[en] This report is a contribution to the actions destined to improve the numerical treatment of the convection term into the Navier-stokes equations using finite elements technique like N3S software. It concerns the study of two alternative methods, different of the characteristic method usually employed. These methods are variational methods, compatible with the treatment of the Stokes problem: a SUPG method (Streamline Upwind Petrov-Galerkin) and an UPWIND one order method directly deduced from finite differences. Actually, the SUPG method is one of the most employed method to obtain a stable numerical treatment of flow dominated by convection effects, using a variational formulation with finite elements. The UPWIND method proposed consists in using an upwind scheme taking into account the upwind element and computing locally in a linear way the information. A combination of both SUPG and UPWIND methods is proposed. Numerical techniques employed are detailed. Results concerning test cases usually used to validate numerical schemes solving a pure convention equation and Navier-Stokes test cases (incompressible and compressible) are then presented. Comparisons with the characteristic method employed into the N3S code are also presented. (authors). 10 refs

[en] The NEPTUNE project aims at building a new two-phase flow thermal-hydraulics platform for nuclear reactor simulation. EDF (Electricite de France) and CEA (Commissariat a l'Energie Atomique) with the co-sponsorship of IRSN (Institut de Radioprotection et Surete Nucleaire) and FRAMATOME-ANP, are jointly developing the NEPTUNE multi-scale platform that includes new physical models and numerical methods for each of the computing scales. One usually distinguishes three different scales for industrial simulations: the 'system' scale, the 'component' scale (subchannel analysis) and CFD (Computational Fluid Dynamics). In addition DNS (Direct Numerical Simulation) can provide information at a smaller scale that can be useful for the development of the averaged scales. The NEPTUNE project also includes work on software architecture and research on new numerical methods for coupling codes since both are required to improve industrial calculations. All these R and D challenges have been defined in order to meet industrial needs and the underlying stakes (mainly the competitiveness and the safety of Nuclear Power Plants). This paper focuses on three high priority needs: DNB (Departure from Nucleate Boiling) prediction, directly linked to fuel performance; PTS (Pressurized Thermal Shock), a key issue when studying the lifespan of critical components and LBLOCA (Large Break Loss of Coolant Accident), a reference accident for safety studies. For each of these industrial applications, we provide a review of the last developments within the NEPTUNE platform and we present the first results. A particular attention is also given to physical validation and the needs for further experimental data. (authors)

[en] In this paper, a local 3D two-fluid model for a turbulent stratified flow with/without condensation, which can be used to predict two-phase pressurized thermal shock, is presented. A modified turbulent K- model is proposed with turbulence production induced by interfacial friction. A model of interfacial friction based on a interfacial sublayer concept and three interfacial heat transfer models, namely, a model based on the small eddies controlled surface renewal concept (HDM, Hughes and Duffey, 1991), a model based on the asymptotic behavior of the Eddy Viscosity (EVM), and a model based on the Interfacial Sublayer concept (ISM) are implemented into a preliminary version of the NEPTUNE code based on the 3D module of the CATHARE code. As a first step to apply the above models to predict the two-phase thermal shock, the models are evaluated by comparison of calculated profiles with several experiments: a turbulent air-water stratified flow without interfacial heat transfer; a turbulent steam-water stratified flow with condensation; turbulence induced by the impact of a water jet in a water pool. The prediction results agree well with the experimental data. In addition, the comparison of three interfacial heat transfer models shows that EVM and ISM gave better prediction results while HDM highly overestimated the interfacial heat transfers compared to the experimental data of a steam water stratified flow

[en] A two-phase CFD modelling approach of the Pressurized Thermal Shock (PTS) problem has been developed and is being validated in the context of PWR life time safety studies. The cold water injection results in strong condensation and complex 3D two-phase phenomena. Direct Contact Condensation (DCC) occurs on the jet and on the free surface of the stratified flow in the leg. These surfaces are much larger than the cells size used in the computational domain, in this sense they can be called large interfaces. DCC depends strongly on the liquid side heat transfer, which is modelled as a function of turbulence, which itself depends on momentum exchange between gas and liquid. A statistical model is used to represent turbulence in each phase. The large interfaces require a special modelling. It has been recently developed and implemented in the NEPTUNE-CFD code which is based on an Eulerian two-fluid model. The present status of this large interface modelling is presented. The validation relies on separate effects experiments such as air-water or steam-water stratified flows and on a more integral experiment, COSI, which represents a cold leg scaled 1/100 for volume and power from a PWR under SBLOCA conditions. The interest of the considered experimental data for PTS CFD is discussed. (authors)

