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Proceedings of the 2017 25th International Conference on Nuclear Engineering
ICONE25
July 2-6, 2017, Shanghai, China
ICONE25-67282
EXPERIMENTAL AND NUMERICAL STUDY OF AIRFLOW DYNAMICS THROUGH
AN OPENING IN A DEPRESSURIZED ENCLOSURE: APPLICATION TO NUCLEAR
DECOMMISSIONING
Salima KAISSOUN
Institut de Radioprotection et de Sûreté
Nucléaire (IRSN)
Gif-sur-Yvette, France
[email protected]
Eric CLIMENT
Institut de Mécanique des Fluides de
Toulouse. Université de
Toulouse CNRS-INPT-UPS, France
Corinne PREVOST
Institut de Radioprotection et de
Sûreté Nucléaire (IRSN)
Gif-sur-Yvette, France
ABSTRACT
In order to understand airflow dynamics through small
openings encountered in containment enclosures used for
nuclear decommissioning operations, the results of
experimental and numerical investigations are analyzed.
The main purposes of this work are to identify the required
conditions likely to generate flow inversions at the studied
opening which lead to pollutant leakage outside depressurized
enclosures, and also to verify the ability of CFD1 simulations to
predict these flow inversions by using U-RANS2 and LES3
approaches. All along this work, we tried to reproduce the
conditions of leakage occurring at the opening in terms of
aerodynamics and openings geometries.
Laser flow visualizations and CFD results show that an
additional flow, such as a turbulent jet in competition with the
directional flow and a disturbed level of pressure inside the
enclosure are among the main causes leading to the leakage
through the opening.
Laurent RICCIARDI
Institut de Radioprotection et de
Sûreté Nucléaire (IRSN)
Gif-sur-Yvette, France
NOMENCLATURE
∆P
pressure drop (Pa)
Φ
passive scalar concentration
k
turbulent kinetic energy (m2/s2)
ω
specific turbulence dissipation rate (s-1)
U0
inlet velocity at the opening (m/s)
V
velocity of the additional flow (m/s)
V0
injection velocity of the additional flow (m/s)
DΦ
kinematic diffusivity of Φ (m2/s)
Sct
turbulent Schmidt number
∆t
time step for the CFD simulations (s)
Q
flow rate (m3/s)
INTRODUCTION
Operations of decommissioning and decontamination
in nuclear facilities require setting up ventilated enclosures
around contaminated equipments in order to prevent leakage of
radioactive materials towards the surrounding environment.
These enclosures are operated with a negative pressure relative
to the room where they are installed, by using exhaust fans. Air
arrives through openings which generates a directional airflow
ensuring the dynamic containment of hazardous material. Due
to operating activities and fluctuating differential pressure on
both sides of the opening, local and unsteady flow inversions
may occur leading to the propagation of contamination
Keywords: Ventilated enclosures, Dynamic containment,
Airflows at openings, Particle image velocimetry,
Computational fluid dynamics.
1
CFD : Computational Fluid Dynamics
U-RANS : Unsteady Reynolds Averaged Navier-Stokes
3
LES : Large Eddy Simulation
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PRELIMINARY EXPERIMENTAL TESTS
First of all, exploratory experimental tests were set up
in order to observe flow inversions phenomena at a rectangular
slit disposed on the front side of an existing airtight box. The
purpose of these preliminary tests was to identify aerodynamic
conditions likely to induce air leakage through the orifice. For
that aim, two flows are involved: the first one represents the
containment inflow generated by an exhaust fan as shown in
Fig. 3 (enclosure extraction units are installed on the enclosure
roof), the second flow is generated by an axial fan mounted
inside the box in order to create additional turbulent
fluctuations which can disturb the initial directional
containment inflow.
outwards. Recommendations of an efficient dynamic
containment in nuclear installations are provided by the ISO
17873 standard [1] where three values of inlet velocity at the
opening are suggested: 1.5 m/s for the tritium, 1 m/s for the
plutonium 238 and 0.5 m/s for any other pollutants. However
the ISO standard indicates that each case must be investigated
specifically. Unlike most investigations on the aerodynamic
containment of laboratory fume cupboards that deal with large
openings [2] [3] and openings obstructed by the presence of a
worker [4] [5], the current study is focusing on airflow
dynamics through small openings, such as rectangular slits
whose hydraulic diameter does not exceed ten centimeters and
where the initial inflow stream is fully turbulent. The specific
purpose of the whole study is to identify the main scenarios
likely to disturb the containment inflow and possibly lead to air
leakage through the studied orifice.
