U.S. patent application number 13/366465 was filed with the patent office on 2012-05-31 for cooling device.
This patent application is currently assigned to MURATA MANUFACTURING CO., LTD.. Invention is credited to Gaku KAMITANI, Hiroaki WADA.
Application Number | 20120134858 13/366465 |
Document ID | / |
Family ID | 40451963 |
Filed Date | 2012-05-31 |
United States Patent
Application |
20120134858 |
Kind Code |
A1 |
WADA; Hiroaki ; et
al. |
May 31, 2012 |
COOLING DEVICE
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.
Inventors: |
WADA; Hiroaki; (Kusatsu-shi,
JP) ; KAMITANI; Gaku; (Kyoto-shi, JP) |
Assignee: |
MURATA MANUFACTURING CO.,
LTD.
Nagaokakyo-shi
JP
|
Family ID: |
40451963 |
Appl. No.: |
13/366465 |
Filed: |
February 6, 2012 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
12722650 |
Mar 12, 2010 |
|
|
|
13366465 |
|
|
|
|
PCT/JP2008/066201 |
Sep 9, 2008 |
|
|
|
12722650 |
|
|
|
|
Current U.S.
Class: |
417/410.2 |
Current CPC
Class: |
H01L 2924/0002 20130101;
H01L 23/467 20130101; G06F 1/20 20130101; F04D 33/00 20130101; H01L
2924/00 20130101; H01L 2924/0002 20130101; H05K 7/20172
20130101 |
Class at
Publication: |
417/410.2 |
International
Class: |
F04B 17/03 20060101
F04B017/03 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 14, 2007 |
JP |
2007-239815 |
Claims
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; the blade includes at least one of a hole or
a cut; and the blade has a bent shape such that a length of the
blade is shortened in a longitudinal direction of the blade.
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
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.
4. 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.
5. 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.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] 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.
[0003] 2. Description of the Related Art
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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 8a, left and right sides 8b and 8c, and a
back 8d, and an open front. The cover element 9 is hermetically
fixed to the upper surface of the casing main body 8.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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 .mu.m. The
shorter-side portions of piezoelectric oscillator and the blade are
connected to each other.
[0014] 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.
[0015] 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
[0016] 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.
[0017] 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.
[0018] 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.
[0019] 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.
[0020] 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.
[0021] 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,
[0022] 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.
[0023] 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.
[0024] 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.
[0025] 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.
[0026] The blade may preferably have a bent shape such that the
blade is shortened in its longitudinal direction.
[0027] With this structure, the overall length of the blade is
increased, and the amplitude is increased. Thus, the cooling
capability is improved.
[0028] 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.
[0029] 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.
[0030] The cooling device may preferably include a fan that
generates a current of air that flows between walls of the heat
dissipating fins.
[0031] 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.
[0032] 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.
[0033] With this configuration, even greater cooling capability is
obtained.
[0034] 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.
[0035] 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
[0036] FIG. 1 illustrates a configuration of a piezoelectric fan of
Japanese Unexamined Utility Model Registration Application
Publication No. 02-127796.
[0037] FIG. 2 illustrates a relationship between air resistance and
amplitude of a piezoelectric fan.
[0038] 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.
[0039] FIGS. 4A and 4B illustrate a configuration of the cooling
device according to a preferred embodiment of the present
invention.
[0040] 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.
[0041] FIG. 6 illustrates how a current of air is produced by
swinging a blade.
[0042] 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.
[0043] 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.
[0044] 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.
[0045] FIG. 10 illustrates the effects of weights and a hole on the
piezoelectric fan.
[0046] 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.
[0047] 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.
[0048] 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.
[0049] FIGS. 14A to 14C illustrate bending modes of the
piezoelectric fan.
[0050] 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.
[0051] FIGS. 16A to 16F illustrate flexural modes and swings of a
blade of the piezoelectric fan.
[0052] FIG. 17 illustrates a configuration of a cooling device
according to an eighth preferred embodiment of the present
invention.
[0053] FIGS. 18A and 18B illustrate a configuration of a cooling
device according to a ninth preferred embodiment of the present
invention.
[0054] 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.
[0055] FIG. 20 illustrates a positional relationship among the heat
dissipating fins, piezoelectric fan, and hole of the blade.
[0056] 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
[0057] 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.
[0058] 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.
[0059] The dimensions of each portion of the piezoelectric fan 31
illustrated in FIG. 3 are described below.
[0060] L1: about 12 mm
[0061] L2: about 18 mm
[0062] W: about 6 mm
[0063] t: about 50 .mu.m
[0064] 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.
[0065] 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 31 can swing between
neighboring heat dissipating fins 30 without coming into contact
with the heat dissipating fins 30.
[0066] 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).
[0067] 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.
