Patent Application
20160298617
The invention disclosed herein relates to devices for transporting fluids. More precisely, it relates to an electro-hydrodynamic device for controlling a fluid flow, as well as methods for manufacturing and controlling such device.
The performance of electronic systems is to a large extent limited by the available cooling techniques for keeping the electronics within an appropriate temperature range. Smaller electronic devices having improved performance are associated with increased heat dissipation over a relatively small area. In other words, there is a growing need for space and energy efficient cooling of electronic devices.
As an example, satellites, such as telecommunication satellites, are approaching the technology limits of existing on-board thermal management systems. The power dissipation of these satellites increases to meet the growing requirements for broadcasting, broadband multimedia and mobile communications services. Micro, nano, or ‘cube’ satellites, which require low-mass heat removal from electronic components (satellite on a chip) are likely to need more compact thermal management systems for maintaining a high performance.
Cooling is also a crucial factor in the design of other electronic devices having heat-generating components, such as power electronics, powerful processors, radio frequency amplifiers, lasers etc.
Bologa et al., “Multistage electrohydrodynamical pump”, proceedings of the 2000 Conference on Electrical Insulation and Dielectric Phenomena, discloses a multistage electrohydrodynamical (EHD) pump with latticed electrodes of parallel wires embedded in a metal ring. According to Bologa et al., a formation of ions in a working medium, comprising a dielectric liquid, occurs at the emitter electrode, whereas the ions recombine at the collector. The ions move under the influence of Coulomb forces, which ions thereby entrain a flow of the working medium.
Even though such EHD pump may be employed in various cooling applications, there is still a need for an improved device and method for controlling the flow of a fluid and for providing improved pumping efficiency. There is also a need for an improved manufacturing method of such devices.
It is an object of the present invention to control the flow of a fluid in such manner that the efficiency of the transportation of the fluid is improved. It is a particular object to improve the capacity of an EHD pump for dielectric fluids.
Accordingly, the invention provides a device and a control method with the features of the independent claims. The dependent claims define advantageous embodiments.
In a first aspect, the device is characterised in a first electrode and a second electrode, wherein the second electrode is offset from the first electrode in a downstream direction of the flow and wherein the electrodes are connectable to a voltage source. The first electrode comprises bridges and joints forming a grid structure, which is arranged to allow the fluid to flow there through. According to this aspect, at least a portion of at least one of the bridges has a maximum height in a direction parallel to the direction of the flow, and a maximum gauge in a direction orthogonal to the direction of the flow, wherein the maximum height is larger than the maximum gauge, preferably at least twice the maximum gauge. The maximum height may also be three, four, five, or six times the maximum gauge, or larger.
In a second aspect, an assembly comprising a plurality of stacked devices according to the first aspect is provided. The assembly may e.g. be a pump assembly or fan assembly.
In a third aspect, the method for controlling the flow of a fluid is characterised in that a first electrode is provided, which comprises bridges and joints forming a grid structure arranged to allow the fluid to flow through the first electrode. At least a portion of at least one of the bridges has a maximum height in a direction parallel to the direction of the flow and a maximum gauge in a direction orthogonal to the direction of the flow, and wherein said maximum height is larger than the maximum gauge, preferably at least twice the maximum gauge. The maximum height may also be three, four, five, or six times the maximum gauge, or larger. Further, a second electrode is arranged offset from the first electrode in the direction of the flow, and an electric potential difference is applied to the first electrode and the second electrode so as to cause the fluid to flow through the first electrode.
In a fourth aspect, a method for controlling the flow of a fluid is provided. The method comprises providing a device according to the first aspect, providing a fluid contacting the first electrode of the device, and applying an electric potential difference between the first electrode and the second electrode.
By a grid it is understood any structure comprising bridges that are joined to each other so as to e.g. a grating, net, or honeycomb structure, etc.
The bridges and the joints define open areas of the grid which admit a fluid flow. In one example, the grid may comprise flanges forming a heat sink. The flanges, which e.g. may be formed of sheet metal, may be arranged in a lamellar structure wherein their edge portions are joined to a peripheral frame of the grid. Further, it is understood that the grid may comprise several bridges having the above specified ratio between their height and gauge. As an example, the whole grid other than its peripheral portions may e.g. be formed of such bridges. In another example, most of or all of bridges of the grid may fulfill the maximum height/gauge relation.
