A heat sink

Abstract

A heat sink 601 comprises a base 603 configured to receive heat from a heat source, and heat exchange elements 605 extending transversely from the base. Each heat exchange element includes multiple helices 607, 609, 611, 613, 615, 617 wound around one another into a multiple helix configuration. At least a portion of each helix has a multi-helical substructure 631. The multi-helical substructure may be a double helix shaped substructure, which may include a selected number of iterations of a self-similar fractal geometry. Different helices in the multiple helix configuration may have different lengths (619, 621, 623, 625, 627, 629, Fig. 7) and the length of adjacent helices may increase in a stepwise manner. Each stepwise increase in length may be accompanied by a stepwise increase the number of iterations of the self-similar fractal geometry in the portion of the helix extending beyond the length of the adjacent helix. Cylindrical crossmembers (811, Fig.14) may be provided between the helices (807,809, Fig.14) to form a DNA-like structure. The cross-members may have perforations 823, Fig.14). A heat conducting member (821, Fig.14) may be provided between the base (803, Fig.14) and each of the heat exchange elements (805, Fig.14).

Description

A HEAT SINK FIELD OF THE INVENTION The invention relates to a heat sink. In particular, it relates to a bio-inspired heat sink that is shaped and configured to optimise the rate of heat transfer to a surrounding fluid medium. BACKGROUND TO THE INVENTION A heat sink is a passive heat exchanger that transfers heat generated by an electronic or a mechanical device to a fluid medium such as air or a liquid coolant to dissipate the heat away from the device. In this manner the heat sink prevents overheating which may lead to premature failure of the device or a component thereof. Therefore, heat sinks improve the reliability and performance of the device they are incorporated into. A heat sink transfers thermal energy from a higher-temperature device to a lower-temperature fluid medium. The fluid medium is frequently air but can also be a liquid coolant such as water, a refrigerant, or oil. Heat sinks must act as a heat reservoir to absorb an amount of heat and have a temperature higher than the surroundings to transfer heat to the surroundings by convection, radiation, and conduction. According to Fourier's law of heat conduction, when there is a temperature gradient in a body, heat transfers from the higher-temperature region to the lower-temperature region. Heat sinks are designed to have a maximum surface area in contact with the fluid medium surrounding it. The performance of a heat sink depends on various factors including the material it is made of, surface treatment, its shape and configuration or fin arrangement, surrounding medium flow rate or air velocity, and attachment methods of the heat sink to the component to be cooled. To improve the thermal conduction of a heat sink it should be made of a material with high heat capacity and thermal conductivity. Gold is considered the best thermal conductor. However, due to its high cost, aluminium, aluminium alloys and copper are often used as heat sink materials. The thermal conductance can also be increased by using special geometries or configurations that optimise heat transfer to the surrounding medium. Bioinspired or nature-inspired heat sink designs often have improved performance compared to conventional heat sinks which include fins that are flat plates or cylindrical pins. For example, monolithic designs of heat sink elements with honeycomb channels are described in US patent application number, US 2013 / 0299148 A1. PCT publication number WO 2013 / 153486 A1 describes a heat sink with several airflow channels having a profile of bio-mimicry shape such as a natural sponge or natural coral shape. US patent number US 11,139,221 B1 describes a double helix pin including two spiral branches defining a double helix shape. The two branches can be joined together by one or more cross-members. The DNA-like double helix shaped pins increase the surface area and the thermal efficiency of the heat exchanger. Heat sink fins with fractal geometries have been described. US patent number US 9,228,785 B2 describes a heatsink comprising a heat exchange device having a plurality of heat exchange elements each having a surface boundary with respect to a heat transfer fluid, having a fractal variation therebetween. US patent number US 11,031,312 B2 describes a heat sink comprising a heat exchange device having a large-scale morphology and a small-scale texture over a scale range, wherein at least one of the large-scale morphology and the small-scale texture has a fractal-like self-similarity over a scale range. There remains a need for a more compact heat sink or heat exchange device with heat exchange elements that efficiently dissipate heat away from a heat source. The preceding discussion of the background to the invention is intended only to facilitate an understanding of the present invention. It should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was part