Experimental and Numerical Evaluation of Different Fluidization Parameters for Successful Target Layering

Kurt J. Boehm1,2

N. B. Alexander2, A. Bozek2, D. Frey2, D. Goodin2, A. R. Raffray1 (1UCSD, 2General Atomics)

Comparison between Numerical modeling and the room temperature loop experiments

Sputter Coating -- Preparation of Studies to Analyze Surface Roughening During Fluidization

Problems and Challenges

• Soft pellet- pellet collisions lead to static charges•Pellets stick to the pan and ultimately lead to a non-uniform layer

• Possible surface damage due to rolling over the dish surface and/or soft particle collisions• Thickness of the sputtered layer can only be estimated.

Gas streamGas streamGas stream

Funnel Frit Swirling FritCone Frit

Angled holes

Particle Movement Particle Movement

Expected particle paths using different frits

Gas stream

Wedge Frit

Fluidized Bed Analysis – Optimizing Fluidization Parameters

Replicable Fluidized Bed Section

Capsules

Frit

Closed loop plumbing circuit

Polonium Strip

The Optimal Frit Design Provides:

- Spin rate and circulation rate higher than a certain threshold value for uniform layering (about 5Hz).

-(The spin rate is the more important performance criterion)- A certain randomness of the orientation of the spin (hard to quantify)

The performance of different frits during fluidization was analyzed to determine the “best design” for the given criteria.

Analysis has been done by post experimental processing of high speed videos (250-500 frames per second)

Frit Performances

Spinning Rate

0

5

10

15

20

25

30

35

40

45

50

0 1 2 3 4 5 6 7

Bed Expansion

Her

tz

STRAIGHT

WEDGE

FUNNEL

SWIRLING BED

CONE TOP VIEW

CONE SIDE VIEW

Circulation Speed

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 1 2 3 4 5 6 7

Bed Expansion

Her

tz

STRAIGHT

WEDGE

FUNNEL

SWIRLING BED

CONE TOP VIEW

CONE SIDE VIEW

Surface damage during bouncing pan experimentsPresented in “High Z coatings for IFE applications” by Abbas Nikroo et alt., HAPL Review Meeting, November 13-14, 2001, Pleasanton, CA

Electro-magnetic shaker

High Z target

Bounce Pan

Previous set up: bouncing pan Proposed design: spinning dish

High Z target

Rotating Dish

Targets roll up on one edge and down though the middle

Wyko surface data taken from a shell coated in the rotating dish set up. The surface roughness before the coating process was unknown.

Numerical Simulation using MFIX

MFIX (Multiphase Flow with Interphase eXchanges) is a general-purpose computer code developed at the National Energy Technology Laboratory (NETL) for describing the hydrodynamics, heat transfer and chemical reactions in fluids-solids systems.

MFIX has two significantly different models of particle movement:Model A - Particle movement averaged computes the average velocity for all particles within one fluid cellModel B - Discrete Element Simulation (DES) each particle motion tracked individually

Random particle path 3

0

5

10

15

20

25

0 5 10 15

X-position of the particle in the bed, cm

Y-p

osi

tio

n o

f th

e p

arti

cle

in t

he

bed

, cm

Pellet path 3

Random particle path 1

0

5

10

15

20

25

0 2 4 6 8 10

X-position of the pellet in the bed, cm

Y-p

osi

tio

n o

f th

e p

elle

t in

th

e b

ed,

cm

Pellet path 1

Random particle path 2

0

2

4

6

8

10

12

14

16

0 5 10 15

X-position of the particle in the bed, cm

Y-p

osi

tio

n o

f th

e p

arti

cle

in t

he

bed

, cm

Pellet path 2

Tracking random individual paths using DES

This run simulates a fluidized bed at 2 BE. The simulated time was 1s, particle diameter 4mm, the diameter of the fluidized bed was 15 cm.

We want to modify MFIX to calculate:- time averaged circulation and spin rates of the pellets- relation between spin and circulation rates at different locations in the bed in experiments only pellets next to the wall can be seen- Explore different bed sizes and fluidization parameters to predict optimized layering- Model the layering process using the heat transfer capabilities- Explore the effects of non-uniform mass density in target (unlayered condition)

The following modifications will be necessary:

- A suitable fluid - particle model for needs to be found.

- Tracking of the rotational motion and angular position of each target- A collision model for unlayered targets (asymmetric mass distribution in capsule)

10pelletofdiameter

bedfluidizedofdiameter

Observations:

- Flow in the lab shows a preference to circulate across the whole width of the bed (at BE > 2.0)- Pellets may behave differently near tube wall than in center of the bed, although- Analysis of top and side view indicate

- similar behavior of the particle motion across the bed- some degree of randomness in the spin orientation

- All bed designs show a spin rate above the threshold of 5Hz.- The circulation rate shows a maximum for all frit designs between 2 and 4 BE.- In the swirling bed, the particles roll along the inside of the tube causing a steep and steady increase in spin rate - The cone and the funnel are expected to provide best randomness of rotation

Schematic example of the observed flow using the cone frit

Air flow

Preferred motion of the pellet

Screenshot of a side and a top view video with a regular straight frit

A white foam ball makes it easier to track a target

The circulation rate vs. bed expansion for the different fluidized bed configurations

The spin rate vs. bed expansion for the different fluidized bed configurations

Using cell averaged model (A) in symmetric cylindrical coordinates

Compared to room temperature experiment under same conditions

At low flow speed:- Strong agreement for the bubbling flow pattern- Bubbling frequency can be predicted quite well

At higher flow speeds:-3-D effect (circulation pattern) dominant in experiment

3D package of MFIX needed

Different particle void fractions in the fluid cells over timeVideo of the room temperature fluidized bed

Bubbles can wander off center

The sputter coater set up with a spinning dish: different dish sizes, angles of incline and the rotational speeds can be chosen.

Goal: Produce a uniform DT Layer on the Inside of the Capsules while Maintaining a Smooth Outer Surface

T = 0.00 s T = 0.04 s T = 0.08 s T = 0.12 s

T = 0.00 s T = 0.04 s T = 0.08 s T = 0.12 s

Nominal Value of 2 BENominal Value of 2 BE

All frits above “critical value” of 5 Hz

Fluidized bed test stand at General Atomics laboratory

Avoides:

Hard particle- particle collisions to preserve smooth surface

Development of a new method to produce a large number of smooth and uniformly coated shells

Current techniques require further work since:- Number of targets produced needs to be increased to 100-1000- Surface roughening during target coating needs to be eliminated

- Reduction of surface damage expected, since only soft collision will occur- Dish can be loaded with larger number of shells

10 m

50 m

Ablation of the gold layer was concluded to be a result of collisions