Note: Descriptions are shown in the official language in which they were submitted.
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IMPROVED SEPARATION OF MULTI-COMPONENT FLUID THROUGH
ULTRASONIC ACOUSTOPHORESIS
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional Patent
Application Serial
No. 61/671,856, filed on July 16, 2012 and to U.S. Provisional Patent
Application Serial
No. 61/754,792, filed January 21, 2013; and is a continuation-in-part of U.S.
Serial No.
13/844,754, filed March 15, 2013, which claimed the benefit of U.S.
Provisional Patent
Application Serial No. 61/754,792, filed January 21, 2013. These applications
are
incorporated herein by reference in their entireties.
BACKGROUND
[0002] Many industrial applications generate wastewater that is
contaminated with
undesirable or hazardous fluid materials, such as oil. These operations
include oil
drilling, mining and natural gas fracking. Also, spills from oil rigs into
seawater generate
emulsified oil in the water that is difficult to separate. The use of methods
such as
hydrocyclones, absorptive media, mechanical filtration, and chemical
dispersion to
separate the oil from the water are both cost prohibitive and possibly
injurious to the
environment.
[0003] Acoustophoresis is the separation of particles using high intensity
sound
waves. It has long been known that high intensity standing waves of sound can
exert
forces on particles. A standing wave has a pressure profile which appears to
"stand" still
in time. The pressure profile in a standing wave varies from areas of high
pressure
(nodes) to areas of low pressure (anti-nodes). Standing waves are produced in
acoustic
resonators. Common examples of acoustic resonators include many musical wind
instruments such as organ pipes, flutes, clarinets, and horns.
[0004] It would be desirable to provide more effective methods of
separating
emulsified oil and other contaminants from the contaminated water at reduced
cost and
low environmental impact.
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BRIEF DESCRIPTION
[0005] The present disclosure relates to systems and devices for
acoustophoresis on
preferably a large scale. The devices use one or more unique ultrasonic
transducers as
described herein, or an array of such transducers. The transducer is driven at
frequencies that produce multi-dimensional standing waves.
[0006] Disclosed in certain embodiments is an acoustophoresis device,
comprising:
one or more device inlets at a first end of the device, the first end having a
first diameter
for receiving fluid flow; a contoured wall downstream of the inlet that
narrows the fluid
flow to a second diameter of a connecting duct; a flow chamber downstream of
the
connecting duct, the flow chamber having: an inlet at a first end for
receiving the fluid
flow, an outlet at a second end opposite the first end, at least one
ultrasonic transducer
located on a wall of the flow chamber, the ultrasonic transducer including a
piezoelectric
material driven by a voltage signal to create a multi-dimensional standing
wave in the
flow chamber, and a reflector located on a wall on the opposite side of the
flow chamber
from the at least one ultrasonic transducer; a first device outlet located at
the first end of
the device and separated from the device inlet by a longitudinal sidewall; and
a second
device outlet located at a second end of the device downstream of the flow
chamber
outlet.
[0007] The device may include a plurality of device inlets spaced about the
first end
of the device, and the longitudinal sidewall is spaced apart from the
contoured wall.
[0008] The piezoelectric material of the at least one ultrasonic transducer
can have a
rectangular shape. The reflector can have a non-planar surface.
[0009] In particular embodiments, the first end of the device has a
circular cross-
section and the flow chamber has a rectangular cross-section.
[0010] The multi-dimensional standing wave generated by the transducer can
result
in an acoustic radiation force having an axial force component and a lateral
force
component that are of the same order of magnitude.
[0011] In embodiments, the transducer comprises: a housing having a top
end, a
bottom end, and an interior volume; and a crystal at the bottom end of the
housing
having an exposed exterior surface and an interior surface, the crystal being
able to
vibrate when driven by a voltage signal.
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[0012] Sometimes, no backing layer is present within the housing, and an
air gap is
present in the interior volume between the crystal and a top plate at the top
end of the
housing.
[0013] In other devices, the transducer further comprises a backing layer
contacting
the interior surface of the crystal, the backing layer being made of a
substantially
acoustically transparent material. The substantially acoustically transparent
material
can be balsa wood, cork, or foam. The substantially acoustically transparent
material
may have a thickness of up to 1 inch.
[0014] The flow chamber can further comprise a transparent window for
viewing the
interior of the flow chamber.
[0015] In particular embodiments, the device has a length L from the at
least one
device inlet to a bottom of the longitudinal sidewall, and a ratio of the
length L to the first
diameter is less than 1.
[0016] These and other non-limiting characteristics are more particularly
described
below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The patent or application file contains at least one drawing
executed in color.
Copies of this patent or patent application publication with color drawing(s)
will be
provided by the Office upon request and payment of the necessary fee.
[0018] The following is a brief description of the drawings, which are
presented for
the purposes of illustrating the exemplary embodiments disclosed herein and
not for the
purposes of limiting the same.
[0019] FIG. 1 is a front top perspective view of an exemplary embodiment of
a
device of the present disclosure.
[0020] FIG. 2 is a front bottom perspective view of the device of FIG. 1.
[0021] FIG. 3 is a right side view of the device of FIG. 1.
[0022] FIG. 4 is a front view of the device of FIG. 1.
[0023] FIG. 5 is a rear view of the device of FIG. 1.
[0024] FIG. 6 is a left side view of the device of FIG. 1.
[0025] FIG. 7 is a top view of the device of FIG. 1.
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[0026] FIG. 8 is a bottom view of the device of FIG. 1.
[0027] FIG. 9 is a right side cross-sectional view of the device of FIG. 1.
[0028] FIG. 10 is a cross-sectional diagram of a conventional ultrasonic
transducer.
[0029] FIG. 11 is a cross-sectional diagram of an ultrasonic transducer of
the present
disclosure. An air gap is present within the transducer, and no backing layer
is present.
[0030] FIG. 12 is a photo of a square transducer and a circular transducer
suitable
for use in the devices of the present disclosure.
[0031] FIG. 13 is a graph of electrical impedance amplitude versus
frequency as a
square transducer is driven at different frequencies.
[0032] FIG. 14 illustrates the trapping line configurations for seven of
the peak
amplitudes of FIG. 13.
[0033] FIG. 15 illustrates a possible array configuration for a group of
transducers.
[0034] FIG. 16 illustrates another possible array configuration for a group
of
transducers.