of the common general knowledge in the art as at the priority date of the application. SUMMARY OF THE INVENTION In accordance with the invention there is provided a heat sink comprising a base configured to receive heat from a heat source; and heat exchange elements extending transversely from the base, each heat exchange element including multiple helices wound around one another into a multiple helix configuration, wherein at least a portion of each helix has a multihelical substructure. The multihelical substructure may be a double helix shaped substructure consisting of two spiralling branches. The spiralling branches may have the same winding ratio. The double helix shaped substructure may include a selected number of iterations of a multiscale self-similar fractal geometry, i.e., a selected number of iterations, levels or stages of a double helix shaped substructure at different size scales. The number of iterations of the self-similar fractal geometry may vary along the length of a helix. The number of iterations of the self-similar fractal geometry may increase along the length of the helix in the direction of its free end so that a portion of the helix at or near the base has a larger volume and lower surface area for heat transfer along the helix to occur mainly via conduction, whereas portions of the helix at or near the end of the helix have a relatively smaller volume but increased surface area (a higher surface area to volume ratio) for heat transfer to occur mainly via convection in use. Different helices in the multiple helix configuration may have different lengths. The length of adjacent helices in the multiple helix configuration may increase in a stepwise manner. A shortest helix in the multiple helix configuration may include the double helix substructure. Each stepwise increase in length of a helix beyond a length of an adjacent helix may be accompanied by a stepwise increase the number of iterations of the self-similar fractal geometry in the portion of the helix extending beyond the length of the adjacent helix. The heat exchange elements may include cylindrical cross-members extending between the multiple helices. The double helix shaped substructure may also include cross-members so that it has a DNA-like structure. The cross-members may be perforated to increase the surface area of the cross-members for increased heat transfer. A solid or continuous heat conducting member may be provided between the base and each of the heat exchange elements. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Figure 1 is a side view of three different heat exchange element configurations: (A) a cylindrical pin; (B) a perforated and hollow cylindrical pin and (C) a DNA-like double helix shaped heat exchange element; Figure 2 is a graph of the normalised temperature distribution over the normalised length for the three different heat exchange element configurations shown in Figure 1; Figure 3 is a three-dimensional view of a heat exchange element with arrows demonstrating heat transfer phenomena via conduction and convection; Figure 4 is a side view of a first embodiment of a heat sink; Figure 5 is a graph of the normalised rate of convection against the normalised surface area of a heat exchange element; Figure 6 is a three-dimensional view of a second embodiment of a heat sink; Figure 7 is a side view of a heat exchange element of the embodiment of Figure 6; Figure 8 is a side view of a heat exchange element of the embodiment of Figure 6 with exploded views showing multihelical and fractal substructures; Figure 9 is a schematic diagram or grid plot which illustrates the variation in the volume of a heat exchange element with an increase in the number of iterations of a fractal geometry; Figure 10 is a graph of the normalised rate of conduction against the normalised volume of a heat exchange element; Figure 11 is a graph illustrating the effect of an increasing number of fractal iterations on the normalised volume and normalised surface area of a heat exchange element; Figure 12 is a graph comparing the thermal energy distribution (temperature) along the normalised length of the heat exchange element of Figures 7 and 8 versus that of a conventional cylindrical pin; Figure 13 is side view of a third embodiment of a heat sink with a heat conducting member between the base and the heat exchange element; and Figure 14 is a side view of a fourth embodiment of a heat sink with a perforated heat exchange element. DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS A heat sink is provided that comprises a heat transmissive base or body configured to receive heat from a heat source and transfer it to heat exchange elements extending transversely from the base. The heat exchange elements may be integrally formed with the base or secured to the base in a manner that does not substantially affect heat transfer from the base to the heat exchange elements. The heat exchange elements are preferably uniformly arranged to be equidistant from one another on a planar surface of the base. Each elongate heat exchange element includes multiple helices wound around one another