US20100155025A1

US20100155025A1 - Collector electrodes and ion collecting surfaces for electrohydrodynamic fluid accelerators

Info

Publication number: US20100155025A1
Application number: US12/642,561
Authority: US
Inventors: Nels Jewell-Larsen; Kenneth A. Honer; Matt Schwiebert; Hongyu Ran; Piyush Savalia; Yan Zhang
Original assignee: Tessera LLC
Current assignee: Panasonic Precision Devices Co Ltd; Adeia Semiconductor Solutions LLC
Priority date: 2008-12-19
Filing date: 2009-12-18
Publication date: 2010-06-24

Abstract

Embodiments of electrohydrodynamic (EHD) fluid accelerator devices utilize collector electrode structures that promote efficient fluid flow and reduce the probability of arcing by managing the strength of the electric field produced at the forward edges of the collector electrodes. In one application, the EHD devices dissipate heat generated by a thermal source in a thermal management system.

Description

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application claims the benefit of U.S. Provisional Application No. 61/139,518, filed Dec. 19, 2008.

BACKGROUND

1. Field
The present application relates to thermal management, and more particularly, to micro-scale cooling devices that use electrohydrodynamic (EHD, also known as electro-fluid-dynamic, EFD) technology to generate ions and electrical fields to control the movement of fluids, such as air, as part of a thermal management solution to dissipate heat.
2. Related Art
In general, electrohydrodynamic (EHD) technology uses corona discharge principles to move fluids (e.g., air molecules). Basic principles of EHD fluid flow are reasonably well understood by persons of skill in the art. Accordingly, a brief illustration of corona discharge principles in a simple two electrode system sets the stage for the more detailed description that follows.
With reference to the illustration in FIG. 14, corona discharge principles include applying a high intensity electric field between a first electrode 11 (often termed the “corona electrode,” the “corona discharge electrode,” the “emitter electrode” or just as the “emitter”) and a second electrode 12. Fluid molecules, such as surrounding air molecules, near the corona discharge region 18 become ionized and form a stream 14 of ions 16 that accelerate toward second electrode 12, colliding with neutral fluid molecules 22. During these collisions, momentum is imparted from the stream 16 of ions 14 to the neutral fluid molecules 22, inducing a corresponding movement of fluid molecules 22 in a desired fluid flow direction, denoted by arrow 13, toward second electrode 12. Second electrode 12 is variously referred to as the “accelerating”, “attracting”, “collector” or “target” electrode. While stream 14 of ions 16 are attracted to, and neutralized by, second electrode 12, neutral fluid molecules 22 move past second electrode 12 at a certain velocity. The movement of fluid produced by corona discharge principles has been variously referred to as “electric,” “corona” or “ionic” wind and has been defined as the movement of gas induced by the repulsion of ions from the vicinity of a high voltage discharge electrode.
Devices built using the principle of the ionic movement of a fluid are variously referred to in the literature as ionic wind machines, electric wind machines, corona wind pumps, electrostatic air accelerators, electro-fluid-dynamics (EFD) devices, electrostatic fluid accelerators (EFA), electrohydrodynamic (EHD) thrusters and EHD gas pumps. Some aspects of the technology have also been exploited in devices referred to as electrostatic air cleaners or electrostatic precipitators.
In the present application, embodiments of the devices illustrated and described herein are referred to as electrohydrodynamic fluid accelerator devices, also referred to in an abbreviated manner herein as “EHD devices”, and are utilized as a component in a thermal management solution to dissipate heat generated by an electronic circuit.

SUMMARY

EHD devices may be employed to motivate flow of air in a thermal management system, such as when employed to exhaust heat dissipated by integrated circuits in computing devices and electronics. For example, in devices such as laptop computers, compact scale, flexible form factor and absence of moving parts can provide design and user advantages over conventional forced air cooling technologies that rely exclusively on fans or blowers. EHD device solutions can operate silently (or at least comparatively so) with reduced volume and mass. In some cases, products incorporating EHD device solutions may be thinner and lighter than those employing conventional forced air cooling technologies. Flexible form factors of EHD devices can facilitate compelling product designs and, in some cases, may provide functional benefits.

Fluid Permeable Ion Collection Surfaces

It has been discovered that, in some EHD device configurations, a fluid-permeable ion collection surface may be provided to promote development of a generally uniform electric field distributed over downstream ion collection surfaces. Accordingly, in some embodiments of the present invention, an apparatus includes an emitter electrode and a collector-radiator assembly including a fluid permeable ion collection surface and an array of heat transfer surfaces extending downstream of the emitter electrode. The fluid permeable ion collection surface spans a major dimension of the heat transfer surface array. The emitter electrode and the collector-radiator assembly are energizable to motivate fluid along a flow path through the fluid permeable ion collection surface and over the heat transfer surfaces of the collector-radiator assembly.
In some embodiments, the fluid permeable ion collection surface is conformal with leading portions of the heat transfer surfaces. In some cases, the fluid permeable ion collection surface is electrically and thermally coupled to the heat transfer surfaces. In some cases, the fluid permeable ion collection surface is electrically isolated from the heat transfer surfaces. In some cases, the fluid permeable ion collection surface is conformal with leading surfaces of a support structure mated with the heat transfer surface array to define the collector-radiator assembly. In some cases, such a support structure includes additional ion collection surfaces downstream of the fluid permeable ion collection surface. In some cases, the fluid permeable ion collection surface presents a curved leading profile displaced from the emitter electrode.
In some embodiments, the fluid permeable ion collection surface includes a mesh or grid, a generally smooth perforated surface, or a spaced apart array of strips or surface portions. In some embodiments, the fluid permeable ion collection surface is at least partially formed of an at least partially conductive material, a metal or a carbon fiber or carbon fiber containing material.
In some embodiments, a fluid permeable ion collection surface presents, at least on surfaces exposed to substantial ion bombardment, a surface comprised of gold (Au) over nickel (Ni), NiPd over Ni or silver (Ag), silver oxide (Ag₂O), an oxide of manganese or an ozone catalytic or reactive material. In some embodiments, a fluid permeable ion collection surface includes, at least on surfaces exposed to substantial ion bombardment, a surface coating that includes an electroplate over injection-molded UL94-VO compliant thermoplastic; an electroplate over die-cast zinc (Zn) or zinc alloy; an electroplated, anodized or alodized die-cast aluminum (Al), aluminum alloy or magnesium (Mg) alloy; or an electroplate over powder injection-molded metal.
In some embodiments, the heat transfer surfaces of the collector-radiator assembly include spaced apart, generally planar portions extending in a direction generally parallel to the flow path. In some embodiments, the emitter electrode has a longitudinal extent than spans a major dimension of the collector-radiator assembly and the generally planar portions of the heat transfer surfaces are oriented generally orthogonally to the longitudinal extent of the emitter electrode. In some embodiments, the planar portions of the heat transfer surfaces are oriented such that the leading portions thereof are generally parallel to a longitudinal extent of the emitter electrode.
In some embodiments, the fluid permeable ion collection surface includes at least a portion positioned upstream of the emitter electrode. In some embodiments, the emitter electrode and the collector-radiator assembly are operatively coupled between terminals of a high voltage source to establish a corona discharge therebetween and to thereby motivate the fluid along the flow path. In some embodiments, the apparatus is disposed in a flow path for ventilating an enclosure and thereby cooling one or more devices within the enclosure, wherein at least the heat transfer surfaces of the collector-radiator assembly are thermally coupled into a heat transfer path from the devices.

