RANS-based CFD Simulations of Wire-Wrapped Fast

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Advanced Thermal
Hydraulics Simulation
– Part II
David Pointer
Carlos Pantano - UIUC
Jeff Smith
Hank Childs – LLNL
Adrian Tentner
Paul Fischer
James Lottes
Aleks Obabko
Yulia Peet
Andrew Siegel
- Argonne National Laboratory
RANS-based simulations
 Commercial CFD codes STAR-CCM+ and STAR-CD
– Finite volume solution of the well-known Reynolds-averaged form of
the Navier-Stokes equations.
 Second order solution using
 Face flux based differencing (COMET)
 SIMPLE predictor-corrector solution algorithm
 Algebraic multi-grid pre-conditioning
 two-equation realizable k-epsilon turbulence model
2
Allowed Mesh Types in Star-CCM+
 Radial cross-sections showing computational mesh distributions in the 7-pin
RANS simulations for:
– (a) the block-structured mesh
– (b) the trimmed cell mesh
– (c) the generic polyhedral mesh.
3
Allowed Mesh Types in Star-CCM+
4
LES vs RANS
 Comparison of velocity magnitude distributions
 LES
 RANS
5
Initial Comparison of LES / RANS Results
 RANS using Star CD k-eps
 Close comparison of results  RANS can predict cross-flow
velocities in the wire-wrap case.
 Pressure-drop comparisons underway.
6
Allowed Mesh Types in Star-CCM+
7
19-pin simulations
 Relative transverse velocity magnitude
8
Comparison of 3 Reynolds Numbers
Relative transverse velocity magnitude
Re = 6000
Re=9000
Re=50000
37-pin Simulations
Relative transverse velocity magnitude
10
Predicted Dimensionless Pressure Loss
Coefficient from RANS Simulations vs.
Correlations
 The dimensionless pressure loss coefficient is the pressure drop
normalized by the dynamic head, so that Cp=f (L/D).
 The Cheng & Todreas correlation assumes that there are three
fundamental sub-channel types: interior, edge, and corner. Each of the
three types of sub-channel frictional losses is calculated separately. The
bundle friction factor is then averaged.
 The Rehme correlation is a simpler single equation formulation based on
representative geometric parameters.
Number of
Pins
7*
19
37
Cheng &
Rehme
Todreas
Correlation
Correlation
1.116 ± 14% 1.179 ± 5%
1.088± 14%
1.041 ± 5%
1.075 ± 14%
0.943 ± 5%
RANS
Simulation
Prediction
2.282
1.199
1.059
* Small 7-pin assemblies are not within the range of applicability of the correlations
11
217-pin heated assembly
 Defined a “typical” SFR assembly for
initial comparisons between CFD code
predictions and SASSYS subchannel
model predictions
– 217 pins
•
•
•
•
8 mm pin outer diameter
Pitch-to-diameter ratio of 1.135
1.03 mm diameter wire wrap
20 cm wire wrap lead length
– Assumed power distribution
• Uniform radial power
• Cosine axial distribution
– Simplified boundary conditions
• Uniform inlet velocity
– 5.8 m/s
• Constant pressure outlet
• Specified wall heat flux on pin surfaces
• Adiabatic can surface
12
Axially coarsened polyhedral meshes
Nominal Cell
Aspect Ratio
Number of Cells
Predicted
Pressure Loss
(kPa)
1:16
7507243
76.01
1:8
9769722
77.00
1:4
14955523
82.01
Changes in pressure loss are mostly form losses, coarsening of surface representation may be introducing artificial surface roughness.
13
Simplifed wire wrap representation
 Minimal effect on flow field
– Slight increase in size of low velocity region on leeward side of wire
Transverse velocity magnitude
(inlet velocity = 5.8 m/s)
14
217 pin SFR Assembly simulation
Polyhedral mesh with simplified wire wrap representation
 Coolant Temperature (K)
 Pin Surface Temperature (K)
15
217 pin SFR Assembly simulation
Polyhedral mesh with simplified wire wrap representation
16
MAX – A Thermal Mixing Experiment
 LDRD funded project
– Define validation requirements
for high-fidelity methods
– Evaluate/Demonstrate
advanced measurement
capabilities
– Develop methods for analysis
and correlation of large 4-D
data sets
• 10-100 TeraBytes per test
– Develop methods and
visualization capabilities for
validation comparisons of
detailed 4-D data
– Provide data set for initial
validation of individual
advanced methods and
integrated multi-scale code
systems
RANS Simulations of MAX Thermal Mixing Experiments
 The computational model of the
nominal geometry was developed
from a simplified CAD
representation
 The surfaces defined by the CAD
model are triangulated and used
as the basis for a tetrahedral
mesh.
 The tetrahedral are then collapsed
into generic polyhedra to form the
mesh shown.
 Boundary Conditions
– uniform velocity, constant
temperature inlets
– constant pressure outlet
– no-slip adiabatic walls
 Short turn-around time allows
RANS to be applied to
configuration studies
RANS Simulations of MAX Thermal Mixing Experiment
 Steady state simulations
– default segregated flow solver
– SIMPLE algorithm
– Rhie- Chow interpolation for
pressure- velocity coupling
– algebraic multi-grid preconditioning.
– 1st-order upwind differencing scheme
– secondary gradient terms in the
diffusion step were retained
– Realizable k-epsilon turbulence model
with a two-layer all y+ wall treatment
(Norris & Reynolds)
 RANS simulations predict the development
of a very sharp interface between the two
jets
 Complex mixing pattern in the surrounding
fluid after the jets have impinged on the
upper surface.
