Patent Application
20140271239
1. Field
The present application relates to devices that generate ions and/or electrical fields to motivate flow of fluids, such as air, and/or charged particulates.
2. Related Art
Many modern electronic devices (including desktop and laptop computers, all-in-one computers, compute tablets, televisions, video displays and projectors) employ forced air flow as part of a thermal management solution. Mechanical air movers such as fans or blowers have conventionally been employed in many such devices. However, in some applications and devices, mechanical air mover operation may result in undesirable levels of noise or vibration that may degrade the user experience. In some cases, physical scale or flow paths that would otherwise be necessary to accommodate a mechanical air mover may be incompatible with, or unacceptably limit, the design, scale or form factor of a particular design. Worse still, at the extremely thin device form factors popular in certain consumer electronics (e.g., laptops, pad-type computers, televisions, smart phones, book readers and media players), mechanical air mover designs (if even accommodatable) tend to exhibit poor cooling efficiencies. As a result, battery life may be adversely affected or, as a practical matter, device performance throttled to a level compatible with passive cooling.
Technologies have been developed that employ electric fields and principles of ionic movement of a fluid to motivate air flow. Devices that operate based on such principles are variously referred to in the literature as ionic wind machines, electric wind machines, corona wind pumps, electro-fluid-dynamics (EFD) devices, electrohydrodynamic (EHD) thrusters and EHD gas pumps. Some aspects of the technology have been exploited in devices referred to as electrostatic air cleaners or electrostatic precipitators and, indeed, some practical large scale device applications of the technology date back to the early 1900s. More recently, researchers have considered the utility of EHD air movers as part of a thermal management solution in consumer electronics devices. See generally, N. E. Jewell-Larsen, H. Ran, Y. Zhang, M. Schwiebert and K. A. Honer, Electrohydrodynamic (EHD) Cooled Laptop, in proceedings of 25th Annual Semiconductor Thermal Measurement and Management Symposium (March 2009).
In some cases, an ion flow or EHD air mover may improve cooling efficiency and thermal management in some devices and/or applications, while reducing noise, vibration and power consumption. Likewise, EHD air mover designs may provide or facilitate systems or devices that have reduced overall device lifetime costs, device size or volume, and/or improved electronic device performance or user experience.
In some cases and or operating environments, sidewalls of a flow channel in which fluid (e.g., air) is motivated may be, or become, susceptible to surface accumulations of contaminants or charge carriers that can (given local strength of an applied electric field) provide a spark or shunting current path between emitter and collector electrodes. In some cases, the electrodes themselves may accumulate contaminants that can likewise contribute to conditions that support or increase likelihood of sparking discharge. These effects can be undesirable for a number of reasons including the acoustic signatures that can be generated, the surface pitting and damage that can occur and excess ozone (03) that can result. Similar design challenges may present in other corona discharge devices, at least at some form factors and in some applications.
Ozone (03), while naturally occurring, can also be produced during operation of various electronics devices including EHD devices, photocopiers, laser printers and electrostatic air cleaners, and by certain kinds of electric motors and generators, etc. At high concentrations, ozone can be undesirable and, accordingly, techniques to reduce ozone concentrations are desired. Indeed, techniques have been developed to catalytically or reactively break down ozone (O3) into the more stable diatomic molecular form (O2) of oxygen. See e.g., U.S. Pat. No. 6,603,268 to Lee and U.S. Patent Application Publication 2010-0116469, naming Jewell Larsen et al. as inventors, each of which is commonly-owned by the assignee of the present application.
Improved techniques are desired for managing conditions that may otherwise lead to sparking discharge. For example, techniques to reduce or mitigate accumulation of contaminants are desired. Likewise, techniques are desired that tend to mitigate impact of contaminants that may nonetheless accumulate and of other contributors to sparking or shunting currents. Finally, designs and techniques are desired for suppressing incipient sparking discharge if and when it occurs.
It has been discovered that by very closely coupling, indeed by directly connecting, ballast to an emitter electrode of an ion generator, e.g., a corona-discharge device, a rapid and self-corrective reduction in emitter-to-collector voltage may be provided responsive to an increase in current characteristic of incipient sparking discharge. Voltage levels in the emitter-to-collector gap can be rapidly reduced based on voltage drop across the ballast that, while negligible under nominal ion (or corona) current conditions, transiently increases in the event of a sparking discharge. As a result, the portion of supply voltage (typically multi-KV supply voltage) across the emitter-to-collector gap is transiently reduced to levels below a current breakdown voltage and, indeed, field intensity proximate to the emitter is transiently reduced below levels otherwise necessary to sustain ion generation.
