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Intrusive sensors are widely employed in numerous industrial applications to determine the phase fraction in gas-liquid two-phase flows. Such applications include those in the chemical, nuclear, and oil industries. This study aims to investigate the use of three distinct intrusive sensor topologies, specifically intended for underwater applications, wherein the detailed characterization of the disperse phase is the primary objective. Examples of such applications include aeration diffusers in wastewater treatment plants (WWTPs) and fish farming facilities., Peer Reviewed

During last years, supermarkets have increased the demand for closed refrigerated display cabinets due to new regulations and trends on energy and sustainability. However, the performance of this type of refrigerated display cabinets is not completely known, being of utmost importance how the airflow distributions could affect over the temperature distributions inside the cabinet. Consequently, this work presents a numerical evaluation of the performance of closed display cabinets focusing on the effect of the sliding doors opening process. The set of computational fluid dynamics (CFD) simulations served to validate the use of immersed solid approach in order to accurately analyze and evaluate transient behavior of the turbulent air flow distributions and temperature evolutions inside the cabinet and over the products, and characterizing the air curtain disruptions and the warm air entrainment during the opening process of the cabinet sliding doors (i.e. from closed doors, through doors opening process and to doors totally open).

[EN] This paper presents a new one-dimensional solver for two-phase flow simulations where boiling is involved. The solver has been implemented within the OpenFOAM® platform. The basic formulation follows the Eulerian description of the Navier¿Stokes equations. Different closure equations for one-dimensional simulations are also included, as well as a subcooled boiling model in order to perform accurate computations of the mass and heat transfer between phases. In addition to the fluid, a domain is included in order to represent the solid structure, so the solver is able to solve conjugate heat transfer problems. Two different test cases are presented in this work, first a single-phase test case in order to verify the conjugate heat transfer, and then a case based on the Bartolomej international benchmark, which consists of a vertical pipe where the fluid runs upwards while it is heated. Transient calculation were performed, and the results were compared to the TRACE system code, and to the experimental data in the corresponding case. With this calculations, the capability of this new solver to simulate one-dimensional single-phase and two-phase flows including boiling is demonstrated. This work is a first step of a final objective, which consists in allowing a 1D¿3D coupling within the CFD platform, avoiding external links.

[EN] In the computational modeling of two-phase flow, many uncertainties are usually faced in simulations and validations with experiments. This has traditionally made it difficult to provide a general method to predict the two-phase flow characteristics for any geometry and condition, even for bubbly flow regimes. Thus, we focus our research on studying in depth the bubbly flow modeling and validation from a critical point of view. The conditions are intentionally limited to scenarios where coalescence and breakup can be neglected, to concentrate on the study of bubble dynamics and its interaction with the main fluid. This study required the development of a solver for bubbly flow with higher resolution level than TFM and a new methodology to obtain the data from the simulation. Part I shows the development of a solver based on the CFD-DEM formulation. The motion of each bubble is computed individually with this solver and aspects as inhomogeneity, nonlinearity of the interfacial forces, bubble-wall interactions and turbulence effects in interfacial forces are taken into account. To develop the solver, several features that are not usually required for traditional CFD-DEM simulations but are relevant for bubbly flow in pipes, have been included. Models for the assignment of void fraction into the grid, seeding of bubbles at the inlet, pressure change influence on the bubble size and turbulence effects on both phases have been assessed and compared with experiments for an upward vertical pipe scenario. Finally, the bubble path for bubbles of different size have been investigated and the interfacial forces analyzed. (C) 2017 Elsevier Ltd. All rights reserved.

[EN] In the computational modeling of two-phase flow, many uncertainties are usually faced in simulations and validations with experiments. This has traditionally made it difficult to provide a general method to predict the two-phase flow characteristics for any geometry and condition, even for bubbly flow regimes. Thus, we focus our research on studying in depth the bubbly flow modeling and validation from a critical point of view. The conditions are intentionally limited to scenarios where coalescence and breakup can be neglected, to concentrate on the study of bubble dynamics and its interaction with the main fluid. This study required the development of a solver for bubbly flow with higher resolution level than TFM and a new methodology to obtain the data from the simulation. In Part II, taking profit of the detailed data provided by the CFD-DEM solver presented in Part I, we propose a novel methodology based on virtual sensor probes, to perform a rigorous validation and to investigate the experimental data. The same approximation used for processing the experimental datasets applies to simulation data, then the same assumptions are considered. In this way we can study an extended number of disperse phase variables as bubble velocity, void fraction, interfacial area concentration, mean chord length and distribution, Sauter mean diameter, bubble frequency and missing ratio, in addition to other variables as bubble size distribution or carrier phase velocity and turbulence. Several upward bubbly flow scenarios from datasets of different authors are used to validate the solver using this methodology. Finally, an axial evolution validation is performed including a discussion motivated by the comparison between experiments and the data from the virtual probes. (C) 2017 Elsevier Ltd. All rights reserved.