Experimental device
Experimental tests were conducted within a
rectangular box with dimensions 0.8 × 0.8 × 0.6 m3 (depth
× height × width). The slit has a surface of 0.2 × 0.03 m2 and
a thickness of 5 mm. Geometric features of both the enclosure
and the opening are detailed in Fig. 1. Initialy, we intended to
study different opening locations and orientations, but we
finally focused on the vertical one located at the top left, given
that more flow inversions were noticed for this configuration.
Incoming air arrives from the studied opening by an exhaust
fan (while the remaining slits are sealed) which maintains a
negative pressure in the enclosure. A smoke generator is
connected to the box; the smoke is used in visualization as a
flow tracer. The whole experimental device is detailed in Fig. 3.
Thanks to the current study, it has been shown that the
containment flow directed from the outside towards the
enclosure might be disturbed by an additional parallel or
perpendicular flow created inside or outside the enclosure
causing flow instabilities in the near field of the opening due to
its sharp edged geometry and the turbulent nature of the flows.
Unfortunately, this phenomenon is not described in the
literature because of the large number of possible
configurations and the difficulties related to its quantification.
In fact, the unsteadiness and three-dimensional aspects of flow
structures make it very difficult to capture quantitatively and
locally the amount of pollutant released. For these reasons, at
the beginning of our study, some preliminary visualization tests
on an existing enclosure formerly used as a glove box were
conducted. Basically, a turbulent flow was created inside the
enclosure by the motion of an axial fan and was meant to
generate velocity fluctuations near the orifice. After that,
Unsteady Reynolds Averaged Navier-Stokes computations of
similar configurations, consisting of replacing the axial fan by
an impinging turbulent jet, were performed using the
commercial software ANSYS-CFX version 16.2. The
turbulence model SST k-ω, was chosen for these computations
since it is considered as the most appropriate for these cases
presenting flow separation and recirculation zones [4]. The
purpose of the preliminary experimental and numerical studies
was to design a reduced scale model dedicated to reproducing
appropriate air flows (in this case, external and internal
turbulent jets) capable of generating air leakage through the
opening. The last section of the paper represents first
experiments and computations conducted on the final reduced
scale model. Experiments based on laser sheet visualizations
were conducted in the case of an external turbulent jet
perpendicular to the containment inflow. U-RANS and Large
Eddy Simulations were performed in the case of an internal jet
blowing directly towards the opening in order to study the
competition between both airflows near the orifice and to
compare U-RANS and LES models for these types of flow
configurations.
Fig. 2 shows the laser flow visualization set up around
the enclosure. The illumination system is a doubled-pulsed Nd:
YAG laser with a head that delivers a green light sheet of
532 nm wavelength oriented towards the central vertical plane
of the slit. A CCD camera is positioned perpendicularly to the
lightening sheet to record images with a 10 Hz frequency ; flow
tracer is filled of 1 µm diameter oil droplets.
Fig. 1. Geometric features of the enclosure and the opening.
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correspond, respectively to inner negative pressure of 0.5 Pa, 1
Pa and 3 Pa. We can observe strong vortex shedding at the slit
which are increasingly significant as the inlet velocity is
reduced. The axial fan motion produces turbulent fluctuations
inside the enclosure and disturbs the inner pressure level which
generates fluctuating velocities nearby the opening. At that
time, it was difficult to quantify the velocity fluctuations
generated by the inner fan.
Fig. 2. Laser flow visualization device.
