US20110064594A1

US20110064594A1 - Cooling device

Info

Publication number: US20110064594A1
Application number: US12/722,650
Authority: US
Inventors: Hiroaki Wada; Gaku Kamitani
Original assignee: Murata Manufacturing Co Ltd
Current assignee: Murata Manufacturing Co Ltd
Priority date: 2007-09-14
Filing date: 2010-03-12
Publication date: 2011-03-17
Also published as: JPWO2009034956A1; WO2009034956A1; JP5083322B2; CN101803011A; TW200930899A; CN101803011B; US20120134858A1; CN102543915A

Abstract

A piezoelectric fan includes a blade that is joined to a piezoelectric oscillator that bends in response to an application of a voltage, and the blade of the piezoelectric fan is arranged to swing in a space between neighboring heat dissipating fins of a heat sink. The formation of a hole in the blade increases the amplitude of the blade and also improves the sweep effect of sweeping high-temperature air in the vicinity of the wall of the heat dissipating fin, and thus, improves the cooling capability of the piezoelectric fan.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a cooling device for ejecting heat from the inside of an electronic apparatus to the outside thereof, the cooling device using a piezoelectric fan.
2. Description of the Related Art
Recently, for a portable electronic apparatus, in particular, with the advancement of miniaturization and high-density mounting, devices for reducing heat inside the electronic apparatus are desired.
One example of a portable electronic apparatus in which the above issue is especially important is a portable personal computer. For portable personal computers, both miniaturization and increased central processing unit (CPU) speeds that improve performance of information processing are advancing. As a result, high-density mounting of parts deteriorates the ventilation inside an electronic apparatus, while heat generated by a CPU increases. This makes it more difficult to dissipate the heat to the outside of the electronic apparatus and thereby reduce or prevent a temperature increase inside the electronic apparatus.
A traditional radiator in which a movable element having an air surrounding structure is disposed between many heat dissipating fins that are aligned at desired intervals on a heat sink that is in contact with a heating portion of a heating element that sends cool air in between the heat dissipating fins by causing the movable elements to rotate or swing to eject warm air between the heat dissipating fins is disclosed in Japanese Unexamined Utility Model Registration Application Publication No. 02-127796.
A piezoelectric fan that includes an air generating oscillator including a piezoelectric oscillator and an outlet and an inlet disposed on the same surface is disclosed in Japanese Unexamined Patent Application Publication No. 2002-339900. The piezoelectric fan including a pair of partition walls extending from a hole of the main body of a casing to an inner portion is disposed so as to sandwich both sides of the air generating oscillator, holes between the partition walls and both sides of the main body of the casing are provided as intakes, and holes sandwiched by the both partition walls are provided as exhausts.
Here, the configuration of the piezoelectric fan illustrated in Japanese Unexamined Patent Application Publication No. 2002-339900 is described with reference to FIG. 1. In FIG. 1, a piezoelectric fan 1 is configured such that a fan main body 5 including a piezoelectric oscillator 3 and an air generating oscillator 4 is included in a fan casing 2 having a flat box shape and an intake 6 (6A, 6B) and an outtake 7 are provided on the same surface of the fan casing 2. The fan casing 2 includes a casing main body 8 and a plate-shaped cover element 9. The casing main body 8 includes a bottom 8 a, left and right sides 8 b and 8 c, and a back 8 d, and an open front. The cover element 9 is hermetically fixed to the upper surface of the casing main body 8.
However, the use of the radiator illustrated in Japanese Unexamined Utility Model Registration Application Publication No. 02-127796 in the portable electronic apparatus without being processed is inconvenient in terms of miniaturization. One approach to address this is to use the piezoelectric fan illustrated in Japanese Unexamined Patent Application Publication No. 2002-339900, in place of the movable elements illustrated in Japanese Unexamined Utility Model Registration Application Publication No. 02-127796.
When the piezoelectric fan is used, its capability to generate air depends on the amount of displacement of the piezoelectric oscillator in the air generating oscillator. The amount of displacement of the piezoelectric oscillator is less than the movement of the movable elements illustrated in Japanese Unexamined Utility Model Registration Application Publication No. 02-127796.
As a result, it is necessary to cool the inside of an electronic apparatus as efficiently as possible. Japanese Unexamined Patent Application Publication No. 2002-339900 discloses that it is desired that the distance between both partition walls is as close as possible to the width of the air generating plate, that is, that the gap between the air generating plate and each of the partition walls is as small as possible.
The movable elements for ejecting warm air between the heat dissipating fins in the radiator illustrated in Japanese Unexamined Utility Model Registration Application Publication No. 02-127796 are arranged to rotate or swing using a strong driving source, such as a motor, even if air resistance to the movable elements is present. Accordingly, the movement of the movable elements is not inhibited by the influence of the air resistance. In contrast, for the air generating oscillator used in the piezoelectric fan of Japanese Unexamined Patent Application Publication No. 2002-339900, if the distance between the heat dissipating fins corresponding to both partition walls and the width of the air generating plate are close to each other, air resistance caused by movement of the air generating oscillator would inhibit displacement.
FIG. 2 illustrates a relationship between air resistance and the amplitude of an end of an air generating plate (hereinafter referred to as “blade”) obtained from an experiment performed by the inventors of the present invention. The dimensions of a piezoelectric oscillator used in the experiment are 6 mm by 12 mm, and the dimensions of the blade are 6 mm by 18 mm by 40 μm. The shorter-side portions of piezoelectric oscillator and the blade are connected to each other.
The air resistance is assumed to be substantially proportional to air density and the air density is assumed to be proportional to pressure, such that this experiment is performed by examining the pressure and the amplitude of the blade end when the blade subjected to a predetermined reduced pressure environment is driven by the piezoelectric oscillator. As shown in FIG. 2, the amplitude of the blade is affected by the pressure, that is, the air resistance, and the amplitude decreases when the air resistance increases.
As described above, a problem exists in that, even if the distance between the heat dissipating fins corresponding to both partition walls and the width of the blade are as close as possible to each other, the air resistance caused by movement of the blade inhibits displacement, and thus, increasing the amplitude of the blade is difficult.