The study and modelling of two-phase flow, even the simplest ones such as the bubbly flow, remains a challenge that requires exploring the physical phenomena from different spatial and temporal resolution levels. CFD (Computational Fluid Dynamics) is a widespread and promising tool for modelling, but nowadays, there is no single approach or method to predict the dynamics of these systems at the different resolution levels providing enough precision of the results. The inherent difficulties of the events occurring in this flow, mainly those related with the interface between phases, makes that low or intermediate resolution level approaches as system codes (RELAP, TRACE, ...) or 3D TFM (Two-Fluid Model) have significant issues to reproduce acceptable results, unless well-known scenarios and global values are considered. Instead, methods based on high resolution level such as Interfacial Tracking Method (ITM) or Volume Of Fluid (VOF) require a high computational effort that makes unfeasible its use in complex systems. In this thesis, an open-source simulation framework has been designed and developed using the OpenFOAM library to analyze the cases from microescale to macroscale levels. The different approaches and the information that is required in each one of them have been studied for bubbly flow. In the first part, the dynamics of single bubbles at a high resolution level have been examined through VOF. This technique has allowed to obtain accurate results related to the bubble formation, terminal velocity, path, wake and instabilities produced by the wake. However, this approach has been impractical for real scenarios with more than dozens of bubbles. Alternatively, this thesis proposes a CFD Discrete Element Method (CFD-DEM) technique, where each bubble is represented discretely. A novel solver for bubbly flow has been developed in this thesis. This includes a large number of improvements necessary to reproduce the bubble-bubble and bubble-wall interactions, tur, El estudio y modelado de flujos bifásicos, incluso los más simples como el bubbly flow, sigue siendo un reto que conlleva aproximarse a los fenómenos físicos que lo rigen desde diferentes niveles de resolución espacial y temporal. El uso de códigos CFD (Computational Fluid Dynamics) como herramienta de modelado está muy extendida y resulta prometedora, pero hoy por hoy, no existe una única aproximación o técnica de resolución que permita predecir la dinámica de estos sistemas en los diferentes niveles de resolución, y que ofrezca suficiente precisión en sus resultados. La dificultad intrínseca de los fenómenos que allí ocurren, sobre todo los ligados a la interfase entre ambas fases, hace que los códigos de bajo o medio nivel de resolución, como pueden ser los códigos de sistema (RELAP, TRACE, etc.) o los basados en aproximaciones 3D TFM (Two-Fluid Model) tengan serios problemas para ofrecer resultados aceptables, a no ser que se trate de escenarios muy conocidos y se busquen resultados globales. En cambio, códigos basados en alto nivel de resolución, como los que utilizan VOF (Volume Of Fluid), requirieren de un esfuerzo computacional tan elevado que no pueden ser aplicados a sistemas complejos. En esta tesis, mediante el uso de la librería OpenFOAM se ha creado un marco de simulación de código abierto para analizar los escenarios desde niveles de resolución de microescala a macroescala, analizando las diferentes aproximaciones, así como la información que es necesaria aportar en cada una de ellas, para el estudio del régimen de bubbly flow. En la primera parte se estudia la dinámica de burbujas individuales a un alto nivel de resolución mediante el uso del método VOF (Volume Of Fluid). Esta técnica ha permitido obtener resultados precisos como la formación de la burbuja, velocidad terminal, camino recorrido, estela producida por la burbuja e inestabilidades que produce en su camino. Pero esta aproximación resulta inviable para entornos reales con la participación, L'estudi i modelatge de fluxos bifàsics, fins i tot els més simples com bubbly flow, segueix sent un repte que comporta aproximar-se als fenòmens físics que ho regeixen des de diferents nivells de resolució espacial i temporal. L'ús de codis CFD (Computational Fluid Dynamics) com a eina de modelatge està molt estesa i resulta prometedora, però ara per ara, no existeix una única aproximació o tècnica de resolució que permeta predir la dinàmica d'aquests sistemes en els diferents nivells de resolució, i que oferisca suficient precisió en els seus resultats. Les dificultat intrínseques dels fenòmens que allí ocorren, sobre tots els lligats a la interfase entre les dues fases, fa que els codis de baix o mig nivell de resolució, com poden ser els codis de sistema (RELAP,TRACE, etc.) o els basats en aproximacions 3D TFM (Two-Fluid Model) tinguen seriosos problemes per a oferir resultats acceptables , llevat que es tracte d'escenaris molt coneguts i se persegueixen resultats globals. En canvi, codis basats en alt nivell de resolució, com els que utilitzen VOF (Volume Of Fluid), requereixen d'un esforç computacional tan elevat que no poden ser aplicats a sistemes complexos. En aquesta tesi, mitjançant l'ús de la llibreria OpenFOAM s'ha creat un marc de simulació de codi obert per a analitzar els escenaris des de nivells de resolució de microescala a macroescala, analitzant les diferents aproximacions, així com la informació que és necessària aportar en cadascuna d'elles, per a l'estudi del règim de bubbly flow. En la primera part s'estudia la dinàmica de bambolles individuals a un alt nivell de resolució mitjançant l'ús del mètode VOF. Aquesta tècnica ha permès obtenir resultats precisos com la formació de la bambolla, velocitat terminal, camí recorregut, estela produida per la bambolla i inestabilitats que produeix en el seu camí. Però aquesta aproximació resulta inviable per a entorns reals amb la participació de més d'unes poques desenes de bambolles. Com a alternativa e

