Note: Descriptions are shown in the official language in which they were submitted.
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EXPANDABLE IMPELLER PUMP
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FIELD OF THE INVENTION
The present invention relates to fluid pumping impellers, in particular to
expandable impellers.
BACKGROUND OF THE INVENTION
Conventional impellers are manufactured with a particular blade
configuration, and significant deformation of the blades is generally
undesirable.
Conventionally, the impeller has the same configuration during storage,
movement to
its operating location, and use. However, there are situations where access to
the
operating location is through a restricted space, or space is otherwise at a
premium
during storage or transport of the impeller, in which case the use of
conventional
impellers can be problematic.
SUMMARY OF THE INVENTION
An impeller according to an embodiment of the present invention comprises a
hub, and at least one blade supported by the hub, the blade having a proximal
end
attached to the hub and a distal end. The impeller has a deployed
configuration in
which the blade extends away from the hub, and a stored configuration in which
the
impeller is radially compressed. In the stored configuration, the distal end
of the blade
is closer to the hub than in the deployed configuration. The impeller may
comprise a
plurality of blades, arranged in blade rows. The blade rows may be spaced
apart along
the hub, and each blade row includes one or more blades. For example, each
blade
row may comprise two or three blades. An example impeller include two blade
rows,
each blade row comprising two blades.
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The impeller may comprise a unitary impeller body, the unitary impeller body
including the hub and the blade. Alternatively, the blades and hub may be
formed
from different materials. Preferably, the blades are flexible, so that they
can be
deformed towards the hub in the stored configuration. The blades may be formed
from a polymer, such as a polyurethane. In example impellers, the blades are
formed
from a polymer having a flexural modulus of approximately 10,000 psi for
operational stresses, such as a low flexural modulus polyurethane. For
example, the
impeller may comprise a unitary body of material having a flexural modulus of
approximately 10,000 psi for operational stresses, the unitary body including
the hub
and blade(s). The modulus is preferably less for deformation of blades into
the stored
state of the impeller. For example, the modulus for operational stresses may
be
approximately 10,000, whereas the modulus for radial compression of the
impeller
from the deployed state to the stored state may be approximately 1,000,
approximately ten times less.
The impeller blade (or blades) may deform during operation, and the optimum
configuration of a blade may only be achieved under deployment and rotation.
For
example, the optimal design configuration of the blade may only be achieved
with
deformation under load. Hence, blade deformation in use, due to flexibility of
the
blade, need not lead to reduced performance. Successful operation can occur
even
when the impeller exhibits significant deflections from a manufactured shape.
The
impeller can be manufactured with allowance for the deflection included in the
design. The configuration of a deformed impeller operating at a predetermined
rotation rate, or within a predetermined operating range, can be optimized.
Hence, in
further embodiments of the present invention, the impeller further has an
operational
configuration, achieved from the deployed configuration by deformation of the
blades
under load, for example upon rotation of the impeller in the deployed
configuration
within a fluid. An improved approach to optimizing a flexible impeller is to
optimize
the operational configuration. Conventionally, the unloaded deployed
configuration is
optimized.
Impellers according to the present invention may further be attached to a
shaft,
such as a flexible shaft, having one end coupled to the impeller. A torque
applied to
the other end of the shaft is used to rotate the impeller. The other end of
the shaft may
be coupled to a rotating member, such as a motor, which powers the rotation of
the
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impeller. The shaft may also be used to deploy the impeller, i.e. to achieve
the
deployed configuration from the stored configuration. For example, the shaft
may be
used to push the impeller out of a storage sleeve, such as a metal tube. The
shaft may
also be used to rotate the impeller within the storage sleeve, and this
twisting of the
impeller within the storage sleeve may help drive the impeller out of the
storage
sleeve, where it achieves the deployed configuration.
Impellers according to the present invention may self-deploy into the deployed
configuration from the stored configuration, driven by stored strain energy of
the
impeller in the stored configuration. For example, if the impeller is pushed
out of the
storage sleeve, stored strain energy within the blades may cause the deployed
configuration to form without any other mechanical intervention.
Impellers according to the present invention may be used for a variety of
applications, such as an axial pump for a fluid (gas or liquid), a motive
force for a
vehicle, or other application.
In further embodiments of the present invention, the impeller has one or more
indentations located proximate to the proximal end of at least one blade of
the
impeller. The impeller may have a plurality of blades, each with at least one
associated indentation. The indentation may be a trench formed in the hub,
surrounding at least part of the proximal end of the blade (the base of the
blade) where
the blade joins the hub. The indentation may be an elongate indentation, such
as a
trench, extending substantially parallel and adjacent to the proximal end of
the blade.