[0035] FIG. 17 is a computer model of an acoustophoretic separator
simulated to
generate FIGS. 18-29.
[0036] FIG. 18 shows a simulation of the axial forces on a particle in an
acoustophoretic separator having a piezoelectric crystal producing a single
standing
wave.
[0037] FIG. 19 shows a simulation of the lateral forces on a particle in an
acoustophoretic separator having a piezoelectric crystal producing a single
standing
wave.
[0038] FIG. 20 shows a simulation of the axial forces on a particle in an
acoustophoretic separator having a piezoelectric crystal in a multi-mode
excitation.
[0039] FIG. 21 shows a simulation of the lateral forces on a particle in an
acoustophoretic separator a piezoelectric crystal in a multi-mode excitation.
[0040] FIG. 22 shows a three dimensional computer generated model of a mode
shape calculation for a circular crystal driven at a frequency of 1 MHz.
[0041] FIG. 23 shows the lateral (horizontal) acoustic radiation force at
1.9964 MHz.
[0042] FIG. 24 shows the axial (vertical) component for a resonance
frequency of
1.9964 MHz.
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[0043] FIG. 25 shows the acoustic pressure amplitude at 1.9964 MHz.
[0044] FIG. 26 shows the lateral force component at a resonance frequency
of
2.0106 MHz.
[0045] FIG. 27 shows the axial acoustic radiation force component at a
resonance
frequency of 2.0106 MHz.
[0046] FIG. 28 shows the lateral force component at a resonance frequency
of 2.025
MHz.
[0047] FIG. 29 shows the axial acoustic radiation force component at a
resonance
frequency of 2.025 MHz.
[0048] FIG. 30 is a picture showing the results of an oil/water separation
experiment.
DETAILED DESCRIPTION
[0049] The present disclosure may be understood more readily by reference
to the
following detailed description of desired embodiments and the examples
included
therein. In the following specification and the claims which follow, reference
will be
made to a number of terms which shall be defined to have the following
meanings.
[0050] The singular forms "a," "an," and "the" include plural referents
unless the
context clearly dictates otherwise.
[0051] As used in the specification and in the claims, the term
"comprising" may
include the embodiments "consisting of" and "consisting essentially of."
[0052] Numerical values should be understood to include numerical values
which are
the same when reduced to the same number of significant figures and numerical
values
which differ from the stated value by less than the experimental error of
conventional
measurement technique of the type described in the present application to
determine the
value.
[0053] All ranges disclosed herein are inclusive of the recited endpoint
and
independently combinable (for example, the range of "from 2 grams to 10 grams"
is
inclusive of the endpoints, 2 grams and 10 grams, and all the intermediate
values).
[0054] As used herein, approximating language may be applied to modify any
quantitative representation that may vary without resulting in a change in the
basic
function to which it is related. Accordingly, a value modified by a term or
terms, such as
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"about" and "substantially," may not be limited to the precise value
specified. The
modifier "about" should also be considered as disclosing the range defined by
the
absolute values of the two endpoints. For example, the expression "from about
2 to
about 4" also discloses the range "from 2 to 4."
[0055] It should be noted that many of the terms used herein are relative
terms. For
example, the terms "upper" and "lower" are relative to each other in location,
i.e. an
upper component is located at a higher elevation than a lower component in a
given
orientation, but these terms can change if the device is flipped. The terms
"inlet" and
"outlet" are relative to a fluid flowing through them with respect to a given
structure, e.g.
a fluid flows through the inlet into the structure and flows through the
outlet out of the
structure. The terms "upstream" and "downstream" are relative to the direction
in which
a fluid flows through various components, Le. the flow fluids through an
upstream
component prior to flowing through the downstream component. It should be
noted that
in a loop, a first component can be described as being both upstream of and
downstream of a second component.
[0056] The terms "horizontal" and "vertical" are used to indicate direction
relative to
an absolute reference, i.e. ground level. However, these terms should not be
construed
to require structures to be absolutely parallel or absolutely perpendicular to
each other.
For example, a first vertical structure and a second vertical structure are
not necessarily
parallel to each other. The terms "top" and "bottom" or "base" are used to
refer to
surfaces where the top is always higher than the bottom/base relative to an
absolute
reference, i.e. the surface of the earth. The terms "upwards" and "downwards"
are also
relative to an absolute reference; an upwards flow is always against the
gravity of the
earth.
[0057] The present application refers to "the same order of magnitude." Two
numbers are of the same order of magnitude if the quotient of the larger
number divided
by the smaller number is a value less than 10.
[0058] Efficient separation technologies for multi-component liquid streams
that
eliminate any waste and reduce the required energy, and therefore promote a
sustainable environment, are needed. Large volume flow rate acoustophoretic
phase
separator technology using ultrasonic standing waves provides the benefit of
having no
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consumables, no generated waste, and a low cost of energy. The technology is
efficient
at removal of particles of greatly varying sizes, including separation of
micron and sub-
micron sized particles. Examples of acoustic filters/collectors utilizing
acoustophoresis
can be found in commonly owned U.S. Patent Application Serial Nos. 12/947,757;
13/085,299; 13/216,049; and 13/216,035, the entire contents of each being
hereby fully
incorporated by reference. Generally, an acoustophoretic system employs
ultrasonic
standing waves to trap (i.e. hold stationary) secondary phase particles,
gases, or liquids
that are suspended in a host fluid stream. The secondary phase can be
continuously
separated out of the host fluid as the mixture flows through the
acoustophoretic system.
[0059] The platform technology described herein provides an innovative
solution that
includes a large volume flow rate acoustophoretic phase separator based on
ultrasonic
standing waves with the benefit of having no consumables, no generated waste,
and a
low cost of energy. Acoustophoresis is a low-power, no-pressure-drop, no-clog,
solid-
state approach to particle removal from fluid dispersions: i.e., it is used to
achieve
separations that are more typically performed with porous filters, but it has
none of the
disadvantages of filters. In particular, the present disclosure provides
systems that
operate at the macro-scale for separations in flowing systems with high flow
rates. The
acoustic resonator is designed to create a high intensity three dimensional
ultrasonic
standing wave that generates three dimensional pressure gradients and results
in an
acoustic radiation force that is larger than the combined effects of fluid
drag and
buoyancy or gravity, and is therefore able to trap (i.e., hold stationary) the
suspended
phase to allow more time for the acoustic wave to increase particle
concentration,
agglomeration and/or coalescence. The present systems have the ability to
create
ultrasonic standing wave fields that can trap particles in flow fields with a
linear velocity
ranging from 0.1 mm/sec to velocities exceeding 1 cm/s. This technology offers
a green
and sustainable alternative for separation of secondary phases with a
significant
reduction in cost of energy. Excellent particle separation efficiencies have
been
demonstrated for particle sizes as small as one micron.
