Piezofan and heat sink system for enhanced heat transfer

An electronic device having enhanced heat dissipation capabilities includes an electronic device, a heat sink, a channel, a piezoelectric element, and a blade. The heat sink is in thermal communication with the electronic device. The channel includes an inlet, an outlet and a constriction disposed along the channel between the inlet and the outlet. The heat sink defines at least a portion of the channel. The blade includes a free end and an attached end. The blade is disposed in the channel and connected to the piezoelectric element. The piezoelectric element is activated to move the blade side to side in the channel to create air vortices. The constriction in the channel and the blade cooperate with one another such that a vortex that is generated as the blade moves toward a first side of the channel is compressed against the first side of the channel and expelled towards the outlet of the channel.

BACKGROUND

Piezoelectric fans operate as a vortex shedding device. U.S. Pat. No. 4,498,851 nicely describes vortex shedding as a process where air is prevented from being sucked around a piezoelectric fan blade tip when its motion reverses. Vortex shedding is based on the fact that air displaced from the front of a moving blade rotates so rapidly that the air is unable to reverse its direction of rotation when the blade reverses its motion. If the rotation is not sufficiently rapid, the vortex can reverse its direction of rotation to be sucked around the blade tip instead of leaving the blade.

The vortex shedding action is illustrated inFIGS. 1A-1I. InFIG. 1A, a blade10of a piezoelectric fan is centered and moving upward at maximum velocity as indicated by arrow12, and air is being sucked downward around the blade tip as indicated by arrow14. While this is happening, a previously shed vortex16is moving to the right below a center line18of the blade (the center line being when the blade10is at rest). InFIG. 1B, the blade10is beginning to curve upward at about one quarter amplitude. The air is being sucked around the blade tip into a vacuum on the back (lower per the orientation inFIG. 1B) side of blade10and the new vortex14ais beginning to form while the old vortex16is moving farther to the right. The blade10nears an upper (per the orientation inFIG. 1C) end of its travel inFIG. 1C, leaving a fully formed vortex14bin its wake, with vortex16still moving outwardly.

InFIG. 1D, blade10has reached its full upward excursion and it has stopped moving and is about to reverse with the fully formed vortex14bstill in its wake and the previously formed vortex16still moving to the right. The blade10then starts downwardly again inFIG. 1E. The vortex14bis rotating too rapidly to reverse this motion and it is therefore expelled from the blade area by the new airflow around the blade10. The new airflow20is moving up around the tip of the blade10towards its wake, while the blade is moving in the direction as shown by arrow22. Upward flow20continues to gain speed as air flows into the vacuum behind (upper per the orientation inFIG. 1F) the blade and the previous vortex14bis now clear of the blade wake and gaining speed. The blade10accelerates towards its center position inFIG. 1Gwhile the air flowing into its wake indicated by arrow20is developing a new vortex. InFIG. 1H, with the blade10centered and moving downward at maximum velocity as indicated by arrow22, the air being drawn into the vacuum of the wake has developed into a full vortex20b. Finally, inFIG. 1Ithe blade10is moved further downward, feeding more air into vortex20bin its wake. The two previous vortices14band16are moved toward the right, rotating in opposite directions, one above the center line18the other below the center line18of blade10. In this way, a line of oppositely rotating vortices is generated resulting in a highly directional stream of air.

U.S. Pat. No. 4,498,851 indicates that if the vortex shedding effect is disturbed by obstructions in the area, then the air flows from the forward surface of the blade around its trailing edge to the rearward surface of the blade when the motion of the blade reverses. Accordingly, there is only circulation around the trailing edge of the blade and very little outward flow.

In some instances it is, however, it is desirable to provide ducts or channels, i.e. obstructions according to U.S. Pat. No. 4,498,851, to direct the air flow. This may be desirable when certain components are to be cooled by the piezoelectric fan. U.S. Pat. No. 4,498,851 does not provide any teaching for directing air flow generated by a piezoelectric fan where ducts and channels are desired.

BRIEF DESCRIPTION

An assembly having enhanced heat dissipation capabilities includes an electronic device, a heat sink, a channel, a fan blade, a piezoelectric element, and a constrictive member. The heat sink is in thermal communication with the electronic device. The heat sink defines a base surface. The base surface of the heat sink at least partially defines the channel. The fan blade is disposed in the channel. The blade is spaced from the base surface of the heat sink and disposed generally perpendicular to the base surface. The blade includes first and second planar surfaces. The piezoelectric element attaches to the blade. The piezoelectric element is activated to cause the blade to oscillate and generate an air flow path in the channel in which air travels generally in a direction from an attached end of the blade toward a free end of the blade. The constrictive member extends into the channel generally towards at least one of the planar surfaces of the blade between the free end and the attached end of the blade.

