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The ablation consecutive to hot jet impingement is a safety issue for future Sodium Fast Reactors (SFR) core-catcher that can occur during relocation of molten fuel (corium) under severe nuclear accident situation. Much is still unknown on the ablation phenomenon especially explanations on cavity shape are lacking. However, these data are required to specify the core-catchers which will be used in future nuclear reactors’ vessel to prevent the corium from drilling through the confinement vessel during severe accident. To tackle this subject, data from experiments are obtained and analyzed to identify first order physical mechanisms at stake, and their links to geometry of the cavity. Two ablation mechanisms are noticed, the film ablation regime, for which liquid exits the cavity as a liquid film followed by the pool effect for which the cavity is filled with liquid. The analysis of results shows that the cavity shape is fixed during the film ablation regime and translates as ablation proceeds. Modes of liquid exit from the cavity are analyzed as well as the shapes the cavity assumes. Liquid/air interface temperature is also determined, in the film ablation regime. An explanation of cavity shape is presented. Conditions based on dimensionless numbers are put forward to differentiate between different cavity shapes and liquid exit modes. A first model for transition between film and pool effect ablation regimes is presented. It is the first time to the best of our knowledge that such analyses are undertaken. These give new tools for ablation risk assessment.

Heat conduction problems are usually solved either with analytical or numerical simulations, or with a reduced model using system identification. The use of polynomial models, often used in automation theory, gained the attention of the thermal community in the last decades to elaborate these reduced models because of their simplicity and performance to characterize an invariable system. They allow, for example, estimating a local temperature with a known input heat source. However, using polynomial models (or identified systems) in inverse conduction problems is not straightforward, usually requiring either a second inversion step. In this paper, we present a novel inverse technique based on the polynomial model ARX (autoregressive with exogenous input) that allows the estimation of an unknown input (like an imposed heat flux on a boundary) using a known output (temperature measurement). This new method, named inverse ARX or IARX, only requires a calibration step as a regular polynomial model and, then, it can estimate the input by a direct calculation with the identified parameters. The difference between IARX and ARX is the presence of future exogenous parameters, which were deduced using the initial discrete form of the ARX model. We present herein a numerical example using IARX of a 1D heat conduction simulation and IARX succeeded to estimate the input heat flux, even with high discontinuities and high measurement noises. Finally, we compare the proposed method with the classical Beck’s function specification method. IARX presented advantages like having no restriction for the number of future terms used in the model and performing the calculation 45% faster and with much less memory space consumption than with Beck’s method.

When the water inventory evaporates during a hypothetical loss of coolant accident in the core of a pressurized water reactor, the fuel rods temperature increases substantially, resulting in the clad ballooning and the formation of blocked sub-channels. During the reflooding phase, where water is injected into the core and a steam-droplets flow is created above the water level, the presence of ballooned zone forces the steam flow to redistribute towards intact sub-channels, which degrades the cooling of ballooned clads within blocked sub-channels. Looking to better understand this flow redistribution process, this study presents experimental results of magnetic resonance velocimetry measurements of three-component velocity fields in several 49-element ballooned bundles to evaluate geometric effects (blockage ratio, length and coplanarity) on the flow dynamics, as well as the flow rate effect (Reynolds number from 1,936 to 9,599). The flow redistribution occurred in the transition zone upstream of the balloon. Moreover, the transverse velocities at this location are higher for higher blockage ratios and they can reach nearly the same magnitude of the axial velocity. The amount of deviated flow is approximately equal to the sub-channel’s blockage ratio and is virtually insensitive to the blockage length or the flow rate. Furthermore, the flow’s axial velocity reduces up- and downstream of blocked sub-channels and this reduction is more substantial as the blockage ratio increases. Results with non-coplanar balloons showed that a less intense flow redistribution takes place; however, a downstream balloon can affect the flow redistribution dynamics caused by an upstream ballooned zone. Finally, for all the tested bundles and tested flow rate ranges, the flow was remarkably homogenized by the downstream mixing spacer grid, restricting the clad ballooning effects only to the portion between the two spacer grids where the ballooned zone is located regardless of the balloon geometry and the flow rate. The present results may be useful as well for other research areas involving flow blockages in heat exchangers.

This paper deals with ablation of a solid by a high temperature liquid jet. This phenomenon is a key issue to maintain the vessel integrity during the course of a nuclear reactor severe accident with melting of the core. Depending on the course of such an accident, high temperature corium jets might impinge and ablate the vessel material leading to its potential failure. Since Fukushima Daiichi accident, new mitigation measures are under study. As a designed safety feature of a future European SFR, bearing the purpose of quickly draining of the corium out of the core and protecting the reactor vessel against the attack of molten melt, the in-core corium is relocated via discharge tubes to an in-vessel core-catcher has been planned. The core-catcher design to withstand corium jet impingement demands the knowledge of very complex phenomena such as the dynamics of cavity formation and associated heat transfers. Even studied in the past, no complete data are available concerning the variation of jet parameters and solid structure materials. For a deep understanding of this phenomenon, new tests have been performed using both simulant and prototypical jet and core catcher materials. Part of these tests have been done at University of Lorraine using hot liquid water impinging on transparent ice block allowing for the visualizations of the cavity formation. Other tests have been performed in Karlsruhe Institute of Technology using liquid steel impinging on steel block.

