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LES ANALYSIS OF THE FLOW IN
A SIMPLIFIED PWR ASSEMBLY
WITH MIXING GRID
U.BIEDER,
P-E ANGELI, A.BURBEAU, C. CALVIN,
G. FAUCHET, M.PEYBERNES
F.FALK
7 MAI 2015
46th Annual Meeting on Nuclear Technology, BERLIN,
14 AVRIL 2015 | PAGE 1 CEA | 10 AVRIL 2012
CONTENT
• Specification of the problem
• The experimental setup (AGATE experiment)
• The numerical approach (Trio_U code)
• Flow in the near wake of the grid
• Flow in the far wake of the grid
• Pressure loss of a mixing grid
• Inter assembly flow
• Conclusion
14 AVRIL 2015 | PAGE 2 CEA | 7 Mai 2015 CEA DEN/DANS/DM2S/STMF/LMSF
SPECIFICATION OF THE PROBLEM
• A nuclear reactor core consists
of about 200 fuel assemblies.
• Each fuel assembly consists of
about 17*17 fuel rods.
• Mixing grids and spacer grids
are placed in the assembly at
axial distances of about 0.6m.
• The mixing grids increase the
turbulence level and enhance
horizontal mixing of the coolant.
• The efficiency of the mixing grid
determines the critical heat flux.
~3.6m
CEA DEN/DANS/DM2S/STMF/LMSF | PAGE 3 CEA | 7 Mai 2015
FEATURES OF MIXING GRIDS
Side view of the mixing grid Top view of the grid
mixing vanes
dimples
springs
grid corps
R
• A mixing grid consists of the grid corps, the mixing vanes, springs
and dimples.
• The springs and dimples clip the fuel rod (® in the top view) to the mixing grid.
• The mixing vanes impose a cross flow and increase the turbulence level. Each vendor has his own grid conception.
CEA DEN/DANS/DM2S/STMF/LMSF | PAGE 4 CEA | 7 Mai 2015
THE AGATE EXPERIMENTS
• The AGATE facility was in operation from 1992-2001.
• The test section consists of a 5x5 rod bundle and a
mixing grid, which is placed within a metallic channel of
a quadratic cross section.
• LDA measurements are placed on one channel side.
• A range of Re between 10.000 and 100.000 has been
analyzed.
• High quality local data of the velocity components and
of the turbulence level were measured. The uncertainty
on the cross flow velocity is below 1%.
More than 30
different mixing
grids have been
characterized
CEA DEN/DANS/DM2S/STMF/LMSF | PAGE 5 CEA | 7 Mai 2015
THE CFD CODE TRIO_U
14 AVRIL 2015
General features
• Application domain: low Mach number, unsteady, turbulent flows.
• Designed for industrial CFD calculations on structured (parallelepipeds) and non-
structured (tetrahedrons) grids of several hundreds of millions of control volumes.
• Based on an object oriented, intrinsically parallel approach, coded in C++.
• Platform independent (runs on PC and HPC machines with up to 10000 cores).
• Open Source
Numerical schemes for LES
Meshing Pure tetrahedral More than 1 billion nodes
Discretization Hybrid FVE Pressure: P0+P1
Velocity: P1NC
Time scheme explicit 2nd order Adams-Bashforth
3rd order Runge-Kutta
Spatial Convection 2nd order centred
Discretisation Diffusion 2nd order centred
Pressure solver PETSc GCP with SSOR
preconditioning
Wall treatment Wall law Reichhardt
Turbulence Sub-grid model WALE
vector and scalar node
pressure node
2D
CEA DEN/DANS/DM2S/STMF/LMSF | PAGE 6 CEA | 7 Mai 2015
THE CAD MODEL AND THE MESHING
• CAD modeling with SALOME
• Pure tetra mesh of the grid and the rod bundle
was created with ICEMCFD
• Two prismatic layers near walls (cut into tetra)
• 300 million velocity calculation points
• 20 days of CPU on 4600 cores of CURIE (TGCC)
Periodic box
Inflow region
Mixing grid
Rod bundle
CEA DEN/DANS/DM2S/STMF/LMSF | PAGE 7 CEA | 7 Mai 2015
EVALUATION OF THE MESH REFINEMENT
14 AVRIL 2015
• Secondary flow structures are developing in bare rod bundles due to the
presence of non-isotropic turbulence (Prandtl’s 2nd mechanism).
