GB2534161B

GB2534161B - Heat Transfer apparatus and method

Info

Publication number: GB2534161B
Application number: GB1500578.8A
Authority: GB
Inventors: Gilmour Sean
Original assignee: PUBBLE HILL Ltd
Current assignee: PUBBLE HILL Ltd
Priority date: 2015-01-14
Filing date: 2015-01-14
Publication date: 2019-09-18
Anticipated expiration: 2035-01-14
Also published as: GB2534161A; GB201500578D0

Description

Heat Transfer Apparatus and Method

This invention relates to an improved heat transfer method and apparatus forcooling a surface, such as a heat sink.

Electronic components, such as microprocessors, generate heat during operation.It is necessary to dissipate this heat to ensure efficient operation of such electroniccomponents and to prevent damage to the components. As electronic componentsare made smaller and more densely packed within devices the need for efficientcooling becomes more critical. It is common to provide electronic components withheat sinks in the form of thermally conductive members having relatively largesurface areas in order to dissipate heat to a heat transfer fluid, such as air or water.To enhance heat transfer from such heat sinks it is known to use fans to create aflow of heat transfer fluid over the surfaces of such heat sinks. However, such fansmay generate noise, have relatively high power consumption and can be bulky. Anobject of the present invention is to provide a more efficient way of enhancing heattransfer from a surface to be cooled, such as a heat sink.

According to a first aspect of the present invention there is provided a heat transferapparatus as claimed in claim 1.

The means for propagating said acoustic wave may comprise a transducer, speakeror any other acoustic device capable of propagating an acoustic wave.

Preferably the surface to be cooled defines an acoustically reflective surface.Alternatively an acoustically reflective surface may be located between the meansfor propagating an acoustic wave and the surface to be cooled.

When a surface is moved forward a high pressure area is created in front of thesurface while a low pressure area is created at the back of the surface. Thedifference in pressure between the front of the surface and the back of the surfacecauses vortices to be formed at the edge of the surface. When the surface is movedbackward the reverse occurs continually creating vortices at the edge of thesurface.

In one embodiment said means for propagating an acoustic wave comprises anultrasonic transducer.

The geometry of the horn may be adapted to control the size, position and/or shapeof the vortices.

In one embodiment said means for propagating an acoustic wave is adapted topropagate a standing wave between the transducer and the surface to be cooled.Preferably the distance between the means for propagating an acoustic wave andthe surface to be cooled is a function of the wavelength of the standing wave. In oneembodiment the distance between the means for propagating an acoustic wave andthe surface to be cooled may be half the wavelength of the standing wave.

The position of the means for propagating an acoustic wave within the cylinder maybe selected to influence the position, intensity, shape and size of the vortices. Anopen end of the cylinder may be spaced from the surface to be cooled by apredetermined distance. The shape and size of the open end of the cylinder maybe adapted to affect the position, intensity, shape and/or size of the vortices. In thecase where the horn is not circular, this could be a different shape and notnecessarily one that is the same shape as the speaker.

In one embodiment the shape of the surface to be cooled may be adapted to affectthe size and position of the vortices, for example, the surface to be cooled may beconcave.

At least one guide shield may be located adjacent the surface in a position toconstrain the size of said vortices generated by the means for propagating anacoustic wave. The at least one guide shield may comprise an annular bodyadapted to bound a substantially toroidal space therewithin.

According to a further aspect of the present invention there is provided a method ofcooling a surface as claimed in claim 16.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which :-

Figure 1 is a schematic diagram of an acoustic fan in accordance with anembodiment of the present invention.

Figure 2 is a schematic view of a modified acoustic fan in accordance with a furtherembodiment of the present invention; and

Figure 3 is a schematic view of an acoustic fan in accordance with a furtherembodiment of the present invention.

As illustrated in Figure 1, an acoustic fan 2 in accordance with an embodiment ofthe present invention comprises a piezoelectric transducer 6 upon which is mountedan acoustic horn 8 for propagating an acoustic wave through a heat transfer fluid,more particularly air (although it is envisaged that such wave may be propagatedthrough other heat transfer fluids, such as water). The outer rim or edge of the horn8 may be circular or may be any other shape. A surface to be cooled 10 defines an acoustically reflective surface located at apredetermined distance from the transducer 4. Alternatively, it is envisaged that anacoustically reflective surface may be located between the transducer 4 and thesurface to be cooled 10.