In some embodiments, a ballast circuit is provided in the form of a circuit board or other electrical assembly with an electrical attach point to which a wire-type emitter electrode may be directly connected (e.g., by solder attachment). In some cases, the ballast board or other electrical assembly is sized and configured for placement within protective cavity into which the wire-type emitter electrode and a high voltage supply lead are both terminated and thereafter “potted” using an ozone resistant potting compound such as polyurethane.
In some embodiments in accordance with the present invention(s), an apparatus includes an elongate wire emitter electrode and at least one collector electrode energizable to establish an ion current therebetween, together with a ballast circuit in direct electrical contact with the elongate wire emitter electrode to close at least a portion of a current path through the emitter and collector electrodes to, or from, a high voltage supply terminal. In some cases, the ballast circuit includes a circuit board with a termination point to which one end of the elongate wire emitter electrode is soldered.
In some cases or embodiments, the apparatus further includes a structural frame formed substantially of dielectric material and including a cavity in which the ballast circuit is positioned. The soldered end mechanically fixes the elongate wire emitter electrode under tension. In some cases, an encapsulating volume of the cavity surrounding the ballast circuit and the soldered end of the elongate wire is substantially filled with potting material. In some cases, the potting material includes an ozone resistant polyurethane.
In some cases or embodiments, the apparatus is configured as an EHD fluid mover to motivate fluid flow past the emitter and collector electrodes. In some cases or embodiments, the apparatus further includes an enclosure having inlet and outlet ventilation boundaries. The EHD fluid mover is disposed within the enclosure to, when energized, motivate air flow along a fluid flow path between the inlet and outlet ventilation boundaries and a heat source is thermally coupled to transfer heat into the motivated air flow.
In some cases or embodiments, the ballast circuit consists essentially of a resistive load providing between 100 KO and 0.5 MΩ of resistance in the current path. In some cases or embodiments, the ballast circuit includes a depletion mode field-effect transistor (FET) coupled into the current path. In some cases or embodiments, a gate terminal of the depletion mode FET is coupled to increase effective resistance of the ballast circuit in correspondence with increased current through the ballast circuit. In some cases or embodiments, the ballast circuit includes a transient voltage suppressor device coupled to provide the depletion mode FET with overvoltage protection. In some cases or embodiments, the ballast circuit includes a hysteresis circuit coupled to allow a resistance increasing bias to develop at the gate terminal more quickly than such a bias may be dissipated to return the depletion mode FET to a fully conductive state.
In some cases or embodiments, the apparatus further includes a high-voltage power supply coupled to supply the emitter and collector electrodes with a nominal energizing voltage in excess of 3 KV. In some cases or embodiments, the apparatus is configured as one of an electrostatic precipitator and an ozone generator.
In some embodiments in accordance with the present invention(s), a method of making an electrohydrodynamic (EHD) fluid mover includes: (i) providing a structural frame defining an open volume with exposed dielectric surfaces and positional registrations to receive an elongate wire emitter electrode and at least one collector electrode, the structural frame further defining a cavity to receive a ballast circuit; (ii) introducing the ballast circuit into the cavity and positionally fixing the ballast circuit therein; (iii) stringing the elongate wire emitter electrode across the open volume and past respective ones of the positional registrations at or adjacent surfaces of the structural frame that define sidewalls of the open volume; and (iv) at a position beyond one of the sidewalls but within the cavity, electrically connecting and mechanically fixing the elongate wire emitter directly to a termination point on the ballast circuit.
In some cases or embodiments, the method further includes: (v) prior to the stringing, fixing a first end of the elongate wire emitter electrode; and (vi) prior to the electrically connecting and mechanically fixing, tensioning the elongate wire emitter electrode strung across the open volume and past respective ones of the positional registrations.
In some cases or embodiments, the method further includes substantially filling the void and thereby substantially encapsulating the ballast circuit, including the termination point thereon and at least a portion of the elongate wire emitter electrode connected directly thereto, with a potting material.