In the scope of safety studies for future sodium cooled fast reactors (SFR), the use of discharge tubes and in-vessel core-catcher are foreseen for severe accident mitigation. During relocation of corium onto the core-catcher, complex thermal hydraulics phenomena can occur. This work focuses on the ablation of the core-catcher by the jet of corium. For that purpose, as experiments using corium and Na are difficult to achieve, we performed experiments using simulant materials: water jet on transparent ice. Transparent ice allows for ablation cavity real-time recording. Two main regimes were identified: a film flowing regime and a pool regime. We also studied the influence of temperature (30, 50, 70 °C) and velocity ( 1 , 2.5 , 5 , 7.5 , 10 m / s ) of the jet. Comparison with existing data have been done and finally, a scaling law in the first regime, for which the ablation rate is constant, has been found.

This paper focuses on the experimental investigation of breakup regimes of a molten fusible metallic droplet in water, at intermediate Weber numbers with emphasis on mass and energy balance. The experiment consists in impacting perpendicularly a molten drop onto the interface of a deep water pool, at a controlled temperature. Using a drop-on-demand device and high-speed shadowgraph, a single drop can be visualized during its evolution. There is a noticeable velocity jump when the droplet crosses the interface that can be modeled using an unsteady Bernoulli equation. As observed for liquid–gas systems, the drop experiences different regimes of fragmentation, depending on its Weber number: oscillations, bag oscillations, prolate drop stretching breakup, and then bowl-shaped bag breakup. However, opposite to the gas–liquid case, a Rayleigh–Taylor instability mechanism seems to be absent and this seems related to the bowl-shaped bag breakup mechanism when compared to the dome-shaped gas–liquid case. Statistics of the daughter droplets are then given, using either image analysis for large droplets size distribution or sieving and weighting of the solidified fragments for measurement of the Sauter mean diameter and surface energy creation. Finally, a simple relation between the Sauter mean diameter and the Weber number is presented based on the energy and mass balances. When comparing with previous higher Weber number results, a viscous transition corresponding to a strong increase in the energy loss is also shown to occur for the higher Weber number.

International audience; In the course of severe accident in a nuclear power plant, the corium mixture may flow down in the reactor pit and start a thermal attack of the basemat concrete. A simple way to terminate the melt progression is to add water on top of it. Among the various physical processes that may participate in the quenching, water may penetrate into the solidified corium through cracks generated by the thermal stress during the solidification. The present paper aims at providing a clarification of the process though an analysis of the few experimental data available using proto-typical corium, namely the SSWICS 1-7 experiments. At first, a general reminder and analysis of the data is given. Then, the thermal-hydraulic aspects are investigated through the construction of a dedicated heat flux correlation and its use via the experimentally measured post-test permeabilities. The analysis is supported by 1D and 2D evaluations with the multi-phase flow code MC3D-PREMIX, slightly modified for the purpose. It is concluded that the heat flux drastically decreases with the amount of added concrete material. Furthermore, 2D border effects are investigated and their importance highlighted in view of the experimental results analysed. These effects should explain the absence of observation of water ingression heat flux in the cases with large concrete amounts.Following, the paper proposes a model for the created permeability. Due to the complexity of the process and to the large uncertainties of the needed material properties in the considered situation with very high temperatures, a semi-empirical model is derived and reproduces the available data to a good approximation.Lastly, the model is adapted to the situation with internal decay heat, although no open experimental data is available to precisely support any model. A complete modeling is out of the scope of the paper, hence the focus is to provide hints for a first analysis of the impact of residual power on the melt progression and its coolability.

L'ablation d'un bloc de glace par un jet d'eau chaude est etudiee dans le but d'obtenir une comprehension physique du phenomene pouvant in fine etre applique au dimensionnement du recuperateur de corium d'un RNR-Na. Le systeme etudie met en oeuvre un jet d'eau et un bloc de glace transparente. Le suivi en temps reel de l'ablation est visualise par une camera rapide. Les resultats presentes montrent la succession des regimes rencontres. Un nouveau regime d'ablation est mis en evidence. L'evolution de la vitesse d'ablation en fonction du temps et de la vitesse du jet est analysee.