[EN] An Eulerian-Eulerian approach was investigated to model adiabatic bubbly flow with CFD techniques. In the framework of the OpenFOAM (R) software, a two-fluid model solver was modified to include a population balance equation, solved with the quadrature method of moments approximation to predict upward bubbly flow in vertical pipes considering the polydisperse nature of two-phase flow. Some progress have been made recently solving population balance equations in OpenFOAM (R) and this research aims to extend its application to the case of vertical pipes under different conditions of liquid and gas velocities. In order to test the solver for nuclear applications, interfacial forces and bubble induced turbulence models were included to provide to this solver the capability to correctly predict the behavior of the continuous and disperse phases. Two-phase flow experiments with different superficial velocities of gas and liquid are used to validate the model and its implementation. Radial profiles of void fraction, gas and liquid velocities, Sauter mean diameter and turbulence intensity are compared to the computational results. These results are in satisfactory agreement with the experiments, showing the capability of the solver to predict two-phase flow characteristics.

[ES] Los denominados códigos CFD o de fluidodinámica computacional se vienen aplicando cada vez con mayor frecuencia al análisis de procesos de sistemas de fluidos, en apoyo de tareas de diseño y análisis de seguridad. Esto sucede en todos los campos de la ingeniería, y la nuclear no es una excepción. ¿Qué nuevo conocimiento aportan estas herramientas en comparación con métodos clásicos?

When a vertical liquid jet plunges into a liquid surface after passing through a surrounding gas phase it entrains a large amount of gas bubbles into the receiving pool, and forms a large submerged two-phase region with a considerable interfacial area. At the intersection of the plunging jet and the liquid surface, free-surface instabilities develop, and gas entrainment may be observed. In this study, a set of experiments were performed on plunging water jets injected vertically downward through short circular nozzles lN/dN 6 5 onto a free water surface. The effect of the operation conditions including initial jet diameters dN, initial jet velocity VN, and jet length x1 on the flow characteristics such as the inception velocity of the gas entrainment Ve, the bubble penetration depth Hp, the gas entrainment rate Qa, the centerline jet velocity Vc, and the axial jet velocity distribution Vx below the free water surface were evaluated. A flow visualization technique using a CCD camera, which allowed simultaneous measurements of several magnitudes, was used to investigate such flows. This technique provided a direct measurement of the interfacial behavior between the entrained air bubbles and the liquid ambient. The results obtained showed that the nondimensional bubble penetration depth Hp/dN decreased with the dimensionless jet length x1/dN up to 25, after this point it was almost constant. Also, the bubble penetration depth was found to increase with the jet velocity and nozzle diameters. The entrainment rate tended to increase when the jet velocity increased and its functional dependence was divided into three regions depending on the jet velocity. The value of Qa was also found to increase as x1 and dN increased for the same jet flow rate. The jet centerline velocity decay Vc was measured and found to be a function of: the jet impact velocity V1 with the plunge water surface, the jet diameter d1 and the plunge depth x. The axial velocity distributions Vx/Vc were fo

Gas injections into liquid are prevalent in the natural environment and are essential in industrial applications, they can lead to complex flow. The flow structure and processes are essentially unsteady and turbulent. In this study, a set of experiments were carried out to investigate the behavior of horizontal round noncondensing gas jets that discharge in a stagnant water ambient, considering subsonic and sonic jet exit conditions. A flow visualization technique using a CCD camera, which allowed simultaneous measurements, was used to investigate such flows. This technique provided a direct measurement of the interfacial behavior between the gas jet and the liquid ambient. Two different methods, the summation and the statistical one were used to obtain and analyze the experimental results, and we have found that both methods yield almost identical results. The results showed that the injector diameter and the Froude number play an important role in dictating both the jet pinch-off and the jet interface unsteadiness. The maximum location before the jet pinchoff is shown to have a logarithmic relation with the Froude number for all the jet diameters. The jet penetration length was measured in the momentum and buoyant regimes respectively and it was found to be strongly influenced by the nozzle diameter and the Froude number as well as by the mass and momentum flow rates of the injected jet. Jet spreading which is indicative of liquid entrainment is also shown to increase with the Froude number and the injector diameters. Also it was found that the Froude number plays an important role in dictating the expansion jet angle and the jet half-width. These magnitudes were obtained from recorded time-averaged images and finally empirical correlations have been developed to predict these parameters.