[0060] This is an important distinction from previous approaches where
particle
trajectories were merely altered by the effect of the acoustic radiation
force. The
scattering of the acoustic field off the particles results in a three
dimensional acoustic
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radiation force, which acts as a three-dimensional trapping field. The
acoustic radiation
force is proportional to the particle volume (e.g. the cube of the radius)
when the particle
is small relative to the wavelength. It is proportional to frequency and the
acoustic
contrast factor. It also scales with acoustic energy (e.g. the square of the
acoustic
pressure amplitude). For harmonic excitation, the sinusoidal spatial variation
of the force
is what drives the particles to the stable positions within the standing
waves. When the
acoustic radiation force exerted on the particles is stronger than the
combined effect of
fluid drag force and buoyancy/gravitational force, the particle is trapped
within the
acoustic standing wave field. The action of the acoustic forces on the trapped
particles
results in concentration, agglomeration and/or coalescence of particles and
droplets.
Particles which are denser than the host fluid are separated through enhanced
gravitational settling, and particles which are less dense than the host fluid
are
separated through enhanced buoyancy.
[0061] Efficient and economic particle separation processes can be useful
in many
areas of energy generation, e.g., producing water, hydro-fracking, and bio-
fuels, e.g,
harvesting and dewatering. Acoustophoretic technology can be used to target
accelerated capture of bacterial spores in water, oil-recovery, and dewatering
of bio-oil
derived from micro-algae. Current technology used in the oil recovery field
does not
perform well in recovery of small, i.e., less than 20 micron, oil droplets.
However, the
acoustophoretic systems described herein can enhance the capture and
coalescence of
small oil droplets, thereby shifting the particle size distribution resulting
in an overall
increased oil capture. To be useful, it is generally necessary to demonstrate
large flow
rates at a level of 15-20 gallons per minute (GPM) per square foot (cross-
sectional
area). Another goal is the increased capture of oil droplets with a diameter
of less than
20 microns. Much prior work on acoustophoretics only occurred at the
microscale, in
MEMS applications in research settings. Industrial processes require high flow
rates
and continuous operation.
[0062] Acoustophoretic separation can also be used to aid such applications
as
advanced bio-refining technology to convert low-cost readily available non-
food biomass
(e.g. municipal solid waste and sewage sludge) into a wide array of chemicals
and
secondary alcohols that can then be further refined into renewable gasoline,
jet fuel, or
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diesel. A water treatment technology is used to de-water the fermentation
broth and
isolate valuable organic salts for further processing into fuels. The
dewatering process
is currently done through an expensive and inefficient ultra-filtration method
that suffers
from frequent fouling of the membranes, a relatively low concentration factor,
and a high
capital and operating expense. Acoustophoretic separation can filter out
particles with
an incoming particle size distribution that spans more than three orders of
magnitude,
namely from 600 microns to 0.3 microns, allowing improvements in the
concentration of
the separated broth with a lower capital and operational expense.
[0063] Acoustophoretic separation is also useful for the harvesting, oil-
recovery, and
dewatering of micro-algae for conversion into bio-oil. Current harvesting, oil
recovery,
and dewatering technologies for micro-algae suffer from high operational and
capital
expenses. Current best estimates put the price of a barrel of bio-oil derived
from micro-
algae at a minimum of $200.00 per barrel. There is a need in the art of micro-
algae
biofuel for technologies that improve harvesting, oil-recovery, and dewatering
steps of
this process. Acoustophoretic separation technology meets this need.
[0064] Some other applications are in the areas of wastewater treatment,
grey water
recycling, and water production. Other applications are in the area of
biopharmaceuticals, life sciences, and medical applications, such as the
separation of
lipids from red blood cells. This can be of critical importance during
cardiopulmonary
bypass surgery, which involves suctioning shed mediastinal blood. Lipids are
unintentionally introduced to the bloodstream when blood is re-transfused to
the body.
Lipid micro-emboli can travel to the brain and cause various neuro-cognitive
disorders.
Therefore, there is a need to cleanse the blood. Existing methods are
currently
inefficient or harmful to red blood cells.
[0065] Particular embodiments focus on the capture and growth of sub 20
micron oil
droplets. At least 80% of the volume of sub-20-micron droplets are captured
and then
grown to droplets that are bigger than 20 microns. The process involves the
trapping of
the oil droplets in the acoustic standing wave, coalescence of many small
trapped
droplets, and eventually release of the larger droplets when the acoustic
trapping force
becomes smaller than the buoyancy force.
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[0066] Desirably, the ultrasonic transducers generate a three-dimensional
standing
wave in the fluid that exerts a lateral force on the suspended particles /
secondary fluid
to accompany the axial force so as to increase the particle trapping
capabilities of a
acoustophoretic system. Typical results published in literature state that the
lateral force
is two orders of magnitude smaller than the axial force. In contrast, the
technology
disclosed in this application provides for a lateral force to be of the same
order of
magnitude as the axial force.
[0067] The present disclosure relates to the use of an acoustic standing
wave
generated by an ultrasonic transducer or transducers to separate oil from
processed
water on a macro scale. The oil may be partially emulsified with the water.
The
separation occurs by trapping the oil particles at the pressure nodes and anti-
pressure
nodes in a standing wave. As the oil is trapped at these nodes, it
agglomerates and,
due to buoyancy, will move to an area of trapped, concentrated oil. The
buoyancy
separation is accomplished through fluid dynamics with the main fluid stream
flowing in
a downward direction and the trapped, agglomerated and coalesced oil particles
floating
upward, due to buoyancy, into a trap.
[0068] The oil particles are separated from the fluid stream at the anti-
pressure
nodes of the acoustic standing wave due to the difference in their acoustic
contrast
factors from the fluid stream. The equation for determining the acoustic
contrast factor
of an oil in a fluid is known, and is related to the density of the fluid, the
density of the oil
in the fluid, the compressibility of the fluid, and the compressibility of the
oil in the fluid.
