150×10 mm2 in Kobe, 250×50 mm2 at FZD) make it possible to point out the right characteristic length for hot leg models with rectangular cross sections. The hydraulic diameter, the channel height, and the Laplace critical wavelength (leading to the Kutateladze number) were tested. A comparison of our own results with similar experimental data and empirical correlations for pipes available in literature shows that the channel height is the characteristic length to be used in the Wallis parameter for channels with rectangular cross sections. However, some limitations were noticed for narrow channels, where CCFL is reached at lower gas fluxes, as already observed in small scale hot legs with pipe cross sections.

Air/Water Counter-Current Flow Experiments in a Model of the Hot Leg of a Pressurized Water Reactor

Different scenarios of small break loss of coolant accident for pressurized water reactors (PWRs) lead to the reflux-condenser mode in which steam enters the hot leg from the reactor pressure vessel (RPV) and condenses in the steam generator. A limitation of the condensate backflow toward the RPV by the steam flowing in counter current could affect the core cooling and must be prevented. The simulation of counter-current flow limitation conditions, which is dominated by 3D effects, requires the use of a computational fluid dynamics (CFD) approach. These numerical methods are not yet mature, so dedicated experimental data are needed for validation purposes. In order to investigate the two-phase flow behavior in a complex reactor-typical geometry and to supply suitable data for CFD code validation, the “hot leg model” was built at Forschungszentrum Dresden-Rossendorf (FZD). This setup is devoted to optical measurement techniques, and therefore, a flat test-section design was chosen with a width of 50 mm. The test section outlines represent the hot leg of a German Konvoi PWR at a scale of 1:3 (i.e., 250 mm channel height). The test section is mounted between two separators, one simulating the RPV and the other is connected to the steam generator inlet chamber. The hot leg model is operated under pressure equilibrium in the pressure vessel of the TOPFLOW facility of FZD. The air/water experiments presented in this article focus on the flow structure observed in the region of the riser and of the steam generator inlet chamber at room temperature and pressures up to 3 bar. The performed high-speed observations show the evolution of the stratified interface and the distribution of the two-phase mixture (droplets and bubbles). The counter-current flow limitation was quantified using the variation in the water levels measured in the separators. A confrontation with the images indicates that the initiation of flooding coincides with the reversal of the flow in the horizontal part of the hot leg. Afterward, bigger waves are generated, which develop to slugs. Furthermore, the flooding points obtained from the experiments were compared with empirical correlations available in literature. A good overall agreement was obtained, while the zero penetration was found at lower values of the gaseous Wallis parameter compared with previous work. This deviation can be attributed to the rectangular cross section of the hot leg model.