In the event of hypothetical accident scenarios in PWR, emergency strategies have to be mapped out, in order to guarantee the reliable removal of decay heat from the reactor core, also in case of component breakdown. One essential passive heat removal mechanism is the reflux condensation cooling mode. This mode can appear for instance during a small break loss-of-coolant-accident (LOCA) or because of loss of residual heat removal (RHR) system during mid loop operation at plant outage after the reactor shutdown. In the scenario of a loss-of-coolant-accident (LOCA), which is caused by the leakage at any location in the primary circuit, it is considered that the reactor will be depressurized and vaporization will take place, thereby creating steam in the PWR primary side. Should this lead to ¿reflux condensation¿, which may be a favorable event progression, the generated steam will flow to the steam generator through the hot leg. This steam will condense in the steam generator and the condensate will flow back through the hot leg to the reactor, resulting in counter-current steam/water flow. In some scenarios, the success of core cooling depends on the behaviour of this counter-current flow. Over several decades, a number of experimental and theoretical studies of counter-current gas¿liquid two-phase flow have been carried out to understand the fundamental aspect of the flooding mechanism and to prove practical knowledge for the safety design of nuclear reactors. Starting from the pioneering paper of Wallis (1961), extensive CCFL data have been accumulated from experimental studies dealing with a diverse array of conditions A one-dimensional two field model was developed in order to predict the counter-current steam and liquid flow that results under certain conditions in the cold leg of a PWR when a SBLOCA (small break loss of coolant accident) in the hot leg is produced. The counter-current model that has been developed can predict the pressure, temperature, velocity

Nuclear fuel bundles include spacers essentially for mechanical stability and to influence the flow dynamics and heat transfer phenomena along the fuel rods. This work presents the analysis of the turbulence effects of a split-type and swirl-type spacer-grid geometries on single phase in a PWR (pressurized water reactor) rod bundle. Various computational fluid dynamics (CFD) calculations have been performed and the results validated with the experiments of the OECD/NEA-KAERI rod bundle CFD blind benchmark exercise on turbulent mixing in a rod bundle with spacers at the MATiS-H facility. Simulation of turbulent phenomena downstream of the spacer-grid presents high complexity issues; a wide range of length scales are present in the domain increasing the difficulty of defining in detail the transient nature of turbulent flowwith ordinary turbulence models. This paper contains a complete description of the procedure to obtain a validated CFD model for the simulation of the spacer-grids. Calculations were performed with the commercial code ANSYS CFX using large eddy simulation (LES) turbulence model and the CFD modeling procedure validated by comparison with measurements to determine their suitability in the prediction of the turbulence phenomena.

The one-group interfacial area transport equation has been coupled to a wall heat flux partitioning model in the framework of two-phase Eulerian approach using the OpenFOAM CFD code for better prediction of subcooled boiling phenomena which is essential for safety analysis of nuclear reactors. The interfacial area transport equation has been modified to include the effect of bubble nucleation at the wall and condensation by subcooled liquid in the bulk that governs the non-uniform bubble size distribution., Im Rahmen des Zweiphasen-Euler-Ansatzes wurde die Eingruppen-Zwischenphasentransportgleichung mit dem Wandwärmestrompartitionsmodell in OpenFOAM gekoppelt. Damit sollten bessere Vorhersage von Phänomen des unterkühlten Siedens erreicht werden. Diese Vorhersagen sind im Rahmen von Sicherheitsanalysen von Kernreaktoren notwendig. Dazu wurde die Zwischenphasentransportgleichung geändert, um die Blasenbildung an der Wand und die Kondensation im Hauptteil der unterkühlten Flüssigkeit zu berücksichtigen. Diese Phänomene bestimmen die ungleichmäßige Verteilung der Blasengrößen.

A local mechanistic model for bubble breakup and coalescence for the one-group interfacial area transport equation has been developed, in agreement and within the limits of the current understanding, based on an exhaustive survey of the theory and of the state of the art models for bubble dynamics simulation. The new model has been tested using the commercial 3D CFD code ANSYS CFX. Upward adiabatic turbulent air-water bubbly flow has been simulated and the results have been compared with the data obtained in the experimental facility PUMA., Ein lokales mechanistisches Modell ist, in Zustimmung und innerhalb der Grenzen des bekannten Wissens, für Blasenzerfall und Koaleszenz für Ein-Gruppen-Transportgleichung der spezifischen Phasengrenzfläche entwickelt worden und basiert auf einer erschöpfenden Literaturübersicht und dem State-of-the-art-Modellen für Blasendynamiksimulation. Das neue Modell wurde mit dem kommerziellen 3D CFD Code ANSYS CFX getestet. Aufwärts adiabatische turbulente Luft-Wasser-Blasenströmung wurde simuliert und die Ergebnisse wurden mit den in der Versuchsanlage PUMA erhaltenen Daten verglichen.