Ion Collection Surfaces Extending Upstream of an Emitter Electrode

It has been further discovered that, in some EHD device configurations, ion collection surfaces may extend upstream of an emitter electrode so as to at least partially surround the emitter. In some cases, such a configuration may protect the emitter from mechanical intrusions and/or human contact with a high voltage emitter. In some cases, such a configuration may tend to shield other electrical components from unwanted electric fields and ion bombardment. In some cases, surface conditioning or coating of upstream surfaces may facilitate accumulation and retention of a surface charge that tends to repel ions from the upstream surfaces.
Accordingly, in some embodiments of the present invention, an apparatus includes an array of generally planar collector electrodes wherein at least a substantial subset thereof include respective hollows defined therein. The apparatus further includes an emitter electrode having a longitudinal extent spanning a major dimension of the collector electrode array and positioned relative to individual ones of the collector electrodes such that the emitter electrode passes through the respective hollows and such that, when the emitter and collector electrodes are energized, generated ions motivate fluid flow in a generally downstream direction toward and past dominant ion collecting surfaces of the collector electrodes that are nearest the emitter electrode. For at least the substantial subset of collector electrodes that include respective hollows, at least some ion collecting surfaces extend upstream of the emitter electrode.
In some embodiments, at least some of the hollows are holes defining ion collecting surfaces that fully surround the emitter electrode. In some embodiments, the hollows define at least a partial Faraday cage around the emitter electrode. In some embodiments, the apparatus is configured as a thermal management assembly wherein collector electrodes constitute convective heat transfer surfaces. In some embodiments, the apparatus is configured as a thermal management assembly, wherein the motivated fluid flow is over at least some convective heat transfer surfaces distinct from the collector electrodes. In some embodiments, the emitter electrode and the collector electrodes operatively coupled between terminals of a high voltage source to establish a corona discharge therebetween and to thereby motivate the fluid in the downstream direction.
In some embodiments, the apparatus further includes at least one additional emitter electrode that also passes through the respective hollows. In some embodiments, the apparatus further includes an additional hollow defined in the respective ones of the collector electrodes and an additional emitter electrode that passes through the additional hollows.
In some embodiments, dominant ion collecting surfaces of the collector electrodes present the emitter electrode with generally curved profiles proximate thereto. In some embodiments, at least the dominant ion collecting surfaces present a surface comprised of gold (Au) over nickel (Ni), NiPd over Ni or silver (Ag), silver oxide (Ag₂O), an oxide of manganese or an ozone catalytic or reactive material. In some embodiments, at least some portions of the collector electrodes other than the dominant ion collecting surfaces are coated with MnO₂or another ozone catalytic or reactive material. In some embodiments, the collector electrodes are formed as an electroplate over injection-molded UL94-VO compliant thermoplastic, an electroplate over die-cast zinc (Zn) or zinc alloy, an electroplated, anodized or alodized die-cast aluminum (Al), aluminum alloy or magnesium (Mg) alloy; or an electroplate over powder injection-molded metal.
In some embodiments, the apparatus is disposed in a flow path for ventilating an enclosure and thereby cooling one or more devices within the enclosure, wherein convective heat transfer surfaces are thermally coupled into a heat transfer path from the devices and wherein the motivated fluid flow is over the convective heat transfer surfaces. In some cases, the convective heat transfer surfaces include surfaces of the collector electrodes.

Resistive Material Conditioning of Selected Ion Collection Surfaces

It has been further discovered that, in some EHD device configurations, ion collection surfaces most closely proximate to an emitter electrode may be preferentially conditioned or coated with a highly-resistive surface. In this way, electrical fields may be advantageously shaped and spark limiting or quenching mechanisms may be provided while still facilitating efficient heat transfer at other downstream surfaces. In some cases, surface conditioning or coating of upstream surfaces may be insulative so as to facilitate accumulation and retention of a surface charge that tends to repel ions from the upstream surfaces.
Accordingly, in some embodiments of the present invention, an apparatus includes an array of collector electrodes and an emitter electrode having a longitudinal extent spanning a major dimension of the collector electrode array and positioned relative to the collector electrodes such that, when the emitter and collector electrodes are energized, generated ions motivate fluid flow in a generally downstream direction toward and past leading surfaces of the collector electrodes that are proximate the emitter electrode. Leading surfaces of the collector electrodes, but not further surfaces downstream of the leading surfaces, are conditioned with a resistive material. In some cases, the resistive material conditioning includes a coating applied to the leading surfaces of the collector electrodes.
In some embodiments, the collector electrodes include spaced apart, generally planar portions that include the downstream surfaces not conditioned with the resistive material, the generally planar portions extending in the downstream direction. In some embodiments, the generally planar portions are oriented such that the leading surfaces thereof are generally parallel to the longitudinal extent of the emitter electrode. In some embodiments, the resistive material conditioned leading surfaces of respective of the collector electrodes are positioned, relative to one another, to present the emitter electrode with a generally curved array of the leading surfaces. In some embodiments, the generally planar portions are oriented generally orthogonally to the longitudinal extent of the emitter electrode. In some embodiments, the resistive material conditioned leading surfaces of individual ones of the collector electrodes present the emitter electrode with a generally curved profile.
In some embodiments, individual ones of the collector electrodes include respective hollows defined in the generally planar portions thereof and the emitter electrode passes through the respective hollows such that, when the emitter and collector electrodes are energized, a dominant portion of ion flow is toward downstream portions of the resistive material conditioned leading surfaces that are closest the emitter electrode. In some cases, the resistive material conditioned leading surfaces of individual ones of the collector electrodes substantially surround the emitter electrode. In some cases, the apparatus includes an additional emitter electrode that also passes through the respective hollows. In some cases, the apparatus includes an additional hollow defined in the generally planar portions of the collector electrodes and an additional emitter electrode that passes through the additional hollows.
In some embodiments, the apparatus is configured as a thermal management assembly, wherein the collector electrodes constitute convective heat transfer surfaces. In some embodiments, the apparatus is configured as a thermal management assembly, wherein the motivated fluid flow is over at least some convective heat transfer surfaces distinct from the collector electrodes. In some embodiments, the emitter electrode and the collector electrodes are operatively coupled between terminals of a high voltage source to establish a corona discharge therebetween and to thereby motivate the fluid in the downstream direction.
In some embodiments, the apparatus is disposed in a flow path for ventilating an enclosure and thereby cooling one or more devices within the enclosure, wherein convective heat transfer surfaces are thermally coupled into a heat transfer path from the devices, and wherein the motivated fluid flow is over the convective heat transfer surfaces. In some embodiments, the convective heat transfer surfaces include surfaces of the collector electrodes downstream from the resistive material conditioned leading surfaces thereof.
Building on the foregoing, we present a variety of embodiments. In some embodiments, collector electrodes of the EHD device are themselves thermally coupled to a heat source such that at least some surfaces thereof act as fins of a heat exchanger. In some embodiments, the EHD device motivates flow of a fluid (typically air) past a heat exchanger that is thermally integrated with the collector electrodes. In some embodiments, multiple EHD device instances are ganged and/or staged so as to increase volume of flow, pressure or both. These and other embodiments will be understood with reference to the description that follows and with respect to the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The description of illustrative embodiments will be understood when read in connection with the accompanying drawings. Drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the structural and fabrication principles of the described embodiments.

