Prediction of bubble size which governs interfacial transfer terms between the two phases is of importance for an accurate prediction of the subcooled boiling flow. In the present work, a mechanistic bubbles size model, S-gamma was examined to enhance the prediction capability of subcooled boiling flows for the CFD (computational fluid dynamics) code. In addition to this, advanced subcooled boiling models such as new wall boiling and two-phase logarithmic wall function models were also applied for an improvement of energy partitioning and two-phase turbulence models, respectively. The benchmark calculation against the DEBORA subcooled boiling data confirms that the S-gamma bubble size model with the two advanced subcooled boiling models shows good prediction results and is applicable to the wide range of flow conditions that are expected in the nominal and postulated accidental conditions of a nuclear power plant. (C) 2011 Elsevier B.V. All rights reserved.

In this paper, experimental data of solid distributions in horizontal pipelines were modelled using CFD and Discrete Element Method (DEM). Experimental data were taken from Gilles et al. (2004), Roco et al. (1983) and Hill (1996) for pipe diameter in the range from 51 mm to 103 mm, the average sand particle size from 90 m to 4400 m and average solid volume fraction from 0.092 to 0.21. Two CFD models available in STAR-CCM+ for Eulerian multiphase flows were used, the simple solid pressure model that uses the exponential formula for the solid pressure force and more sophisticated model based on the kinetic theory of granular flows that takes into account bulk, collisional, kinetic and frictional viscosities, which result from the presence of solid particles. In both models the turbulent flow was modelled using the standard k- model. The drag force acting on the particles was modelled using the Gidaspow formula. The granular flow model was compared with the DEM implemented in STAR-CCM+. The DEM formulation used in STAR-CCM+ is based on the soft-particle formulation where particles are allowed to develop an overlap. The non-slip Hertz-Mindlin contact model was used to account for particle-particle interactions and the Gidaspow drag formula for liquid-particle interaction. CFD and DEM predictions of solids concentration were compared against experimental data. The comparisons were made in terms of accuracy and computational time. NOMENCLATURE A constant in solid pressure force equation D ij A linearized drag coefficient c concentration of solid particles average solid concentration CD drag coefficient CL lift coefficient d particle diameter D pipe diameter e coefficient of restitution F force   i int F internal forces g gravitational acceleration g0 radial distribution function I2D second invariant of deviator of the strain rate tensor Mi interphase momentum transfer p pressure v velocity vr relative velocity between phases  volume fraction d volume fraction of dispersed phase d,max maximum volume fraction of dispersed phase(packing limit)  angle of internal friction  dynamic viscosity p granular viscosity C p  collisional part of granular viscosity f p  frictional part of granular viscosity K p  kinetic part of granular viscosity f p max ,  maximum frictional viscosity  kinematic viscosity t c  turbulent viscosity  density  turbulent Prandtl number i stress tensor INTRODUCTION Transport of minerals like coal, copper, iron, phosphate and oil sand requires good understanding of slurry behaviour in the pipelines. This paper presents a CFD model of solid distribution in pipelines that gives better insight into the sedimentation process. In this work two approaches were used, the Eulerian multiphase model and the Discrete Element Method (DEM). The Eulerian multiphase model treats phases as interpenetrating continua. Each phase is characterized by its own physical properties, velocity and temperature. The pressure is shared by the phases and the amount of a given phase in a computational cell is given by the phase volume fraction. The DEM model is based on explicit modelling of the collisions between the particles. This modelling technique uses the Lagrangian approach. Eulerian multiphase model STAR-CCM+ solves the mass conservation equation for phase i:     0 vi     i i i i t       (1) and momentum balance takes form:

In order to assess the accuracy and validity of subchannel, system, and computational fluid dynamics codes, the Paul Scherrer Institut has participated in the OECD/NRC PSBT benchmark with the thermal-hydraulic system code TRACE5.0 developed by US NRC, the subchannel code FLICA4 developed by CEA, and the computational fluid dynamic code STAR-CD developed by CD-adapco. The PSBT benchmark consists of a series of void distribution exercises and departure from nucleate boiling exercises. The results reveal that the prediction by the subchannel code FLICA4 agrees with the experimental data reasonably well in both steady-state and transient conditions. The analyses of single-subchannel experiments by means of the computational fluid dynamic code STAR-CD with the CD-adapco boiling model indicate that the prediction of the void fraction has no significant discrepancy from the experiments. The analyses with TRACE point out the necessity to perform additional assessment of the subcooled boiling model and bulk condensation model of TRACE.