The indentation can reduce flow vortices formed as the fluid moves relative to
the impeller. The indentation may also facilitate compression of the impeller
into the
stored configuration, for example by facilitating movement of the distal end
of the
blade towards the hub. A typical blade has a pressure face and a suction face,
and the
indentation may comprise a trench parallel and adjacent to one or both of
these faces.
A blade may have an airfoil cross-section, the indentation being a curved
trench
formed in the impeller parallel to the curved proximal end of the blade.
In further embodiments of the present invention, an impeller includes a blade
supported on a hub, and the distal end of the blade relative to the hub (the
blade tip)
has a winglet. The winglet increases the cross-sectional area of the blade at
the distal
end of the blade. A winglet may extend tangentially from the blade, the
tangent being
to a circular motion path of the blade tip on rotation of the impeller. The
winglet may
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extend in the direction of motion of the impeller (from the pressure face of
the
impeller), in the opposed direction from the direction of motion (from the
suction face
of the impeller), or in both directions. The blade provides a pressure face
and a
suction face as the blade rotates, the blade having a blade rotation
direction, and the
winglet may extend parallel to the blade rotation direction in either or both
directions
(the winglet and the blade may generally forming an approximate T-shape in
cross-
section). If the blade has a distal thickness between the pressure face and
the suction
face at the distal end of the blade, the winglet may have a winglet width
between
approximately one and three times the distal thickness of the blade, measured
in a
direction parallel to the blade rotation direction. If the blade has a chord
length, the
winglet may have a length approximately equal to the chord length. Further,
the
winglet may form a hydraulic bearing for rotation of the impeller with the
inner
surface of a sleeve through which fluid flows.
In embodiments of the present invention, the distal end of the blade, either
with or without a winglet, is located proximate to the interior surface of a
cylindrical
sleeve, the distance (tip gap) between the blade distal end and the inner
diameter of
the sleeve being approximately 10 to 50 percent of the maximum thickness of
the
distal end of the blade.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURES 1A and 1B show an impeller in the deployed and stored
configuration, respectively;
FIGURE 2 schematically illustrates deployment of an impeller;
FIGURE 3A illustrates an impeller in a stored configuration, within a storage
sleeve;
FIGURE 3B illustrates an impeller self-deploying after emerging from a
storage sleeve;
FIGURE 4 illustrates deployed and operational configurations of an impeller;
FIGURE 5 illustrates an impeller design having a low Reynolds number;
FIGURES 6A and 6B illustrate an impeller having three blade rows;
FIGURES 7A and 78 illustrate an impeller blade having a winglet;
FIGURES 8A ¨ 8C illustrate possible winglet configurations;
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FIGURE 8D illustrates possible winglet edge geometries;
FIGURES 9A ¨ 9D illustrate an end view of an impeller blade, further
illustrating possible winglet configurations;
FIGURES 10A and 10B illustrate a blade having a trench around the
proximate end of the blade;
FIGURE 11 is a photograph of a molded polymer impeller;
FIGURE 12 shows a stress-strain curve for an impeller blade material; and
FIGURES 13 and 14 show normalized average fluid shearing stresses as a
function of tip gap sizes.
DETAILED DESCRIPTION OF THE INVENTION
An impeller according to an embodiment of the present invention comprises a
hub, and at least one blade supported by the hub. Embodiments of the present
invention include impellers having at least one flexible blade, having a
deployed
configuration in which the blade extends away from the hub, and a stored
configuration in which the impeller is radially compressed. For example, the
blade
may be folded in towards the hub, and held there by a storage sleeve such as a
metal
tube or cammla. In the stored configuration, the distal end of the blade is
closer to the
hub than in the deployed configuration, and the radius can be significantly
less, such
as less than half that of the radius in the deployed state. The sleeve may
comprise a
non-expandable portion, in which the impeller is stored, and an expandable
portion,
into which the impeller can be moved for deployment. The impeller deploys
within
the expanded portion of the sleeve.
Impellers according to the present invention may comprise a plurality of
blades, arranged in blade rows. The blade rows may be spaced apart along the
hub,
each blade row including one or more blades. For example, each blade row may
comprise two or three blades. Achieving the stored configuration is
facilitated by
providing multiple blade rows, rather than, for example, a single long blade
curving
around the hub. A single long blade can be considerably more difficult to fold
in
towards the hub.