An electronic device having enhanced heat dissipation capabilities includes an LED device, a heat sink, a channel, a piezoelectric element, and a blade. The heat sink is in thermal communication with the LED device. The channel includes an inlet, an outlet and a constriction disposed along the channel between the inlet and the outlet. The heat sink defines at least a portion of the channel. The blade includes a free end and an attached end. The blade is disposed in the channel and connected to the piezoelectric element. The piezoelectric element is activated to move the blade side to side in the channel to create air vortices. The constriction in the channel and the blade cooperate with one another such that a vortex that is generated as the blade moves toward a first side of the channel is compressed against the first side of the channel and expelled towards the outlet of the channel.

A method for cooling an electronic device includes the following steps: placing a heat sink in thermal communication with an electronic device; oscillating a fan blade adjacent to the heat sink to generate an air vortex over the heat sink; and compressing the air vortex against a surface. The surface is configured to urge the vortex further downstream as the vortex is being compressed against the surface.

DETAILED DESCRIPTION

FIGS. 2A-2Ddepict a blade30of a piezoelectric fan disposed in a channel32defined by a first side wall34, a second side wall36and a base wall (not numbered) that the side walls extend upwardly from. The blade is driven by a piezoelectric element (not shown), which will be described later. InFIG. 2A, the blade30of the piezoelectric fan is centered and moving upward as indicated by arrow42, and air is being sucked toward the second wall36around the blade tip as indicated by arrow44. The blade30nears its maximum stroke of its travel inFIG. 2B, leaving a nearly fully formed vortex44ain its wake. The blade30then starts downwardly again inFIG. 2Cas indicated by arrow46. A fully formed vortex44cis compressed against a constriction (formed by a constrictive member48extending into the channel32from the second side wall36) and is expelled from an outlet52of the channel as seen inFIG. 2Das the blade30continues to move toward the second side wall36. The constrictive member48is shown attached to the second side wall36; however, the constrictive member can simply extend upwardly into the channel32from the base or the constrictive member may depend downwardly from a lid that at least partially covers the channel. An example of a lid will be described in more detail below.

In the embodiment depicted inFIGS. 2A-2D, one outlet52is defined between a baffle54and the second side wall36. An additional outlet56, which can operate as an inlet (the first mentioned outlet52can also operate as an inlet) is defined between the baffle54and the first side wall34. The baffle can also depend downwardly from a lid that at least partially covers the channel. The vortex44ais shaped in the channel32to increase the velocity of the air leaving the channel, which allows more heat to escape from the channel. The constriction reduces the cross-sectional area (Ac) of the channel at the constriction as compared to the cross-sectional area of the channel both upstream of and downstream from the constriction. The baffle54further limits the cross-sectional area of the channel where the baffle is located (Ao). Because of the conservation of momentum and that the air is not traveling quickly enough to be compressed, the velocity of the air moving through the outlet52is much quicker than if the baffle54were not present. Nevertheless, if desired the baffle54need not be present. The constriction in the channel32precludes the air vortex from moving further to the left (as per the orientation ofFIGS. 2A-2D), thus avoiding the problem of recirculation with very little outward flow as discussed in U.S. Pat. No. 4,498,851.

With reference toFIG. 3, a device100having enhanced heat transfer capabilities includes a heat sink102, an electronic device104(or a plurality of electronic devices) in thermal communication with the heat sink, a pair of fan blades106connected to the heat sink, and a pair of piezoelectric elements108attached to a respective blade. The heat sink102includes a plurality of walls defining a pair of channels112(FIG. 4) through which air flows to transfer heat generated by the electronic devices104. The components and configuration of each channel112depicted inFIG. 3are the same except that one channel and the elements associated with it are rotated 90° with respect to the other. The blades106can oscillate 180° out of phase with each other such that the complementary back and forth motion of the two blades106provides balancing and prevents vibration of the device100. The blades have a generally rectangular configuration having opposite planar surfaces.

The electronic devices104depicted inFIG. 3are light emitting diode devices (“LEDs”). Other electronic devices that generate heat, in addition to or in lieu of LEDs, can also be attached to the heat sink102. In the depicted embodiment, the heat sink102includes a base120. The base120includes an upper planar surface122and a lower planar surface124. Alternatively, the base120need not be planar. The LEDs104attach to the lower surface124. A thermally conductive support, such as a metal core printed circuit board, can be interposed between the LEDs104and the lower planar surface124. The circuit board, or other similar device, includes circuitry in electrical communication with a power source (not shown) to provide electricity to the LED or other electrical device.

Outer side walls126extend upwardly from the base120. Inlet end walls128also extend upwardly from the base120adjacent to an attached end of the blade106. Outlet end walls132extend upwardly from the base120adjacent to a free end of the blade106. The inlet end walls128and the outlet end walls132are generally perpendicular to both the base120and the outer side walls126. An inner wall134is positioned between each blade106and extends upwardly from the base120. The inner wall134is disposed generally parallel to each of the outer side walls126and perpendicular to the base120and the end walls128and132.