International audience; Reflooding experiments with rod bundles at Loss Of Coolant Accident (LOCA) conditions usually use intrusive methods with limited access and, consequently, their data are not always adequate and comprehensive to validate simulation codes. For this reason, sub-channel scale experiments are useful to obtain detailed data in a more controlled environment for an accurate thermal–hydraulics investigation. In this study, we present experiments of the cooling phase with an internal steam-droplets flow in a vertical pipe simulating an undamaged sub-channel in a nuclear reactor. Simultaneous measurements were performed of the wall temperature and droplets characteristics (velocity, diameter and temperature) using only optical techniques. An analysis is made on how the steam flow rate and maintained heating power during the cooling phase affect the wall heat dissipation, wall rewetting and droplets dynamics. Results show that wall rewetting normally occurs from bottom to top, and the temperature at minimum heat flux is highly affected by the droplets dynamics. Furthermore, droplets are accelerated when passing through the heated tube, especially at higher wall temperatures, and their temperature is nearly the same up- and downstream of the test section. Results with heating during the cooling phase show that wall rewetting takes place at higher wall temperatures and advances slower with the increase in the maintained heating power. Moreover, for the time period and flow conditions used in this work, wall rewetting does not occur for maintained powers higher than 1.5 kW/m.

Much effort has been devoted by the nuclear community to develop mechanistic models to predict the heat dissipation of fuel rods during a loss of coolant accident (LOCA). However, there is still the challenge to find correlations that accurately predict the rates of heat transfer corresponding to each involved phenomena and gather them all in a single physical model. Looking for improving thermal-hydraulics calculations in LOCA conditions, this work introduces a novel mechanistic model implemented in a new code named NECTAR to calculate heat and mass transfer phenomena and droplets dynamics in a polydispersed flow film boiling. The simulation results are compared with experimental measurements using three different geometries that represent the cladding ballooning at sub-channel scale. NECTAR can predict with great accuracy the heat dissipation by the internal steam-droplets flow with less than 8% mean deviation for all the test cases. The droplets characteristics downstream of the heated tube are also well predicted by NECTAR. In a detailed analysis of each heat transfer process, we found that wall-to-steam convection plays the major role in the internal heat dissipation, followed by the heat removed by the droplets impact onto the wall in spite of the very low volume fraction of droplets. Moreover, droplets impact and steam-to-droplets convection are also the main responsible for the droplets evaporation. Finally, considering the droplets diameter polydispersed distribution into thermal-hydraulics calculations (instead of a single mean diameter) has an effect only for higher volume fractions of droplets.

During a loss of coolant accident (LOCA), blocked sub-channels may appear due to the swelling of the fuel rods’ cladding, which results in flow redistribution during the reflooding phase. For this reason, special attention has been paid to the effect of fuel rods ballooning on the thermal-hydraulics in LOCA conditions. Due to the practically impossible physical or optical access to blocked sub-channels, no experiment so far has performed precise three-component velocity field measurements in the presence of ballooned regions. In this study, we used magnetic resonance velocimetry (MRV) to obtain three-component velocity fields of water flow within two 7 × 7 fuel rods bundles built mainly in plastic, one regular and one containing sixteen ballooned fuel rods with 90% blockage ratio and 240 mm blockage length. We present herein results with 50 lpm water flow rate, which corresponds to a Reynolds number of 1936. With the regular bundle, the performance of spacer grids’ mixing vanes to homogenize the flow was notable. With the ballooned bundle, we observed transverse velocities upstream of the ballooned zone that are as intense as the bulk mean velocity. Furthermore, there are substantial decreases in the axial velocity within blocked sub-channels up- and downstream of the ballooned zone, reaching near-zero and even negative values downstream, indicating flow recirculation. Although the flow is highly affected by the ballooned zone, the mixing spacer grid placed downstream remarkably homogenized the flow and effects of the flow redistribution disappeared. Finally, with the present ballooned bundle configuration, about 90% of the flow that should pass through blocked sub-channel deviates towards less resistant regions, which suggests a predominant geometric effect on the flow redistribution.

International audience; This paper presents experimental results on the fragmentation of a low meting point liquid metallic alloy jet into water. The liquid is Field's metal whose melting point is 62 °C. Data are obtained using high-speed camera acquisition and the solidified particles are sieved, a size Probability Distribution Function (PDF) is obtained from their mass distribution. These results are compared to separate data acquisitions obtained using a phase Doppler anemometer (PDA used in reflexion regime). Injection diameter range from 1 mm to 5 mm and injection velocity from 2.28 m/s to 4.97 m/s resulting in a Weber number ranging from 26 to 309 and a Reynolds number ranging from 4577 to 24875. The conclusion is that for these intermediate Weber and Reynolds numbers, the size of the droplets can mainly be related to Kelvin-Helmholtz instabilities. However, there exists also a long tail of small sized droplet whose distribution can be attributed to turbulent re-agglomeration of ligaments. This part of the distribution is very close to a log-Lévy law thanks to a model developed by [Rimbert & Castanet, PhysRevE, 84, 016318, 2011] By estimating the different turbulent scales, it is even possible to construct the small size distribution of droplet without using any fitting parameter.