Three-dimensional computational fluid dynamics (CFD) method was used to model the boiling two-phase flow in one of the PSBT 5-by-5 rod bundle tests. The rod bundle with all the spacers was modeled explicitly using unstructured computational grids. The six-equation, two-fluid model with the wall boiling model was used to model the boiling two-phase flows in the bundle. The computed void fractions compare well with the measured data at the measuring plane. In addition to the averaged void data, the CFD results give a very detailed picture of the flow and void distributions in the bundle and how they are affected by solid structures in the flow paths such as the spacer grids and mixing vanes.

The commercial CFD code STAR-CD 4.02 is used as a numerical simulation tool for flows in the supercritical water-cooled nuclear reactor (SCWR). The basic heat transfer element in the reactor core can be considered as round tubes and tube bundles. Reactors with vertical or horizontal flow in the core can be found. In a vertically oriented core, symmetric characters of flow and heat transfer can be found and two-dimensional analyses are often performed. However, in a horizontally oriented core the flow and heat transfer are fully three-dimensional due to the buoyancy effect. In this paper, horizontal tubes and tube bundles at SCWR conditions are studied. Special STAR-CD subroutines were developed by the authors to correctly represent the dramatic change in physical properties of the supercritical water with temperature. From the study of single round tubes, the Speziale quadratic non-linear high-Re k–ɛ turbulence model with the two-layer model for near wall treatment is found to produce the best results in comparison with experimental data. In tube bundle simulations, it is found that the temperature is higher in the top half of the bundle and the highest tube wall temperature is located at the outside tubes where the flow rate is the lowest. The secondary flows across the bundle are highly complex. Their main effect is to even out the temperature over the area within each individual recirculation region. Similar analysis could be useful in design and safety studies to obtain optimum fuel rod arrangement in a SCWR.

The commercial CFD code STAR-CD v4.02 is used as a numerical simulation tool for flows in the supercritical water-cooled nuclear reactor (SCWR). The basic heat transfer element in the reactor core can be considered as round rods and rod bundles. Reactors with vertical or horizontal flow in the core can be found. In vertically oriented core, symmetric characters of flow and heat transfer can be found and two-dimensional analyses are often performed. However, in horizontally oriented core the flow and heat transfer are fully three-dimensional due to the buoyancy effect. In this paper, horizontal rods and rod bundles at SCWR conditions are studied. Special STAR-CD subroutines were developed by the authors to correctly represent the dramatic change in physical properties of the supercritical water with temperature. In the rod bundle simulations, it is found that the geometry and orientation of the rod bundle have strong effects on the wall temperature distributions and heat transfers. In one orientation the square bundle has a higher wall temperature difference than other bundles. However, when the bundles are rotated by 90° the highest wall temperature difference is found in the hexagon bundle. Similar analysis could be useful in design and safety studies to obtain optimum fuel rod arrangement in a SCWR.