El estudio del comportamiento del flujo multifásico y su modelado ha sufrido un gran cambio con la introducción de los programas CFD, y el avance en los sistemas de medida. La resolución temporal y espacial de los programas de cálculo y la instrumentación se ha incrementado notablemente. Así, se hace indispensable disponer de nuevos resultados experimentales que permitan desarrollar nuevos modelos adaptados a estas capacidades, y a su vez validar las actuales herramientas de cálculo, especialmente los códigos CFD. La Universitat Jaume I de Castelló y la Universidad Politécnica de Valencia poseen una larga trayectoria en la experimentación y modelado de sistemas bifásicos. Fruto de ello se ha desarrollado en la UJI una instalación experimental, un loop vertical de 6 metros para estudiar el flujo bifásico en tuberías, con abundante instrumentación como sondas de conductividas, LDA y cámaras rápidas. En este trabajo se presentarán los resultados para tuberías verticales de 52 mm de diámetro en régimen de bubbly y slug. La instalación experimental está totalmente automatizada y es capaz de medir distribución de fracción de huecos, velocidad de la fase gas y líquido, turbulencia de la fase líquida, densidad de área interfacial y distribución de población de burbujas y tamaños. Se han desarrollado las sondas de conductividad de 4 sensores y el software para el procesado e la señal obtenida. Se han ensayado en el rango de velocidad del líquido de 0.5 a 3 m/s y fracciones de gas medias de 5 a 30%, así como con temperaturas y tensiones interraciales del fluido distintas. Se han tomado medidas en tres puertos axiales y en cada uno de ellos 15 puntos radiales. Con todo ello se ha creado una base de datos detallada que permite a los investigadores desarrollar nuevos modelos o validar los códigos actuales con garantías. En ella se estudia el flujo en régimen bubbly, la transición hacia slug y el inicio del slug. Cabe destacar el estudio realizado mediante LDA de la fase líquido

The one-group interfacial area transport equation has been coupled to a wall heat flux partitioning model in the framework of two-phase Eulerian approach using the OpenFOAM CFD code for better prediction of subcooled boiling phenomena which is essential for safety analysis of nuclear reactors. The interfacial area transport equation has been modified to include the effect of bubble nucleation at the wall and condensation by subcooled liquid in the bulk that governs the non-uniform bubble size distribution., Im Rahmen des Zweiphasen-Euler-Ansatzes wurde die Eingruppen-Zwischenphasentransportgleichung mit dem Wandwärmestrompartitionsmodell in OpenFOAM gekoppelt. Damit sollten bessere Vorhersage von Phänomen des unterkühlten Siedens erreicht werden. Diese Vorhersagen sind im Rahmen von Sicherheitsanalysen von Kernreaktoren notwendig. Dazu wurde die Zwischenphasentransportgleichung geändert, um die Blasenbildung an der Wand und die Kondensation im Hauptteil der unterkühlten Flüssigkeit zu berücksichtigen. Diese Phänomene bestimmen die ungleichmäßige Verteilung der Blasengrößen.

Air-water two-phase flow is present in natural and industrial processes of different nature as nuclear reactors. An accurate local prediction of the boiling flow could support safety and operation analyses of nuclear reactors. A new Eulerian-Lagrangian approach is investigated in this contribution. A new solver has been developed and implemented in the framework of the open source package OpenFOAM R and based on the PIMPLE algorithm coupled with the Lagrangian equation of motion has been implemented for computing incompressible bubbly flows. Each bubble is divided in equivolumetric volumes and tracked into the Eulerian mesh for an appropriate assignment of the effect of the bubble in the cell without resolving the interface. The coupling between phases is done considering in the momentum equation the interfacial forces and bubble induced contribution along the bubble path during an Eulerian time step. The bouncing of the bubbles between themselves and the wall is modeled with a dynamic soft sphere model. The computational results obtained for different flow conditions are validated with the recently released experimental data on upward pipe flow. The test section used is a 52 mm pipe of 5500 mm of length maintained under adiabatic conditions with air and water circulating fluids working with inlet velocity ranges of 0-2 m/s and 0-0.3 m/s for the continuous and dispersed phase respectively. Averaged results of radial distribution for void fraction, chord length, turbulence kinetic energy, dispersed and continuous velocity profiles show a good agreement among different flow conditions.

La presente ponencia presenta un análisis realizado con el código TRACE del transitorio RESA que tuvo lugar en la Central Nuclear de Trillo en el año 2001. La comparación de los resultados de TRACE con los de planta puede considerarse satisfactoria, por lo que el código TRACE es una buena herramienta predictiva para este tipo de transitorios.