FIG. 1A depicts a side cross-sectional view, consistent with certain EHD device configurations, of a corona discharge electrode and several generally planar collector electrodes that are arranged to present a curved array of leading surfaces, where each of the leading surfaces is oriented generally parallel to the longitudinal extent of the corona discharge electrode. FIG. 1B depicts a perspective view of a collector electrode assembly consistent with the arrangement of FIG. 1A. FIG. 1C depicts a perspective view of a corona discharge electrode and collector electrode assemblies consistent with the arrangement.

FIG. 2A depicts a side cross-sectional view, consistent with certain EHD device configurations in which a collector electrode assembly includes a fluid permeable ion collection surface electrically coupled to and conformal with an arrangement of collector electrodes. In the illustrated configuration, a curved array of leading surfaces are provided, each oriented generally parallel to the longitudinal extent of a corona discharge electrode. FIG. 2B depicts a perspective view of a collector electrode assembly consistent with the arrangement of FIG. 2A. FIG. 2C depicts a perspective view of a corona discharge electrode and collector electrode assemblies consistent with the arrangement. FIG. 2D depicts a side cross-sectional view of a variation in which fluid permeable ion collection surfaces extend upstream of, and fully surround the corona discharge electrode.

FIG. 3 is a side cross-sectional view of still another variation in which several instances of the EHD device configuration illustrated with reference to FIGS. 2A, 2B and 2C are integrated into a ganged structure.

FIG. 4A is a perspective view of a variation in which a pair of collector electrode assemblies such as illustrated with reference to FIG. 1B integrated into a ganged structure for use in an EHD device configuration. FIGS. 4B, 4C, 4D and 4E illustrate thermal pathways consistent with several design variations in which collector electrode surfaces are thermally coupled (via a thermal conduit) to a heat source and dissipate heat into a fluid flow motivated by operation of the EHD device.

FIG. 5A depicts a side cross-sectional view, consistent with certain EHD device configurations, of a corona discharge electrode and several columnar or rod-shaped collector electrodes positioned between generally planar collector electrodes. Leading surfaces of collector electrodes are arranged to present a curved array, where each of the leading surfaces is oriented generally parallel to the longitudinal extent of the corona discharge electrode. FIG. 5B depicts a perspective view of a collector electrode assembly consistent with the arrangement of FIG. 5A. FIG. 5C depicts a perspective view of a corona discharge electrode and collector electrode assemblies consistent with the arrangement.

FIGS. 6A, 6B and 6C are side cross-sectional views of several generally planar collector electrode surfaces suitable for use in certain EHD device configurations in which ion collection surfaces of the collector electrode extend upstream and define a hollow that fully surrounds the corona discharge electrode. FIG. 6D is a perspective view of one such EHD device configuration utilizing the collector electrode structure of FIG. 6C and illustrated as a component of a thermal management system in which a thermal conduit acts as a heat transfer path to collector electrode surfaces which, in turn, dissipate heat into a motivated fluid flow.

FIG. 7A is an end-on view of an EHD device configuration in which an array of generally planar collector electrodes present generally curved profiles of leading surfaces to a corresponding corona discharge electrodes. FIG. 7B is a cross-sectional view of the EHD device of FIG. 7A.

FIG. 8A is a side cross-sectional view of another generally planar collector electrode configuration in which an array of such surfaces are oriented orthogonally to a longitudinal extent of a corona discharge electrode and in which the generally planar collector electrode present the corona discharge electrode with a generally curved leading surface profile. FIG. 8C is a perspective view of a collector electrode assembly consistent with geometries illustrated in FIG. 8A, in which a fluid permeable ion collection surface is electrically coupled to and conformal with the generally curved leading surfaces. FIG. 8B is an enlarged perspective view of a portion of the generally planar collector electrode illustrated in FIG. 8A.

FIGS. 9A and 9B are perspective views of representative EHD device deployments relative to a pair of ventilated boundary surfaces of representative enclosures, illustrating respective fluid flow paths therethrough.

FIGS. 10A, 10B and 10C illustrate representative variations on certain previously illustrated collector electrodes or assemblies in which semi-conductive or resistive coating (or surface conditioning) is provided at surfaces substantially exposed to ion impingement during EHD device operation. FIG. 10A is a perspective view of the collector electrode array of FIG. 1B, illustrating the use of a semi-conductive or resistive coating or surface treatment at leading surfaces thereof. FIG. 10B is a partial cross-sectional view of a representative portion of a generally planar collector electrode, illustrating the disposition of a resistive coating thereon. FIG. 10C is a cross-sectional view of the collector electrode of FIG. 6B, illustrating the use of a resistive coating or surface treatment at ion collection surfaces of a hollow that fully surrounds a corona discharge electrode.

FIG. 11 is a perspective view of an EHD device having a certain geometry expressed as a ratio of length to height, or length to depth.

FIG. 12 is a table of design parameters for implementing the various embodiments of EHD devices described and illustrated herein.

FIG. 13A is an elevated cross-sectional side view of an EHD device disposed within an enclosure and illustrating several dimensions that may be useful in designing a device with improved fluid flow performance. FIG. 13B is a top plan view of a portion of the collector electrode array of the EHD device of FIG. 13A illustrating additional dimensions that may be useful in designing a device with improved fluid flow performance.

FIG. 14 is a graphical depiction of certain basic principles of corona-induced electrohydrodynamic (EHD) fluid flow.

Use of the same reference symbols in different drawings indicates similar or identical items.