[en] This paper deals with the modeling and the numerical simulation of cavitation phenomena. The cavitation nuclei come from wall nucleation or are pre-existing in the flow. Vapor bubbles generated are carried by the flow and expand in the regions where the local pressure is below the saturation with a tendency to agglomerate into slug bubbles. Compressible, unsteady, turbulent 3D two-phase flow is computed by the NEPTUNE CFD solver, developed jointly by EDF R and D and CEA. The numerical approach is based on a finite volume co-located cell-centered approach and makes use of an original pressure-based multi-field coupling algorithm. The model predictions compared with experimental data on enough selective local variables showed that satisfactory agreement could be obtained without any floating parameter to adjust the data. (authors)

[en] A three-dimensional (3-D) two-fluid model for a turbulent stratified flow with and without condensation is presented, in view of investigating pressurized thermal shock (PTS) scenarios when a stratified two-phase flow takes place in the cold legs of a pressurized water reactor. A modified turbulent K-[curly epsilon] model is proposed with turbulence production induced by interfacial friction. A model of interfacial friction based on an interfacial sublayer concept and three interfacial heat transfer models - namely, a model based on the small eddies-controlled surface renewal concept, a model based on the asymptotic behavior of the eddy viscosity, and a model based on the interfacial sublayer concept - are implemented into a preliminary version of the NEPTUNE code based on the 3-D module of the CATHARE code. As a first step, the models are evaluated by comparison of calculated profiles of velocity, turbulent kinetic energy, and turbulent shear stress with data in a turbulent air-water stratified flow in a rectangular channel and with data for a water jet impacting the free surface of a water pool. Then, a turbulent steam-water stratified flow with condensation is calculated, and some first conclusions are drawn on the interfacial heat transfer modeling and on the applicability of the model to PTS investigations

[en] Based on experience gained at EDF and CEA, a more general and robust 3D multiphase flow solver is being currently developed for over three years. This solver, based on an elliptic oriented fractional step approach, is able to simulate multicomponent/multiphase flows. Discretization follows a 3D full unstructured finite volume approach, with a collocated arrangement of all variables. The non linear behaviour between pressure and volume fractions and a symmetric treatment of all fields are taken into account in the iterative procedure, within the time step. It greatly enforces the realizability of volume fractions (i.e 0<□<1), without artificial numerical needs. Applications to widespread test cases are shown to assess the accuracy and the robustness of the flow solver in different flow conditions, encountered in nuclear reactor pipes

[en] EDF is preparing a new generation of two-phase CFD codes, able to handle the whole range of void fraction and flow configurations. In order to achieve this goal, new numerical methods are being developed and tested: an elliptic based one and an hyperbolic based one. In this paper, first results are presented. (author)

[en] This paper focuses on the modeling and the numerical simulation with the NEPTUNE-CFD code of cavitation phenomena and boiling bubbly flows. Compressible, unsteady, turbulent 3D two-phase flow is computed by the NEPTUNE-CFD solver, developed jointly by EDF R and D and CEA. The numerical approach is based on a finite volume co-located cell-centered approach and makes use of an original pressure-based multi-field coupling algorithm [15]. The cavitation nuclei come from wall nucleation or are pre-existing in the flow. Generated vapor bubbles are advected by the flow and expand in the regions where the local pressure is below the saturation with a tendency to agglomerate into slug bubbles. The model predictions compared with experimental data on enough selective local variables showed that satisfactory agreement could be obtained without any floating parameter to fit the data. After cavitation flows, the second part of the paper deals with boiling bubbly flow through a mixing device representing the effect of a fuel assembly spacer grid equipped with mixing blades (DEBORA-mixing experiment, CEA, Grenoble). Local measurements of the void fraction are provided downstream the mixing enhancer. The computations compare favourably with the experimental results, in particular the global effect of the mixing blades was observed. A modification of the classical nucleate boiling model is proposed to overcome the strong model sensitivity with respect to near wall grid refinement. (authors)

[en] For PWR-type reactors, Departure from Nucleate Boiling (DNB) is one of the key limiting phenomena since safety requires that occurrence of DNB is precluded under normal or incidental operating conditions. It is necessary to develop new simulation tools at smaller scales than those used in current industrial tools (system codes, sub-channel codes). Based on a better understanding of local flow processes, such CFD (Computational Fluid Dynamics) tools should help developing more general DNB-type CHF (Critical Heat Flux) predictions, they aim at replacing industrial methods by the Local Predictive Approach (LPA) which consists in developing DNB predictors based on local flow parameters, as soon as these CFD tools are validated for bubbly boiling flows in fully representative conditions (complex geometries and real flow parameters). Grid effects, non-uniform heat flux impact, channel shape/size effect should be predicted in a more generalized way. Based on a fully unstructured finite volume solver and on state-of-the-art physical modeling, the NEPTUNE-CFD code is currently being developed and validated against numerous experimental data. This paper presents the current status of the NEPTUNE-CFD code with respect to boiling bubbly flow modeling. It also details the related physical validation plan that was drawn up, along with the new associated experimental programs to be set up. Examples of validation results in simple geometry and first computations of real fuel assembly geometries are also provided