In some cases or embodiments, the method further includes (vii) prior to the introduction of the ballast circuit into the cavity, at least partially filling the cavity with a first volume of uncured potting material; and (viii) after the electrical connection and mechanical fixation of the elongate wire emitter directly to a termination point on the ballast circuit, substantially filling remaining unfilled portions of the cavity with a second volume of the uncured potting material and thereafter curing at least the second volume of potting material to thereby encapsulate the ballast circuit, including the termination point thereon and at least the portion of the elongate wire emitter electrode connected directly thereto within cured potting material that, together with the encapsulated ballast circuit fills the substantial entirety of the cavity.
In some embodiments in accordance with the present invention(s), an electrohydrodynamic (EHD) fluid mover assembly includes a structural frame, a ballast circuit, and an elongate wire emitter electrode. The structural frame defines an open volume with exposed dielectric surfaces and positional registrations to receive the elongate wire emitter electrode and at least one collector electrode. The structural frame further defines a cavity to receive the ballast circuit. The ballast circuit is positioned within the cavity. The elongate wire emitter electrode is strung across the open volume and past respective ones of the positional registrations at or adjacent surfaces of the structural frame that define sidewalls of the open volume and, at a position beyond one of the sidewalls but within the cavity, is electrically connected and mechanically fixed directly to a termination point on the ballast circuit. In some cases or embodiments, an encapsulating volume of the cavity substantially surrounding the ballast circuit and the termination point thereon is filled with an ozone resistant potting material.
The present invention may be better understood, and its numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings.
The use of the same reference symbols in different drawings indicates similar or identical items. System and device exploitations of the various configurations described and illustrated herein will be appreciated by persons of ordinary skill in the art having benefit of the present disclosure.
As will be appreciated, many of the designs and techniques described herein have particular applicability to the thermal management challenges of densely-packed devices and small form-factors typical of modern consumer electronics. Indeed, some of the EHD fluid/air mover designs and techniques described herein facilitate active thermal management in electronics whose thinness or industrial design precludes or limits the viability of mechanical air movers such as fans, blowers, etc. In some embodiments, such EHD fluid/air movers may be fully integrated in an operational system such as a pad-type or laptop computer, a projector or video display device, a set-top box, etc. In other embodiments, such EHD fluid/air movers may take the form of subassemblies or enclosures adapted for use in providing such systems with EHD motivated flows.
In general, a variety of scales, geometries and other design variations are envisioned for electrostatically operative surfaces that provide field shaping or that functionally constitute a collector electrode, together with a variety of positional interrelationships between such electrostatically operative surfaces and the emitter and/or collector electrodes of a given EHD device. For purposes of illustration, we focus on certain exemplary embodiments and certain surface profiles and positional interrelationships with other components. For example, in much of the description herein, generally planar collector electrodes are formed as or on respective parallel surfaces that define opposing walls of a fluid flow channel and which are positioned proximate to a corona discharge-type emitter wire that is displaced (upstream) from leading portions of the respective collector electrodes. Nonetheless, other embodiments may employ other configurations or other ion generation techniques and will nonetheless be understood in the descriptive context provided herein.
For purposes of illustration and not limitation, contents of U.S. Provisional Application No. 61/612,892, filed Mar. 19, 2012, entitled “OPERATIONAL CONTROL OF ELECTROHYDRODYNAMIC (EHD) AIR MOVER AND ELECTRODE CONDITIONING MECHANISM” and of U.S. patent application Ser. No. 13/737,464, filed Jan. 9, 2013, entitled “ELECTROHYDRODYNAMIC (EHD) AIR MOVER CONFIGURATION WITH FLOW PATH EXPANSION AND/OR SPREADING FOR IMPROVED OZONE CATALYSIS” and naming Jewell-Larsen, Lee, Honer and Humpston as inventors are incorporated herein by reference. The '892 provisional application and the '464 application illustrate and describe certain laptop and display device deployments of EHD air movers. In addition, the '892 and '464 applications illustrate and describe variations on electrode geometries that, based on teachings herein, may be adapted to convey benefits and advantages analogous to those described herein. Further alternative EHD fluid mover configurations are detailed in application Ser. No. 13/310,676, filed Dec. 2, 2011, entitled “ELECTROHYDRODYNAMIC (EHD) FLUID MOVER WITH FIELD SHAPING FEATURE AT LEADING EDGE OF COLLECTOR ELECTRODES” and naming Jewell-Larsen as inventor, which is also incorporated herein by reference.