The base120and the walls126,128,132, and134generally define the channels112. For each channel112, a first opening142is defined between the inlet end wall128, the base120and the outer side wall126. For each channel112, a second opening144is defined between the internal wall134, the base120and the inlet end wall128. The first opening142and the second opening144generally act as inlets for the channel112. For each channel, a third opening146is defined between the outer side wall126, the base120and the outlet end wall132. For each channel, a fourth opening148is defined generally between the central wall134, the base120and the outlet end wall132. The third opening146and the fourth opening148act generally as outlets for the channel112. As described below, the third opening146and the fourth opening148can also act as inlets.

A plurality of fins160extend inwardly from the outer side walls126and the internal side wall134. The fins160are disposed nearer to the attached end of the blade106than the free end of the blade. A pair of angled walls162also extends into the channel112to provide a constriction to limit the cross-sectional area of the channel112in the area of the constriction. For each channel112, one of the angled walls162extends inwardly from the outer wall126and another extends inwardly from the internal wall134. The angled walls162are disposed at an obtuse angle with respect to the upstream portion of the respective wall (either outer wall126or internal wall134) to encourage vortices that contact the angled walls to be urged towards their respective outlets146and148as will be described in more detail below. In the depicted embodiment, a baffle164also extends inwardly from the outlet end wall132. The baffle164extends in a plane that is generally coplanar with the blade106when the blade is at rest, as seen inFIG. 4.

The blade106attaches to a pedestal170that extends upwardly from the base120. In the depicted embodiment, the pedestal170is disposed adjacent the inlet end wall128; however, the pedestal170can be placed elsewhere. The blade106is made of a flexible material, preferably a flexible metal. An unattached or free end of the blade106cantilevers away from the pedestal170and over the upper surface122of the base120. The blade106mounts to the pedestal170so that the blade does not contact the upper surface122of the base120. If desired, the blade can attach to the pedestal at a central location along the blade such that the blade would have two free ends.

The piezoelectric material108attaches to the blade106opposite the free end (and in the depicted embodiment adjacent to pedestal170). Alternatively, the piezoelectric material108can run the length or a portion of the length of the blade106. The piezoelectric material108comprises a ceramic material that is electrically connected to the power source (not shown) in a conventional manner. As electricity is applied to the piezoelectric material108in a first direction, the piezoelectric material expands, causing the blade106to move in one direction. Electricity is then applied in the alternate direction, causing the piezoelectric material108to contract thus moving the blade106back in the opposite direction. Alternating current causes the blade106to move back and forth continuously in the channel112. The blade106and the angled walls162are configured such that the blade does not contact the angled walls as it moves back and forth in the channel112.

During operation of the device, the LEDs104(or other heat generating device) generate heat. The LED device104includes a die (not visible) that allows conduction of the heat generated by the LED to transfer into the heat sink102. Meanwhile, an alternating current is supplied to the piezoelectric material108causing the blade106to move back and forth in the channel112, which results in a fluid (typically air) current moving generally through the channel112.

With specific reference toFIG. 4, air generally enters into the channel112through the inlet openings142and144and moves through the channel and is finally expelled through the outlet openings146and148. As per the orientation depicted inFIG. 4, air generally moves from right to left in the upper channel112and from left to right in the lower channel112. Such a configuration allows for LEDs104(or other electronic devices) to be placed in any location on the lower surface124(FIG. 3) of the base120of the heat sink102. The angled walls162extend into the channel112to provide a constriction in the channel. The area of the channel112upstream of the angled walls162can be referred to as a vortex shaping zone180. As the blades106move back and forth in the channel112, vortices are formed via the shedding action that is described with reference toFIGS. 1 and 2. The angled walls162inhibit airflow movement in a direction going from a free end of the blade106towards the attached end of the blade as depicted by arrow182(FIG. 4). The angled walls162act as a sort of nozzle that urges the vortex (as depicted by arrows182) towards the respective outlets146and148thus expelling hot air from the channel112. Because of the conservation of momentum, the smaller cross-sectional outlet openings146and148, as compared to the portion of the channel just upstream from the outlets, results in high velocity flow through the outlet openings146and148thus expelling a greater amount of hot air from the channel112more quickly than if the outlet end walls132were not provided. As most clearly seen inFIG. 4, the distal ends (innermost ends) of the angled walls162are disposed between the free end of the blade106and the attached end thus encouraging the formation of the vortex shaping zone180.

