The Ultrasonic Scrubber: Description,
Performance and Operating Mechanism
John R. Saylor
Department of Mechanical Engineering
Clemson University
Clemson, SC, USA
Collaborators:
Weiyu Ran (Cummins Inc.)
Steven Fredericks (Clemson Univ.)
Tyler Merrell (Clemson Univ.)
R. Glynn Holt (Boston Univ.)
Financial Support:
NSF
NIOSH
Air pollution (particulate)
Alex Hofford Photography
Wikipedia
•Ship track
NASA, J. Allen & R. Simmon
Wikimedia Commons
% Increase in Deaths
Fine particles (PM 2.5) and daily death rate
PM2.5 (mg/m3)
J. Schwartz et. al. Environ Health Perspect. 2002 October; 110(10)
So, O(1) micron particles are dangerous!
Technologies to control particulate pollution
Wet scrubber
Electrostatic Precipitator
Source: EPA
Fabric Filter
Wet scrubber
• Advantages




Simultaneously collect particulate matter and gaseous pollutants.
Treat particulate matter regardless of its composition and condition.
Compact.
Low energy cost.
• Disadvantage
 Has limited capability to remove fine particles (order of 1 μm).
Wet scrubber
100
10−1
E
10−2
10−3
10−4
10−3 10−2 10−1
EPA
Scavenging coefficient : 𝐸 =
𝑛𝐶
𝑛𝑇
𝑛𝐶 : Number of particles removed by the device.
𝑛 𝑇 : Total number of particles enter the device
100
101
102
d (μm)
H. T. Kim, C. H. Jung, S. N. Oh, and K. W. Lee. Particle removal efficiency of gravitational
wet scrubber considering diffusion, interception, and impaction. Environmental
engineering science, 18(2):125–136, 2001.
Motivation
- How can we get wet scrubbers to do a
better job of removing O(1mm) particles
from pollution?
(Wikipedia)
- How can we get water sprays to do a
better job of removing O(1mm) particles?
Wet scrubber
Which really means…
(EPA)
Why are sprays bad at O(1um) particles?
𝑑 ≫ 1μm
𝑑 ≪ 1μm
𝑑~1μm
Particle scavenging mechanism
Big Particles
𝑑 ≫ 1μm
𝑑 ≪ 1μm
𝑑~1μm
Particles scavenged
by inertial processes
Particle scavenging mechanism
Big Particles
𝑑 ≫ 1μm
Particles scavenged
by inertial processes
Small Particles
𝑑 ≪ 1μm
Particles scavenged
by diffusive processes
𝑑~1μm
Particle scavenging mechanism
Big Particles
𝑑 ≫ 1μm
Particles scavenged
by inertial processes
Small Particles
𝑑 ≪ 1μm
Particles scavenged
by diffusive processes
Intermediate Size
Particles
𝑑~1μm
Neither scavenging
process works well !
How Do We Solve This Problem?
We need some force that pushes particles
and drops together.
Our approach was to use ultrasonics (the
acoustic radiation force, Far).
Ultrasonic Standing Wave Fields
It has long been known that an Ultrasonic
Standing Wave Field (USWF) can be used
to levitate objects.
How Does This Work?
L
Ultrasonic transducer
Reflector
Pressure Nodes
Sound pressure distribution
How Does This Work?
Levitation and Accretion in USWF
Hypothesis
We hypothesized that we could get drops
and particles to accrete in an USWF and
thereby be removed from the air.
The Ultrasonic Scrubber
E: Scavenging Coefficient
E = (Particles In – Particles Out)/(Particles In)
150mm
300mm
Successful Results
𝑛𝐶
𝐸=
𝑛𝑇
I
𝑛𝐶 : Number of particles removed by the device.
𝑛 𝑇 : Total number of particles entering the device
 Ew  Ewo 
Ewo
 100%
Ew: with ultrasonics
Ewo: without ultrasonics
I was as large as 140%
Results
𝑑𝑝 =0.9±0.2μm
Results
𝐸𝑤 − 𝐸𝑤𝑜
𝐼=
× 100%
𝐸𝑤𝑜
𝐸𝑤 : Scavenging coefficient with ultrasonics.
𝐸𝑤𝑜 : Scavenging coefficient without ultrasonics.
Results
Results
7
But why? What is the mechanism?
Particle entrained in wake?
Drop
Particle
Node (Accretion Disk)
Particles and drops
moved?
Particles agglomerate,
then moved?
Acoustic radiation force (Settnes & Bruus)
𝑘𝜋 3
𝐹𝑎𝑟 =
𝑑 𝐸𝑎𝑐 ϕ sin 2𝑘𝑦
6
1
3
1
2
ϕ = 𝑓1 + 𝑓2
𝜅0
2 1−𝛾
𝑓2 = ʀ
𝜌𝑝
−1
𝜌0
(N)
𝜌𝑝
+1−3𝛾
𝜌0
2
𝛾=−
3
2
𝛿=
2𝜈
𝜔
1+𝑖 1+
2𝛿
𝑑
2𝛿
𝑑
𝐹𝑎𝑟
𝑓1 = 1 −
𝜅𝑝
𝑘 : Wave number
𝑑 : Particle diameter
y: location in the standing wave field
𝐸𝑎𝑐 : Acoustic energy density
𝜅𝑝 : Compressibility of the particle
𝜅0 : Compressibility of the air
𝜌𝑝 : Density of the particle
𝜌0 : Density of the air
𝜈 : Kinematic viscosity of the air
𝜔 : Angular frequency of the
acoustic wave
Simulations
Drag force
𝐹𝑎𝑟
Drop
𝑚𝑑 𝑔
Drag force
𝐹𝑎𝑟
Particle
The trajectories of spray drops and partiles
dp = 0.9 mm
dd = 87 mm
Ug = 2.8 cm/s
Ql = 0.92 ml/s
Results
(1) Droplet Reynolds numbers
were all O(1), so there is no wake
(and no particle entrainment).
Particle concentration in the accretion disk
𝑊0
𝐶𝑦 = 𝐶0
𝑊𝑦
𝐶𝑦 : Concentration of particles
at location 𝑦
𝑊𝑦
𝐶0 : Concentration of particles
at y=0
𝑊0
Maximum 𝐶0 ~107 /m3
𝑊0
Maximum
~104
𝑊𝑦
11
3
𝐶𝑚𝑎𝑥 ~10 /m
Particle-particle interactions in accretion disk
𝐶𝑚𝑎𝑥 ~1011 /m3
1
𝐸𝑎 = 1 −
× 100%
1 + 𝐶𝑚𝑎𝑥 𝐾𝑡
𝐸𝑎 : Percent decrease of particle
number due to particle-particle
agglomeration
𝐾: Particle agglomeration coefficient
𝑡 : Time particles reside in the accretion disk
𝐸𝑎 is only 0.1% of observed scavenging in the accretion disk.
Results
(1) Droplet Reynolds numbers
were all O(1), so there is no
wake (and no particle
entrainment).
(2) Particle-particle agglomeration
can be eliminated.
Conclusions
(1) An un-optimized ultrasonic scrubber is
able to improve spray drop scavenging by
as much as 140%.
(2) The mechanism is movement of both
drops and particles to the node where dropparticle interactions result in particle
removal.
(3) For particle concentrations like those in
actual industrial emissions (C = 1010 /m3 –
1013 /m3), particle-particle may be
comparable to particle-drop.
Questions?
Thank you!