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International audience; In the design of every new nuclear reactor, the possibility of a core meltdown is studied. In the case of ASTRID fast breeder a core-catcher will be placed beneath the core. To ensure that it performs as planed its ablation by a liquid jet of corium is investigated. To do so, an experimental set up was designed where the ablation of a solid by a liquid jet is studied using transparent ice as solid and water as liquid. The aim is to visualize the evolution of the cavity shape, and the ablation rate, during the ablation process. We observed that the cavity first assumes a conical shape, and the ablation rate stays constant. Then the cavity shape changes, and the ablation rate decreases with time. The aims to clarify the influence of the temperature, diameter and speed of the jet are studied as well as the inclination of the solid block on the ablation rate and mechanisms. A correlation giving the Nusselt number as a function of the Reynolds and the Prandtl number was obtained for our system. This will be used as a base to identify and model the mechanisms at stake. Introduction

International audience; This experimental study deals with the impingement of sprays on a heated wall in close to semi-industrial conditions. Therefore, an experimental setup was specifically designed: high liquid mass flux (up to 8 kg/m 2 /s), large droplets (up to 300 lm) and high surface temperature widely above the Leidenfrost point (up to 800 °C). The surface to be cooled is a 175 mm diameter nickel disk with 5 mm thickness heated by electromagnetic induction. The spraying devices consist in full cone sprays. Several studies, performed under these particular conditions, has allowed characterizing the heat flux removed by the spray by using techniques based on thermocouples or infrared thermography (IRT). However, to investigate more finely the heat transfer mechanism, it is necessary to measure the droplet temperature but, to date, no measurement for sprays are available in the literature. Therefore, the objective of the present work is to demonstrate the ability of the three-color Laser Induced Fluorescence (3cLIF) to meet this challenge. The combination of the 3cLIF with a Phase Doppler system allows also determining the droplet temperature per size class before and after impact. By adding the IRT measurements combined to an inverse heat conduction model, an energy balance is done in order to estimate the evaporated liquid mass. The influence of the normal Weber number on the liquid vaporization is investigated by using three sprays (Normal Weber numbers up to about 1500). Thereby, main results show that the droplets temperature after impact increases with the incident Weber number before to reach a plateau, attributed to the apparition of the splashing regime. The analysis of the energy balance shows that the mass of evaporated liquid decreases with the incident Weber number and the droplet heating. These first results are in well agreement with previous works conducted with single calibrated droplets, validating as the same time the use of combined 3cLIF-PDA measurements in the case of sprays.

To understand how tube bending occurs during the quenching by a water jet, an experimental device has been designed to measure heat transfer, tube bending and to visualize the hydrodynamics of the liquid jet during the cooling of a hot steel tube (grade: 42CrMo4, length: 500 mm, diameter: 76 mm, thickness: 6 mm). The bending resulting from the thermal gradients and phase transformations inside the tube is also measured all along its cooling. Knowing the heat transfer at the wall as a function of the wall surface temperature allowed us to compute the time evolution of temperature, phase transformations and deformations using SYSWELD code. This coupled analysis allows us to better understand why bending can occur in industrial processes.

International audience; During a Loss of Coolant Accident (LOCA) in a Pressurized Water Reactor, partial or complete drying of fuel assemblies may take place. In these conditions, the fuel temperature increases and may lead to substantial deformation of the fuel rod cladding and a partial blockage of the fluid sub-channels. These regions could significantly affect the cooling capacity of the nuclear core during the reflooding phase by the emergency core cooling systems. Understanding the cooling process and the thermal-hydraulic characteristics of the flow in these deformed regions is decisive to guarantee nuclear safety. Looking to provide valuable experimental data to validate existing and new models, this work is an experimental study on the thermal hydraulics during the cooling of a vertical tube with an internal steam-droplets flow, representing the LOCA conditions at sub-channel scale. The effect of the blockage ratio on the tube temperature, heat dissipation, wall rewetting, and droplets characteristics is evaluated by testing three configurations (0%, 61% and 90%). Optical techniques were used for a comprehensive characterization of the process, being them infrared thermography, phase-Doppler analyzer and three-color laser induced fluorescence thermometry. In general, wall rewetting in the test section occurs from bottom to top, although there is a discontinuity in the rewetting front with the 90% blockage ratio configuration. The droplets diameter reduces downstream of the test section because of evaporation. Droplets breakup was specifically observed with 90% blockage ratio. In all the cases, the droplets temperature was approximately the same up-and downstream of the test section, which indicates they are in nearly thermal equilibrium state and, therefore, representative of a LOCA situation.