In the present application, some aspects of embodiments illustrated and described herein are referred to as electrohydrodynamic fluid accelerator devices, also referred to as “EHD devices,” “EHD fluid accelerators,” “EHD fluid movers,” “ion fluid movers” and the like. For purposes of illustration, some embodiments are described relative to particular EHD device configurations in which a corona discharge at, or proximate to, an emitter electrode operates to generate ions that are accelerated in the presence of an electrical field, thereby motivating fluid flow. While corona discharge-type fluid acceleration devices provide a useful descriptive context, it will be understood (based on the present description) that spark suppression ballast techniques described herein may also be employed in other corona discharge devices, such as ion generators and electrostatic precipitators.
Focusing illustratively on EHD fluid accelerator embodiments and thermal management system use cases, it will be understood that heat dissipated by electronics (e.g., microprocessors, graphics units, etc.) and/or other components can be transferred into an EHD motivated fluid flow and exhausted from an enclosure through a ventilation boundary. 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.
For illustration, heat transfer fins are depicted with respect to various exemplary embodiments. However, as will be appreciated based on the description herein, in some embodiments, conventional arrays of heat sink fins need not be provided and EHD motivated fluid flow over exposed interior surfaces, whether proximate a heat generating device (such as a processor, memory, RF section, optoelectronics or illumination source) or removed therefrom, may provide sufficient heat transfer. In each case, provision of ozone catalytic or reactive surfaces/materials on heat transfer surfaces may be desirable. Typically, heat transfer surfaces, field shaping surfaces and dominant ion collecting surfaces of a collector electrode present differing design challenges and, relative to some embodiments, may be provided using different structures or with different surface conditioning. However, in some embodiments, a single structure may be both electrostatically operative (e.g., to shape fields or collect ions) and provide heat transfer into an EHD motivated fluid flow.
Basic principles 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.
Summarizing briefly with reference to the illustration in
EHD fluid mover designs illustrated herein generally include a single elongate wire, corona discharge type emitter electrode, although (more generally) multiple emitter electrodes and other emitter geometries may be employed. Typically, corona discharge type emitter electrodes include a portion (or portions) that exhibit(s) a small radius of curvature and may take the form of a wire, rod, edge or point(s). Other shapes for corona discharge electrodes 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, a corona discharge type emitter electrode is formed of Palladium Nickel (PdNi) plated Tungsten (W) wire with a Rhodium (Rh) coating. U.S. application Ser. No. 13/302,811, filed Nov. 22, 2011, which is incorporated herein by reference, describes certain layered structures of drawn wire that, at diameters of 10-50 μm (typically less that about 25 μm) are suitable for EHD air mover devices of the illustrated designs with KV range emitter-to-collector voltages and scales typical of modern consumer electronics. 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 emitter electrodes that may be employed in some corona discharge-type embodiments. In general, a high voltage power supply creates the electric field between corona discharge electrodes and collector electrodes.
EHD fluid mover designs illustrated herein include ion collection surfaces positioned downstream of one or more corona discharge electrodes. Often, ion collection surfaces of an EHD fluid mover portion include leading surfaces of generally planar collector electrodes extending downstream of the corona discharge electrode(s). In small form factor designs that seek to minimize flow channel height, collector electrode surfaces may be positioned against, or may partially define opposing walls of, the flow channel. In some cases, a collector electrode may do double-duty as heat transfer surfaces. In some cases, a fluid permeable ion collection surface may be provided.
In general, collector electrode surfaces may be fabricated of, or with, any suitable conductive material or surface, such as aluminum or copper. Alternatively, as disclosed in U.S. Pat. No. 6,919,698 to Krichtafovitch, collector electrodes (referred to therein as “accelerating” electrodes) may be formed 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.
Given the substantial voltage differential and short distances involved between emitter electrode 191 and leading surfaces of collector electrodes 192 (perhaps 5 mm or less, depending on EHD fluid mover scale), a strong electrical field is developed which imposes a net downstream motive force on positively charged ions (or particles) in the fluid. Field lines illustrate (generally) spatial aspects of the resulting electric field and spacing of the illustrated field lines is indicative of field strength.