SUMMARY OF THE INVENTION

To overcome the problems described above, preferred embodiments of the present invention provide a cooling device having an improved cooling capability by increasing the amplitude of a blade, and thus improving the capability to move air and improving the heat dissipation effect produced by heat dissipating fins.
Warm air to be ejected is air that is warmed by heat generated by heat dissipating fins. Therefore, a temperature distribution of a space between the heat dissipating fins is not uniform, and a high-temperature portion is concentrated in the vicinity of the walls of the heat dissipating fins.
When a flow velocity distribution when air is blown between the heat dissipating fins is taken into account, the velocity of air flow is relatively fast in the central portion between the heat dissipating fins, whereas the velocity of air flow decreases towards the surface of the wall of each of the heat dissipating fins because viscous drag of air is caused by the wall of the heat dissipating fin.
That is, causing air to flow alone can eject relatively low-temperature air existing in the central portion between the heat dissipating fins, but cannot sufficiently eject high-temperature warm air existing in the vicinity of the walls of the heat dissipating fins.
The inventors of the present invention have determined, from various experiments and simulations, that moving a blade so as to sweep warm air in the vicinity of the walls of the heat dissipating fins and thereby moving the warm air towards the central portion between the heat dissipating fins to facilitate ejecting the warm air enables heat inside the electronic apparatus to be efficiently ejected to the outside without trying to eject all of the air existing between the heat dissipating fins.
A preferred embodiment of the present invention provides a cooling device that includes a piezoelectric fan including a piezoelectric oscillator that bends in accordance with an application of a voltage and a blade that is connected to or provided integrally with the piezoelectric oscillator and that is arranged to swing via the piezoelectric oscillator and a heat sink including at least two heat dissipating fins,
The blade preferably has an elongated shape that extends from the piezoelectric oscillator, the piezoelectric oscillator and the blade are preferably arranged at a position that enables the blade to swing without coming into contact with the heat dissipating fins in a space between the neighboring heat dissipating fins.
The blade preferably has a hole or a cut provided therein. With this structure, air resistance is reduced by the hole or the cut, and the amplitude of the blade is increased. Although the overall amount of generated air is reduced by the hole or cut, the effect of sweeping warm air in the vicinity of the walls of the heat dissipating fins is not reduced, such that the overall cooling capability is improved along with the increase in the amplitude. Additionally, a current of warm air in the vicinity of the heat dissipating fins separates and moves in the direction of the central portion in an undulating manner, and the warm air in the vicinity of the heat dissipating fins is transferred outward. Therefore, the heat dissipation effect and the cooling capability are improved.
In the cooling device, a weight may preferably be disposed at or in the vicinity of an end of the blade that is remote from the piezoelectric oscillator.
With this structure, a moment of inertia is increased by the weight, and driving the blade at a resonant frequency of the blade with the weight increases the amplitude of the blade. Thus, the cooling capability is improved.
The blade may preferably have a bent shape such that the blade is shortened in its longitudinal direction.
With this structure, the overall length of the blade is increased, and the amplitude is increased. Thus, the cooling capability is improved.
The piezoelectric oscillator may preferably be arranged so as to sandwich an end of the blade from both sides, and the piezoelectric oscillator and the blade may preferably define a bimorph oscillator.
With this configuration, the amount of bending displacement with respect to an applied voltage is increased, and the amplitude is increased. Thus, the cooling capability is further improved.
The cooling device may preferably include a fan that generates a current of air that flows between walls of the heat dissipating fins.
With this configuration, a current of warm air separating from the vicinity of the walls of the heat dissipating fins and moving toward the direction of the central portion while undulating by the presence of the hole or cut efficiently flows outward by the additional fan, and the overall cooling capability is improved.
The hole may preferably have a long shape extending along a longitudinal direction of the blade, and a dimension from a longitudinal-direction side of the blade to a side of the hole that is parallel or substantially parallel to the longitudinal-direction side may preferably be greater than a dimension of a gap between the blade and one of the heat dissipating fins.
With this configuration, even greater cooling capability is obtained.
With preferred embodiments of the present invention, the amplitude is increased, and the cooling capability is improved. In addition, the heat dissipation effect from the heat dissipating fins and the cooling capability are improved.
Other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a configuration of a piezoelectric fan of Japanese Unexamined Utility Model Registration Application Publication No. 02-127796.

FIG. 2 illustrates a relationship between air resistance and amplitude of a piezoelectric fan.

FIG. 3 is a perspective view of a piezoelectric fan for use in a cooling device according to a first preferred embodiment of the present invention.

FIGS. 4A and 4B illustrate a configuration of the cooling device according to a preferred embodiment of the present invention.

FIG. 5 illustrates effects of a hole of the piezoelectric fan used in the cooling device according to the first preferred embodiment of the present invention.