[0068] As described above, the heat sink 40 and the plurality of
piezoelectric fans 31 define a cooling device 100.
[0069] 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.
[0070] 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.
[0071] 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.
[0072] 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.
[0073] 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.
[0074] 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.
[0075] 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.
[0076] 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.
[0077] 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.
[0078] 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.
[0079] 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.
[0080] 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
[0081] 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.
[0082] 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.
[0083] In the example shown in FIG. 8B, a piezoelectric fan is
configured such that the blade 21 having a plurality of holes 22a
to 22d is connected to the piezoelectric oscillator 20.
[0084] 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.
[0085] 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.
[0086] 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.
[0087] 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.
[0088] 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.
[0089] 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
[0090] 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 24a and 24b disposed at the end and the
piezoelectric oscillator 20 define a piezoelectric fan 34.
[0091] The weights 24a and 24b 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 24a and 24b 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 .mu.m,
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.
[0092] 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 24a and 24b 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.
[0093] As described above, the weights 24a and 24b 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.
[0094] 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.
[0095] Therefore, with the increase in amplitude of the blade
caused by the weights 24a and 24b, the sweep effect caused by the
hole 22 can be further enhanced.
Fourth Preferred Embodiment
[0096] 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 22b and 22c 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.
[0097] In this example, the blade 21 is bent into three segments
indicated by 21a, 21b, and 21c, the segment 21a near the
piezoelectric oscillator 20 does not include a hole, and the
segments 21b and 21c include the holes 22b and 22c, respectively.
The holes 22b and 22c 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 21b and 21c (in particular, the amplitude of the
segment 21c) is increased. Accordingly, swinging is similar to
movement of a round fan, such that a high cooling capability is
obtained.
[0098] The stress does not concentrate on the hole, so the
reliability in long-time driving can be improved.
[0099] 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
[0100] 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 20a and
20b arranged so as to sandwich an end of the blade 21 from both
surfaces thereof, and the piezoelectric elements 20a and 20b and
the blade 21 define a bimorph oscillator.
[0101] Each of the piezoelectric elements 20a and 20b includes an
electrode film provided on its surface. Applying a driving voltage
corresponding to the polarization direction of each of the
piezoelectric elements 20a and 20b between the blade and each of
the electrodes expands and contracts the piezoelectric elements 20a
and 20b in opposite directions, thus driving the piezoelectric
elements 20a and 20b as a bimorph piezoelectric oscillator.
[0102] 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 20a and 20b, so the amplitude
of the blade 21 can be more efficiently increased.
[0103] 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
[0104] 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 26a and 26b 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 26a with a spacer
29 disposed therebetween. In this example, the blade 21 includes
the hole 22.
[0105] It is noted that the spacers 28 and 29 are not
necessary.
[0106] 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 26a and 26b 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.
[0107] Here, an end of the lower piezoelectric oscillator 26b is
fixed, such that an end of the upper piezoelectric oscillator 26a
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.
[0108] 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
[0109] 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 27a, 27b, and 27c 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.
[0110] FIGS. 16A to 16C illustrate flexural modes of the above
E-shaped piezoelectric oscillator unit portion. FIG. 16D to FIG.
16F illustrate swings of the blade of the above piezoelectric fan
38.
[0111] FIG. 16A illustrates a state in which a voltage applied to
the piezoelectric oscillators 27a, 27b, and 27c 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.
[0112] Here, an end of each of the piezoelectric oscillators 27a
and 27b is fixed, such that an end of the central piezoelectric
oscillator 27c 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.
[0113] 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
[0114] 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.
[0115] 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.degree., 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.
[0116] 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.
[0117] 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
[0118] 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.
[0119] 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.
[0120] 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.
[0121] 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
[0122] FIGS. 19A and 19B are plan views of piezoelectric fans
according to a tenth preferred embodiment.
[0123] 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.
[0124] 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.
[0125] 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.
[0126] 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.
[0127] 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.
[0128] A positional relationship and dimensional relationship among
portions of a cooling device according to the above preferred
embodiments are discussed below.
[0129] 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.
[0130] 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.
[0131] 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.
[0132] 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.
[0133] 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.
[0134] Also for these reasons, it is preferable that the hole 22 be
disposed only at the central portion of the blade 21.
[0135] 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.
[0136] 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.sub.A, Air Resistance in x.varies.f.sup.2h.sup.2r.sub.A and
the one in which air resistance at each cross section is integrated
over the length of the blade is air resistance of the whole.
[0137] It is noted that [Cross-section Ratio]=([Blade Width]-[Hole
Width])/[Distance between Heat Dissipating Fins].
[0138] 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.
[0139] 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.
[0140] 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.
[0141] 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.
[0142] 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).
[0143] 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.
[0144] 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.
[0145] 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.
[0146] 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.
[0147] 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.
[0148] 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.
* * * * *