U.S. patent application number 10/242247 was filed with the patent office on 2003-01-23 for electrostatic atomizers.
Invention is credited to Kelly, Arnold J..
Application Number | 20030015594 10/242247 |
Document ID | / |
Family ID | 26812500 |
Filed Date | 2003-01-23 |
United States Patent
Application |
20030015594 |
Kind Code |
A1 |
Kelly, Arnold J. |
January 23, 2003 |
Electrostatic atomizers
Abstract
An electrostatic atomizer includes a body defining an interior
space and a discharge orifice communicating with the interior
space. An emitting electrode or electron gun is disposed inside the
body so as to apply charges to the fluid passing through the
interior space. A counter electrode is disposed outside the body.
The exposed surfaces on the interior of the body are formed from a
dielectric material so that there is no substantial electric field
between exposed conductive elements on the inside of the body. This
arrangement minimizes soot buildup and plugging of the orifice. The
device may include a single element defining numerous orifices and
formed by micro-machining techniques such as those used in
fabrication of semiconductor devices. Orifice sizes as small as a
few micrometers can use successfully to provide controllable
atomization at extremely low flow rates.
Inventors: |
Kelly, Arnold J.; (Princeton
Junction, NJ) |
Correspondence
Address: |
LERNER, DAVID, LITTENBERG,
KRUMHOLZ & MENTLIK
600 SOUTH AVENUE WEST
WESTFIELD
NJ
07090
US
|
Family ID: |
26812500 |
Appl. No.: |
10/242247 |
Filed: |
September 12, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10242247 |
Sep 12, 2002 |
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09476246 |
Dec 30, 1999 |
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6474573 |
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60114727 |
Dec 31, 1998 |
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Current U.S.
Class: |
239/3 ; 239/690;
239/695; 239/696; 239/704 |
Current CPC
Class: |
F02M 61/166 20130101;
F02M 61/165 20130101; F02M 61/1846 20130101; F02M 51/06 20130101;
F02M 61/186 20130101; B05B 5/0255 20130101; F02K 9/52 20130101;
F02M 61/1826 20130101; F23D 11/32 20130101 |
Class at
Publication: |
239/3 ; 239/690;
239/695; 239/696; 239/704 |
International
Class: |
B05B 005/025 |
Goverment Interests
[0002] Applicant's invention was supported in part by Department of
Defense (Army) Contract No. DAAN002-98-P-8570. Therefore, the
Government may have certain rights in the invention.
Claims
1. A method of atomizing a fluid comprising the steps of: (a)
passing the fluid through an interior space within a body and out
of the interior space through a discharge orifice; and (b) applying
a net charge to the fluid so that the fluid passing out of the
discharge orifice will be atomized under the influence of said net
charge, said step of applying a net charge being performed without
exposing the fluid to an electric field between exposed conductive
surfaces within said interior space in excess of 1000 V/mm.
2. A method as claimed in claim 1 wherein said step of applying an
net charge is performed by maintaining an electric field between a
charge injection electrode within said interior space and a counter
electrode outside of said interior space and separated therefrom by
a dielectric.
3. A method as claimed in claim 2 wherein said discharge orifice
has a diameter of less than about 100 .mu.m.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a divisional of U.S. patent
application Ser. No. 09/476,246, filed Dec. 30, 1999, which
application claims benefit of U.S. Provisional Patent Application
Serial No. 60/114,727, filed Dec. 31, 1998, the disclosures of
which are hereby incorporated by reference herein.
BACKGROUND OF THE INVENTION
[0003] The present invention relates to electrostatic
atomizers.
[0004] As described in U.S. Pat. No. 4,255,777, the disclosure of
which is incorporated by reference herein, a fluid can be atomized
by injecting an electrically charge into it. In certain embodiments
taught in the '777 patent, the fluid passes through a housing and
out of a discharge orifice defined by a wall of the housing. An
electron emitter electrode, also referred to as a charge injection
electrode, is disposed immediately upstream of the discharge
orifice. Typically, the wall defining the orifice itself is a
metallic body and serves as a second electrode. The second
electrode which is maintained at a different electrical potential
from the charge injection electrode or emitter. Under these
conditions, electric charges leave one of the electrodes and move
towards the other electrode through the fluid. For example, where
the emitter is maintained at a negative potential with respect to
the second or aperture electrode, electrons leave the emitter
electrode and move towards the second electrode through the flowing
fluid. Because the charges have a finite, limited velocity within
the flowing fluid, some or all of the charge is carried out through
the orifice with the flowing fluid before such charge reaches the
second electrode. The stream of fluid passing out of the orifice
thus carries a net charge. Because the fluid has a net charge, the
various portions of the fluid repel one another. Such repulsion
causes the fluid to break apart or atomize. The charges are
ultimately discharged by a third, ground electrode outside of the
orifice.