Embodiments of the present invention may further include a sleeve, at least
part of the impeller being located within the sleeve, and the fluid flowing
through the
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sleeve when the impeller rotates. The sleeve may be expandable, the sleeve
having an
expanded configuration when the impeller is in the deployed configuration, and
a
stored configuration when the impeller is in the stored configuration. The
sleeve may
act to constrain the impeller in the stored configuration. Alternatively, a
separate
storage sleeve may be provided, with the impeller and expandable sleeve both
expanding when pushed out of the storage sleeve. An expandable sleeve may
comprise a metal framework, for example comprising a shape memory alloy. An
elastic polymer film may be disposed over the metal framework. Impeller blades
may
have a winglet at the distal end of the blade, the winglet and the sleeve
providing a
hydraulic bearing for rotation of the impeller. For example, the sleeve may
have a
cylindrical inner surface inside which the impeller rotates, the fluid flowing
through
the sleeve, with the winglet of each blade moving proximate to the cylindrical
inner
surface as the impeller rotates, the fluid between the winglet and cylindrical
inner
surface forming the hydraulic bearing for rotation of the impeller.
An impeller may be stored in a storage sleeve, and deployed in a fluid pipe,
through which fluid flows when the impeller is rotated. The storage sleeve may
have a
diameter approximately equal to or less than half the diameter of the fluid
pipe. The
storage sleeve may be a metal tube, in which the impeller is stored prior to
deployment. The fluid pipe may be a utility pipe (water, gas, sewage, and the
like),
bodily vessel (such as a blood vessel), portion of a thrust unit for a
vehicle, or other
structure through which a fluid may flow. The impeller may be conveyed to a
desired
location in a stored configuration, then self-deploy to an expanded, deployed
state.
The stored configuration facilitates conveyance of the impeller to the desired
location,
enabling it to be passed through openings less than the diameter of the
deployed state.
The fluid pipe may be an expanded form of the storage sleeve, expansion of
the storage sleeve allowing the impeller to deploy. In this case, the impeller
does not
need to be pushed out of the sleeve to achieve the deployed configuration. For
example, an impeller according to an example of the present invention can be
inserted
in the stored configuration through a small entrance hole into a pipe of
larger
diameter. The impeller can be deployed by causing the impeller to move out of
the
storage sleeve using the drive shaft. The impeller then unfolds into the
deployed state
using stored strain energy in the blade material.
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Rotation of the impeller may further change the blade configuration to an
operating configuration. An impeller may have flexible blades that deform into
an
optimized hydrodynamic shape when rotating and operating under design load
conditions. =
Embodiments of the present invention include impellers having at least one
blade having a winglet. In the operating state, the winglet can improve
hydrodynamic
performance of the impeller and reduces shear stresses that exist within the
fluid.
Impellers may include a plurality of blades that facilitate the folding of the
blades into
the storage state. The blades may be arranged in a plurality of rows of blades
that
facilitates the folding of the blades into the storage state, compared with
folding a
single blade extending a similar distance along the hub. The blades and
(optionally)
the hub may be constructed of a low modulus material such as a polymer. The
impeller can be a unitary structure, with the blades and impeller formed from
the
same material, for example by molding a polymer.
An impeller with a plurality of blade rows also facilitates the input of large
values of fluid head or pressure rise. The specific speed of an axial flow
impeller
according to the present invention may be comparable to the specific speed of
mixed
flow pumps.
An impeller can be inserted into a pipe in a folded state and subsequently
deployed. The impeller, when deployed in a fluid flow pipe, may further deform
into
an operating configuration when the fluid is being pumped by impeller
rotation. At
the end of the operation of the impeller, the impeller can be radially
compressed back
into the stored configuration, for example by re-folding the flexible blades,
and
extracted through an entrance hole having a diameter less than that of the
fluid pipe or
deployed configuration. For example, the blades can be refolded and the
impeller
extracted into a cylindrical storage cavity by means of an attached rotary
drive shaft
or guide wire.
An impeller according to the present invention can operate in a low Reynolds
number pipe flow, where the pipe boundary layer comprises a majority of the
flow in
the pipe. The Reynolds number of the relative flow over the blades can be low,
compared to conventional impellers and pumps.
The impeller can be optimized to operate in a non-Newtonian fluid. The
impeller can be optimized to operate in a fluid containing delicate particles
(such as
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emulsion droplets, cells, and the like) that are damaged by excessive shearing
stress in
the fluid. The impeller can be designed so that the operational configuration
is
optimized, not necessarily the same as the deployed configuration under no
loading.
An impeller with an indentation in the hub about the blade root can have
reduced internal mechanical stresses within the blades when in the stored
configuration. The indentation may also be used to further reduce fluid shear
stress
induced by the impeller in the operating state.
