Dirk Lucas

Iztok Tiselj

Yassin A. Hassan

Fabio Moretti

Computational Fluid Dynamics for Gas-Liquid Flows

Due to the importance of boiling heat transfer in general, and boiling crisis in particular, for the analysis of operation and safety of both nuclear reactors and conventional thermal power systems, extensive efforts have been made in the past to develop a variety of methods and tools to evaluate the boiling heat transfer coefficient and to assess the onset of temperature excursion and critical heat flux (CHF) at various operating conditions of boiling channels. The objective of this paper is to present mathematical modeling concepts behind the development of mechanistic multidimensional models of low-quality forced convection boiling, including the mechanisms leading to temperature excursion and the onset of CHF.

Michael Z. Podowski

Raf M. Podowski

Mechanistic Multidimensional Modeling of Forced Convection Boiling Heat Transfer

A commercial CFD code was applied, for validation purposes, to the simulation of a slug mixing experiment carried out at OKB “Gidropress” scaled facility in the framework of EC TACIS project R2.02/02: “Development of safety analysis capabilities for VVER-1000 transients involving spatial variations of coolant properties (temperature or boron concentration) at core inlet.” Such experimental model reproduces a VVER-1000 nuclear reactor and is aimed at investigating the in-vessel mixing phenomena. The addressed experiment involves the start-up of one of the four reactor coolant pumps (the other three remaining idle), and the presence of a tracer slug on the starting loop, which is thus transported to the reactor pressure vessel where it mixes with the clear water. Such conditions may occur in a boron dilution scenario, hence the relevance of the addressed phenomena for nuclear reactor safety. Both a pretest and a posttest CFD simulations of the mentioned experiment were performed, which differ in the definition of the boundary conditions (based either on nominal quantities or on measured quantities, resp.). The numerical results are qualitatively and quantitatively analyzed and compared against the measured data in terms of space and time tracer distribution at the core inlet. The improvement of the results due to the optimization of the boundary conditions is evidenced, and a quantification of the simulation accuracy is proposed.

F. Moretti

D. Melideo

A. Del Nevo

F. D'Auria

T. Höhne

E. Lisenkov

CFD Analysis of a Slug Mixing Experiment Conducted on a VVER-1000 Model

Two-dimensional axisymmetric simulations of pressurized thermal shock (PTS) phenomena through Neptune&#x5f;CFD module are presented aiming at two-phase models validation against experimental data. Because of PTS complexity, only some thermal-hydraulic aspects were considered. Two different flow configurations were studied, occurring when emergency core cooling (ECC) water is injected in an uncovered cold leg of a pressurized water reactor (PWR)&#x2014;a plunging water jet entering a free surface, and a stratified steam-water flow. Some standard and new implemented models were tested: modified turbulent <svg style="vertical-align:-0.13794pt;width:8.6000004px;" id="M1" height="10.7375" version="1.1" viewBox="0 0 8.6000004 10.7375" width="8.6000004"  xmlns="http://www.w3.org/2000/svg">
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</svg> models with turbulence production induced by interfacial friction, models for the drag coefficient, and interfacial heat transfer models. Quite good agreement with experimental data was achieved with best performing models for both test cases, even if a further improvement in phase change modelling would be suitable for nuclear technology applications.

Maria Cristina Galassi

Pierre Coste

Christophe Morel

Two-Phase Flow Simulations for PTS Investigation by Means of Neptune&#x0005F;CFD Code

The NURESIM Project of the 6th European Framework Program initiated the development of 
a new-generation common European Standard Software Platform for nuclear reactor simulation. The
 thermal-hydraulic subproject aims at improving the understanding and the predictive capabilities of 
 the simulation tools for key two-phase flow thermal-hydraulic processes such as the critical heat 
 flux (CHF). As part of a multi-scale analysis of reactor thermal-hydraulics, a two-phase CFD tool is
  developed to allow zooming on local processes. Current industrial methods for CHF mainly use 
  the sub-channel analysis and empirical CHF correlations based on large scale experiments having
   the real geometry of a reactor assembly. Two-phase CFD is used here for understanding some boiling
    flow processes, for helping new fuel assembly design, and for developing better CHF predictions in both 
    PWR and BWR. This paper presents a review of experimental data which can be used for validation of 
    the two-phase CFD application to CHF investigations. The phenomenology of DNB and
     Dry-Out are detailed identifying all basic flow processes which require a specific modeling in CFD
      tool. The resulting modeling program of work is given and the current state-of-the-art of the modeling
       within the NURESIM project is presented.