[0005] Other electrostatic atomization systems described in certain
preferred embodiments of U.S. Pat. Nos. 5,093,602 and 5,378,957,
the disclosures of which are hereby incorporated by reference
herein, utilize electron beams to introduce charge into the fluid.
Systems of this type have a small electron gun mounted adjacent to
discharge orifice. Typically, the electron gun incorporates a
housing having an interior space maintained under vacuum. An
electron-transmissive window is provided over on opening in the
electron-gun housing. A cathode and accelerating electrodes within
the electron gun form an electron beam which is directed through
the window into the fluid as the fluid passes into and through the
orifice. Here again, a net charge is introduced into the fluid and
the fluid is atomized by mutual repulsion between charged portions
of the fluid.
[0006] Electrostatic atomization systems as discussed above offer
numerous advantages over conventional atomization systems. In
particular, the degree of atomization is controlled by the amount
of charge introduced into the fluid. The preferred systems
described in the aforementioned patents can apply substantial net
charge and can provide very effective atomization. These systems do
not depend upon mechanical action for atomization. Thus, it is
possible to dispense with many of the elements commonly found in
mechanical atomization systems. For example, there is no need to
force the fluid through a fine orifice at a high flow rate to
induce atomization by shear, and no need to supply high-velocity
jets of compressed gas to induce atomization. The system can
operate at low fluid pressures and with any desired flow rate.
These features facilitate construction of simple, light weight
atomization systems. Moreover, because droplet size is strongly
controlled by the amount of charge injected, the system can achieve
the desired degree of atomization despite variations in fluid flow
rate and fluid properties such as viscosity. The systems can
operate with small amounts of electrical power. Systems of these
types can be used to atomize numerous different materials. However,
one significant application for such systems has been in
atomization of liquid fuels such as fuel oil, diesel oil, kerosene
and jet engine fuel in engine and combustion applications. For
example, systems of this type can be used in place of conventional
fuel injectors in diesel engines and in gas turbine engines.
[0007] Because electrostatic atomization of this type can provide
effective atomization even with very low flow rates, fuel can be
atomized at flow rates appropriate to provide a flame having few
watts to a few hundred watts of heat output. As described in
co-pending, commonly assigned U.S. patent application Ser. No.
09/237,583, filed Jan. 26, 1999, the disclosure of which is hereby
incorporated by reference herein, a small burner which provides
such a flame can be used, for example, as the heating element in a
small, simple cooking stove for use by an individual soldier or
camper. As described in greater detail in the '583 application,
such a low flow atomizer typically includes a small orifice at the
atomization nozzle for regulating the fluid flow. The nozzle may be
of variable size to provide variable flow rate.
[0008] Despite these and other improvements in electrostatic
atomization, still further improvement would be desirable.
Electrostatic atomization systems using small orifices, and
especially those using orifices less than about 100 .mu.m in
diameter, can become clogged with sooty particulates. Although the
present invention is not limited by any theory of operation, it is
believed that this soot arises from some side effects of the
electric fields applied to the fluid such as the field applied
between the emitter electrode and the second or counter electrode.
Thus, it is believed that phenomena associated with injection of
charge into the fluid to be atomized cause chemical reactions to
occur in the vicinity of the charge injection electrode or electron
gun. Such reactions may cause polymerization of the fluid,
particularly where the fluid is a organic liquid such as a liquid
fuel. Regardless of the cause however, sooty particles tend to form
inside the atomization device. These particles generally do not
pose a problem in systems using relatively large discharge
orifices, such as those above about 100 microns in diameter and
particularly above about 500 mm in diameter. However, small
apertures, particularly those below about 100 microns in diameter,
can become clogged in as little as an hour of operation. Reducing
the applied voltage can increase the time required for clogs to
form. However, this does not provide a complete solution to the
problem and limits the capability of the system. Thus, a better
solution to the clogging problem would be desirable.
[0009] Moreover, it would be desirable to provide arrays of
electrostatic spray nozzles. For example, in a small burner, it
would be desirable to provide multiple plumes of atomized liquid
fuel to provide multiple, small flames. In particular, it would be
desirable to provide such an array in a form which can be
manufactured readily at low cost.
SUMMARY OF THE INVENTION
[0010] The present invention addresses these needs.
[0011] One aspect of the present invention provides an
electrostatic atomizer incorporating a body defining an interior
space, and an exterior surface. The body also defines a fluid entry
port communicating with the interior space remote from the orifice.
A charge injection structure is disposed within the interior space
in the vicinity of the orifice. The charge injection structure may
be an emitting electrode or an electron gun as discussed above.