Fig. 4. Laser visualizations of tracer leakage through the opening. (a)
∆P = -0.5 Pa, (b) ∆P = -1 Pa, (c) ∆P =-3Pa.
Discussion
Exploratory experiments have set the starting point of
an approach aiming at understanding the phenomenon of flow
inversions at the thin opening. It has been shown that turbulent
fluctuations inside the enclosure generated by the axial fan
motion is likely to disturb the directional containment flow and
lead to unsteady flow inversions at the opening. The results of
these flow inversions are shown in Fig. 4 as tracer leakage
through the orifice. However flow patterns and directions of the
additional flow created by the axial fan were not controlled. For
that reason, a more controlled flow was set up for the following
CFD simulations related to the same enclosure. The additional
perturbating flow corresponds to an upstream flow competing
with the initial one which is generated by a recirculation loop
consisting of blowing and extraction ducts (Fig. 5). The
blowing duct provides a turbulent jet impacting against the
facing wall perpendicular to the the investigated opening; it
constitutes, with the extraction duct, a closed loop so that the
same quantity of air injected is extracted.
Fig. 3. Details of the experimental set-up.
Experiments and results
Experiments presented below highlight the
competition between the existing inflow at the opening and a
second flow created inside the enclosure, which induces
turbulent flow dynamics in the near field of the opening.
Experiments of air exhaust were conducted first, in order to
obtain inlet velocity values at the opening and the associated
enclosure pressure. Then, the perturbating flow was included by
putting an axial fan inside the enclosure, perpendicularly to the
opening (Fig. 3) processed at a constant output speed equal to
20 m/s. Inlet velocities vary between 0.65 m/s and 2.4 m/s at
the opening, driven by a range of negative pressure between 0.5
Pa and 7 Pa. Note that velocity magnitude is measured at the
central point of the opening by a hot-wire probe.
Fig. 4 illustrates instant laser tomographic
visualizations of the flow tracer leakage outside the enclosure
in the central vertical plane of the slit when both flows are
enabled. Three values of initial inlet velocity at the opening are
presented in Fig. 4: 0.65 m/s, 0.92 m/s and 1.57 m/s, which
PRELIMINARY NUMERICAL SIMULATIONS
This section is dedicated to numerical simulations,
performed with the commercial CFD software ANSYS-CFX, of
airflows in the enclosure and the slit presented above. Airflows
in the following computations are, on the one hand, the
containment inflow at the opening, and on the other hand, the
impinging jet created at the wall adjacent to the slit. The
objective of these simulations is to show the ability of U-RANS
computations to predict the temporal and spatial details of this
type of unsteady flow. The CFD results will permit to describe
the airflow dynamics inside the enclosure and around the
opening.
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Pre-processing
A description of the computational domain and mesh
will be provided in this part as well as a presentation of the
computational assumptions. Fig. 5 shows the three-dimensional
view of the geometry performed by ANSYS WORKBENCH
which is a global simulation system of ANSYS that integrates
all necessary software programs from the CAD to the
computation. The mesh was generated by ANSYS-ICEMCFD.
A non-uniform, hybrid (structured/non structured) mesh
containing 10,810,519 cells (10,484,026 tetra and 326,493
hexahedral cells) was employed. Fig. 6 shows horizontal and
vertical planes of the mesh used for these computations
centered on the investigated opening. Finer cells were used on
the edges and around the opening to enable capture of flow
recirculation; refinements were also applied along the adjacent
walls to the opening. Considering the significant size of the
preliminary computational domain (Fig. 5), a hybrid mesh was
performed: hexahedral cells were created at the opening and
adjacent walls and tetrahedral cells were positioned in the inner
volume of the enclosure and the external domain. Thereafter,
the geometry and size of the final reduced scale model will be
improved; the enclosure will be chosen smaller and the external
domain will be significantly reduced in order to have optimized
structured meshes with a reasonable number of cells.
Inlet and outlet (Fig. 5) are the boundary conditions related to
the blowing and extraction of the additional flow. Outlet flow
rate boundary conditions were imposed at the extraction of the
enclosure and of the additional flow. Inlet flow rate boundary
condition was imposed at the blowing of the secondary flow.