The blades can be formed from polymer materials, such as polyurethane. A
polymer, such as polyurethane, having a modulus of 10,000 psi can be used. In
some
examples, the blades may have a stiffiiess approximating that of a thick
rubber band.
Hence, the blades have some stiffness but will deform under operating load.
For
example, the material can be chosen so as to have a linear modulus at
operational
stresses, allowing predictable deformation under load, and a non-linear
modulus at the
higher stresses used to fold the blades into the stored configuration.
Figure IA shows an impeller in a deployed configuration, the impeller
comprising a hub 10 and blades such as blade 12. The impeller has a radius R1,
as
measured from the central long axis of the hub to the outermost blade tip.
Also shown
is a fluid flow sleeve 14, through which fluid flows relative to the impeller.
The
impeller may be used as an axial pump, to pump fluid through the sleeve.
Alternatively, the impeller may be used as a motive force provider for a
vehicle. For
example, the impeller may power a boat, such as jet-boat, or other water
craft, the
sleeve being a tube immersed in the water surrounding the vehicle. In this
configuration, the blades are deployed.
Figure 1B shows the impeller in a stored configuration, with blade 12 folded
or otherwise deformed towards the hub 10. The radius R2 is less than the
radius Ri
shown in Figure IA.
An impeller according to an embodiment of the present invention has flexible
blades that can be folded such that the maximum diameter of the impeller in
the
folded state is approximately half, or less than half, the diameter of the
impeller in the
operating state. Referring to Figures IA and 1B, this corresponds to R2
(R1/2).
Figure 2 is a schematic illustrating deployment of the impeller. The impeller
has hub 20 and blades such as 22, and is retained in the stored configuration
by
storage sleeve 24. A rotating shaft 30 is used to drive the impeller. The
figure also
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shows a guide wire 28 within the rotating shaft, which can be used to position
the
impeller, and also to help push the impeller out of the storage sleeve. The
storage
sleeve may be, for example a metal tube. Rotation of the shaft may also assist
deploying the impeller, for example through twisting the impeller out of the
storage
sleeve if the inner surface of the storage sleeve has a threaded texture. On
the left, a
fluid pipe 26 is shown, through which fluid flows when the impeller is
deployed and
rotated
An impeller in the stored configuration can be stored in a cylindrical cavity
formed by storage sleeve 24 of diameter approximately equal to or less than
half the
diameter of the fluid pipe 26.
The storage sleeve may be a metal tube, in which the impeller is stored prior
to
deployment. The fluid pipe 26 is any structure through which a fluid may flow
relative to the impeller, such as a tube or bodily vessel. The impeller may be
conveyed to the desired location within the fluid pipe in the stored
configuration, then
self-deploy to an expanded, deployed state. The stored configuration allows
the
impeller passed through openings having an area less than the area of the
deployed
state, as swept out by the rotating blades.
Alternatively, the fluid pipe 26 may be an expanded form of the storage sleeve
24, expansion of the constraining sleeve allowing the impeller to deploy. In
this case,
the impeller does not need to be pushed out of the sleeve to achieve the
deployed
configuration. For example, an impeller can be inserted into a fluid pipe
through a
smaller hole, such as a smaller branch pipe or hole in the pipe wall. The
impeller can
then be deployed by causing the impeller to move out of the storage sleeve
using the
drive shaft. Deployment may occur without any outside energy input, using
stored
strain energy in the blades when the blades are in the stored configuration.
Figure 3A further illustrates an impeller in a stored configuration, showing
blades such as blade 34, and hub 30. The blades are kept folded against the
hub by the
storage sleeve 36. Figure 3B shows the impeller pushed out of the storage
sleeve and
self-deployed. The impeller has two rows of blades, as is seen more clearly in
the
deployed state, the first row including blade 34 and the second row including
blade
32.
Figure 4 shows an impeller comprising hub 60 and a plurality of blades, the
blades being shown in both the deployed and operating configurations. The
deployed
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configuration is the blade configuration under no load, and the operating
configuration is the configuration when the impeller rotates at the
operational rotation
speed. The blades are shown at 62A, 64A, and 66A for the deployed
configuration.
When under load, such as rotating in a fluid, the blades deform to an
operational
configuration, with the blades at 62B, 64B, and 66B. Rotation of the impeller
changes
the blade configuration from the deployed configuration (for no load) to an
operating
configuration. The flexible blades can deform into an optimized hydrodynamic
shape
when rotating and operating under design load conditions.