Desirably, a counter electrode is disposed in the vicinity of the
orifice. According to this aspect of the present invention, a
dielectric structure is disposed between the counter electrode and
the interior space, so that the counter electrode is electrically
insulated from the interior space. For example, the body may
incorporate a dielectric material and the dielectric material of
the body may serve as the dielectric structure which insulates the
counter electrode from the interior space. The counter electrode
desirably overlies the exterior surface of the body on the vicinity
of the orifice. The dielectric material of the body may define the
orifice and the exterior surface and the counter electrode may be
in the form of an electrically conductive coating on the exterior
surface of the body.
[0012] Atomizers according to this aspect of the invention
desirably are substantially devoid of electrically conductive
surfaces exposed to the interior space other than the electrically
conductive surfaces of the charge injection structure itself which
are at the same electrical potential as the charge injection
structure. Stated another way, the atomizer desirably does not
apply electric fields in excess of about 1000 V/mm between
electrically conductive surfaces exposed to the interior space, and
most desirably does not apply any electric fields between
electrically conductive surfaces exposed to the interior space.
There may be substantial electric fields between the charge
injection structure and a counter electrode outside of the
body.
[0013] Atomizers according to this aspect of the invention
incorporate the discovery that formation of large soot particles
which cause plugging within the atomizer can be suppressed by
insulating electrically conductive structures such as the second or
counter electrode from the flowing fluid. Despite the dielectric
exposed between the electrode and the flowing fluid, the electrode
still performs the required function for atomization. For example,
in a triode-type device, the electric field between the emitter
electrode and the second electrode is imposed through the fluid and
through the dielectric structure. Atomization proceeds
substantially in the same way as where the second electrode is on
contact with the flowing fluid. However, large soot particles do
not form inside the atomizer and do not clog the discharge orifice,
even if the orifice is of small diameter. Here again, although the
present invention is not limited by any theory of operation, it is
believed that by limiting or eliminating electrical fields between
conductive elements, electrical currents flowing between conductive
elements inside the interior space are reduced, and that this in
turn reduces the tendency of sooty material formed during charge
injection to agglomerate or settle on surfaces of the apparatus. A
further aspect of the present invention provides an atomizer
incorporating a body having one or more interior spaces and a first
wall bounding the one or more interior spaces. The first wall
defines an exterior surface of the body and a plurality of orifices
extending between the exterior surface and the one or more interior
spaces. The body also has one or more fluid inlets communicating
with the one or more interior spaces remote from the orifices. A
plurality of charge injection electrodes desirably are mounted in
the one or more interior spaces and are disposed adjacent the
orifices. One or more counter electrodes are also disposed adjacent
the orifices. Desirably, the one or more counter electrodes are
disposed on the exterior surface of the first wall, and the first
wall includes a dielectric structure facing the interior space or
spaces to insulate the one or more counter electrodes from the
interior space or spaces and provide the advantages discussed
above. The body desirably includes a second wall extending
generally parallel to the first wall and internal structure
extending between the first and second walls. The one or more
interior spaces are disposed between the first and second wall. The
charge injection electrodes may be mounted to the second wall in
alignment with the orifices in the first wall. Atomizers of this
type may include large numbers or orifices and may include small
orifices as, for example, orifices less than about 200 microns
meters in diameter. The orifices may be spaced apart from one
another by less than about 500 micrometers. In a particularly
preferred arrangement, the orifices are less than about 50 microns
in diameter and are spaced apart from one another by less than
about 125 micrometers. As further discussed below, structures of
this type can be formed by microstructure fabrication techniques
similar to those commonly employed to a manufacture semiconductor
chips and the like. As also discussed below, the optimum spacing
between the charge injection electrodes and the discharge orifices
is directly related to orifice size. The use of numerous small
orifices instead of a single larger orifice thus permits the use of
smaller spacing, which in turn allows operation at lower voltages.
Operation at lower voltages permits the use of simpler, more
economical power supplies. Preferred atomization devices according
to this aspect of the invention can be made to provide essentially
any desired flow rate at any specified fluid inlet pressure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a fragmentary, diagrammatic sectional view
depicting an atomizer in accordance with one embodiment of the
invention in conjunction with components of the stove.
[0015] FIG. 2 is fragmentary, sectional view on an enlarged scale
depicting the area indicated in FIG. 1.
[0016] FIG. 3 is a fragmentary, partially sectional diagrammatic
perspective view depicting an atomizer in accordance with a further
embodiment of the invention.
[0017] FIG. 4 is a sectional view along line 4-4 in FIG. 3, on a
reduced scale showing a larger portion of the atomizer than FIG.
3.
[0018] FIG. 5 is a fragmentary, diagrammatic perspective view
depicting a portion of an atomizer according to a further
embodiment of the invention.