D. Bestion

H. Anglart

D. Caraghiaur

P. P&#xe9;turaud

B. Smith

M. Andreani

B. Niceno

E. Krepper

D. Lucas

M. C. Galassi

J. Macek

L. Vyskocil

B. Koncar

G. Hazi

Review of Available Data for Validation of Nuresim Two-Phase CFD Software 
                        Applied to CHF Investigations

Within the European Integrated Project NURESIM, the simulation of PTS is investigated. Some accident scenarios for Pressurized Water Reactors may cause Emergency Core Coolant injection into the cold leg leading to PTS situations. They imply the formation of temperature gradients in the thick vessel walls with consequent localized stresses and the potential for propagation of possible flaws present in the material. This paper focuses on two-phase conditions that are potentially at the origin of PTS. It summarizes recent advances in the understanding of the two-phase phenomena occurring within the geometric region of the nuclear reactor,that is, the cold leg and the downcomer, where the “PTS fluid-dynamics" is relevant. Available experimental data for validation of two-phase CFD simulation tools are reviewed and the capabilities of such tools to capture each basic phenomenon are discussed. Key conclusions show that several two-phase flow subphenomena are involved and can individually be simulated at least at a qualitative level, but the capability to simulate their interaction and the overall system performance is still limited. In the near term, one may envisage a simplified treatment of two-phase PTS transients by neglecting some effects which are not yet well controlled, leading to slightly conservative predictions.

E. Bodèle

P. Coste

M. Scheuerer

D. Mazzini

I. Tiselj

A. Martin

D. Lakehal

J.-M. Seynhaeve

R. Kyrki-Rajamäki

M. Ilvonen

An Overview of the Pressurized Thermal Shock Issue in the Context of the NURESIM Project

In this work, we report on Euler-Euler large eddy simulation (EELES) of dispersed bubbly flow in 
a square cross-sectional bubble column. Simulations are performed using the Neptune&#x0005f;CFD 
package, and results processed using the SALOME platform. The motivation to undertake this study is 
to check our implementation of the Smagorinsky subgrid-scale (SGS) model into Neptune&#x0005f;CFD. We 
outline all the physical models used, and we present instantaneous realizations of velocity and void fraction fields
 in order to illustrate the structure of the turbulence field, and long-time averaged results, to compare with 
 analogous simulations performed using the CFX-4 code and experimental data. The same physical models 
 and constants have been used in both the CFX-4 and Neptune&#x0005f;CFD codes, except the SGS 
 model, which is Smagorinsky in case of Neptune&#x0005f;CFD and a one-equation model in CFX-4. The 
 results obtained with EELES compare reasonably well with experiment, meaning in particular that the
  implementations have been successful. Some perspectives on the further use of EELES are also given.

B. Ni&#269;eno

M. Boucker

B. L. Smith

Euler-Euler Large Eddy Simulation of a Square Cross-Sectional Bubble
                         Column Using the Neptune&#x0005f;CFD Code

This paper describes the modeling of boiling multisize bubbly flows and its application to
                  the simulation of the DEBORA experiment. We follow the method proposed originally by 
                  Kamp, assuming a given mathematical expression for the bubble diameter pdf. The original model
                   is completed by the addition of some new terms for vapor compressibility and phase change. The
                    liquid-to-interface heat transfer term, which essentially determines the bubbles condensation rate 
                    in the DEBORA experiment, is also modeled with care. First numerical results realized with the 
                    Neptune&#x0005F;CFD code are presented and discussed.

J&#233;r&#244;me M. Lavi&#233;ville

Modeling of Multisize Bubbly Flow and Application to the 
                        Simulation of Boiling Flows with the Neptune_CFD Code

This paper concerns the model of a polydispersed bubble population in the frame of an ensemble averaged two-phase flow formulation. The ability of the moment density approach to represent bubble population size distribution within a multi-dimensional CFD code based on the two-fluid model is studied. Two different methods describing the polydispersion are presented: (i) a moment density method, developed at IRSN, to model the bubble size distribution function and (ii) a population balance method considering several different velocity fields of the gaseous phase. The first method is implemented in the Neptune_CFD code, whereas the second method is implemented in the CFD code ANSYS/CFX. Both methods consider coalescence and breakup phenomena and momentum interphase transfers related to drag and lift forces. Air-water bubbly flows in a vertical pipe with obstacle of the TOPFLOW experiments series performed at FZD are then used as simulations test cases. The numerical results, obtained with Neptune_CFD and with ANSYS/CFX, allow attesting the validity of the approaches. Perspectives concerning the improvement of the models, their validation, as well as the extension of their applicability range are discussed.

