Pressure Drop and Multi-scale Flow Structure Measurements in a Pebble Bed

Reactor

Yassin A. Hassan

Elvis Domingue Ontiveros

Multiphase Flow Research Laboratory

Department of Nuclear Engineering

Texas A&M University

NRC/OSU Review Meeting

College Station

February 24, 2009

Introduction

In the Advanced Gas Cooled Pebble Bed Reactors for nuclear power

generation, the fuel is presented in the form of spherical coated

particles.

The energy transfer phenomenon requires detailed understanding of

the flow and temperature fields around the spherical fuel pebbles.

Many of the macroscopic processes that affect material transport in

porous media are manifestations of the flow behavior of the system at

the microscopic scale ( at the scale of an individual pore volume)

Dispersion, for example, is the result of the cumulative effects of a number

of micro-scale phenomena, including the mixing caused by solid obstructions

in the flow path, the incomplete connectivity of the medium, eddies in the

medium, and recirculation caused by regional pressure gradients.

Predictive macroscopic transport theory uses volume averages of micro-

scale flow.

Multipoint measurements with spatial resolution are necessary to permit

visualization of complex flow patterns and to provide data at high enough

spatial density to allow volume averaging

Pebble bed Scales

System scale

Particle Image Velocimetry (PIV)

Optical method ( Non-intrusive )

Velocity fields

Spatial resolution

Time resolution ( DPIV)

Capability of studying two-phase flows

Frame 1

Frame 2

Frame 1 Frame 2

T

Velocity field

Air Water P-cymene

Streamwise velocity component Re=50

Pore 1 Pore 2 Pore 3

Streamwise velocity component Re=85

Pore 1 Pore 2 Pore 3

Streamwise velocity component Re=221

Pore 1 Pore 2 Pore 3

Streamwise velocity component Re=500

Pore 1 Pore 2 Pore 3

New Packed Bed& Proposed Studies

Scaling

Scale1:5 Length 1:4 DiameterD/d=10Annular configurationSimilar porosity

Scaling

Sphere packing type in packed bed

dp

dp

dp

In-line lattice packing

Cubic close packing

Wall region

Wall influence

Annular core D/d=10 Cylindrical core D/d=28

Multi-scale measurements

Ergun

Carman

Etc. (From Re. Hayes paper)

Re. Hayes :

Where,

Ex) Comparing correlations by using graph

Ex) Comparing correlations by using graph

Ex) Comparing correlations by using graph

Lower plenum studies

Heat transfer studies

An Induction furnace is an electrical furnace in which the heat is generated through electro-magnetic induction in an electrically conductive medium (usually a metal).

Convection and conduction studies

Induction furnace1.5-150 kHz

Metal beadPlastic beads

*CFD Simulation of a Coolant Flow and a Heat Transfer in a Pebble Bed Reactor

Yassin A. Hassan*

Wang-Kee In

* This work was performed before-Presented here for information only

Objective

• Examine a coolant (helium gas) flow structure in the Pebble Bed Reactor(PBR) core

• Estimate the temperature variation of a fuel pebble in the PBR core

CFD-PBR (Track 7) 27

CFD Model 1 (Fluid+Solid)

Unstaggered 3x3 Pebble Array(Ball dia.=60mm, Porosity=0.46)

Pebble Size Number

Full 1

Half 6

Quarter 12

Octant 8

Total 8

UHe,in = 15 m/s

THe,in = 1000 K Qfuel,gen = 9.0 MW/m3

P=Const.

Cyclic Side B.C

11

9

- TRISO(~1mm Dia.)

- Fuel(UO2) core

- Graphite coating

- 450,000 fuel pebbles

in PBMR400 core

(core porosity 0.39)

CFD-PBR (Track 7) 28

CFD Model 2 (Fluid Only)

Pebble Size Number

Full 14

Half 24

Quarter 24

Total 3220

1.6

201.6

128Cyclic B.C

Symmetric

Side B.C

Staggered Pebble Array(Ball dia.=60mm, Porosity=0.3)

CFD-PBR (Track 7) 29

Computational Method

• Turbulence Model– Standard k-e(Epsilon) model

– SST k-w(Omega) model

• Convection Scheme: Second-order Upwind

• Reynolds Number (Dpebble, Vbulk,coolant)– 65000 for CFD model 1

– 46000, 290 for CFD model 2

• CFD Codes: STAR-CD(4.0), STAR-CCM+(V2.08)

CFD-PBR (Track 7) 30

Velocity Vector around Pebble

Side View

Bottom View

Multiple vortices

Re=46000, Standard k-e

CFD-PBR (Track 7) 31

Center Pebble Temperature

• Large temperature variation on pebble surface as well as

in pebble core(UO2 layer)

• Temperature variations are estimated as 180K in pebble core

and 15K on pebble surface

Pebble center

High temperature

behind pebble contact

Pebble Array Temperature

• Higher surface temperature of the downstream pebbles than the upstream ones

• Significantly high temperature near the pebble contact region

Flowdirection

Pebble contact

Central Pebble Surface Temp.

0 30 60 90 120 150 180730

740

750

760

770

Streamwise

Circumferential

Te

mp

era

ture

[oC

]Angle, [deg]

Side view Top view

Bottom view Hemispherical view

• Low temperature in the large flow region and high temp. in the narrow flow region

• Maximum variation of a pebble’s surface temperature is 25 oC

• The peak-to-peak variations are approx. 20 oC and 10 oC in the streamwise and

circumferential directions with a peak value near the bottom of a pebble, i.e., =160

Summary

• Multiple vortices were predicted to occur at the bottom as well as at the side downstream of a pebble due to a flow separation.

• Temperature variation at a central pebble is approximately 340 oC and 25 oC in the pebble core and the pebble surface, respectively.

• The coolant flow structure in a PBR core appears to largely depend on the in-core distribution of the pebbles. In the future, a CFD model should be developed to more adequately represent the pebbles randomly stacked in the PBR core.

Angelo FrisaniAkshay GandhirYassin Hassan

2/18/2009

Pebble Bed ReactorComputational Approach

• Simple Cubic

– Symmetric

• Face Centered Cubic (FCC)

– Staggered

• Body Centered Cubic (BCC)

– Staggered

• Void Space

– Missing Sphere

Scenarios

• Schematic

• Velocity Inlet– 1 m/s (Re =1890)

– 20 m/s (Re =37,800)

– 36 m/s (Re =68,040)

• Model– K-epsilon (current)

– K-omega

– Reynolds Stress Turbulence

– Spalart-Allmaras

Simple Cubic

Velocity Inlet

Symmetry Wall

• Sphere Diameter=60mm

• Porosity=0.55

CFD Model – Fluid Region

Pebble Size Number Complete

Spheres

Octant 8 1

Quarter 36 9

Half 54 27

Full 27 27

Total 125 64

Mesh

Name Number of Prism

Layers

Cell Number

No.1 2 563,283

No.2 2 737,735

No.3 2 1,068,483

No.4 2 2,583,764

No.5 5 4,536,619

No.6 2 10,272,720

Prism Layers

5 Prism Layers

Contact

with

other

sphere

Uin=1m/s

Pressure Distribution

Bottom View

Contact with

other sphere

Velocity Around the Sphere

Multiple Vortices Around

Sphere

Bottom View

Vortices

in the

contact

region

Vorticity

Multiple

vortices in the

contact region

Uin=1m/s

Streamline around the cener block

Future Computational Work

Simulation of other geometries

Body Centered Cubic

Face Centered Cubic

Void Space (Missing Sphere)

Data Analysis

Compare results with existing correlations.