LES ANALYSIS OF THE FLOW IN A SIMPLIFIED PWR ...... 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

www.cea.fr

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.


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


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


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


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


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



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.


Velocity vectors AGATE Velocity vectors LES

Velocity vectors k Anonymized TKE spectra LES

II

II

I


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:


k- with 1 prism layer:


k- with 2 prism layers:



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


• 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


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


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.


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


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

Documents

LES ANALYSIS OF THE FLOW IN A SIMPLIFIED PWR ...... 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