Figure 4 compares the deployed blade shape with the operating blade shape.
For a hub and blades formed from the same polymer, simulations showed that the
hub
also deflects slightly in a rotational manner, as the second blade row is
rotated at the
root compared to the first blade row. In general, the blades deflect forward
as the lift
on the blades is such that they create thrust, a force directed towards the
left side of
the figure, moving the blades toward the right side of the picture. The
leading edge of
the second blade row is obscured. There are two blade rows, each with two
identical
blades.
Blade shapes can be optimized using standard computational fluid dynamics
analysis (CFD). However, conventionally, the non-rotating, non-loaded
configuration
is optimized. (If the impeller is not expandable, the deployed shape is the
shape of the
impeller when not rotating, and there is no stored configuration). An improved
impeller has an optimized operational configuration, and an improved method of
designing an impeller includes optimizing the operational configuration. A
structural
computation determines an Allowance for deformation under load from the
deployed
state.
Figure 5 illustrates an impeller design having a low Reynolds number. The
impeller comprises hub 80, and two rows of blades having two blades each. The
first
row includes blades 82 and 84, and the second row includes blades 86 and 88.
This illustration shows the design elements of low Reynolds number impeller,
where the thickness of the boundary layer on the fluid pipe walls is as thick
as the
diameter of the pipe. The impeller has highly curved leading and trailing edge
lines
where the blade pitch angles are adjusted for the local values of relative
flow angle.
The second row blades have a groove-like feature that takes a helical path
from the
leading edge to the trailing edge. This is due to variations in the spanwise
loading, and
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allows an axial flow pump using this impeller to achieve a head rise similar
to that of
a mixed flow pump. The middle of the span of the blade is relatively highly
loaded,
leading to this feature. The second row blades may be further split into two
separated
blade rows, and this general feature will still present but not so apparent.
Figures 6A and 6B illustrate end and side views of an impeller, respectively.
The impeller comprises hub 100, a first row of blades comprising blades 102
and 104,
a second row of blades comprising blades 106 -and 108, and a third row of
blades
comprising blades 110 and 112.
For a mixed flow impeller of similar performance, the hub diameter is
typically much larger, so that folding into a stored diameter half the
deployed
diameter is impossible.
Figures 7A and 7B show a side and end view of a blade 120 having a winglet
122 at the distal end. Figure 7A shows the distal cross section of the blade
as a dashed
line. Figure 7B shows the winglet moving proximate to the inner surface of a
fluid
flow sleeve, a configuration which may be used as a hydraulic bearing for the
impeller.
Impellers may have at least one blade having a winglet. In some embodiments,
all blades within a blade row include a winglet; other blades may or may not
have a
winglet. A winglet can improve hydrodynamic performance of the impeller. A
winglet may also reduce shear stresses that exist within the fluid, for
example
reducing degradation of biological structures such as cells that may exist
within the
fluid.
Figures 8A ¨ 8C show possible winglet configurations. An impeller blade
typically has a pair of opposed faces: a pressure face inducing relative
motion of the
fluid through pressure as the blade rotates through the fluid; and a suction
face
inducing fluid motion by suction. The blade also has a leading edge cutting
though the
fluid as the blade rotates, a trailing edge, and an outer edge (which may also
be
referred to as a blade tip or edge of the distal end of the blade). The
winglet is
supported by the outer edge or blade tip, which has an airfoil shape. As
shown, the
suction side of the blade is on the right, and the pressure side is on the
left.
Figure 8A shows a suction side winglet, the winglet 142 extending from the
suction side of the outer edge of blade 140. This is a view from the leading
edge, in
cross-section, so that the blade rotates towards the direction of viewing.
Figure 8B
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shows a pressure side winglet 146, extending from the pressure face of the
blade 144.
The parameters may be similar to the suction side winglet. The function of the
pressure side winglet is to reduce flow through the gap. There is less effect
of creating
a hydrodynamic bearing, but the pressure side winglet "scrapes" low momentum
fluid
off the inner surface of the fluid pipe and prevents this fluid from entering
the gap and
subsequently being used in the core of a tip vortex. This can reduce shearing
stresses
in the bulk of the fluid flow.
Figure 8C illustrates a combined winglet, extending from both the pressure
and suction sides of the outer edge. Embodiments of the present invention
include the
configurations shown in Figures 8A-8C. Numerical methods can be used to design
the
winglet configurations. Where the blade chord lengths are long and the blade
has a
significant helical extent, the geometry and shape of the blade tip and the
winglet can
become complex.