into a multiple helix configuration. The terms “multiple helices” or “multihelical” as used herein is meant to refer to at least two or more helices. At least a portion of a helix, preferably a portion of all the helices in the multiple helix configuration, has a multihelical substructure. The term “multihelical substructure” refers to the larger scale helix structure including a smaller-scale morphology, geometrical structure or texture in the form of two or more intertwined spiralling branches. The multihelical substructure is in essence a first stage “self-similar fractal” in that it has a similar (not necessarily identical) shape to the larger morphological multiple helix configuration of the heat exchange element, but at a smaller scale. A fractal is defined as a rough or fragmented geometric shape that can be split into parts, each of which is (at least approximately) a reduced-size copy of the whole. Therefore, there is an iterative or recursive algorithm that describes a fractal structure. When the multihelical substructure has two intertwined spiralling branches it is referred to herein as a double helix shaped substructure. The spiralling branches may have a uniform winding ratio or a non-uniform winding ratio. The non-uniform winding ratio, may for example having a midpoint radius and a base radius, wherein the midpoint radius is smaller than the base radius. The multihelical substructure portion of a helix has an increased surface area in comparison to the portion of the helix without a substructure. The rate at which heat is transferred from a surface, either through convection or through radiation, is typically related to, and increases with the surface area. Accordingly, the multihelical substructure is included to increase heat transfer via convection or radiation. Preferably, the ends of the heat exchange elements, and therefore the end portions of the multiple helices, include the muiltihelical substructure. In such an embodiment, the portion of the helix near the base consisting only of the larger scale helix structure has a larger volume and enhanced rate of conduction of heat away from the base, whereas the portion consisting of the larger scale helix carved or otherwise formed or reduced into a multihelical substructure has a smaller volume for heat transfer by conduction but a larger surface area for heat exchange to the surrounding fluid to occur via convection. The heat sink accordingly has heat exchange elements that are shaped and configured to optimise the rate of conduction along a portion thereof that is at or near the base and to optimise the rate of convection and radiation along another, preferably an end portion thereof. In an alternative embodiment, the entire length of one or more helices in the multiple helix heat exchange element consists of the multihelical substructure, preferably the double helix shaped substructure. A solid or continuous heat conducting member may be provided between the base and each of the heat exchange elements. The heat conducting member may be a rectangular- or cylindrical- shaped with a flat first side in contact with the base and an opposite flat side supporting the heat exchange element. The heat exchange element thus extends from the heat conducting member and it may be integrally formed with the heat conducting member. The heat conducting member provides a highly conductive, solid (non-perforated) interface between the base and the heat exchange element. The heat conducting member is configured to improve the heat transfer between the base and the heat transfer elements by conducting the heat current in the direction of the heat transfer elements in an optimal manner. ‘The multihelical substructure present in one or more of the helices of the heat exchange elements may have a fractal geometric structure. In one embodiment, the double helix shaped substructure or a portion thereof comprises a selected number of iterations of a multiscale self-similar fractal geometry, i.e., a selected number of iterations, levels or stages of a double helix shaped substructure at different size scales. As the number of iterations of the fractal geometry increase in a helix or a portion thereof, the volume of the helix or the portion thereof is less, but its surface area is increased. ‘The number of iterations of the self-similar fractal geometry (i.e., the multiscale fractal variation of the double helix shaped substructure) may vary along the length of a helix. Accordingly, there is a fractal variation along the length of the helices in the multiple helix heat exchange element. The number of iterations of the self-similar fractal geometry may increase along the length of the helix in the direction of its free end so that the portion of the heat exchange element at or near the base has a larger volume and lower surface area for heat transfer along the heat exchange element to occur mainly via conduction, whereas the portions at or near the end of the helix have a relatively smaller volume but increased surface area for heat transfer to occur mainly via convection in use. In some embodiments, each helix in the multiple