• The formation of these secondary flow structures are an indicator for a correct
physical modeling and an adequate numerical scheme and meshing.
• The secondary flow structures are present but not very well established.
• This meshing is the most coarse mesh possible to capture the secondary flow.
RSTM of a single tube in a tube bundle LES of the inlet box
CEA DEN/DANS/DM2S/STMF/LMSF | PAGE 8 CEA | 7 Mai 2015
Velocity vectors AGATE Velocity vectors LES
Velocity vectors k- Anonymized TKE spectra LES
AGATE CALCULATION: VELOCITY
14 AVRIL 2015
Results: Near grid cross flow
• 20 mm downwind of the grid
• The measurements show a
main cross flow at the 45°
diagonal.
• LES and RANS modeling show
the formation of the 45° flow
direction.
• The LES approach shows much
more tiny swirls than the RANS
approach.
• The TKE energy spectra follows
the -5/3 slope for about ½
decade of frequencies with an
accumulation of energy at high
frequencies
CEA DEN/DANS/DM2S/STMF/LMSF | PAGE 9 CEA | 7 Mai 2015
AGATE CALCULATION: VELOCITY
14 AVRIL 2015
k- calculation at z=3.4dh LES calculation at z=3.4dh
k- calculation at z=8.5dh LES calculation at z=8.5dh
Results: Near grid cross flow
• Profiles of the cross flow
velocities are compared.
• LES and RANS modeling
give results of similar quality.
• The RANS approach
overestimates the axial
velocity (well known
shortcoming of the k-
model). The over-estimation
is reduced in coarse
meshing.
CEA DEN/DANS/DM2S/STMF/LMSF | PAGE 10 CEA | 7 Mai 2015
Velocity vectors AGATE Velocity vectors LES
Velocity vectors k Anonymized TKE spectra LES
II
II
I
AGATE CALCULATION: VELOCITY
14 AVRIL 2015
Results: Far grid cross flow
• 200 mm downwind of the grid
• The measurements show a
reorganization of the main
cross flow (developing the 135°
diagonal).
• The LES approach shows a
reorganization of the cross flow
(formation of dominant swirls in
sub channels).
• RANS modeling keeps the 45°
diagonal flow direction.
• The TKE energy spectra follows
the -5/3 slope for almost 1
decade of frequencies with an
reduction of the former
accumulated energy at high
frequencies.
| PAGE 11 CEA | 7 Mai 2015
AGATE CALCULATION: PRESSURE DROP
14 AVRIL 2015
DP Grid
Pressure Drop of the
Mixing Grid [Pa]
Pressure Gradient of the
Rod Bundle [Pa/m]
EPRI benchmark (McAdams) - 22600
LES (periodic box) - 24800
LES (AGATE) 9000 27170
k- pure tétra 8300 38000
k- prism layers: 1 8500 35000
k- prism layers: 2 8900 36200
Analysis of central sub-channel
LES:
300 Millions control volumes
k- pure tetrahedrons:
10 Millions control volumes
k- with 1 prism layer:
13 Millions control volumes
k- with 2 prism layers:
16 Millions control volumes
CEA DEN/DANS/DM2S/STMF/LMSF | PAGE 12 CEA | 7 Mai 2015
INTER-ASSEMBLY CROSS FLOW
Objectives
• Predicting the viscous and pressure
forces acting on the rods and the
mixing grid.
