US20140216696A1

US20140216696A1 - Cooling device and a cooling assembly comprising the cooling device

Info

Publication number: US20140216696A1
Application number: US13/757,006
Authority: US
Inventors: Brian Donnelly; Nicholas Jeffers; Jason Stafford
Original assignee: Alcatel Lucent SAS
Current assignee: Alcatel Lucent SAS
Priority date: 2013-02-01
Filing date: 2013-02-01
Publication date: 2014-08-07
Also published as: TW201441575A; WO2014118623A2; WO2014118623A3

Abstract

A cooling device comprises a heat sink having fins and a piezofan having a piezoelectric element attached to a planar body. The planar body is capable of oscillating at a movable end in response to applying alternating electric current to the piezoelectric element, thereby generating an air flow toward an output of the cooling device. A plurality of fins is located between the movable end of the planar body and the output end of the cooling device. Each fin has a planar form with a main surface being substantially aligned with the air flow. A cooling assembly comprising such cooling device is also disclosed.

Description

TECHNICAL FIELD

The present invention is directed, in general, to cooling techniques.

BACKGROUND

This section introduces aspects that may be helpful in facilitating a better understanding of the disclosure. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
Piezoelectric devices are known. Typically a piezoelectric device comprises a body of a solid material having the property of accumulating electrical charge when a mechanical stress such as a pressure is applied thereupon. Conversely, such piezoelectric body exhibits a mechanical movement in response to an electric current applied thereto. Some examples of materials exhibiting piezoelectric property are certain crystals or ceramics. One implementation of a piezoelectric device is a piezofan. A piezofan is typically made of a piezoelectric element which is physically attached, e.g. bonded, to an end of a planar, typically thin, body (sometimes referred to as blade or cantilever) the other end of the body being free and movable. When an alternating electric current is applied to the piezoelectric element, the latter exhibits an oscillating movement, causing the opposite end of the planar body to move. If the frequency of the alternating current is equal to the resonant frequency of the planar body, the latter produces an oscillating movement at the free end thereof. The oscillation of the free end of the planar body produces an air flow in a similar manner as a conventional hand fan.

SUMMARY

Some embodiments of the disclosure feature a cooling device comprising a heat sink and a piezofan; the heat sink comprising a plurality of fins of a thermally conductive material and the piezofan comprising a piezoelectric element attached to a planar body, the planar body being configured to oscillate at a movable end in response to applying alternating electric current to said piezoelectric element and generating an air flow from an input end of the device to an output end of the device, wherein a plurality of fins are located between the movable end of the planar body and the output end of the cooling device; each fin having a planar form with a main surface being substantially aligned with said air flow.
According to some specific embodiments, the piezofan and the plurality of fins are located between side walls.
According to some specific embodiments, a side wall comprises a protuberant portion having a surface configured for directing said air flow toward said plurality of fins.
According to some specific embodiments, the piezoelectric element and the planar body are mounted on a support element attached to the heat sink with a damping material positioned between the support element and heat sink configured for isolating or reducing a vibration caused by the operation of piezofan.
According to some specific embodiments, the support element is a rigid rail.
Some embodiments of the disclosure feature a cooling assembly comprising a heat sink and a piezofan; the heat sink comprising a plurality of fins of a thermally conductive material and the piezofan comprising a piezoelectric element attached to a planar body, the planar body being configured to oscillate at a movable end in response to applying alternating electric current to said piezoelectric element and generating an air flow from an input end of the device to an output end of the device, wherein a plurality of fins are located between the movable end of the planar body and the output end of the cooling device; each fin having a planar form with a main surface being substantially aligned with said air flow.
According to some specific embodiments a first cooling assembly with a first piezofan is assembled with a second cooling assembly with a second piezofan, wherein the first piezofan is configured to oscillate out of phase with respect to the second piezofan.