helix configuration has a different length. In particular, the length of adjacent helices in the multiple helix configuration increase in a stepwise manner. Each stepwise increase in length of a helix beyond a length of an adjacent helix may be accompanied by a stepwise increase the number of iterations of the self-similar fractal geometry in the portion of the helix extending beyond the length of the adjacent helix. For example, the shortest helix in the multiple helix configuration may have the double helix shaped substructure. A helix adjacent the shortest helix includes a first portion near the base having a double helix shaped substructure (similar to the shortest helix), and a second end portion extending beyond the length of the shortest helix having a double helix shaped substructure which includes a further, smaller scale double helix shaped substructure (i.e., a first iteration of a self-similar fractal). With each helix a further iteration of the self-similar fractal structure is introduced into the portion the helix extending beyond the adjacent helix. For example, in an exemplary heat exchange element consisting of six intertwined helices, the first helix includes a double helix shaped substructure (stage 1 substructure). The second helix consists of two distinct portions, the first portion nearest the base has the double helix shaped substructure like the first helix, and the second portion which is the portion of the helix extending beyond the length of the first helix, includes one iteration of the double helix shaped fractal structure (stage 2 substructure). The third helix can be divided in three portions, a first portion nearest the base that includes the double helix shaped substructure, a second portion extending beyond the length of the first helix and including one iteration of the double helix shaped fractal structure (stage 2 substructure) and a third portion extending beyond the length of the second helix and including two iterations of the double helix shaped fractal structure (stage 3 substructure). This pattern repeats until the end portion of the sixth helix extending beyond the length of the fifth helix includes five iterations of the self-similar fractal, i.e., six differently scaled levels of the double helix shaped substructure (stage 6 substructure). This fractal variation along the length of each helix ensures that the overall heat transfer rate can be maintained by increasing the surface area of the heat exchange element whilst decreasing its volume in a direction away from the base. The fractal variation may be described more generally as follows in respect of a heat exchange element consisting of N helices wound around a common central axis into a multiple helix configuration. The longest helix of the N number of helices with its free end extending the furthest from the base may be defined as the first helix. The adjacent second helix has a shorter length than the first helix, the third helix adjacent the second helix has a shorter length than the second helix and so forth. The stepwise decrease in length in adjacent helices repeat with the N™ helix being the shortest helix having its free end closest to the base. In one embodiment the shortest helix (N™) has the double helix shaped substructure. The (N-1)" helix includes a first iteration of the self-similar fractal in the portion of the (N-1)™" helix that is longer than the N* helix, whilst the base portion of the (N-1)" helix has the double helix shaped substructure. The (N-2)" helix includes two iterations of the self-similar fractal in the portion of the (N-2)" helix extending beyond the (N-1)" helix, one iteration of the self-similar fractal in the portion of the (N-2)™ helix extending beyond the length of the N' helix and the double helix shaped substructure in the portion of the helix nearest the base (i.e., the base portion). The (N-3)™ helix includes three iterations of the self- similar fractal in the portion of the (N-3)™" helix extending beyond the length of the (N-2)" helix, two iterations of the self-similar fractal in the portion of the (N-3)™" helix extending beyond the (N- 1)* helix, one iteration of the self-similar fractal in the portion of the (N-3)" helix extending beyond the length of the N* helix and the double helix shaped substructure in the portion of the helix nearest the base (i.e., the base portion). This fractal pattern repeats until the first, longest helix includes N iterations of the self-similar fractal in its end portion that is longer than all the other helices. N may be an integer between 2 and 20, preferably between 2 and 10. It will be appreciated by those skilled in the art that the number of iterations of the self-similar fractal (repeats of the same morphology at increasingly smaller scales) that are possible in a heat sink will be dictated by manufacturing / fabricating capabilities when producing the heat exchange elements. The heat exchange elements may include interconnecting cylindrical strands or cylindrical cross- members extending between the multiple helices. The double helix shaped substructure may also include cross-members, giving