As will be understood by persons of ordinary skill in the art, corona discharge principles may be employed to generate ions in the intense electric field closely proximate the surface of a corona-discharge type emitter electrode. Thus, in corona discharge type embodiments in accord with
Because corona discharge devices such as the EHD fluid mover configuration of
The resulting voltage drop across the ballast (e.g., across ballast circuit 270) robs the emitter-to-collector gap of some of the voltage necessary to sustain a sparking discharge across the gap and tends to suppress the corona discharge-based process by which carriers are generated in the gap. By very closely coupling ballast circuit 270 to emitter electrode 191, time constants (RC- or LC-related) can be minimized such that temporal response of the emitter-to-collector gap voltage reduction is improved. In addition, stored energy available and peak sparking current (based on now negligible parasitic capacitance downstream of the ballast circuit 270) are minimized. Accordingly, acoustics of any sparking that does occur are likewise minimized. Description that follows (particularly that related to
As will be appreciated by persons of ordinary skill in the art having access to the present description, additional techniques may be applied to reduce the incidence of sparking discharge and shunting currents in EHD fluid movers. Additional techniques include the use of one or more cleaning and/or conditioning mechanisms and use of field blunting structures, collector shaping and/or flow baffles as summarized below and further detailed in documents incorporated by reference herein. In some embodiments, closely-coupled ballast techniques described and illustrated herein are used in combination with such additional techniques.
In the illustrated cross-sectional view of
Further details regarding cleaning/conditioning mechanisms for in situ application of a consumable catalyst that includes silver (Ag) to an emitter electrode of certain EHD air mover configurations such as illustrated herein may be found in U.S. patent application Ser. No. 13/602,256, filed Sep. 3, 2012, entitled “System and Method for In-Situ Conditioning of Emitter Electrode with Silver” and naming Jewell-Larsen, Honer, Gao and Schwiebert as inventors, which is incorporated herein by reference.
In the illustrated perspective view of
Based on the present description, it will be understood that certain surfaces in an EHD air mover design (at least in compact designs at or below the scales illustrated and described herein) are particularly susceptible to accumulation of airborne contaminants and charge, which (in the presence of substantial electric fields) may, in turn, create conditions suitable for sparking discharge. Sidewalls of an EHD air mover channel adjacent to emitter and collector electrodes (or into which the lateral extent of such electrodes impinges) are one example of such susceptible surfaces. Another example, in some embodiments, is sidewalls of an electrode conditioning carriage configured to travel across an EHD air mover channel. EHD air mover designs illustrated and described herein employ several strategies to mitigate these susceptibilities which may otherwise lead to undesirable sparking discharge. In general, mitigation strategies may be used individually, or in combination, in various contemplated embodiments. Accordingly, mitigation strategies are described individually, but combined use will also be understood and appreciated based on the description herein.
A first such strategy includes use of structures to “blunt” the electric field in a spatially selective way near such susceptible surfaces. In general, the physical design of a corona discharge type emitter electrode seeks to focus electric field strength so as to establish and maintain a region of corona discharge closely proximate to the emitter electrode surface. However, by physically augmenting or scaling some of the electrode (or other physically or capacitively-coupled) surfaces that are coupled (or charge up) to emitter supply voltage, it is possible to locally increase the effective cross-section or radius of the emitter electrode at locations adjacent to the aforementioned susceptible surfaces. In this way, the strength of the electric field closely proximate to such susceptible surfaces can be reduced in an engineered and spatially selective way to below that necessary for corona onset. In such designs, ion generation and flux in the region closely proximate susceptible surfaces can be reduced and it has been discovered that, as a result, incidence of sparking discharge along otherwise susceptible sidewall surfaces may be effectively managed. Note that in some embodiments, the corona may be locally suppressed using field blunting structures while, in others, corona discharge may simply be reduced.
As explained elsewhere herein and/or as will be appreciated by persons of ordinary skill in the art having benefit of the present disclosure, corona discharge is, in general, sustainable in a region adjacent an emitter electrode and in which field strength is of sufficient intensity to ionize air molecules. For example, in embodiments such as described and illustrated herein (given a typical voltage of about 6 KV, emitter wire diameters of about 20 μm and emitter-to-collector electrode distances of about 2 mm), a corona may be sustained in conditions (e.g., temperature, pressure, humidity and constituents) typical for consumer electronics out to a few tens of microns from collector facing surfaces of the emitter wire. By distributing high voltage potential over an effectively larger emitter cross-section, field strength can be reduced in a spatially selective way near surfaces susceptible to sparking discharge proximate to locally suppress (or at least reduce) corona discharge. Thus, to locally provide an effectively-larger emitter cross-section, a sidewall-positioned field blunting structure 42 and a carriage-positioned, field blunting structure 42A are provided in the form of conductive metal tabs.