The aim of the chapter is to give physical insights regarding solid/liquid interfaces and solid/liquid nanocomposites via atomistic simulations and improved macroscopic analytical approaches. Particularly, the use of molecular dynamics for the study of a nanodroplet located on a solid substrate will be discussed. The impact of line tension, Tolman length and adsorption layer on the wetting characteristic will be considered in details.

The strategy used in ASTRID Sodium Fast Reactor Demonstrator to mitigate the consequences of a postulated severe accident relies on the presence of dedicated corium discharge tubes between the active core region and the lower plenum and an in-vessel core catcher in the lower plenum to collect and cool the molten fuel. In this situation, the corium progression from the core towards the core catcher could lead to jet impingement on the core catcher surface and a subsequent degradation of the core-catcher material(s). After a review of the existing experimental database, it appeared necessary to define a dedicated experimental R&D program related to the long-lasting jet impingement on thick material plates. A particular behaviour will be studied when a molten pool is created (named “pool effect”) at the impingement point that reduces the heat transfer at the jet-material interface. In this scope, experimental tests with simulants and prototypic materials will be carried out by using the most possible representative conditions (e.g. Reynolds and Prandtl numbers) for ESFR (European Sodium Fast Reactor) severe accident conditions. Three R&D program have been proposed: (i) Ice-water jet impingement, the few results in the literature show a good agreement between ice-water jet system and metal wall/metal jet system which justifies the use of such simulant materials for investigating the impingement behaviour. The JOLO facility will be designed at LEMTA Nancy University. (ii) Molten steel jet impingement on the thick wall. The MOCKA facility in the Karlsruhe Institute of Technology will be used. (iii) Dedicated test section PLINIUS-2 IMPACT will be designed in future PLINIUS 2 large-mass prototypic-corium experimental platform at CEA Cadarache. First experiments have been carried out in the JOLO facility and provided dynamic ablation data before and during the formation of the pool. Additional JOLO tests and the MOCKA tests will be carried out in the frame of the ESFR-SMART European project starting by the end of 2017. The MOCKA tests will be performed with steel at higher temperature than in JOLO, before running core catcher qualification tests with prototypical corium (depleted UO2 and steel) and the ASTRID core catcher selected materials in PLINIUS-2 platform. New correlations and models will be developed from the experimental results in order to estimate the heat transfer data between the jet and the impacted material taking into account the “pool effect” phenomenon.

We present an efficient technique for the evaluation of the Gibbs adsorption of a liquid on a solid substrate. The behavior of a water nanodroplet on a silicon surface is simulated with molecular dynamics. An external field with varying strength is applied on the system to tune the solid-liquid interfacial contact area. A linear dependence of droplet's volume as a function of the contact area is observed. We introduce a modified Young-Laplace equation to explain the influence of the Gibbs adsorption on the nanodroplet volume contraction. Fitting of the molecular dynamics results with the analytical approach allows us to evaluate the number of atoms per unit area adsorbed on the substrate, which quantifies the Gibbs adsorption. Thus, a threshold of a droplet size is obtained, for which the impact of the adsorption is crucial. For instance, a water droplet with 5 nm radius has 3% of its molecules adsorbed on silicon substrate, while for droplets less than 1 nm this amount is more than 10%. The presented results could be beneficial for the evaluation of the adsorption impact on the physical-chemical properties of nanohybrid systems with large surface-to-volume ration.

International audience; Spherical and cylindrical water droplets on silicon surface are studied to tune the silicon–oxygen interaction. We use molecular dynamics simulations to estimate the contact angle of two different shaped droplets. We found that the cylindrical droplets are independent of the line tension as their three phases curvature is equal zero. Additionally, we compare an analytical model, taking into account or not the Tolman length and we show that for spherical small size droplets, this length is important to be included, in contrast to cylindrical droplets in which the influence of the Tolman length is negligible. We demonstrate that the usual convenient way to exclude linear tension in the general case can give wrong results. Here, we consider cylindrical droplets, since their contact angle does not depend on the droplet size in the range of few to 10ths of nanometres. The droplets are stabilised due to the periodic boundary conditions. This allows us to propose a new parameterisation for nanoscale droplets, which is independent the size of the droplets or its shape, minimising at the same time the calculation procedure. With the proposed methodology, we can extract the epsilon parameter of the interaction potential between a liquid and a solid from the nanoscaled molecular simulation with only as input the macrosized experimental wetting angle for a given temperature.