it a DNA-like structure. The cross-members may be perforated to increase the surface area available for heat exchange. The effect of heat exchange element shape and configuration on heat transfer performance is demonstrated in Figures 1 and 2. Figure 1 shows three different heat exchange element or fin geometries mounted to a conductive base plate (101) of a heat sink. A solid cylindrical pin (103), a perforated and hollow cylindrical pin (105) and DNA-like double helix shaped heat exchange element (107) with cylindrical crossmembers (109) between the helices. The side wall (111) of the hollow cylindrical pin (105) includes perforations (113) to accelerate the heat dissipating ability of the pin (105). Figure 2 is a plot of the normalised temperature distribution along the normalised length of the elongate heat exchange elements shown in Figure 1. The top line (203) shows the temperature distribution along the length of the solid cylindrical pin (105). The middle line (205) shows the temperature distribution along the length of the perforated and hollow cylindrical pin (105). The bottom line (207) shows the temperature distribution along the length of the DNA-like double helix shaped heat exchange element (107). The plot shows that the DNA-like double helix heat exchange element (107) has a larger temperature drop along its length in comparison to the conventional cylindrical pin (103) and the hollow, perforated pin (105). The DNA-like bicinspired heat exchange element (107) has a higher heat dissipation rate and is more efficient than cylindrical heat exchange elements (103, 105). Figure 3 illustrates the phenomenon of conduction and convection in a typical heat exchange element or device. The heat exchange element (301) is in the form of a rectangular plate with rectangular cut-out perforations (303) which may be either longitudinally (vertically) or laterally (horizontally) aligned. When a bottom end (305) of the heat exchange element (301) is heated, the heat is conducted along the heat conductive heat exchange element according to the heat vector distribution shown as arrows (307). The convective heat transfer vectors are shown as the set of arrows (309). The continuous, solid portions of the heat transfer element (301) enable conductive heat transfer, whilst the perforations (303) aid in heat transfer via natural or forced convection. A first embodiment of a heat sink (401) which includes a bio-inspired double fractal DNA- structured heat sink is shown in Figure 4. The heat sink has improved heat exchange efficiency due to the larger surface area it has from the double fractal geometry. The heat sink (401) comprises a base (403) with a DNA-like double helix shaped heat exchange element (405) extending transversely from the base (403). Only one heat exchange element (405) is shown in Figure 4, but it will be appreciated that multiple heat exchange elements are to be distributed evenly over a planar surface of the base (403). The heat exchange element (405) comprises two interwound helices (407, 409) joined by cylindrical crossmembers (411) which extend laterally between the helices (407, 409). Each helix (407, 408) comprises a two-stage fractal geometry with a double helix substructure present at two different size scales shown as exploded, window views of a portion of the helix (407) in Figure 4. The first fractal stage of the helices (407, 409) is in the form of a self-similar double helix substructure (413) consisting of two spiralling, or interwound helical branches (415, 417). Each of the branches (415, 417) include a further, second self-similar double helix substructure (419) or a second stage fractal. For purpose of illustration, the smaller-scale substructures or fractal stages are only shown in the respective exploded, window views in Figure 4. It should be appreciated that the helices (407, 409) and branches (415, 417) illustrated as cylindrical tubes are actually formed from spiralling, interwound helical substructures. The fractal stages may be achieved by carving previously solid helices (407, 409) such that a double helix DNA-like substructures are generated in each of two helices in (407, 409). The “carved-out” material is removed which reduces the volume of the helix and the heat exchange element as a whole. However, the surface area of the helices (407, 409) is increased to enhance the convection heat transfer rate. The two fractal stages in the heat exchange element increases the surface area exposed to the surrounding fluid. It will be appreciated that the number of fractal iterations or stages can extended to the N'' stage to increase the convection heat transfer rate. The number of fractal stages may just be limited by contemporary manufacturing capabilities. Figure 5 demonstrates the effect of increasing the surface area (A) of a heat exchange element by including multiple fractal stages (1 to N stages) in the helices on the rate of convection. As the surface area of the heat exchange element is increased (As to An) by an increasing number of fractal stages or iterations, the rate of convective heat transfer also increases. The convective heat transfer rate is directly dependent on the normalised surface area of the fractal heat transfer element as is evident from Figure 5. An embodiment of a heat sink (601) with a plurality of heat exchange elements (605) in a multiple helix configuration is shown in Figure 6. The heat sink (601) comprises a heat transmissive base (603) in the form of a rectangular plate and uniformly spaced muilti-helix heat exchange elements (605) or fins extending transversally therefrom. The number of heat exchange elements (605) on the base plate (603) and the spacing between them may be selected depending on various factors, including the specific heat dissipating requirements of the heat sink, the type and flow rate of the surrounding fluid and the intended use of the heat sink. Each heat exchange element (605) consists of six interwound helices (607, 609, 611, 613, 615, 617). Each helix (607, 609, 611, 613, 615, 617) includes a multihelical substructure, and more particularly a multihelical substructure with a fractal geometry. It will be appreciated by those skilled in the art that the number of helices that heat exchange element consists of and the number of fractal stages in the substructure of the helices may vary depending upon the heat dissipation requirements of the heat sink. A single heat exchange element (605) on a base plate (603) is illustrated in Figures 7 and 8. The heat exchange element (605) consists of multiple helices, more particularly six helices (607, 608, 611, 613, 615, 617), wound around a common central axis, but differing by a translation along the axis. Each helix is circular, i.e., has a constant radius. The helices have the same pitch, but each helix has a different length so that they extend to different heights from the base (603). The first helix (607) is the longest helix with its free end extending a first distance (619) from the base (603). The lengths of the second helix (609), third helix (611), fourth helix (613), fifth helix (615) and sixth helix (617) decrease in similar sized steps so that the distances (621, 623, 625, 627, 629) of the respective free ends of the helices from the base (603) decrease in a stepwise manner. The sixth, shortest helix is the Nth helix is this embodiment (N = 6). The helices (607, 609, 611, 613, 615, 617) have a double helix substructure and in most helices the substructure follows a fractal pattern which is present in different stages or iterations along the length of the helix as illustrated in Figure 8. The sixth, shortest helix (617) has a double helix substructure (631). All portions of the other five helices (607, 609, 611, 613, 615) that are near the base (603) and extend to the same height or distance (629) from the base (603) as the sixth helix (617) have the same double helix substructure (631) as the sixth, shortest helix (617). The end portion of the fifth helix (615) adjacent to the sixth helix (617) and all portions of the other four helices extending beyond the height / distance (629) of the sixth helix (617) to the height or distance (627) of the fifth helix (615) from the base (603) includes a double fractal double helix substructure (a first iteration of a self-similar fractal geometry) in that the double helix substructure (631) consists of a further double helix substructure (633) at a smaller scale. This pattern repeats so that each stepwise increase in length of a helix beyond a length of an adjacent helix is accompanied by a stepwise increase the number of iterations of the self-similar fractal geometry in the portion of the helix extending beyond the length of the adjacent helix. In essence the double helix shape is repeated at increasingly smaller scales with the end portion of the first helix (607) extending beyond the length of the second helix (609) consists of the double helix shaped substructure having five iterations of the fractal geometry (which repeats at 5 different increasingly smaller size scales). The portions of the helices corresponding to the differences between the distances (619, 621, 623, 625, 627, 629) therefore follow a fractal pattern in stages with the first, longest helix having the most iterations of the fractal geometry in its end portion and the shortest, N*" helix in a multiple helix configuration having just the double helix-shaped substructure. The portion (635) of the heat exchange element (605) nearest the base (603) and extending to the distance (619) from the base (603) and in which each helix of the multiple helix configuration of N helices have a double helix shaped substructure may be referred to generally as the N* stage of the heat exchange element, where the volume of the multiple helix heat exchange element is at its maximum. There are six helices in the portion (635) of the heat exchange element (605) of the embodiment illustrated in Figures 7 and 8, the Nth stage as defined above is the sixth stage in the embodiment. The sixth stage (or generally the Nth stage) optimises the heat transfer rate in terms of conduction from base plate (603) to the heat exchange element (605) since in this stage the portion of the heat transfer element comprises the maximum number of helices and the helices have a relatively larger volume compared to the carved-out fractal geometries of the other stages of the heat exchange element. The portion (637) of the heat exchange element may be referred to as the fifth stage (or more generally the (N-1)" stage), where the volume of the portion (837) is slightly less than the sixth (or more generally the N*' stage). Similarly the portions (639, 641, 643, 645) of the heat exchange element may also be referred to as the fourth, third, second, and first stage of the heat exchange element, each stage being accompanied by a decrease in volume of the heat exchange element as it extends away from the base due to the fractal geometry described herein. The variation in volume among the different stages defined along the length of the heat exchange element can be illustrated by means of a grid plot shown in Figure 9. At the Sy stage number, the volume Vy is maximum. As the stage number decreases (further along the length of the heat exchange element from the base), the heat exchange element occupies a decreasing volume, with stage Sw.1 having a smaller volume than stage Sn, and stage Sw.z having a smaller volume than stage Sn.1 etcetera. At stage Si., the volume of the heat exchange element portion becomes as its minimum in the plot. The plot illustrates the trend of an decreasing volume of the heat exchange element with a decrease in the number of stages. Therefore, at stage one, the volume (V1) of the heat exchange element is minimum and thus the conduction rate is minimum as shown in Figure 10. As the volume of the heat exchange element decreases, the rate of conduction decreases. Accordingly, with an increase in number of stages (in the direction of the base), the volume of the heat exchange element portion is increasing, and the rate of conduction will be at its maximum at N*" stage of the fractal heat exchange element. To optimise the overall heat transfer rate of the heat exchange element (605) the effect of the decrease in volume of the heat exchange element on the rate of conduction at increasing distances from the base is counteracted by the increase in the surface area of the heat exchange element portions by way of the fractal variation along the length of the heat exchange element. The increasing surface area of the heat exchange element at increasing distances from the base along the length of the element due to the multi-stage fractals optimises convective heat transfer. Therefore, both the conductive and the convective heat transfer rates are optimised with the multiple helix fractal heat exchange elements described herein. As the number of helices and thus the number of fractal iterations or stages in a heat exchange element is increased, the efficiency of the heat sink in terms of heat dissipation increases. Figure 11 demonstrates the effect of an increasing number of fractal stages on the surface area and volume of the heat exchange element. The normalised volume (1001) decreases with an increase in the number of fractal stages, whereas the normalised surface area (1003) increases with an increase in the number of fractal stages. At the N" iteration of the fractal geometry, the surface area is maximum, but the volume is minimum. The increased surface area allows better convective heat transfer. The thermal distribution (temperature) along the length of a multi-stage fractal heat exchange element (605) as described herein is compared to that of a conventional solid cylindrical pin (103) in Figure 12. The first plot (1001) is of the normalised temperature of the cylindrical pin (103) heat exchange element against its normalised length and the second plot (1003) is of the normalised temperature of the multi-stage fractal heat exchange element (605) against its normalised length. The larger temperature difference at the end of the multi-stage fractal heat exchange element (605) demonstrates that the multi-stage fractal heat exchange element (605) performs better in terms of heat dissipation than a conventional cylindrical pin (103). To increase the contact area of the heat exchange element with the base of a heat sink, each heat exchange element may be mounted on or extend from a heat conducting member. The increased contact area allows for more efficient conductive heat transfer between the base and the heat exchange element. The heat conducting member increases the rate of conduction in the region of the heat sink near the base, whilst the heat exchange elements are shaped and configured to improve the rate of convection. Figure 13 illustrates an embodiment of a heat sink (701) which includes a heat exchange element (705) extending from a heat conducting member (721) in the form of a solid cylindrical pin. Figure 13 shows just one heat exchange element for clarity, but it will be appreciated that the heat sink includes a plurality of substantially identical elements or fins spaced across its surface. The heat conducting member (721) is between the base (703) and the heat exchange element (705). In the illustrated embodiment of Figure 13, the heat exchange element (705) has the same