Influence des déformations successives alternées de la paroi sur l'accroissement des performances d'échange d'un tube : application aux échangeurs multifonctionnels

The work presented here is focused on the numerical study of specific successive wall deformations in alternate directions, applied to a tubular geometry. Those deformations help modifying the flow structure and thus its heat transfer and mixing properties. One of the main aims of the study is to apply those deformations to multifunctional exchangers which are heat exchangers and chemical reactors at the same time. The study is mainly focused on laminar flows and all the numerical calculations were performed using the CFD code ANSYS Fluent. The first step of the study is to assess the secondary flow created by the wall deformations. The influence of several deformation geometrical parameters has also been studied. In order to enhance the mixing in the deformed tube, the wall deformations have been applied to coaxial configurations (often used in the industry). Two kinds of annular configurations have been evaluated. At first, the wall deformations are applied to the external and internal walls of the coaxial tube. The effect on the heat transfer enhancement of the longitudinal and angular phase-shifting between the two deformations has been specifically assessed. The second configuration considered combines the alternate deformations on its external walls and a swirled internal wall. This particular annular configuration creates chaotic advection in laminar flows, therefore helping increasing the mixing. This geometry is used as a solar captor and helps increasing the global performances when compared with a smooth tube usually used. The last part of the presented work is focused on the experimental validation of the numerical results. Techniques such as PIV and LDA are used to measure local velocity fields in a plane duct with alternate deformations applied to its lower wall.Les travaux de thèse sont consacrés à l’étude numérique de l’application de macro-déformations successives alternées a la paroi d’un tube. La modification de l’écoulement du fait des déformations permet de modifier ses propriétés en termes de transfert thermique et de mélange. L’objectif de l’étude d’un tel dispositif est entre autre de l’appliquer pour des configurations d’échangeurs multifonctionnels, qui sont à la fois échangeurs de chaleur et réacteurs chimiques. L’étude s’intéresse principalement aux écoulements laminaires. Les calculs sont réalisés avec le code ANSYS Fluent. L’étude est tout d’abord consacrée à la caractérisation de l’écoulement secondaire créé par les déformations ainsi qu’à l’influence des différents paramètres de déformation. Afin d’améliorer le mélange dans l’écoulement, l’étude d’une configuration coaxiale déformée a été envisagée (cette géométrie correspond de plus à une configuration d’écoulement utilisée dans l’industrie). Deux configurations annulaires ont été considérées. Dans un premier temps, les déformations pariétales ont été appliquées aux tubes interne et externe : différents déphasages longitudinaux et angulaires entre ces deux déformations ont été étudiés pour optimiser les performances thermo-hydrauliques. La seconde configuration combine des déformations sur la paroi externe et un swirl sur la paroi interne de la géométrie. Cette configuration particulière permet en régime laminaire d’augmenter significativement le mélange du fait de l’apparition d’advection chaotique dans l’écoulement. Cette dernière géométrie est appliquée dans le cas d’un échangeur solaire à concentration et permet d’améliorer les performances par rapport à un tube lisse dans des conditions similaires. La dernière partie de l’étude est consacrée à une validation expérimentale des résultats numériques lorsque les déformations sont appliquées à une plaque. Des mesures par PIV et LDA ont été réalisées pour mesurer la vitesse locale de l’écoulement

Successive alternate wall deformations effect on the transfer performances of a tube : application to multifunctional heat exchangers