DETAILED DESCRIPTION

Some embodiments of thermal management systems described herein employ EHD devices to motivate flow of a fluid, typically air, based on acceleration of ions generated as a result of corona discharge. Other embodiments may employ other ion generation techniques and will nonetheless be understood in the descriptive context provided herein. Using heat transfer surfaces that may or may not be monolithic or integrated with collector electrodes, heat dissipated by electronics (e.g., microprocessors, graphics units, etc.) and/or other components can be transferred to the fluid flow and exhausted. Typically, when a thermal management system is integrated into an operational environment, heat transfer paths (often implemented as heat pipes or using other technologies) are provided to transfer heat from where it is dissipated (or generated) to a location (or locations) within the enclosure where air flow motivated by an EHD device (or devices) flows over heat transfer surfaces. Of course, while some embodiments may be fully integrated in an operational system such as a laptop or desktop computer, a projector or video display device, etc., other embodiments may take the form of subassemblies.
Often, heat transfer surfaces and dominant ion collecting surfaces of a collector electrode can present differing design challenges and, relative to some embodiments, may be provided using different structures or with different surface conditioning. In some embodiments, a monolithic structure may act as a collector electrode and provide heat transfer surfaces. In some embodiments, collector electrodes and dominant heat transfer surfaces are provided (or at least fabricated) as separate structures that may be mated, integrate or more generally proximate each other in operational configurations. These and other variations will be understood even with respect to embodiments described, for simplicity, with collector electrode assemblies that include portions that operate as ion collection surfaces and as heat transfer surfaces.
In general, a variety of scales, geometries and other design variations are envisioned for collector electrodes and/or the dominant ion collection surfaces that functionally constitute a collector electrode, together with a variety of positional interrelationships between corona discharge and collector electrodes of a given EHD device. For concreteness of description, we focus on certain illustrative embodiments and certain illustrative surface profiles and positional interrelationships with other components. For example, in much of the description herein, plural planar collector electrodes are arranged in a parallel, spaced-apart array proximate to a corona discharge wire that is displaced from leading surfaces of the respective collector electrodes. In some embodiments, planar portions of the collector electrodes are oriented generally orthogonally to the longitudinal extent of a corona discharge wire. In other embodiments, orientation of collector electrodes is such that leading surfaces thereof are generally parallel to the longitudinal extent of a corona discharge wire.
In some embodiments, a fluid permeable ion collection surface is provided that is conformal with and electrically coupled to leading surfaces of an array of collector electrodes. In some embodiments, such a fluid permeable ion collection surface is conformal with leading surfaces of support structure that need not participate substantially in ion collection. In some embodiments, such a fluid permeable ion collection surface may be electrically isolated from downstream heat transfer surfaces.
In some embodiments, leading surfaces (whether of collector electrodes of an array or fluid permeable ion collection surfaces) present a curved arrangement or profile to a corona discharge electrode (or electrodes). In some embodiments, leading surfaces present other (e.g., non-curved) arrangements or profiles to a corona discharge electrode (or electrodes). In some thermal management system embodiments, collector electrodes provide significant heat transfer to fluid flows motivated therethrough or thereover. In some thermal management system embodiments, heat transfer surfaces that do not participate substantially in EHD fluid acceleration may provide substantial, even dominant, heat transfer.
It will be understood that particular EHD design variations are included for purposes of illustration and, persons of ordinary skill in the art will appreciate a broad range of design variations consistent with the description herein. In some cases, and particularly in the illustration of flow paths, EHD designs are illustrated simply as a corona discharge electrode assembly and a collector electrode assembly proximate each other; nonetheless, such illustrations within the broad context of a full range of EHD design variations are described herein.
Although embodiments of the present invention are not limited thereto, much of the description herein is consistent with geometries, air flows, and heat transfer paths typical of laptop-type computer electronics and will be understood in view of that descriptive context. Of course, the described embodiments are merely illustrative and, notwithstanding the particular context in which any particular embodiment is introduced, persons of ordinary skill in the art having benefit of the present description will appreciate a wide range of design variations and exploitations for the developed techniques and configurations. Indeed, EHD device technologies present significant opportunities for adapting structures, geometries, scale, flow paths, controls and placement to meet thermal management challenges in a wide range of applications and systems. Moreover, reference to particular materials, dimensions, electrical field strengths, exciting voltages, currents and/or waveforms, packaging or form factors, thermal conditions, loads or heat transfer conditions and/or system designs or applications is merely illustrative. In view of the foregoing and without limitation on the range of designs encompassed within the scope of the appended claims, we now describe certain illustrative embodiments.