Several advantages are associated with the invention. Firstly, by forming a grid of bridges that have a relatively large height in relation to their gauge, the grid may be relatively rigid in terms of its ability to carry loads in the height direction of the bridges, or the direction of the flow. Thereby, a relatively rigid electrode is enabled, which is less prone to bend or deform, especially in the direction of the flow, and hence the risk for e.g. short-circuiting of the device may be reduced. Further, the relatively rigid and stable grid may still have a relatively large open area which may provide a relatively low flow resistance being met by the fluid passing through the grid. Further, the relatively high and narrow bridges may reduce the amount of material required for forming a relatively stable and rigid grid, which may reduce both weight and cost of the device. By using a relatively rigid grid, the need for additional support structures may be reduced and a relatively well defined and constant spacing between the first and second electrodes may be achieved. The spacing may e.g. be within the range of 10-2000 μm, and more preferably in the range of 50-1000 μm.
With their relatively large height, the bridges also provide a relatively large contact surface between the grid structure and the passing fluid, which may facilitate any interactions between the electrode and the fluid, such as e.g. diffusion of material and/or injection of ions or electrons.
The distance, or spacing, between the first and the second electrode may be varied so as to control the strength of the electric field being induced between the electrodes. Experiments have shown that a smaller gap, and thus a stronger induced electric field, may enable increased pump efficiency, or flow rate, as compared to devices having a larger gap and being supplied with the same electric power.
According to an embodiment, at least one of the bridges of the first electrode comprises a tapered portion forming an edge or tip that is directed towards the second electrode. The present embodiment is based on the insight that by providing the first electrode with beaked or pointed portions, the injection of electrons per unit area of the first electrode into the fluid may be improved. Increasing the emitting of electrons may enhance the electrohydrodynamic effect, increase the flow through the device, and thus improve its pumping efficiency. Further, increasing the electron emitting efficiency from the first electrode may advantageously allow for a reduction of the flow resistance through the grid, since the open area of the grid, i.e. the grid area through which the fluid may pass, may be reduced without necessarily reducing the injected current.
According to an embodiment, at least one of the bridges comprises a tapered portion forming an edge or tip directed away from the second electrode. In other words, the tapered portion is directed anti-parallel to the flow of the fluid, which advantageously may streamline the upstream portion of the grid so as to reduce the flow resistance and enhance the efficiency of the device.
According to an embodiment, the second electrode comprises bridges and joints forming a grid structure that allows the fluid to flow through the second electrode. At least one of the bridges comprises a portion having a maximum height in a direction parallel to the direction of the flow and a maximum gauge in a direction orthogonal to the direction of the flow, wherein the maximum height is larger than the maximum gauge, preferably at least twice the maximum gauge. The present embodiment is associated with similar advantages and effects as described with reference to the structure of the grid of the first electrode.
According to an embodiment, the second electrode comprises a structured surface portion facing the first electrode. The structured surface portion may comprise micro- and/or nanostructures which may increase the area of the surface portion. The microstructures and/or nanostructures may for example include the geometrical form of hills, ridges, paraboloids, pillars, or trenches. Increasing surface area of the second electrode is advantageous in that it may improve the ability of collecting, or absorbing, electrons and hence improve the efficiency of the electrode. Further, by increasing the surface area by means of micro- and /or nanostructures, a relatively higher active surface area can be achieved on a relatively small surface portion. This advantageously allows for a relatively larger active surface area and a relatively lower flow resistance. A relatively larger active area may also increase the lifetime of the second electrode, since it may then be less sensitive to contaminants passivating the surface.
According to an embodiment, the second electrode comprises a concave surface portion facing the first electrode. A concave surface portion is advantageous in that it may provide an increased surface area as compared to a flat surface portion, thereby enhancing the ability to collect electrons, e.g., electrons emitted by the first electrode. The concave surface may e.g. conform to an arc of a circle, or a surface of a sphere or of a cylinder, having its centre or symmetry axis at an edge or tip of the first electrode. Thereby, a homogenous electric field may be achieved between the first electrode and the second electrode.
According to an embodiment, at least one of the bridges and/or joints of the second electrode comprises a channel, or a plurality of channels, adapted to allow the fluid to flow through said channel. By arranging a passage through the material of the grid, the flow may be increased and/or the fluid resistance reduced.
