TSI Incorporated Seeding of Flows TSI LDV/PDPA Workshop & Training Presented by Joseph Shakal Ph.D.

TSI Incorporated

Seeding of FlowsSeeding of Flows

TSI LDV/PDPA Workshop & Training

Presented by Joseph Shakal Ph.D.

Copyright 2008 TSI Incorporated

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SeedingSeeding

• Most neglected aspect in LDV

• Proper seeding can significantly increase measurement accuracy and rate

• Key parameter to optimize SNR

• Guidelines for selection

• Related issues

TSI Particle Generators


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Solid Seed ParticlesSolid Seed Particles25um & 40um Glass BeadsPSL

TiO2 10-40um Silver Coated HGB4um Nylon


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RequirementsRequirements

Single Particle Parameters– Aerodynamic size– Signal strength (surface

properties)– Safety and Compatibility with

Test Equipment

Composite Parameters– Concentration– Monodispersity– Size of Flow System

TSI 3400A Fluidized Bed Particle Generator


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RequirementsRequirements

Safety Parameters– Use Olive Oil

– Toxicity: see MSDS

– Flammability

– System Compatibility: will moving parts be damaged by the particles?

TSI 9307 Particle Generator


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Particle Size Particle Size DistributionsDistributions

0 5 10 15 Diameter

20

Volume (Mass) Based

VMD = 5 m

Number BasedNMD = .4 m

Diameter0.1 1 10 100

Diameter0 20 40 60

0.01 0.1 1 10 100

diameterDiameter

(Linear)

(Linear)

(Log)


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Seeding Techniques for Air Seeding Techniques for Air FlowsFlows

• Generate liquid droplets– Direct use of generated droplets– Start with solution- leaves solid particle of solute when dry– Start with mixture of high & low vapor pressure materials (e.g., oil

and alcohol)– Disperse solid particles suspended in a liquid by drying droplets

• Disperse dry powders• Evaporation and condensation of a liquid

– (Evaporation condensation generator)– TSI 9308 ‘Fog Generator’


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Seed Particle RequirementsSeed Particle RequirementsWater and AirWater and Air

Water (n = 1.33)– Forward-scatter

• Generally no seeding is required

– Backscatter

• Seeding often necessary

Air (n = 1.0)– Seeding usually desirable


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Seeding Water for Seeding Water for BackscatterBackscatter

Conditions

Laser - Argon-ion Beam spacing - 35 mmPower - 0.7 W Beam expansion - 3.75xF.L. - 450 mm Maximum velocity - 10 m/sAperture - 150 mm Spherical particles

SNR (dB) Settling Velocity (µm/s)Particle TiO2 PSL TiO2 PSLDiameter (n = 2.9) (n = 1.59) (s.g. = 4.26) (s.g. = 1.05)

0.25 35.8 0.10

0.54 49.8 21.8 0.47 0.008

1.00 29.3 0.027


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Particle Motion Relative to Particle Motion Relative to FluidFluid

Settling velocity:

When (gases - continuous regime)

Time constant of particle:

Step change: particle

( gas

Frequency: particle

( gas

V d g

dt

u

ue

V V

V V

u

u t f

ft

ps p c g

g p

p p

g

t t p pi

g gi

db

2

2

1

12 1 2

31

18

18

1

1 2

1

2

1

/

( )

)

( )

)

/

/

Ref: Feller & Myers (1976)

Note that these relations could easily be made into a spreadsheet for to help with seeding particle selection.


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Particle Response in AirParticle Response in Air

0.5 µm 1 µm 2 µm

f3db f3db f3db

Particles in Air n t1(µs) (kHz) t1(µs) (kHz) t1(µs) (kHz)

Silicon Carbide 2.6 3.3 2.55 63 10.2 16 41 3.9Alumina 1.76 3.8 2.93 54 11.7 14 47 3.0Polystyrene 1.6 1.05 0.81 196 3.24 52 13 13 Peanut Oil 1.47 0.91 0.70 227 2.81 57 11.2 14 Microballoons — 0.23 0.18 897 0.71 224 2.84 56

For air, settling velocity = 9.8 x Time Constant (t1) in m/s

Time Constant and Frequency Response


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Particle Response in WaterParticle Response in Water

Settling Velocity (µm/s)

Particles m 0.5 µm 1 µm 2 µm 5 µm

Silicon Carbide 1.95 0.31 1.2 5.0 30

Alumina 1.32 0.38 1.4 6.0 38

Polystyrene 1.2 0.007 0.027 0.11 0.68

Relative refractive index m = np/nw


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Seed Particles - Air and WaterSeed Particles - Air and WaterParticles Medium Density Refractive Dia

(g/cc) Index (m)

Silicon Carbide Air/Water 3.2 2.65 1.5

Nylon Air/Water 1.14 1.53 4.0

PSL Air 1.05 1.55-1.6 0.5

TiO2 Air/Water 4.2 2.6 3-5

Hollow Glass Beads (HGB) Air 1.05 - 1.15? 1.5 8-12

Silver Coated HGB Water 1.65 0.21 (R) 142.62 (I)

Olive Oil Air 0.92 1.47 ---


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Other CommentsOther Comments• Anti-static additives can help prevent particles from sticking to glass or

acrylic windows. Larostat is commercially available.

• Hydrophobic (Anti-fogging) coatings are now available for windows. Abrisa Corp can supply this type of window, also with AR coating.

• ‘Transparent’ conductive film is available to prevent static buildup on windows, & thus prevent particles from sticking. Initial results appear promising. Customer at Brigham-Young Univ. has done testing.


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SummarySummary• Careful selection of particles can offer much improved signals in

backscatter measurements in water

• In air, TSI atomizers are a proven liquid particle generation technique

• Solid Particles:

– No universal solution

– Acceptable particles and dispersion technique normally available for a given application.

TSI Incorporated Seeding of Flows TSI LDV/PDPA Workshop & Training Presented by Joseph Shakal Ph.D.

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