Development and Assessment of a Physics-based Model for Subcooled Flow Boiling with Application to CFD PDF Download

Author: Ravikishore Kommajosyula
Publisher:
ISBN:
Category :
Languages : en
Pages : 148

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Boiling is an extremely efficient mode of heat transfer and is the preferred heat removal mechanism in power systems in general and, more recently, in electronics cooling. Physics-based models that describe boiling heat transfer, when coupled with Computational Fluid Dynamics (CFD), can be an invaluable tool to increase the performance of such systems. Existing modeling approaches do not incorporate all relevant heat transfer mechanisms at the wall, limiting their predictive capability and general applicability. These shortcomings restrict the application of CFD in the design process. For the nuclear industry, this means having to rely on expensive experimental campaigns to develop and license new reactor designs. A second-generation mechanistic heat flux partitioning framework developed in our group provides an enhanced physical description of flow boiling. It introduces several mechanisms not accounted for in previous formulations, such as 1) bubbles sliding on the heater surface, 2) interaction of nucleation sites and 3) microlayer evaporation. The framework requires describing the complete bubble ebullition cycle, including bubble nucleation, growth, and departure through closure models, which are currently lacking. This thesis extends the framework into a closed-formulation by developing closure models that adequately represent the underlying physics. New models for predicting the bubble departure diameter and frequency are developed based on insights gathered from experiments and direct numerical simulations. An assessment against existing approaches to model boiling heat transfer demonstrates the model’s ability to predict over 80% of the boiling curves within a 20% error, while also capturing the correct trends with flow conditions. The model implementation in a commercial CFD software is demonstrated using data from the Bartolomei experiment. The extendability of the model to novel heater surfaces is further demonstrated for a sapphire heater substrate, where fewer cavities for nucleation shift the boiling curves to considerably higher wall superheats. This mechanistic representation of boiling heat transfer has the potential to support predictive design with optimal boiling heat transfer for improved system efficiency, with the specific objective to accelerate the development of novel nuclear fuel concepts.

Author: Lindsey Anne Gilman
Publisher:
ISBN:
Category :
Languages : en
Pages : 188

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Book Description
Advanced modeling capabilities were developed for application to subcooled flow boiling through this work. The target was to introduce, and demonstrate, all necessary mechanisms required to accurately predict the temperature and heat flux for subcooled flow boiling in CFD simulations. The model was developed using an experimentally based mechanistic approach, where the goal was to accurately capture all physical phenomena that affect heat transfer and occur at the heated surface to correctly predict surface temperatures. The proposed model adopts a similar approach to the classical heat partitioning method, but captures additional boiling physical phenomena. It introduces a new evaporation term, to truly capture the evaporation occurring on the surface while also tracking the bubble crowding effect on the boiling surface. This includes evaporation from the initial bubble inception and evaporation through the bubble microlayer. The convection term is modified to account for increased surface roughness caused by the presence of the bubbles on the heated surface. The quenching term accounts for bringing the bubble dry spot back to the wall superheat prior to bubble inception. In addition to the changes to these three classic components, a sliding conduction term is added to capture the increased heat transfer due to bubble sliding on the heated surface prior to lift-off. The sliding conduction component includes all heat removal associated with transient conduction caused by disruption of the thermal boundary layer. The new method extends the generality and applicability of boiling models in CFD through a fully mechanistic representation. The new model also tracks the dry surface area during boiling for possible application in DNB predictions. A statistical tracking method for bubble location on the heated surface provides information on the bubble merging probability and prevents the active nucleation site density from reaching un-physical values. The model was implemented in the CFD software STAR-CCM+, and the wall temperature predictions were recorded and compared against the standard model's predictions and experimental data for a range of mass fluxes, heat fluxes, inlet subcoolings, and pressures. In general, the new model predicts wall temperatures closer to experimental data for both low and high pressures when compared against the standard model. The new model also converges at higher heat fluxes and greater subcoolings than the standard model.

Author:
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ISBN:
Category :
Languages : en
Pages :

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Abstract : The main objective of the current work is to achieve a better understanding - through modeling and simulation/programming activities - of annular flow-boiling and its applications based on a synthesis of Computational Fluid Dynamics (CFD) simulations, existing correlations, and experiments. A unique and rigorous 2-D CFD simulations technique was developed for annular flow-boiling to propose a correlation for convective component of Heat Transfer Coefficient (HTC) - defined here as flow-boiling in the absence of nucleation. To provide a context for the correlation structure of convective component of HTC, flow-physics details of annular flow-boiling and correlation structure (based on fundamental considerations) for HTC have also been discussed. Further, other existing correlations for Nusselt number, void-fraction, flow-regime transition, and pressure-drop have been used to develop a general but first order engineering estimates-methodology for design of annular flow-boilers and flow-condensers. The first order estimates-methodology, thus developed for annular flow-boiler operations, was used to: (i) make a priori estimates of flow predictions towards choosing suitable instrumentations for the design of a particular test-section and associated flow-loop needed for a new high heat-flux flow-boiling experiments involving water as a working fluid), and (ii) define a range of experimental operating conditions - for a low heat-flux test-section and flow-loop (involving FC-72 as working fluid) facility from which experimental data needed to be obtained to throw light on the flow-physics being modeled by the CFD code. Furthermore, preliminary results from a different low heat-flux experiments are briefly discussed here and then compared with rigorous CFD simulations to achieve better understanding of the flow-physics.

