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

Authors: D. Bestion; H. Anglart; D. Caraghiaur; P. Péturaud; B. Smith; M. Andreani; B. Niceno; E. Krepper; D. Lucas; F. Moretti; M. C. Galassi; J. Macek; L. Vyskocil; B. Koncar; G. Hazi
Journal: Science and Technology of Nuclear Installations
 (2009)

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## Abstract

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.


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## Body

## 1. Introduction

The NURESIM
Integrated Project of the
6th European Framework Programme is envisaged to
provide the initial step towards a common European Standard Software Platform
for modeling, recording, exchanging, and recovering data for nuclear reactors
simulations. Key objectives of NURESIM include the integration of advanced
physical models in a shared, open software platform, incorporating the latest
advances in reactor core physics, thermal hydraulics, and coupled multiphysics
modeling. The specific objectives of NURESIM are to initiate the development
of the next generation of experimentally validated, “best-estimate” tools with
improved prediction capabilities, standardization, and robustness to address
current and future needs of industry, reactor safety organizations, academic,
government, and private institutions.The overall
objective of NURESIM thermal-hydraulic subproject is to improve the understanding
and the predictive capabilities of the simulation tools for key two-phase flow
thermal-hydraulic processes that can occur in nuclear reactors, focusing on two
high priority issues, the critical heat flux (CHF), and the pressurized thermal
shock (PTS). This overall objective has resulted from the conclusions of the
EUROFASTNET [1] concerted action, which established a priority list of 44
industrial needs, and the results of the ASTAR [2], and ECORA [3] projects of
the 5th Framework Programme are considered as inputs for NURESIM. The initial
framework for performing the tasks is provided by the Neptune [4–6] two-phase CFD module, which is being developed by CEA and EDF, under
the cosponsorship of AREVA-NP and IRSN. Other CFD tools such as CFX or FLUENT
are also used within the NURESIM project. This paper focuses on the CHF
investigations and primarily on the use of the two-phase CFD for both departure
from nucleate boiling (DNB) and dryout investigations.This paper
presents a review of existing experimental data bases which can be used for
validation of the two-phase CFD application to critical heat flux (CHF)
investigations with respect to nuclear reactors. The phenomenology of DNB and dryout
is detailed identifying all basic flow processes which require a specific modeling
in CFD tool. The resulting programme of work is given, and the current state of
the art of the modeling is presented.


## 2. The Multiscale Analysis of CHF

Four basic spatial scales encountered in
thermal-hydraulic phenomena relevant to nuclear power plants:(i)
system scales, which are addressed
by zero- and one-dimensional flow models for pipes, pumps, valves, breaks, and
control systems together with CFD methods for porous media;(ii)
component-scales, which are
addressed by CFD methods for porous media (typically for the core of a reactor
or for the steam generators with a minimum spatial resolution in the case of
the subchannel analysis);(iii)
mesoscales, which are addressed by
computational fluid dynamics (CFDs) methods in open medium, including
turbulence models, using either Reynolds-averaged simulations (RANSs) or large
eddy simulation (LES);(iv)
microscales, which are addressed
by direct numerical simulation (DNS) and interface tracking methods (ITMs) that
focus on a very small domain (e.g., a domain containing a few bubbles or
droplets).In CHF investigations, the present industrial methods mainly
use the component scale with 3D modeling of core assemblies using in the hot
assembly the subchannel analysis. Large-scale experiments having the real
geometry of the reactor assembly are used to develop empirical correlation for
the CHF as function of flow variables which are averaged over the cross-section
of a subchannel. The NURESIM-TH activities regarding CHF aim at using two-phase
CFD as a tool for understanding boiling flow processes, in order to
subsequently help new fuel assembly design and to develop better CHF
predictions in both PWR and BWR. A “local
predictive approach” may be envisaged for the long term where CHF
correlations would be based on local (mesoscale) T/H parameters provided by CFD.
If the processes leading to DNB and dryout are well understood, the CHF correlation
will be physically based, but one may also develop empirical correlations if
some phenomena are not clearly identified.Considering the rather low maturity
of two-phase CFD, a general methodology was proposed by a Writing Group of the
OECD-CSNI (see Bestion et al. 2006 [7]) to apply it to a reactor issue with successive steps:(i)
identification of all important
flow processes of the application,(ii)
selecting a basic model,(iii)
filtering turbulent scales and
two-phase intermittency scales,(iv)
identification of local interface structure,(v)
modeling interfacial transfers,(vi)
modeling turbulent transfers,(vii)
modeling wall transfers,(viii)
use
of finer scale simulations for modeling,(ix)
identification of validation and verification
test cases with possibly some demonstration test cases.The choice of a
validation test matrix and of the basic modeling approach should be consistent
with each other since there must be enough measured physical parameters to be
able to validate separately each sensitive process modeled in the equations.The identification
of the basic flow processes related to both DNB and dryout and a review of available
experimental data were performed before selecting a basic model and defining a
development and validation programme. Next sections will present this initial
work and will conclude on the present state of the art in the modeling within
the NURESIM project.


## 3. Departure from Nucleate Boiling

### 3.1. The DNB Phenomenology

Departure from nucleate boiling is
the main governing critical heat flux mechanism for pressurized water reactors. A huge amount of work has been
devoted to the DNB in the past decades but the evaluation of the CHF still
relies on fully empirical methods.Rod bundles with spacer grids are
tested in real conditions with the fuel assembly geometry and the same flow T/H
conditions as in the reactor. Such experiments are very expensive and time
consuming but necessary to determine the CHF behaviour of any new fuel assembly
design.The reason of this situation is
that the phenomenology of convective boiling and DNB is very complex, and many
small-scale processes are not well understood. It is very likely that phenomena
occurring at various scales play a role; one can distinguish three scales for
reactor DNB phenomenology.(i)
Themacroscale refers to
phenomena at the scale of the subchannel (e.g., about 1 cm).
Macroscale phenomena are modeled in subchannel analysis codes.(ii)
Themesoscale refers to flow
processes responsible for the profiles of the main flow parameters within
subchannels (e.g., about
0.1 or 1 mm). Mesoscale flow processes can be modeled in two-phase CFD
simulation tools.(iii)
Microscale phenomena occur at the scale of the smallest bubbles or nuclei and can only be
numerically simulated by direct numerical simulation (DNS) tools and interface tracking methods (ITMs).A nonexhaustive list of flow
processes at the various scales is given here below.MICROSCALE Phenomena
(i)
Activation of nucleation sites.(ii)
Evolution of active sites density
with increasing power.(iii)
Growing of attached bubbles.(iv)
Sliding of attached bubbles along
heating wall.(v)
Coalescence of attached bubbles.(vi)
Extension of dry patch.(vii)
Effects of wall conductivity and heat capacity.(viii)
Detachment of bubbles.(ix)
Rewetting after detachment.(x)
Mutual influence of neighboring
nucleation sites.(xi)
Influence of flow characteristics on
local processes: external convective velocity.(xii)
Behaviour of detached bubbles:
coalescence, migration.(xiii)
Interactions between detached
bubbles.(xiv)
Forces between detached bubbles and
liquid flow.(xv)
Formation of high-void layer if
bubbles cannot escape due to counter current flow limitation (CCFL) type
phenomenon and behaviour of the thin liquid film which vaporizes below the
bubble layer.MESOSCALE Phenomena
(i)
Wall to fluid heat transfer in
subcooled boiling: liquid heating, vaporization, quenching.(ii)
Transport and dispersion of bubbles.(iii)
Vaporization-condensation of
bubbles.(iv)
Coalescence and breakup of bubbles.(v)
Turbulent transfers of heat and momentum
within liquid.(vi)
Effects of polydispersion of bubbles
on interfacial transfers(vii)
Local effects of grids: enhanced
turbulence and flow rotation.MACROSCALE Phenomena
(i)
Mixing between subchannels,
cross-flows, turbulence.(ii)
Grid spacers effects on mixing
between sub-channels.(iii)
Effects of cross-sectional averaged pressureP, mass flux, G, and quality Xth, on DNB occurrence.(iv)
Effects of nonuniform heat flux on
DNB occurrence.(v)
Effects of spacer grids on DNB
occurrence.
Two-phase CFD predictions should be
compared to relevant experimental data in order to validate all mesoscale flow
processes, in geometrical and T/H conditions preferably representative of the
industrial ones. This will bring a better understanding of the effects of the
mesoscale phenomena on the CHF occurrence. Moreover, microscale flow phenomena
should also be better understood for developing physically based closure laws
in the CFD approach. In this purpose, any experimental information on such
microscale phenomena or any DNS simulations may be used to improve the CFD
simulation tool. However, this project did not bring enough information to
build a physically based DNB criterion. Nevertheless, CFD simulations of
boiling flowup to DNB have the potentiality to predict some mesoscale effects
on flow conditions at the wall such as the development of two-phase boundary
layers, or spacer grid effects, which are not seen by the subchannel analysis
and current empirical CHF models. One may at least expect that the effects of
nonuniform axial heat flux, which are now empirically modeled, may be simply
seen by local conditions resulting from CFD predictions. Also the effects of
spacer grid design on flow conditions seen by the wall may be described at the
CFD scale whereas subchannel analysis can only describe the associated pressure
loss, the additional mixing between neighboring subchannels and the effect on
CHF when experimental data are available.


### 3.2. Review of the Data Basis for DNB

The following data sources were
reviewed and analysed with respect to their interest for validating CFD tools
used in DNB investigations. Table2 summarizes the characteristics of the
experiments, the measured parameters, and the correspondence between mesoscale
phenomena and the available data. Some of these experiments provide data which
may be far from flow conditions encountered in reactors when CHF occurs. However,
they allow a separate effect validation with increasing complexity of the
phenomenology. The “local predictive approach”
requires that all local (mesoscale) T/H parameters be correctly predicted by
CFD since the CHF criterion will be expressed as a function of them.There are single phase liquid data
(AGATE) which may be used as a first step in the validation of turbulence
models in a rod bundle with spacer grids. Some air-water bubbly flow data
(DEDALE, TOPFLOW) may be used as a first step in the validation of models for
bubble transport and dispersion, coalescence and breakup, effects of
polydispersion on interfacial forces, and momentum turbulent transfers. Boiling
flow data in simple geometry (DEBORA, ASU, Purdue data, KAERI data) either in
steam-water of Freon (R12, R113) may then be used to further validate in more
representative conditions (pressure is either atmospheric or similar to reactor
conditions) the models already investigated in air-water conditions, with additional
effects of wall heat transfers, turbulent heat transfers and interfacial heat,
and mass transfers due to vaporization and condensation. Some DEBORA data
were recorded in conditions which were very close to CHF occurrence. Effects of
spacers are also validated in boiling flow conditions with the DEBORA-Promoter data.
BFBT data are used to validate the void distribution of a steam-water boiling
flow in a real BWR rod bundle geometry. These data are unique and can also be
used to some extent for DNB investigations if one considers the low quality data.
LWL data in a real-rod bundle of a WWER reactor finally allow a global
validation of the boiling flowup to DNB.


#### 3.2.1. DEDALE Air-Water Bubbly Flow Tests

DEDALE is an adiabatic air-water two-phase
experimental programme performed at EDF/DER [8]. DEDALE aimed at analyzing the
axial development of a bubbly flow in a vertical pipe up to the transition to
slug flow and creating an accurate and reliable data base with local
information for the validation of dynamics-related models in CFD tools [9, 10].


#### 3.2.2. DEBORA Boiling Flow Tests in a Heated Pipe

The DEBORA experiment [11] was
carried out at the Commissariat à l’Energie Atomique, Grenoble, France, to
provide a reliable local data base on boiling phenomena (up to DNB) in PWR T/H
condition ranges, in order to eventually achieve a better understanding and
prediction of DNB-type boiling. Calculations
and analysis with Neptune are reported in [12].The test section is an electrically
heated vertical tube with upward R12 boiling flow simulating PWR in-core T/H
conditions, with local measurements along a diameter within the outlet tube
cross section of both steam phase characteristics (void fraction, interfacial
area concentration, bubble size, and mean axial velocity) and liquid phase
parameter (temperature).


#### 3.2.3. DEBORA Tests in a Heated Pipe with a Turbulence
Promoter/Enhancer (Swirl Flows)

The “DEBORA-Promoter” tests (see Figure1) with a vane type turbulence promoter/enhancer were carried out in addition
to the previous ones, to characterize the two-phase boiling flow behaviour in a
complex geometry representing the industrial one. The test section is similar
to the previous one, with addition of a turbulence promoter/enhancer located
inside the tube either 23.5D or 10D upstream from the end of the heated
length.Figure 1
“DEBORA-promoter” geometry.Validation of CFD tools on these
tests provides additional information on the effect of spacer grid wake on the
mixing of bubbles generated at the wall and on the effects of the flow rotation
on the void repartition; simulations of such tests with Neptune_CFD were
presented [13].