BRIEF DESCRIPTION

For a more complete understanding of the present disclosure, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 is an exemplary schematic representation of a front view of a cooling device according to an embodiment;

FIGS. 2 a and 2 b are exemplary schematic representations of a cooling device according to an embodiment in front view and perspective view respectively; and

FIG. 3 is an exemplary schematic representation of a cooling assembly comprising a plurality of cooling devices according to some embodiments.

DETAILED DESCRIPTION

A trend in producing electronic devices is toward reducing their size while enabling such devices of processing increasingly larger volumes of data. Processing larger volumes of data typically involves higher levels of heat generation which in turn would require stronger and more efficient cooling mechanisms.
One typical option for dissipating heat loads within electronic components is the use of passive heat sinks that combine natural convection and radiation heat transfer modes. As component heat loads increase, the size and therefore weight of these heat sinks would typically need to increase to keep component temperatures below their threshold values (i.e. highest value of the temperature at which a particular device operates normally). However, increasing the size of the heat sinks would cause an increase in the overall size of the device which is typically undesirable for modern devices. Therefore, passive cooling systems involve physical limits as to their suitability for use in modern electronic devices.
Active cooling systems, e.g. the use of rotating fans, constitute another known solution which, compared to passive cooling systems, is typically capable of removing larger quantities of heat from an electronic component. However, such active cooling systems typically suffer from relatively short lifetimes (typically about 5-7 yrs) and therefore poor reliability.
As mentioned above, future electronic products may require higher performance and functionality coupled with lower product volume. Therefore, there is a need for active cooling without the typical inherent unreliability of fans lifetime.
Embodiments of the disclosure feature the use of a piezofan for providing active cooling, together with a heat sink for providing passive cooling.
Some solutions directed to the use of a piezofan together with a passive heat sink are known.
However, in certain applications it may be desired to ensure a smooth and undisturbed flow of the air from the region where the blade is oscillating toward the output of the device. One example of such applications is where various cooling devices are to be assembled adjacent each other, for example in the form of a stack. Indeed in such cases, it is preferred that the air flow is aligned toward the output end of the device substantially undisturbed and as much as possible free of perturbation.
Herein, the term align as it is referred to air flow is to be understood to refer to a condition in which air flows in parallel layers and assists the natural buoyancy driven flow within the heat sink when the device is in use, with no or negligible flow disruption between the layers of the flow and with absence of vortices in these layers.
Referring now to FIG. 1, there is provided a cooling device 1 comprising a piezofan generally represented by reference numeral 11 and a convection heat sink generally represented by reference numeral 12. The piezofan 11 comprises a base portion 111 and a planar portion 112, herein also referred to as a cantilever or a blade. The base portion may be a piezoelectric element, i.e. made of a piezoelectric material such as for example ceramic. The planar portion 112 is attached at a first end to the base portion 111 and has a second end opposite to the first end which is free and movable. The mounting point for the planar portion 112 may also be on an external rail or solar shield. The heat sink 12 comprises a plurality of fins 121 disposed at convenient locations within the cooling device 1. The cooling device 1 may further comprise side walls 122. The side walls may also have geometry to assist the movement of air within the cooling device as will be described further below. The device further comprises an input end 123 from which air used for cooling may flow inside the device 1 and an output end 124 from which such air exits the device after being fanned by the operation of the piezofan 11.
In operation, when it is desired to cool a heat load of an electronic equipment, the piezofan may be activated. The heat loads of individual electronic components may be spread across the body of the heat sink base material in similar fashion as done in the case of known equipment. As described further above, by applying an alternating current (not shown) to the base portion 111, the latter which is made of piezoelectric material exhibits an oscillating displacement which is then transferred to the cantilever 112. If the frequency of the alternating current is the same as the resonant frequency of the cantilever, it may cause the latter to oscillate at such resonant frequency. Such oscillation is maximum in amplitude at the free and movable end of the cantilever and may cause air flow in the surrounding air forcing the latter to flow toward the output end 124 of the device 1. The oscillation of the movable end of the cantilever 112 is shown in FIG. 1 by double-headed arrow OS.
The air flow is forced to move (pushed) away from the region where the movable end of the cantilever 112 is oscillating as a result of the oscillating action of said movable end. This effect is shown in FIG. 1 by means of arrows A1, A2 and A3, where arrows A1 illustrate the direction of the flow of the air into the cooling device 1 at the input end 123 thereof, arrows A2 show the direction of flow of the air after entering in the device 1 and before reaching the oscillating end of the cantilever 112 and arrows A3 show the direction of flow of the air between the oscillating end of the cantilever 112 and the output end 124 of the cooling device 1.