With reference to the upper channel112depicted inFIG. 4(the lower channel112would act in much the same way) as the blade106moves toward the outer side wall126, a vacuum is formed in the channel on a side of the blade106that generally faces the inner wall134. This vacuum draws air from an area of the channel112adjacent the second inlet opening144and also through the second outlet opening148, thus making the second outlet opening an additional inlet opening. Similarly, as the blade106moves towards the inner wall134, a vacuum is formed on a side of the blade that generally faces the external wall126. This vacuum draws air from an area of the channel112near the first inlet opening142and also draws air through the first outlet opening146, thus making the first outlet opening an additional inlet.

The fins160are provided nearer to the attached end of the blade106as compared to the free end. The air velocity through the portion of the channel112where the fins160are located will be generally lower than the vortex shaping area180of the channel112. Accordingly, additional heat can be dissipated from the LEDs104using the fins as additional heat dissipating members. Accordingly, the fins, as well as the walls126,128,132,162, and164can be made of a heat dissipating material to further increase the heat transfer from the LEDs104into the ambient, i.e., the area outside of the channel.

With reference toFIG. 5, an alternative embodiment of a heat dissipating electronic device200is disclosed. The electronic device200includes a heat sink202that is similar to the heat sink102described above. Electronic devices (not visible, but similar to the electronic devices disclosed above) attach to the heat sink202. A pair of blades206(similar to blades106) also connect to the heat sink. Piezoelectric material208that is driven by an alternating current attaches to the blades206so that when current is applied to the piezoelectric material the blades oscillate within channels212disposed adjacent to (and in the depicted embodiment formed integrally with) the heat sink202.

The heat sink202includes a base220having an upper surface222and a lower surface224. The electronic device is attached to the lower surface224. A pair of outer walls226extend upwardly from the upper surface222of the base220. A curved upstream barrier wall232extends upwardly from the upper surface222of the base220and is disposed upstream from a free end of each blade206. In the embodiment depicted inFIG. 5, the upstream barrier member232is generally curved following a radius of curvature that generally coincides with the radius of curvature that the free end of the blade206travels when oscillating back and forth in the channel212. An interior wall member234extends upwardly from the upper surface222of the base220generally between each of the blades206. Accordingly, the channel212is generally defined between one of the outer walls226, the upper surface222of the base220and a respective side of the interior wall member234.

Air generally travels through the channel212from an end of the channel adjacent the attached end of the blade206towards an end of the channel adjacent the free end of the blade. Each barrier member232includes wings236that extend in the same general direction (although not exactly parallel) as the outer wall226and the inner wall member234to form outlet openings238for the channel212. The outlet openings238can also act as additional inlets similar to the openings146and148described above. The barrier member232restricts the cross-sectional area of the channel212adjacent the outlet openings238as compared to a portion of the channel that is located upstream from the outlet openings. As explained above, due to the conservation of momentum, increased velocity of air can be achieved through the outlet openings thus expelling more hot air from the channel212.

A plurality of fins260extend upwardly from the upper surface222of the base220in an upstream portion of the channel222. Air traveling through the portion of the channel212that includes the fins260generally travels at a slower speed as compared to the area near the outlet openings238. Accordingly, more heat can be transferred because more surface area is provided in the area that includes the fins260.

The internal wall member234and the outer walls236are appropriately shaped to constrict the channel212in an area between the free end of the blade206and the attached end of the blade. In an embodiment depicted inFIG. 5, the exterior wall226extends inwardly at a protuberance262and the internal wall member234also extends inwardly into the channel212at a protuberance264. The protuberances262and264act as a sort of nozzle similar to the angled walls162described with reference to the embodiment disclosed inFIGS. 3 and 4. Accordingly, the protuberances act to urge air vortices formed in a vortex shaping zone280of the channel and urges the vortices out the outlets238. To further enhance heat dissipation, in addition to the heat sink202, the outer walls226, the interior wall member234, the barrier member232and the fins260can all be made from a highly thermally conductive material such as metal.

With reference toFIG. 6, a lid300can attach to the walls226and234of the heat sink. InFIG. 6, the lid300is shown only covering half of the heat sink; this is shown for reasons for clarity. The lid300, or lids, can cover the entire heat sink202. The lid can also include openings302that can provide further inlets and outlets to the channel212.

In the depicted embodiment, the lid is non-planar. The lid is non-planar in that it can include an apex304that is disposed at a distance greater from the fan blade206as compared to other portions throughout the lid. The apex304can align with the constriction that is defined by the protuberances262and264(FIG. 5). The raised area adjacent the protuberances allows for air to move upwardly (i.e., towards the lid) as the vortex is compressed against the respective wall226or234. If desired, the base220can also take a non-planar shape that is similar to that of the lid300.

An electronic device having enhanced dissipating features has been described with reference to the above-described embodiments. Modifications and alterations will occur to those upon reading and understanding the preceding detailed description. The invention is not limited to only the embodiments disclosed above. Instead, the invention is defined by the appended claims and the equivalents thereof.