In the embodiment(s) illustrated in
In embodiments without a carriage, instances of field blunting structure 42 may be provided at opposing sidewalls. In embodiments in which a movable carriage (e.g., carriage 32) may be parked at one end (or the other) to, in effect, define a lateral sidewall of the EHD channel, a carriage-positioned, field blunting structure 42A may be provided. In embodiments in which it is desirable to energize electrodes (e.g., emitter electrode 91 and collector electrodes 92) when a movable carriage (e.g., carriage 32) is positioned, or is travelling, in the channel away from sidewalls (e.g., laterally mid-channel in the EHD air mover), instances (e.g., a pair) of carriage-positioned, field blunting structures 42A may be provided on opposing sides of the movable carriage and instances (e.g., a pair) of sidewall-positioned field blunting structures 42 may be provided at, or adjacent to, sidewalls of the EHD channel.
Although conductive metal tabs are illustrated (relative to
In addition, an optional and, in the illustrated embodiment, smaller one (42B) of the field blunting structures is provided on the opposing sidewall of carriage 32. Although field blunting structure 42B is not readily discernible in the views of
Note that in the embodiments of carriage 32 illustrated in
Referring to
In general, the physical design and relative positioning of emitter and collector electrodes seek to maximize a portion of the flow channel in which an electric field can accelerate charged airflow constituents (e.g., ions and/or charged particulate resulting from corona discharge). However, by creating or accentuating an emitter-to-collector gap near susceptible sidewall surfaces, it is possible to reduce the field conditions that can contribute to sparking discharge. Specifically, by locally increasing an emitter-to-collector gap (e.g., by providing collector taper 44 at or near sidewall surfaces and thereby locally increasing the distance between emitter 91 and collector 92 surfaces), both magnitude and shaping of the electric field can be defined in an engineered spatially selective way to guide ions and charged particulate away from sidewall surfaces and to reduce field strength at the sidewalls. In this way, both the propensity of susceptible sidewall surfaces to accumulate contaminants and charge and the susceptibility of such surfaces (even with some charge or contaminants accumulated thereon) to sparking discharge can be reduced.
Furthermore, and as described elsewhere herein, in those embodiments in which in situ electrode cleaning and conditioning is provided for either or both of an emitter electrode and a collector electrode (e.g., for emitter electrode 91 and collector electrodes 92 using carriage 32), it will be understood by persons of ordinary skill in the art having access to the present disclosure that the ability frictionally engage and effectively remove debris from electrode surfaces all the way up to an impingement on sidewall surfaces may be limited. See
By providing collector tapers 44 such as illustrated in
Likewise, although a particular taper in leading surfaces of an upper/lower collector electrode configuration is illustrated, persons of ordinary skill in the art will appreciate a range of variations on shaping and presentation of leading surfaces to provide similar effect. In some embodiments, recesses may be provided in sidewalls of the flow channel and/or sidewall-positioned field blunting structure 42 and carriage-positioned, field blunting structure 42A may be just slightly offset relative to one another (e.g., in a vertical dimension) to allow more complete side-to-side travel of carriage 32. Note that in the embodiments illustrated in
Still yet a third strategy may be employed in some embodiments to complement either or both of the field blunting and collector shaping strategies described above to mitigate susceptibilities to contaminant and charge accumulation, which may (as previously described) otherwise lead to undesirable sparking discharge. Specifically, and again referring back again to
To mitigate device inefficiencies that could otherwise result from a low-pressure-well-induced vortex and to limit contaminant delivering vortex-type flows at sidewalls, a fluid flow baffle is provided downstream of leading surfaces of the collector electrode. By interrupting a backflow path into the portion of the EHD channel adjacent to the sidewalls (or effective sidewall(s)), fluid flow baffles such as provided by surfaces 46 effectively prevent formation of a vortex involving peripheral lateral portions of the flow channel in which spatially selective corona suppression (or reduction) and/or field reduction tends to locally quench the motive EHD forces that are otherwise active in the major interior portion of the EHD channel.