[EN] In the event of a hypothetical core disruptive accident in nuclear power plants, the molten core may flow out the reactor vessel and interact with the cold water. The evolution of the accident is strongly affected by the fragmentation of the jet of molten metallic fuel due to its interaction with the water (i.e. this situation is known as fuel coolant interaction, FCI). In order to evaluate and predict the various consequences of a FCI, many researches are conducted with either corium or high melting temperature molten metal, where premixing stage evolves with an important production of steam. This steam production that is unavoidable because the high temperature of corium leads to difficulties for using optical diagnostics. Hence, in our case, we use a eutectic alloy (Field’s metal) with a low melting point (62°C) in order to be able to visualize correctly the droplet fragmentation processes. The present work focuses on the fragmentation of a single Field’s metal liquid droplet with mass equals to 0.27g (±0.01g). The liquid droplet interacts with a water pool whose temperature range between 20°C to 60°C. According to its Weber number, it fragments in different ways. For each experiment, a single droplet has been visualized using a high-speed camera (at 8000 fps). All measurements (drop size, velocity, impact parameter and geometrical properties of the drops after the penetration) into the pool are evaluated using an open source image processing. Solidified fragments can then be sieved and the size PDF determined. Focus of the present work is put on the evolution of the Sauter Mean Diameter with increasing Weber number and varying bath pool temperature. It is shown that using a simple crust model during solidification and defining an effective Weber number which include the crust elasticity all the curves collapse on the same master curve for all the water bath temperature considered., This work has been funded by French Government through “programme d'investissements d'avenir” AnR RSNR ICE under the grant n° ANR-10-RSNR-01. The authors would also like to mention support from EU-COST Action MP1305 “Flowing Matter”.

National audience; Cette étude porte sur la compréhension fine des phénomènes de fragmentation liquide-liquide et a pour objectif principal l'analyse et la modélisation de l'interaction corium-eau qui est une possibilité dans la progression d'un accident grave d'une centrale nucléaire. Des expériences de fragmentation de gouttes de métal liquide dans de l'eau sont menées dans le but de comprendre l'influence des transferts thermiques et de la solidification sur les processus. Une visualisation rapide permet de suivre l'atomisation des gouttes ; les fragments solides sont ensuite tamisés pour déterminer des statistiques de taille. Les résultats montrent clairement une influence importante de la solidification sur le processus de fragmentation et la distribution finale des gouttes.

International audience; This work is an experimental study aiming at characterizing the heat transfers induced by the impingement of water droplets (diameter 80–180 μm) on a thin nickel plate heated by electromagnetic induction. The temperature of the rear face of the nickel sample is measured by means of an infrared camera and the heat removed from the wall due to the presence of the droplets is estimated using a semi-analytical inverse heat conduction model. In parallel, the temperature of the droplets is measured using the two-color Laser-Induced Fluorescence thermometry (2cLIF) which has been extended to imagery for the purpose of these experiments. The measurements of the variation in the droplet temperature occurring during an impact allow determining the sensible heat removed by the liquid. Measurements are performed at wall conditions well above the Leidenfrost temperature. Different values of the Weber numbers corresponding to the bouncing and splashing regimes are tested. Comparisons between the heat flux removed from the wall and the sensible heat gained by the liquid allows estimating the heat flux related to liquid evaporation. Results reveal that the respective level of the droplet sensible heat and the heat lost due to liquid vaporization can vary significantly with the droplet sizes and the Weber number.

SI:NURETH-14; International audience; This study focuses on thermal-hydraulic simulations, at sub-channel scale, of a damaged \PWR\ reactor core during a Loss Of Coolant Accident (LOCA). The aim of this study is to simulate the thermal-hydraulics to provide the thermal-mechanical code \DRACCAR\ with an accurate wall heat transfer law. This latter code is developed by the Institut de Radioprotection et de Sûreté Nucléaire (IRSN) to evaluate the thermics and deformations of fuel assemblies within the core. The present paper first describes the use of \CFD\ to perform analysis at sub-channel scale. Then we study the capabilities of existing codes CATHARE-3 and \CESAR\ to simulate dispersed droplet flows.

16th ESAFORM Conference on Material Forming (ESAFORM 2013), Univ Aveiro, Aveiro, PORTUGAL, APR 22-24, 2013; International audience; Cooling from impinging jet is nearly compulsory in steel industry processing especially in Run Out Table processing and steel tubes production because of the high heat transfer rates provided by the boiling of the subcooled water jet. As far as metallurgical phase transformations, residual stresses and deformations in the workpiece are concerned, the temperature drop during cooling must be perfectly controlled thanks to a fully understanding of the heat transfer mechanisms. In a previous study [1] the effect of surface to jet velocity ratio on heat transfer has been characterized and it has been shown that this parameter has a significant influence on shoulder of flux collapse. In order to understand the effect of more industrial quench process parameters, an innovative experimental quenching device has been designed and built. It allowed us to make heat transfer measurements at the surface of a hot nickel cylinder impinged by a subcooled water jet, according to several angles of impact and three jet directions against gravity. The results clearly highlight an effect of these two parameters on the heat transfer mechanisms at the surface of the tube. These results allow a better understanding of the origins of temperature heterogeneities inside the tube during the quench.