Both oil and emulsified oil typically have a negative contrast factor (4)).
[0069] In the present disclosure, a 3-D acoustic standing wave is generated
by
causing the ultrasonic transducer to act in a "drumhead" fashion as opposed to
a
"piston" fashion. The "drumhead" operation of the piezoelectric element in the
ultrasonic
transducer causes multiple standing waves to be generated in a 3-D space. This
is
opposed to the action of the piezoelectric crystal in the ultrasonic
transducer acting in a
"piston" fashion n where a single standing wave is produced. Through the use
of a 3-0
multi-standing wave, macro-scale trapping of oil particles may be
accomplished. This
allows for high volumes of processed water to be treated and the oil to be
separated
from the water,
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[0070] The piezoelectric crystal in the ultrasonic transducer may be
directly
interfaced with the fluid stream or may have a protective layer or matching
layer over
the surface of the piezoelectric crystal that is interfaced with the fluid
stream, The
protective layer may be a coating, such as a polyurethane or epoxy. The
protective
layer may also be plated onto the surface of the piezoelectric crystal that is
interfaced
with the fluid stream. The plated layer may be added to the surface of the
piezoelectric
crystal through either electrolytic or electroless plating. The plating
material may be
nickel, chrome, copper, indium or combination of layers of these materials.
Also, the
secondary material or matching layer may be adhered to the surface of the
piezoelectric
crystal such that the matching layer is now interfaced with the fluid stream.
The
matching layer may be a material such as a stainless steel that is adhered to
the
piezoelectric crystal through the use of a two-part epoxy system.
[0071] FIGs. 1-9 show various views of an acoustophoresis device of the
present
disclosure. Generally, the acoustophoresis device uses the ultrasonic
transducer to
separate suspended oil particles/droplets in a fluid stream into ordered,
coalesced and
agglomerated particles trapped in a standing wave of the acoustophoresis
device. The
flow of the fluid stream is from the upper end downward (i.e. with gravity).
The fluid
stream can enter the device through one of many inlets that surround a central
trapping
device for the agglomerated and separated oil. The fluid stream flows into the
acoustophoresis separation device from a pump through the inlet. The
agglomerated
and coalesced oil gains buoyancy and rises into the central oil trapping
device. The
device is shown here in an orientation where the flow direction is downwards,
which is
used for separating less-dense particles from the host fluid. However, the
device may
be essentially turned upside down to allow separation of particles which are
heavier
than the host fluid. Instead of a buoyant force in an upward direction, the
weight of the
agglomerated particles due to gravity pulls them downward.
[0072] The initial fluid stream is made up of a host fluid (e.g. water) and
a suspended
phase (e.g. oil droplets/articles). The fluid stream enters the device 200
through one or
more device inlets 206 into an annular plenum 220 at a first end 202 of the
device. The
first end 202 includes an outer sidewall 222 and an inner longitudinal
sidewall 224. An
end wall 212 is also visible, from which the longitudinal sidewall extends.
The term
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"annular," as used herein, only designates the area or volume between the
outer
sidewall and the inner longitudinal sidewall, and should not be construed as
requiring
the first end of the device to have a circular cross-section. However, in
contemplated
embodiments the first end of the device has a circular cross-section. The
annular
plenum has an inner diameter 225 and an outer diameter 227. This construction
guides
the fluid stream flow downwards in the direction of the centerline, i.e. with
little to no
radial or circumferential motion component. This helps to create laminar/plug
flow later
downstream. One device inlet 206 is shown here, with three other inlets spaced
about
the first end being shown in dotted line. It is contemplated that any number
of inlets
may be provided as desired. In particular embodiments, four inlets are used.
The inlets
are radially oriented.
[0073] A contoured nozzle wall 230 reduces the outer diameter of the flow
path,
which generates higher velocities near the wall and reduces turbulence,
producing near
plug flow as the fluid velocity profile develops and the fluid passes through
the
connecting duct and into a flow/separation chamber. The contoured wall also
adds a
radial motion component to the suspended particles, moving the particles
closer to the
centerline of the device and generating more collisions with rising, buoyant
agglomerated particles. This radial motion will allow for optimum scrubbing of
the
particles from the fluid in the connecting duct prior to reaching the
separation chamber.
The term scrubbing is used to describe the process of particle/droplet
agglomeration,
aggregation, clumping or coalescing, that occurs when a larger
particle/droplet travels in
a direction opposite to the fluid flow and collides with smaller particles, in
effect
scrubbing the smaller particles out of the suspension. The contoured nozzle
wall directs
the fluid in a manner that generates large scale vortices at the entrance of
the first
device outlet to also enhance particle collection. Generally, the flow area of
the device is
designed to be continually decreasing from the device inlets to the separation
chamber
to assure low turbulence and eddy formation for better particle separation,
agglomeration, and collection. Put another way, the contoured wall 230 has a
wide end
232 and a narrow end 234. The first end of the device / the wide end of the
nozzle wall
has a first diameter 235, and the narrow end of the nozzle wall has a second
diameter
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237. The second diameter is less than the first diameter. The connecting duct
240 is
downstream of the nozzle wall and connects to the inlet 256 of the flow
chamber 250.
[0074] The flow/separation chamber 250 is downstream of the connecting duct
240
and has an inlet 256 at a first end 252, and an outlet 258 at a second end 254
opposite
the first end. At least one ultrasonic transducer 270 is present on a wall
260, and a
reflector 272 is located on a wall 262 opposite the transducer. Multiple
transducers can
be used, as desired. In use, standing waves are created between the transducer
270
and reflector 272. These standing waves can be used to agglomerate particles,
and
this orientation is used to agglomerate particles that are buoyant (e.g. oil).
Fluid,
containing residual particles, then exits through the flow chamber outlet 258
and
through a second device outlet 210 located at a second end 204 of the device
opposite
the first end 202 of the device. Also shown here is a transparent window 274
on a third
wall 264 of the flow chamber. It is contemplated that in particular
embodiments, the
flow chamber has a rectangular cross-section. The flow chamber inlet and
outlets have
a circular cross-section for interfacing with the other components of the
device.