Electrohydrodynamic (EHD) Fluid Acceleration, Generally

Basic principals of electrohydrodynamic (EHD) fluid flow are well understood in the art and, in this regard, an article by Jewell-Larsen, N. et al., entitled “Modeling of corona-induced electrohydrodynamic flow with COMSOL multiphysics” (in the Proceedings of the ESA Annual Meeting on Electrostatics 2008) (hereafter, “the Jewell-Larsen Modeling article”), provides a useful summary. Likewise, U.S. Pat. No. 6,504,308, filed Oct. 14, 1999, naming Krichtafovitch et al. and entitled “Electrostatic Fluid Accelerator” describes certain electrode and high voltage power supply configurations useful in some EHD devices. U.S. Pat. No. 6,504,308, together with sections I (Introduction), II (Background), and III (Numerical Modeling) of the Jewell-Larsen Modeling article are hereby incorporated by reference herein for all that they teach.
Note that the simple illustration of corona-induced electrohydrodynamic fluid flow shown in FIG. 14 (which has been adapted from the Jewell-Larsen Modeling article and discussed above) includes shapes for first electrode 10 and second electrode 12 that are particular to the simple illustration thereof. Likewise, the electrode configurations illustrated in U.S. Pat. No. 6,504,308 and aspects of the power supply design are particular thereto. Accordingly, such illustrations, while generally useful for context, are not intended to limit the range of possible electrode or high voltage power supply designs in any particular embodiment of the present invention.
EHD device embodiments described herein include one or more corona discharge electrodes. In general, such corona discharge electrodes include a portion that exhibits a small radius of curvature and may take the form of a wire or rod or edge. Other shapes for the corona discharge electrode are also possible; for example, the corona discharge electrode may take the shape of barbed wire, wide metallic strips, and serrated plates or non-serrated plates having sharp or thin parts that facilitate ion production at the portion of the electrode with the small radius of curvature when high voltage is applied. In general, corona discharge electrodes may be fabricated in a wide range of materials. For example, in some embodiments, compositions such as described in U.S. Pat. No. 7,157,704, filed Dec. 2, 2003, entitled “Corona Discharge Electrode and Method of Operating the Same” and naming Krichtafovitch et al. as inventors may be employed. U.S. Pat. No. 7,157,704 is incorporated herein for the limited purpose of describing materials for some corona discharge electrodes that may be employed in some embodiments. In general, a high voltage power supply creates the electric field between corona discharge electrodes and collector electrodes.
EHD device embodiments described herein include ion collection surfaces positioned downstream of one or more corona discharge electrodes. Often such ion collection surfaces include leading surfaces of generally planar collector electrodes extending downstream of the corona discharge electrode(s). In some cases, a fluid permeable ion collection surface is provided. In some cases, such a fluid permeable ion collection surface is disposed at leading edges of, and electrically connected to, generally planar collector electrodes that extend downstream of the corona discharge electrode. In general, a collector electrode (and/or fluid permeable ion collection surfaces described herein) may be fabricated of any suitable metal material, such as aluminum or copper.
As disclosed in U.S. Pat. No. 6,919,698 to Krichtafovitch, collector electrodes (referred to therein as “accelerating” electrodes) may be formed in aerodynamically “friendly” shapes that provide a low coefficient of drag for the fluid (air). As further disclosed in the '698 patent, collector electrodes may be made of a body of high resistivity material that readily conducts a corona current, but for which a result voltage drop along current paths through the body of high resistivity collector electrode material provides a reduction of surface potential, thereby damping or limiting an incipient sparking event. Examples of such relatively high resistance materials include carbon filled plastic, silicon, gallium arsenide, indium phosphide, boron nitride, silicon carbide, and cadmium selenide. U.S. Pat. No. 6,919,698 is incorporated herein for the limited purpose of describing materials for some collector electrodes that may be employed in some embodiments. Note that in some embodiments described herein, a surface conditioning or coating of high resistivity material (as contrasted with bulk high resistivity) may be employed.
Typically, configurations described and illustrated herein include an array of collector electrodes (and/or fluid permeable surfaces) that constitute the dominant ion collection surfaces during EHD device operations. The number of, and distances between, such collector electrodes and surfaces shown (as shown in the Figures) is merely exemplary and generally not to scale. Indeed, numbers and distances may vary from what is shown according to device specifications and the type of fluid being moved. The distance between a corona discharge electrode and a collector electrode is referred to as the “gap” or “air gap” (see, e.g., gap 102 in FIG. 1C) and is determined by the particular shapes of the corona discharge electrode(s) and the collector electrode(s).
Although not shown in the Figures that illustrate the embodiments herein, a high voltage power supply is electrically connected to, and creates the electric field between, the corona discharge electrode and the collector electrode, generating an ion stream that moves ambient fluid toward the collector electrode.
EHD device embodiments described herein may be implemented in a repeated adjacent plural configuration in order, for example, to improve fluid flow efficiency, or to fit into a specific space within an enclosure. Likewise, embodiments of EHD devices described herein may also be implemented in combination with a different embodiment of an EHD device in a plural adjacent configuration. In addition, while not illustrated and described herein, it is understood that any one of the embodiments of the EHD devices described herein may also be implemented in a plural configuration in which two or more individual EHD devices of the type described herein are sequentially disposed along a desired fluid flow direction. Each individual EHD device may then be referred to as a stage, and the entire configuration is referred to as a multi-stage EHD device. In operation, each individual EHD device stage may be operated simultaneously and synchronously with the others in order to produce increased volume and pressure of fluid flow in the desired direction, thereby sequentially accelerating a fluid through the multiple stages. Synchronous operation of a multi-stage EHD device is defined herein to mean that a single power supply, or multiple synchronized and phase-controlled power supplies, provide high voltage power to each EHD device stage such that both the phase and amplitude of the electric power applied to the same type of electrodes in each stage (i.e., the corona discharge electrodes or the collector electrodes) are aligned in time. U.S. Pat. No. 6,727,657, entitled “Electrostatic Fluid Accelerator for and a Method of Controlling a Fluid Flow” provides a discussion of the configuration and operation of several embodiments of a multi-stage EHD device, including computing an effective inter-stage distance and exemplary designs for a high voltage power supply for powering neighboring EHD device stages with respective synchronous and syn-phased voltages. U.S. Pat. No. 6,727,657 is incorporated by reference herein in its entirety for all that it teaches.
Embodiments of EHD devices illustrated herein may be used to dissipate heat from a thermal source housed in an enclosure, as part of a thermal management system. The thermal management system may further comprise one or more additional elements that efficiently transports heat generated by the thermal source to the collector electrode component of the EHD device, thereby heating the collector electrode component. The operational EHD device causes a substantial amount of the fluid entering the enclosure that houses the thermal source to generally follow along a fluid flow path through or over the heated collector electrode component and then exit the enclosure, thereby dissipating heat accumulating in the air above and in the vicinity of the EHD device, and in particular, the collector electrode component.
As a preliminary matter, the front perspective views of the embodiments of EHD devices and collector electrode arrays illustrated herein are situated in a three-dimensional coordinate system 101 (FIG. 1B) in which the x-y plane respectively designates the width and depth of the embodiment of the EHD device illustrated in the Figure. The z direction indicates the height of the device. The coordinate system 101 is shown with the embodiment of the collector electrode array illustrated in FIG. 1B and is not repeated in the other Figures.
FIG. 1A is a cross-sectional view of a first embodiment of an EHD device 100 comprising corona discharge electrode 110 and collector electrode array 120. Collector electrode array 120 comprises several collector electrodes 122, 124, 126 and 128 in the shape of substantially flat plates of any desirable length and thickness. Collector electrodes 124 and 126, disposed between top and bottom collector electrodes 122 and 128, are recessed away from corona discharge electrode 110 in a manner such that the ends of the collector electrodes form a generally curved shape 106. Each collector electrode is illustrated as having a rounded end 123, which may reduce the strength of the electric field in this area of the collector electrode and beneficially promote fluid flow past collector electrode structure. It is understood, however, that the ends may have sharp edges as well. When EHD device 100 is operational, the EHD forces generated between corona discharge electrode 110 and collector electrode array 120 force fluid in the direction of arrow 130 between the collector electrodes. FIG. 1B is a perspective view of collector electrode array 120 of FIG. 1A showing individual collector electrodes 122, 124, 126 and 128 attached to support members 142. FIG. 1C is a perspective view of EHD device 100 showing corona discharge electrode 110 supported by frame 112 positioned a distance 102 from collector electrode array 120, and illustrating the fluid flow direction produced by the device 100 when it is operational. Although corona discharge electrode 110 and collector electrode array 120 are shown as discrete structures, a person of skill in the art will recognize that EHD device 100 may be constructed as a unitary structure by, for example, extending frame 112 to include support members 142.
FIGS. 2A, 2B and 2C illustrate various views of EHD device 200 which is a variation of EHD device 100 of FIGS. 1A, 1B and 1C. In EHD device 200, a fluid-permeable, electrically conductive element 240 is disposed as a dominant ion collection surface in front of the forward end portions 215 of the collector electrodes. Element 240 comprises pores or openings of sufficient size to permit the fluid being moved to pass minimally impeded through collector electrode array 120 in fluid flow direction 130. Suitable fluid-permeable elements include a metal mesh structure made of metal, a carbon fiber material or any at least partially conductive materials such as a plastic with carbon fiber elements or the like.
When EHD device 200 is operational, the EHD forces generated between corona discharge electrode 110 and collector electrode array 120 force fluid in the direction of arrow 130, through fluid-permeable element 240 and between the collector electrodes. The presence of fluid-permeable element 240 promotes the development of a uniform electric field at the forward leading edges 215 of the collector electrode structure, which in turn reduces the electric field strength at these leading edges 215. The reduced electric field strength in this portion of the collector electrode structure may reduce the probability of back corona or arcing originating from leading edges 215. The presence of a uniform electric field at the collector electrodes may in turn cause a more uniform discharge of ions from all portions of corona discharge electrode 110 resulting in improved fluid flow in the direction of arrow 130.
