Folie 1

1
Progress in AREVA NPs LWR Thermal Hydraulics
Methodology
F. Wehle R. Reinders T. Salnikova M.Glück AREVA
NP GmbH
2
AREVA NPs Thermal Hydraulics Methodology Part
1 Sub-Channel Analysis

• Background
• Dryout modeling phenomena
• The individual film model of F-COBRA-TF
• Conclusions and outlook
• Part 2 CFD Modeling of Two Phase
  Flows

3
Challenges for design and operation of advanced
LWR fuel assemblies and cores

• Customer

Optimized fuel utilization for safe, reliable and
highly flexible reactor operation

• Advanced fuel designs
• Modern core loading concepts
• High operational flexibility

• AREVA NP

• Design Tools

• Qualified methods
• Comprehensive physical modeling
• Validated and predictive
• (e.g. F-COBRA-TF, STAR-CD,
  S-RELAP/RAMONA-5)

4
Measurement background for code validation
Thermal Hydraulic Test Loop KATHY
5
Classification of TH computer codes

• Spatial discretization
• Fuel assembly codes
• Sub-channel codes
• Computational Fluid Dynamics (CFD) codes
• Time discretization
• Steady-state codes
• Transient codes (explicit or implicit time
  discretization)
• Two-phase flow modeling
• Homogeneous equilibrium model
• Drift-flux model
• Two-fluid model
• Two-fluid / Three-field model

6
Classification of TH computer codes

• Spatial discretization
• Fuel assembly codes
• Sub-channel codes
• Computational Fluid Dynamics (CFD) codes
• Time discretization
• Steady-state codes
• Transient codes (explicit or implicit time
  discretization)
• Two-phase flow modeling
• Homogeneous equilibrium model
• Drift-flux model
• Two-fluid model
• Two-fluid / Three-field model

F-COBRA-TF
7
Basic equations in F-COBRA-TF

• F-COBRA-TF solves the conservation equations
  separately for three different fields
• continuous liquid (e.g., as film)
• entrained liquid (droplets)
• vapor

8
Optimization of fuel assembly components
BWR
PWR
Typical dimensions
Tools
Single-phase Two-phase CFD
Spacer vanes, dimples, strip thickness
Millimeters
Spacer positions
Centimeters Meters
Two-phase sub- channel codes
Part length fuel rods
Water channel
9
Dryout modeling phenomena (1)

• Entrainment / Deposition

Entrainment
Deposition

• Turbulent fluctuations in lateral direction
• Swirl flow due to spacer grids

• Bubble movement from the wall to the film surface
  (in case of nucleate boiling)
• Dispersion of film wave peaks by faster moving
  gas core
• Creation of droplets by bursting of liquid
  bridges at the transition from churn to annular
  flow

10
Dryout modeling phenomena (2)

• Lateral exchange between sub-channels

Diversion cross-flow

• due to lateral pressure gradient

Void drift

• tendency of vapor to go to less obstructed
  sub-channels with higher mass velocity (e.g. due
  to lift force)

Turbulent mixing

• gradient diffusion of vapor

11
The individual film model of F-COBRA-TF

• The new model
• considers separate mass balances for each
  individual film with respect to
• Evaporation / Condensation
• Entrainment / Deposition
• keeps the common momentum and energy equations
  for continuous liquid.

Vapor (bubbl. / vapor core)
Continuous liquid (liquid core / film)
Entrained liquid (- / droplets)
Gas (non-cond.)
Mass equation 1
Mass equation 2
Mass equation 3
Mass equation 4
Rod
Rod
Unheated Structure
Momentum eq. 2
Momentum equation 3
Momentum eq. 1
Energy equation 1
Energy equation 2
12
Evaporation / Condensation

• Separate evaporation of all rod surface films
• Additional evaporation from all surface films
• and from the droplets
• (in case of dryout or sudden pressure decrease)

Partitioning according to rod power ratios
Partitioning according to film and droplet
surface ratios
Heated Rod 1
Unheated structure Str 1
Evaporation at the rod surface
Evaporation at the entrained liquid-vapor
interface
Evaporation at the continuous liquid-vapor
interface
Convective heat input to the liquid
Heated Rod 2
13
Entrainment / Deposition (1)

• Separate entrainment from and deposition onto all
  surface films

Rod 1
Heated Rod 1
Unheated structure Str 1
Deposition of droplets
Entrainment from liquid film
Rod 2
Heated Rod 2
14
Entrainment / Deposition (2)

• Disappearance of film at the end of a part length
  rod

Full length rod
Full length rod
Additional entrainment at the end of a PLR
Part length rod
15
Dryout prediction with F-COBRA-TF

• ATRIUM10 sample case Film thicknesses of
  individual films
• (side sub-channel with 2 FLR and 1 water
  channel segment)

mm
Film thickness at unheated water channel
Average film thickness in sub-channel
Dryout
Film thickness at heater rods
16
Conclusions and outlook