Les travaux de thèse sont consacrés à l’étude numérique de l’application de macro-déformations successives alternées a la paroi d’un tube. La modification de l’écoulement du fait des déformations permet de modifier ses propriétés en termes de transfert thermique et de mélange. L’objectif de l’étude d’un tel dispositif est entre autre de l’appliquer pour des configurations d’échangeurs multifonctionnels, qui sont à la fois échangeurs de chaleur et réacteurs chimiques. L’étude s’intéresse principalement aux écoulements laminaires. Les calculs sont réalisés avec le code ANSYS Fluent. L’étude est tout d’abord consacrée à la caractérisation de l’écoulement secondaire créé par les déformations ainsi qu’à l’influence des différents paramètres de déformation. Afin d’améliorer le mélange dans l’écoulement, l’étude d’une configuration coaxiale déformée a été envisagée (cette géométrie correspond de plus à une configuration d’écoulement utilisée dans l’industrie). Deux configurations annulaires ont été considérées. Dans un premier temps, les déformations pariétales ont été appliquées aux tubes interne et externe : différents déphasages longitudinaux et angulaires entre ces deux déformations ont été étudiés pour optimiser les performances thermo-hydrauliques. La seconde configuration combine des déformations sur la paroi externe et un swirl sur la paroi interne de la géométrie. Cette configuration particulière permet en régime laminaire d’augmenter significativement le mélange du fait de l’apparition d’advection chaotique dans l’écoulement. Cette dernière géométrie est appliquée dans le cas d’un échangeur solaire à concentration et permet d’améliorer les performances par rapport à un tube lisse dans des conditions similaires. La dernière partie de l’étude est consacrée à une validation expérimentale des résultats numériques lorsque les déformations sont appliquées à une plaque. Des mesures par PIV et LDA ont été réalisées pour mesurer la vitesse locale de l’écoulement.The work presented here is focused on the numerical study of specific successive wall deformations in alternate directions, applied to a tubular geometry. Those deformations help modifying the flow structure and thus its heat transfer and mixing properties. One of the main aims of the study is to apply those deformations to multifunctional exchangers which are heat exchangers and chemical reactors at the same time. The study is mainly focused on laminar flows and all the numerical calculations were performed using the CFD code ANSYS Fluent. The first step of the study is to assess the secondary flow created by the wall deformations. The influence of several deformation geometrical parameters has also been studied. In order to enhance the mixing in the deformed tube, the wall deformations have been applied to coaxial configurations (often used in the industry). Two kinds of annular configurations have been evaluated. At first, the wall deformations are applied to the external and internal walls of the coaxial tube. The effect on the heat transfer enhancement of the longitudinal and angular phase-shifting between the two deformations has been specifically assessed. The second configuration considered combines the alternate deformations on its external walls and a swirled internal wall. This particular annular configuration creates chaotic advection in laminar flows, therefore helping increasing the mixing. This geometry is used as a solar captor and helps increasing the global performances when compared with a smooth tube usually used. The last part of the presented work is focused on the experimental validation of the numerical results. Techniques such as PIV and LDA are used to measure local velocity fields in a plane duct with alternate deformations applied to its lower wall

Interfacial heat transfer with non-condensable gas in ASTEC V2.2: Application to severe accidents study during PWR cold shutdown states

International audienceIn a diphasic flow, the presence of non-condensable gas has an important impact on interfacial heat transfer, especially at low global pressure. In severe accidents studies, such flow can be commonly encountered. For example, in pressurised water reactors, high concentration of non-condensable gas can be found in the reactor coolant system in case of late core reflooding with a high oxidation rate (high hydrogen concentration) or in case of accidents happening during the cold shutdown of the reactor, when the reactor coolant system has been partly drained (high air concentration). Such flows with non-condensable gas are challenging to compute for severe accident system codes. A new model is implemented in ASTEC v2.2 to improve the interfacial heat transfer calculation in presence of non-condensable gas. The model gives a more accurate estimation of the heat transfer by assuming that it is mainly driven by vapour diffusion in the gas phase. The new model is applied to study cold shutdown states for 1300 MWe pressurized water reactors. A complete calculation of the cooling, depressurisation and draining of the reactor can be successfully performed. In order to show ASTEC new capabilities, a first accident scenario with loss of the residual heat removal system is also presented

Calculating steam-water flows at large non-condensible gas concentration with severe accident code astec v2.2

International audienceLarge amount of non-condensible gas in a flow has an important impact on condensation and vaporisation process. In accident studies, such flow can be commonly encountered in various installations such as spent fuel pools but also in the primary loop of a Reactor Coolant System (RCS) of a pressurised water reactor. For example, high concentration of air can be found in case of accidents occurring during the cold shutdown of the reactor, throughout mid-loop operations, when the reactor coolant system has been partly drained. Such configurations are challenging to simulate with nuclear safety codes and require specific modelling.To accurately represent the interfacial heat and mass transfer with non-condensible gas, the model presented in this paper, based on the diffusion layer approach, assumes that the interfacial heat transfer is mainly driven by the vapour diffusion in the gas phase. This model has been implemented in the severe accident code ASTEC v2.2.Results of small scale experiments [1] are first used to test and validate the new model. The code new capability to calculate accident during cold shutdown states is then specifically assessed using experimental results obtained through the global scale facility BETHSY [2]. For the different considered experimental results, calculations with the new model are shown be in satisfactory agreement