#### 3.2.4. AGATE Single-Phase Tests

The AGATE experiment has been developed in CEA Grenoble.
Two-test sections
were used:(i)
“AGATE-Grid” consists of a5×5 rod bundle inside
a squared-section housing with a mixing vane grid;(ii)
“AGATE-Promoter” with a similar
geometry as “DEBORA-Promoter” one (i.e., pipe with a 3-vane turbulence
enhancer).Nonheated water flows upwardly in
the vertical test section, and velocity measurements are made using laser Doppler
anemometry (LDA). Both the mean velocity and velocity fluctuations are measured
in order to investigate the effects of the grid or promoter.The data allow to validate the
turbulence modeling with spacer grid (or turbulence promoter/enhancer) effects
in single-phase conditions. They were used for validation of a 1D model withk&ε model
[14].


#### 3.2.5. QLOVICE Visualisation Tests

QLOVICE tests are being performed
by CEA in order to investigate basic processes associated with DNB. QLOVICE is
a visualization of pool boiling with high-speed video-camera.(i)
A transparent heated bottom wall
allows to visualise the bubble nucleation and detachment.(ii)
A side window allows to see bubble
behaviour after detachment.First tests were
performed and have clearly shown the dry patch evolutions. It was observed(i)
bubble sliding along the heating
wall before detachment,(ii)
sudden large size dry patch
extension observed followed by a wall rewetting,(iii)
many bubble clusters,(iv)
interactions between neighbouring
nucleation sites.Two main processes are assumed to
play a significant (dominant) role on the DNB occurrence: a sudden extension of
dry patch up to DNB or a CCFL type phenomenon with bubbles which cannot escape
from wall after detachment. However, no conclusion can be presently drawn on
the dominant process.


#### 3.2.6. Arizona State University (ASU)
Tests of Boiling Flow in a Heated Annular Channel

Experiments of turbulent subcooled
flow in a vertical annular channel were carried out at the Arizona State University
[15–18]
to provide detailed information on average flow structure, temperature, and gas
and liquid flow fields in fully developed nucleate boiling, as well as on
turbulent variables controlling transport mechanisms. In the experiment, R-113
was the working fluid.Validation of CFD tools on ASU
tests provides information on the steam production at the wall in subcooled
boiling, on the interfacial forces responsible for the void profiles, on
interfacial heat and mass transfers, on interfacial area concentration
evolution, and on turbulence in the bubbly boundary layer.Measurements used simultaneously a
two-component laser Doppler velocimetry for liquid velocity and a fast response cold-wire for
temperature field, as well as a dual-sensor fiber optic probe for the vapour
fraction and vapour axial velocity.A comparison of Neptune simulations with the early tests was presented in [17].


#### 3.2.7. Purdue University (PU/NE) Tests of Boiling Flow in a Heated Annular
Channel

Experiments have been carried out
at the School of Nuclear Engineering of Purdue University in an internally
heated annulus to provide local measurements of void fraction, interfacial area
concentration, and interfacial velocity in subcooled boiling [19–22]. Water
at atmospheric pressure was the working fluid. Influence of inlet liquid
temperature, heat flux, and inlet liquid velocity on local flow parameters was
specially investigated. The chosen geometry and set of conditions were aimed at
scaling the conditions of a BWR. Although properties at 70 bar could not be
represented, geometrical, hydrodynamic, and thermal similarities for the flow
boiling processes were preserved.Earlier tests [19, 20] include
information on the axial evolution of the measured variables, and preliminary
studies [19] addressed the dependence of bubble size before detachment on the
axial position.Visual observations of the boiling
processes provided essential information on the displacement between the
location of net vapor generation (NVG) and the location of bubble detachment [19].
More recent photographic studies of bubble lift-off diameters have been
presented by Situ et al. [22–24].A few analyses to test the validity
of CFD codes have been carried out using the earlier series of test data [25, 26].


#### 3.2.8. KAERI Tests of Boiling Flow in a Heated Annular
Channel

Experiments have been carried out
at the Korea Atomic Energy Research Institute (KAERI) in an internally heated
annulus to provide local measurements of void fraction and phase velocities in
subcooled boiling [27–29].
Water at low pressure (1 to 2 bar) is the working
fluid. The aim was to provide a database for subcooled boiling modeling,
including aspects such as force balances for departing vapour bubbles and bubble
population balance.Measurements of void fraction and
bubble velocity were taken using a double-sensor conductivity probe. Liquid
velocities were measured by a Pitot tube, correcting for the effect of bubbles
[30]. Data included radial distributions of void fraction, axial liquid, and
vapour velocity, interfacial area concentration (three tests only, [28]), Sauter mean diameter (three tests in [28], two more in
[30]), bubble concentration (bubbles/unit volume, three tests only, [28]).Tests have been used for assessing
the CFX-4 code [27–31] especially the performance of an extension
to 15 bubble classes of the MUSIG model.


#### 3.2.9. Experimental Data on TOPFLOW Loop on Two Phase
Flow in a Vertical Tube

The structure of an adiabatic air-water and of
steam-water flow with reduced condensation and with slight subcooling in a
vertical pipe of 195.3 mm inner diameter (DN200) was studied using wire-mesh
sensors. The experiments were performed at the two-phase FLOW test
facility (TOPFLOW) [32] of Safety Research of Forschungszentrum
Dresden-Rossendorf e.V., which can be operated for pressure up to 7 MPa and
temperature up to 286°C. Air-water data at ambient conditions and steam-water data
under nearly adiabatic conditions as well as with slightly subcooled water are
available for pressures of 1 and 2 MPa. Wire-mesh sensors can characterize the
shape of large bubbles, since they acquire the phase distribution in the entire
cross-section. By changing the injecting position of the gas supply during the
next test, it is possible to study the evolution of the flow structure along
the flow path in the DN200 vertical pipe.Function and construction of wire-mesh sensors are
described in [33]. Cross-section averaged gas fractions as well as radial gas
fraction profiles can be calculated [34]. Radial gas velocity profiles were
obtained by means of a point-to-point cross-correlation between the signals of
both sensors placed in a distance of 63 mm behind each other [35]. Bubble size
distributions were extracted from the measuring data using the algorithm
described by [36].A technique to analyse the evolution of the flow
structure is the calculation of radial gas fraction profiles decomposed
according to bubble size classes [34]. The method was used to decompose the
radial gas fraction profiles into 4 bubble size classes: class 1 from 0 to 4.8 mm equivalent diameter, class 2 from 4.8 to 5.8 mm, class 3 from 5.8 to
7.0 mm, and class 4
above 7.0 mm. Here, 5.8 mm is the critical diameter for the inversion of the
lift-force according to Tomiyama [37] for air bubbles in water at ambient
temperature. It decreases in case of steam-water flow with increasing pressure.A visualisation (see Figure2) is done by generating
virtual side projections and side views of virtual central cuts from the mesh-sensor data
according to the algorithms described in [38]. For each mesh-sensor data set,
virtual side views and virtual centre cuts are combined in the same image. The
height-to-width relation of the depicted bubbles is nearly respected in this
image. It is visible how bubbles injected at the periphery move towards the
centre of the pipe in case of the reference experiment without subcooling,
while in the experiment with condensation the bubble density decreases with
growing distance from the injection device.Figure 2
Virtual side projections (left halves of the columns) and side
views of virtual central cuts (right halves) of the mesh-sensor data (from
[39]).The data can be used to test the complex interaction
of local bubble distributions, bubble size distributions, and local heat
and mass transfer. The lateral motion of the bubbles in a shear flow, bubble
coalescence, and
breakup and the phase transfer can be observed by measurements along the pipe. For
example, the radial distribution of bubbles strongly depends on their diameter.
For a vertical cocurrent upwards flow,
smaller bubbles tend to move towards the wall, while large bubbles are
preferably found in the centre. Details on the steam-water experiments and
investigations on the modeling of such flows are  presented by Lucas and Prasser [39].


#### 3.2.10. BFBT Data on Void Fraction Distribution in BWR
Fuel Assembly

Experimental tests for measuring the void fraction distribution inside boiling
water reactor (BWR) fuel assemblies have been conducted by the Nuclear Power
Engineering Corporation (NUPEC), Tokyo,
Japan, by the use of an experimental facility
referred to as BFBT (BWR Full-size Fine-mesh Bundle Tests). Data provided by
such facility have been initially used for subchannel code assessment [40] and
are currently being used for CFD code assessment in the framework of an
OECD-NEA/US-NRC Benchmark. X-ray
CT
scanner and X-ray densitometers are employed to
measure the void fraction distribution in a BWR full-scale fuel assembly under
steady-state and transient conditions.The test loop has a full range of steady-state void fraction testing
capabilities over BWR operating conditions. Unsteady characteristics, flow
changes, power changes, and complicated BWR operational transients are
simulated too.The test section consists of a full-scale BWR fuel assembly simulator,
which is made of electrically heated rods able to reproduce the actual power
profiles generated by nuclear fission. The instrumentation allows measurements
of temperature, flow rate, pressure and, mainly, void fraction.An X-ray CT  scanner, consisting of an X-ray tube and 512 detectors, is employed to measure
the void fraction in the upper part of the test section in steady-state
conditions. The void fraction data have a0.3×0.3mm2 resolution.
Such a high resolution makes those data useful for CFD code validation.


#### 3.2.11. Large Water Loop Experimental Test Facility

The large water loop has been built
at the NUCLEAR MACHINERY PLANT,ŠKODA, Plzen Ltd.,
Plzen, Czech Republic. The loop is a nonactive
pressurized-water equipment with technological and thermal parameters
corresponding to those of PWR. The possible parameter ranges are suitable for
all types of pressurized water reactors. The CHF experimental facility (a part
of large water loop) has been designed for the research of CHF in water flow
through a bundle of electrically heated rods.The test sections were formed by 7
or 19 parallel electrically heated rods with external diameters of 9 mm. Axial
and radial uniform or nonuniform heat flux distribution and water up flow were
used in the tests. The rods were with direct heating were specially
manufactured with axially varying wall thickness while maintaining a constant
outside diameter to achieve nonuniform axial heat flux. The rods (3500 mm long)
were placed in regular hexagonal geometry with a pitch of 12.5–13 mm. Critical
conditions were obtained under constant thermal-hydraulic conditions by
gradually increasing heat input.


### 3.3.
Development and Validation Programme of Work

Based on data and manpower
availability, the following programme of validation was planed to be performed
within the NURESIM project (see Table1). Validation (V) tests allow to draw
conclusions on the validity of some models whereas demonstration (D) tests check
the capability of a software to simulate a complex process.Table 1
Planed validation and demonstration calculations within
NURESIM project.
Validation tests and demonstration
testsValidation DemonstrationMain interest of
validationDEBORAVInvestigations of wall heat transfer
modelsDEBORA tests close to CHF conditionsVLooking for processes responsible
for void accumulation close to the wallDEBORA and/or TOPFLOW polydispersed
dataVValidation of the method of
statistical momentsTOPFLOW polydispersed dataVValidation of the MUSIG methodDEBORA polydispersed dataVValidation of the MUSIG methodDEDALEVEvaluation of LES simulation of
bubbly flowASU boiling water experimentVValidation of wall function Evaluation of LES simulation of
boiling bubbly flowBFBT experimentsVDEvaluation of models controlling
void distribution in actual core geometryDEBORAVInvestigations of wall heat transfer
modelsLarge water loop (LWL)VDEvaluation of CHF prediction CFD in
actual core geometryTable 2
Correspondence between the data sources relative to DNB investigations and the
basic phenomena at the mesoscale.
α: local void fraction, Ai: local interfacial
area density, δb: local average bubble diameter, Fδ: bubble size distribution, Vlz:
axial mean liquid velovity, vlz′: rms axial liquid velovity, Vbz:
axial mean bubble velovity, Tl: liquid temperature, Tw:
wall temperature, Tw′: rms value of wall temperature, Vlx:
radial component of mean liquid velovity, vlx′: rms radial component
of liquid velovity, ϕcrit:
critical heat flux.
DEDALEDEBORADEBORA PromoterAGATE (&AGATE-promoter)ASUPURDUE boilingKAERI boilingTOPFLOWBFBTLWLGeometryTubeL = 6 m
D = 0.0381 mTubeD = 0.0192 mTube
+ promoterRod bundle (tube  +promoter)AnnulusDi = 15.78 mm
De = 38.02 mAnnulusDi = 19.1 m
De = 38.1 mAnnulusDi = 19 mm
De = 37.5 mmTubeD = 195.3 mmRod
bundleRod
bundleFluid/flow conditionsAir-water adiabaticR12R12waterR113Steam-waterSteam-waterAir-water and steam-waterSteam/ waterSteam/ waterLocal measur-ementsα, Ai, δb, Fδ, Vlz, vlz′, Vbz  at Z/D=8, 55 & 155α, Ai, Tl, Tw, δb,
Vbz,α, Ai, Tl, Tw, δb,
Vbz,Vlz, vlz′Vlx, vlx′α,  Vlz, vlz′, vlx′,
Vbz, Tw, Tw′, turbulent heat fluxesα, Ai, δb, Vbz at z/D=52.6
+visual observationsα, Ai, δb, Vlz, Vbzα, δb, Vbz, FδαϕcritϕcritWall
to fluid heatXXXXBubbles transport and dispersionXXXXXXVaporization-condensationXXXXXCoalescence and breakup of bubblesXXXXXXTurbulent Transfers of heat and momentumXXXXXXXXEffects
of polydispersionXXEffects
of gridsXXCombined
effects in real geometryBWRPWR
WWERTable2 presents the correspondence
between the above data sources and the basic phenomena at the mesoscale.The present data basis is not
sufficient to validate all phenomena of interest, and the main defaults are the
lack of turbulence data in high void bubbly flow and the lack of data for
validation of the heat flux partitioning at the wall in convective nucleate
boiling. More generally no data can provide information on microscale phenomena
which makes the development of physically based models in the near wall region difficult.