Author: Jyeshtharaj Joshi
Publisher: Woodhead Publishing
ISBN: 0081023383
Category : Science
Languages : en
Pages : 888

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Book Description
Advances of Computational Fluid Dynamics in Nuclear Reactor Design and Safety Assessment presents the latest computational fluid dynamic technologies. It includes an evaluation of safety systems for reactors using CFD and their design, the modeling of Severe Accident Phenomena Using CFD, Model Development for Two-phase Flows, and Applications for Sodium and Molten Salt Reactor Designs. Editors Joshi and Nayak have an invaluable wealth of experience that enables them to comment on the development of CFD models, the technologies currently in practice, and the future of CFD in nuclear reactors. Readers will find a thematic discussion on each aspect of CFD applications for the design and safety assessment of Gen II to Gen IV reactor concepts that will help them develop cost reduction strategies for nuclear power plants. Presents a thematic and comprehensive discussion on each aspect of CFD applications for the design and safety assessment of nuclear reactors Provides an historical review of the development of CFD models, discusses state-of-the-art concepts, and takes an applied and analytic look toward the future Includes CFD tools and simulations to advise and guide the reader through enhancing cost effectiveness, safety and performance optimization

Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 19

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Because of its practical importance and various industrial applications, the process of subcooled flow boiling has attracted a lot of attention in the research community in the past. However, the existing models are primarily phenomenological and are based on correlating experimental data rather than on a first-principle analysis of the governing physical phenomena. Even though the mechanisms leading to critical heat flux (CHF) are very complex, the recent progress in the understanding of local phenomena of multiphase flow and heat transfer, combined with the development of mathematical models and advanced Computational Fluid Dynamics (CFD) methods, makes analytical predictions of CHF quite feasible. Various mechanisms leading to CHF in subcooled boiling have been investigated. A new model for the predictions of the onset of CHF has been developed. This new model has been coupled with the overall boiling channel model, numerically implemented in the CFX 4 computer code, tested and validated against the experimental data of Hino and Ueda. The predicted critical heat flux for various channel operating conditions shows good agreement with the measurements using the aforementioned closure laws for the various local phenomena governing nucleation and bubble departure from the wall. The observed differences are consistent with typical uncertainties associated with CHF data.

Author: Monica V. Pham
Publisher:
ISBN:
Category :
Languages : en
Pages : 148

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Book Description
Multiphase Computational Fluid Dynamics (M-CFD) has the potential to provide high fidelity simulation of complex boiling phenomena in Light Water Reactors (LWRs), thereby accelerating the development cycle and reducing the need for expensive large-scale experiments. M-CFD relies on two-phase closure models to consistently represent the relevant physical phenomena in flow boiling. However, the still incomplete understanding of the ability of these closures to accurately capture the underlying physics limits the adoption of M-CFD in reactor development and design optimization. Due to the interaction of complex physical phenomena present in subcooled flow boiling, local measurements are necessary to assess the performance of existing closures. Additionally, because previous validation was performed using low pressure data, measurements at high pressure are needed to understand the performance of multiphase closures at PWR conditions.

Author: Dochan Kwak
Publisher: Springer Science & Business Media
ISBN: 9400701934
Category : Technology & Engineering
Languages : en
Pages : 294

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Book Description
This monograph is intended as a concise and self-contained guide to practitioners and graduate students for applying approaches in computational fluid dynamics (CFD) to real-world problems that require a quantification of viscous incompressible flows. In various projects related to NASA missions, the authors have gained CFD expertise over many years by developing and utilizing tools especially related to viscous incompressible flows. They are looking at CFD from an engineering perspective, which is especially useful when working on real-world applications. From that point of view, CFD requires two major elements, namely methods/algorithm and engineering/physical modeling. As for the methods, CFD research has been performed with great successes. In terms of modeling/simulation, mission applications require a deeper understanding of CFD and flow physics, which has only been debated in technical conferences and to a limited scope. This monograph fills the gap by offering in-depth examples for students and engineers to get useful information on CFD for their activities. The procedural details are given with respect to particular tasks from the authors’ field of research, for example simulations of liquid propellant rocket engine subsystems, turbo-pumps and the blood circulations in the human brain as well as the design of artificial heart devices. However, those examples serve as illustrations of computational and physical challenges relevant to many other fields. Unlike other books on incompressible flow simulations, no abstract mathematics are used in this book. Assuming some basic CFD knowledge, readers can easily transfer the insights gained from specific CFD applications in engineering to their area of interest.