[0019] FIG. 6 is a fragmentary, diagrammatic elevational view
depicting a portion of the atomizer shown in FIG. 5.
[0020] FIG. 7 is a fragmentary, diagrammatic sectional view
depicting a portion of an atomizer according to a further
embodiment of the invention.
[0021] FIG. 8 is a fragmentary, diagrammatic sectional view
depicting a portion of an atomizer according to yet another
embodiment of the invention.
[0022] FIG. 9 is a view similar to FIG. 3 but depicting an atomizer
according to a further embodiment of the invention.
DETAILED DESCRIPTION
[0023] An atomizer in accordance with one embodiment of the
invention incorporates a hollow housing 20 defining an interior
space 22 and a first wall 24 at a downstream end of the housing.
The first wall 24 has an exterior surface 28 facing outwardly away
from the interior space 22 and has an orifice 26 extending through
the first wall, between the interior space 22 and exterior surface
28. Housing 20 further defines an opening at its upstream end 30
forming a fluid inlet to interior space 22. Housing 20 desirably is
formed from a dielectric material such as a polymer as, for
example, a poly (amide-imide) polymer of the type sold under the
trademark TORLON or from any other polymer having suitable chemical
resistance and thermal properties sufficient to withstand the
temperatures which will be encountered during service. The housing
can be formed by injection molding or other melt-processing
techniques or else can be made by machining a preexisting slab,
billet or rod. Alternatively, the housing can be formed from a
ceramic such as a crystalline or partially crystalline ceramic or
an amorphous, glassy ceramic.
[0024] A porous filter 32 is mounted in inlet 30. The porous filter
may include, for example, a sintered or expanded polymer filter
having a pore size smaller than the diameter of discharge orifice
26. For example, the filter 32 may be a 20 .mu.m pore size expanded
polyethylene filter. Filter 32 is retained in housing 20 by
crimping the wall of the housing slightly as indicated at 34. An
interior insulator and spacer 36 having numerous passages 38 is
disposed inside of interior space 22. Passages 38 extend from
filter 32 to a plenum or antechamber 40 within interior space 22,
just upstream from orifice 26. Insulator 38 may be formed from any
convenient dielectric material resistant to the fluid to be
atomized and resistant to the temperatures encountered in service.
For example, a castable acrylic or epoxy may be employed to form
this element.
[0025] An emitter electrode 44 is mounted in the interior space 22
of the housing. Emitter electrode 44 includes a conductive metal
element 46 such as a metal pin having a setaceous element 48 (FIG.
2) mounted a downstream end of the pin 46. Setaceous element 48 has
numerous small points to facilitate emission of electrical charges
from this element. For example, setaceous element 48 may be formed
from a material such as ytrria-stabilized zircona-tungsten
eutectic. Alternatively, the tip 48 of the emitter may be a
non-setaceous element such as a thoriated tungsten rod. A rod of 2%
thorium in tungsten can be used. Insulator 36 covers the metallic
surfaces of pin 46, leaving only a small portion of the pin and tip
element 48 exposed within interior space 22. For example, the
insulator 36 may be cast in place around pin 46. The insulator and
filter 32 support the emitter electrode 44 so that the tip 48 is
aligned with orifice 26 and positioned just slightly upstream of
the orifice.
[0026] As best seen in FIG. 2, a counter electrode 50 is disposed
on the exterior surface 28 of first wall 24 and surrounds orifice
26. The counter electrode may be a metallic washer formed
separately from wall 24, or else may be a metallic or other
conductive coating applied on the exterior surface of the wall.
Electrode 50 has an opening 52 in its center, aligned with orifice
26. Where electrode 50 is formed as a coating on wall 24, such a
coating may be deposited by plating procedures such as electroless
plating followed by electroplating to build up the desired
thickness. The thickness of electrode 50 is exaggerated for clarity
of illustration in FIG. 2. No minimum thickness is required beyond
that required for electrical continuity.
[0027] Orifice 26 has an interior diameter d less than about 200
micrometers, desirably less than about 100 micrometers and
preferably about 50 micrometers or less. The length or axial extent
of orifice 26 is equal to the thickness of wall 24 at the orifice.
The spacing between the tip of 48 of the charge injection electrode
and the upstream end of discharge orifice 26 desirably is about 0.5
to about 2.0 times the diameter of the orifice. The thickness of
wall 24 should be as small as possible to minimize the distance
between the counter electrode 50 and charge injection electrode 44,
and thereby minimize the voltage required to produce a given
electric field between these two electrodes. The minimum thickness
of wall 24 is set by mechanical strength requirements and the need
to provide a pinhole-free wall which withstands the applied
voltages without dielectric breakdown.