### 3.4. State of the Art in DNB Modeling
Within the NURESIM Project

The
following state of the art on the modeling of two-phase flow up to DNB occurrence
results from the ongoing work in NURESIM which mainly addressed flow conditions
before DNB.(1)
Basic model: as boiling bubbly flows are encountered, the two-fluid
model is naturally used in this flow conditions to benefit from the possibility
to model all interfacial forces acting on the bubbles such as drag, lift,
turbulent dispersion, virtual mass, and wall forces which control the void
repartition in a boiling channel. The choice of the method to model
poly-dispersion effects remains partly open.(2)
Averaging or filtering equations: considering flow in a PWR core in
conditions close to nominal, when boiling occurs, a high velocity steady flow
regime takes place with times scales associated to the passage of bubbles being
very small (10−4, 10−3s) and with bubble diameter being
rather small (10−5 to 10−3m) compared to the hydraulic
diameter (about 10−2m). These are perfect conditions to use a time
average or ensemble average of equations as usually done in the RANS approach.
All turbulent fluctuations and two-phase intermittency scales can be filtered
since they are significantly smaller than scales of the mean flow. The use of a
large eddy simulation (LES) approach may allow to simulate bubble dispersion by
liquid turbulence instead of modeling it. This LES approach has been used with
success in bubble plume simulations but cannot replace the RANS approach for convective
boiling flows.(3)
Identification of local interface structure: there is a unique
interfacial structure corresponding to a dispersed gas phase in a continuous
liquid. As long as bubbly flow is encountered, there is no need to develop an
identification of the local flow regime and there is no need to use an ITM.
Going to DNB occurrence, a gas layer appears and a criterion must be
implemented for identifying this occurrence.  A very simple criterion based on the local
void fraction was applied to LWL tests. However, the description of the
interface structure may require addition of transport equations such as
interfacial area transport (IAT) or bubble number density transport. More
generally, the method of the statistical moments (MMSs) can be used to
characterise the poly-dispersion of the vapour phase with a bubble size
spectrum. Another approach of the poly-dispersion is to use a multigroup model (MUSIG
method) with mass (and momentum) equations written for several bubble sizes.
These two methods are being used, evaluated, and compared on both DEBORA and
TOPFLOW tests. The MUSIG method with several mass equations for different
bubble sizes and at least two momentum equations has shown good capabilities
for capturing all qualitative effects in TOPFLOW vertical pipe tests. The MMS
has been applied to a subcooled boiling DEBORA test, demonstrating a
significant effect of polydispersion on the condensation predictions. MUSIG and
MMS still have to be further evaluated.(4)
Momentum transfer control the void distribution and it is necessary to
model all the forces acting on the bubbles. The virtual mass force is not
expected to play a very important role, and rather reliable models exist for
the drag force. More effort should be paid to the modeling and validation of
both lift and turbulent dispersion forces since available models are still
often tuned. In particular, since the lift force may depend on the bubble size,
it is now necessary to model poly-dispersion to take this into account.(5)
Turbulent transfers: liquid turbulence plays a very important role in
boiling flows. It influences liquid temperature diffusion, bubble dispersion, bubble
detachment, bubble coalescence, and breakup which affect the interfacial area. Then,
the liquid turbulent scales have to be predicted correctly to model all these
processes and this will require additional transport equations. Thek-epsilon
or SST method was used with some success in DEBORA and TOPFLOW. A bubble column
was simulated with some success using the NURESIM platform with a SGS model by
Niceno et al. [41]. However, LES was not found well adapted to DEDALE test
simulations or boiling flow simulations.(6)
Wall-to-fluid transfers: modeling of velocity profiles in the near-wall
boiling region was improved by implementing the two-phase wall function in
momentum equations. Models were validated on ASU boiling flow tests [42]. More
specific wall functions need to be developed for boiling flow for energy
equations. Such wall functions should be able to provide converged solution
with a reasonably coarse nodalization close to a heating wall. Present versions
of CFD tools provide models for heat flux partitioning into convection to
liquid, vaporization, and quenching. Such models are using several correlations
for density of nucleation sites, bubble departure diameter, and frequency of
bubble departure, which are not separately validated by the present data basis.
The comparison of void fraction close to the wall with measurements in DEBORA
and ASU tests gives an indication that the amount of vaporization is reasonably
predicted and the comparison of Sauter mean bubble diameter close to the wall
in DEBORA tests gives an indication that the bubble departure diameter is also reasonably
predicted, but more detailed experimental data in prototypical convective
boiling conditions would be necessary for a more rigorous validation. The
absence of a physically based DNB criterion is also a difficulty, and one may
argue that microscale effects may affect the CHF even more than the mesoscale
effects which are modeled. In the far future, such microscale phenomena might
be clarified by microvisualisation techniques of by DNS prediction. In medium
term, an empirical DNB criterion may be envisaged which will require final
validation on very representative conditions. Today a simple criterion based on
a limit void fraction at the wall is implemented in Neptune_CFD which allows a
switch from nucleate boiling to film boiling heat transfer model but it is not
satisfactory and did not predict LWL CHF tests very well.(7)
First demonstration test cases were performed with Neptune_CFD
calculations of critical heat flux tests in the LWL loop which is prototypical
of WWER type core assemblies. Computational grid consists of 150 000 hexahedral
cells. Although the simulation is not fully successful quantitatively, Neptune showed the capability to model boiling flow in a
complex industrial geometry and in reactor flow conditions up to CHF. CHF
occurrence was predicted at the right location but with errors from 1% to 25%
on the heat flux, which shows how far we still are from the final goal of the “local
predictive approach.”


## 3.1. The DNB Phenomenology

Departure from nucleate boiling is
the main governing critical heat flux mechanism for pressurized water reactors. A huge amount of work has been
devoted to the DNB in the past decades but the evaluation of the CHF still
relies on fully empirical methods.Rod bundles with spacer grids are
tested in real conditions with the fuel assembly geometry and the same flow T/H
conditions as in the reactor. Such experiments are very expensive and time
consuming but necessary to determine the CHF behaviour of any new fuel assembly
design.The reason of this situation is
that the phenomenology of convective boiling and DNB is very complex, and many
small-scale processes are not well understood. It is very likely that phenomena
occurring at various scales play a role; one can distinguish three scales for
reactor DNB phenomenology.(i)
Themacroscale refers to
phenomena at the scale of the subchannel (e.g., about 1 cm).
Macroscale phenomena are modeled in subchannel analysis codes.(ii)
Themesoscale refers to flow
processes responsible for the profiles of the main flow parameters within
subchannels (e.g., about
0.1 or 1 mm). Mesoscale flow processes can be modeled in two-phase CFD
simulation tools.(iii)
Microscale phenomena occur at the scale of the smallest bubbles or nuclei and can only be
numerically simulated by direct numerical simulation (DNS) tools and interface tracking methods (ITMs).A nonexhaustive list of flow
processes at the various scales is given here below.MICROSCALE Phenomena
(i)
Activation of nucleation sites.(ii)
Evolution of active sites density
with increasing power.(iii)
Growing of attached bubbles.(iv)
Sliding of attached bubbles along
heating wall.(v)
Coalescence of attached bubbles.(vi)
Extension of dry patch.(vii)
Effects of wall conductivity and heat capacity.(viii)
Detachment of bubbles.(ix)
Rewetting after detachment.(x)
Mutual influence of neighboring
nucleation sites.(xi)
Influence of flow characteristics on
local processes: external convective velocity.(xii)
Behaviour of detached bubbles:
coalescence, migration.(xiii)
Interactions between detached
bubbles.(xiv)
Forces between detached bubbles and
liquid flow.(xv)
Formation of high-void layer if
bubbles cannot escape due to counter current flow limitation (CCFL) type
phenomenon and behaviour of the thin liquid film which vaporizes below the
bubble layer.MESOSCALE Phenomena
(i)
Wall to fluid heat transfer in
subcooled boiling: liquid heating, vaporization, quenching.(ii)
Transport and dispersion of bubbles.(iii)
Vaporization-condensation of
bubbles.(iv)
Coalescence and breakup of bubbles.(v)
Turbulent transfers of heat and momentum
within liquid.(vi)
Effects of polydispersion of bubbles
on interfacial transfers(vii)
Local effects of grids: enhanced
turbulence and flow rotation.MACROSCALE Phenomena
(i)
Mixing between subchannels,
cross-flows, turbulence.(ii)
Grid spacers effects on mixing
between sub-channels.(iii)
Effects of cross-sectional averaged pressureP, mass flux, G, and quality Xth, on DNB occurrence.(iv)
Effects of nonuniform heat flux on
DNB occurrence.(v)
Effects of spacer grids on DNB
occurrence.
Two-phase CFD predictions should be
compared to relevant experimental data in order to validate all mesoscale flow
processes, in geometrical and T/H conditions preferably representative of the
industrial ones. This will bring a better understanding of the effects of the
mesoscale phenomena on the CHF occurrence. Moreover, microscale flow phenomena
should also be better understood for developing physically based closure laws
in the CFD approach. In this purpose, any experimental information on such
microscale phenomena or any DNS simulations may be used to improve the CFD
simulation tool. However, this project did not bring enough information to
build a physically based DNB criterion. Nevertheless, CFD simulations of
boiling flowup to DNB have the potentiality to predict some mesoscale effects
on flow conditions at the wall such as the development of two-phase boundary
layers, or spacer grid effects, which are not seen by the subchannel analysis
and current empirical CHF models. One may at least expect that the effects of
nonuniform axial heat flux, which are now empirically modeled, may be simply
seen by local conditions resulting from CFD predictions. Also the effects of
spacer grid design on flow conditions seen by the wall may be described at the
CFD scale whereas subchannel analysis can only describe the associated pressure
loss, the additional mixing between neighboring subchannels and the effect on
CHF when experimental data are available.


## 3.2. Review of the Data Basis for DNB

The following data sources were
reviewed and analysed with respect to their interest for validating CFD tools
used in DNB investigations. Table2 summarizes the characteristics of the
experiments, the measured parameters, and the correspondence between mesoscale
phenomena and the available data. Some of these experiments provide data which
may be far from flow conditions encountered in reactors when CHF occurs. However,
they allow a separate effect validation with increasing complexity of the
phenomenology. The “local predictive approach”
requires that all local (mesoscale) T/H parameters be correctly predicted by
CFD since the CHF criterion will be expressed as a function of them.There are single phase liquid data
(AGATE) which may be used as a first step in the validation of turbulence
models in a rod bundle with spacer grids. Some air-water bubbly flow data
(DEDALE, TOPFLOW) may be used as a first step in the validation of models for
bubble transport and dispersion, coalescence and breakup, effects of
polydispersion on interfacial forces, and momentum turbulent transfers. Boiling
flow data in simple geometry (DEBORA, ASU, Purdue data, KAERI data) either in
steam-water of Freon (R12, R113) may then be used to further validate in more
representative conditions (pressure is either atmospheric or similar to reactor
conditions) the models already investigated in air-water conditions, with additional
effects of wall heat transfers, turbulent heat transfers and interfacial heat,
and mass transfers due to vaporization and condensation. Some DEBORA data
were recorded in conditions which were very close to CHF occurrence. Effects of
spacers are also validated in boiling flow conditions with the DEBORA-Promoter data.
BFBT data are used to validate the void distribution of a steam-water boiling
flow in a real BWR rod bundle geometry. These data are unique and can also be
used to some extent for DNB investigations if one considers the low quality data.
LWL data in a real-rod bundle of a WWER reactor finally allow a global
validation of the boiling flowup to DNB.