Influence du nombre de Reynolds sur les performances thermo-hydrauliques d'un tube annulaire à parois macro-déformées combinant deux écoulements secondaires croisés

National audienceDans une configuration d’écoulement annulaire, la combinaison de déformations radiales successives et alternées sur la paroi externe, associées à une géométrie engendrant un mouvement de swirl sur la paroi interne améliore grandement le mélange par rapport à un tube annulaire lisse classique. Dans cette communication, nous étudions plus précisément l'influence du nombre de Reynolds (90< Re < 860) sur les performances thermiques d'une telle configuration annulaire. Les simulations sont réalisées pour des écoulements laminaires, incompressibles et stationnaires. A partir d'une condition de température constante sur les deux parois, le transfert de chaleur convectif est calculé et les performances thermo-hydrauliques (PEC) sont évaluées. Les résultats montrent que le transfert de chaleur est amélioré par rapport au cas d'un échangeur annulaire à tubes lisses : le facteur de performance obtenu est compris entre 1,25 et 1,58 sur la gamme étudiée

Calculating steam-water flows at large non-condensible gas concentration with severe accident code astec v2.2

International audienceLarge amount of non-condensible gas in a flow has an important impact on condensation and vaporisation process. In accident studies, such flow can be commonly encountered in various installations such as spent fuel pools but also in the primary loop of a Reactor Coolant System (RCS) of a pressurised water reactor. For example, high concentration of air can be found in case of accidents occurring during the cold shutdown of the reactor, throughout mid-loop operations, when the reactor coolant system has been partly drained. Such configurations are challenging to simulate with nuclear safety codes and require specific modelling.To accurately represent the interfacial heat and mass transfer with non-condensible gas, the model presented in this paper, based on the diffusion layer approach, assumes that the interfacial heat transfer is mainly driven by the vapour diffusion in the gas phase. This model has been implemented in the severe accident code ASTEC v2.2.Results of small scale experiments [1] are first used to test and validate the new model. The code new capability to calculate accident during cold shutdown states is then specifically assessed using experimental results obtained through the global scale facility BETHSY [2]. For the different considered experimental results, calculations with the new model are shown be in satisfactory agreement

BEPU analysis of a 2-in DVI break in a generic IRIS SMR by ASTEC -RAVEN coupling

International audienceAdvanced light-water Small Modular Reactors (SMRs) have acquired great interest in the international framework due to recognized advantages in terms of flexibility, capital cost and especially safety features, achieved by their designs. The "inherent safety" of SMRs is guaranteed by lower decay-heat, allowing the use of integral configuration for the primary coolant system and of passive safety systems. In the framework of NUGENIA TA-2 ASCOM collaborative project, coordinated by IRSN, an ASTEC code generic input-deck, based on an IRIS-like reactor, has been recently developed with the aim of studying the code capability to simulate SMR designs in challenging conditions. In the present paper, a Best-Estimate Plus Uncertainty (BEPU) method has been applied to study the safety criteria in a Design Basis Accident (DBA) due to a 2-in guillotine break of a Direct Vessel Injection (DVI) line and by assuming the operability of the passive safety systems of the generic IRIS. The uncertainty quantification study is performed through the propagation of the input uncertainty methodology, by implementing the RAVEN-ASTEC coupling on a multi-core cluster. The input uncertainty parameters perturbing the system are selected among the main reactor's initial and boundary conditions as well as related to passive safety systems. The statistical study of the reactor response in terms of output variation of the main safety Figures Of Merit (FOMs) is carried out by analyzing the sensitivity of the FOMs, with respect to the variation of the input uncertainties. The study is aimed to provide information regarding the role played by passive safety systems in the mitigation strategy; to characterize the thermal-hydraulic response of the code model and its capability to simulate the main natural-driven phenomena of passive advanced SMRs; and to develop a first uncertainty analysis regarding ASTEC application to SMR