The free-stream turbulence intensity is fixed as 5% of the inlet
velocity. An ‘opening’ boundary condition was imposed on the
surfaces of the external domain, specified with a static relative
pressure value of 0 Pa. Blue arrows in Fig. 5 were added to
show the boundaries between the physical wall related to the
inner domain and the surface related to the external domain
where an ‘opening’ boundary condition is imposed.
In these U-RANS calculations, Shear Stress Transport (SST) kω model was used. Physical quantities related to Air at 25°C
were chosen. To simulate the flow tracer injected inside the
enclosure from the blowing duct, an additional turbulent
transport equation for the variable (passive scalar) Φ is solved..
The kinematic diffusivity for the passive scalar DΦ is fixed to
10−5 m2 /s (correponding to smoke in air [6]) and the turbulent
Schmidt number defined as the ratio of the turbulent viscous
diffusion to the turbulent mass diffusion was chosen as Sct =
1 for the following studies.
Fig. 5. Computational domain with boundary conditions.
Fig. 6. Views of the computational mesh centered at the opening, (a)
vertical x-y plane, (b) horizontal x-z plane.
Computations and Results
This part focuses on the CFD computations carried out
to characterize airflow dynamics when the containment flow is
disturbed by the secondary flow added inside the box. A
description of the secondary flow alone is presented first, and
then computations including both airflows are detailed.
Dynamics of the additional flow
In this part, only the additional flow is considered, the
outlet boundary condition in the enclosure exhaust has been
replaced by a no-slip wall. Flow rate at the inlet and the outlet
of the additional flow loop is set to 0.1 m3 / which is
equivalent to a mean velocity of 20 m/s through the 5.10−3 m2
blowing and extraction ducts. A time step ∆t of 10−2 s is chosen
for the simulation.
Fig. 7 shows three-dimensional velocity vectors when the flow
goes out of the opening with a maximum speed of 5 m/s; the
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can notice that the flow tracer goes outside the enclosure
through the opening, visibly further, in the case of V0 = 20 m/s
than the case of V0 = 6.5 m/s. In both cases, flow inversions
occur at the upper part of the rectangular slit as illustrated by
the exploratory experiments in Fig. 4 and for the
characterization of the additional flow in Fig. 7; this can be
explained by the presence of the enclosure roof very close to
the slit and the adjacent perpendicular wall which drive the
flow towards the top of the slit in the opposite direction of the
containment flow. The additional flow in Fig. 7 impinges at the
opening at a speed nearly equal to 5 m/s.
turbulent jet (Re=10,000) injected from the inlet duct impinges
against the face wall, follows it and arrives in front of the
opening; in this case the additional flow is parallel to the initial
one. Both vertical and horizontal planes given in Fig. 8 show
the flow tracer outside the enclosure. We can notice that the
additional flow, when it reaches the opening, is parallel to the
axis of the containment flow in the opposite direction (countercurrent flows). This specific result will allow us to focus on the
competition between the containment flow and a second one
directly blowing towards the opening.
Fig. 7. Description of the additional flow - three dimensional velocity
vectors of the outflow; vertical plane in the near field of the opening.
Fig. 9. Passive scalar fields (tracer flow) centered in the opening,
vertical x-y plane; (a)  = .  / , (b)  =  /.
Fig. 8. Description of the additional flow - passive scalar fields (tracer
flow) centered at the opening, (a) vertical x-y plane, (b) horizontal x-z
plane.
Fig. 10. Velocity profiles at the vertical central x-y plane.
Competition between two flows
The competition between these flows at the opening is
displayed in Fig. 9 which shows tracer spatial distribution.
Velocity profiles at the central plane of the opening are given in
Fig. 10 for the containment flow (dash red line) as well as in
the case of competition between the two flows; V0 = 6.5 m/s
(solid blue line) and V0 = 20 m/s (solid black line). We can
notice that the initial inflow has changed its direction when
adding an additional perturbating flow coming at the upper part
of the slit. At the same time, the flow has been recovered inside
the enclosure through the slit bottom part.