En las instalaciones nucleares existe una larga trayectoria en el uso de códigos específicos como Trace, TRAC/BF1 o Relap5 para su simulación, validación y licenciamiento. Sin embargo, cada vez más, se usan los códigos de CFD para estudiar problemas más concretos. OpenFOAM es un programa de CFD orientado a objetos, libre y de código abierto desarrollado bajo el lenguaje de programación C++. Se trata de una herramienta completa que permite al usuario modificarla para mejorar sus aplicaciones En este trabajo se plantea evaluar el potencial del código OpenFOAM para la simulación de flujos de fluido típicos en los reactores PWR, y en concreto para el estudio del estrés térmico presurizado en estos reactores. Se ha simulado el test Tl-1 dentro del proyecto OECD ROSA, con el objetivo de evaluar las prestaciones del código OpenFOAM y de los modelos de turbulencia que tiene implementados para capturar los efectos de las fuerzas de empuje en el caso de estudio. En este artículo se ha simulado el Test 1-1 del proyecto OECD ROSA que investiga la estratificación de temperatura en las ramas frías y en el downcomer de un PWR durante la inyección de agua de los ECCS bajo condiciones de circulación natural monofásica. Se presenta una comparativa de dos códigos: el código comercial Ansys-CFX y el código abierto OpenFOAM. En ambos códigos el trabajo se centra en los modelos de turbulencia RANS (Reynodls Average Navier-Stokes), siendo el modelo de 2 ecuaciones k-E (ampliamente utilizado), el modelo SST (shear stress transport) yelmodelo de siete ecuaciones RSM (Reynolds Stress Model) los modelos estudiados. Ambos códigos proporcionan buenos resultados para el estudio del estrés térmico presurizado, siendo el código OpenFOAM, libre y de código abierto.

[EN] A set of two phase flow experiments for different conditions ranging from bubbly flow to cap/slug flow have been performed under isothermal concurrent upward air-water flow conditions in a vertical column of 3m height. Special attention in these experiments was devoted to the transition from bubbly to cap/slug flow. The interfacial velocity of the bubbles and the void fraction distribution was obtained using 2 and 4 sensors conductivity probes. Numerical simulations of these experiments for bubbly flow conditions were performed by coupling a Lagrangian code with an Eulerian one. The first one tracks the 3D motion of the individual bubbles in cylindrical coordinates (r, phi, z) inside the fluid field under the action of the following forces: buoyancy, drag, lift, wall lubrication. Also we have incorporated a 3D stochastic differential equation model to account for the random motion of the individual bubbles in the turbulent velocity field of the carrier liquid. Also we have considered the deformations undergone by the bubbles when they touch the walls of the pipe and are compressed until they rebound. The velocity and turbulence fields of the liquid phase were computed by solving the time dependent conservation equations in its Reynolds Averaged Transport Equation form (RANS). The turbulent kinetic energy k, and the dissipation rate e transport equations were simultaneously solved using the k, epsilon model in a (r, z) grid by the finite volume method and the SIMPLER algorithm. Both Lagrangian and Eulerian calculations were performed in parallel and an iterative self-consistent method was developed. The turbulence induced by the bubbles is an important issue considered in this paper, in order to obtain good predictions of the void fraction distribution and the interfacial velocity at different gas and liquid flow conditions.

A set of air-water experiments has been performed under isothermal upward concurrent flow conditions, in a vertical column. The interfacial velocity, the bubble interfacial area and the void fraction distributions have been measured. Numerical simulation of these experiments were performed by coupling a Lagrangian code which tracks the 3D motion of the individual bubbles, with an Eulerian one. In the Eulerian solver the velocity and turbulence fields of the liquid phase were computed by solving the time dependent conservation equations in its Reynolds Averaged Transport Equation form (RANS). The turbulent kinetic energy k, and the dissipation rate transport equations were simultaneously solved by using the k, epsilon model in a (r, z) grid by the finite volume method and the SIMPLER algorithm. Both Lagrangian and Eulerian calculations were performed in parallel and an iterative self-consistent method was developed. The turbulence induced by the bubbles is an important issue considered in this paper, in order to obtain good predictions of the void fraction distribution and the interfacial velocity at different gas and liquid flow conditions. The Eulerian Code was upgraded from an axisymmetric 2D code to a 3D code in order to improve the turbulence solution. The results of the 3D CFD code have been tested and show a good agreement with the experimental results. In this paper special attention is given to the coupling between the different models.