Coolability of a Corium Pool in a Debris Bed: Impact of Debris Size, Steam, and Liquid Flow Rate; Tilting Angle; and Pressure on Critical Heat Flux—First Numerical Evaluations

Selected papers from the NURETH-20 special issueInternational audienceIn case of severe accident in a light water reactor, a debris bed may form in the core and possibly melt, as it happened in TMI-2. Knowledge about the coolability of such molten pool surrounded by debris is crucial to investigate the possibility of stabilizing a part of the fuel inside the vessel. In particular, it is of primary interest to determine the maximum size of a molten pool surrounded by debris which may be stabilized under water. The maximum heat flux (CHF) that may be extracted from the pool boundary by water flowing within the debris bed is a key parameter. A facility was built at IRSN to determine the CHF under various conditions. A heated copper surface simulates the boundary of the pool and is placed in contact with a debris bed (monodisperse steel balls), under water. In this article, we first complete previous experimental work with steam and liquid flowrate, ball diameter, tilting angle of the heated surface and pressure up to 2.5 bar abs. A general CHF correlation depending on the tilting angle and the steam flowrate is derived and an example of the use of this correlation to evaluate the maximum mass of corium pool that can be stabilized under water is given, for some typical reactor conditions. In the last part, the accuracy of the ASTEC code is evaluated based on first calculations intending to reproduce the experimental impact of liquid and steam flowrate

PERFORMANCES THERMO-HYDRAULIQUES D’UN TUBE EN U AUX PAROIS MACRO-DEFORMEES

International audienceUne déformation successive alternée appliquée à la paroi d’un tube permet d’augmenter ses performances thermo-hydrauliques de façon significative en régime laminaire. Ce type de configuration présente donc un intérêt particulier pour les échangeurs de chaleur. Afin de pouvoir évaluer les performances du tube déformé en configuration d’échangeur incluant notamment des coudes, l’objet de cette communication est d’étudier ce tube dans une géométrie en U. Les performances d’un tube, constitué de deux tronçons rectilignes aux parois déformées et reliés par une jonction coudée à 180°, sont déterminées. L’influence du rayon de courbure de la partie coudée sur les résultats est analysée par voie de simulations numériques. Il est montré que l’utilisation du tube à déformations pariétales permet d’augmenter significativement les performances thermo-hydrauliques par rapport à un tube lisse équivalent. De ce fait, la géométrie particulière proposée ici présente une solution intéressante pour améliorer le transfert thermique et applicable dans le cas d’une configuration en U

NUMERICAL ANALYSIS OF HEAT TRANSFER INCREASE IN A TUBE WITHALTERNATE SUCCESSIVE GRADUAL WALL DEFORMATIONS

International audienceFrom previous studies, it is known that the use of walldeformations in alternate directions while keeping a quasiconstantcross-section is an efficient way to enhance the heattransfer in a laminar flow regime inside a tube. In the presentstudy, the tube cross-section shape gradually changes along thetube length while keeping the same cross section area toprevent flow separation areas thereby limiting pressure drops.These wall deformations create vortical macro-structures insidethe flow that significantly modify the transfer properties. Twogeometrical parameters characterize the tube wall shape: theradial deformation amplitude and its streamwise wavelength.Through a numerical study, the effects of the variation of thesetwo parameters on the flow and on the heat transfer have beenstudied. An important finding is that the ratio between thewavelength and the amplitude holds a significant impact on theobserved results: both the friction and the heat transfer increaseas this deformation ratio decreases. At the same time, a localanalysis of the flow mechanisms has been performed to outlinethe modifications that occur in the flow pattern when the walldeformations are increased. Flow in the entrance region hasalso been specifically considered: it has been found thatgeometrical parameters do have an influence on the lengthneeded for the flow to get fully hydrodynamically andthermally established. Finally a performance analysis has beenconducted to assess, for a given performance criterion, thedeformation parameters that give optimal results. Through thisparametric study, for given Reynolds and Prandtl numbers, analternate wall deformed tube geometry that maximizes the heattransfer without significantly increasing the pressure drops canthus be define