Here both the containment flow and the additional one
are considered. The flow rate imposed as a boundary condition
in the enclosure exhaust is fixed to 0.004 m3 /s which is
equivalent to an inflow velocity at the opening of 0.66 m/s.
Two flow rates are chosen for the secondary flow: 0.032 m3 /s
and 0.1 m3 /s, respectively, corresponding to 6.5 m/s and
20 m/s as mean input velocities through the 5.10−3 m2
blowing and extraction ducts. A time step ∆t of 5. 10−2 s is
chosen for these unsteady simulations. These computations
were initialized by the results of a stationary calculation with
the containment flow only. A uniform negative pressure equal
to −0. 6 Pa is given by the results of the containment flow
simulation.
Discussion
Unsteady computations of airflows encountered in the
preliminary experiments have been performed with the CFD
commercial software ANSYS-CFX by applying the SST k- ω
turbulence model. Thanks to these computations, we were able
to describe airflow dynamics inside the enclosure in the case of
a counter current competition between two flows. Vortices
escaping through the investigated slit illustrated through laser
flow visualizations in Fig. 4 could not be captured by the U-
Fig. 9 shows the vertical central plane near the
opening at a given time corresponding to maximum tracer
going through the opening for the two output velocities of the
additional flow. The flow is convected by the flow dynamics of
the additional turbulent jet injected inside of the enclosure; we
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RANS calculations. In fact CFD analysis of flow tracer
illustrated in Fig. 9 shows smooth passive scalar escapes
without any eddy structures. Given the large number of cells
used for these computations (more than 10,000,000 cells) and
the complexity of the unstructured mesh, a new approach was
adopted for next CFD calculations. For that aim, a smaller three
dimensional domain was used to simulate the competition
between the directional flow and the additional one; this latter
is either parallel to the initial one or directed towards the
opening, both cases were investigated.
opening or perpendicular to it as shown in Fig. 13 and in the
computational domain in Fig. 16.
Both the front face containing the opening and the rear face
containing the extract units are removable for simple access to
the enclosure. A flow visualization set-up (Fig. 12) consisting
of a dual Nd : YAG laser delivering a 532 nm wavelength green
light sheet and a 10 Hz CCD camera with 2048 × 2048 pixel
resolution is implemented. A smoke generator delivering 1 µm
diameter oil particles as flow tracer is connected to the
enclosure.
FINAL REDUCED SCALE MODEL
In the light of the previous numerical and experimental
studies, we were able, at that stage, to design a final reduced
scale model on which the future investigations will be
conducted. Experimental and numerical investigations on the
final reduced scale model have started and first results will be
presented in the following section. In this section is presented
the final enclosure designed in order to reproduce the flows
investigated above. The enclosure size and geometry have been
carefully designed to ensure the best description of the
investigated phenomenon; the studied enclosure is averagesized and its geometry allows a structured computational grid
with a reasonable number of cells. The purpose of designing the
reduced scale model is to be able to study several airflows
competitions on a simple enclosure geometry. In this regard, we
intend to study three flow configurations : the turbulent jet
perpendicular to the containment inflow, either internal or
external to the enclosure, and the internal turbulent jet in front
of the orifice is also studied. The influence of the opening
dimension and thickness is also intended to be investigated as
prospects for further work. In the following section, we will
present first experimental results of an external turbulent jet
perpendicular to the containment inflow and the U-RANS –
LES comparison in the case of a turbulent jet blowing towards
the orifice directly competing with the initial containment
inflow.
Flow visualizations and PIV results
A thin (1-3 mm) vertical light sheet is oriented towards
the opening central y-z plane. The doubled pulsed laser-CCD
camera facility is set in order to record laser tomography
images as flow visualizations on both sides of the opening and
also to conduct PIV post-processing on the successive particles
images. Particle image velocimetry is a non-intrusive method
which consists in capturing two images on two separate frames
and performing multistep cross-correlation analysis [7].