[EN] A computational code system called Coupled Solver ANSYS CFX/PARCS (CSAP) based on coupling the 3D neutron diffusion code PARCS v2.7 and the ANSYS CFX 13.0 Computational Fluid Dynamics (CFD) code has been developed as a tool for nuclear reactor systems simulations. This paper presents the coupling methodology between the CFD and the neutronic code. The methodology to simulate a 3D-neutronic problem coupled with 1D thermalhydraulics is already a mature technology, being part of the regular calculations performed to analyze different kinds of Reactivity Insertion Accidents (RIA) and asymmetric transients in Nuclear Power Plants, with state-of-the-art coupled codes like TRAC-B/NEM, RELAP5/PARCS, TRACE/PARCS, RELAP3D, RETRAN3D, etc. The transport of neutrons depends on several parameters, like fuel temperature, moderator temperature and density, boron concentration and fuel rod insertion. These data are calculated by the CFD code with high local resolution and used as input to the neutronic code to calculate a 3D nodal power distribution that will be returned and remapped to the CFD code control volumes (cells). Since two different nodalizations are used to discretize the same system, an averaging and interpolating procedure is needed to realize an effective data exchange. These procedures have been developed by means of the ANSYS CFX “User Fortran” interface; a library with several subroutines has been developed for calculation and synchronization purposes. The data exchange is realized by means of the Parallel Virtual Machine (PVM) software package. In this contribution, steady-state and transient results of a quarter of PWR fuel assembly with cold water injection are presented and compared with obtained results from a RELAP5/PARCS v2.7 coupled calculation. A simplified model for the spacers has been included. A methodology has been introduced to take into account the pressure drop and the turbulence enhancement produced by the spacers.

This paper constitutes a first approach of the assessment of the ANSYS-CFX code to simulate the TRIGA research reactor. Due to computational limits simplifications of the core and of some other geometrical details have been introduced. Results obtained show that the CFD model for the IPR-R1 TRIGA reproduces the actual steady-state behavior in good agreement. The results described here confirm the efficiency of natural convection in removing the heat produced in the reactor core by nuclear fission.

Una nueva aproximación euleriana-lagarangiana, en su forma de acople en dos vías, para la simulación de flujo de burbujas, agua-aire es presentada en la tesis, en la que se incluyen los efectos de las colisiones entre burbujas, así como las posibles roturas o coalescencia de burbujas. Esta aproximación utiliza el modelo Continuous Random Walk, CRW, para tener en cuenta las fluctuaciones de la velocidad. Esta aproximación se enmarca dentro de un modelo de turbulencia k-epsilon para la fase continua del líquido. En esta tesis se estudiarán los métodos para realizar el acople entre ambas aproximaciones, el efecto de la fuerza lift y de la dispersión turbulenta sobre la distribución de la fracción de huecos, así como los modelos de coalescencia y rotura de burbujas que puedan ser empleados en este tipo de aproximación. Se ha partido de un código euleriano para simular la parte continua, y sobre él se ha acoplado la aproximación lagrangiana. Para que ese acople afecte a la fase continua sobre su solver ser han añadido fuentes de momento y turbulencia. Además se ha modificado el volumen computacional de cada celda para que tenga en consideración el volumen ocupado por la fase dispersa. El acople en doble vía hace que los perfiles de velocidad y turbulencia de la fase continua se modifiquen notablemente y que se aproximen a los reales, lo que resulta básico para la correcta simulación de las fuerzas interfaciales. La colisión entre burbujas, y burbujas y pared se ha incluido. Este efecto es necesario como paso previo a incluir los procesos de rotura o coalescencia de burbujas, aunque la colisión en sí tenga efectos limitados en la distribución de la fracción de huecos. El proceso de coalescencia se basa en el modelo de Chester ( 1991 ) , el modelo compara el tiempo de colisión con el tiempo de drenaje de la película entre burbujas para determinar si existe o no coalescencia. El modelo de rotura se basa en el modelo de Martínez-Bazán. Uno de los principales hitos de

[EN] It has been observed by many authors that for ascending bubbly flows, the gas fraction distributes radially in different manners. Depending on the flow conditions and channel characteristics, the bubbles may accumulate in the center of the pipe (core peak) or near the walls (wall peak). The radial distribution of gas fraction significantly affects the friction coefficient and the heat transfer. Therefore, it is important to be able to predict which distribution will appear under certain flow conditions. In this note, by analyzing a vast experimental data base, a criterion to identify the transition from core to wall peak is proposed. The transition occurs when both Reynolds and Weber numbers reach critical values (Rcrit = 1500, Wcrit = 8). This criterion is validated with our experimental results and with those found in the literature.

En este estudio se han construido dos diferentes instalaciones para investigar primero los chorros de gas horizontales y en segundo lugar los chorros verticales de agua que impactan sobre superficies libres de fluido, también se ha desarrollado un modelo numérico integral para predecir las trayectorias de estos jets y sus parámetros más importantes, validándose con los resultados experimentales obtenidos. En la primera parte de este trabajo, se han realizado experimentos para investigar el comportamiento de chorros de gas horizontales penetrando en agua. Los resultados experimentales indicaron que la longitud de penetración de los chorros de gas está fuertemente influenciada por el diámetro de la boquilla y el número de Froude, así como con el flujo de masa de de entrada y su momento. Aumentar el número de Froude y el diámetro del inyector lleva a aumentar la inestabilidad de jet. Además, la máxima ubicación antes de jet pinch-off se muestra que mantiene una relación logarítmica con el número de Froude para todos los diámetros de jet. Se han desarrollado correlaciones empíricas para predecir estos parámetros. Se ha desarrollado un modelo basado en la integración de las ecuaciones de conservación para que resulte útil en el diseño de aplicaciones en las que participen chorros horizontales así como para asistir a la investigación experimental. Las predicciones del modelo integral se comparan con los datos de los datos experimentales obtenidos con muy buenos resultados. En la segunda parte de este trabajo, se realizaron una serie de experimentos con de chorros de agua, inyectados verticalmente hacia abajo, a través de toberas circulares que impactan sobre una superficie de agua. Los resultados obtenidos mostraron que la profundidad de penetración de la burbuja disminuye con la longitud del chorro, pero que después de ciertas condiciones se mantiene casi constante. Además ésta aumenta con los diámetros de la boquilla y la velocidad del chorro. La velocidad de arrastre

