Heat transfer device

Abstract

A heat transfer device 10 comprising a body defining a fluid circuit containing a ferrofluid, passing in proximity to a first surface 112 and a second surface 114, transferring heat between the surfac

Description

FIELD OF THE INVENTION This invention relates to a heat transfer device. More particularly, but not exclusively, the invention relates to a heat transfer device for use in missile systems, the device having a thermal conductivity that varies in dependence on the direction of heat transfer. BACKGROUND] There are many systems comprising complex electronic equipment that implement increasingly demanding functions and which must often operate over considerable amounts of time. As the complexity of the requirements for electronics rise, and the amount of time over which those electronics must be operated rises, the requirements for heat generated by those electronics to be appropriately managed become more significant. Many electronic components function optimally within a limited temperature range. Heat generated by their operation must be removed if these components are to remain within that limited temperature range over an extended period of time. In some applications, it is possible to use an environmental heat dump in certain operating conditions, but not in others. For example, in missile systems, it is possible in some circumstances for an outer skin or airframe to provide a heat dump for this generated heat. However, the missile skin can heat significantly when the missile moves at very high speeds. As a result the missile airframe can also heat significantly. In these conditions the skin becomes a source of heat, rather than a possible heat sink, and it can be necessary to insulate the electronics from the skin and airframe so that the components do not overheat. There arises a need for a heat transfer device that is suitable for use in a missile application, and that can enable one or both of the missile skin and airframe to be used as a heat dump when possible, whilst insulating the internal electronic components from excess heat when the missile is subject to frictional heating due to high speed travel. SUMMARY OF THE INVENTION In accordance with a first aspect of the present invention there is provided a heat transfer device for transferring heat from a first surface to a second surface, the device comprising: a body defining a fluid circuit, the fluid circuit containing a ferrofluid and being arranged to pass in proximity to the first and second surfaces; and a magnet arranged, when magnetised, to produce a magnetic field in a part of the fluid circuit; such that, when the magnet is magnetised, the ferrofluid is transported around the fluid circuit so as to transfer heat from the first surface to the second surface. Such a device can provide high thermal conduction when the magnet is magnetised, and low thermal conduction otherwise. The high thermal conduction across the device arises since the magnet, when magnetised, creates a pumping force on the ferrofluid. It can therefore be used to transfer heat under certain circumstances, and not under other circumstances. The magnet may be arranged to switch between a magnetised state and a non-magnetised state at a predetermined temperature. In such a way the device can be arranged to switch between a state in which heat is transferred, and a state in which heat is not transferred, in dependence on a predetermined temperature criterion. This is appropriate for cases where an environmental heat dump can be used but is not always available, such as in the case of a missile as described above. The magnet may be an electromagnet. An electromagnet can be readily switched between magnetised and unmagnetised states so as to control the themal conductivity of the device. The device may further comprise a temperature sensor and a controller operable to switch the magnet from the magnetised state to the non-magnetised state when the temperature sensor senses that the temperature of the device has risen above the predetermined temperature. The magnet may be a permanent magnet. For example, the magnet may be a neodymium magnet. The predetermined temperature may be the Curie temperature of the magnet. At temperatures above the Curie temperature, the magnet loses its permanent magnetism and so the pumping effect ceases. This provides a particularly simple manner in which to switch the thermal conductivity of the device between high and low thermal conductivity states. It may be particularly appropriate for missile applications. The first and second surfaces may be opposed surfaces of the body. The body may further comprise a cavity between the first and second surfaces. This limits the thermal conductivity of the body. The cavity may be evacuated. This further limits the thermal conductivity of the body. The fluid circuit may comprise a first section following an extended path proximate to the first surface, a second section following an extended path proximate to the second surface, and a link between the first and second sections. The extended paths support good heat exchange between the body of the device and the ferrofluid close to the first and second surfaces. The device may further comprise a plurality of fluid circuits, each of the fluid circuits containing a ferrofluid and being arranged to pass in proximity to the first and second surfaces, and wherein the magnet is arranged to produce a magnetic field in a part of each of the fluid circuits. A first of the fluid circuits may be generally parallel to a second of the fluid circuits. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, of which: Figure 1 is a schematic illustration of a device according to a first embodiment of the present invention; Figure 2 is a cut-away view of the device of Figure 1; Figure 3 is a further cut-away view of the device of Figure 1; Figure 4 is an exploded view of the device of Figure 1; Figure 5 is a schematic illustration of the device of Figure 1 in use on a missile; and Figure 6 is a cut-away view of a device according to a second embodiment of the invention. DETAILED DESCRIPTION A heat transfer device 100 in accordance with a first embodiment of the invention is illustrated schematically in Figures 1 to 4. As shown in Figure 1, heat transfer device 1 is in the form of a mechanical washer, being generally cylindrical with an annular cross section, so as to define a central hole 11, and having first and second generally parallel surfaces 112 and 114. First surface 112 is on the upper side of the device as shown in Figure 1, and second surface 114 is on the lower side of the device as shown in Figure 1. As will be appreciated, in use, a bolt or other mechanical fastening may pass through the central hole to provide a fastening between first and second structures, the head of the bolt applying pressure through one of the structures on one of the parallel surfaces of the device, and the other surface of the device abutting against one of the structures. In one exemplary application, the first and second structures may be a missile outer skin and a missile airframe. The heat transfer device 10 defines three fluid circuits for a ferrofluid. The fluid circuits are illustrated schematically in Figures 2 and 3, which show the heat transfer device 10 in different cut-away views. Each of the fluid circuits comprises a first section proximate to surface 112, and a second section proximate to surface 114. The first section follows an extended path proximate to the first surface, and the second section follows an extended section proximate to the second surface 114. As illustrated in Figure 3, the first sections of the paths each follow a circumferential path proximate to the first surface 112, the outer such circumferential path being labelled with reference numeral 122. The second sections similarly follows a circumferential path proximate to the second surface 114. The first and second sections are linked by relatively short sections, such as section 124, extending between the first and second sections and perpendicular to the first and second surfaces. Device 10 also comprises a magnet 120 which, when magnetised, operates to pump the ferrofluid around the fluid circuits, as described further below. Heat transfer then occurs as a result of motion of the ferrofluid around the fluid circuits. When upper surface 112 is hotter than lower surface 114, for example, and the magnet 120 is magnetised, the ferrofluid absorbs heat from the upper surface 112 in the first sections of the fluid circuits, and loses heat through lower surface 114 in the second sections of the fluid circuits. The ferrofluid may be a colloidal liquid comprising magnetic particles suspended in a carrier fluid. The magnetic particles may comprise iron or a compound containing iron, such as magnetite or hematite. The magnetic particles may comprise graphite, a combination of graphite and iron, or a combination of graphite and compound containing iron. The carrier fluid may comprise water, organic solvents, surfactants, oil, or some combination thereof. It will be appreciated by the skilled person that the ratio of carrier fluid to magnetic particles will depend on the desired thermal and / or magnetic characteristics of the ferrofluid. The magnetic particles may be nanoparticles, having dimensions around 1-100nm. The magnetic susceptibility of a ferrofluid varies with temperature. The magnetic susceptibility increases as temperature decreases. A lower temperature ferrofluid therefore as a higher magnetic susceptibility than a higher temperature ferrofluid. As a result, a ferrofluid that is subject to both a temperature gradient and a magnetic field will experience a force biasing the lower temperature ferrofluid towards regions of higher magnetic field. In the fluid circuits of device 10 these effects result in a pumping force operating to pump the ferrofluid around the fluid circuits when magnet 120 is magnetised. Magnet 120 is a neodymium magnet having a Curie temperature of approximately 80°C. Magnet 120 is positioned in the upper part of the device 10 as is shown in Figures 1 to 3, and the fluid circuits pass through the magnet. When the temperature of the upper part of the device exceeds the Curie temperature of the magnet 120 (i.e. above approximately 80°C), the magnet 120 will transition into a non-magnetised state and the pumping effect caused by the magnetic field ceases. Heat transfer as a result of fluid motion therefore also ceases. The device therefore operates to transfer heat between surfaces 112 and 114 of the device when the temperature of the upper surface is below a threshold temperature. Above the threshold temperature, heat transfer is limited to conduction through