The combined effect of the oscillation of the movable end of the cantilever and the flow of the air out of the cooling device may further generate a vacuum effect in the region of the air flow upstream the oscillating end of the cantilever (i.e. prior to reaching the oscillating end) which in turn contributes to drawing more outside air inside the cooling device 1 in the direction of arrows A1 and A2.
As it is clearly shown in FIG. 1 the plurality of fins that are located between the movable end of the planar body 112 and the output end 124 of the cooling device serve for aligning the air flow A3 due to their planar form and in particular to the shape of the main surface 112 a of each fin which assists a smooth and undisturbed exit of the air out of the device 1. This region is identified in FIG. 1 by reference numeral R1. In the example of FIG. 1, the device is shown to be in vertical position with the gravity vector illustrated by Arrow G. Therefore, it may be observed that the direction of flow A3 of the air is aligned with buoyancy force which in operation opposes the gravity vector G in FIG. 1.
In the context of the present disclosure the main surface of a fin, having planar form, is to be understood to refer to the surface with the highest area in the three dimensional body of the fin.
In order to ensure an optimum undisturbed air flow in region R1, it is preferred that the shapes of fins, or any other intervening structure present in said region, be designed such that the generation of vortices in the upstream air flow is avoided and flow is aligned with driving buoyancy forces so that heat transfer is enhanced, and quality of airflow entering any further devices installed upstream a particular device (as shown in the example of FIG. 3) is improved.
In FIG. 1, the surfaces 121 a of the fins 121 are shown to be planar and parallel to each other. However, this is only exemplary and other configurations of the surfaces may also be used as long as they are within the scope of the claimed invention such surfaces facilitate a natural convection of the air flow and increased heat transfer surface are from the oscillating end of the cantilever 112 to the output end of the cooling device 1 substantially without vortices travelling out of the output end of the device (e.g. into upstream devices). For example, such surfaces may have a curved shape or the fins may be arranged in a non-parallel manner.
Advantageously, the conduction of the air flow from the input end of the cooling device 1 to the oscillating end of the cantilever 112 (arrows A1 and A2) is also free of vortices at least in a first portion of such air flow. This region is identified in FIG. 1 by reference numeral R2.
In this manner, air is actively made to flow inside the cooling device 1, through the fins 121 thereby cooling the fins and then made to exit the cooling device 1 from the output end 124.
FIG. 2 a shows a front view of an exemplary cooling device according to a further embodiment of the disclosure and FIG. 2 b shows a perspective view of the device of FIG. 2 a. In FIGS. 2 a and 2 b, unless otherwise provided, like elements have been given like reference numeral as those of FIG. 1. The embodiment shown in FIGS. 2 a and 2 b is similar to that of FIG. 1 with the exception that the cooling device of FIGS. 2 a and 2 b further comprises one or more protuberant portions 125, 126 on corresponding side walls 122 a and 122 b.
The use of such protuberant portions may be advantageous as they may contribute, due to their specific geometry and location, to an improved flow of the air in region R1, as long as such geometry does not hinder the buoyancy driven air flow path, at least between the movable end of the cantilever 112 and the output end 124 of the cooling device 1.
For example in FIG. 2 a, incoming air may be drawn inside the cooling device as shown by arrows A1. However when such air reaches the vicinity of the protuberant portions 125 and 126, the specific shape of these protuberant portions may deflect the direction of the flow of the air slightly toward the center of the air flow (close to the middle of the cooling device where the cantilever 112 is located in FIG. 2 a). Such deflection of the flow of the air is shown in FIG. 2 a by arrows A2 which in this case, contrary to the arrows A2 of FIG. 1 which are substantially straight, present certain angle toward the center of the flow. The overall effect of the deflection may therefore be considered as causing air flow to be more concentrated (centered) toward the center of the oscillation of the movable end of the cantilever 112. As the cantilever 112 oscillates, it pushes at least a part of the concentrated air to the sides which in turn is reflected from the corresponding surfaces 125 a and 126 a toward the fins 121 upstream the protuberant portions 125 and 126, thereby contributing to a stronger or faster air flow toward the output end 124 of the cooling device 1.
In the example shown in FIGS. 2 a and 2 b the protuberant portions 125 and 126 present curved surfaces projecting inside the cooling device from the side walls 122 a and 122 b respectively. However this is only exemplary and the protuberant portions 125 and 126 may have any other convenient shape as long as such shape contributes to an improved flow of the air in region R1 and is not invasive to buoyancy flows when the device is in operation. For example, the protuberant portions may have plane sloped surfaces on each side instead if curved surfaces as shown in FIGS. 2 a and 2 b; likewise they may be located closer or farther away from the output end 124 according to the specific design required.