Nuclear fuel bundles contain spacers essentially for mechanical stability and to influence the flow dynamic and heat transfer phenomena along the fuel rods. This work presents the analysis of the turbulence effects of a split-type and swirl-type spacer grid geometries on single-phase in a PWR (Pressurized Water Reactor) rod bundle. Various Computational Fluid Dynamics (CFD) calculations have been performed and the results validated with the experiments of the OECD/NEA-KAERI Rod Bundle CFD Benchmark Exercise on Turbulent Mixing in a Rod Bundle with Spacers at the MATiS-H facility. The aim of this Benchmark is to provide validated CFD analysis tools providing a firm basis for quantifying the CHF Margin reliably for normal operation and operational transients conditions and allowing eventually the use of CMFD Codes for predicting DNB under accidental conditions [1]. Simulation of turbulent phenomena downstream of the spacer grid presents high complexity issues. A wide range of length scales are present increasing the difficulty of defining in detail the transient nature of turbulent flow. Calculations were performed with the commercial code ANSYS R CFX R and CFD modelling using Large Eddy Simulation (LES) turbulence models by comparison with measurements to determine their suitability in the prediction of the turbulence phenomena. One of the most important aspects to be taken into account in order to properly simulate the flow downstream of the spacer grids is the use of a suitable turbulence model. Time-averaged values for all three velocity components, timeaveraged RMS values of the fluctuating component of all three velocity components in several cross-planes downstream of the spacer grid and circulation data in a selected sub-channel are in good agreement with the measured data. These results could be of great value for future studies of spacer grid including heat transfer from the rods and as a basis of spacer grid simplifications.

[ES] El uso de los códigos de Dinámica de Fluidos Computacional (CFD) 3D para predecir el flujo estacionario y transitorio en los reactores nucleares está creciendo rápidamente y constituye una nueva tendencia en su estudio. En este trabajo se presenta un modelo de un reactor de entrenamiento TRIGA (Training Research Isotopes General Atomic) tipo IPR-R1 desarrollado con el código de CFD ANSYS-CFX. El reactor nuclear TRIGA IPR-R1 está ubicado en el Centro de Desarrollo de Tecnología Nuclear (CDTN/CNEN) de Brasil y consiste en un reactor de investigación de 250 kW, moderado y refrigerado con agua ligera, con reflectores de grafito y de piscina abierta. En el trabajo se muestra el desarrollo del modelo de CFD y se muestran los resultados de la simulación en estado estacionario operando a 50 kW de potencia.

En años recientes se han realizado esfuerzos para incrementar la compresión de los fenómenos asociados al flujo bifásico líquido - gas, para lo cual se han establecido modelos matemáticos que intentan reflejar el comprotamiento del flujo, como es el caso del modelo de los Dos Fluídos. Una causa de la complejidad que representa el modelado es la transferencia de masa, momento y energía entre fases debida a la interacción entre fases. Actualmente los modelos de cálculo empleados en CFD y en códigos termohidraúlicos confían en correlaciones experimentales altamente dependientes del régimen de flujo para determinar el área interfacial, sin embargo este procedimiento no refleja la naturaleza física y se presentan irregularidades en la zona de la interfase debidas a fenómenos de transporte no descritos por estos modelos. En este sentido se ha propuesto recientemente una teoría de transporte de area interfacial que parece ser una solución viable al problema de la obtención de ecuaciones constitutivas del area interfacial y con ella, el cierre del modelo. Esta ecuación consta de términos convectivos y temporales que pueden ser resueltos analíticamente, por otro lado contiene términos fuente y sumidero que representan la creación y destrucción de burbujas que aún no han sido completamente modelados; para la comprension de dichos términos es requerida información experimental, misma que este trabajo intenta generar mediante la obtención de una base de datos experimentales que aporten información útil para el modelado de sistemas bifásicos. Para la obtención de la base de datos fue necesario diseñar, construir e instrumentar una instalación experimental que permitiera la formación de flujos bifásicos agua-aire con velocidades superficiales de fase líquida y gas similares a las existentes en los actuales reactores nucleares. El sistema de formación de mezcla bifásica que se diseñó y construyó permite la obtención de varios regímenes de flujo, en su funcionamiento emplea medio