Extracted Data
 To facilitate quantitative comparisons of characteristic data from simulations
of different design options, data was extracted along lines at the mid-plane
of the “fish tank”
Extracted Data - Nominal Configuration
Mesh resolution studies
1.4
Finer
Velocity Magnitude (m/s)
1.2
Fine
Nominal
1
Coarse
Coarser
0.8
0.6
0.4
0.2
0
-1.4
-0.9
-0.4
Position (m)
0.1
0.6
22
MAX Experiment for TH Validation
 Initial RANS / LES Comparisons:
– Average velocity distributions on two centerline cuts
• RANS: 2 million gridpoints, steady state
• LES: 23 million gridpoints, unsteady (still in initial transient)
– TKE comparisons similar
A
B
A
B
Pointer, Lomperski, Fischer., Validation of CFD Methods for Advanced SFR
Design: Upper Plenum Thermal Striping and Stratification, ICONE17-75740,
2009
23
Other Leveraged Efforts
 Very High Temperature Reactor Integrated Multi-Physics Simulation
 2-Phase Boiling Model
 Coarse Finite Element Design Simulations
24
VHTR Single Fuel Block Column
 Completed coupled CFD/Neutronics
simluations of a single column of
prismatic VHTR fuel blocks
– DeCART 2D/1D MOC
– STAR_CD Steady RANS
 CFD model uses 8.8 million
computational cells
– Polyhedral elements allow
conformal meshing of solid and
fluid components
• Improved numerical
performance for conjugate heat
transfer
– Includes upper and lower plenum
volumes
– Flow splits between channels are
simulated rather than specified
25
DeCART/STAR-CD Mesh Mapping
 Initial mapping utility uses a simple approach in which DeCART zones are
associated with all STAR-CD cells whose centroid falls within that zones.
 Global conservation is enforced within any single material across the entire
domain
26
Coupled Full Block Model Results
T
q’’’
 Power distribution from DeCART,
reflects temperature feedback
from CFD
 Temperature feedback
exaggerated, due to greatly
increased F/M ratio for single
block
 After CFD initialization, coupled
simulation required 4.2 hours on
32 cores for 9 data exchanges
27
2-Phase Boiling CFD
 Generation 1:
• Bubbly flow topology used in all cells.
• Spherical vapour bubbles surrounded
by continuous liquid phase.
• Used for previous BFBT calculations
 Generation 2:
• Bubbly flow, Mist flow and Transition
topologies
• Bubbly flow topology: Spherical
bubbles surrounded by liquid
• Mist flow topology: Spherical droplets
surrounded by vapor
• Preliminary liquid film models
• Used for current BFBT calculations
 Generation 3:
• Bubbly flow, Mist Flow, Transition,
and Sharp Interface topologies
• Uses 2-D Topology Map based on α
and α gradients
• Inter-phase surface transport planned
• Developed jointly by Adapco,
Argonne, Sarov Labs, VNIIEF
BFBT BWR Benchmark Simulation Results
Exit Quality = 25%
Microscopic
Distribution
(CT Scan
Data)
Channel
Averages
(Calculated
from CT
Scan Data)
Predicted
void fraction
Percent Error
Predicted
void fraction
Percent Error
Coarse Finite Element Design Simulations
 Solve ConvectionDiffusion
 Use imposed flow field
based on RANS or LES
simulations
 Relatively fast running
– A few minutes on a
few hundred nodes
 High-order numerical
accuracy
 High-order spatial
accuracy
30
Duplicate spacer grid design experiment methodology
31
Off-Set Injection Point
No Spacers
Effect of
Nominal Spacer
Effect of Spacer
With 0.5 * H/D
Thermal
Diffusion x 2
Thermal
Diffusion x 4
32
CONCLUSIONS – RANS (and other)
 The applicability of commercial RANS-based CFD tools to hydrodynamic
analysis of wire-wrapped sodium-cooled fast reactor fuel assemblies has been
demonstrated
 RANS-based CFD predictions compare well with LES predictions
 Sensitivity of predictions to Reynolds number is low
– Re=9000 is probably sufficient for hydrodynamic studies
 Broad axial and radial mesh density sensitivity studies have been completed for
7-, 37-, and 217-pin models
 Sensitivity of bulk predictions to mesh structure is low, but mesh structure
impacts resolution of jet structures and recirculation regions on leeward side of
pins.
 Completed initial benchmarking study comparing predicted pressure drop versus
lumped parameter correlations and experimental data
 Completed study comparing predicted temperature distributions versus
conventional sub-channel models based on lumped parameter correlations
 Demonstrated initial course mesh conduction diffusion solver
33
Current/Future Work – RANS (and other)
 Comparisons with legacy experimental data
in progress
– Turbulent diffusion in HEDL experiments
 217 pin simulations with conjugate heat
transfer in fuel, cladding and sodium bond
gap
 Simulations of alternate spacer options
– Engineered can walls
– Reduced diameter edge row wire wrap
 Implementing thermophysical property
functions for liquid sodium, clad and fuel.
 Beginning to extend simulations to ex-core
regions
 Initiate development of standalone finite
element subchannel simulation tool and finite
element network flow solver
34
Questions?
35
Extra Slides
36
Central Injection Point
No Spacers
Effect of
Nominal Spacer
Effect of Spacer
With 0.5 * H/D
Thermal
Diffusion x 2
Thermal
Diffusion x 4
37
Side Channel Injection Point
No Spacers
Effect of
Nominal Spacer
Effect of Spacer
With 0.5 * H/D
Thermal
Diffusion x 2
Thermal
Diffusion x 4
38
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