[0028] The atomizer is mounted in the base 54 of a stove or other
device which employs an atomized fluid. Housing 20 is received in a
bore 56 of base 54 and an O-ring 58 is provided for making a seal
around the exterior of the housing. The downstream end of the
housing, including wall 24 and orifice 26, is exposed to the
exterior of the base 54. A passage 60 in base 54 communicates with
bore 56 and hence communicates with the fluid inlet or opening 30
of body 20. Base 54 may also be formed from any convenient
material, including dielectric materials such as polymer or a
ceramic. Metals and other conductive materials can be used if
appropriate insulation is provided for the high voltage connection
discussed below.
[0029] A high voltage connection clip 62 is attached to the pin 46
of emitter electrode 44. Clip 62 is attached to a high voltage lead
66, which is connected to one terminal of a high voltage source 68.
A metal ground clip 70 is mounted to base 54. Clip 70 bears on
counter electrode or second electrode 50 (FIG. 2) and on the
exterior surface 28 of the housing wall so that the clip retains
housing 20 in base 54. Clip 70 is also in electrical contact with
second electrode or counter electrode 50. Clip 70 and hence the
second electrode are connected through a resistor 72 to a ground
node 74. Resistor 72 may be omitted and clip 70 may be connected
directly to ground. The ground node 74 is connected to the third or
ground electrode 76 of the system. The ground electrode 76 may be
disposed remote from the other electrodes and remote from the
discharge orifice 26. For example, in a stove the ground electrode
may be a metallic part on the stove base such as a lid, a hinge or
a part of the base. Ground node 74 is connected to the ground
connection of high voltage source 68.
[0030] In operation, a fluid such as a liquid fuel as, for example,
diesel fuel, jet fuel, kerosene or home heating oil is supplied
through inlet passage 60 in the base under a low pressure as, for
example, approximately 100 kPa (14.5 psig) or less and desirably as
low as 20 kPa (about 3 psig). The fluid flows downstream through
filter 32 and within passages 38 through the interior space 22 of
housing 20. The flowing fluid reaches plenum 40 and passes within
the plenum in a generally radial direction, between tip 48 of the
emitter electrode 44 and wall 24. Source 68 is operated to apply an
atomizing voltage between emitter electrode 44 and the second
electrode or counter electrode 50. This voltage appears as an
electric field between the tip 48 and the counter electrode 50
(FIG. 2). The field of course extends through the dielectric of
wall 24 and through the fluid itself. The desired atomizing voltage
depends on the physical configuration of the system. Typically,
this voltage is selected to provide an electric field on the order
of 5 kV/mm or more, and most typically about 15 kV/mm, between the
emitter electrode and the counter electrode. The operating voltage
required to provide such field depends on the spacing between
emitter electrode tip 48 and counter electrode 50. That spacing is
directly related to the orifice size. Thus, with relatively small
orifices, it is practical to use a small spacing between the
emitter and the upstream end of the orifice, which in turn
minimizes the distance between the emitter electrode and counter
electrode.
[0031] Under the influence of the applied voltage, charges leave
the charge injection or emitter electrode 44 at tip 48 and enter
the fluid flowing radially inwardly towards orifice 26. The charges
move toward counter electrode 50, but cannot reach the counter
electrode before the fluid passes out of orifice 26. The fluid
exiting from the orifice is thus electrically charged. The
electrically charged fluid breaks apart under the influence of the
charges. The charges are dissipated into ground electrode 76, into
counter electrode 50, or both.
[0032] The use of a dielectric wall 24 segregating the second
electrode or counter electrode 50 from the flowing fluid
substantially reduces the inter-electrode current flowing in the
device. That is, essentially all of the charge leaving the charge
injection or emitter electrode 44 is ultimately incorporated into
the fluid exiting through orifice 26 and performs useful work in
atomizing the fluid. The inter electrode current between emitter
electrode 44 and second electrode or counter electrode 50 is
substantially lower than the comparable current in a system using a
counter electrode having a conductive portion exposed to interior
space 22 as, for example, a solid metal conductive orifice used as
an electrode.
[0033] The system according to this embodiment of the present
invention is essentially immune to soot clogging. Orifice 26 does
not become clogged due to soot formed within the atomizer. Filter
32 prevents clogging due to particulates in the incoming fluid. To
preserve cleanliness within the housing during shipment and
storage, orifice 26 may be covered with a temporary closure such as
a piece of adhesive tape overlying the second electrode 50. Such a
tape is removed manually before the device is used. Alternatively,
the orifice may be plugged with a grease or gel soluble in the
fluid to be atomized. The initial fluid supplied to the device
washes the plug out of the orifice.