### 3.2.1. DEDALE Air-Water Bubbly Flow Tests

DEDALE is an adiabatic air-water two-phase
experimental programme performed at EDF/DER [8]. DEDALE aimed at analyzing the
axial development of a bubbly flow in a vertical pipe up to the transition to
slug flow and creating an accurate and reliable data base with local
information for the validation of dynamics-related models in CFD tools [9, 10].


### 3.2.2. DEBORA Boiling Flow Tests in a Heated Pipe

The DEBORA experiment [11] was
carried out at the Commissariat à l’Energie Atomique, Grenoble, France, to
provide a reliable local data base on boiling phenomena (up to DNB) in PWR T/H
condition ranges, in order to eventually achieve a better understanding and
prediction of DNB-type boiling. Calculations
and analysis with Neptune are reported in [12].The test section is an electrically
heated vertical tube with upward R12 boiling flow simulating PWR in-core T/H
conditions, with local measurements along a diameter within the outlet tube
cross section of both steam phase characteristics (void fraction, interfacial
area concentration, bubble size, and mean axial velocity) and liquid phase
parameter (temperature).


### 3.2.3. DEBORA Tests in a Heated Pipe with a Turbulence
Promoter/Enhancer (Swirl Flows)

The “DEBORA-Promoter” tests (see Figure1) with a vane type turbulence promoter/enhancer were carried out in addition
to the previous ones, to characterize the two-phase boiling flow behaviour in a
complex geometry representing the industrial one. The test section is similar
to the previous one, with addition of a turbulence promoter/enhancer located
inside the tube either 23.5D or 10D upstream from the end of the heated
length.Figure 1
“DEBORA-promoter” geometry.Validation of CFD tools on these
tests provides additional information on the effect of spacer grid wake on the
mixing of bubbles generated at the wall and on the effects of the flow rotation
on the void repartition; simulations of such tests with Neptune_CFD were
presented [13].


### 3.2.4. AGATE Single-Phase Tests

The AGATE experiment has been developed in CEA Grenoble.
Two-test sections
were used:(i)
“AGATE-Grid” consists of a5×5 rod bundle inside
a squared-section housing with a mixing vane grid;(ii)
“AGATE-Promoter” with a similar
geometry as “DEBORA-Promoter” one (i.e., pipe with a 3-vane turbulence
enhancer).Nonheated water flows upwardly in
the vertical test section, and velocity measurements are made using laser Doppler
anemometry (LDA). Both the mean velocity and velocity fluctuations are measured
in order to investigate the effects of the grid or promoter.The data allow to validate the
turbulence modeling with spacer grid (or turbulence promoter/enhancer) effects
in single-phase conditions. They were used for validation of a 1D model withk&ε model
[14].


### 3.2.5. QLOVICE Visualisation Tests

QLOVICE tests are being performed
by CEA in order to investigate basic processes associated with DNB. QLOVICE is
a visualization of pool boiling with high-speed video-camera.(i)
A transparent heated bottom wall
allows to visualise the bubble nucleation and detachment.(ii)
A side window allows to see bubble
behaviour after detachment.First tests were
performed and have clearly shown the dry patch evolutions. It was observed(i)
bubble sliding along the heating
wall before detachment,(ii)
sudden large size dry patch
extension observed followed by a wall rewetting,(iii)
many bubble clusters,(iv)
interactions between neighbouring
nucleation sites.Two main processes are assumed to
play a significant (dominant) role on the DNB occurrence: a sudden extension of
dry patch up to DNB or a CCFL type phenomenon with bubbles which cannot escape
from wall after detachment. However, no conclusion can be presently drawn on
the dominant process.


### 3.2.6. Arizona State University (ASU)
Tests of Boiling Flow in a Heated Annular Channel

Experiments of turbulent subcooled
flow in a vertical annular channel were carried out at the Arizona State University
[15–18]
to provide detailed information on average flow structure, temperature, and gas
and liquid flow fields in fully developed nucleate boiling, as well as on
turbulent variables controlling transport mechanisms. In the experiment, R-113
was the working fluid.Validation of CFD tools on ASU
tests provides information on the steam production at the wall in subcooled
boiling, on the interfacial forces responsible for the void profiles, on
interfacial heat and mass transfers, on interfacial area concentration
evolution, and on turbulence in the bubbly boundary layer.Measurements used simultaneously a
two-component laser Doppler velocimetry for liquid velocity and a fast response cold-wire for
temperature field, as well as a dual-sensor fiber optic probe for the vapour
fraction and vapour axial velocity.A comparison of Neptune simulations with the early tests was presented in [17].


### 3.2.7. Purdue University (PU/NE) Tests of Boiling Flow in a Heated Annular
Channel

Experiments have been carried out
at the School of Nuclear Engineering of Purdue University in an internally
heated annulus to provide local measurements of void fraction, interfacial area
concentration, and interfacial velocity in subcooled boiling [19–22]. Water
at atmospheric pressure was the working fluid. Influence of inlet liquid
temperature, heat flux, and inlet liquid velocity on local flow parameters was
specially investigated. The chosen geometry and set of conditions were aimed at
scaling the conditions of a BWR. Although properties at 70 bar could not be
represented, geometrical, hydrodynamic, and thermal similarities for the flow
boiling processes were preserved.Earlier tests [19, 20] include
information on the axial evolution of the measured variables, and preliminary
studies [19] addressed the dependence of bubble size before detachment on the
axial position.Visual observations of the boiling
processes provided essential information on the displacement between the
location of net vapor generation (NVG) and the location of bubble detachment [19].
More recent photographic studies of bubble lift-off diameters have been
presented by Situ et al. [22–24].A few analyses to test the validity
of CFD codes have been carried out using the earlier series of test data [25, 26].


### 3.2.8. KAERI Tests of Boiling Flow in a Heated Annular
Channel

Experiments have been carried out
at the Korea Atomic Energy Research Institute (KAERI) in an internally heated
annulus to provide local measurements of void fraction and phase velocities in
subcooled boiling [27–29].
Water at low pressure (1 to 2 bar) is the working
fluid. The aim was to provide a database for subcooled boiling modeling,
including aspects such as force balances for departing vapour bubbles and bubble
population balance.Measurements of void fraction and
bubble velocity were taken using a double-sensor conductivity probe. Liquid
velocities were measured by a Pitot tube, correcting for the effect of bubbles
[30]. Data included radial distributions of void fraction, axial liquid, and
vapour velocity, interfacial area concentration (three tests only, [28]), Sauter mean diameter (three tests in [28], two more in
[30]), bubble concentration (bubbles/unit volume, three tests only, [28]).Tests have been used for assessing
the CFX-4 code [27–31] especially the performance of an extension
to 15 bubble classes of the MUSIG model.


### 3.2.9. Experimental Data on TOPFLOW Loop on Two Phase
Flow in a Vertical Tube

The structure of an adiabatic air-water and of
steam-water flow with reduced condensation and with slight subcooling in a
vertical pipe of 195.3 mm inner diameter (DN200) was studied using wire-mesh
sensors. The experiments were performed at the two-phase FLOW test
facility (TOPFLOW) [32] of Safety Research of Forschungszentrum
Dresden-Rossendorf e.V., which can be operated for pressure up to 7 MPa and
temperature up to 286°C. Air-water data at ambient conditions and steam-water data
under nearly adiabatic conditions as well as with slightly subcooled water are
available for pressures of 1 and 2 MPa. Wire-mesh sensors can characterize the
shape of large bubbles, since they acquire the phase distribution in the entire
cross-section. By changing the injecting position of the gas supply during the
next test, it is possible to study the evolution of the flow structure along
the flow path in the DN200 vertical pipe.Function and construction of wire-mesh sensors are
described in [33]. Cross-section averaged gas fractions as well as radial gas
fraction profiles can be calculated [34]. Radial gas velocity profiles were
obtained by means of a point-to-point cross-correlation between the signals of
both sensors placed in a distance of 63 mm behind each other [35]. Bubble size
distributions were extracted from the measuring data using the algorithm
described by [36].A technique to analyse the evolution of the flow
structure is the calculation of radial gas fraction profiles decomposed
according to bubble size classes [34]. The method was used to decompose the
radial gas fraction profiles into 4 bubble size classes: class 1 from 0 to 4.8 mm equivalent diameter, class 2 from 4.8 to 5.8 mm, class 3 from 5.8 to
7.0 mm, and class 4
above 7.0 mm. Here, 5.8 mm is the critical diameter for the inversion of the
lift-force according to Tomiyama [37] for air bubbles in water at ambient
temperature. It decreases in case of steam-water flow with increasing pressure.A visualisation (see Figure2) is done by generating
virtual side projections and side views of virtual central cuts from the mesh-sensor data
according to the algorithms described in [38]. For each mesh-sensor data set,
virtual side views and virtual centre cuts are combined in the same image. The
height-to-width relation of the depicted bubbles is nearly respected in this
image. It is visible how bubbles injected at the periphery move towards the
centre of the pipe in case of the reference experiment without subcooling,
while in the experiment with condensation the bubble density decreases with
growing distance from the injection device.Figure 2
Virtual side projections (left halves of the columns) and side
views of virtual central cuts (right halves) of the mesh-sensor data (from
[39]).The data can be used to test the complex interaction
of local bubble distributions, bubble size distributions, and local heat
and mass transfer. The lateral motion of the bubbles in a shear flow, bubble
coalescence, and
breakup and the phase transfer can be observed by measurements along the pipe. For
example, the radial distribution of bubbles strongly depends on their diameter.
For a vertical cocurrent upwards flow,
smaller bubbles tend to move towards the wall, while large bubbles are
preferably found in the centre. Details on the steam-water experiments and
investigations on the modeling of such flows are  presented by Lucas and Prasser [39].


### 3.2.10. BFBT Data on Void Fraction Distribution in BWR
Fuel Assembly

Experimental tests for measuring the void fraction distribution inside boiling
water reactor (BWR) fuel assemblies have been conducted by the Nuclear Power
Engineering Corporation (NUPEC), Tokyo,
Japan, by the use of an experimental facility
referred to as BFBT (BWR Full-size Fine-mesh Bundle Tests). Data provided by
such facility have been initially used for subchannel code assessment [40] and
are currently being used for CFD code assessment in the framework of an
OECD-NEA/US-NRC Benchmark. X-ray
CT
scanner and X-ray densitometers are employed to
measure the void fraction distribution in a BWR full-scale fuel assembly under
steady-state and transient conditions.The test loop has a full range of steady-state void fraction testing
capabilities over BWR operating conditions. Unsteady characteristics, flow
changes, power changes, and complicated BWR operational transients are
simulated too.The test section consists of a full-scale BWR fuel assembly simulator,
which is made of electrically heated rods able to reproduce the actual power
profiles generated by nuclear fission. The instrumentation allows measurements
of temperature, flow rate, pressure and, mainly, void fraction.An X-ray CT  scanner, consisting of an X-ray tube and 512 detectors, is employed to measure
the void fraction in the upper part of the test section in steady-state
conditions. The void fraction data have a0.3×0.3mm2 resolution.
Such a high resolution makes those data useful for CFD code validation.


### 3.2.11. Large Water Loop Experimental Test Facility

The large water loop has been built
at the NUCLEAR MACHINERY PLANT,ŠKODA, Plzen Ltd.,
Plzen, Czech Republic. The loop is a nonactive
pressurized-water equipment with technological and thermal parameters
corresponding to those of PWR. The possible parameter ranges are suitable for
all types of pressurized water reactors. The CHF experimental facility (a part
of large water loop) has been designed for the research of CHF in water flow
through a bundle of electrically heated rods.The test sections were formed by 7
or 19 parallel electrically heated rods with external diameters of 9 mm. Axial
and radial uniform or nonuniform heat flux distribution and water up flow were
used in the tests. The rods were with direct heating were specially
manufactured with axially varying wall thickness while maintaining a constant
outside diameter to achieve nonuniform axial heat flux. The rods (3500 mm long)
were placed in regular hexagonal geometry with a pitch of 12.5–13 mm. Critical
conditions were obtained under constant thermal-hydraulic conditions by
gradually increasing heat input.


