Adrian Tentner

Advances in Computational Fluid Dynamics Modeling of Two Phase Flow in a Boiling Water Reactor Fuel Assembly

A new code, CFD-BWR, is being developed for the simulation of two-phase flow phenomena inside a BWR fuel bundle. These phenomena include coolant phase changes and multiple flow regimes which directly influence the coolant interaction with fuel assembly and, ultimately, the reactor performance. CFD-BWR is a specialized module built on the foundation of the commercial CFD code STAR-CD which provides general two-phase flow modeling capabilities. New models describing the inter-phase mass, momentum, and energy transfer phenomena specific for BWRs have been developed and implemented in the CFD-BWR module. A set of experiments focused on two-phase flow and phase-change phenomena has been identified for the validation of the CFD-BWR code and results of two experiment analyses focused on the radial void distribution are presented. The close agreement between the computed results, the measured data and the correlation results provides confidence in the accuracy of the models. (authors)

Computational Fluid Dynamics Modeling of Two-Phase Flow Topologies in a Boiling Water Reactor Fuel Assembly

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.Copyright

A lattice Boltzmann framework to simulate boiling water reactor core hydrodynamics

This paper presents a consistent LBM formulation for the simulation of a two-phase water-steam system. Results of initial model validation in a range of thermodynamic conditions typical for Boiling Water Reactors (BWRs) are also shown. The interface between the two coexisting phases is captured from the dynamics of the model itself, i.e., no interface tracking is needed. The model is based on the Peng-Robinson (P-R) non-ideal equation of state and can quantitatively approximate the phase-coexistence curve for water at different temperatures ranging from 125 to 325 ^@?C. Consequently, coexisting phases with large density ratios (up to ~1000) may be simulated. Two-phase models in the 200-300 ^@?C temperature range are of significant importance to nuclear engineers since most BWRs operate under similar thermodynamic conditions. Simulation of bubbles and droplets in a gravity-free environment of the corresponding coexisting phase until steady state is reached satisfies Laplace law at different temperatures and thus, yield the surface tension of the fluid. Comparing the LBM surface tension thus calculated using the LBM to the corresponding experimental values for water, the LBM lattice unit (lu) can be scaled to the physical units. Using this approach, spatial scaling of the LBM emerges from the model itself and is not imposed externally.

Prediction of Boiling Water Reactor Assembly Void Distributions Using a Two-Phase Computational Fluid Dynamics Model

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.© 2008 ASME

Computational Fluid Dynamics Modeling of Two-Phase Boiling Flow and Critical Heat Flux

This paper presents recent advances in the modeling of two-phase boiling flow and critical heat flux that have been implemented in the Extended Boiling Framework (EBF) [1, 2, 3]. The EBF code was developed as a customized module built on the foundation of the commercial Computational Fluid Dynamics (CFD) code STAR-CD, which provides general two-phase flow modeling capabilities, for the detailed analysis of the two-phase flow and heat transfer phenomena that occur in Boiling Water Reactor (BWR) fuel assemblies. These phenomena include coolant phase changes and multiple flow regimes that directly influence the coolant interaction with the fuel pins and, ultimately, the reactor performance. An effort to expand the EBF two-phase models and to explore their applicability to other CFD codes is currently underway.The paper presents results of recent CFD analyses of Critical Heat Flux (CHF) experiments that have measured the axial distribution of wall temperature in two-phase upward flow in a vertical channel with a heated wall. The experiments were designed to produce the onset of CHF in the upper half of the heated channel. The simulated axial distribution of wall temperature is compared with experimental data, illustrating the ability of the extended EBF model to capture the onset of CHF for a wide range of thermal-hydraulic conditions relevant for BWRs. The paper concludes with a discussion of results and plans for future work.Copyright

Development and Validation of a Computational Fluid Dynamics Model for the Simulation of Two-Phase Flow Phenomena in a Boiling Water Reactor Fuel Assembly

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.Copyright

Validation of a Short Kinetic Mechanism for Jet A-Air Detonations

Progress on comprehensive validation of the short chemical kinetic mechanism for the JetA/air mixtures under conditions, typical of Pulsed Detonation Engines (PDE) operating conditions, is reported. The reduced (13 reactions, 15 species) kinetic mechanism (under development in Kurchatov Institute for the high fidelity simulations of the PDEs) is subjected to a specially designed, multi-step validation procedure. The results of the zerodimensional thermodynamic- and kinetic-based simulations are briefly described. This report discusses the procedures and the results of the one-dimensional reactive CFD validation of the reduced kinetic mechanism. For the 1-dimensional validation of the mechanism (as well as any other kinetic mechanism for any fuel), implemented in the computational fluid dynamics software (CFD software), the test data of detonation initiation behind reflected shock waves are proposed as proper detonation initiation validation measurements. The predictions show acceptable agreement with the available experimental measurements on detonation initiation in Jet-A/air mixtures. Additional investigation of the mechanisms of detonation formation behind reflected shock wave in 1-dimensional simulations allows to explain several experimental observations, which could not be explained before.

Computational Modeling of Blood Hydrodynamics and Blockage Formation Phenomena in the Human Cardiovascular System

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.

First‐Principles‐Based Development of Kinetic Mechanisms in Chemically Active Light‐Emitting Nonthermal Plasmas and Gases

Recent progress in several related research areas such as first‐principles electronic‐structure calculations of atoms and diatomic molecules, theory of elementary processes, kinetics, and numerical engineering, and also continued exponential growth in computational resources enhanced by recent advances in massively parallel computing have opened the possibility of directly designing kinetics mechanisms to describe chemical processes and light emission in such complex media as nonequilibrium plasmas and reacting gases. It is important that plasma and combustion kinetics can be described in the framework of this direct approach to a sufficiently high accuracy, which makes it an independent predictive research tool complementary to experimental techniques. This paper demonstrates the capabilities of the first‐principles based approach to develop kinetic mechanisms. Two examples are discussed in detail: (1) the mechanism of hydrocarbon fuel combustion at high temperatures and (2) light emission in non‐thermal...