FIG. 2D is a cross-sectional view of EHD device 210 which is a variation of EHD device 200 of FIGS. 2A, 2B and 2C. In EHD device 210, a fluid permeable, electrically conductive element 250 defines a dominant ion collection surface disposed in front of the forward end portions 215 of collector electrodes 120 and extends in such a manner as to surround corona discharge electrode 110. Element 250 comprises pores or openings of sufficient size to permit the fluid being moved to pass unimpeded through collector electrode array 120 in fluid flow direction 130. While element 250 may have any suitable shape and length, L, it may be preferable for element 250 to have a shape that permits corona discharge electrode 110 to be positioned a greater distance away from inner surface 251 than from inner surface 252 in order to promote fluid flow in the direction of arrow 130. Element 250 may function as a type of EMI shield and as protection against damage or handling of corona discharge electrode 110.
FIG. 3 is a cross-sectional view of an EHD device 300 comprised of several EHD devices 200 a, 200 b and 200 c of the type illustrated in FIG. 2A having fluid-permeable electrically conductive element 240 disposed in front of collector electrode array 120. Note that in FIG. 3, EHD device 200 a shares collector electrode 128 with EHD device 200 b. It is understood by those of skill in the art that the EHD device 100 illustrated in FIG. 1A may also be ganged in a similar manner. In one embodiment, it may be preferable to form blunted edge 351 at the junction of any two ganged EHD devices.
In addition to ganged collector electrode arrays that together form a collector electrode array such as that shown in FIG. 3, collector electrode arrays may also be ganged as illustrated in FIG. 4A. FIG. 4A is a perspective view of a pair of collector electrode arrays 120 positioned adjacent to one another and sharing a center support member 143 to form one collector electrode array 420.
FIGS. 4B through 4E are perspective views of various embodiments of collector electrode array 420 of FIG. 4A when collector electrode array 420 is used as heat dissipation component in an EHD device that is part of a thermal management system for dissipating heat generated by one or more thermal sources. In each of FIGS. 4B through 4E, the corona discharge electrode(s) are omitted from the Figure. FIG. 4B illustrates heat dissipation component 440 which comprises collector electrode arrays 446. Collector electrode arrays 446 are made of both a thermally-conductive and electrically conductive material and function both as collector electrodes and as a heat sink. As defined herein, a heat sink is an object that absorbs and dissipates heat from another object using conductive, convective or radiant thermal contact. Heat dissipation component 440 further comprises thermally conductive vertical support members 442 and thermal conduit 445. Vertical support members 442 support collector electrode arrays 446 and are in thermal contact with thermal conduit 445. Thermal conduit 445 transports heat from a thermal source disposed in the interior of the enclosure to vertical support members 442 which, in turn, transfers heat to the collector electrodes. In each of FIGS. 4B through 4E, the extent of the path and configuration of thermal conduit 445 from the thermal source within the enclosure are omitted. Small horizontal and vertical arrows 447 in FIG. 4B indicate the direction of heat movement and heat transfer from thermal conduit 445 to vertical support members 442 and the individual horizontal collector electrodes that comprise heat dissipation component 440. In operation, the EHD device generates EHD forces in the direction of arrow 441 to move ambient air within the enclosure over the collector electrodes of heat dissipation component 440.
FIGS. 4C, 4D and 4E illustrate variations of heat dissipation component 440 of FIG. 4B in which the thermal conduit that transports heat from the thermal source to the vertical support members is disposed in different positions within the heat dissipation component, with small horizontal and vertical arrows indicating the direction of heat movement and heat transfer from a thermal conduit to vertical support members, and the larger arrows indicating fluid flow direction. In all other respects, the description of heat dissipation component 440 of FIG. 4B above is equally applicable to the heat dissipation components in these Figures. FIG. 4C is a front perspective view of heat dissipation component 450 which comprises collector electrode arrays 456, thermally conductive vertical support members 452 and thermal conduit 455. In FIG. 4C, thermal conduit 455 is disposed at the top of heat dissipation component 450 and heat transfer to vertical support members 452 is in the downward direction, as indicated by small vertical arrows 457. FIG. 4D is a front perspective view of heat dissipation component 460 which comprises collector electrode arrays 466, thermally conductive vertical support members 462 and thermal conduit 465. In FIG. 4D, thermal conduit 465 is disposed in an interior position of heat dissipation component 450 between individual collector electrodes. Heat transfer to vertical support members 462 is both in the upward and downward direction, as indicated by small vertical arrows 467. FIG. 4E is a front perspective view of heat dissipation component 470 which comprises collector electrode arrays 476, thermally conductive vertical support members 472 and thermal conduit 475. In FIG. 4E, thermal conduit 475 is disposed at the bottom of heat dissipation component 470, as in heat dissipation component 440 of FIG. 4B. Thermal conduit 475 includes a portion that is perpendicularly oriented to heat dissipation component 470. Heat transfer to vertical support members 472 is in the upward direction, as indicated by small vertical arrows 477.
FIG. 5A is a cross-sectional view of another embodiment of an EHD device 500 comprising corona discharge electrode 110 and collector electrode array 520. Collector electrode array 520 comprises a pair of horizontally disposed collector electrodes 522 and 524 in the shape of substantially flat plates of any suitable length and thickness, and several wire- or rod-shaped collector electrodes 526, 528 and 530. Collector electrodes 526, 528 and 530, disposed between collector electrodes 522 and 524, are recessed away from corona discharge electrode 110 in a manner such that the edges of the collector electrodes closest to corona discharge electrode 110 form a parabolic shape 106. Collector electrodes 522 and 524 are illustrated as having a rounded end 523, but it is understood that the ends may be flat as well. When EHD device 500 is operational, the EHD forces generated between corona discharge electrode 110 and collector electrode array 520 force fluid in the direction of arrow 130 between the collector electrodes. FIG. 5B is a front perspective view of collector electrode array 520 of FIG. 5A showing individual collector electrodes 522, 524, 526, 528 and 538 attached to support members 542. FIG. 5C is a front perspective view of EHD device 500 showing corona discharge electrode 110 supported by frame 112 positioned a distance 502 from collector electrode array 520, and illustrating the fluid flow direction 130 produced by the device 500 when it is operational. Although corona discharge electrode 110 and collector electrode array 520 are shown as discrete structures, a person of skill in the art will recognize that EHD device 500 may be constructed as a unitary structure by, for example, extending frame 112 to include support members 542.
FIGS. 6A, 6B and 6C are cross-sectional views of variations of a collector electrode structure which may be constructed of a thermally- and electrically-conductive material in any suitable length and thickness. Each of the collector electrode structures 622, 624 and 626 of FIGS. 6A, 6B and 6C comprises at least one opening through which a corona discharge electrode (not shown) will pass. In operation, an EHD device comprising any one of the collector electrode structures 622, 624 or 626 will force fluid in the direction of arrow 630. Collector electrode structure 622 of FIG. 6A comprises at least one substantially round opening 612, while collector electrode structures 624 and 626 comprise at least one substantially oblong opening 614. Collector electrode structure 626 of FIG. 6C further comprises cutout portion 616 which may accommodate a support member or other structure not shown in the Figure.
FIG. 6D is a front perspective view of a fourth embodiment of an EHD device using the collector electrode structure of FIG. 6C and illustrated as a component of a thermal management system for dissipating heat generated by one or more thermal sources. EHD device 600 comprises corona discharge electrodes 110 and collector electrode array 620 of plural collector electrodes 626 of FIG. 6C. Collector electrodes 626 are made of both a thermally-conductive and electrically conductive material and function both as collector electrodes and as a heat sink. EHD device 600 further comprises thermal conduit 645 which is in thermal contact with collector electrodes 626. Thermal conduit 645 transports heat from a thermal source in the general direction or small arrows 647 to collector electrodes 626 of array 620. The extent of the path and configuration of thermal conduit 645 from the thermal source are omitted. In operation, EHD device 600 generates EHD forces in the direction of arrow 630 to move ambient air in the vicinity of the EHD device between collector electrodes 626.
FIG. 7A is a top plan view of a fifth embodiment of an EHD device comprising an array of corona discharge electrodes 710 and an array 720 of collector electrodes. Individual collector electrodes 721 are supported by at least one support member 714. Frame members 712 support corona discharge electrodes disposed in parallel with the collector electrodes 720. While EHD device 700 is illustrated as having a relatively shallow depth (in the y direction), it is understood that EHD device 700 may be configured in a variety of aspect ratios (width-to-height relationships) to suit a particular purpose. In operation, EHD device 700 generates EHD forces in the downward z direction to move a fluid in the vicinity of the EHD device between collector electrodes 720. FIG. 7B is a cross-sectional view of the EHD device of FIG. 7A taken at dashed line 702 of FIG. 7A, and illustrating fluid flow direction 730. It can be seen from this view that the height of the collector electrodes varies in a pattern. For example, collector electrodes 722 are shorter than collector electrodes 721 and form parabolic shape 706 near each corona discharge electrode 710.
FIG. 8A is a cross-sectional view of a collector electrode structure 822 which may be constructed of a thermally- and electrically-conductive material in any suitable length and thickness. Collector electrode structure 822 is shown relative to a corona discharge electrode 810 and comprises a curved forward surface 804 at leading edge 815. In operation, an EHD device comprising collector electrode structure 822 will force fluid in the direction of arrow 830. In the embodiment of collector electrode structure 822 shown in FIG. 8A, the lower and upper leading ends of collector electrode 822 as called out by leading end 816 are illustrated as having sharp edges. FIG. 8B is an enlarged partial perspective view of collector electrode 823, which is a variation of the collector electrode structure 822 of FIG. 8A in which the leading end 817 of collector electrode structure 823 is blunted or curved in shape, which may reduce the strength of the electric field in this area of the collector electrode and beneficially promote fluid flow past collector electrode structure 823 in direction 830.