FIG. 6 illustrates how a current of air is produced by swinging a blade.

FIGS. 7A and 7B illustrate examples of temperature distributions of a current of air flowing between heat dissipating fins for with and without a hole in the blade.

FIGS. 8A and 8B are perspective views of piezoelectric fans for use in a cooling device according to a second preferred embodiment of the present invention.

FIG. 9 is a perspective view of a piezoelectric fan for use in a cooling device according to a third preferred embodiment of the present invention.

FIG. 10 illustrates the effects of weights and a hole on the piezoelectric fan.

FIG. 11 is a perspective view of a piezoelectric fan for use in a cooling device according to a fourth preferred embodiment of the present invention.

FIG. 12 is a perspective view of a piezoelectric fan for use in a cooling device according to a fifth preferred embodiment of the present invention.

FIG. 13 is a perspective view of a piezoelectric fan for use in a cooling device according to a sixth preferred embodiment of the present invention.

FIGS. 14A to 14C illustrate bending modes of the piezoelectric fan.

FIG. 15 is a perspective view of a piezoelectric fan for use in a cooling device according to a seventh preferred embodiment of the present invention.

FIGS. 16A to 16F illustrate flexural modes and swings of a blade of the piezoelectric fan.

FIG. 17 illustrates a configuration of a cooling device according to an eighth preferred embodiment of the present invention.

FIGS. 18A and 18B illustrate a configuration of a cooling device according to a ninth preferred embodiment of the present invention.

FIGS. 19A and 19B are plan views of piezoelectric fans for use in a cooling device according to a tenth preferred embodiment of the present invention.

FIG. 20 illustrates a positional relationship among the heat dissipating fins, piezoelectric fan, and hole of the blade.

FIG. 21 illustrates a difference of a temperature gradient with respect to a distance from a heat dissipating fin, depending on with and without a piezoelectric fan.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Preferred Embodiment

FIG. 3 is a perspective view of a piezoelectric fan for use in a cooling device according to a first preferred embodiment of the present invention. In FIG. 3, a piezoelectric fan 31 includes a blade 21 and a piezoelectric oscillator 20. The blade 21 includes a hole 22. The piezoelectric oscillator is preferably a bimorph piezoelectric oscillator in which a piezoelectric element is arranged on each of two opposed surfaces of a metal plate defining intermediate electrode. That is, each of the piezoelectric elements on both surfaces of the metal plate defining the intermediate electrode of the piezoelectric oscillator 20 includes an electrode film provided on its surface, and polarization is performed such that each of the piezoelectric elements is caused to oscillate by being bent in the longitudinal direction (L1 dimension direction) by the application of a driving voltage corresponding to a polarization direction of the piezoelectric element to the electrode and the metal plate defining the intermediate electrode.
The blade 21 preferably includes a hole 22 having a rectangular or substantially rectangular shape, for example, that is punched in a stainless steel plate, and an end connected to an end of the piezoelectric oscillator 20.
The dimensions of each portion of the piezoelectric fan 31 illustrated in FIG. 3 are described below.
L1: about 12 mm
L2: about 18 mm
W: about 6 mm
t: about 50 μm
The dimensions of the hole 22 are preferably about 12 mm by about 2 mm, for example. An end of the hole 22 near the base is aligned with the end of the piezoelectric oscillator 20.
FIG. 4A is a perspective view of a main portion of a cooling device in which the piezoelectric fans 31, each being illustrated in FIG. 3, are arranged in a heat sink. The piezoelectric fans 31 are arranged in the heat sink such that a plurality of heat dissipating fins 30 illustrated in FIGS. 4A and 4B protrude in parallel or substantially in parallel with one another and the blade 21 of each of the piezoelectric fans can swing between neighboring heat dissipating fins 30 without coming into contact with the heat dissipating fins 30.
FIG. 4B is a front view of the entire cooling device as seen from the direction in which the heat dissipating fins extend (the direction facing an air sending direction).
The plurality of heat dissipating fins 30 extending in parallel or substantially in parallel with one another is disposed in a heat sink 40. In this example, a heating element (heating component) 110, e.g., a CPU, is mounted on the upper portion of a circuit board 120, and the heat sink 40 is arranged such that its bottom is thermally coupled to the upper surface of the heating element 110.
As described above, the heat sink 40 and the plurality of piezoelectric fans 31 define a cooling device 100.
FIG. 5 illustrates changes in amplitude of a blade end with and without the hole 22 in the blade 21 of the piezoelectric fan 31 illustrated in FIGS. 3, 4A, and 4B and with and without the heat dissipating fins 30.
As illustrated, the amplitude of the blade end when the heat dissipating fins exist on both sides of the blade is less than that when the space is open. For example, a comparison when an input voltage of about 30 V is applied and no hole exists is discussed below. The amplitude of the blade end is about 9.2 mm when no heat dissipating fins exist, whereas the amplitude of the blade end is reduced to about 5.5 mm when the heat dissipating fins exist, for example. When the hole 22 is disposed, the amplitude of the blade end is increased to about 7.5 mm, for example.
This is because the presence of the hole 22 reduces a substantial area of the blade 21, and facilitates air to escape through the hole 22, which is described below, and thus, air resistance is reduced correspondingly.
As described above, it is clear that the hole 22 in the blade enables the blade to swing with a relatively large amplitude even in a space between the heat dissipating fins.
FIG. 6 illustrates the effects of the hole 22 disposed in the blade 21. In the piezoelectric fan 31, the blade 21 swings in the directions indicated by the arrows u and d illustrated in the drawing about a support portion of the piezoelectric oscillator 20 (the left end in the drawing). This causes air in a space sandwiched by the heat dissipating fins 30 to flow as a current of air indicated by the arrows AF. At this time, currents of air flowing from up to down and from down to up through the hole 22 occur, and the currents of air flowing above and below the blade 21 are mixed. At the same time, a current of warm air in the vicinity of the walls of the heat dissipating fins 30 is separated at the side edge of the blade, and the warm air is moved in the direction of the hole 22, that is, in the direction of the central portion in the space sandwiched by the two neighboring heat dissipating fins 30.
As described above, the hole 22 not only increases the amplitude of the blade 21 by reducing air resistance but also acts so as to separate a current of warm air in the vicinity of the walls of the heat dissipating fins 30 and sweep it together with the currents of air sandwiched by the two neighboring heat dissipating fins 30 toward the outside. (Hereinafter, this operational effect is referred to as “sweep effect.”) Accordingly, the heat dissipation effect is improved.
FIGS. 7A and 7B are illustrations describing the operation of a piezoelectric fan that includes a blade having a hole. In FIGS. 7A and 7B, a low-brightness portion indicates a low-temperature region, and a high-brightness portion indicates a high-temperature region.
Three-dimensional simulation is required calculate a temperature distribution of a current of air flowing in a space between the heat dissipating fins in a cooling device having the structure illustrated in FIGS. 4A and 4B. However, three-dimensional simulation requires an enormous amount of calculation, so the simulation is replaced with two-dimensional simulation.
FIG. 7A illustrates a temperature distribution when the two neighboring heat dissipating fins 30 are at a predetermined high temperature and cool air having a predetermined temperature flows in the space sandwiched by these heat dissipating fins 30 from left to right in the drawing.
As illustrated, when the current of air flowing in the space sandwiched by the heat dissipating fins 30 is a laminar flow, the velocity of flow of air increases toward the central portion of the space, and, in theory, it is zero at the walls of the heat dissipating fins 30 due to viscous drag of air. Accordingly, the quantity of cool air that is supplied from the left end and then flows toward the right end through the central portion in the space sandwiched by the heat dissipating fins 30 while remaining cool is relatively large, so the heat dissipation effect of the heat dissipating fins 30 is relatively low.
FIG. 7B illustrates a temperature distribution when a paddle P in the vicinity of the walls of the heat dissipating fins 30 is oscillated. The other conditions are substantially the same as in FIG. 7A. The paddle P corresponds to the segments of both side portions of the hole 22 of the blade 21 illustrated in FIGS. 4A and 4B.
As described above, the warm air (high-temperature air layer) distributed over the walls of the heat dissipating fins 30 undulates and moves towards the central portion of the space, and is swept out by a current of air flowing through the entire space. Accordingly, the overall heat dissipation effect is improved.