[0075] As the buoyant particles agglomerate, they eventually overcome the
combined effect of the fluid flow drag forces and acoustic radiation force,
and their
buoyant force is sufficient to cause the buoyant particles to rise upwards. In
this regard,
a first device outlet or collection duct 208 is present at the first end of
the device 202,
and is surrounded by the longitudinal sidewall 224, or put another way is
separated from
the device inlets 206 by the longitudinal sidewall 224, or put yet another way
the first
device outlet is a hole in the end wall 212. The agglomerated buoyant
particles exit the
device through the first device outlet 208. The first device outlet and the
second device
outlet are on opposite ends of the device.
[0076] It should be noted that the buoyant particles formed in the
separation
chamber 250 subsequently pass through the connecting duct 240. This causes the
incoming fluid stream flow from the device inlets 206 to flow over the rising
agglomerated particles due to the inward radial motion imparted by the
contoured wall
230. This allows the rising particles to also trap smaller particles in the
incoming flow,
increasing scrubbing effectiveness. The length of the connecting duct and the
contoured
nozzle wall thus increase scrubbing effectiveness. Especially high
effectiveness is
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found for particles with a size of 0.'1 microns to 10 microns, where
efficiency is very low
for conventional methods. As noted here, the distance from the device inlets
206 to the
bottom of the longitudinal sidewall 224 is marked as length (L). The first
diameter is
marked as D1 (reference numeral 235). This length-to-diameter ratio here (i.e.
L/D1) is
less than 1.
[0077] The design here results in low flow turbulence at the flow chamber
inlet, a
scrubbing length before (i.e. upstream of) the flow chamber to enhance
particle
agglomeration and/or coalescence before acoustic separation, and the use of
the
collection vortices to aid particle removal upstream of the flow chamber.
[0078] The ultrasonic transducer(s) are arranged to cover the entire cross-
section of
the fluid stream flowpath. In certain embodiments, the flow chamber has a
square cross
section of 6 inches x 6 inches which operates at flow rates of up to 3 gallons
per minute
(GPM), or a linear velocity of 8 mm/sec. The transducer can be a PZT-8 (Lead
Zirconate Titanate) transducer with a 1-inch diameter and a nominal 2 MHz
resonance
frequency. Each transducer consumes about 28 W of power for droplet trapping
at a
flow rate of 3 GPM. This translates in an energy cost of 0.25 kW hr/ m3. This
is an
indication of the very low cost of energy of this technology. Desirably, when
multiple
transducers are present, each transducer is powered and controlled by its own
amplifier. This device shifts the particle size distribution in the host fluid
through
agglomeration of smaller oil droplets into larger oil droplets.
[0079] FIG. 10 is a cross-sectional diagram of a conventional ultrasonic
transducer.
This transducer has a wear plate/protective layer 50 at a bottom end, epoxy
layer 52,
piezoelectric material 54 (made of, e.g. PZT), an epoxy layer 56, and a
backing layer
58. The epoxy layer 56 attaches backing layer 58 to the crystal 54. The entire
assembly
is contained in a housing 60 which may be made out of, for example, aluminum.
A
connector 62 provides connection for wires to pass through the housing and
connect to
leads (not shown) which attach to the piezoelectric material 54. Typically,
backing layers
are designed to add damping and to create a broadband transducer with uniform
displacement across a wide range of frequency and are designed to suppress
excitation
at particular vibrational eigen-modes. Wear plates are usually designed as
impedance
transformers to better match the characteristic impedance of the medium into
which the
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transducer radiates, and face in the direction in which the wave is generated.
The
piezoelectric material can be, for example, a ceramic crystal.
[0080]
FIG. 11 is a cross-sectional view of an ultrasonic transducer 81 of the
present
disclosure, which can be used with the acoustophoretic device of FIGS. 1-9.
Transducer
81 has an aluminum housing 82. A PZT crystal 86 defines the bottom end of the
transducer, and is exposed from the exterior of the housing. The crystal is
supported on
its perimeter by the housing, with a small elastic layer, e.g. silicone or
similar material,
located between the crystal and the housing.
[0081]
Screws (not shown) attach an aluminum top plate 82a of the housing to the
body 82b of the housing via threads 88. The top plate includes a connector 84
to pass
power to the PZT crystal 86. The bottom and top surfaces of the PZT crystal 86
each
contain an electrode. A wrap-around electrode tab 90 connects to the bottom
electrode
and is isolated from the top electrode. Electrical power is provided to the
PZT crystal 86
through the electrodes, with the wrap-around tab 90 being the ground
connection point.
Note that the crystal 86 has no backing layer or epoxy layer as is present in
Figure 5.
Put another way, there is an air gap 87 in the transducer between aluminum top
plate
82a and the crystal 86. A minimal backing may be provided in some embodiments.
[0082]
The transducer design can affect performance of the system. A typical
transducer is a layered structure with the ceramic crystal bonded to a backing
layer and
a wear plate. Because the transducer is loaded with the high mechanical
impedance
presented by the standing wave, the traditional design guidelines for wear
plates, e.g.,
half wavelength thickness for standing wave applications or quarter wavelength
thickness for radiation applications, and manufacturing methods may not be
appropriate. Rather, in one embodiment of the present disclosure the
transducers,
there is no wear plate or backing, allowing the crystal to vibrate in one of
its eigenmodes
with a high Q-factor. The vibrating ceramic crystal/disk is directly exposed
to the fluid
flowing through the flow chamber.
[0100]
Removing the backing (e.g. making the crystal air backed) also permits the
ceramic crystal/piezoelectric material to vibrate higher order modes of
vibration (e.g.
higher order modal displacement) with little damping. In a transducer having a
crystal
with a backing, the crystal vibrates with a more uniform displacement, like a
piston.
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Removing the backing allows the crystal to vibrate in a non-uniform
displacement mode.
The higher order the mode shape of the crystal, the more nodal lines the
crystal has.
The higher order modal displacement of the crystal creates more trapping
lines,
although the correlation of trapping line to node is not necessarily one to
one, and
driving the crystal at a higher frequency will not necessarily produce more
trapping
lines. In the present disclosure, the transducers are driven so that the
piezoelectric
crystal vibrates in higher order modes of the general formula (m, n), where m
and n are
independently 1 or greater. In practice, the transducers of the present
disclosure will
vibrate at higher orders than (1,2).