## 3.2.1. DEDALE Air-Water Bubbly Flow Tests

DEDALE is an adiabatic air-water two-phase
experimental programme performed at EDF/DER [8]. DEDALE aimed at analyzing the
axial development of a bubbly flow in a vertical pipe up to the transition to
slug flow and creating an accurate and reliable data base with local
information for the validation of dynamics-related models in CFD tools [9, 10].


## 3.2.2. DEBORA Boiling Flow Tests in a Heated Pipe

The DEBORA experiment [11] was
carried out at the Commissariat à l’Energie Atomique, Grenoble, France, to
provide a reliable local data base on boiling phenomena (up to DNB) in PWR T/H
condition ranges, in order to eventually achieve a better understanding and
prediction of DNB-type boiling. Calculations
and analysis with Neptune are reported in [12].The test section is an electrically
heated vertical tube with upward R12 boiling flow simulating PWR in-core T/H
conditions, with local measurements along a diameter within the outlet tube
cross section of both steam phase characteristics (void fraction, interfacial
area concentration, bubble size, and mean axial velocity) and liquid phase
parameter (temperature).


## 3.2.3. DEBORA Tests in a Heated Pipe with a Turbulence
Promoter/Enhancer (Swirl Flows)

The “DEBORA-Promoter” tests (see Figure1) with a vane type turbulence promoter/enhancer were carried out in addition
to the previous ones, to characterize the two-phase boiling flow behaviour in a
complex geometry representing the industrial one. The test section is similar
to the previous one, with addition of a turbulence promoter/enhancer located
inside the tube either 23.5D or 10D upstream from the end of the heated
length.Figure 1
“DEBORA-promoter” geometry.Validation of CFD tools on these
tests provides additional information on the effect of spacer grid wake on the
mixing of bubbles generated at the wall and on the effects of the flow rotation
on the void repartition; simulations of such tests with Neptune_CFD were
presented [13].


## 3.2.4. AGATE Single-Phase Tests

The AGATE experiment has been developed in CEA Grenoble.
Two-test sections
were used:(i)
“AGATE-Grid” consists of a5×5 rod bundle inside
a squared-section housing with a mixing vane grid;(ii)
“AGATE-Promoter” with a similar
geometry as “DEBORA-Promoter” one (i.e., pipe with a 3-vane turbulence
enhancer).Nonheated water flows upwardly in
the vertical test section, and velocity measurements are made using laser Doppler
anemometry (LDA). Both the mean velocity and velocity fluctuations are measured
in order to investigate the effects of the grid or promoter.The data allow to validate the
turbulence modeling with spacer grid (or turbulence promoter/enhancer) effects
in single-phase conditions. They were used for validation of a 1D model withk&ε model
[14].


## 3.2.5. QLOVICE Visualisation Tests

QLOVICE tests are being performed
by CEA in order to investigate basic processes associated with DNB. QLOVICE is
a visualization of pool boiling with high-speed video-camera.(i)
A transparent heated bottom wall
allows to visualise the bubble nucleation and detachment.(ii)
A side window allows to see bubble
behaviour after detachment.First tests were
performed and have clearly shown the dry patch evolutions. It was observed(i)
bubble sliding along the heating
wall before detachment,(ii)
sudden large size dry patch
extension observed followed by a wall rewetting,(iii)
many bubble clusters,(iv)
interactions between neighbouring
nucleation sites.Two main processes are assumed to
play a significant (dominant) role on the DNB occurrence: a sudden extension of
dry patch up to DNB or a CCFL type phenomenon with bubbles which cannot escape
from wall after detachment. However, no conclusion can be presently drawn on
the dominant process.


## 3.2.6. Arizona State University (ASU)
Tests of Boiling Flow in a Heated Annular Channel

Experiments of turbulent subcooled
flow in a vertical annular channel were carried out at the Arizona State University
[15–18]
to provide detailed information on average flow structure, temperature, and gas
and liquid flow fields in fully developed nucleate boiling, as well as on
turbulent variables controlling transport mechanisms. In the experiment, R-113
was the working fluid.Validation of CFD tools on ASU
tests provides information on the steam production at the wall in subcooled
boiling, on the interfacial forces responsible for the void profiles, on
interfacial heat and mass transfers, on interfacial area concentration
evolution, and on turbulence in the bubbly boundary layer.Measurements used simultaneously a
two-component laser Doppler velocimetry for liquid velocity and a fast response cold-wire for
temperature field, as well as a dual-sensor fiber optic probe for the vapour
fraction and vapour axial velocity.A comparison of Neptune simulations with the early tests was presented in [17].


## 3.2.7. Purdue University (PU/NE) Tests of Boiling Flow in a Heated Annular
Channel

Experiments have been carried out
at the School of Nuclear Engineering of Purdue University in an internally
heated annulus to provide local measurements of void fraction, interfacial area
concentration, and interfacial velocity in subcooled boiling [19–22]. Water
at atmospheric pressure was the working fluid. Influence of inlet liquid
temperature, heat flux, and inlet liquid velocity on local flow parameters was
specially investigated. The chosen geometry and set of conditions were aimed at
scaling the conditions of a BWR. Although properties at 70 bar could not be
represented, geometrical, hydrodynamic, and thermal similarities for the flow
boiling processes were preserved.Earlier tests [19, 20] include
information on the axial evolution of the measured variables, and preliminary
studies [19] addressed the dependence of bubble size before detachment on the
axial position.Visual observations of the boiling
processes provided essential information on the displacement between the
location of net vapor generation (NVG) and the location of bubble detachment [19].
More recent photographic studies of bubble lift-off diameters have been
presented by Situ et al. [22–24].A few analyses to test the validity
of CFD codes have been carried out using the earlier series of test data [25, 26].


## 3.2.8. KAERI Tests of Boiling Flow in a Heated Annular
Channel

Experiments have been carried out
at the Korea Atomic Energy Research Institute (KAERI) in an internally heated
annulus to provide local measurements of void fraction and phase velocities in
subcooled boiling [27–29].
Water at low pressure (1 to 2 bar) is the working
fluid. The aim was to provide a database for subcooled boiling modeling,
including aspects such as force balances for departing vapour bubbles and bubble
population balance.Measurements of void fraction and
bubble velocity were taken using a double-sensor conductivity probe. Liquid
velocities were measured by a Pitot tube, correcting for the effect of bubbles
[30]. Data included radial distributions of void fraction, axial liquid, and
vapour velocity, interfacial area concentration (three tests only, [28]), Sauter mean diameter (three tests in [28], two more in
[30]), bubble concentration (bubbles/unit volume, three tests only, [28]).Tests have been used for assessing
the CFX-4 code [27–31] especially the performance of an extension
to 15 bubble classes of the MUSIG model.


## 3.2.9. Experimental Data on TOPFLOW Loop on Two Phase
Flow in a Vertical Tube

The structure of an adiabatic air-water and of
steam-water flow with reduced condensation and with slight subcooling in a
vertical pipe of 195.3 mm inner diameter (DN200) was studied using wire-mesh
sensors. The experiments were performed at the two-phase FLOW test
facility (TOPFLOW) [32] of Safety Research of Forschungszentrum
Dresden-Rossendorf e.V., which can be operated for pressure up to 7 MPa and
temperature up to 286°C. Air-water data at ambient conditions and steam-water data
under nearly adiabatic conditions as well as with slightly subcooled water are
available for pressures of 1 and 2 MPa. Wire-mesh sensors can characterize the
shape of large bubbles, since they acquire the phase distribution in the entire
cross-section. By changing the injecting position of the gas supply during the
next test, it is possible to study the evolution of the flow structure along
the flow path in the DN200 vertical pipe.Function and construction of wire-mesh sensors are
described in [33]. Cross-section averaged gas fractions as well as radial gas
fraction profiles can be calculated [34]. Radial gas velocity profiles were
obtained by means of a point-to-point cross-correlation between the signals of
both sensors placed in a distance of 63 mm behind each other [35]. Bubble size
distributions were extracted from the measuring data using the algorithm
described by [36].A technique to analyse the evolution of the flow
structure is the calculation of radial gas fraction profiles decomposed
according to bubble size classes [34]. The method was used to decompose the
radial gas fraction profiles into 4 bubble size classes: class 1 from 0 to 4.8 mm equivalent diameter, class 2 from 4.8 to 5.8 mm, class 3 from 5.8 to
7.0 mm, and class 4
above 7.0 mm. Here, 5.8 mm is the critical diameter for the inversion of the
lift-force according to Tomiyama [37] for air bubbles in water at ambient
temperature. It decreases in case of steam-water flow with increasing pressure.A visualisation (see Figure2) is done by generating
virtual side projections and side views of virtual central cuts from the mesh-sensor data
according to the algorithms described in [38]. For each mesh-sensor data set,
virtual side views and virtual centre cuts are combined in the same image. The
height-to-width relation of the depicted bubbles is nearly respected in this
image. It is visible how bubbles injected at the periphery move towards the
centre of the pipe in case of the reference experiment without subcooling,
while in the experiment with condensation the bubble density decreases with
growing distance from the injection device.Figure 2
Virtual side projections (left halves of the columns) and side
views of virtual central cuts (right halves) of the mesh-sensor data (from
[39]).The data can be used to test the complex interaction
of local bubble distributions, bubble size distributions, and local heat
and mass transfer. The lateral motion of the bubbles in a shear flow, bubble
coalescence, and
breakup and the phase transfer can be observed by measurements along the pipe. For
example, the radial distribution of bubbles strongly depends on their diameter.
For a vertical cocurrent upwards flow,
smaller bubbles tend to move towards the wall, while large bubbles are
preferably found in the centre. Details on the steam-water experiments and
investigations on the modeling of such flows are  presented by Lucas and Prasser [39].


## 3.2.10. BFBT Data on Void Fraction Distribution in BWR
Fuel Assembly

Experimental tests for measuring the void fraction distribution inside boiling
water reactor (BWR) fuel assemblies have been conducted by the Nuclear Power
Engineering Corporation (NUPEC), Tokyo,
Japan, by the use of an experimental facility
referred to as BFBT (BWR Full-size Fine-mesh Bundle Tests). Data provided by
such facility have been initially used for subchannel code assessment [40] and
are currently being used for CFD code assessment in the framework of an
OECD-NEA/US-NRC Benchmark. X-ray
CT
scanner and X-ray densitometers are employed to
measure the void fraction distribution in a BWR full-scale fuel assembly under
steady-state and transient conditions.The test loop has a full range of steady-state void fraction testing
capabilities over BWR operating conditions. Unsteady characteristics, flow
changes, power changes, and complicated BWR operational transients are
simulated too.The test section consists of a full-scale BWR fuel assembly simulator,
which is made of electrically heated rods able to reproduce the actual power
profiles generated by nuclear fission. The instrumentation allows measurements
of temperature, flow rate, pressure and, mainly, void fraction.An X-ray CT  scanner, consisting of an X-ray tube and 512 detectors, is employed to measure
the void fraction in the upper part of the test section in steady-state
conditions. The void fraction data have a0.3×0.3mm2 resolution.
Such a high resolution makes those data useful for CFD code validation.


## 3.2.11. Large Water Loop Experimental Test Facility

The large water loop has been built
at the NUCLEAR MACHINERY PLANT,ŠKODA, Plzen Ltd.,
Plzen, Czech Republic. The loop is a nonactive
pressurized-water equipment with technological and thermal parameters
corresponding to those of PWR. The possible parameter ranges are suitable for
all types of pressurized water reactors. The CHF experimental facility (a part
of large water loop) has been designed for the research of CHF in water flow
through a bundle of electrically heated rods.The test sections were formed by 7
or 19 parallel electrically heated rods with external diameters of 9 mm. Axial
and radial uniform or nonuniform heat flux distribution and water up flow were
used in the tests. The rods were with direct heating were specially
manufactured with axially varying wall thickness while maintaining a constant
outside diameter to achieve nonuniform axial heat flux. The rods (3500 mm long)
were placed in regular hexagonal geometry with a pitch of 12.5–13 mm. Critical
conditions were obtained under constant thermal-hydraulic conditions by
gradually increasing heat input.