Author: Adela Ionescu
Publisher: BoD – Books on Demand
ISBN: 9535137905
Category : Computers
Languages : en
Pages : 412

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Book Description
This book is the result of a careful selection of contributors in the field of CFD. It is divided into three sections according to the purpose and approaches used in the development of the contributions. The first section describes the "high-performance computing" (HPC) tools and their impact on CFD modeling. The second section is dedicated to "CFD models for local and large-scale industrial phenomena." Two types of approaches are basically contained here: one concerns the adaptation from global to local scale, - e.g., the applications of CFD to study the climate changes and the adaptations to local scale. The second approach, very challenging, is the multiscale analysis. The third section is devoted to "CFD in numerical modeling approach for experimental cases." Its chapters emphasize on the numerical approach of the mathematical models associated to few experimental (industrial) cases. Here, the impact and the importance of the mathematical modeling in CFD are focused on. It is expected that the collection of these chapters will enrich the state of the art in the CFD domain and its applications in a lot of fields. This collection proves that CFD is a highly interdisciplinary research area, which lies at the interface of physics, engineering, applied mathematics, and computer science.

Author: V. K. Dhir
Publisher:
ISBN:
Category :
Languages : en
Pages : 5

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At present, guidelines for fuel cycle designs to prevent axial offset anomalies (AOA) in pressurized water reactor (PWR) cores are based on empirical data from several operating reactors. Although the guidelines provide an ad-hoc solution to the problem, a unified approach based on simultaneous modeling of thermal-hydraulics, chemical, and nuclear interactions with vapor generation at the fuel cladding surface does not exist. As a result, the fuel designs are overly constrained with a resulting economic penalty. The objective of present project is to develop a numerical simulation model supported by laboratory experiments that can be used for fuel cycle design with respect to thermal duty of the fuel to avoid economic penalty, as well as, AOA. At first, two-dimensional numerical simulation of the growth and departure of a bubble in pool boiling with chemical interaction is considered. A finite difference scheme is used to solve the equations governing conservation of mass, momentum, energy, and species concentration. The Level Set method is used to capture the evolving liquid-vapor interface. A dilute aqueous boron solution is considered in the simulation. From numerical simulations, the dynamic change in concentration distribution of boron during the bubble growth shows that the precipitation of boron can occur near the advancing and receding liquid-vapor interface when the ambient boron concentration level is 3,000 ppm by weight. Secondly, a complete three-dimensional numerical simulation of inception, growth and departure of a single bubble subjected to forced flow parallel to the heater surface was developed. Experiments on a flat plate heater with water and with boron dissolved in the water were carried out. The heater was made out of well-polished silicon wafer. Numbers of nucleation sites and their locations were well controlled. Bubble dynamics in great details on an isolated nucleation site were obtained while varying the wall superheat, liquid subcooling and flow velocity parametrically. Concentration variation of boron near the liquid-vapor interface was detected successfully with a newly developed miniature concentration sensor. The measured concentration variations at different radial locations from the center of cavity have the same trend as given by the numerical simulations. The deposition of boron was found near the nucleation site on the heater surface, which validates the numerical simulation. Subcooled flow boiling experiments at three pressures were performed on a nine-rod bundle with water and with boron dissolved in the water. The test runs were conducted with a wide range of mass fluxes (186 to 2800 kg/m2s) and heat fluxes (1.0 to 30.0 W/ cm2). Not only the variables required to develop mechanistic models for subcooled flow boiling were measured, but also the crud formation during boiling and its effect on the heat transfer process were investigated. (B204).

Author: Francesco D'Auria
Publisher: Elsevier
ISBN: 0323856098
Category : Technology & Engineering
Languages : en
Pages : 818

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Book Description
Handbook on Thermal Hydraulics of Water-Cooled Nuclear Reactors, Volume 3, Procedures and Applications includes all new chapters which delve deeper into the topic, adding context and practical examples to help readers apply learnings to their own setting. Topics covered include experimental thermal-hydraulics and instrumentation, numerics, scaling and containment in thermal-hydraulics, as well as a title dedicated to good practices in verification and validation. This book will be a valuable reference for graduate and undergraduate students of nuclear or thermal engineering, as well as researchers in nuclear thermal-hydraulics and reactor technology, engineers working in simulation and modeling of nuclear reactors, and more. In addition, nuclear operators, code developers and safety engineers will also benefit from the practical guidance provided. Presents a comprehensive analysis on the connection between nuclear power and thermal hydraulics Includes end-of-chapter questions, quizzes and exercises to confirm understanding and provides solutions in an appendix Covers applicable nuclear reactor safety considerations and design technology throughout