Figure 8D shows possible winglet edge geometries which may be used. The
figure shows a radius edge 150, sharp edge 152, and chisel edges 154 and 156.
Figures 9A-9D further illustrate winglet configurations, the blade supporting
the winglet retaining the same shape in these examples. Figure 9A illustrates
the outer
edge of a blade 160, not having a winglet.
Figure 9B shows a pressure side winglet extending the pressure side of the
outer blade edge, extending over portion 164. The portion 162 of the winglet
corresponds to the original outer edge area of the blade shown in Figure 9A.
Figure 9C shows a suction side winglet, the portion 166 extending from the
suction side of the outer edge of the blade, and the portion 168 corresponding
to the
original outer edge of the blade. In embodiments of the present invention, the
pressure
side of the blade will have a radius of approximately 1/3 to 1/2 the blade
thickness or
width. The extent of the winglet may be from 1/2 to 3 times the blade
thickness. A
thickness approximately equal to the blade thickness is shown. The winglet is
mostly
positioned to the downstream half of the blade as shown. The purpose is to
create a
hydrodynamic bearing where the outer face of the winglet is in close proximity
to the
inner surface of the fluid pipe in which the blade is operating. Flow in the
gap is
reduced in strength, and a tip vortex is less likely to form. This reduces
shearing
stresses in the fluid. The gap can be between approximately 10 to
approximately 25
percent of the base blade maximum thickness. The gap is a mostly parallel
surface to
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the pipe of casing. It can be a cylindrical, conical, or curved side cylinder
where the
radius is a function of the axial position of the blade element. Parameters
for the
pressure side and combined (described below) winglets may be similar.
Figure 9D shows a combined pressure side and suction side winglet extending
from both the pressure face and the suction faces of the blade, the portion
170
extending from the pressure face, the portion 174 extending from the suction
face, and
a portion 172 corresponding to the original outer edge of the blade.
The winglets are preferably aerodynamically smooth shapes. The winglets
have leading edges where flows impact the edges of the winglets, and trailing
edges
where flow is discharged from the winglet surfaces. Winglets preferably have
smooth
aerodynamic cross sections, generally in the direction of the mean flow, which
is
parallel to the flow direction along the blade tip surfaces.
Figures 10A and 10B illustrates provision of an indentation, in this case a
trench, proximate to the base of a blade. Figure 10A shows blade 180,
surrounded by
trench 182. The trench is formed in hub 184, and is parallel with and adjacent
to the
proximal edge of the blade, the proximal edge of the blade extending around
the base
of the blade where it joins the hub. Figure 10B is a sectional view, showing
the trench
182 and blade 180. The indentation may also be referred to as a "diner.
An indentation close to the blade root, such as a tench around some or all of
the blade root, can help reduce internal mechanical stresses in the blades
when the
blades are in the stored configuration, for example folded against the hub.
The
indentation may also be used to reduce fluid shear stress in the operating
state.
Figure 11 is a photograph of an impeller molded to a configuration according
to an embodiment of the present invention. The impeller is a polyurethane
impeller
taken from a mold, having two blades rows of three blades each.
Figure 12 is a stress-strain relationship of a non-linear material that can be
used to form an impeller according to the present invention. The left (low
stress) and
right (high stress) filled circles correspond to the impeller operating point
and storage
conditions, respectively. The stress/strain relationship is approximately
linear at the
impeller operating point. The storage condition, where the blades are folded
against
the hub, is within the high strain non-linear portion of the material property
curve.
This allows the stored configuration to be achieved without passing the
material
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tensile failure point. In example impellers, the maximum material elongation
in the
stored configuration is approximately 75 percent.
Preferably, a non-linear property material is used for the blades. The blade
material can be relatively stiff at operating loads, and the same material
relatively
flexible at higher strains, for example when the blades are folded in the
stored
condition. For example, the strain might be 10 percent at operating loads and
75
percent while folded, and the stress/strain curve has high modulus (e.g.
10000) at
operating loads, and low modulus (e.g. 1000) at higher loads associated with
folding.
The stress-strain curve may have two approximately linear regions with a break
point
between the operating point and the folded point strains.
Figures 13 and 14 illustrate optimization for fluid shear stress for an
example
impeller. The distal end of the impeller blade moves proximate to the interior
surface
of a cylindrical sleeve, the tip gap between the blade distal end and the
inner diameter
of the sleeve being approximately 10 to 50 percent of the maximum thickness of
the
distal end of the blade.