## 3.3.
Development and Validation Programme of Work

Based on data and manpower
availability, the following programme of validation was planed to be performed
within the NURESIM project (see Table1). Validation (V) tests allow to draw
conclusions on the validity of some models whereas demonstration (D) tests check
the capability of a software to simulate a complex process.Table 1
Planed validation and demonstration calculations within
NURESIM project.
Validation tests and demonstration
testsValidation DemonstrationMain interest of
validationDEBORAVInvestigations of wall heat transfer
modelsDEBORA tests close to CHF conditionsVLooking for processes responsible
for void accumulation close to the wallDEBORA and/or TOPFLOW polydispersed
dataVValidation of the method of
statistical momentsTOPFLOW polydispersed dataVValidation of the MUSIG methodDEBORA polydispersed dataVValidation of the MUSIG methodDEDALEVEvaluation of LES simulation of
bubbly flowASU boiling water experimentVValidation of wall function Evaluation of LES simulation of
boiling bubbly flowBFBT experimentsVDEvaluation of models controlling
void distribution in actual core geometryDEBORAVInvestigations of wall heat transfer
modelsLarge water loop (LWL)VDEvaluation of CHF prediction CFD in
actual core geometryTable 2
Correspondence between the data sources relative to DNB investigations and the
basic phenomena at the mesoscale.
α: local void fraction, Ai: local interfacial
area density, δb: local average bubble diameter, Fδ: bubble size distribution, Vlz:
axial mean liquid velovity, vlz′: rms axial liquid velovity, Vbz:
axial mean bubble velovity, Tl: liquid temperature, Tw:
wall temperature, Tw′: rms value of wall temperature, Vlx:
radial component of mean liquid velovity, vlx′: rms radial component
of liquid velovity, ϕcrit:
critical heat flux.
DEDALEDEBORADEBORA PromoterAGATE (&AGATE-promoter)ASUPURDUE boilingKAERI boilingTOPFLOWBFBTLWLGeometryTubeL = 6 m
D = 0.0381 mTubeD = 0.0192 mTube
+ promoterRod bundle (tube  +promoter)AnnulusDi = 15.78 mm
De = 38.02 mAnnulusDi = 19.1 m
De = 38.1 mAnnulusDi = 19 mm
De = 37.5 mmTubeD = 195.3 mmRod
bundleRod
bundleFluid/flow conditionsAir-water adiabaticR12R12waterR113Steam-waterSteam-waterAir-water and steam-waterSteam/ waterSteam/ waterLocal measur-ementsα, Ai, δb, Fδ, Vlz, vlz′, Vbz  at Z/D=8, 55 & 155α, Ai, Tl, Tw, δb,
Vbz,α, Ai, Tl, Tw, δb,
Vbz,Vlz, vlz′Vlx, vlx′α,  Vlz, vlz′, vlx′,
Vbz, Tw, Tw′, turbulent heat fluxesα, Ai, δb, Vbz at z/D=52.6
+visual observationsα, Ai, δb, Vlz, Vbzα, δb, Vbz, FδαϕcritϕcritWall
to fluid heatXXXXBubbles transport and dispersionXXXXXXVaporization-condensationXXXXXCoalescence and breakup of bubblesXXXXXXTurbulent Transfers of heat and momentumXXXXXXXXEffects
of polydispersionXXEffects
of gridsXXCombined
effects in real geometryBWRPWR
WWERTable2 presents the correspondence
between the above data sources and the basic phenomena at the mesoscale.The present data basis is not
sufficient to validate all phenomena of interest, and the main defaults are the
lack of turbulence data in high void bubbly flow and the lack of data for
validation of the heat flux partitioning at the wall in convective nucleate
boiling. More generally no data can provide information on microscale phenomena
which makes the development of physically based models in the near wall region difficult.


## 3.4. State of the Art in DNB Modeling
Within the NURESIM Project

The
following state of the art on the modeling of two-phase flow up to DNB occurrence
results from the ongoing work in NURESIM which mainly addressed flow conditions
before DNB.(1)
Basic model: as boiling bubbly flows are encountered, the two-fluid
model is naturally used in this flow conditions to benefit from the possibility
to model all interfacial forces acting on the bubbles such as drag, lift,
turbulent dispersion, virtual mass, and wall forces which control the void
repartition in a boiling channel. The choice of the method to model
poly-dispersion effects remains partly open.(2)
Averaging or filtering equations: considering flow in a PWR core in
conditions close to nominal, when boiling occurs, a high velocity steady flow
regime takes place with times scales associated to the passage of bubbles being
very small (10−4, 10−3s) and with bubble diameter being
rather small (10−5 to 10−3m) compared to the hydraulic
diameter (about 10−2m). These are perfect conditions to use a time
average or ensemble average of equations as usually done in the RANS approach.
All turbulent fluctuations and two-phase intermittency scales can be filtered
since they are significantly smaller than scales of the mean flow. The use of a
large eddy simulation (LES) approach may allow to simulate bubble dispersion by
liquid turbulence instead of modeling it. This LES approach has been used with
success in bubble plume simulations but cannot replace the RANS approach for convective
boiling flows.(3)
Identification of local interface structure: there is a unique
interfacial structure corresponding to a dispersed gas phase in a continuous
liquid. As long as bubbly flow is encountered, there is no need to develop an
identification of the local flow regime and there is no need to use an ITM.
Going to DNB occurrence, a gas layer appears and a criterion must be
implemented for identifying this occurrence.  A very simple criterion based on the local
void fraction was applied to LWL tests. However, the description of the
interface structure may require addition of transport equations such as
interfacial area transport (IAT) or bubble number density transport. More
generally, the method of the statistical moments (MMSs) can be used to
characterise the poly-dispersion of the vapour phase with a bubble size
spectrum. Another approach of the poly-dispersion is to use a multigroup model (MUSIG
method) with mass (and momentum) equations written for several bubble sizes.
These two methods are being used, evaluated, and compared on both DEBORA and
TOPFLOW tests. The MUSIG method with several mass equations for different
bubble sizes and at least two momentum equations has shown good capabilities
for capturing all qualitative effects in TOPFLOW vertical pipe tests. The MMS
has been applied to a subcooled boiling DEBORA test, demonstrating a
significant effect of polydispersion on the condensation predictions. MUSIG and
MMS still have to be further evaluated.(4)
Momentum transfer control the void distribution and it is necessary to
model all the forces acting on the bubbles. The virtual mass force is not
expected to play a very important role, and rather reliable models exist for
the drag force. More effort should be paid to the modeling and validation of
both lift and turbulent dispersion forces since available models are still
often tuned. In particular, since the lift force may depend on the bubble size,
it is now necessary to model poly-dispersion to take this into account.(5)
Turbulent transfers: liquid turbulence plays a very important role in
boiling flows. It influences liquid temperature diffusion, bubble dispersion, bubble
detachment, bubble coalescence, and breakup which affect the interfacial area. Then,
the liquid turbulent scales have to be predicted correctly to model all these
processes and this will require additional transport equations. Thek-epsilon
or SST method was used with some success in DEBORA and TOPFLOW. A bubble column
was simulated with some success using the NURESIM platform with a SGS model by
Niceno et al. [41]. However, LES was not found well adapted to DEDALE test
simulations or boiling flow simulations.(6)
Wall-to-fluid transfers: modeling of velocity profiles in the near-wall
boiling region was improved by implementing the two-phase wall function in
momentum equations. Models were validated on ASU boiling flow tests [42]. More
specific wall functions need to be developed for boiling flow for energy
equations. Such wall functions should be able to provide converged solution
with a reasonably coarse nodalization close to a heating wall. Present versions
of CFD tools provide models for heat flux partitioning into convection to
liquid, vaporization, and quenching. Such models are using several correlations
for density of nucleation sites, bubble departure diameter, and frequency of
bubble departure, which are not separately validated by the present data basis.
The comparison of void fraction close to the wall with measurements in DEBORA
and ASU tests gives an indication that the amount of vaporization is reasonably
predicted and the comparison of Sauter mean bubble diameter close to the wall
in DEBORA tests gives an indication that the bubble departure diameter is also reasonably
predicted, but more detailed experimental data in prototypical convective
boiling conditions would be necessary for a more rigorous validation. The
absence of a physically based DNB criterion is also a difficulty, and one may
argue that microscale effects may affect the CHF even more than the mesoscale
effects which are modeled. In the far future, such microscale phenomena might
be clarified by microvisualisation techniques of by DNS prediction. In medium
term, an empirical DNB criterion may be envisaged which will require final
validation on very representative conditions. Today a simple criterion based on
a limit void fraction at the wall is implemented in Neptune_CFD which allows a
switch from nucleate boiling to film boiling heat transfer model but it is not
satisfactory and did not predict LWL CHF tests very well.(7)
First demonstration test cases were performed with Neptune_CFD
calculations of critical heat flux tests in the LWL loop which is prototypical
of WWER type core assemblies. Computational grid consists of 150 000 hexahedral
cells. Although the simulation is not fully successful quantitatively, Neptune showed the capability to model boiling flow in a
complex industrial geometry and in reactor flow conditions up to CHF. CHF
occurrence was predicted at the right location but with errors from 1% to 25%
on the heat flux, which shows how far we still are from the final goal of the “local
predictive approach.”


## 4. The DryOut

### 4.1. The DryOut Phenomenology

Annular
flow pattern usually is the predominant flow regime in upper core regions in boiling
water reactors. The limitation of the total power obtained from each assembly
is the occurrence of dryout. Increasing the heat flux above some critical value
can lead to dryout that is associated with a sudden increase in the wall
temperature, which, in turn, can destroy the cladding material and allow the
radiation releases into the primary system. The phenomenology of dryout in
annular mist flow was described in [43].The
liquid phase exists as a liquid film, which is attached to walls, and as
droplets, which are carried in the central part of the channel by the vapour
phase.The
mass flow rate in the liquid film is changing due to several mass transfer
mechanisms.(i)
Due to hydrodynamic forces acting on
the liquid film surface, certain amount of liquid from liquid film is entrained
into the vapour core.(ii)
Another mechanism that is causing
liquid film depletion is associated with evaporation due to heating applied to
walls.(iii)
These two mechanisms must be
counterbalanced by drop deposition from the vapour core to the liquid film
surface to avoid film dryout.There
are several possible mechanisms that have been postulated for dryout (Hewitt,
1982 [44]).(i)
The liquid film dries by progressive
entrainment and evaporation, which are prevailing in comparison to deposition,
and dryout occurs when the film has gone.(ii)
Formation of a dry patch within the
liquid film, causing such wall temperature increase that cannot be rewetted. In
some situations a sudden disruption of liquid film may occur beyond which the wall
surface is dry. The disruption mechanism is not fully understood yet, however,
hydrodynamic mechanisms for the disruption are postulated.
(a)
For very thin liquid films. dryout
occurs when the rate of evaporation at the surface exceeds the rate at which droplets
arrive at the surface due to deposition.(b)
For thicker liquid films, it is
postulated that dryout may occur due to vapour film formation under the liquid
film. The mechanism of forming this vapour film might be of the same type as
described for the DNB mechanisms.Annular
regime in boiling flow is characterized by a thin liquid film flowing on the
channel walls and a gas core flowing in the central part of the channel. The
droplets in the gas core represent a larger interfacial area than the liquid film
and thus can dominate heat and mass transport between the phases. System
pressure drop is increased by droplet acceleration in the gas core, and
depositing droplets contribute to corrosion by increasing local wall friction.To some
extent, the dryout is a more simple process than the DNB since one cannot list so many
microscale phenomena which may play a role. In particular, if one first focuses
on the first dryout scenario with entrainment and evaporation prevailing in
comparison to deposition, only mesoscale phenomena have to be considered.The
most important mesoscale phenomena and parameters in annular flow affecting the
occurrence of dryout are(1)
drop size,(2)
deposition of droplets,(3)
entrainment of droplets, and(4)
film thickness.Drop Size in Disperse Two-Phase Flow
Drop
size is an important parameter which affects the deposition rates and thus the
dryout phenomenon. It can be described by a size PDF,fd(d) defined as the
probability that a droplet from the distribution will have a diameter of d. It
is often required that drop size distribution is represented by a single
weighted mean size.Deposition Rate
Liquid
droplets carried by a turbulent gas stream will deposit on bounding walls.
Clearly, deposition rate will have an important influence on the dryout
occurrence.
It may
depend on several unresolved issues, such as turbulence-particle interactions
and drop breakup and coalescence.
Deposition
rate will depend on drop dispersion in turbulent flow where particle motion is
primarily governed by interactions with eddies of various scales. Depending on
the ratio of the particle response time to the eddy characteristic time, the
dispersion can have different characters. If this ratio is very small,
particles are following the continuous flow structure. When the ratio is close
to 1 (the time constants of eddies and particles are of the same range of
magnitude), the dispersion of drops can be even bigger than that observed in
the carrier fluid. Finally, for high values of the ratio particles remain
largely unaffected by eddies.
Typically,
drop deposition is associated with two mechanisms: the diffusion process and
the free-flight to the wall. For proper prediction of the deposition rate of
droplets, both these mechanisms have to be taken into account. In addition,
impinging conditions of a drop on a liquid surface have to be considered. When
a single droplet impinges a liquid film, various phenomena can occur. The
droplet can bounce from the surface or merge with the liquid film. Splash can
occur when the drop kinetic energy is high enough. For conditions typical for
BWRs, the liquid film is thin and the velocity of droplets is high, thus
splashing and mergence are the key phenomena involved.Entrainment Rate
Several
mechanisms of drop entrainment from the liquid film have been identified. The
dynamic impact of gas core causes generation of waves on the film surface, with
droplets being separated and entrained from the crests of these waves. The
creation and breakup of the disturbance waves play important roles in the drop
entrainment process. Another entrainment mechanism is associated with splashing
associated with drop deposition, as already mentioned in the previous section.
Finally, in a heated channel with nucleate boiling in the film, entrainment can
occur due to the action of vapour bubbles which induce splashing.Liquid Film Thickness
Calculation
of the liquid film thickness is necessary to predict the occurrence of dryout.
To obtain the liquid film thickness and velocity, it is necessary to solve the
mass and momentum conservation equations of the film in order to determine the
film flowrate and pressure drop. This requires proper modeling of deposition,
entrainment, and evaporation in mass equation and of the wall friction and
interfacial friction in the momentum equation which depend on the wave
structure of the film interface.