The transducer 4 is adapted to generate an acoustic wave perpendicular to thesurface to be cooled 10. The transducer 4 vibrates the acoustic horn 6,compressing air on one side of the horn 6 while expanding air on the other side andvisa versa as the transducer flexes. Since there is no acoustic isolation between thehigh and low pressure areas of the transducer, air moves to equalise pressure atthe outer edge of the horn. This generates vortices A,B adjacent the surface to becooled 10. Such vortices can entrain a flow of fluid across the surface to be cooled,enhancing heat transfer from such surface.

In one embodiment the transducer 4 may be adapted to generate a standing wavebetween the horn 6 and the surface to be cooled 10, such standing wave having awavelength equal to twice the distance between the transducer 4 and the surface tobe cooled 10. If a standing wave is present, the pressure difference between thefront and rear sides of the horn 6 is greater, causing more intense vortices. A streaming effect caused by the transducer 4 can also cause air movement at thesurface to be cooled 10. Vortices caused by acoustic streaming as well as theacoustic streaming itself can contribute to the cooling effect at 10.

Using an open type piezoelectric ultrasonic transducer, wherein a horn is mountedon a vibrating plate, has the effect of causing streaming in different directions andcauses pressure differences at different locations within the piezoelectric ultrasonictransducer. These interactions are significantly attenuated when a transducer isencapsulated in the standard casing such as that in which open transducers aretypically provided. However, when the transducer is not enclosed, the acousticstreaming interactions and pressure differences produced are significantlyenhanced when compared to those seen where a flat vibrating plate is used. Suchvortices are more intense when a standing wave is created between the vibrating surface and an acoustically reflective surface. The inventor has realised that suchvortices can be used to enhance cooling of a surface to be cooled.

Acoustic cooling devices exist where objects to be cooled are placed in the area ofa standing wave that has most air movement. This requires an acousticallyreflective surface and an object to be cooled to be placed within the acoustic field.There are also implications as to the size of the object to be cooled and thewavelength of sound and size of the reflective surface making this a less desirablesolution.

In an acoustic fan in accordance with the present invention, the acousticallyreflective surface is also the surface to be cooled. This reduces the number ofcomponents required to provide acoustic cooling.

Traditional fans blow turbulent air in one direction, typically against a hot surface orover hot components. The acoustic fan creates fast moving vortices that mix heatfrom a hot surface into the air. Traditional fans have moving parts. The acoustic fandoes not. Unlike traditional fans, the acoustic fan does not generate any audiblenoise. Acoustic fans are more suitable for miniaturisation.

As shown in Figure 2, in a further embodiment the transducer 4 of the acoustic fan2A may be at least partially inserted into an open-ended cylinder or tube 12. Byadjusting the position of the transducer 4 within the cylinder 12 and/or adjusting thedistance of an open end of the cylinder 12 from the surface to be cooled 10 theposition and size of the vortices A,B may be altered in order to optimise the flow ofheat transfer fluid across the surface to be cooled 10. The outer edges of thecylinder 12 may force the vortices A,B closer to the surface to be cooled 10 and/orcloser to the centre. This can significantly improve cooling. Changing the shape andsize of the opening of the cylinder 12 or the shape of profile of the cylinder 12 itselfcan also affect the position, intensity, shape and size of the vortices A,B. A shown in Figure 3, in a further embodiment the geometry of the surface to becooled 10 of the acoustic fan 2B may influence the position, intensity, shape and size of the vortices A,B and thus may optimise the flow of heat transfer fluid acrossthe surface to be cooled 10.

The geometry of the horn 6 of the transducer 4 may also influence the position,intensity, shape and size of the vortices A,B. Similarly the geometry of thetransducer 4 with respect to the horn 6 may cause the vortices A,B to changeposition, intensity, shape and size.

The vortices A,B may be enclosed in a manner that will entrain and cause air to bemoved in a linear, directional manner. In one embodiment (not shown) at least oneguide shield may be located adjacent the surface in a position to constrain the sizeof said vortices generated by said acoustic wave. The at least one guide shieldmay comprise an annular body, or body of some other geometry, adapted to bounda substantially toroidal space therewithin.