The curves are double normalized, the design point values both being 1.0, the
scales being read as percent of design flow and a factor times the value of
stress at the
design point. For example, Figure 13 illustrates that at 70 percent of the
design flow,
the shear stress is 1.3 times the value at the design condition. Figure 14
shows that
making the tip gap smaller makes the shear stress higher, whereas making the
gap
bigger reduces the stress, by a smaller factor. Therefore, the fluid shear
stress can be
reduced without significantly impacting pumping efficiency.
Impellers according to embodiments may be compressed and packaged into a
storage sleeve, such as a metal tube, catheter, or other structure, for
insertion into an
object. For an object such as a living subject, the diameter of the storage
sleeve can be
approximately three to four millimeters, or less. Having inserted the device,
the
impeller can be deployed in situ into a geometry that may be approximately six
to
seven millimeters in diameter. The impeller then can be rotated using a
flexible drive
shaft coupled to an external drive motor external to the subject. Impellers
according to
the present invention can be inserted in the stored state, then deploy into an
expanded
configuration (relative to the stored state) and be capable of pumping 4
liters per
minute, for example as a medical assist device. In a representative example of
such a
device, the impeller rotates at approximately 30,000 RPM. The impeller may
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comprise two or more airfoil shaped blades that form an axial flow pump. The
impeller may be positioned using a guide wire and rotated using a flexible
shaft. The
guide wire may run within a hollow center of the flexible shaft, and the
hollow center
may also convey saline solution or other fluid, for infusion, cooling, and/or
lubrication purposes. The guide wire may be removed, if desired. Implantation
into a
living subject can be achieved using through a cannula having a diameter of 3-
4 mm,
without surgical intervention. For medical implantation, a drive shaft
comprising
comprise a metal braid, or a polymer or composite material braid, can be used,
and the
drive shaft diameter may be of the order 11/2 to 2 millimeters, and may be
hollow to
allow a guide wire to pass through.
In further embodiments, the sleeve has expandable and non-expandable
portions. The impeller is stored within the non-expandable portion for
insertion.
When the impeller is located at or near the desired location, the impeller is
then urged
out of the non-expandable portion of the sleeve into the expandable portion.
The
stored elastic energy within the flexible blades of the impeller induces self-
deployment of the impeller, and also the expansion of the expandable portion
of the
sleeve. The expanded sleeve then may have the role of a fluid flow pipe,
through
which fluid flows when the impeller is rotated. The expandable sleeve may
comprise
a metal or polymer mesh, or woven fibers, and a smooth sheathing to provide a
flexible, expandable tube.
An expandable sleeve may comprise a mesh formed from a flexible material,
such as polymers, metals, or other material. In one example, the mesh is made
from
nitinol, a memory metal alloy. A thin sheet or cylinder of the metal, of a
thickness on
the order of a thousandth of an inch, is cut using a laser so as to leave a
mesh
structure. Alternatively, the mesh can be formed from a polymer. Other
suitable
materials for the mesh include other metals (such as alloys, including memory
metal
alloy), polymers, and the like. A coating, such an elastic coating, is then
provided
over the mesh. For example, an elastic polymer such as Estanerm can be used,
or other
polyurethane.
Hence, the expandable sleeve may comprise a mesh, such as matrix of woven
wires, or a machined metal cylinder with laser cut voids representing the
spaces
between wires, or another material that when deformed in one direction would
elongate in the perpendicular direction. The mesh can then be covered with a
thin film
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of elastane to form a fluid flow pipe through which the fluid flows. The mesh
can be
formed as a cylinder with flow entrance voids at the distal end and flow
discharge
voids at the proximal end, the proximal end being closer to the point of
insertion into
an object, such as a pipe or living subject. As the sleeve is shortened in
length, for
example using a guide wire, the cylinder expands to a greater diameter,
allowing a
greater flow rate. A hexagonal cell matrix design, or other design, can be
used for the
mesh. A coating (for example, biocompatible, corrosion resistant, or flow
improving)
can be applied to the sleeve, for example by solution casting.
The orientation of the mesh or woven fibers of the sleeve can be chosen to
allow two stable configurations, stored and deployed. In one example, designed
for
subject implantation in the stored position, the expandable sleeve in the
deployed
configuration was approximately 20- 30 cm long and the diameter was
approximately
6-7 mm. This diameter allowed for higher fluid flow rate and reduced friction
pressure losses. In the stored configuration, the expandable portion was
elongated by
approximately 30 percent relative to the deployed configuration, and the
diameter was
approximately 3 mm. The fmal portion (distal end) of the assembly comprises a
second set of openings and plates, providing an inlet or opening for the
influx of fluid
to be pumped. The sleeve may also provide a guide wire attachment openings for
fluid discharge. A short (such as 1 cm) section of the sleeve may contain
linear
elements (vanes) arranged about the central axis of the sleeve, through which
fluid is
discharged. The vanes may act as stationary stator blades and remove swirl
velocity
from the impeller discharge flow. The vanes may be manufactured with airfoil
type
cross sections. Applications of an impeller deploying within an expandable
sleeve
include a collapsible fire hose with an integral booster pump, a collapsible
propulsor,
a biomedical pump for a biological fluid, and the like.