Second Preferred Embodiment

In the first preferred embodiment, the blade 21 includes the single hole 22 extending along the longitudinal direction of the blade. FIGS. 8A and 8B illustrate two configurations of a piezoelectric fan that are different from that of the first preferred embodiment.
The example shown in FIG. 8A illustrates a piezoelectric fan 32 in which the blade 21 preferably includes a cut 23 extending along the longitudinal direction of the blade 21 at an end thereof (farther from the piezoelectric oscillator 20). In the structure illustrated in FIG. 8A, the hole is arranged at the end of the blade 21. As described above, when the cut 23 is disposed at the end of the blade 21, the effect of increasing the amplitude of the blade 21 resulting from a reduction in air resistance and the effect of sweeping high-temperature air on the walls of the heat dissipating fins are obtained.
In the example shown in FIG. 8B, a piezoelectric fan is configured such that the blade 21 having a plurality of holes 22 a to 22 d is connected to the piezoelectric oscillator 20.
Differences in operational effect depending on the location of each of the holes and the number of holes in the blade 21 are described below.
First, as illustrated in FIG. 3, when the hole 22 is disposed near the base of the blade (adjacent to the piezoelectric oscillator 20), the hole 22 is present at a location at which the displacement of the blade 21 during swinging is relatively small. Accordingly, the effect of reducing air resistance is relatively low and the effect of increasing the amplitude of the blade 21 is relatively low, whereas the air blowing performance is relatively high because the hole does not impair the effect of pushing out air by the end of the blade 21.
In contrast, as illustrated in FIG. 8A, when the cut is disposed near the end of the blade 21 (or the hole is disposed near the end), the effect of reducing air resistance is relatively high, such that the amplitude of the blade 21 is relatively large. Accordingly, the sweep effect of sweeping high-temperature air on the walls of the heat dissipating fins is improved. However, the effect of pushing a current of air from the blade end is reduced, such that the performance of blowing air is reduced.
As described above, there is, to some extent, a trade-off between the performance of sending air and the sweep effect, so the shape, location, and size of the hole can be determined so as to achieve a maximum cooling capability.
Also, when a plurality of holes are provided, as illustrated in FIG. 8B, the size and location of each of the holes and the number of holes can be determined based on the performance of sending air and the sweep effect.
It is noted that, when the hole is disposed near the base of the blade (adjacent to the piezoelectric oscillator 20), although the substantial width of the section at which the bending stress of the blade 21 is relatively large is small, the shape of the hole 22 extending in the longitudinal direction of the blade 21, not in the width direction, reduces the concentration of the bending stress, such that reliability in long-time driving can be improved.