### 4.2. DryOut Data Basis

Early
experiments were focused on the measurements of the total power, which was
necessary for the dryout occurrence in a heated channel. A vast number of these
experiments were performed for different conduit geometries in different flow
conditions. The measurements for steam-water were done in round ducts, annuli,
and rod clusters. Measurements in annuli covered the pressures of 30, 50, and
70 bar (Becker and
Letzer [45]; Persson [46]). For a validation of models based on the analysis
of wall film flows, experimental data of pressure drops, including wall shear
stress and interfacial shear stress, which characterize
liquid film thickness and the onset of entrainment, respectively, are required.
Also, actual measurements of film flows, film thickness, wave amplitude,
frequencies, and wave velocities are needed for the validation. Moreover,
because complete physical models for droplet entrainment and droplet deposition
are still not available, experimental data of these are needed to develop
reliable correlations and/or computational models.Würtz [47] has reported more than 2700 pressure drop
measurements for steam-water and reviewed film flow measurements in steam-water
mixtures in annular flow (see also Cousins & Hewitt [48]). The film flows
were measured both in tubes and in annuli and recently, Adamson and Anglart
[49] provided high-pressure steam-water data.An extensive review of existing measurements of deposition
rate has been presented by Okawa et al. [50]. The deposition rates were
predominantly measured in air-water systems with low pressures (see also Govan
et al. [51]). The techniques employed are the double film extraction, thermal
method, and tracer methodIt was experimentally proven that the mode of the
deposition is dependent on the droplet size. Observations of droplet motion
(Andreussi [52]) show that larger droplets travel across the gas core at about
their initial velocity in a constant direction until they are deposited. This
mechanism of deposition has been called direct impaction. At higher gas
velocities where the droplets are comparatively smaller, the effect of the
initial momentum on droplet motion becomes negligible. In this case, the eddy
diffusion mechanism of deposition prevails. Bates and Sheriff [53] have
presented a summary of the previous work done on droplet size/velocity in
vertical annular air-water two-phase flow. The drop size distribution was
investigated by Fore et al. [54] and Fore and Dukler [55]. When a gas phase is
flowing over a liquid film, several different flow regimes are possible
depending on the magnitude of the gas velocity. For a very small gas velocity,
the interface is relatively stable, however, as the gas velocity increases the
interfacial waves appear. The amplitude and irregularity of waves become
pronounced as the gas velocity is further increased. At sufficiently high gas
flow, the capillary waves transform into large-amplitude roll waves
(disturbance waves). Near the transition to the roll wave or at a still higher
gas velocity, the onset of entrainment occurs.One way to measure entrainment is to reach a
quasiequilibrium state in the system where it is considered that deposition
rate is equal to the entrainment rate. Okawa et al. [50] presented a summary of
existing experiments for the equilibrium entrainment rate.Table3 summarizes the available data base for
annular-mist flow which can be used to validate CFD tools for dryout
investigations.Table 3
Data sources relative
to dryout investigations.Qfilm: film flowrate, δf: film thickness, ΔP: pressure drop, fw: wave frequency, Cw: wave velocity, E: fraction of entrained liquid, Eeq: equilibrium entrainment rate, Qdep: deposition
rates.
ReferenceMeasured
valueGeometryFluid/heatingFlow
conditionsWürtz, 1978 [47]ΔPQfilmδffwCw:Tube test section:Di  = 10 mm, Lh = 9.0 m Annulus section:
Di = 17 mm De = 26 mm, Lh = 8.0 m; 3.5 mSteam-water adiabatic and
diabaticP: 30, 50, 70, and 90 bar G: 500–3000 kg/m2sAndreussi, 1983 [52]ΔPPlexiglass tubeDi =  24 mm L =  5 mAir-water adiabaticP: atmospheric
Gl: 9.73–200.3 g/s Gg:  17.5–50.3 g/sGovan et al., 1989 [51]ΔPτwτIQdepLOTUS test rig tubular sectionDi = 31.8 mm L = 23 mAir-water adiabaticP: 2.4 bar
Gl: 100, 200, 300, 500 kg/m2sGg: 70–240 kg/m2s Temperature ambientCousins and Hewitt, 1968 [48]QfilmCwEacrylic resin tubeDi:  9.525 mmAir-water adiabaticP: 2 bar Gg: 18.14 & 31.75 kg/h
Temperature ambientAdamsson and Anglart, 2005
[49]QfilmTube test sectionDi:  13.9 mm
Lh: 3.65 m various power profilesSteam-water diabaticP: 70 bar
G:  500–1750 kg/m2sOkawa et al., 2005 [50]Deposition
mass transfer coefficients
Droplet
concentrationEeqStainless steel tubeDi: 5 mm L: 3670 mmAir-waterP: 1.4–7.6 bar Temperature ambient
Gl: 201–1264 kg/m2sGg: 173–627 kg/m2sFore et al., 2002 [54]Drop size distributionStainless steel ductDi: 9.67 mm
L: 3.4 mNitrogen-waterPressure 3.4 and 17 bar
Temperature38°C Ql: 0.0157 and 0.126 kg/s
Jg: 5, 7, 17 and 23 m/sFore and Dukler, 1995 [55]Drop size distributionVertical
tubeDi:
50.8 mm
L:
7.6 mAir-water air-water+glycerine (50%
mix) (6 cP liquid)Andreussi,
1983 [52]E, Rate of liquid interchangePlexiglass tubeDi: 24 mm
L: 5 mAir-water
adiabaticPressure: atmosphericQl: 9.73–200.3 g/s
Gg: 17.5–50.3 g/s


### 4.3. State of the Art in DryOut Modeling
within the NURESIM Project

The
following state of the art on the modeling of dryout by two-phase CFD results from the ongoing work in
NURESIM.(1)Basic model approach: in annular flows, the gas is a
continuous phase and the liquid phase is split into a film which is
continuous field and droplets as a dispersed field. The three-field model
is naturally used in this flow conditions to benefit from the possibility
to model separately the two liquid fields which have very different behaviours
since the droplets have a high interfacial area and no wall friction
whereas the film has a low interfacial area and has a friction along the
wall. A simplified three-field model can be easily implemented in a
two-fluid code by adding the film balance equations only in meshes along
walls. The films are then treated as in a 1D model with mass momentum and
energy equations written with a unique velocity component along vertical
direction and a film thickness function of the vertical position only.(2)Filtering or averaging procedure: considering flow in a BWR
core in conditions close to nominal, a high velocity steady flow regime
takes place with times scales associated to the passage of droplets being
very small (10–4, 10–3 seconds) and with droplet diameter being rather
small (10–5 to 10–3 m) compared to the hydraulic diameter (about 10–2 m).
These are perfect conditions to use a time average or ensemble average of
equations as usually done in the RANS approach. All turbulent fluctuations
and two-phase intermittency scales can be filtered since they are
significantly smaller than scales of the mean flow. There may be a
difficulty if film waves have to be simulated since it is not clear how
the averaging of the RANS approach may filter or damp the disturbance
waves.(3)Identification of local interface structure: is necessary to
select the adequate interfacial transfer laws and to determine the
interfacial area. Here, there are two interfacial structures corresponding
either to a dispersed liquid phase in a continuous gas in the core flow or
a film surface with waves and with droplet entrainment of deposition along
walls. A simple way to identify the two situations is to consider that the
latter only takes place in meshes along the walls while the former takes place
everywhere else. The characterisation of the droplet field may require the
use of additional transport equations for the droplet number density, or
the interfacial area of any statistical moment of the droplet size
distribution function. Another approach of the poly-dispersion is to use a
multigroup model with mass (and momentum) equations written for several droplet
sizes. A more simple characterisation of the droplet field by using an
algebraic expression of an average drop diameter will be used and
evaluated during the project.(4)Interfacial transfers: mass transfers affect the film
thickness and it is necessary to model at least the droplet deposition,
the entrainment and the vaporisation. A new droplet deposition model was
proposed and models for entrainment and vaporization were proposed to be
evaluated. Momentum transfers affect the film thickness, and it is
necessary to model gravity, wall friction, and interfacial friction.
Models for these forces were proposed to be evaluated. Energy transfers also
affect the film thickness, and it is necessary to model the wall heat
flux, the interfacial transfer, the evaporation, and the energy transfer
due to deposition and entrainment. Models for these transfers are proposed
to be evaluated. Interfacial heat and mass transfer also affect the
droplet field, and models are necessary for the convective heat flux from
steam to droplet interface and the radiation heat flux from walls to the
droplets. The mechanical behaviour of the droplets is mainly controlled by
gravity and interfacial friction. Again the drop size and poly-dispersion effects
play an important role on these transfers. Models have still to be
developed for these transfers on the droplet-vapour interface.(5)Turbulent transfers: liquid turbulence plays a very important
role in annular flows in a BWR core. It influences droplet deposition,
droplet coalescence, and breakup which affects the drop size and
consequently the deposition. Then, the vapour turbulent scales have to be
predicted correctly to model all these processes and this will require
additional transport equations to the three-field model. The k-epsilon
method was used in a Eulerian-Lagrangian approach to investigate the
deposition