[0083] In some embodiments, the crystal may have a backing that minimally
affects
the Q-factor of the crystal (e.g. less than 5%). The backing may be made of a
substantially acoustically transparent material such as balsa wood, foam, or
cork which
allows the crystal to vibrate in a higher order mode shape and maintains a
high Q-factor
while still providing some mechanical support for the crystal. In another
embodiment,
the backing may be a lattice work that follows the nodes of the vibrating
crystal in a
particular higher order vibration mode, providing support at node locations
while
allowing the rest of the crystal to vibrate freely. The goal of the lattice
work or
acoustically transparent material is to provide support without lowering the Q-
factor of
the crystal or interfering with the excitation of a particular mode shape.
[0084] Placing the crystal in direct contact with the fluid also
contributes to the high
Q-factor by avoiding the dampening and energy absorption effects of the epoxy
layer
and the wear plate. Other embodiments may have wear plates or a wear surface /
protective layer to prevent the PZT, which contains lead, contacting the host
fluid. This
may be desirable in, for example, biological applications such as separating
blood. Such
applications might use a wear layer such as chrome, electrolytic nickel, or
electroless
nickel. Chemical vapor deposition could also be used to apply a layer of
poly(p-
xylxyene) (e.g. Parylene) or other polymer. Organic and biocompatible coatings
such as
silicone or polyurethane are also contemplated for use as a wear surface.
[0085] FIG. 12 illustrates two different ultrasonic transducers that can be
used in the
devices of the present disclosure. The transducer on the right shows a
circular-shaped
PZT-8 crystal 110 that is 1 inch in diameter. The transducer on the right
shows a
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rectangular-shaped crystal, which here is a square 1 inch by 1 inch crystal.
The effect
of transducer shape on oil separation efficiency was investigated, and Table 1
shows
the results.
Table 1: Results of Investigation of Round and Square Transducer Shape
Transducer Total Power Flowrate Duration Capture Efficiency
Shape Input (ml/min) (min) (%)
(Watts)
Round 20 500 45 59%
Square 20 500 30 91%
[0086] The results indicate that the square transducer 112 provides better
oil
separation efficiencies than the round transducer 110, explained by the fact
that the
square transducer 112 provides better coverage of the flow channel with
acoustic
trapping forces, and that the round transducer only provides strong trapping
forces
along the centerline of the standing wave.
[0087] The size, shape, and thickness of the transducer determine the
transducer
displacement at different frequencies of excitation, which in turn affects oil
separation
efficiency. Typically, the transducer is operated at frequencies near the
thickness
resonance frequency (half wavelength). Gradients in transducer displacement
typically
result in more places for oil to be trapped. Higher order modal displacements
generate
three-dimensional acoustic standing waves with strong gradients in the
acoustic field in
all directions, thereby creating equally strong acoustic radiation forces in
all directions,
leading to multiple trapping lines, where the number of trapping lines
correlate with the
particular mode shape of the transducer.
[0088] FIG. 13 shows the measured electrical impedance amplitude of the
transducer as a function of frequency in the vicinity of the 2.2 MHz
transducer
resonance. The minima in the transducer electrical impedance correspond to
acoustic
resonances of the water column and represent potential frequencies for
operation.
Numerical modeling has indicated that the transducer displacement profile
varies
significantly at these acoustic resonance frequencies, and thereby directly
affects the
acoustic standing wave and resulting trapping force. Since the transducer
operates
near its thickness resonance, the displacements of the electrode surfaces are
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essentially out of phase. The typical displacement of the transducer
electrodes is not
uniform and varies depending on frequency of excitation. As an example, at one
frequency of excitation with a single line of trapped oil droplets, the
displacement has a
single maximum in the middle of the electrode and minima near the transducer
edges.
At another excitation frequency, the transducer profile has multiple maxima
leading to
multiple trapped lines of oil droplets. Higher order transducer displacement
patterns
result in higher trapping forces and multiple stable trapping lines for the
captured oil
droplets.
[0089] To investigate the effect of the transducer displacement profile on
acoustic
trapping force and oil separation efficiencies, an experiment was repeated ten
times,
with all conditions identical except for the excitation frequency. Ten
consecutive
acoustic resonance frequencies, indicated by circled numbers 1-9 and letter A
on FIG.
13, were used as excitation frequencies. The conditions were an experiment
duration of
30 min, a 1000 ppm oil concentration, a flow rate of 500 ml/min, and an
applied power
of 20W.
[0090] As the emulsion passed by the transducer, the trapping lines of oil
droplets
were observed and characterized. The characterization involved the observation
and
pattern of the number of trapping lines across the fluid channel, as shown in
FIG. 14, for
seven of the ten resonance frequencies identified in FIG. 13.
[0091] The effect of excitation frequency clearly determines the number of
trapping
lines, which vary from a single trapping line at the excitation frequency of
acoustic
resonance 5 and 9, to nine trapping lines for acoustic resonance frequency 4.
At other
excitation frequencies four or five nodal trapping lines are observed.
Different
displacement profiles of the transducer can produce different (more) trapping
lines of
the standing waves, with more gradients in displacement profile generally
creating
higher trapping forces and more trapping lines.
[0092] Table 2 summarizes the findings from an oil trapping experiment
using a
system similar to FIGS. 1-9. An important conclusion is that the oil
separation efficiency
of the acoustic separator is directly related to the mode shape of the
transducer. Higher
order displacement profiles generate larger acoustic trapping forces and more
trapping
lines resulting in better efficiencies. A second conclusion, useful for
scaling studies, is
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that the tests indicate that capturing 5 micron oil droplets at 500 ml/min
requires 10
Watts of power per square-inch of transducer area per 1" of acoustic beam
span. The
main dissipation is that of thermo-viscous absorption in the bulk volume of
the acoustic
standing wave. The cost of energy associated with this flow rate is 0.667 kWh
per cubic
meter.
Table 2: Trapping Pattern Capture Efficiency Study
Resonance Total Power # of
Flowrate Duration Capture Efficiency
Peak Input Trapping
(ml/min) (min) (%)
Location (Watts) Lines
4 20 9 500 30 91%
8 20 5 500 30 58%
A 20 4 500 30 58%
9 20 2 500 30 37%
[0093] In larger systems, different transducer arrangements are feasible.
FIG. 15
shows a transducer array 120 including three square 1"x1" crystals 120a, 120b,
120c.