Basically two experimental campaigns have been conducted;
the containment inflow has been investigated first for a mean
input speed U0 = 1m/s equivalent to an exhaust flow rate
of 10.8 m3 /h. Then, interaction between the containment flow
and the external perpendicular turbulent jet has been studied.
Initial injection speed from the nozzle was fixed to V0 =
25.5 m/s; the layout of the secondary flow injection nozzle is
illustrated in Fig. 13. Two distances between the slit center and
the injection nozzle were chosen: 45 cm and 24 cm. In this way
the turbulent jet impacts perpendicularly to the opening at two
speeds, V0 = 5 m/s and V0 = 6.8 m/s. These two values were
measured at the opening central point by a hot-wire
anemometer probe.
Design of the experimental device
A final reduced scale model has been designed.
Investigations are carried out on a rectangular small opening
centered in the enclosure front wall in order to investigate the
dynamics of flows involved without having wall effects that
could change flow structures above. The enclosure is 0.5 ×
0.5 × 1.2 m3 (see Fig. 11) and the slit has a surface of
0.1 × 0.03 m2 and a thickness of 5 mm. Two square (a =
0.05 m) extract units, located at the rear face parallel to the
front wall containing the opening, are chosen for the enclosure
exhaust. The square geometry is specifically chosen to keep
computational meshes structured. The extract units are centered
at the rear face and symmetrically opposed with respect to the
slit axis in order to ensure a homogenous internal pressure field
and a directional inflow. Enclosure depth (0.5 m) and width
(1.2 m) have been carefully chosen to allow an internal
additional flow, basically as a turbulent jet injected from a
rectangular small 0.1 × 0.01 m2 nozzle located in front of the
Fig. 11. Geometric features and dimensions of the reduced scale
model.
Fig. 12. Experimental device and PIV set-up.
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Considering the cases of competition between the
initial flow and the external perpendicular turbulent jet, leakage
flow visualizations outside the enclosure (Fig. 14) show more
significant vortices in the case where the injection nozzle is
positioned the nearest to the opening (Fig. 14 (b)). The time
averaged velocity vector fields shown in Fig. 15 present more
intense inversion speeds for the case when the jet is the nearest
to the opening; 0.3 m/s is globally the mean velocity at which
airflow goes out of the enclosure when the perturbating velocity
is about 6.8 m/s (Fig. 15 (b)) perpendicularly to the opening
and 0.2 m/s in the case of an additional flow of 5 m/s (Fig. 15
(a)).
Fig. 15. Particles images and time-averaged velocity vector fields;
competition between two flows-enclosure inner plane. (a) =  / ,
(b)  = .  /.
Numerical simulations: U-RANS vs LES
Here are commented the first CFD results about
simulations conducted on the final reduced scale geometry.
In this part containment flow simulation is described in addition
to the competition between this initial airflow and an internal
turbulent jet blowing towards the opening. The U-RANS
assumptions (working fluid, turbulence modeling, and transport
equation) are kept identical all along the article; however new
Large Eddy Simulations are conducted. Structured grids are
highly refined and time steps are smaller (especially for the
‘LES’ computations). The sub-grid scale model chosen for the
Large Eddy Simulations is the Smagorinsky-Lilly model
available in ANSYS CFX software.
The U-RANS mesh is about 11 million hexahedral cells and the
‘LES’ one is more than 29 million cells. Meshes are refined in
the surrounding volume of the opening including opening sides
and adjacent walls; cells are also refined in the depth of the
opening. The grid horizontal and vertical planes are illustrated
in Fig. 17.
Unsteady simulations are initialized by the RANS
computations results of the containment flow only. Given that a
mean velocity of 1 m/s is required, an outlet speed boundary
condition was imposed at the extract units equal to 0.6 m/s.