Note that, in some embodiments, fluid flow baffles may be formed integrally with the trailing edge of collector electrode 92, such as with the baffle 46 instance illustrated in cutaway detail in
Notwithstanding the aforementioned complexities, embodiments are envisioned in which a fluid flow baffle (or an additional fluid flow baffle) is provided closer to (or indeed with) the emitter-to-collector gap. In some cases, a flexible baffle formed of dielectric material in of close to the emitter-to-collector gap may be provided and still accommodate electrostatic design goals and/or electrode surface cleaning/conditioning requirements. In some cases, a fluid flow baffle (or an additional fluid flow baffle) may be provided upstream of the emitter electrode.
More generally, desirable sizing and placement of fluid flow baffles is a function of localized corona current reductions or suppression (e.g., at sidewalls or adjacent a movable carriage) in a particular EHD device configuration. Based on the description herein, persons of ordinary skill in the art will appreciate a range of variations on shaping and placement of baffles to provide similar effect.
In addition to flow baffles at channel sidewalls (or effective sidewalls), it will be appreciated by persons of ordinary skill in the art having benefit of the present disclosure that similar issues present if/when the EHD air mover supports operational modes in which emitter and collector electrodes 91, 92 are energized with high, though possibly reduced, voltage sufficient to establish a corona discharge even during carriage traversal across the flow channel. In such embodiments or operational modes, the pair of field blunting structures 42A, 42B, provide corona suppression or reduction adjacent to both sidewall surfaces of carriage 32 and tend to locally quench the motive EHD forces that are active elsewhere in the EHD channel. As a result, it has been found to be advantageous to provide a back flow impeding baffle that travels with carriage 32. For example, in embodiments illustrated in
Referring back to
Note that, for ease of viewing, an upper surface or lid of frame 48 assembly is omitted in the illustrated views. Also omitted from the illustrated views is encapsulation of ballast PCB 47 within a suitable potting compound such as an ozone resistant polyurethane. As will be appreciated, after emitter wire 91 and the illustrated high voltage supply are directly connected to ballast PCB 47 at suitable attachment points thereon, otherwise exposed high voltage surfaces may be encapsulated within non-conductive “potting” material. In some embodiments, for example, the substantial entirety of the volume of cavity 49 surrounding ballast PCB 47 may be filled with potting compound so as to isolate conductors that will, during operation of the illustrated EHD fluid mover, be energized to voltages of at least several KV.
Because ballast PCB 647 is directly connected to a terminal end of emitter electrode 91, time constants are minimized and the response (i.e., voltage drop across ballast PCB 647) rapidly reduces emitter-to-collector voltage below that necessary to sustain an incipient spark. Typically, the corresponding rapid reduction in strength of the electric field proximate the emitter electrode 91 is to levels below that necessary to sustain corona discharge and generation of ions. As a result, an incipient sparking discharge is typically quenched or at least reduced in magnitude and duration. In the illustrated ballast PCB 647 circuit, an optional transient voltage suppression device 698 protects depletion mode FET 696 and an optional hysteresis or clamping circuit 697 facilitates rapid transition of depletion mode FET 696 from a maximally conductive state (ON) to a resistive state (OFF) while imposing a recovery time constraint on return to the maximally conductive state (ON) state.
Any of a number of commonly available electrical components may be used in the illustrated ballast PCB 647 circuit without substantially departing from the operational characteristics described. For purposes of illustration, and without limitation, several variations on the foregoing circuit realization are illustrated in
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. For example, while particular field blunting structures, collector tapers and baffle placements have been illustrated and described, persons of ordinary skill in the art having benefit of the present disclosure will appreciate that other suitable implementations are also contemplated. In some cases, field blunting structures or collector tapers or baffles illustrated and described herein may be omitted while still preserving some of the structures and/or advantages described herein.
Although operative embodiments have been illustrated and/or described herein with respect to a particular illustrative power supply voltage configuration in which emitter electrodes are coupled to high positive voltage, field shaping dielectric surfaces accumulate positive charge, and collector electrodes are coupled to ground, it will be appreciated by persons of ordinary skill in the art having access to the present disclosure that other configurations are also possible. Grounded emitter embodiments are contemplated, as are embodiments in which voltages coupled to emitter and collector electrodes straddle a ground potential or have different polarity.
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.