Two squares are parallel to each other, and the third square is offset to form
a triangular
pattern and get 100% acoustic coverage. FIG. 16 shows a transducer array 122
including two rectangular 1" x 2.5" crystals 122a, 122b arranged with their
long axes
parallel to each other. Power dissipation per transducer was 10 W per 1"x1"
transducer
cross-sectional area and per inch of acoustic standing wave span in order to
get
sufficient acoustic trapping forces. For a 4" span of an intermediate scale
system, each
1"x1" square transducer consumes 40 W. The larger 1"x2.5" rectangular
transducer
uses 100W in an intermediate scale system. The array of three 1"x1" square
transducers would consume a total of 120 W and the array of two 1"x2.5"
transducers
would consume about 200 W. Arrays of closely spaced transducers represent
alternate
potential embodiments of the technology. Transducer size, shape, number, and
location
can be varied as desired to generate desired three-dimensional acoustic
standing
waves.
[0094] Figure 17 is a computer model of an acoustophoretic separator 92
simulated
to produce Figures 18-29. The piezo ceramic crystal 94 is in direct contact
with the fluid
in the water channel 96. A layer of silicon 98 is between the crystal 94 and
the
aluminum top plate 100. A reflector 102 reflects the waves to create standing
waves.
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The reflector is made of a high acoustic impedance material such as steel or
tungsten,
providing good reflection. For reference, the Y-axis 104 will be referred to
as the axial
direction. The X-axis 106 will be referred to as the radial or lateral
direction. The
acoustic pressure and velocity models were calculated in COMSOL including
piezo-
electric models of the PZT transducer, linear elastic models of the
surrounding structure
(e.g. reflector plate and walls), and a linear acoustic model of the waves in
the water
column. The acoustic pressure and velocity was exported as data to MATLAB. The
radiation force acting on a suspended particle was calculated in MATLAB using
Gor'kov's formulation. The particle and fluid material properties, such as
density, speed
of sound, and particle size, are entered into the program, and used to
determine the
monopole and dipole scattering contributions. The acoustic radiation force is
determined
by performing a gradient operation on the field potential U, which is a
function of the
volume of the particle and the time averaged potential and kinetic energy of
the acoustic
field.
[0095] FIGs. 18-21 show simulations of the difference in trapping pressure
gradients
between a single acoustic wave and a multimode acoustic wave. FIG. 18 shows
the
axial force associated with a single standing acoustic wave. FIG. 19 shows the
lateral
force due to a single standing acoustic wave. FIG. 20 and FIG. 21 show the
axial force
and lateral force, respectively, in a multi-mode (higher order vibration modes
having
multiple nodes) piezoelectric crystal excitation where multiple standing waves
are
formed. The electrical input is the same as the single mode of FIG. 18 and
FIG. 19, but
the trapping force (lateral force) is 70 times greater (note the scale to the
right in FIG. 19
compared to FIG. 21). The figures were generated by a computer modeling
simulation
of a 1MHz piezo-electric transducer driven by 10 V AC potted in an aluminum
top plate
in an open water channel terminated by a steel reflector (see FIG. 17). The
field in FIG.
18 and FIG. 19 is 960 kHz with a peak pressure of 400 kPa. The field in FIG.
20 and
FIG. 21 is 961 kHz with a peak pressure of 1400 kPa. In addition to higher
forces, the
961 kHz field has more gradients and focal spots.
[0096] FIG. 22 shows a three dimensional computer generated model of a mode
shape calculation showing the out-of-plane displacement for a circular crystal
driven at
a frequency of 1 MHz.
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[0097] FIGS. 23-29 are based on the model of FIG. 17 with a PZT-8 piezo-
electric
transducer operating at 2 MHz. The transducer is 1" wide and 0.04" thick,
potted in an
aluminum top plate (0.125" thick) in a 4"x 2" water channel terminated by a
steel
reflector plate (0.180" thick). The acoustic beam spans a distance of 2". The
depth
dimension, which is 1", is not included in the 2D model. The transducer is
driven at 15V
and a frequency sweep calculation is done to identify the various acoustic
resonances.
The results of the three consecutive acoustic resonance frequencies, i.e.,
1.9964 MHz
(FIGS. 23-25), 2.0106 MHz (FIG. 26 and FIG. 27), and 2.025 MHz (FIG. 28 and
FIG.
29), are shown. The acoustic radiation force is calculated for an oil droplet
with a radius
of 5 micron, a density of 880 kg/m3, and speed of sound of 1700 m/sec. Water
is the
main fluid with a density of 1000 kg/m3, speed of sound of 1500 m/sec, and
dynamic
viscosity of 0.001 kg/msec.
[0098] FIG. 23 shows the lateral (horizontal) acoustic radiation force.
FIG. 24 shows
the axial (vertical) component for a resonance frequency of 1.9964 MHz. FIG.
25 shows
the acoustic pressure amplitude. FIG. 23 and FIG. 24 show that the relative
magnitude
of the lateral and axial component of the radiation force are very similar,
about 1.2e-10
N, indicating that it is possible to create large trapping forces, where the
lateral force
component is of similar magnitude or higher than the axial component. This is
a new
result and contradicts typical results mentioned in the literature.
[0099] A second result is that the acoustic trapping force magnitude
exceeds that of
the fluid drag force, for typical flow velocities on the order of mm/s, and it
is therefore
possible to use this acoustic field to trap the oil droplet. Of course,
trapping at higher
flow velocities can be obtained by increasing the applied power to the
transducer. That
is, the acoustic pressure is proportional to the driving voltage of the
transducer. The
electrical power is proportional to the square of the voltage.
[0100] A third result is that at the frequency shown, high trapping forces
associated
with this particular trapping mode extend across the entire flow channel,
thereby
enabling capture of oil droplets across the entire channel width. Finally, a
comparison
of the minima of the acoustic trapping force field, i.e., the locations of the
trapped
particles, with the observed trapping locations of droplets in the standing
wave shows
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good agreement, indicating that COMSOL modeling is indeed an accurate tool for
the
prediction of the acoustic trapping of particles. This will be shown in more
detail below.
[0101] FIG. 26 shows the lateral force component at a resonance frequency
of
2.0106 MHz, and FIG. 27 shows the axial acoustic radiation force component at
a
resonance frequency of 2.0106 MHz. FIG. 26 and FIG. 27 exhibit higher peak
trapping
forces than FIG. 23 and FIG. 24. The lateral acoustic radiation forces exceed
the axial
radiation force. However, the higher trapping forces are located in the upper
part of the
flow channel, and do not span the entire depth of the flow channel. It would
therefore
represent a mode that is effective at trapping particles in the upper portion
of the
channel, but not necessarily across the entire channel. Again, a comparison
with
measured trapping patterns indicates the existence of such modes and trapping
patterns.