Encasing the created vortices A,B into various ducts may cause additional air to beentrained into the system. In this configuration, a net airflow could be achieved. Thismay create a situation where the acoustic fan could blow air in a single direction likeconventional fans. Furthermore, encasing the created vortices A,B into variousducts may cause vortex shedding. In this configuration, the air within the vorticeswill be exchanged at a greater rate. This may enhance the cooling effect.

If the amplitude of the signal driving the transducer is too high, it may cause thevortices to collapse, driving too low a signal into the transducer will prevent thevortices from forming at a useful intensity. The appropriate drive signal will varyfrom device to device.

The ideal distance between the transducer 4 and the surface to be cooled 10 isapproximately half the wavelength of the acoustic wave generated by thetransducer 3 to ensure that a standing wave is generated between the transducer 4and the surface to be cooled 10. The wavelength of a 40kHz sound wave in air atnominal pressure and temperature is approximately 8.575mm. The ideal position toplace the transducer 4 in this case is 4.29mm away from the surface to be cooled10 (to create a standing wave).

These techniques can be used in liquids in the same way they can be used in air.

This technique may be used to move other mechanical bodies that may move air ina linear, directional manner.

The invention is not limited to the embodiment(s) described herein but can beamended or modified without departing from the scope of the present invention.

Claims

1. A heat transfer apparatus comprising a surface to be cooled by a heat transferfluid, means for propagating an acoustic wave through the heat transfer fluid in adirection substantially perpendicular to said surface, wherein the means forpropagating an acoustic wave includes a horn mounted on a driver, whereinvortices are generated in the heat transfer fluid at an outer peripheral edge of suchhorn by the interaction of high and low pressure areas behind and in front of thehorn, or wherein the means for propagating an acoustic wave is located within anopen ended cylinder or tube such that said vortices are produced or propagatedfrom at an open end of said cylinder, said vortices leading to a flow of said fluid oversaid surface, enhancing heat transfer between the surface and the heat transferfluid.

2. An apparatus as claimed in claim 1, wherein said means for propagating saidacoustic wave comprises a transducer, speaker or any other acoustic devicecapable of propagating an acoustic wave.

3. An apparatus as claimed in claim 1 or claim 2, wherein the surface to be cooleddefines an acoustically reflective surface.

4. An apparatus as claimed in any preceding claim, wherein said means forpropagating an acoustic wave comprises an ultrasonic transducer.

5. An apparatus as claimed in any preceding claim, wherein the geometry of thehorn is adapted to control the size, position and/or shape of the vortices.

6. An apparatus as claimed in any preceding claim, wherein said means for propagating an acoustic wave is adapted to propagate a standing wave betweenthe driver and the surface to be cooled.

7. An apparatus as claimed in claim 6, wherein the distance between the means forpropagating an acoustic wave and the surface to be cooled is a function of thewavelength of the standing wave.

8. An apparatus as claimed in claim 7, wherein the distance between the means forpropagating an acoustic wave and the surface to be cooled is half the wavelength ofthe standing wave.

9. An apparatus as claimed in claim 1, wherein the position of the means forpropagating an acoustic wave within the cylinder is selected to influence theposition, intensity, shape and size of the vortices.

10. An apparatus as claimed in claim 1 or claim 9, wherein an open end of thecylinder is spaced from the surface to be cooled by a predetermined distance.

11. An apparatus as claimed in claim 1, claim 9 or claim 10, wherein the shape andsize of the open end of the cylinder is adapted to affect the position, intensity, shapeand/or size of the vortices.

12. An apparatus as claimed in any preceding claim, wherein the shape of thesurface to be cooled is adapted to affect the size and position of the vortices.

13. An apparatus as claimed in claim 12, wherein the surface to be cooled isconcave.

14. An apparatus as claimed in any preceding claim, wherein at least one guideshield is located adjacent the surface in a position to constrain the size of saidvortices generated by the means for propagating an acoustic wave.

15. An apparatus as claimed in claim 14, wherein the at least one guide shieldcomprises an annular body adapted to bound a substantially toroidal space therewithin.

16. A method of cooling a surface comprising propagating an acoustic wave in aheat transfer fluid in a direction substantially perpendicular to said surface such thatvortices are generated in said fluid, said vortices leading to a flow of said fluid oversaid surface, enhancing heat transfer between the surface and the heat transferfluid, wherein the means for propagating an acoustic wave includes a horn mountedon a driver, wherein said vortices are generated at an outer peripheral edge of suchhorn by the interaction of high and low pressure areas behind and in front of thehorn.