International audience

International audience

International audience; Cooling from impinging jet is nearly compulsory in steel industry processing especially in Run Out Table processing and steel tube production because of the high heat transfer provided by the boiling of the subcooled water jet. As far as metallurgical phase transformations, residual stresses and deformations in the workpiece are concerned, the temperature drop during cooling must be controlled thanks to a full understanding of the heat transfer mechanisms. One of the main characteristic using jet impingement is that the transition boiling regime may exist for very high superheat and thus the Leidenfrost temperature is higher than in pool boiling; consequently, boiling curves generally have a particular shape in the transition boiling regime which is usually called “shoulder of flux”. In this study, an innovative experimental quenching device has been used for analyzing the effect of the wall velocity of the surface to be cooled on the boiling curves (i.e. heat transfer) and we especially point out that the “shoulder of flux” (i.e. transition boiling regime) is strongly dependent on the surface to jet velocity ratio (r*). We found that a very small increase of the wall velocity has a high influence on shoulder of flux collapse.

Dans le cas d'un APRP (Accident de Perte de Refrigerant Primaire) sur un REP, les assemblages de crayons combustibles peuvent presentes des zones localement tres deformees. Le transitoire se termine par une injection d'eau dans le coeur par l'action des systemes de securite. Le refroidissement de ces zones depend de l'importance du blocage qui affectera l'ecoulement diphasique les traversant. La plupart des modeles de refroidissement de ces zones assechees a hautes temperatures, ne tiennent compte que de l'echange par convection avec la vapeur. Cependant, un regime d'ecoulement disperse est genere par le renoyage du coeur. Il est donc possible qu'une part non-negligeable du refroidissement soit due aux interactions gouttes-parois dans l'assemblage et en particulier dans les zones ballonnees. Aux hautes temperatures de surface de l'assemblage (au dela du point de Leidenfrost), le regime de rebond semble etre le regime predominant. Jusqu'a present, aucun modele tenant compte des caracteristiques de la goutte et de la paroi (vitesse, diametre, temperature, orientation) ne permet d'estimer avec precision le flux extrait a l'impact. Une experimentation permettant l'estimation de ce flux avec une marge d'erreur acceptable est mise en oeuvre au LEMTA ; on montre que l'estimation du flux de chaleur extrait lors de l'impact de la goutte est possible en utilisant un modele inverse parametre par les caracteristiques de l'impact (temps de contact, etalement de la goutte...). L'objectif de la collaboration entre LEMTA et IRSN est de pouvoir implanter des modeles de refroidissement realistes dans le code NEPTUNE_CFD, co-financee par EDF, CEA, AREVA et IRSN.

International audience; The solution of the linear, inverse, transient heat conduction problem (IHCP) in a cylindrical geometry is analysed. The rotating cylinder under investigation is experiencing boiling convection induced by the impingement of a water jet. The initial temperature is known, additional temperature measurements in time are taken with sensors positioned at a constant radius within the solid material, and the estimation of the wall heat flux at the external radius is sought. First, simulated temperature measurements inside the cylinder are processed in order to be used to estimate the wall heat flux. When noise is present in the data, some of the simulated results obtained using the least squares method exhibit oscillatory behavior, but these large oscillations are substantially reduced by the implementation of a regularization technique. Real experimental data are also used for the wall condition estimation and for the subsequent building of local boiling curves are plotted and discussed. The question of the possible effect of a temperature dependent conductivity on the reconstructed wall condition is also considered.

This work aims to verify the feasibility of the estimation of heat fluxes during the cooling of a rotating cylinder by an impinging jet. A semi-analytical method has been developed for this two-dimensional inverse heat conduction problem (IHCP) using Laplace and Fourier transforms technique. The simulations of inversion for two representative test cases show that the estimated surface heat flux is not biased and can therefore be reconstructed.

International audience; Large panels of studies have been performed in order to investigate the dynamic behavior of a droplet during the impingement onto a rigid wall. Most of them are dealing with single droplets and only fewer are devoted to sprays, for which several diagnostics were implemented. Moreover, when sprays are involved, the liquid mass flux is generally quite low (on the order of several kg/m²/s) and the case of a cold surface was mainly considered. Therefore, the present experimental work aims at investigating the impingement of sprays onto hot surfaces (up to 800°C) with liquid mass flux up to 13 kg/m²/s. Specifically, the main objective is to characterize the secondary droplets as a function of the surface temperature, starting from the Leidenfrost regime. To that purpose, a 2D Phase Doppler Analyzer (PDA) is synchronized with an infrared camera in order to measure simultaneously the time evolution of the local droplets size and velocity distributions with the surface temperature. Six full cone sprays were used in order to obtain a wide range of impingement conditions (normal incident Weber and liquid mass flux). A comparison of the impingement characteristics is also performed when the surface remains at room temperature. Another important point concerns also the effect on the spray flow of the presence of the surface. Results show clearly that the incident droplet trajectories are modified by the presence of the solid wall itself and depend strongly on the temperature surface. The in-depth investigation of the impingement characteristics of the spray is mainly focused on the description of the statistical mean diameter and the mass flux after the impingement. The main results highlight that the mean diameter value of the secondary droplets does not change during the Leidenfrost regime and increases for lower surface temperature. This behavior is strongly correlated to the liquid film formation for temperatures lower than the Leidenfrost point. The expelled mass flux exhibits a similar behavior but a decrease is observed after a given temperature, which can be attributed to the deepening of the liquid, leading to a reduction of the number of expelled droplets.