An international collaborative effort to develop a computational fluid dynamics (CFD) model of the human cardiovascular system (HCVS) has been initiated in 2008. The HCVS model is designed to describe (a) the blood flow hydrodynamics and associated heat transport phenomena, (b) the blood flow interactions with the essential organs, and (c) the vessel blockage formation associated with atherosclerosis and thrombosis. The CFD-HCVS model is being developed as a new specialized software module using as a foundation the CFD code, STAR-CD, that is developed and distributed by CD-adapco, Ltd., a member of the project team. The CFD-HCVS module includes the following components and capabilities. (1) A simplified 3D coarse mesh CFD model of the HCVS, which allows the simulation of hemodynamic transient phenomena. The circulatory system model is closed with porous-media flow components having a hydraulic resistance equivalent to the lumped flow resistance of the smaller vessels, including microcirculation. Both hydrodynamic and thermodynamic phenomena are described, allowing the study of blood flow transients in the presence of temperature changes. (2) Simplified zero-dimensional models of the essential organs (e.g., heart, kidneys, brain, liver, etc.) describing the time-dependent consumption or production of various blood components of interest. The organ models exchange information with the CFD system model through interfaces designed to allow their replacement, in the future, with more complex 3D organ models. (3) Selected sections of the circulatory system can be replaced by realistic 3 fine mesh vessel models allowing the detailed study of the 3D blood flow field and the vascular geometry changes due to blockage formation. (4) Models of local blockage formation due to atherosclerosis and thrombosis. Three HCVS models of increasing complexity have been designed. These models contain 27 vessels, 113 vessels, and 395 vessels. The initial CFD-HCVS model development is based on the medium HCVS model with 113 vessels. A closed circuit CFD model describing the major vessels and containing 0D models of the heart and kidneys has been developed. The CFD-HCVS model includes porous-media models describing the blood flow in the smaller vessels and capillaries. Initial simulations show that the calculated blood flow rates in the vessels modeled are in reasonably good agreement with the corresponding physiological values. A simplified model of thrombosis has also been developed. Current development efforts are focused on the addition of new vessels and 0D organ models and the development of atherosclerosis models. The HCVS model provides a flexible and expandable modeling framework that will allow the researchers from universities, research hospitals and the medical industry to study the impact of a wide range of phenomena associated with diseases of the circulatory system and will help them develop new diagnostics and treatments.

Numerical simulations of gas-liquid two-phase flow with high superficial velocity in a vertical pipe were conducted with the use of the commercial software package STAR-CD 3.27. The change in bubble size due to breakup and coalescence was modelled by the Sγ model. The applicability and performance of the Sγ model in modelling of gasliquid bubbly flow were studied. The sensitivity of the Sγ model to the distribution moment γ, and the drainage mode were investigated. The numerical results were compared with the experimental data of Hibiki et al., (2001). Good agreement was achieved for axial velocities and void fraction for all tested cases. It was found in this work that the Sγ model is capable of predicting with reasonable accuracy the bubble size and its distribution even in high void fraction. Except in the near wall region, the simulated bubble size and therefore the interfacial area density fit well with the experiment measurements. It was observed that the predicted bubble size and interfacial area density obtained from both the S0 and S2 models are more or less the same, indicating that the numerical results are independent of the distribution moment γ. It was further found that, the drainage mode greatly affects the bubble size: an increase in mobility of the bubble surface enhances the coalescence and leads to an over-prediction of the bubble size in the pipe centre. The bubble size increases with the increase of the gas phase superficial velocity while the variation of the interfacial area density is smaller as it is a combined function of the bubble size and local gas hold-up.

This paper presents the current status in the development and validation of an advanced Computational Fluid Dynamics (CFD) model, CFD-BWR, which allows the detailed analysis of the two-phase flow and heat transfer phenomena in Boiling Water Reactor (BWR) fuel assemblies under various operating conditions. The CFD-BWR model uses an Eulerian Two-Phase (E2P) approach, and is also referred to as the E2P modeling framework. It is being developed as a customized module built on the foundation of the commercial CFD-code STAR-CD which provides general two-phase flow modeling capabilities. The integral validation efforts have focused on the analysis of the NUPEC Full-Size Boiling Water Reactor Test (BFBT) within the framework of the OECD/NRC benchmark exercise.The paper reviews the two-phase models implemented in the CFD-BWR code, and emphasizes recently implemented models of inter-phase and coolant-cladding momentum and energy exchanges. Results of recent BFBT experiment simulations using these models are presented and the effects of the new models on the calculated void distribution are discussed. The paper concludes with a discussion of future model development and validation plans.

