Bulk Motion of Aerosols

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Bulk Motion of Aerosols

• by David Ayres

www.mattbauza.com/coolpics
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Examples of Bulk Motion
www.chaos.org.uk
www.soest.hawaii.edu/HIGEAR
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What is a bulk aerosol?

• Region of high aerosol concentration w/ a
  definite boundary in a much larger region of
  clean air.
• a.k.a. Cloud
• Examples
• Cigarette smoke ring 30 g/m3
• Cloud (H2O) 1 g/m3

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Cloud Settling

• VTS Settling velocity of one particle.
• VC Settling velocity of the cloud.

VTS
VTS
VC
VTS lt VC
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Cloud Settling How can Vc gt VTS?

• Path of Least Resistance
• depends on Cm

S FD

• Low Cm
• Wind moves through.

FD
VTS
VC
Group discussion High Cm?
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Cloud SettlingHow can Vc gt VTS?

• Now consider high Cm.
• Wind moves around.

S FD
FD, C
VTS

• Before calculating, must know cloud properties

VC
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Cloud properties

• Depend on the bulk properties of the aerosol, not
  the individual particles.
• Viscosity
• Density
• Net Density

?C Cloud viscosity ?g Gas viscosity Cv
Particle volume concentration Cm Particle mass
concentration ?g Gas density ?C, abs Cloud
absolute density ?C Cloud net
density

• Advantage of net density?

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Cloud Settling Velocity

• Settling velocity

• Should we calculate CD in the Stokes or Newton
  region? Why is this a problem?

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Cloud Drag Coefficient

• Use the equation
• Use Table 3.4 to find Re as a f(CDRe2).
• Solve for CD.
• For 4.4x105 lt CDRe2 lt 1.76x1010,
• CD 0.44.
• For Re lt 1,
• CD 24/Re.

Table 3.4 (Excerpt)a
Re f(99000) 403
a Source Hinds, 1999
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Example Problem

• A spherical cloud of water droplets 5 m in
  diameter has a mass concentration of 1 g/m3.
  What is the terminal settling velocity of the
  cloud?

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Cloud vs. Particle Settling

• Recall
• Then, the ratio of cloud settling velocity to
  particle settling velocity is

in the Stokes region (Re lt1)
Slip correction for the particles.
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Minimum settling concentration
Table 17.1 Minimum Concentration for Cloud
Settling (G 1) at Standard Conditionsa

• How does minimum concentration change with
  increasing cloud size? Does this make sense?
• Are these settling velocities significant?

a Source Hinds, 1999
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Settling velocity 1 cm/s
Table 17.2 Concentrations for a Cloud Settling
Velocity of 0.01 m/s 1 cm/s at Standard
Conditionsa

• How does mass conc. change with increasing cloud
  size? Why?
• What about conc.?

a Source Hinds, 1999
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Life as a Cloud

• Assume clouds start like this

www.flickr.com, Esther17
but most end up like this.

• What gives?

www.icteacher.co.uk
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Life as a Cloud
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Bonus! (Non-quizzable)

• Rayleigh-Taylor instability
• Heavy fluid lies on top of light fluid.
• Heavy fluid breaks through.
• Light fluid accelerates
• heavy fluid.
• Needs a force field. What
• type is in the picture?

en.wikipedia.org
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Reflection
But wait, theres more
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Dust ExplosionsThe Basics

• by David Ayres

http//www.cdc.gov/niosh/nas/mining/
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The more you know

• 40 dust explosions annually in U.S.
• Require 3 things
• Oxygen
• Ignition
• Fuel
• Coal
• Sawdust
• Powdered
• milk

Screen capture from Science Made Easy, by Henry
Brouwer
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Dust vs. Gas
Figure 18.2 Range of explosive concentrations in
air.a

• Based on the figure, why is dust more dangerous
  than gas?

a Source Hinds, 1999
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Dust vs. Gas (cont.)

• Gas Detonation Shock wave due to very fast
  flame speed.
• Dust Deflagration Slow flame catches newly
  suspended dust kicked up by faster sound wave.

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General Properties
Figure 18.3 Effect of particle size on minimum
explosive concentration.a
Minimum explosive concentration, g/m3
Particle diameter, µm

• Minimum conc. decreases as ease of volatilization
  increases.

a Source Hinds, 1999
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General Properties (cont.)
Table 18.1 Explosive characterists of selected
dusts (excerpt).a

• What makes a material more hazardous?

a Source Hinds, 1999
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Prevention

• Control ignition sources.
• Properly ground equipment.
• Frequently clean up dust.
• Ventilation and monitoring.
• Segregate dangerous operations.

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Reflection