• Quantification of the influence of the
inter-assembly gap on the flow field
14 AVRIL 2015
Vx=-1.6 m/s
• Same approach as for the
AGATE study has been used
• to model the flow and
• to mesh the geometry
• More than 1.1 billion velocity
control volumes (550M tetra)
• 400 M CV for periodic box
• 700 M CV for the assembly
• 10K cores on the TGCC
machine CURIE
Inter assembly
gap
Fuel rods
Left
grid
Right
grid
CEA DEN/DANS/DM2S/STMF/LMSF | PAGE 13 CEA | 7 Mai 2015
• A new calculation procedure
has been developed to couple
the Periodic Box to the Main
Domain (assembly)
• Sequential procedure
• Parallel procedure
MESH REFINEMENT AND PARALLELISM
10k cores reserved for the calculation
3600 for P.Box 6400 for the assembly
10k cores reserved for the calculation
6600 cores for P.Box
10k cores for assembly
3400 cores unused
Save 30% of CPU time
| PAGE 14 CEA | 7 Mai 2015
Trio_U
Trio_U
Mixing vane Guide tap
RESULTS: INSTANTANEOUS VELOCITY
Horizontal cup at z=0.03m
CEA | AUGUST 2014
Inter-assembly gap
Guide tubes
Big vortex
Small
vortices
Guide
tube
CEA DEN/DANS/DM2S/STMF/LMSF | PAGE 15 CEA | 7 Mai 2015
Vertical cut planes
Localisation of a
control rod
High Frequency
Fluctuations
Turbulent
Fluctuations
• Turbulent fluctuations
are especially present:
o In the wake of guide
taps
o In the wake of control
rods
• Strong impact of the
control rods on the flow
behavior
RESULTS: INSTANTANEOUS VELOCITY
Localisation of
guide tap
CEA DEN/DANS/DM2S/STMF/LMSF | PAGE 16 CEA | 7 Mai 2015
RESULTS: VELOCITY AND DYNAMIC PRESSURE
Horizontal cut plane at z=0.01m
• Almost constant
horizontal distribution
of the dynamic
pressure
• Points of low press-
ure are located in the
center of the vortices o In the inter-assembly
gap in the wake of
guide taps,
o In the wake of
mixing vanes.
P*=P/r-g*z-Fx
Low Pressure
Low Pressure
The geometry is turned by 90°
• Low influence of the
cross flow
| PAGE 17 CEA | 7 Mai 2015
RESULTS: VELOCITY AND DYNAMIC PRESSURE
Horizontal cut plane at z=0.10m
• A dominant cross flow is
present
P*=P/r-g*z-Fx
Low Pressure
Low Pressure
• Presence of vertical
bands of the dynamic
pressure
• Points of low press-
ure are located in the
center of the vortices o In the inter-assembly
gap in the wake of
guiding vanes,
o Between control
rods
The geometry is turned by 90°
| PAGE 18 CEA | 7 Mai 2015
RESULTS: STREAMLINES
14 AVRIL 2015
Streamlines in
the wake of a
rod
Streamline
leaving the
wake of a rod
Streamlines in
the main flow
Streamlines
entering the
wake of a rod
• Définition: Streamlines are
a family of curves that are
instantaneously tangent to
the velocity vector of the
flow. They are not time
dependent.
• Two types of streamlines
can be distinguished
• Lines inclined by 23°
representing the main flow,
• Lines generally not inclined
(0°) in the wake of a rod.
CEA DEN/DANS/DM2S/STMF/LMSF | PAGE 19 CEA | 7 Mai 2015
CONCLUSION
• The flow in a 5x5 rod bundle with mixing grid has been analyzed
experimentally (AGATE experiments) and numerically (Trio_U code).
• Close downstream of the mixing grid, LES and RANS modeling predict mean
cross flow velocities of similar quality (confirmation of MATHIS_H benchmark).
• Far downstream of the mixing grid some mixing grids have shown
experimentally a reorganization of the main cross flow. The cross flow switches
from the 45° diagonal close to the grid to the 135° diagonal far from the grid.
• Only the LES approach shows a reorganization of the cross flow (formation of
dominant swirls in sub channels) whereas the RANS modeling keeps the initial
45° diagonal flow direction in the whole domain.
• The flow field changes globally in the presence of cross flow:
• High frequency fluctuations exist particularly in the wake of control rods and guide
taps
• Spatially and temporally persistent vortices are present between control rods and in
the wake of guide taps
• Straight and inclines streamlines coexist in the rod bundle
CEA DEN/DANS/DM2S/STMF/LMSF | PAGE 20 CEA | 7 Mai 2015
Direction de l’Energie Nucléaire
Département de Modélisation des
Systèmes et Structures
Service de Thermohydraulique et de
Mécanique des Fluides
Commissariat à l’énergie atomique et aux énergies alternatives
Centre de Saclay | 91191 Gif-sur-Yvette Cedex
T. +33 (0)1 69 08 76 78 | F. +33 (0)1 69 08 52 42
Etablissement public à caractère industriel et commercial | R.C.S Paris B 775 685 019 14 AVRIL 2015
| PAGE 21
CEA | 10 AVRIL 2012
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