shape and configuration as the heat exchange element (405) described with reference to Figure 4. However, it will be appreciated that the heat conducting member may be provided between the base and any differently configured multiple helix heat exchange element as described herein, including the embodiment of a heat exchange element (605) of Figures 7 and 8, for example. A further embodiment of a heat sink (801) is shown in Figure 14 and includes a base (803) with a heat exchange element (805) having a DNA-like double helix structure and crossmembers (811) between the two helices (807, 809) of the double helix structure. A cylindrical heat conducting member (821) is between the base (803) and the heat exchange element (805). The helices (807, 809) and crossmembers (811) include perforations (823) to increase the effective surface area of the heat exchange element and accelerate the heat transfer process. The perforations (823) may be circular, elliptical, star-shaped, cross-shaped, square-shaped, rectangular, or have a DNA-like shape. The location of the perforations may be limited to the part or parts of the helices that do not include the multihelical substructure and / or fractal geometry. Advantageously, the multihelical substructure in helices of the heat exchange elements of the heat sink described herein results in improved heat exchange efficiency. The efficiency is particularly improved in embodiments that have a self-similar repeating pattern (fractal) in the helices of the heat exchange elements. Each fractal iteration serves to increase the surface area in the three-dimensional DNA-shaped fractal. A large physical surface area to volume ratio is achieved by way of the fractal design of the heat exchange elements or fins of the heat sink to improve efficiency. Further, the multihelical configuration of the heat exchange elements are elongate, essentially not taking up more space than conventional cylindrical pin shaped fins. Accordingly, the surface area and thus efficiency of the heat sink is improved relative to cylindrical pin shaped fins without expanding the geometry or size of the heat exchange elements themselves. The heat exchange elements comprising multiple helices with decreasing lengths and a fractal variation along the length of each helix has improved heat exchange efficiency whilst occupying less volume than a heat sink fin with just a fractal geometry. The foregoing description has been presented for the purpose of illustration; it is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above disclosure. The language used in the specification has been principally selected for readability and instructional purposes, and it may not have been selected to delineate or circumscribe the inventive subject matter. It is therefore intended that the scope of the invention be limited not by this detailed description, but rather by any claims that issue on an application based hereon. Accordingly, the disclosure of the embodiments of the invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims. Finally, throughout the specification and accompanying claims, unless the context requires otherwise, the word ‘comprise’ or variations such as ‘comprises’ or ‘comprising’ will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Claims

CLAIMS:

9. A heat sink comprising a base configured to receive heat from a heat source; and heat exchange elements extending transversely from the base and configured to transfer heat received from the base to a surrounding fluid, each heat exchange element including multiple helices wound around one another into an intertwined multiple helix configuration, wherein at least a portion of each helix has a multihelical substructure. 2 The heat sink as claimed in claim 1, wherein the multihelical substructure is a double helix shaped substructure.

3. The heat sink as claimed in claim 2, wherein the double helix shaped substructure includes a selected number of iterations of a self-similar fractal geometry. 4, The heat sink as claimed in any one of claims 1 to 3, wherein different helices in the multiple helix configuration have different lengths.

5. The heat sink as claimed in claim 4, wherein the length of adjacent helices in the multiple helix configuration increase in a stepwise manner.

0. The heat sink as claimed in claim 4 or claim 5, wherein a helix in the multiple helix configuration with the shortest length has the double helix shaped substructure.

7. The heat sink as claimed in any one of claims 4 to 6, wherein each stepwise increase in length of a helix beyond a length of an adjacent helix is accompanied by a stepwise increase the number of iterations of the self-similar fractal geometry in the portion of the helix extending beyond the length of the adjacent helix.

8. The heat sink as claimed in claim any one of claims 1 to 7 which includes cylindrical cross- members between the helices.

9. The heat sink as claimed in claim 8, wherein the cross-members are perforated.

10. The heat sink as claimed in any one of claims 1 to 9 which includes a heat conducting member between the base and each of the heat exchange elements.