[0034] Desirably, any metallic elements or electrically conductive
parts other than the tip of the emitter electrode disposed within
the interior space 22 are covered by a dielectric layer and
separated from the flowing fluid by the dielectric layer. Stated
another way, the interior surfaces of the housing bounding interior
space 22 and other elements such as insulator 36 disposed to the
interior space are substantially devoid of exposed electrically
conductive surfaces so that the only exposed conductive surfaces
are the surfaces of the charge injection electrode itself, at and
near the point of charge injection. Thus, the only electrically
conductive surfaces exposed to the interior space are at the same
potential as the charge injection electrode. There is no path to
ground through a conductive surface exposed to interior space 22.
There is no electrical potential difference between conductive
surfaces exposed to interior space 22. Although the present
invention is not limited by any theory of operation, it is believed
that in the absence of potential difference between exposed
conductive surfaces, agglomeration and deposition of soot within
the chamber is substantially reduced or eliminated.
[0035] Some modification of the foregoing principles can be
employed. For example, an exposed conductive surface within
interior space 22 at a great distance from the charge injection
electrode or at a potential only slightly different from the charge
injection electrode would be expected to produce only minimal
deposition or agglomeration of soot. Thus, although it is preferred
to practice the invention with no potential differences between
exposed conductive surfaces on the inside of the chamber, some
potential differences between such exposed conductive surfaces can
be tolerated. The maximum potential difference that can be
tolerated depends upon factors such as the composition of the fluid
and its tendency to form soot; the geometry of the apparatus; and
the required lifetime before soot plugging occurs. In general,
however, if any potential differences between conductive surfaces
exposed to the interior space occur, the electric field between
such exposed conductive surfaces desirably is less than about 1000
V/mm, more preferably less than 500 V/mm and most preferably less
than 100 V/mm.
[0036] Apparatus according to a further embodiment of the
invention, depicted in FIGS. 3 and 4 includes a body 120 having a
first wall 124 and a second wall 125 generally parallel to the
first wall but spaced therefrom. The first wall 124 defines a
plurality of discharge orifices 126. Wall 124 is formed from a
dielectric material such as silicon dioxide and defines an exterior
surface 128. A common external electrode 150 is formed on the
exterior surface 128 by depositing a coating of an electrically
conductive material such as a metal on this surface. First wall 124
and second wall 125 are held apart from one another by internal
structure 121 in the form of a plurality of walls subdividing the
space between the walls into a large number of hexagonal chambers
or internal spaces 122. Hexagonal spaces 122 are disposed on center
with orifices with 126, so that each orifice is aligned with the
center of one hexagonal space. Emitter electrodes 144 are mounted
to second wall 125 in alignment with orifices 126. Second wall 125
incorporates a dielectric layer 123 and a conductive layer 127
electrically connected to all of the emitter electrodes 144. The
second wall 125 has a large number of fluid inlet orifices 130
extending through it. These orifices form a filter. The relative
size of the orifices 130 is exaggerated in FIGS. 3 and 4 for
quality of illustration. In practice, the orifices 130 should be
considerably smaller than discharge orifices 126 so as to provide a
filter at the fluid inlet of the device.
[0037] Atomization devices according to this embodiment of the
invention can be fabricated using micro-mechanical fabrication
techniques, similar to the techniques used for forming
semiconductor chips and related devices. For example, the first
wall 124 and internal structures 121 can be fabricated as a unit
from a single wafer of silicon using photo-etching techniques and
the silicon can be oxidized to form silicon dioxide dielectric.
Similarly, the dielectric portion 123 of second wall 125 can be
fabricated by photo-etching techniques, whereas the metal layer 127
can be applied by plating, vacuum deposition or other conventional
metal-application techniques used in semiconductor fabrication. The
emitter electrodes can be formed by etching and/or deposition on
the same wafer or other mass of material used to form the second
wall 125. For example, tungsten emitters can be formed by
sputtering, by vapor deposition or by chemical vapor deposition. In
a variant of this technique, the internal structure 121 can be
fabricated together with the second wall 125 so that the internal
structure is integral with the second wall. Also, although the
internal structure is shown as completely dividing the space
between walls 124 and wall 125 into entirely separate spaces 122,
these spaces may communicate with one another. For example, as
shown in FIG. 9, the internal structure may be formed as spaced
apart columns 12' rather than as continuous walls, so that the
entire space between walls 124 and walls 125 is a single, unitary
interior space.
[0038] The device shown in FIGS. 3 and 4 is used in a manner
similar to the device discussed above with reference to FIG. 1.
Thus, emitter electrodes 144 are connected to a high voltage
terminal of a power supply, whereas the second electrode 150 is
connected to a lower potential, preferably by connecting the second
electrode to ground through as a resistor as discussed above with
reference to FIG. 1. A third, grounded electrode (not shown), is
provided remote from the device. Second wall 125 is exposed to a
plenum or manifold containing the fluid to be atomized so that the
fluid passes into interior spaces 122 through a fluid entry holes
130 and passes out through discharge orifices 126. Here again, the
electric field between emitter electrode 144 and the second or
external electrode 150 causes injection of electrical charge into
the fluid passing downstream into discharge orifices 126. The
injected electrical charge causes atomization of the fluid.