The impeller blade design can be designed so as to minimize destruction of
delicate particles (such as emulsion droplets, suspensions, biological
structures such
as cells, and the like) within a fluid. A CFD model was used to simulate the
shear
stresses experienced by particles passing through a simulated impeller. Time
integrations of intermediate shear stresses experienced by the particles were
used to
provide an estimated probability of cell destruction in a biomedical
application. A
split blade design, in which there are a plurality of blade rows such as
discussed
above, reduces the residency time that cells remain in intermediate shear
stress
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regions, allowing an advantageous reduction in cell or other particle
destruction
compared with conventional impeller designs.
Impeller blades may, for example, occupy as much as 95% of the compressed
volume of the impeller when the impeller is in the stored state. The blades
may be
formed from a rubbery, elastic, or other resilient material that has
sufficient resilience
to expand when ejected from a sleeve. In other examples the blades may be
formed
from other flexible polymers, an expandable foam optionally with a skin, or
other
compressible or deformable materials including metals.
Impellers according to embodiments of the present invention may have
multiple separate sets of blades, rather than a long, continuous, spiral
blade. Prior art
impellers typically have a continuous long helical blade that is difficult to
fold up
against the hub. By splitting a long blade into two or three shorter sections,
the blade
can be more easily folded into a cylindrical volume or space and subsequently
deployed when properly located. The number of blade rows can be one, two,
three,
four, five, or higher. The twist pitch angles may be variable.
One approach to impeller design provides a two blade impeller with blades
exhibiting a significant degree of wrap around the central hub. However, the
three
dimension shape of the blades limits the degree to which they can be folded
without
deforming or breaking. By breaking a single blade row into two, three (or
possibly
more) rows of blades that exhibit minimum wrap around the hub, the blades have
a
more two-dimensional shape, allowing easier bending during the storage
process. The
combination of three or two blade rows can produce the same flow and pressure
as a
single blade row. An axial pump was designed with two blade rows was designed,
and
CFD (computational fluid dynamics) analysis indicated that this pump design
was
adequate for use in a medical assist application. A model was constructed of a
flexible
polyurethane material and successfully folded into a metal sleeve.
Impellers can be used with flows of very small Reynolds number, for example,
the pumping of relatively viscous fluids at low velocity or flow rate. Very
small
impeller pumps, on the order of 6 mm diameter, may be fabricated from a
polymer
and extracted from a precision mold. This allows production of impellers at
very low
cost. The use of polymer blades allows the pump impellers to be extracted from
molds
with becoming mold-locked, and allows the use of one piece molds, instead of
multi-
part, or split molds. This can be advantageous for pumping small quantities of
bio-
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fluids. Impellers may be used for flows of typical Reynolds numbers as well.
Impeller
diameters can also be in the range of several inches to several feet.
Applications of the improved impeller designs described include pumps for
chemical engineering, propellers for airborne or maritime vessels, water
pumps, and
the like. Improved impeller designs are useful for any application where an
impeller is
to be stored in a compact configuration. Impellers may be formed from metal
sheets,
plastic, and non-resilient materials, for example in foldable configurations.
Deployment may include the use of motors or other mechanical devices to unfold
blades, automatic deployment induced by centrifugal forces, and the like.
Examples of
the present invention include a device locatable inside a subject so as to
pump a fluid,
the device being inserted into the subject in an insertion configuration
having an
insertion cross-section, the device operating inside the subject in an
operating
configuration having an operating cross-section, wherein the operating cross
section is
greater than the insertion cross section. The operating diameter (of the
largest circle
swept out by the outer edge of the impeller blade as it rotates) may be over
50%
greater than the insertion diameter of the impeller, and may be over 100%
greater than
the insertion diameter.
The invention is not restricted to the illustrative examples described above.
Examples are not intended as limitations on the scope of the invention.
Methods,
apparatus, compositions, and the like described herein are exemplary and not
intended
as limitations on the scope of the invention. Changes therein and other uses
will occur
to those skilled in the art. The scope of the invention is defined by the
scope of the
claims.
Having described our invention, we claim:
