GB2534161A

GB2534161A - Heat Transfer apparatus and method

Info

Publication number: GB2534161A
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: 2016-07-20
Anticipated expiration: 2035-01-14
Also published as: GB2534161B; GB201500578D0

Abstract

The heatsink comprises an ultrasonic transducer 6 for propagating acoustic waves through a coolant. Vortices and a fluid flow are generated in the coolant by the acoustic waves to transfer heat from the surface 10 to be cooled. The acoustic wave generator may be positioned to propagate a standing wave between it and the hot surface. The position, intensity, shape and size of the vortices may be controlled by a horn or an open cylinder 12 mounted on the acoustic transducer.

Description

Heat Transfer Apparatus and Method This invention relates to an improved heat transfer method and apparatus for cooling 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 electronic components and to prevent damage to the components. As electronic components are made smaller and more densely packed within devices the need for efficient cooling becomes more critical. It is common to provide electronic components with heat sinks in the form of thermally conductive members having relatively large surface 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 a flow of heat transfer fluid over the surfaces of such heat sinks. However, such fans may generate noise, have relatively high power consumption and can be bulky. An object of the present invention is to provide a more efficient way of enhancing heat transfer 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 transfer 20 apparatus comprising a surface to be cooled by a heat transfer fluid, means for propagating an acoustic wave through the heat transfer fluid in a direction substantially perpendicular to said surface, wherein said transducer is adapted to generate vortices in said fluid, said vortices leading to a flow of said fluid over said surface, enhancing heat transfer between the surface and the heat transfer fluid. 25 The means for propagating said acoustic wave may comprise a transducer, speaker or any other acoustic device capable of propagating an acoustic wave.

Preferably the surface to be cooled defines an acoustically reflective surface. 30 Alternatively an acoustically reflective surface may be located between the means for propagating an acoustic wave and the surface to be cooled.

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

Preferably the means for propagating an acoustic wave includes a horn mounted on a driver, wherein said vortices are generated at an outer peripheral edge of such horn by the interaction of high and low pressure areas behind and in front of the 5 horn.

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

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

In one embodiment the means for propagating an acoustic wave is located within an open ended cylinder or tube such that said vortices are produced or propagated from at an open end of said cylinder. The position of the means for propagating an acoustic wave within the cylinder may be selected to influence the position, intensity, shape and size of the vortices. An open end of the cylinder may be spaced from the surface to be cooled by a predetermined distance. The shape and size of the open end of the cylinder may be adapted to affect the position, intensity, shape and/or size of the vortices. In the case where the horn is not circular, this could be a different shape and not necessarily one that is the same shape as the speaker.

In one embodiment the shape of the surface to be cooled may be adapted to affect the size and position of the vortices, for example, the surface to be cooled may be 30 concave.

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

According to a further aspect of the present invention there is provided a method of cooling a surface comprising propagating an acoustic wave in a heat transfer fluid in a direction substantially perpendicular to said surface such that vortices are generated in said fluid, said vortices leading to a flow of said fluid over said surface, enhancing heat transfer between the surface and the heat transfer fluid.

Preferably the means for propagating an acoustic wave includes a horn mounted on a driver, wherein said vortices are generated at an outer peripheral edge of such horn by the interaction of high and low pressure areas behind and in front of the horn.

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 an embodiment of the present invention.

Figure 2 is a schematic view of a modified acoustic fan in accordance with a further embodiment of the present invention; and Figure 3 is a schematic view of an acoustic fan in accordance with a further 25 embodiment of the present invention.

As illustrated in Figure 1, an acoustic fan 2 in accordance with an embodiment of the present invention comprises a piezoelectric transducer 4 upon which is mounted an acoustic horn 6 for propagating an acoustic wave through a heat transfer fluid, more particularly air (although it is envisaged that such wave may be propagated through other heat transfer fluids, such as water). The outer rim or edge of the horn 6 may be circular or may be any other shape.

A surface to be cooled 10 defines an acoustically reflective surface located at a predetermined distance from the transducer 4. Alternatively, it is envisaged that an acoustically reflective surface may be located between the transducer 4 and the surface to be cooled 10.

The transducer 4 is adapted to generate an acoustic wave perpendicular to the surface 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 and visa versa as the transducer flexes. Since there is no acoustic isolation between the high and low pressure areas of the transducer, air moves to equalise pressure at the outer edge of the horn. This generates vortices A,B adjacent the surface to be cooled 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 wave between the horn 6 and the surface to be cooled 10, such standing wave having a wavelength equal to twice the distance between the transducer 4 and the surface to be cooled 10. If a standing wave is present, the pressure difference between the front 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 the surface to be cooled 10. Vortices caused by acoustic streaming as well as the acoustic streaming itself can contribute to the cooling effect at 10.

Using an open type piezoelectric ultrasonic transducer, wherein a horn is mounted on a vibrating plate, has the effect of causing streaming in different directions and causes pressure differences at different locations within the piezoelectric ultrasonic transducer. These interactions are significantly attenuated when a transducer is encapsulated in the standard casing such as that in which open transducers are typically provided. However, when the transducer is not enclosed, the acoustic streaming interactions and pressure differences produced are significantly enhanced when compared to those seen where a flat vibrating plate is used. Such vortices are more intense when a standing wave is created between the vibrating surface and an acoustically reflective surface. The inventor has realised that such vortices 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 of a standing wave that has most air movement. This requires an acoustically reflective 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 the wavelength of sound and size of the reflective surface making this a less desirable solution.

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

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

As shown in Figure 2, in a further embodiment the transducer 4 of the acoustic fan 2A may be at least partially inserted into an open-ended cylinder or tube 12. By adjusting the position of the transducer 4 within the cylinder 12 and/or adjusting the distance of an open end of the cylinder 12 from the surface to be cooled 10 the position and size of the vortices A,B may be altered in order to optimise the flow of heat transfer fluid across the surface to be cooled 10. The outer edges of the cylinder 12 may force the vortices A,B closer to the surface to be cooled 10 and/or closer to the centre. This can significantly improve cooling. Changing the shape and size of the opening of the cylinder 12 or the shape of profile of the cylinder 12 itself can 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 be cooled 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 across the surface to be cooled 10.

The geometry of the horn 6 of the transducer 4 may also influence the position, 5 intensity, shape and size of the vortices A,B. Similarly the geometry of the transducer 4 with respect to the horn 6 may cause the vortices A,B to change position, intensity, shape and size.

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

Encasing the created vortices A,B into various ducts may cause additional air to be entrained into the system. In this configuration, a net airflow could be achieved. This may create a situation where the acoustic fan could blow air in a single direction like conventional fans. Furthermore, encasing the created vortices A,B into various ducts may cause vortex shedding. In this configuration, the air within the vortices will 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 the vortices to collapse, driving too low a signal into the transducer will prevent the 25 vortices from forming at a useful intensity. The appropriate drive signal will vary from device to device.

The ideal distance between the transducer 4 and the surface to be cooled 10 is approximately half the wavelength of the acoustic wave generated by the transducer 3 to ensure that a standing wave is generated between the transducer 4 and the surface to be cooled 10. The wavelength of a 40kHz sound wave in air at nominal pressure and temperature is approximately 8.575mm. The ideal position to place the transducer 4 in this case is 4.29mm away from the surface to be cooled 10 (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 in 5 a linear, directional manner.

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

Claims

Claims 1. A heat transfer apparatus comprising a surface to be cooled by a heat transfer fluid, means for propagating an acoustic wave through the heat transfer fluid in a direction substantially perpendicular to said surface, wherein said transducer is adapted to generate vortices in said fluid, said vortices leading to a flow of said fluid over said surface, enhancing heat transfer between the surface and the heat transfer fluid.
2. An apparatus as claimed in claim 1, wherein said means for propagating said acoustic wave comprises a transducer, speaker or any other acoustic device capable of propagating an acoustic wave.
3. An apparatus as claimed in claim 1 or claim 2, wherein the surface to be cooled 15 defines an acoustically reflective surface.
4. An apparatus as claimed in any preceding claim, wherein said means for propagating an acoustic wave comprises an ultrasonic transducer.
5. An apparatus as claimed in any preceding claim, wherein the means for propagating an acoustic wave includes a horn mounted on a driver, wherein said vortices are generated at an outer peripheral edge of such horn by the interaction of high and low pressure areas behind and in front of the horn.
6. An apparatus as claimed in claim 5, wherein the geometry of the horn is adapted to control the size, position and/or shape of the vortices.
7. An apparatus as claimed in any preceding claim, wherein said means for propagating an acoustic wave is adapted to propagate a standing wave between 30 the transducer and the surface to be cooled.
8. An apparatus as claimed in claim 7, wherein the distance between the means for propagating an acoustic wave and the surface to be cooled is a function of the wavelength of the standing wave.
9. An apparatus as claimed in claim 8, wherein the distance between the means for propagating an acoustic wave and the surface to be cooled is half the wavelength of the standing wave.
10. An apparatus as claimed in any preceding claim, wherein the means for propagating an acoustic wave is located within an open ended cylinder or tube such that said vortices are produced or propagated from at an open end of said cylinder.
11. An apparatus as claimed in claim 10, wherein the position of the means for propagating an acoustic wave within the cylinder is selected to influence the position, intensity, shape and size of the vortices.
12. An apparatus as claimed in claim 10 or claim 11, wherein an open end of the 15 cylinder is spaced from the surface to be cooled by a predetermined distance.
13. An apparatus as claimed in of claims 10 to 12, wherein the shape and size of the open end of the cylinder is adapted to affect the position, intensity, shape and/or size of the vortices.
14. An apparatus as claimed in any preceding claim, wherein the shape of the surface to be cooled is adapted to affect the size and position of the vortices.
15. An apparatus as claimed in claim 13, wherein the surface to be cooled is 25 concave.
16. An apparatus as claimed in any preceding claim, wherein at least one guide shield is located adjacent the surface in a position to constrain the size of said vortices generated by the means for propagating an acoustic wave.
17. An apparatus as claimed in claim 16, wherein the at least one guide shield comprises an annular body adapted to bound a substantially toroidal space therewithin.
18. A method of cooling a surface comprising propagating an acoustic wave in a heat transfer fluid in a direction substantially perpendicular to said surface such that vortices are generated in said fluid, said vortices leading to a flow of said fluid over said surface, enhancing heat transfer between the surface and the heat transfe 5 fluid.
19. A method as claimed in claim 17, wherein the means for propagating an acoustic wave includes a horn mounted on a driver, wherein said vortices are generated at an outer peripheral edge of such horn by the interaction of high and 10 low pressure areas behind and in front of the horn.