This paper presents recent advances in the development and validation of the two-phase flow topology models implemented in CFD-BWR, an advanced Computational Fluid Dynamics (CFD) computer code that allows the detailed analysis of the two-phase flow and heat transfer phenomena in Boiling Water Reactor (BWR) fuel assemblies under various operating conditions. The local inter-phase surface topology plays a central role in determining the mass, momentum, and energy exchanges between the liquid and vapor phases and between the two-phase coolant and the fuel pin cladding. The paper describes the topology map used to determine the local inter-phase surface topology and the role of the local topology in determining the inter-phase mass, momentum, and energy transfer. It discusses the relationship between the local interphase surface topology and the traditional channel flow regimes and presents results of experiment analyses in which computed local topologies are aggregated into flow regimes and compared with experimental observations.

This paper presents recent results obtained as part of the on-going integral validation of an advanced Eulerian-Eulerian two-phase (E2P) computational fluid dynamics based boiling model that allows the detailed analysis of the two-phase flow and heat transfer phenomena in a Boiling Water Reactor (BWR) fuel assembly. The code is being developed as a customized module built on the foundation of the commercial CFD-code STAR-CD which provides general two-phase flow modeling capabilities. Simulations of a prototypic BWR fuel assembly experiment have been completed as an initial assessment of the applicability of the E2P model to realistic BWR geometries and conditions. Initial validation has focused on comparison with measured sub-channel averaged data to enable the benchmarking of the accuracy of the E2P against the current predictive capabilities of the sub-channel methods. The paper will discuss the effects of modeling assumptions, assumed coefficient values and the computational mesh structure used to describe the fuel assembly geometry on the accuracy of the sub-channel averaged void fraction.

This paper presents recent advances and validation results obtained during the development of an advanced two-phase Computational Fluid Dynamics (CFD) computer model (CFD-BWR) that allows the detailed analysis of the two-phase flow and heat transfer phenomena in a Boiling Water Reactor (BWR) fuel bundle. The CFD-BWR code is being developed as a customized module built on the foundation of the commercial CFD-code STAR-CD which provides general two-phase flow modeling capabilities. We have described in (1, 2) the model development strategy that has been adopted by the development team for the prediction of boiling flow regimes in a BWR fuel bundle. The strategy includes the use of local inter-phase surface topology maps and flow topology specific phenomenological models in conjunction with an interface transport and inter-phase surface topology transport approach. The initial boiling models have been developed for a bubbly flow topology and have been described in (1, 2). The model validation strategy and results of experiment analyses focused on individual two-phase phenomena in the presence of bubbly flow topologies have been presented in (3). As a result of a collaborative effort between Argonne National Laboratory and CD-adapco Group the boiling model has been expanded to allow the modeling new flow topologies in addition to the original bubbly flow topology, with the goal of allowing the modeling of the entire spectrum of flow topologies present in a BWR fuel assembly. The expanded boiling model relies on a simplified flow topology map that allows the selection use of: a bubbly flow topology where the continuous phase is liquid, a droplet flow topology where the continuous phase is vapor, or a transition flow topology for each computational cell, depending primarily on the local void fraction. For near-wall cells the formation of a liquid film is also allowed, leading to a special liquid film topology under certain conditions. Inter-phase mass, momentum, and energy exchanges and wall-fluid momentum and energy exchanges have been implemented for the topologies listed above and will be discussed in the paper. Preliminary integral validation efforts have focused on the analysis of the NUPEC Full-Size Boiling Water Reactor Test (BFBT) within the framework of the OECD/NRC benchmark exercise. Results of calculations, including the detailed two-dimensional void fraction distribution in the measuring plane will be presented and compared with the measured data. The development of the extended boiling model which allows the presence of flow topologies with either liquid or vapor continuous phase is a significant advance in the development of the CFD-BWR code and provides the foundation for the future implementation of a more complex inter-phase surface topology map.

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