In some embodiments, the protuberant portions may be made of inserts that may be added to the base structure of the cooling device when needed or removed therefrom when such need no longer exists. In some embodiments such inserts may be made of plastic or other suitable material. This option has the advantage of providing cost optimization as the heat sink may first extruded by standard methods of mass manufacturing and then according to a specific requirement and case by case inserts may be used as air deflectors.
The cooling device as proposed herein further provides the capability of being assembled as is described further below.
FIG. 3 is an exemplary schematic representation of a cooling assembly 10 comprising a plurality of cooling devices according to some embodiments. In the example shown in FIG. 3 twelve individual cooling devices D1-D12 are shown assembled in two rows L1, L2 and six columns C1-C6. However, the disclosure is not so limited and any convenient number of cooling devices may be assembled to form the cooling assembly as proposed herein.
As shown in FIG. 3, an output end of a cooling device located in the lower row L2 is coupled to an input end of an adjacent cooling device located in the upper row L1. For example with reference to the cooling device D6 in the upper row L1 and the cooling device D12 in the lower row L2, it may be observed that these two cooling devices are assembled vertically with respect to each other and that the output end OE-D12 of the cooling device D12 is coupled to the input end IE-D6 of the cooling device D6.
Herein the term coupling is to be understood to refer to any suitable joining or connecting of the output end of a first cooling device to the input end of a second cooling device, irrespective of whether or not such joining or connecting involves a direct physical contact between the two devices, as long as such joining or connecting allows for a flow of air from the output end of the first cooling device to the input end of the second cooling device.
In operation, incoming air may enter the corresponding input ends of the cooling devices D7-D12 located in the lower row L2, as shown by arrows A1. The activation of the piezofans provided inside each cooling device may generate an air flow toward the respective output ends of each cooling device in similar fashion as was already described with reference to FIGS. 1, 2 a and 2 b. The gravity vector is shown by arrow G. As the air flow exits the output ends of each of the cooling devices D7-D12, it is then entered in the corresponding cooling devices D1-D6 through the respective input ends thereof. For example air flow exiting the output end OE-D12 of cooling device D12 may enter the input end IE-D6 of cooling device D6. As mentioned with reference to FIGS. 1 2 a and 2 b, the air flow exiting the lower cooling device D12 and entering the upper cooling device D6 has been aligned with natural convention which is advantageous as it enables an easy and smooth flow of the air from the lower cooling device to the upper cooling device. Furthermore, the absence of additional obstructive structures along the path of the air flow at the interface between the output end of the lower cooling device and the input end of the upper cooling device ensures a substantially undisturbed mixed convection which is advantageous in particular where a high number of cooling devices are assembled.
The cooling device as disclosed here in may comprise additional elements to isolate or substantially reduce vibration caused by the operation of the piezofan. For example such isolation element may be made of a rigid rail whereupon the piezofan may be mounted. Such rigid rail may be fixed to the heat sink or the solar shield arrangement with additional damping material positioned between the rail and heat sink or solar shield.
Another solution to reduced vibration and structure borne noise when an assembly of devices is used may be achieved by operating adjacent oscillating blades out of phase, for example at 90 degrees with respect to each other, thereby reducing or cancelling such unwanted phenomena.
Furthermore, the above two solutions may be combined.
The proposed solution according to the various embodiments disclosed herein enables a combined action of an active cooling mechanism with a passive cooling mechanism while the use of conventional fans is avoided thereby improving reliability, size, lifetime and power consumption.
Another advantage of the cooling device as proposed in the various embodiments provided herein is that the use of the active cooling mechanism, i.e. activating the piezofan, may be made in a selective manner. In other words, the piezofan 11 may be turned off when the cooling device can satisfactorily operate to cool an electronic component using solely the heat sink 12 by natural convection(passive cooling); and be turned on only when an active cooling is required in combination with the natural convection heat sink. This possibility allows for saving in energy consumption and increases the lifetime of the device which would otherwise occur earlier as the device would wear out earlier due to continuous usage.
The introduction of the piezofans in a cooling device may not degrade heat transfer due to natural convection, but rather is buoyancy assisting which improves heat transfer considerably by increasing the total air flow rate, thereby optimizing the thermal and cost aspects of overall cooling device.
Those skilled in the art to which this application relates will appreciate that other and further additions, deletions, substitutions and modifications may be made to the described embodiments.
It is to be noted that the list of structures as recited in the claims is not exhaustive and that one skilled in the art understands that equivalent structures can be substituted for the recited structure without departing from the scope of the disclosure.
Furthermore, the various embodiments of the present disclosure may be combined as long as such combination is compatible and/or complimentary.

Claims

What is claimed is:

1. A cooling device comprising a heat sink and a piezofan; the heat sink comprising a plurality of fins of a thermally conductive material and the piezofan comprising a piezoelectric element attached to a planar body, the planar body being configured to oscillate at a movable end in response to applying alternating electric current to said piezoelectric element and generating an air flow from an input end of the device to an output end of the device, wherein a plurality of fins are located between the movable end of the planar body and the output end of the cooling device; each fin having a planar form with a main surface being substantially aligned with said air flow.

2. The cooling device of claim 1, wherein the piezofan and the plurality of fins are located between side walls.

3. The cooling device of claim 2, wherein a side wall comprises a protuberant portion having a surface configured for directing said air flow toward said plurality of fins.

4. The cooling device of claim 1, wherein the piezoelectric element and the planar body are mounted on a support element attached to the heat sink with a damping material positioned between the support element and heat sink configured for isolating or reducing a vibration caused by the operation of piezofan.

5. The cooling device of claim 4, wherein the support element is a rigid rail.

6. A cooling assembly comprising a heat sink and a piezofan; the heat sink comprising a plurality of fins of a thermally conductive material and the piezofan comprising a piezoelectric element attached to a planar body, the planar body being configured to oscillate at a movable end in response to applying alternating electric current to said piezoelectric element and generating an air flow from an input end of the device to an output end of the device, wherein a plurality of fins are located between the movable end of the planar body and the output end of the cooling device; each fin having a planar form with a main surface being substantially aligned with said air flow.

7. The cooling assembly of claim 6, wherein a first cooling assembly with a first piezofan is assembled with a second cooling assembly with a second piezofan, wherein the first piezofan is configured to oscillate out of phase with respect to the second piezofan.