• Validation of F-COBRA-TF
• In-house pressure drop, CHF, and void
  measurements
• Steady-state exercises in NUPEC BWR benchmark
• Void distribution
• Standard mixing experiments
• Methods of CHF prediction
• PWR DNB correlations, look-up tables
• BWR Mechanistic dryout models
• New individual film model in F-COBRA-TF
• Attempt of a mechanistic dryout model
• First promising results achieved
• Further calibration necessary and ongoing

• Critical power

• Pressure loss

17
Part 2 CFD Modeling of Two Phase Flows
18
Swirl in a Simplified Rod Bundle Geometry-Single
Phase-

• Swirl and cross flow have a strong impact on the
  CHF performance
• At AREVA NP studies on subchannel flow under
  swirl conditions were carried out

Outgoing swirl intensity vs. flow deflection at
inlet

• An indicator for DNB behavior could be derived
  from single phase conditions
• Looking for maximum swirl at the outlet
• Variation of vane deflection
• Here Optimum vane deflection at about 30-40

19
Results for Bubble Flow under Swirl
Conditions-without heat and mass transfer-
450 mm
250 mm
72 mm
Void distribution in a quarter section of a
simplified subchannel at different altitudes

• In the pictures the void fraction was limited to
  lt5 to show some details
• 72 mm after the inlet the maximum void is focused
  to the center of the subchannel
• Halfway of the spacer-span the void is spreading
  out by diffusive forces
• Close to the outlet most of the void is in the
  center of the subchannels, but a pocket of
  bubbles is formed on the surface of the rod

20
Bubble Distribution in the Subchannel under Swirl
Max void on the rod surface
Quarter section results mounted to a full
subchannel

• The high void fraction in the center is shaped
  like a tilted square
• At the inlet centrifugal forces focus void to the
  center, later diffusion phenomena spread it
  continuously
• The void fraction of the bubble pocket depends
  strongly on the incoming void fraction

21
Void Distribution in the Subchannel
Void fractions at different local positions

• Impact of the declination angle (air / water)
• High void concentration on the axis ? no impact
  on local heat transfer at the rod surface
• At the rod surface depending on the azimuthal
  position increased void fraction could occur

22
Boiling in Subchannels under Swirl Conditions
Void distribution under Swirl
Void distribution without Swirl

• Taking into account Two-phase heat transfer at
  the rod surface, condensation in the bulk flow
• under certain conditions bubble pockets
• Risk for DNB only in the region of the pocket
• Without swirl the void fraction at the rod
  surface is much higher

23
Validation for Boiling Model in Pipe Flow
STAR-CD calculation
Experiment Garnier

• Experiment for subcooled boiling
• Radial distribution of void
• Curves for different qualities
• Agreement quite good, but still some problems in
  modeling tests with high qualities
• Bubble physics might be not more adequate
• More detailed considerations are necessary

24
Transient Simulation of a Film / Steam
InterfaceFree Surface Flow

• VOF method was applied to model the interaction
  between a liquid film and a steam core flow
• Waves are created on the film / steam
  interface
• Some droplets are pinched off by acceleration and
  surface tension
• Too much numerical diffusion ? later the waves
  were smoothed out
• With improved methods and finer mesh, results
  could be improved to some extend

Droplets pinched off from the tip of the wave
Example from jet atomization Double flash
picture showed fragmentation of Ligaments
25
Droplet flow under Swirl ConditionsCatchment
Volume

• Analytical consideration for an heuristic model
  of droplet deposition
• The axial velocity determines the residence time
  in a spacer span
• In this time droplets have to travel towards the
  rod surface for deposition
• The lateral droplet speed can be estimated from
  the swirl
• Only droplets out of a limited volume can
  approach the surface (catchment volume)
• Qualitatively Droplets with higher diameter
  contribute more to deposition

Axial flow and swirl determine roughly a volume
in the subchannel from where droplets can hit the
rod surface (film)
26
Droplet Distribution under Swirl Conditions
Droplet concentration 70mm downstream the spacer

• Droplets are accumulating locally at the rod
  surface
• Deposition proportional to droplet concentration
• Under swirl besides diffusive deposition a
  significant convective component is present
• Regions with reduced droplet concentration could
  indicate an imbalance of entrainment and
  deposition

27
Droplet Distribution in a 2x2 Rod Bundle Section

• The locally high volume fractions of droplets
  will coincide with locally increased film
  thickness
• the film should be thicker on the luff side (with
  respect to the swirl direction) of the rod
  surface
• This simple model indicates that Dry Out could be
  most probable slightly offset from the gap
  between the rods

The previous results are mounted to a 2x2 rod
model
28
Conclusion

• Two-phase modeling in CFD has much improved
• Together with higher computing power finer meshes
  can be applied to give sufficient resolution
• Most of the work is done on bubbly flow or spray
  flow
• High volume fractions have to be treated
• E.g. high void accumulations at heated walls with
  high heat flux have to be considered in more
  detail
• With the available methods indicators for CHF can
  be derived
• Very fine meshes are required to take into
  account all details of the swirl promoting
  components
• Results are promising and very useful for
  engineering judgment