FIG. 8C is a top perspective view of an array 820 of collector electrode structures 822 which may be attached to or formed integrally with a base support member not shown in the Figure. Array 820 of collector electrode structures 822 further comprises a fluid-permeable, electrically conductive element 840 disposed (as a dominant ion collection surface) in front of the leading edge portions 815 of collector electrodes 822. Element 840 comprises pores or openings of sufficient size to permit the fluid being moved to pass minimally impeded through collector electrode array 820 in fluid flow direction 830. Suitable fluid-permeable elements include a metal mesh material.
FIGS. 9A and 9B illustrate exemplary enclosures and positions of EHD devices disposed therein. FIG. 9A is a perspective view of EHD device 900 disposed within enclosure 915 between opposing boundary surfaces with ventilated portions 904 and 906. In operation, EHD device 900 moves ambient air both within enclosure 915 as well as air drawn from outside of the enclosure, and produces a flow path in the direction of arrow 960. If enclosure 915 is typically used in the orientation as shown in FIG. 9A while supported on a flat surface, then enclosure 915 may have support structures (not shown) on its bottom surface to raise enclosure 915 up and away from the supporting flat surface in order to allow air to circulate under enclosure 915. EHD device 900 may be any one of the embodiments of EHD devices described above, oriented within enclosure 915 in a manner that produces air flow in the direction of arrow 960.
By way of another example, FIG. 9B is a perspective view of EHD device 1400 disposed within enclosure 1415 between adjoining boundary surfaces with ventilated portions 1404 and 1405. In the orientation of enclosure 1415 illustrated in FIG. 9B, ventilated portion 1404 is part of a rear side boundary and ventilated portion 1404 is part of a bottom boundary of enclosure 1415. Top boundary 1418 opens from the rest of enclosure 1415 by way of hinge 1416. Enclosure 1415 may also have an interior fixed or movable boundary surface, not shown in the Figure, which is disposed between EHD device 1400 and top boundary surface 1418. In operation, EHD device 1400 produces an air flow path in the direction of the arrows shown in the interior of enclosure 1415. If enclosure 1415 is typically used in the orientation as shown in FIG. 9B while supported by flat surface 1420, then enclosure 1415 may have support structures (not shown) on its bottom surface to raise enclosure 1415 up and away from supporting flat surface 1420 in order to allow air to circulate under enclosure 1415, for example, in the direction of arrow 1440. EHD device 1400 may be any one of the embodiments of EHD devices described above, oriented within enclosure 1415 in a manner that produces air flow in the direction of the arrows as shown in the Figure.
FIGS. 10A, 10B and 10C illustrate the use of a semi-conductive or resistive coating on the edges of a collector electrode. FIG. 10A is a front perspective view of collector electrode array 120 of FIG. 1B with semi-conductive or resistive coating 1012 applied to the edges of individual collector electrodes 128. Area 1022 of collector electrodes 128 is enlarged to show that semi-conductive or resistive coating 1012 may extend to the front and side edges 1014 of electrode 128. FIG. 10B illustrates one embodiment in which semi-conductive or resistive coating 1012 may extend to the bottom surface 1024 of electrode 128. FIG. 10C is a side elevation view of collector electrode 626 of collector electrode array 620 of FIG. 6D and illustrating semi-conductive or resistive coating 1032 applied on the front-facing visible surface 625 of collector electrode 626 around the edges of openings 614. While not shown in the Figure, semi-conductive or resistive coating 1032 is also applied on the rear-facing surface of collector electrode 626 in the same manner.
The presence of the resistive coating on the leading edges of the collector electrodes is one mechanism for managing the electric field strength of the portion of the collector electrodes closest to the corona discharge electrode. The presence of the resistive coating serves to physically dull the edges of the collector electrodes that are proximate to the corona discharge electrode and eliminate sharp edges at the collector electrode ends that could cause ions attracted to the collector electrodes to tend to collect only at the collector electrode ends which would adversely affect the performance of the fluid movement through the collector electrode array. One benefit of having ions and fluid move through collector electrode array 120 is that charged particles, such as dust, in the fluid may accumulate in areas of the collector electrodes that are more distant from the corona discharge electrode. The presence of the resistive coating in certain areas of the collector electrodes thus reduces the electric field strength in these areas. The presence of the resistive coating may also prevent arcing between the corona discharge electrode and the ends of the collector electrodes.
While many of the collector electrode structures illustrated in the Figures herein are illustrated as having substantially smooth and uniformly even surfaces, a person of skill in the art will recognize that they need not be so limited. It may be advantageous in some implementations of EHD devices for the sides of the collector electrodes that are parallel to the fluid flow direction to have non-planar, bumpy or uneven surfaces. By way of one example, FIG. 10D illustrates a partial top plan view of collector electrode 1050 with side surfaces 1052 a and 1052 b parallel to fluid flow direction 1030 each having a wave-like shape. Other side surface configurations that depart from a uniformly planar surface are also possible.
The EHD devices illustrated herein are suitable for dissipating heat generated by a thermal source, as a component in a thermal management system for an electronic circuit in an electronic apparatus. As defined herein, an electronic circuit is defined as one or more electronic components. When there is more than one electronic component, the electronic components are in mutual electromechanical contact, usually by being soldered to a printed circuit board (PCB). An electronic component is any physical entity in an electronic system whose intention is to affect the electrons or their associated fields in a desired manner consistent with the intended function of the electronic system. Electronic components may be packaged singly or in more complex groups as integrated circuits. Some common electronic components are capacitors, resistors, diodes and transistors. As used herein, an “electronic apparatus” is an apparatus that comprises one or more electronic circuits.
The EHD devices illustrated herein may be constructed in a wide range of sizes in order to meet the requirements of a particular thermal heat management solution. By way of one example, when EHD device 100 of FIG. 1C is configured for dissipating heat from an electronic circuit in an electronic apparatus, corona discharge electrode 110 may have a height, h, in the range of 0.5 mm to 30 mm, and a length, l, chosen to meet the needs of the particular enclosure within which the EHD device will operate. When the corona discharge electrode component comprises multiple corona discharge electrodes such as shown in FIG. 3, the distance between adjacent corona discharge electrodes 110 may be approximately 2-4 mm. Such a device may be suitable for use in an electronic device having a thin form factor. Note that the scale of the individual components shown in the Figures herein is solely for illustration purposes; each component may have height, width and depth dimensions that are different from the relative dimensions shown in a particular Figure.
The various embodiments of EHD devices illustrated herein, when used as a component in a thermal management system, may be designed to achieve a target fluid flow rate that is sufficient to dissipate a target heat quantity generated by a particular one or more thermal sources contained within an enclosure, while operating the EHD device under the constraint of a given pressure head range. Once the target fluid flow rate and target heat quantity are known, the design of such an EHD device begins with determining the geometry and fluid flow resistance of the device that will permit the operation of the EHD device within the desired pressure head range. In one embodiment, the EHD device may be configured to operate with a pressure head in the range of 1-50 Pa. For some thermal management applications, the desired, or target, pressure head range may be a range of 3-20 Pa.
Each of the various embodiments of EHD devices illustrated herein may be configured to have a high aspect ratio and positioned within an enclosure proximate to one or more ventilated surface boundaries in order to minimize resistance along the fluid flow path. With reference to FIG. 11, assume that structure 1100 is an EHD device of the type illustrated in FIG. 6D comprising collector electrode array 620 that also functions as a heat sink and a sub-component 645 that functions as a thermal conduit (e.g., a heat pipe) that directs heat from a thermal source to array 620 of collector electrodes. A high-aspect ratio EHD device is defined herein as having at least one of the following relationships:
5<L/H<300 Equation (1), or
5<L/D<150 Equation (2),
where L is the length of the device, H is the height of the device, and D is the depth of the device, where the depth of the device is along the fluid flow path. In some embodiments of an EHD device, these relationships may preferably be stated as:
10<L/H<40 Equation (3), or
10<L/D<30 Equation (4).
When the EHD device is configured according to the relationships of any one of Equations (1)-(4), and the EHD device is positioned proximate to one or more ventilated boundary surfaces in an enclosure, as illustrated, for example, in FIGS. 8 and 9 herein, to produce a substantially compact fluid flow path, the EHD device should operate within the desired pressure head range. The operational pressure head of the EHD device will produce a fluid flow velocity through the collector electrode array component and at the output of the device sufficient to achieve the target fluid flow rate that is needed to dissipate the target heat quantity. Given the pressure head ranges and EHD device aspect ratios recited above, the fluid flow velocity is expected to be in the range of 0.1-3 m/s in some embodiments, and preferably in the range of 0.2-1.5 m/s in other embodiments. FIG. 12 summarizes the ranges of the several factors discussed above.
In addition, in some configurations of an EHD device according to these designs, the device operation will maximize the dissipation of heat from the thermal source while maintaining a substantial equilibrium of the fluid flow velocity through the enclosure within which the EHD device operates. That is, the device will maintain a fluid flow velocity at an intake ventilated boundary surface that is substantially equal to the fluid flow velocity at an outgoing ventilated boundary surface. Note that EHD device 1100 of FIG. 11 is represented in a substantially rectangular form. It is understood that this is for illustration purposes only; the actual shape of the EHD device is dependent on the configuration of the collector electrodes and the position of any associated thermal management components such as a heat pipe. It is sufficient that the EHD device have a length, height and depth by which the ratios of Equations (1) through (4) may be satisfied.
The discussion above in conjunction with FIGS. 11 and 12 relates to achieving improved fluid flow performance by optimizing the aspect ratio of the entire EHD device. Fluid flow performance improvements may also be achieved by optimizing characteristics of the relationship of a corona discharge electrode to one or more collector electrodes. Assume, for example, that EHD device 1400 of FIG. 9B disposed in enclosure 1415 is configured to have one or more corona discharge electrodes in the shape of a thin wire and the collector electrode structure illustrated in FIGS. 8A and 8B. FIGS. 13A and 13B illustrate some exemplary dimensions of such an EHD device. These are referenced and described in Table 1 below:

TABLE 1

Ref. No
or
Symbol	Dimension	Dimension description

1302	d₁	Diameter of corona discharge wire
1304	d2	Distance from corona discharge wire to
		leading edge of collector electrode, also
		referred to as the air gap
H	Height	Height of collector electrode
L	Length	Length of collector electrode
1306	d3	The center-to-center distance between
		collector electrodes, or pitch
1308	T	The thickness of the collector electrode
1310	d4	A portion of the length, L, of the collector
		electrode closest to the corona discharge
		electrode, referred to as the leading edge
		length.
Henc	Height	Height of the enclosure in which the EHD
		device operates

FIG. 13A is a cross-sectional side elevation view of an EHD device. In FIG. 13A, corona discharge electrode 810 has a diameter, d₁, and collector electrode 823 has a length, L, and a width, W. The air gap distance 1304 between corona discharge electrode 810 and collector electrode structure 823 is denoted as d₂. When the EHD device of FIG. 13A is operational, fluid travels in the direction of arrow 830. FIG. 13B is a top plan view of collector electrode array 1320 which is a portion of the collector electrode array of the EHD device illustrated in FIG. 13A. In FIG. 13B, fluid flow is along the length of collector electrode 823 in the direction of arrow 830. The center-to-center distance 1306 between collector electrodes, or pitch, is denoted as d₃, and the thickness 1308 of each collector electrode 823 is denoted as T.
Designing an EHD device using the dimensions referenced in Table 1 and illustrated in FIGS. 13A and 13B and consistent with the relationships stated in the following equations may improve fluid flow performance:
L/d3<20 Equation (5),
0.1<T/d3<0.5 Equation (6), and
1.25<d2/d3<10 Equation (7).
The ratio of the length, L, of the collector electrode to the pitch, d₃, should preferably be less than 20. The ratio of the thickness, T, of each collector electrode to the pitch, d₃, should preferably be between 0.1 and 0.5. The ratio of the air gap distance, d₂, to the pitch, d₃, should preferably be between 1.25 and 10. In addition, as a general principle, an EHD device will achieve efficient fluid flow performance in a thermal management system when the pitch of the collector electrodes is greater than or equal to 0.5 mm and less than or equal to 2 mm. Boundary layer disruption is another characteristic to be considered. More significant boundary layer disruption along the side surface of collector electrode is likely to be achieved in the distance d₄of the electrode (i.e., the portion of the length, L, of the collector electrode closest to the corona discharge electrode, referred to as the leading edge length). The extent of distance d₄is affected by the length, L, of a collector electrode as well as the pitch, d₃. Decreasing the pitch of the collector electrodes may affect the electric field strength at the leading edges of the collector electrodes.
Some of the embodiments of electrohydrodynamic fluid accelerator devices illustrated and described herein are discussed in the context of a thermal management solution to dissipate heat generated by a thermal source. However, the devices are not limited in their use to that context. Embodiments of the devices illustrated and described herein may be suitable for use in any type of device that requires the movement of a fluid, such as, for example, electrostatic precipitators, and electrostatic air cleaners and purifiers.

Other Embodiments

While the techniques and implementations of the EHD devices discussed herein have been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the appended claims. In addition, many modifications may be made to adapt a particular situation or material to the teachings without departing from the essential scope thereof. Therefore, the particular embodiments, implementations and techniques disclosed herein, some of which indicate the best mode contemplated for carrying out these embodiments, implementations and techniques, are not intended to limit the scope of the appended claims.

Claims

1. An apparatus comprising:

an emitter electrode; and

a collector-radiator assembly including a fluid permeable ion collection surface and an array of heat transfer surfaces extending downstream of the emitter electrode, the fluid permeable ion collection surface spanning a major dimension of the heat transfer surface array,

the emitter electrode and the collector-radiator assembly energizable to motivate fluid along a flow path through the fluid permeable ion collection surface and over the heat transfer surfaces of the collector-radiator assembly.

2. An apparatus comprising:

an array of generally planar collector electrodes wherein at least a substantial subset thereof include respective hollows defined therein; and

an emitter electrode having a longitudinal extent spanning a major dimension of the collector electrode array and positioned relative to individual ones of the collector electrodes such that the emitter electrode passes through the respective hollows and such that, when the emitter and collector electrodes are energized, generated ions motivate fluid flow in a generally downstream direction toward and past dominant ion collecting surfaces of the collector electrodes that are nearest the emitter electrode,

wherein for at least the substantial subset of collector electrodes that include respective hollows, at least some ion collecting surfaces extend upstream of the emitter electrode.

3. An apparatus comprising:

an array of collector electrodes; and

an emitter electrode having a longitudinal extent spanning a major dimension of the collector electrode array and positioned relative to the collector electrodes such that, when the emitter and collector electrodes are energized, generated ions motivate fluid flow in a generally downstream direction toward and past leading surfaces of the collector electrodes that are proximate the emitter electrode,

wherein the leading surfaces of the collector electrodes, but not further surfaces downstream of the leading surfaces, are conditioned with a resistive material.