International audience; The purpose of the present paper is to study experimentally a free impinging axisymmetric jet on a moving surface. Comparisons of the experimental data and the numerical simulations using Star CD software have been made in order to validate the numerical procedure. Since heat transfer efficiency of cooling process is controlled by hydrodynamic field, the first step is to check if numerical results are in agreement with experimental data. The final goal is to simulate the cooling of rolling process in steel making. Experiments have been carried out with tap water and with two nozzle diameters (17 and 20 mm).

International audience; Hydrodynamic and thermal analysis of suspension flows is a difficult but fundamental task for the sterilization problems of particle charged fluids in food industries and also in any process where we can find flow with seeded particles. Nowadays, the influence of solid suspended particles on the flow is not yet a well known problem. Nevertheless, the solid particles have a strong influence on the rheological mixture's behaviour. The new approach of these solid-liquid suspension flows that we propose is based on experimental data and theoretical considerations. We use hard sphere approach in order to model the pressure drop assuming the mixture as an effective continuous medium. On these bases, we present a model of the heat transfer at the wall.

International audience; The effect of hydrodynamic conditions on the enhancement of heat transfer for single phase flow is studied using a combination of experimental data and numerical data. Experiments have been conducted for a wide range of Reynolds numbers, (0

International audience

International audience; An experimental test-rig was designed to study the cooling of hot surfaces under semi-industrials conditions. Two cooling methods were compared: liquid jet and full cone sprays. The wall is a 175 mm diameter nickel disk heated by electromagnetic induction up to 800 °C. The heat removed during the cooling phase was investigated with the use of infrared thermography while the droplet properties, in the case of sprays, were characterized simultaneously with a Phase Doppler system. In the case of spray cooling, the Phase Doppler technique was mainly applied to study the statiscal properties of the droplets in the vicinity of the heated surface. The influence of the liquid mass flux distribution as well as the droplets properties on the cooling are studied by using five different spray nozzles. The liquid mass flux distribution is also measured with the help of patternator. The main results show that even if higher heat flux can be removed with the liquid jet the efficiency cooling is poor because of its non uniformity. When spray cooling is considered, the results are in qualitative agreement with previous studies performed in similar conditions.

International audience; This experimental work aims at investigating the cooling of hot surfaces by using full cone sprays; comparison with the use of a liquid jet is also considered. The wall is a 175 mm diameter nickel disk and 5 mm thickness heated by electromagnetic induction up to about 800 °C. In the case of the sprays, the goal is to link the spray properties with the heat flux removed from the heated surface. For the spray, the influence of the mass flux distribution as well as the droplets properties on the cooling are studied by using three different spray nozzles. The mass flux distribution for each of the sprays is measured with the help of a patternator. The heat removed during the cooling phase is investigated with the use of infrared thermography while the droplet properties are characterized simultaneously by a Phase Doppler system. The Phase Doppler technique is mainly applied to study the statistical properties of the outcom-ing droplets in the vicinity of the heated surface. A decrease of the outcoming droplets size compared to the incoming one is well observed. Moreover, Phase Doppler device has also highlighted that the presence of the surface may have a significant influence on the upstream spray flow. Compared to the liquid jet, heat flux measurements have clearly demonstrated that the sprays lead a more spatial uniformity of the extracted heat flux and a better cooling efficiency. When spray cooling is considered, the influence of the mass flux on the heat flux and the cooling efficiency is in qualitative agreement with previous studies performed in similar conditions. In addition, a comparative study of performances, in term of liquid consumption and cooling duration, is performed by considering a similar surface temperature decrease for the three sprays and the liquid jet. Among the four experiments, the liquid jet has the longest cooling time as well as the highest liquid consumption. Furthermore, the best performances are reached for the spray having the highest mass flux value.

Experiments have been carried out for adiabatic concurrent gas–liquid flow of nitrogen and water through two corrugated test sections. This study examines flow patterns as the gas fluxes are increased. At low gas fluxes two main patterns have been observed: stratified flow for low liquid fluxes and bubble flow for greater ones. The mean wall shear stress of the two-phase flow has been measured with the electrochemical method (polarography); its analysis reveals the flow structure modification from monophasic to two-phase flow.