[0102] FIG. 28 shows the lateral force component at a resonance frequency
of 2.025
MHz, and FIG. 29 shows the axial acoustic radiation force component at a
resonance
frequency of 2.025 MHz. The acoustic field changes drastically at each
acoustic
resonance frequency, and therefore careful tuning of the system is critical.
At a
minimum, 2D models are necessary for accurate prediction of the acoustic
trapping
forces.
[0103] 2D axisymmetric models were developed to calculate the trapping
forces for
circular transducers. The models were used to predict acoustic trapping forces
on
particles, which can then be used to predict particle trajectories in
combination with the
action of fluid drag and buoyancy forces. The models clearly show that it is
possible to
generate lateral acoustic trapping forces necessary to trap particles and
overcome the
effects of buoyancy and fluid drag. The models also show that circular
transducers do
not provide for large trapping forces across the entire volume of the standing
wave
created by the transducer, indicating that circular transducers only yield
high trapping
forces near the center of the ultrasonic standing wave generated by the
transducer, but
provide much smaller trapping forces toward the edges of the standing wave.
This
further indicates that the circular transducer only provides limited trapping
for a small
section of the fluid flow that would flow across the standing wave of the
circular
transducer, and no trapping near the edges of the standing wave.
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[0104] FIG. 30 is a picture showing the separation attained by an apparatus
of FIGs.
1-9 after 30 minutes of operation. This picture is taken in a column attached
to the first
device outlet. An air layer is present at the top, followed by an oil layer
and a water
column. The oil is clearly separated from the water column.
[0105] The acoustophoretic devices of the present disclosure create a three
dimensional pressure field which includes standing waves perpendicular to the
fluid
flow. The pressure gradients are large enough to generate acoustophoretic
forces
orthogonal to the standing wave direction (i.e., the acoustophoretic forces
are parallel to
the fluid flow direction) which are of the same order of magnitude as the
acoustophoretic forces in the wave direction. This permits better particle
trapping and
collection in the flow chamber and along well-defined trapping lines, as
opposed to
merely trapping particles in collection planes as in conventional devices. The
particles
have significant time to move to nodes or anti-nodes of the standing waves,
generating
regions where the particles can concentrate, agglomerate, and/or coalesce.
[0106] In some embodiments, the fluid flow has a Reynolds number of up to
500, i.e.
laminar flow is occurring. For practical application in industry, the Reynolds
number is
usually from 10 to 500 for the flow through the system. The particle movement
relative
to the fluid motion generates a Reynolds number much less than 1Ø The
Reynolds
number represents the ratio of inertial flow effects to viscous effects in a
given flow field.
For Reynolds numbers below 1.0, viscous forces are dominant in the flow field.
This
results in significant damping where shear forces are predominant throughout
the flow.
This flow where viscous forces are dominant is called Stokes flow. The flow of
molasses
is an example.
[0107] Wall contouring and streamlining have very little importance to the
flow of
very viscous fluids or the flow in very tiny passages, like MEMS devices. The
flow of the
particles relative to the fluid in MEMS devices will be Stokes flow because
both the
particle diameters and the relative velocities between the particles and fluid
are very
small. On the other hand, the Reynolds number for the flow through the present
system
will be much greater than 1.0 because the fluid velocity and inlet diameter
are much
larger. For Reynolds numbers much greater than 1.0, viscous forces are
dominant only
where the flow is in contact with the surface. This viscous region near the
surface is
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called a boundary layer and was first recognized by Ludwig Prandtl (Reference
2). In
duct flow, the flow will be laminar if the Reynolds number is significantly
above 1.0 and
below 2300 for fully developed flow in the duct. The flow velocity starts off
uniform. As
the flow moves down the duct, the effect of wall viscous forces will diffuse
inward
towards the centerline to generate a parabolic velocity profile. This
parabolic profile will
have a peak value that is twice the average velocity. The length required for
the
parabolic profile to develop is a function of the Reynolds number. For a
Reynolds
number of 20, the development length will be 1.2 duct diameters. Thus, fully
developed
flow happens very quickly. This peak velocity in the center can be detrimental
to
acoustic particle separation. Also, turbulence can occur and so flow surface
contouring
is very important in controlling the flow. Thus, the shape of the contoured
nozzle wall
will have a large effect on the final velocity profile. The area convergence
increases the
flow average velocity, but it is the wall contour that determines the velocity
profile. The
nozzle wall contour will be a flow streamline, and is designed with a small
radius of
curvature.
[0108] The transducer(s) is/are used to create a pressure field that
generates forces
of the same order of magnitude both orthogonal to the standing wave direction
and in
the standing wave direction. When the forces are roughly the same order of
magnitude,
particles of size 0.1 microns to 300 microns will be moved more effectively
towards
regions of agglomeration ("trapping lines"). Because of the equally large
gradients in the
orthogonal acoustophoretic force component, there are "hot spots" or particle
collection
regions that are not located in the regular locations in the standing wave
direction
between the transducer and the reflector. Such hot spots are located in the
maxima or
minima of acoustic radiation potential. Such hot spots represent particle
collection
locations which allow for better wave transmission between the transducer and
the
reflector during collection and stronger inter-particle forces, leading to
faster and better
particle agglomeration.
[0109] In biological applications, many parts, e.g. the tubing leading to
and from the
device, may all be disposable, with only the transducer and reflector to be
cleaned for
reuse. Avoiding centrifuges and filters allows better separation of cells
without lowering
the viability of the cells. The form factor of the acoustophoretic device is
also smaller
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than a filtering system, allowing cell separation to be miniaturized. The
transducers may
also be driven to create rapid pressure changes to prevent or clear blockages
due to
agglomeration of cells. The frequency of the transducers may also be varied to
obtain
optimal effectiveness for a given power.
[0110] The present disclosure has been described with reference to
exemplary
embodiments. Obviously, modifications and alterations will occur to others
upon
reading and understanding the preceding detailed description. It is intended
that the
present disclosure be construed as including all such modifications and
alterations
insofar as they come within the scope of the appended claims or the
equivalents
thereof.