Third Preferred Embodiment

FIG. 9 is a perspective view of a piezoelectric fan for use in a cooling device according to a third preferred embodiment of the present invention. In this example, the blade 21 including the hole 22 and weights 24 a and 24 b disposed at the end and the piezoelectric oscillator 20 define a piezoelectric fan 34.
The weights 24 a and 24 b are preferably made of the same or substantially the same stainless steel as the blade 21 and are joined thereto by adhesive, for example. The dimensions L3 and d of the weights 24 a and 24 b illustrated in the drawing are preferably about 2 mm and about 0.5 mm, respectively, for example. The thickness dimension of the blade 21 preferably is about 100 μm, for example. The other dimensions L1, L2, and W are the same or substantially the same as in the first preferred embodiment illustrated in FIG. 3: L1=about 12 mm, L2=about mm, and W=about 6 mm, for example. The location and dimensions of the hole 22 are the same or substantially the same as in FIG. 3.
FIG. 10 illustrates the effects of the weights and the hole of the cooling device including the piezoelectric fan 34 illustrated in FIG. 9. It is clear that the weights 24 a and 24 b and the hole 22 enables a larger amplitude, as compared to when a piezoelectric fan that includes no weights or hole is oscillated in an open space. For example, when an input voltage of about 30 V is applied, in the case of a piezoelectric fan that does not include any weights or hole, the amplitude of the blade end is about 5.5 mm, whereas in the case of a piezoelectric fan including the weights and the hole, as illustrated in FIG. 10, the amplitude of the blade end is increased to about 9.5 mm.
As described above, the weights 24 a and 24 b provided to the end of the blade 21 increases moment of inertia, and driving the blade with the weights at a resonant frequency increases the amplitude of the blade 21 even when the piezoelectric fan 34 is arranged in a space sandwiched by the heat dissipating fins, as illustrated in FIG. 4. Accordingly, the cooling capability can be improved.
The addition of the weights and the formation of the hole have a synergistic effect. This is because, due to the addition of the weights and the formation of the hole, the center of mass of the blade is moved closer to the end, and thus, the moment of inertia per mass of the blade is increased.
Therefore, with the increase in amplitude of the blade caused by the weights 24 a and 24 b, the sweep effect caused by the hole 22 can be further enhanced.

Fourth Preferred Embodiment

FIG. 11 is a perspective view of a piezoelectric fan according to a fourth preferred embodiment. In this example, the blade 21 including the holes 22 b and 22 c and the piezoelectric oscillator 20 connected to the blade 21 define a piezoelectric fan 35. The blade 21 is relatively long and is bent so as to be shortened in its longitudinal direction. With this structure, the overall length of the blade 21 is relatively long, and driving the blade 21 so as to resonate at the fundamental frequency increases the amplitude, and thus, the cooling capability is improved. Additionally, although the overall length of the blade 21 is relatively long, the dimension of the blade 21 in the longitudinal direction can be reduced, such that the cooling capability can be improved without a large increase in the size of the entire cooling device.
In this example, the blade 21 is bent into three segments indicated by 21 a, 21 b, and 21 c, the segment 21 a near the piezoelectric oscillator 20 does not include a hole, and the segments 21 b and 21 c include the holes 22 b and 22 c, respectively. The holes 22 b and 22 c do not overlap the bent portions. With this structure, the base adjacent to the piezoelectric oscillator is more elastic, the end is less elastic, and the amplitude of each of the segments 21 b and 21 c (in particular, the amplitude of the segment 21 c) is increased. Accordingly, swinging is similar to movement of a round fan, such that a high cooling capability is obtained.
The stress does not concentrate on the hole, so the reliability in long-time driving can be improved.
It is noted that a weight as illustrated in FIG. 9 may be attached to an end or at a predetermined position of the blade having the above bent structure.

Fifth Preferred Embodiment

FIG. 12 is a side view of a piezoelectric fan according to a fifth preferred embodiment. The foregoing preferred embodiments illustrate types in which the piezoelectric oscillator 20 is connected to a first surface of the blade 21. A piezoelectric fan 36 illustrated in FIG. 12 includes piezoelectric elements 20 a and 20 b arranged so as to sandwich an end of the blade 21 from both surfaces thereof, and the piezoelectric elements 20 a and 20 b and the blade 21 define a bimorph oscillator.
Each of the piezoelectric elements 20 a and 20 b includes an electrode film provided on its surface. Applying a driving voltage corresponding to the polarization direction of each of the piezoelectric elements 20 a and 20 b between the blade and each of the electrodes expands and contracts the piezoelectric elements 20 a and 20 b in opposite directions, thus driving the piezoelectric elements 20 a and 20 b as a bimorph piezoelectric oscillator.
As described above, the bimorph type increases the displacement of flexure of the blade 21 with respect to an applied voltage by the piezoelectric elements 20 a and 20 b, so the amplitude of the blade 21 can be more efficiently increased.
FIG. 12 illustrates a piezoelectric fan in which the hole 22 is disposed in the blade 21, for example. The present preferred embodiment is also applicable to a piezoelectric fan having including a weight that is attached to an end of the blade 21, as illustrated in FIG. 9, or the structure in which the blade 21 is bent, as illustrated in FIG. 11.

Sixth Preferred Embodiment

FIG. 13 is a perspective view of a piezoelectric fan for use in a cooling device according to a sixth preferred embodiment. As illustrated in FIG. 13, a piezoelectric fan 37 is configured such that first ends of two piezoelectric oscillators 26 a and 26 b are joined together with a spacer 28 disposed therebetween so as to form a U-shaped piezoelectric oscillator unit and the blade 21 is joined to an end of the piezoelectric oscillator 26 a with a spacer 29 disposed therebetween. In this example, the blade 21 includes the hole 22.
It is noted that the spacers 28 and 29 are not necessary.
FIGS. 14A to 14C illustrate flexural modes of the U-shaped piezoelectric oscillator unit when a voltage is applied thereto. FIG. 14A illustrates a state in which a voltage applied to the two piezoelectric oscillators 26 a and 26 b is zero; FIG. 14B illustrates a state in which a positive voltage is applied thereto; FIG. 14C illustrates a state in which a negative voltage is applied thereto.
Here, an end of the lower piezoelectric oscillator 26 b is fixed, such that an end of the upper piezoelectric oscillator 26 a can swing at an angle that is approximately twice as large as when a single piezoelectric oscillator is used. Accordingly, the amplitude of the blade 21 illustrated in FIG. 13 can be further increased.
It is noted that, although FIG. 13 illustrates a piezoelectric fan in which the blade 21 includes the hole 22, as an example, the present preferred embodiment is also applicable to a piezoelectric fan having a structure in which a weight that is attached to an end of the blade 21, as illustrated in FIG. 9, or a structure in which the blade 21 is bent, as illustrated in FIG. 11.

Seventh Preferred Embodiment

FIG. 15 is a perspective view of a piezoelectric fan for use in a cooling device according to a seventh preferred embodiment. As illustrated in FIG. 15, a piezoelectric fan 38 is configured such that a piezoelectric oscillator unit including three piezoelectric oscillators 27 a, 27 b, and 27 c and having a substantial E shape as a whole is joined to the blade 21 with the spacer 29 disposed therebetween. In this example, the blade 21 includes the hole 22.
FIGS. 16A to 16C illustrate flexural modes of the above E-shaped piezoelectric oscillator unit portion. FIGS. 16D to FIG. 16F illustrate swings of the blade of the above piezoelectric fan 38.
FIG. 16A illustrates a state in which a voltage applied to the piezoelectric oscillators 27 a, 27 b, and 27 c is zero; FIG. 16B illustrates a state in which a positive voltage is applied thereto; FIG. 16C illustrates a state in which a negative voltage is applied thereto.
Here, an end of each of the piezoelectric oscillators 27 a and 27 b is fixed, such that an end of the central piezoelectric oscillator 27 c can swing at an angle that is approximately twice as large as when a single piezoelectric oscillator is used. Accordingly, the amplitude of the blade 21 illustrated in FIG. 15 can be further increased.
It is noted that, although FIG. 15 illustrates a piezoelectric fan in which the blade 21 includes the hole 22 as an example, the present preferred embodiment is also applicable to a piezoelectric fan having the structure in which a weight is attached to an end of the blade 21, as illustrated in FIG. 9, or a structure in which the blade 21 is bent, as illustrated in FIG. 11.

Eighth Preferred Embodiment

FIG. 17 illustrates a configuration of a cooling device according to an eighth preferred embodiment. This cooling device 101 includes the piezoelectric fan 31, the heat sink 40, and a blower fan 50. In the first to eighth preferred embodiments, a piezoelectric fan and a heat sink define a cooling device, and the piezoelectric fan dissipates heat by sweeping air in a space surrounded by heat dissipating fins of the heat sink. In the example illustrated in FIG. 17, air in a space between the heat dissipating fins 30 of the heat sink 40 is stirred with the piezoelectric fan 31, and the blower fan 50 causes the air move outward.
The blade 21 including the hole 22 and the piezoelectric oscillator 20 define the piezoelectric fan 31, and the piezoelectric fan 31 is substantially the same as the piezoelectric fan illustrated in FIG. 3. However, in this example, the orientation of the blade 21 is preferably inclined approximately 45°, for example, toward the longitudinal direction of the heat dissipating fin 30. This allows the support portion (fixed portion) of the piezoelectric oscillator 20 to be disposed outside the heat sink 40, so as to facilitate the attachment of the piezoelectric fan 31.
A component that faces a current of air caused by the blower fan 50 is increased, and the conditions of the hole 22 are similar to those of the simulation illustrated in FIG. 7B. Thus, the sweep effect of sweeping high-temperature air on the walls of the heat dissipating fins 30 can be improved.
It is noted that, although FIG. 17 illustrates a piezoelectric fan in which the blade 21 includes the hole 22 as an example, the present preferred embodiment is also applicable to a piezoelectric fan having a structure in which a weight is attached to an end of the blade 21, as illustrated in FIG. 9, or a structure in which the blade 21 is bent, as illustrated in FIG. 11.

Ninth Preferred Embodiment

FIG. 18A is a perspective view that illustrates a configuration of a cooling device according to a ninth preferred embodiment. FIG. 18B is a plan view of a piezoelectric fan used therein.
As illustrated in FIG. 18B, a piezoelectric fan 39 includes a metal plate 19 from which the plurality of blades 21 having an integral base protrude. Each of the blades 21 includes the hole 22. A piezoelectric oscillator 25 is connected on the metal plate 19 at the base of the blades 21. The metal plate 19 is attached to a support member 41 preferably with screws 42, for example. Applying alternating voltage to the piezoelectric oscillator 25 causes the metal plate 19 and the blades 21 to swing using the position of the support member 41 as a pivot.
As illustrated in FIG. 18A, a cooling device 102 is configured such that the piezoelectric fan 39 is arranged at a predetermined height from the bottom of the heat sink 40 (at a height corresponding to the approximate center of an overall height or at a height below the approximate center). The heat sink 40 includes the plurality of heat dissipating fins 30 extending in parallel or substantially in parallel to one another, and the piezoelectric fan 39 is arranged such that each of the blades 21 of the piezoelectric fan 39 can swing between the neighboring heat dissipating fins 30 without coming into contact with the heat dissipating fins 30.
As described above, the cooling device 102 including the piezoelectric fan in which the plurality of blades can be arranged to swing by the single piezoelectric oscillator is configured.

Tenth Preferred Embodiment

FIGS. 19A and 19B are plan views of piezoelectric fans according to a tenth preferred embodiment.
In the example of FIG. 19A, the metal plate 19 is divided into three regions of the left region L, central region C, and right region R, on which piezoelectric oscillators 25L, 25C, and 25R are disposed, respectively. The three regions can independently swing.
Similarly, in the example of FIG. 19B, the metal plate is divided into two regions of the left region L and right region R, on which piezoelectric oscillators 25L and 25R are disposed, respectively. The two regions can independently swing.
With these configurations, the blade 21 in any region can independently swing depending upon the desired purpose. For example, in FIG. 19A, driving the piezoelectric oscillators 25L and 25R with a positive-phase voltage and driving the piezoelectric oscillator 25C with a negative-phase voltage reduces a reaction force received by the support member 41. Similarly, in the case of FIG. 19B, driving the piezoelectric oscillator 25L with a positive-phase voltage and driving the piezoelectric oscillator 25R with a negative-phase voltage reduces a reaction force received by the support member 41.
As described above, oscillation of a member to which the support member 41 is attached can be suppressed and prevented, so noise can be minimized and eliminated.
It is noted that the number of regions and the number of blades in each region are not limited to the ones illustrated in FIG. 19. They may be set such that the metal plate 19 and each of the blades 21 oscillate in a desired oscillation mode.
A positional relationship and dimensional relationship among portions of a cooling device according to the above preferred embodiments are discussed below.
A piezoelectric fan includes a blade that is disposed between heat dissipating fins of a heat sink and the blade pulls hot air on the surface of the heat dissipating fins and sweeps it to facilitate cooling. To facilitate cooling, an increase in the area from which hot air is pulled is important. To this end, it is necessary to increase the surface area of the heat dissipating fins of the heat sink and to increase the amplitude of the blade, and preferably, the elongated blade may preferably be disposed in the gap between the neighboring heat dissipating fins.
FIG. 20 is a plan view that illustrates a state in which the blade 21 is arranged in the gap between the heat dissipating fins 30 of the heat sink. Here, the x direction is referred to as the longitudinal direction, the y direction is referred to as the width direction, and the z direction is referred to as the thickness direction.
To pull hot air on the surface of each of the heat dissipating fins 30 by the blade 21, it is preferable that the gap G between the heat dissipating fin 30 and the blade 21 be relatively small. However, if the gap G is small, air resistance when moving the blade 21 would be large and the amplitude of the blade 21 would be small. In terms of the purpose of pulling air on the surface of the heat dissipating fin 30, it is not overly important to pull air at the central portion between the heat dissipating fins 30 (the region approximately indicated by the letter B illustrated in FIG. 20). Thus, it is preferable to include the hole 22 at the central portion of the blade 21 in order to reduce air resistance.
It is preferable that the hole 22 be disposed over the overall length of the blade 21. However, in practice, the hole is disposed only at the central portion of the blade 21 (the position indicated by the letter A in FIG. 20) for the reasons described below.
First, when the base of the blade 21 is considered, because the amplitude of the base is relatively small, air resistance is also relatively small, such that there is no need to provide the hole. Next, when the end of the blade 21 is considered, because the blade 21 does not extend beyond the end, pushed air can easily escape to a wide space. As a result, at the end of the blade 21, the effect of reducing air resistance by the hole is relatively small. If the hole extends to the end of the blade 21, two thin slender blades would move in a narrow gap between heat dissipating fins. However small air resistance and an unstable oscillation may occur because of the interaction between the movements of the blades through air. As a result, it is preferable that the end of the blade 21 is defined by a rigid coupled portion. Additionally, to prevent heat build-up between the heat dissipating fins 30 of the heat sink, air flow is necessary to some extent. The end at which the amplitude is the largest greatly affects the effect of air flow, and the hole being disposed adjacent to the base, not at the end, facilitates production of a stable one-way current of air.
Also for these reasons, it is preferable that the hole 22 be disposed only at the central portion of the blade 21.
As for the shape of the hole 22, for the reasons described below, the hole in the longitudinal direction can be configured in a wide range except for the base and the end, whereas the hole in the width direction is required to have a minimum dimension E from the longitudinal-direction side of the blade 21 to the side of the hole 22 that is parallel or substantially parallel to that longitudinal-direction side. Because of this and the fact that the blade 21 is elongated, the hole 22 is also elongated.
When the end-surface effect is neglected, air resistance at a point in the x direction is assumed to be proportional to the square of the speed and the cross-section ratio. That is, where the amplitude is h(x), the frequency is f, and the cross-section ratio is r_A, Air Resistance in x∝f²h²r_Aand the one in which air resistance at each cross section is integrated over the length of the blade is air resistance of the whole.
It is noted that [Cross-section Ratio]=([Blade Width]−[Hole Width])/[Distance between Heat Dissipating Fins].
As described above, because air resistance is proportional to the square of the amplitude, air resistance in the vicinity of the base whose amplitude is relatively small is negligible. As a result, there is no reason to provide the hole in the vicinity of the base.
As for the end, even if no hole is provided, a wide space is present beyond the end. As a result, pushed air can flow toward the end direction without passing through the narrow gap between the blade 21 and the heat dissipating fin (the portion G in FIG. 20). Accordingly, air resistance is less than the estimation in the above-described case in which the end-surface effect is ignored. In theory, the range of this end-surface effect is substantially the same as the width of the blade 21. As a result, at least in a range whose length from the end is equal to or substantially equal to the width of the blade, there is no need to include a hole in terms of air resistance.
FIG. 21 illustrates a difference of a temperature gradient with respect to a distance from a heat dissipating fin with and without a piezoelectric fan. The thick line indicates a case without a piezoelectric fan, and the thin line indicates a case with a piezoelectric fan. A temperature distribution between heat dissipating fins of a heat sink is represented by the thick line in FIG. 21. When a piezoelectric fan is provided and operates, because air at both ends of the blade is mixed, as indicated by the thin line in the drawing, the temperature at the blade portion (the portion E in the drawing) is substantially uniform (the temperature gradient is gentle). Consequently, the temperature at the gap portion between the heat dissipating fin and the blade (the portion G in the drawing) decreases, and the temperature gradient at the surface of the heat dissipating fin is steep. This means that more heat moves out from the surface of the heat dissipating fin, that is, the cooling effect is improved because heat flux is proportional to the temperature gradient.
Because of a mechanism of improving the cooling capability, if the difference between the temperatures at both ends of the blade (the position G and the position G+E in FIG. 20) is not sufficient, no sufficient improvement in cooling capability can be obtained. Accordingly, it is necessary to set G in a region of a large temperature difference (that is, to arrange both side portions of the blade 21 as close as possible to the heat dissipating fins 30), and in addition to this, the dimension E is required to be a dimension that enables a sufficient temperature difference between both ends.
When the distance from the heat dissipating fin 30 is sufficiently small, the temperature distribution is a substantially straight line. Where the slope of this straight line (=temperature gradient) is k and the temperature at the surface of the heat dissipating fin is To, the temperature at a position that is remote by the dimension G from the wall of the heat dissipating fin is To+k*G and the temperature at a position that is remote by the dimension (G+E) is To+k*(G+E). If the temperature at the position of the blade 21 is perfectly uniform, the temperature at this blade 21 is To+k*(G+E/2), so the temperature gradient can be estimated at k*(1+0.5*E/G).
That is, if E=G, the improvement in the cooling capability can be estimated at about 50% at a maximum. In view of the impossibility of perfectly uniform temperature distribution of the blade portion and the possibility that the temperature distribution may deviate from a linear range, setting E>G is effective to achieve distinct improvement in cooling capability.
In some of the preferred embodiments described above, a unimorph piezoelectric fan is preferably arranged such that an end of a piezoelectric oscillator is connected to an end of a blade. However, the entire surface of a piezoelectric oscillator may be connected to an end of a blade.
Some of the preferred embodiments described above illustrate an example in which a weight is connected to an end of a blade. The weight and the blade may be integrally provided. In addition, the weight may be disposed in the vicinity of an end, instead of at the distal end.
In the preferred embodiments described above, other than stainless steel, a highly elastic metal plate, such as one made of phosphor bronze, and a resin plate, for example, may also preferably be used as a blade.
Additionally, in the examples, except for the configuration illustrated in FIG. 12, a bimorph piezoelectric oscillator is connected to a single side of a blade. However, a simple piezoelectric element may be used as a piezoelectric oscillator to be connected to a single side of a blade, and the piezoelectric element and the blade may form a unimorph oscillator.
While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.

Claims

What is claimed is:

1. A cooling device comprising:

a piezoelectric fan including a piezoelectric oscillator arranged to bend in response to an application of a voltage and a blade that is connected to or provided integrally with the piezoelectric oscillator and that is arranged to swing by the piezoelectric oscillator; and

a heat sink including at least two heat dissipating fins; wherein

the blade has an elongated shape that extends from the piezoelectric oscillator;

the piezoelectric oscillator and the blade are arranged at a position that allows the blade to swing without coming into contact with the heat dissipating fins in a space between the neighboring heat dissipating fins; and

the blade includes at least one of a hole or a cut.

2. The cooling device according to claim 1, wherein a weight is disposed at or in a vicinity of an end of the blade that is remote from the piezoelectric oscillator.

3. The cooling device according to claim 1, wherein the blade has a bent shape such that a length of the blade is shortened in a longitudinal direction of the blade.

4. The cooling device according to claim 1, wherein the piezoelectric oscillator is arranged so as to sandwich an end of the blade from both sides, and the piezoelectric oscillator and the blade define a bimorph oscillator.

5. The cooling device according to claim 1, further comprising a fan arranged to generate a current of air that is directed to flow through the space between the heat dissipating fins.

6. The cooling device according to claim 1, wherein the hole has an elongated shape extending along a longitudinal direction of the blade, and a dimension from a longitudinal-direction side of the blade to a side of the hole that is substantially parallel to the longitudinal-direction side is greater than a dimension of a gap between the blade and one of the heat dissipating fins.