According to an embodiment, the device comprises a support structure arranged to separate the second electrode from the first electrode in the direction of the flow. The support structure may e.g. be electrically non-conductive and have a well defined thickness so as to maintain a desired spacing between the first and second electrodes. The support structure may e.g. be formed as a grid or a spacer comprising e.g. ceramics or polymers, and the first and/or second electrode(s) may be connected to or arranged on the support structure by means of e.g. welding, gluing, soldering, brazing, glazing or sintering. The support structure may comprise an alignment structure adapted to align the first electrode with the second electrode, and/or to align several stacked device with each other. The alignment structure may e.g. comprise a protruding member and a receiving member, such as a depression or recess, wherein the protruding member is adapted to cooperate with a corresponding receiving member of another support structure, and vice versa. Thereby the assemblage and alignment of the electrodes and/or the device may be facilitated.
According to further embodiments, at least one of the first electrode, the second electrode and the support structure comprises a deformation structure arranged to compensate for, or absorb, e.g. thermally induced stresses, particularly in a plane orthogonal to the direction of the fluid flow. The deformation structure may e.g. be formed by a bridge being curved in the plane orthogonal to the direction of the flow. As the bridge is exposed to stresses or torsional torques in the plane orthogonal to its height (i.e. the flow direction), the bridge may due to its relatively large height and small gauge tend to deform in that plane rather than in the flow or height direction. This advantageously allows for a device being less sensitive to thermally induced stresses and thermal expansion. Thereby a device having relatively well defined dimensions and a relatively reliable shape may be achieved. Furthermore, the deformation structure may allow materials having different coefficients of thermal expansion (CTE) to be combined. As an example, the first and/or the second electrodes may be formed of a material having a first CTE whereas the support structure, to which the first and/or the second electrodes may be attached, may have another CTE. In such case, a deformation structure may be provided in the electrodes and/or the support structure so as to enable any internal thermal stresses that may be caused by the difference in CTE to be absorbed by the deformation structure being deformed in the plane orthogonal to the direction of the fluid flow. Thus, the deformation structure may enable a more reliable device having a prolonged life.
According to an embodiment, the first electrode and/or the second electrode and/or the support structure is formed of a material that is selectively deposited so as to form the desired structure. The material may e.g. comprise a stacked structure of one or several metals. The depositing method may e.g. comprise molding, plating, screen printing, glazing, sputtering, evaporation or sintering.
Alternatively, or additionally, the manufacturing may comprise removal of material, e.g. by selectively removing material from a substrate. Examples of suitable techniques may include cutting, milling, etching, and abrasive blasting.
The first and/or second electrodes may advantageously comprise a material that has a relatively good ability of emitting electrons and is chemically stable, or inert, in relation to the pumped fluid. Further, the material may have a relatively high temperature resistance. Examples of such materials may include e.g. Pt, Au, and stainless steel.
According to an embodiment, the applied electric potential difference between the first and the second electrodes may be varied as a function of time. Experiments have shown that by e.g. alternating the potential difference between a first, positive value and zero, and/or between a positive and a negative value, the fluid flow per unit area, and hence the pump efficiency, may be improved.
Examples of fluids, i.e. liquids and gases, that can be pumped by means of embodiments of the inventions includes e.g. dielectrics such as acetone, alcohols, helium, nitrogen, and fluorocarbon-based fluids such as e.g. Fluorinert™ or Novec™.
In the present specification, the term “pump” or “pump assembly” may include any device capable of creating a movement, current or flow of a fluid within and/or through the device. The term may also be understood as a fan or fan assembly, in particular in case the fluid comprises a gaseous material.
Further objectives of, features of and advantages with the present invention will become apparent when studying the following detailed disclosure, the drawings and the appended claims. Those skilled in the art realise that different features of the present invention, even if recited in different claims, can be combined into embodiments other than those described in the following.
The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non-limiting detailed description of embodiments of the present invention. Reference will be made to the appended drawings, on which:
All the figures are schematic, generally not to scale, and generally only show parts which are necessary in order to elucidate the invention, whereas other parts may be omitted or merely suggested.
The emitter 110 and the collector 120 may be arranged spaced apart from each other in the flow direction by a positive distance d. The spacing may e.g. be maintained by a support arrangement, or grid spacer 130 (not shown in
It will also be realised that the grid may have one of a broad variety of shapes, wherein the edges and the joints e.g. may form a grating, a net, a hole pattern, a honeycomb structure, or other structures or patterns suitable for admitting a flow through the emitter 110 and/or collector 120.