Then for the unsteady simulations, an inlet velocity boundary
condition of 10 m/s was imposed at the injection nozzle
positioned inside the enclosure as shown in Fig. 16. In order to
avoid any internal over pressure due to mass contribution of the
airflow additionally injected inside the enclosure, the same air
flow rate injected was extracted through the exhaust so that
outlet boundary condition at extract units became equal to 2.6
m/s. In the U-RANS computations, time step was c equal
to 10−2 s. Concerning ‘LES’ computations, a time step of
10−3 s was adopted in simulating the containment flow only
and 10−4 s for both the containment flow and the turbulent jet.
The Reynolds number of the jet is equal to 10,000.
Fig. 13. Additional flow experimental set-up, (a) layout of the
injection nozzle, (b) plane view of the investigated cases.
Fig. 14. Laser tomography visualization of leakage flow at the vertical
plane outside the enclosure. (a) V = 5 m/s , (b) V = 6.8 m/s .
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the same time t = 2 s. It is noticeable that flow tracer could
reach the opening and leave the enclosure in the form of eddy
structures driven by the dynamics of the turbulent jet in the case
of LES computation. This phenomenon is not observed in the
U-RANS computation where we can see that flow tracer was
not able to go beyond the opening. In fact, in the developed
region of the turbulent jet, a three dimensional mixing zone is
obtained where vortices structures disrupt the inflow at the
opening and lead to flow inversions. These eddy structures are
not captured by the U-RANS approach due to averaging; URANS results can give us information on mean velocities but
not local and instantaneous velocity fluctuations. The analysis
of U-RANS results shows that in general U-RANS
computations are unable to predict the flow leakage through the
orifice which is an important feature for further analysis.
Fig. 16. View of the computational domain.
Fig. 17. Views of the computational mesh centered at the opening, (a)
horizontal x-z plane, (b) vertical y-z plane.
Fig. 19. Passive scalar fields at the vertical plane, (a) ‘RANS’
computation, (b) ‘LES’ computation.
Fig. 20. Passive scalar fields at the horizontal plane, (a) ‘RANS’
computation, (b) ‘LES’ computation.
DISCUSSION
Due to the few investigations conducted in the
exploration of flow inversions phenomena at rectangular slits in
a depressurized enclosure, the approach adopted was to try to
reproduce the phenomenon under different controlled
configurations and to characterize its dynamics. It has been
found that an additional turbulent flow (typically a jet) either
inside the enclosure or outside is the main cause leading to the
leakage at the opening. Vortex structures highlighted in our
experiments by laser visualizations are not captured by CFD URANS computations even though the meshes used are
adequately refined. However U-RANS simulations can provide
global information on general flow patterns such as mean
velocities and pressure fields. LES computations are able to
describe turbulent structures and consequently capture flow
Fig. 18. Velocity fields of the containment flow- horizontal and
vertical planes, (a) ‘RANS’ computation, (b) ‘LES’ computation.
Results of containment flow calculations show for
both RANS and LES cases a potential core region of a
maximum velocity about 1.55 m/s (Fig. 18). The containment
inflow impacts against the rear wall of the enclosure and goes
along the adjacent walls. LES velocity fields evidence turbulent
structures in a region far from the opening. LES computations
results are more interesting when the second turbulent flow is
competing with the initial one. In this regard, passive scalar
fields are illustrated in both the U-RANS and LES
computations in Fig. 19 and Fig. 20; the comparison is made at
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leakages at the opening. Unfortunately, this type of calculations
are highly CPU demanding; almost three weeks of calculation
were needed to compute a turbulent case with 29 million cells
and a time step of 10−4 s on 48 parallel processors.
As prospects for further work, a wide range of V0 /U0 ,
the ratio of the additional flow velocity at injection to the
containment flow velocity, will be studied thanks to the final
experimental set-up in the cases of both the internal
perpendicular jet and the parallel one (the external
perpendicular jet will also be investigated). Lastly, the quantity
of outflow leakage will be measured by a helium tracer-gas
detection technique.
ACKNOWLEDGMENTS
This work was conducted as part of a PhD equally
supported by IRSN and EDF. The contribution of Luc
Lafanechere from EDF is gratefully acknowledged by the
authors, particularly for fruitful discussions.
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9
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