P. Ruyer

M. Beyer

H.-M. Prasser

N. Seiler

CFD Simulation of Polydispersed Bubbly Two-Phase Flow around an Obstacle

In our work in 2008, we evaluated the aptitude of the code Neptune_CFD to reproduce the incidence of a structure topped by vanes on a boiling layer, within the framework of the Neptune project. The objective was to reproduce the main effects of the spacer grids. The turbulence of the liquid phase was modeled by a first-order <svg style="vertical-align:-0.0pt;width:13.3125px;" id="M1" height="11.175" version="1.1" viewBox="0 0 13.3125 11.175" width="13.3125" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"><g transform="matrix(.017,-0,0,-.017,.062,11.113)"><path id="x1D43E" d="M764 650l-7 -26q-61 -10 -91.5 -22.5t-76.5 -48.5l-235 -184q104 -152 208 -276q31 -37 54.5 -48.5t68.5 -16.5l-7 -28h-156q-138 170 -213 284q-19 29 -33.5 35t-33.5 -7l-29 -173q-15 -75 -5.5 -90.5t74.5 -20.5l-5 -28h-260l7 28q58 5 74 21.5t30 89.5l70 378&#xA;q12 68 2 83t-72 22l6 28h257l-7 -28q-62 -7 -76 -21.5t-27 -83.5l-32 -172q29 11 78 46q112 84 217 179q18 16 23.5 25.5t-1 14.5t-26.5 8l-30 4l5 28h249z" /></g></svg>-<svg style="vertical-align:-0.1638pt;width:7.0999999px;" id="M2" height="7.9499998" version="1.1" viewBox="0 0 7.0999999 7.9499998" width="7.0999999" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"><g transform="matrix(.017,-0,0,-.017,.062,7.675)"><path id="x1D700" d="M387 375q0 -16 -17 -31t-29 -15q-9 0 -12 13q-13 74 -80 74q-36 0 -61 -24.5t-25 -56.5t24 -51.5t68 -19.5q33 0 47 2l2 -7l-32 -42q-20 2 -54 2q-45 0 -74 -21.5t-29 -60.5q0 -41 28 -65.5t73 -24.5q78 0 145 67l17 -23q-33 -47 -84.5 -75t-111.5 -28q-70 0 -114.5 35.5&#xA;t-44.5 92.5q0 46 38 78t95 45v2q-35 10 -54.5 33t-19.5 52q0 55 53 88.5t122 33.5q67 0 98.5 -23.5t31.5 -49.5z" /></g></svg> model. We show in this paper that this model is unable to describe the turbulence of 
rotating flows, in accordance with the theory. The objective of this paper is to improve the turbulence 
modeling of the liquid phase by a second turbulence model based on a <svg style="vertical-align:-5.76988pt;width:19.975px;" id="M3" height="18.362499" version="1.1" viewBox="0 0 19.975 18.362499" width="19.975" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"><g transform="matrix(.017,-0,0,-.017,.062,11.113)"><path id="x1D445" d="M627 18l-10 -26q-79 6 -116 27t-69 76q-41 71 -71 138q-13 29 -27.5 39t-42.5 10h-46l-27 -145q-13 -74 -2.5 -88.5t78.5 -20.5l-6 -28h-271l5 28q66 6 82.5 21.5t30.5 87.5l71 387q12 66 2 78.5t-77 19.5l8 28h233q102 0 147 -29q65 -43 65 -129q0 -69 -45.5 -117&#xA;t-115.5 -72q40 -86 66 -133q39 -68 65 -101q28 -37 73 -51zM491 483q0 67 -33.5 101t-91.5 34q-35 0 -51 -10q-13 -8 -20 -48l-45 -245h49q71 0 113 28q79 52 79 140z" /></g><g transform="matrix(.012,-0,0,-.012,11.013,15.188)"><path id="x1D456" d="M244 607q0 -25 -15.5 -43t-37.5 -18q-19 0 -32 13t-13 35q0 21 15 41t39 20q20 0 32 -14t12 -34zM222 91q-29 -33 -79 -68t-75 -35q-13 0 -19 7.5t-6 31t10 65.5l62 253q5 26 -1 26q-21 0 -72 -43l-13 24q43 40 91 68t71 28q30 0 10 -78l-71 -274q-8 -30 3 -30&#xA;q16 0 76 48z" /></g><g transform="matrix(.012,-0,0,-.012,14.261,15.188)"><path id="x1D457" d="M394 607q0 -25 -15.5 -43t-39.5 -18q-19 0 -32 13t-13 35t16 41.5t40 19.5q20 0 32 -13.5t12 -34.5zM360 349l-57 -283q-38 -188 -122 -263q-72 -64 -114 -64q-16 0 -30 10.5t-14 22.5q0 24 21 45q15 13 26 6q30 -20 54 -20q41 0 65 95q37 147 86 415q6 34 6.5 42.5&#xA;t-5.5 11.5q-18 0 -79 -54l-14 25q48 48 96.5 79t71.5 31q29 0 9 -99z" /></g></svg>-<svg style="vertical-align:-0.1638pt;width:7.0999999px;" id="M4" height="7.9499998" version="1.1" viewBox="0 0 7.0999999 