## 4.1. The DryOut Phenomenology

Annular
flow pattern usually is the predominant flow regime in upper core regions in boiling
water reactors. The limitation of the total power obtained from each assembly
is the occurrence of dryout. Increasing the heat flux above some critical value
can lead to dryout that is associated with a sudden increase in the wall
temperature, which, in turn, can destroy the cladding material and allow the
radiation releases into the primary system. The phenomenology of dryout in
annular mist flow was described in [43].The
liquid phase exists as a liquid film, which is attached to walls, and as
droplets, which are carried in the central part of the channel by the vapour
phase.The
mass flow rate in the liquid film is changing due to several mass transfer
mechanisms.(i)
Due to hydrodynamic forces acting on
the liquid film surface, certain amount of liquid from liquid film is entrained
into the vapour core.(ii)
Another mechanism that is causing
liquid film depletion is associated with evaporation due to heating applied to
walls.(iii)
These two mechanisms must be
counterbalanced by drop deposition from the vapour core to the liquid film
surface to avoid film dryout.There
are several possible mechanisms that have been postulated for dryout (Hewitt,
1982 [44]).(i)
The liquid film dries by progressive
entrainment and evaporation, which are prevailing in comparison to deposition,
and dryout occurs when the film has gone.(ii)
Formation of a dry patch within the
liquid film, causing such wall temperature increase that cannot be rewetted. In
some situations a sudden disruption of liquid film may occur beyond which the wall
surface is dry. The disruption mechanism is not fully understood yet, however,
hydrodynamic mechanisms for the disruption are postulated.
(a)
For very thin liquid films. dryout
occurs when the rate of evaporation at the surface exceeds the rate at which droplets
arrive at the surface due to deposition.(b)
For thicker liquid films, it is
postulated that dryout may occur due to vapour film formation under the liquid
film. The mechanism of forming this vapour film might be of the same type as
described for the DNB mechanisms.Annular
regime in boiling flow is characterized by a thin liquid film flowing on the
channel walls and a gas core flowing in the central part of the channel. The
droplets in the gas core represent a larger interfacial area than the liquid film
and thus can dominate heat and mass transport between the phases. System
pressure drop is increased by droplet acceleration in the gas core, and
depositing droplets contribute to corrosion by increasing local wall friction.To some
extent, the dryout is a more simple process than the DNB since one cannot list so many
microscale phenomena which may play a role. In particular, if one first focuses
on the first dryout scenario with entrainment and evaporation prevailing in
comparison to deposition, only mesoscale phenomena have to be considered.The
most important mesoscale phenomena and parameters in annular flow affecting the
occurrence of dryout are(1)
drop size,(2)
deposition of droplets,(3)
entrainment of droplets, and(4)
film thickness.Drop Size in Disperse Two-Phase Flow
Drop
size is an important parameter which affects the deposition rates and thus the
dryout phenomenon. It can be described by a size PDF,fd(d) defined as the
probability that a droplet from the distribution will have a diameter of d. It
is often required that drop size distribution is represented by a single
weighted mean size.Deposition Rate
Liquid
droplets carried by a turbulent gas stream will deposit on bounding walls.
Clearly, deposition rate will have an important influence on the dryout
occurrence.
It may
depend on several unresolved issues, such as turbulence-particle interactions
and drop breakup and coalescence.
Deposition
rate will depend on drop dispersion in turbulent flow where particle motion is
primarily governed by interactions with eddies of various scales. Depending on
the ratio of the particle response time to the eddy characteristic time, the
dispersion can have different characters. If this ratio is very small,
particles are following the continuous flow structure. When the ratio is close
to 1 (the time constants of eddies and particles are of the same range of
magnitude), the dispersion of drops can be even bigger than that observed in
the carrier fluid. Finally, for high values of the ratio particles remain
largely unaffected by eddies.
Typically,
drop deposition is associated with two mechanisms: the diffusion process and
the free-flight to the wall. For proper prediction of the deposition rate of
droplets, both these mechanisms have to be taken into account. In addition,
impinging conditions of a drop on a liquid surface have to be considered. When
a single droplet impinges a liquid film, various phenomena can occur. The
droplet can bounce from the surface or merge with the liquid film. Splash can
occur when the drop kinetic energy is high enough. For conditions typical for
BWRs, the liquid film is thin and the velocity of droplets is high, thus
splashing and mergence are the key phenomena involved.Entrainment Rate
Several
mechanisms of drop entrainment from the liquid film have been identified. The
dynamic impact of gas core causes generation of waves on the film surface, with
droplets being separated and entrained from the crests of these waves. The
creation and breakup of the disturbance waves play important roles in the drop
entrainment process. Another entrainment mechanism is associated with splashing
associated with drop deposition, as already mentioned in the previous section.
Finally, in a heated channel with nucleate boiling in the film, entrainment can
occur due to the action of vapour bubbles which induce splashing.Liquid Film Thickness
Calculation
of the liquid film thickness is necessary to predict the occurrence of dryout.
To obtain the liquid film thickness and velocity, it is necessary to solve the
mass and momentum conservation equations of the film in order to determine the
film flowrate and pressure drop. This requires proper modeling of deposition,
entrainment, and evaporation in mass equation and of the wall friction and
interfacial friction in the momentum equation which depend on the wave
structure of the film interface.


## 4.2. DryOut Data Basis

Early
experiments were focused on the measurements of the total power, which was
necessary for the dryout occurrence in a heated channel. A vast number of these
experiments were performed for different conduit geometries in different flow
conditions. The measurements for steam-water were done in round ducts, annuli,
and rod clusters. Measurements in annuli covered the pressures of 30, 50, and
70 bar (Becker and
Letzer [45]; Persson [46]). For a validation of models based on the analysis
of wall film flows, experimental data of pressure drops, including wall shear
stress and interfacial shear stress, which characterize
liquid film thickness and the onset of entrainment, respectively, are required.
Also, actual measurements of film flows, film thickness, wave amplitude,
frequencies, and wave velocities are needed for the validation. Moreover,
because complete physical models for droplet entrainment and droplet deposition
are still not available, experimental data of these are needed to develop
reliable correlations and/or computational models.Würtz [47] has reported more than 2700 pressure drop
measurements for steam-water and reviewed film flow measurements in steam-water
mixtures in annular flow (see also Cousins & Hewitt [48]). The film flows
were measured both in tubes and in annuli and recently, Adamson and Anglart
[49] provided high-pressure steam-water data.An extensive review of existing measurements of deposition
rate has been presented by Okawa et al. [50]. The deposition rates were
predominantly measured in air-water systems with low pressures (see also Govan
et al. [51]). The techniques employed are the double film extraction, thermal
method, and tracer methodIt was experimentally proven that the mode of the
deposition is dependent on the droplet size. Observations of droplet motion
(Andreussi [52]) show that larger droplets travel across the gas core at about
their initial velocity in a constant direction until they are deposited. This
mechanism of deposition has been called direct impaction. At higher gas
velocities where the droplets are comparatively smaller, the effect of the
initial momentum on droplet motion becomes negligible. In this case, the eddy
diffusion mechanism of deposition prevails. Bates and Sheriff [53] have
presented a summary of the previous work done on droplet size/velocity in
vertical annular air-water two-phase flow. The drop size distribution was
investigated by Fore et al. [54] and Fore and Dukler [55]. When a gas phase is
flowing over a liquid film, several different flow regimes are possible
depending on the magnitude of the gas velocity. For a very small gas velocity,
the interface is relatively stable, however, as the gas velocity increases the
interfacial waves appear. The amplitude and irregularity of waves become
pronounced as the gas velocity is further increased. At sufficiently high gas
flow, the capillary waves transform into large-amplitude roll waves
(disturbance waves). Near the transition to the roll wave or at a still higher
gas velocity, the onset of entrainment occurs.One way to measure entrainment is to reach a
quasiequilibrium state in the system where it is considered that deposition
rate is equal to the entrainment rate. Okawa et al. [50] presented a summary of
existing experiments for the equilibrium entrainment rate.Table3 summarizes the available data base for
annular-mist flow which can be used to validate CFD tools for dryout
investigations.Table 3
Data sources relative
to dryout investigations.Qfilm: film flowrate, δf: film thickness, ΔP: pressure drop, fw: wave frequency, Cw: wave velocity, E: fraction of entrained liquid, Eeq: equilibrium entrainment rate, Qdep: deposition
rates.
ReferenceMeasured
valueGeometryFluid/heatingFlow
conditionsWürtz, 1978 [47]ΔPQfilmδffwCw:Tube test section:Di  = 10 mm, Lh = 9.0 m Annulus section:
Di = 17 mm De = 26 mm, Lh = 8.0 m; 3.5 mSteam-water adiabatic and
diabaticP: 30, 50, 70, and 90 bar G: 500–3000 kg/m2sAndreussi, 1983 [52]ΔPPlexiglass tubeDi =  24 mm L =  5 mAir-water adiabaticP: atmospheric
Gl: 9.73–200.3 g/s Gg:  17.5–50.3 g/sGovan et al., 1989 [51]ΔPτwτIQdepLOTUS test rig tubular sectionDi = 31.8 mm L = 23 mAir-water adiabaticP: 2.4 bar
Gl: 100, 200, 300, 500 kg/m2sGg: 70–240 kg/m2s Temperature ambientCousins and Hewitt, 1968 [48]QfilmCwEacrylic resin tubeDi:  9.525 mmAir-water adiabaticP: 2 bar Gg: 18.14 & 31.75 kg/h
Temperature ambientAdamsson and Anglart, 2005
[49]QfilmTube test sectionDi:  13.9 mm
Lh: 3.65 m various power profilesSteam-water diabaticP: 70 bar
G:  500–1750 kg/m2sOkawa et al., 2005 [50]Deposition
mass transfer coefficients
Droplet
concentrationEeqStainless steel tubeDi: 5 mm L: 3670 mmAir-waterP: 1.4–7.6 bar Temperature ambient
Gl: 201–1264 kg/m2sGg: 173–627 kg/m2sFore et al., 2002 [54]Drop size distributionStainless steel ductDi: 9.67 mm
L: 3.4 mNitrogen-waterPressure 3.4 and 17 bar
Temperature38°C Ql: 0.0157 and 0.126 kg/s
Jg: 5, 7, 17 and 23 m/sFore and Dukler, 1995 [55]Drop size distributionVertical
tubeDi:
50.8 mm
L:
7.6 mAir-water air-water+glycerine (50%
mix) (6 cP liquid)Andreussi,
1983 [52]E, Rate of liquid interchangePlexiglass tubeDi: 24 mm
L: 5 mAir-water
adiabaticPressure: atmosphericQl: 9.73–200.3 g/s
Gg: 17.5–50.3 g/s


## 4.3. State of the Art in DryOut Modeling
within the NURESIM Project

The
following state of the art on the modeling of dryout by two-phase CFD results from the ongoing work in
NURESIM.(1)Basic model approach: in annular flows, the gas is a
continuous phase and the liquid phase is split into a film which is
continuous field and droplets as a dispersed field. The three-field model
is naturally used in this flow conditions to benefit from the possibility
to model separately the two liquid fields which have very different behaviours
since the droplets have a high interfacial area and no wall friction
whereas the film has a low interfacial area and has a friction along the
wall. A simplified three-field model can be easily implemented in a
two-fluid code by adding the film balance equations only in meshes along
walls. The films are then treated as in a 1D model with mass momentum and
energy equations written with a unique velocity component along vertical
direction and a film thickness function of the vertical position only.(2)Filtering or averaging procedure: considering flow in a BWR
core in conditions close to nominal, a high velocity steady flow regime
takes place with times scales associated to the passage of droplets being
very small (10–4, 10–3 seconds) and with droplet diameter being rather
small (10–5 to 10–3 m) compared to the hydraulic diameter (about 10–2 m).
These are perfect conditions to use a time average or ensemble average of
equations as usually done in the RANS approach. All turbulent fluctuations
and two-phase intermittency scales can be filtered since they are
significantly smaller than scales of the mean flow. There may be a
difficulty if film waves have to be simulated since it is not clear how
the averaging of the RANS approach may filter or damp the disturbance
waves.(3)Identification of local interface structure: is necessary to
select the adequate interfacial transfer laws and to determine the
interfacial area. Here, there are two interfacial structures corresponding
either to a dispersed liquid phase in a continuous gas in the core flow or
a film surface with waves and with droplet entrainment of deposition along
walls. A simple way to identify the two situations is to consider that the
latter only takes place in meshes along the walls while the former takes place
everywhere else. The characterisation of the droplet field may require the
use of additional transport equations for the droplet number density, or
the interfacial area of any statistical moment of the droplet size
distribution function. Another approach of the poly-dispersion is to use a
multigroup model with mass (and momentum) equations written for several droplet
sizes. A more simple characterisation of the droplet field by using an
algebraic expression of an average drop diameter will be used and
evaluated during the project.(4)Interfacial transfers: mass transfers affect the film
thickness and it is necessary to model at least the droplet deposition,
the entrainment and the vaporisation. A new droplet deposition model was
proposed and models for entrainment and vaporization were proposed to be
evaluated. Momentum transfers affect the film thickness, and it is
necessary to model gravity, wall friction, and interfacial friction.
Models for these forces were proposed to be evaluated. Energy transfers also
affect the film thickness, and it is necessary to model the wall heat
flux, the interfacial transfer, the evaporation, and the energy transfer
due to deposition and entrainment. Models for these transfers are proposed
to be evaluated. Interfacial heat and mass transfer also affect the
droplet field, and models are necessary for the convective heat flux from
steam to droplet interface and the radiation heat flux from walls to the
droplets. The mechanical behaviour of the droplets is mainly controlled by
gravity and interfacial friction. Again the drop size and poly-dispersion effects
play an important role on these transfers. Models have still to be
developed for these transfers on the droplet-vapour interface.(5)Turbulent transfers: liquid turbulence plays a very important
role in annular flows in a BWR core. It influences droplet deposition,
droplet coalescence, and breakup which affects the drop size and
consequently the deposition. Then, the vapour turbulent scales have to be
predicted correctly to model all these processes and this will require
additional transport equations to the three-field model. The k-epsilon
method was used in a Eulerian-Lagrangian approach to investigate the
deposition


## 5. Conclusion

While current industrial methods for CHF still use the subchannel
analysis and empirical CHF correlations, the use of CFD already proved its
potential interest in fine-scale investigations of boiling flows for a better
understanding of sensitive flow processes. The “local predictive approach”
where CHF empirical correlations would be based on local T/H parameters
provided by CFD is not yet available but, with the present state of the modeling,
CFD can already be used to subsequently help new fuel assembly design and to
develop better CHF predictions in both PWR and BWR.



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*Source: 214512-2008-10-14.xml*