[0039] Although devices as shown in FIGS. 3 and 4 can be fabricated
in any size, this configuration is particularly well-suited to
provide numerous small-diameter orifices. Thus, the discharge
orifices 126 desirably are less than about 500 .mu.m in diameter,
more preferably less than about 200 .mu.m in diameter, and most
preferably less than about 50 .mu.m in diameter. The spacing
between adjacent orifices 126 desirably is about 500 .mu.m or less,
and more preferably about 125 .mu.m or less. The axial distance (in
the direction along the axis of the discharge orifice) between the
tip of each emitter electrode 144 and the upstream end of the
associated orifice 126 desirably is between about one half to about
two times the diameter of the orifice. Lesser spacing is desirable
provided that it does not result in dielectric breakdown and
shorting between the emitter electrode and the counter
electrode.
[0040] The use of multiple orifices in the device provides several
significant advantages. First, plugging or other problems affecting
one orifice will not cause complete failure of the device. For
example, where the device is used to atomize fuel for combustion,
an acceptable flame can be maintained even if one or a few of the
orifices becomes plugged or otherwise unusable. Further, for a
given flow rate and pressure, the individual orifices in a
multi-orifice device can be of smaller diameter. As discussed
above, the preferred spacing between the emitter electrode and the
orifice is directly related to the orifice diameter. By reducing
the orifice diameter, one can reduce the spacing between the
emitter electrode and the orifice, and hence the spacing between
the emitter electrodes 144 and counter electrode 150. Moreover,
smaller orifice size favors atomization with lower voltages for
reasons associated with the configuration of the electric fields at
a droplet exiting from the orifice. All of these factors facilitate
operation with lower voltages and hence facilitate the use of
simpler, more compact and less expensive high voltage power
supplies. For example, an atomization device incorporating thirteen
15 .mu.m diameter discharge orifices 26 at 114 micron spacings
provides a total orifice area equal to a single 54 micron diameter
orifice within a chip about 2 mm by 2 mm, and accommodates flow
rates on the order of 0.01-0.02 milliliters per second of the
diesel fuel, jet fuel or kerosene. Combustion of fluid at this rate
provides about 400 watts total thermal output. The device can
operate with fluid inlet pressure on the order of 70 kPa or 10
psig, although considerably higher pressures can be used to provide
higher flow rates without damaging the device. The operating
voltage is on the order of 3 kV or less, and desirably about 1 kV
or less. Also, any number of orifices can be used to provide a
device with greater or lesser flow capability without altering the
other operating characteristics of the device.
[0041] Atomizing apparatus partially depicted in FIGS. 5 and 6
includes a body 220 defining an interior space (not shown) and a
discharge orifice 226 similar to the corresponding elements
discussed above with reference to FIGS. 1 and 2. Here again, a
second electrode or counter electrode 250 is disposed on the
exterior surface of the downstream or first wall 228 of the body
and surrounds orifice 226. A gate 201 overlies the exterior surface
228 of the first wall and overlies counter electrode 250. Gate 201
is mounted to body 220 for rotation about an axis 203 spaced apart
from the axis 226 so that the gate can be moved relative to body
220 between the retracted position illustrated in solid lines, the
partially extended position illustrated at 201' in FIG. 6 and the
fully extended position illustrated in broken lines at 201" in FIG.
6. In the fully extended position 201" , the gate completely blocks
orifice 226 whereas in the partially extended position 201' of the
gate partially occludes the orifice and thus reduces its effective
diameter. As explained in greater detail in co-pending, commonly
assigned U.S. patent application Ser. No. 09/237,583, filed Jan.
26, 1999, the disclosure of which is hereby incorporated by
reference herein, a variable size orifice such as an orifice
provided with a movable gate can be used to regulate the flow rate
of the fluid to be atomized through the device. Other means for
providing a variable size orifice may be employed. For example, as
shown in greater detail in the '583 application, the discharge
orifice may be defined by a pair of elements movable towards and
away from one another to vary the size of the orifice.
[0042] As in the embodiments discussed above, the interior space
within the housing is substantially devoid of exposed conductive
surfaces, and, in particular, exposed conductive surfaces at a
potential different from the potential at the charge injection
device, so that there are no substantial electric field between
exposed conductive surfaces exposed to the interior space. This
arrangement is particularly valuable when used with a variable-size
orifice. When such an orifice is partially closed, it acts like a
small-diameter orifice and is particularly prone to clogging due to
soot inside the chamber.