7.9499998" width="7.0999999" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"><g transform="matrix(.017,-0,0,-.017,.062,7.675)"><use xlink:href="#x1D700" /></g></svg> approach. Results obtained on typical single-phase cases highlight the improvement of the prediction for all computed values. We tested the turbulence model <svg style="vertical-align:-5.76988pt;width:19.975px;" id="M5" height="18.362499" version="1.1" viewBox="0 0 19.975 18.362499" width="19.975" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"><g transform="matrix(.017,-0,0,-.017,.062,11.113)"><use xlink:href="#x1D445" /></g><g transform="matrix(.012,-0,0,-.012,11.013,15.188)"><use xlink:href="#x1D456" /></g><g transform="matrix(.012,-0,0,-.012,14.261,15.188)"><use xlink:href="#x1D457" /></g></svg>-<svg style="vertical-align:-0.1638pt;width:7.0999999px;" id="M6" height="7.9499998" version="1.1" viewBox="0 0 7.0999999 7.9499998" width="7.0999999" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"><g transform="matrix(.017,-0,0,-.017,.062,7.675)"><use xlink:href="#x1D700" /></g></svg> implemented in the code versus typical adiabatic two-phase flow experiments. We check that the simulations with the Reynolds stress transport model (RSTM) give satisfactory results in a simple geometry as compared to a <svg style="vertical-align:-0.0pt;width:13.3125px;" id="M7" height="11.175" version="1.1" viewBox="0 0 13.3125 11.175" width="13.3125" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"><g transform="matrix(.017,-0,0,-.017,.062,11.113)"><use xlink:href="#x1D43E" /></g></svg>-<svg style="vertical-align:-0.1638pt;width:7.0999999px;" id="M8" height="7.9499998" version="1.1" viewBox="0 0 7.0999999 7.9499998" width="7.0999999" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"><g transform="matrix(.017,-0,0,-.017,.062,7.675)"><use xlink:href="#x1D700" /></g></svg> model: this point is crucial before calculating rod bundle geometries where the <svg style="vertical-align:-0.0pt;width:13.3125px;" id="M9" height="11.175" version="1.1" viewBox="0 0 13.3125 11.175" width="13.3125" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"><g transform="matrix(.017,-0,0,-.017,.062,11.113)"><use xlink:href="#x1D43E" /></g></svg>-<svg style="vertical-align:-0.1638pt;width:7.0999999px;" id="M10" height="7.9499998" version="1.1" viewBox="0 0 7.0999999 7.9499998" width="7.0999999" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"><g transform="matrix(.017,-0,0,-.017,.062,7.675)"><use xlink:href="#x1D700" /></g></svg> model may fail.

S. Mimouni

F. Archambeau

J. Laviéville

C. Morel

A Second-Order Turbulence Model Based on a Reynolds Stress Approach
                         for Two-Phase Flow—Part I: Adiabatic Cases

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<p><em>Science and Technology of Nuclear Installations</em> publishes research on issues related to the nuclear industry, particularly the installations of nuclear technology, and aims to promote development in the area of nuclear sciences and technologies.</p>


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<p>Professor Michael I. Ojovan is the Chief Editor of the journal, and is currently based at the University of Sheffield, UK. He is known for many innovations in nuclear research, including metallic and glass-composite materials for nuclear waste immobilisation.</p>


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Science and Technology of Nuclear Installations

Computational Fluid Dynamics for Gas-Liquid Flows

Computational Fluid Dynamics for Gas-Liquid Flows

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