The cross section of the bridges 111 of the emitter 110 may comprise a downstream portion 113 having a tapered shape forming an edge or a point 114 facing the collector 120. The tapered shape may e.g. be manifested as an edge or narrow end 114 extending along the downstream portion 113 of the bridge 111, or one or several protrusions having a shape conforming to e.g. a tip, needle, pyramid, dome, etc. As the emitter 110 is subjected to an electric potential difference, there may be an electric field concentration at the edge 114 of the tapered portion 113 which may facilitate or promote emission of electrons.
Correspondingly, the portion of the bridges 121 of the collector 120 which face the emitter 110 may be provided with a dedicated shape or surface structure for enhancing collection of the emitted electrons. The bridges 121 and/or joints 122 of the collector 120 may e.g. be provided with a concave surface portion 123 increasing the surface area, and/or a structured surface comprising microscopic protrusions and/or recesses 124 increasing the active surface area. The structures 124 may e.g. be formed by molding, electroplating, surface treatment or by selectively adding and/or removing material by e.g. abrasive blasting, etching, milling, grinding, etc.
The devices 100 in
The electric contact portions 119 may e.g. comprise protrusions which are integrally formed with the emitter 110, and which may be adapted to engage with a corresponding structure of e.g. a support structure 130 and/or stacking structure 140 (not shown in
In
Even though the embodiments described with reference to
The stacking structure 140 may comprise alignment structures 142 for facilitating alignment of the stacked devices 100. The alignment structures 142 of the stacking structure may 140 e.g. comprise a protruding portion adapted to fit into a recess of a corresponding alignment structure of a below device of the stack. Correspondingly, the alignment structure 140 may comprise a recess adapted to receive a protruding portion of a alignment structure of an above device in the same stack. Thereby the alignment of a stack of a plurality of devices 100 may be facilitated.
The deformation structure 115, 125 may be provided in an emitter 110 and/or collector 120 that is attached to a support structure 130, wherein in the support structure 130 may have a coefficient of thermal expansion (CTE) that differs from the CTE(s) of the emitter 110 and/or collector 120. In case the emitter 110 and/or collector 120 is/are rigidly attached to the support structure 130, the risk for deformations, such as e.g. bending and flexures, and damages such as fractures, disconnected or loosening joints etc. may be reduced by the deformation structure 115, 125. Thereby, reliability and useful life of the device 100 may be increased.
The grid spacer 130 may e.g. be formed as a grid supporting the emitter 110 and the collector 120. As illustrated in
The spacing d of the emitter and collector may be determined by the height of the bridges of the grid spacer 130, which may hence determine the magnitude of the electric field induced between the emitter 110 and the collector 120. The distance d between the emitter 110 and the collector 120 may e.g. be within the range of 10 μm and 1000 μm.
Further, the grid spacer 130 may comprise an alignment structure for facilitating alignment of the emitter 110 and the collector 120, and/or alignment of the devices 100 of the stack.
The pump assembly 200 may also comprise a stacked structure with stage spacers 140 arranged to maintain a distance between the emitter 110 of a first device and the collector 120 of a second device. The stacking structure 140 may also comprise an alignment structure 142 (not shown in
The grid spacer 130 and/or the stacking structure 140 may e.g. comprise a ceramic material, such as Al2O3 or Macor™.
As shown in
Advantageously, several or all of the emitters 110 and/or collectors 120 of the stack can be connected in the same manufacturing step.
To further improve relaxation, a pulse-reverse current may be introduced between the pulses described with reference to
From a design point of view, it is an advantage to confine charged particles, such as e.g. ions, between a portion of a bridge 111 and/or joint 112 of the emitter 110 and a corresponding portion of a bridge 121 and/or joint 122 of the collector 120. Outside this volume, i.e. between the open portions of the respective grid, the charged ions may have a limited effect on the pumping action. The time duration of the positive pulses may be selected such that negatively charged ions, created at the emitter, may just reach the collector 120. Hence, if the time duration is sufficiently short, the spreading of unwanted ions into the liquid loop may be limited. This time length can be calculated from the ion mobility, where a range from 2×10−8 to 2×10−7 m2/Vs is known form the prior art. For a pump having an electrode spacing of 100 μm, this may correspond to pulse duration of around 1 ms. The zero or negative pulse may advantageously be sufficiently long to allow recombination of ions or charged particles.
As outlined above, the method for controlling the flow of a fluid as illustrated by
| Number | Date | Country | Kind |
|---|---|---|---|
| 1351440-1 | Dec 2013 | SE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/SE2014/051426 | 12/1/2014 | WO | 00 |