[0043] According to a further embodiment of the invention,
counterelectrode 250 may be omitted and gate 201 may act as the
counterelectrode in addition to acting as a flow restricting
element. Thus, provided the gate is formed from a conductive
material and connected to ground or another potential source, the
gate can perform the same functions as counter electrode 250. Here
again, the conductive electrode or gate overlies the exterior
surface of downstream wall 228, and is electrically insulated from
the interior space within the body.
[0044] A device according to another embodiment of the invention
utilizes a second electrode 350 in the form of a small tube
extending partially into the dielectric front wall 324 so that the
second electrode defines a portion of the orifice 326. In other
respects, this structure is identical to the structure discussed
above with reference to FIGS. 1 and 2. A structure as illustrated
in FIG. 7 is less preferred as some soot buildup may occur in the
tubular second electrode itself. Nonetheless, because the second
electrode is not directly exposed within the interior space 322 of
the device is less prone to soot buildup than a structure wherein
the second electrode is exposed to the interior space.
[0045] A device according to a further embodiment of the invention
includes a housing 420 defining an interior space 422 and a
discharge orifice 426 in a front or downstream wall 428 as
discussed above. Here, however, the charge injection electrode is
replaced by an electron beam gun 401 having an electron
transmissive beam window 403 facing interior space 422 just
upstream of nozzle 426. As explained in greater detail in commonly
assigned U.S. Pat. Nos. 5,378,957 and 5,093,602, the disclosures of
which are incorporated by reference herein, such a beam gun
typically includes an electron accelerating tube 405, a cathode 407
mounted within the tube and one or more accelerating electrodes
409. The space within the tube 405 typically is maintained under
vacuum. A beam power source 411 is connected to the cathode 407 and
to the electron accelerating electrodes 409. Electrons leave
cathode 407 and are directed in a beam through window 403 into the
interior space 422 within housing 420 and into the fluid as it
passes through orifice 426. Here again, the interior space within
the housing is devoid of conductive elements other than the beam
window 403, which may be electrically conductive. As discussed
above in connection with the embodiments using electron-emitting
electrodes, the absence of electric fields between conductive
elements exposed to the interior space 422 minimizes soot
deposition within the interior space and minimizes plugging of the
orifice. As described in greater detail in the aforementioned
patents, one or more electrodes 413 may be provided in proximity to
orifice 426. Such electrodes may be provided, for example, at
ground potential, or at other potentials different from the
potential at the electron window 403. Preferably, such electrodes
are disposed outside of the body as, for example, on the exposed,
outwardly facing surface 428 of the front wall.
[0046] Numerous variations and combinations of the features
discussed above can be utilized without departing from the present
invention as defined by the claims. For example, the high voltage
sources used to power the devices incorporating charge-emitting
electrodes may have any polarity, and may have alternating
polarity. The counter electrode need not be disposed on an exterior
surface of the device. For example, a counter electrode can be
disposed inside a wall of the device and embedded in the dielectric
material of the wall. Such an embedded electrode will still be
isolated from the interior space. In other embodiments, the body of
the device, including the downstream or front wall, may be
fabricated from a metal structural material and coated with a
dielectric material on its interior surfaces so as to isolate the
metal surfaces from the interior space. For example, a dielectric
coating can be applied by spin-coating a polymer precursor such as
an uncured polyimide on the metal surface and then curing the
polymer precursor; by sputtering; or by chemical vapor deposition.
Because such a dielectric coating need not provide structural
strength, it can be considerably thinner than a dielectric
structural wall. This, in turn, reduces the distance between the
charge injection electrode and the counter electrode in a
triode-type device, and thus reduces the operating voltage required
to yield a given electric field.
[0047] The constructional features discussed above with reference
to FIGS. 3 and 4, including multiple orifices in a single device,
can be employed without using the other features of the invention.
For example, multiple orifices may be provided in a device having
exposed conductive surfaces in the interior space as, for example,
where the front wall of the device is formed entirely from a metal
so that the front wall itself acts as the second electrode. Such an
arrangement is markedly less preferred as it sacrifices the
advantages discussed above with reference to prevention of soot
buildup. However, it can be employed in the case where the fluid is
particularly resistant to soot formation or where the device need
only operate for a brief time as, for example, in certain
disposable devices.
[0048] The particular materials discussed above are merely
illustrative. For example, other dielectric materials such as
undoped diamond can be employed. Also, conductive materials other
than metals can be employed in the electrodes.
[0049] As these and other variations and combinations of the
features as discussed above can be utilized without departing from
the present invention, the foregoing description of the preferred
embodiments should be taken by way of illustration rather than by
way of limitation of the invention as defined by the claims.
* * * * *