the body of device 10, and conduction through the (non-moving) ferrofluid. The device 10 is fabricated in a number of separate component parts, each of which may be fabricated separately before being assembled to form device 10. The first sections of the fluid circuits are fabricated in component 140. Component 140 receives magnet 120. Holes are drilled through magnet 120 so as to define a number of passages that form a part of the first sections of the fluid circuits. The second sections of the fluid circuits are fabricated in component 150. Components 140 and 150 are assembled with the magnet 120 and a body 160. Body 160 has the form of two concentric cylinders joined by a number of ribs such as rib 165. The ribs provide structural support to the device 10. One of the ribs, rib 168, includes ducts linking the first and second sections of the fluid circuits, thus providing the short sections of the fluid circuits running perpendicular to the generally opposed surfaces 112 and 114. Components 140 and 150 are assembled to the body to rest on the ribs, such that there is a space between them in the assembled device 10. Components 140 and 150 are fabricated of a material having a relatively high thermal conductivity. For example, components 140 and 150 may be made from a metallic material having a relatively high thermal conductivity, such as copper. A relatively higher thermal conductivity supports transfer of heat to and from the opposed external surfaces 112, 114 of the device to the ferrofluid in the fluid circuits. Components 140 and 150 may alternatively be made from titanium, stainless steel, aluminium, or aluminium alloys. Components 140 and 150 may be made from the same material, or from different materials. Body 160 may be fabricated from a material selected to have a low thermal conductivity. This reduces thermal conduction through the device itself, thus enhancing the directionality of heat transfer. Suitable materials for the body include ceramic, plastic or composite material, or a low thermal conductivity metal such as titanium or stainless steel. In some examples it may be that the body is made from a material selected for compatibility with the materials of components 140 and 150, or for compatibility with the ferrofluid, or for other reasons apart from ensuring low thermal conductivity. In such cases the thermal conductivity of the body can be kept low by evacuating the hollow space between the components 140 and 150. The body, components and magnet can be assembled together such that seals are formed between them. In this way it is ensured that the ferrofluid is preserved in the fluid circuits described above. The seals also enable a vacuum to be maintained in the cavity defined within the device. Each structure may for example be fabricated by additive manufacturing, such as by powder bed fusion or directed energy deposition, which is suited to construction of features such as the ribs of the body, and the channels formed to define the fluid circuits. Subtractive manufacturing techniques, such as milling or laser cutting, may also be used, for example to define the channels in the magnet 120 forming a part of the fluid circuits. Suitable joining techniques, such as soldering, brazing, welding, or adhesive bonding can be used in dependence on the materials used for the structures. For example, where both the body and components 140 and 150 are fabricated from titanium, the two structures can be welded or brazed together. Where the components 140 and 150 are made from a different metal, such as copper, brazing may be used. Where the body is made from a ceramic, adhesive bonding can be used to join components 140 and 150; or the ceramic may be metal plated so that the components 140 and 150 can be brazed onto the body. Magnet 120 and component 140 can be joined by brazing. In one application, the device 10 may be used as part of a thermal link between a missile electronics unit and a missile outer skin. In many circumstances, the missile outer skin can function as an effective heat dump for excess heat generated in an electronics unit. As is illustrated schematically in Figure 5, device 10 can be used as part of a mechanical fastening between the enclosure 54 of the missile electronics unit and the missile outer skin 52. Bolt 56 passes through the central hole of device 10. The head of bolt 56 abuts against the missile outer skin, whilst the opposite end of the bolt is threaded into the enclosure 54. The device thus acts as a washer. A further washer 58 is provided between the head of the bolt 56 and the outer skin 52. In one example, suitable to function as a washer for an M4 bolt (a bolt with a 4 mm diameter), the diameter of the device is 13 mm, whilst the height of the device is 4.3 mm. Either or both of bolt 56 and washer 58 are fabricated from a thermally insulating material. This prevents heat from the skin being transported through the bolt to the enclosure 54. Device 10 is positioned such that the first surface 112 faces inwardly, and is in contact with the missile outer skin 52, whilst the second surface 114 of the device 10 abuts the enclosure 54. Magnet 120 is therefore on the inner side of the device 10. In this way, effective thermal transfer is achieved from the enclosure of the electronics unit to the outer skin via device 10 when magnet 120 is magnetised, and the skin can act as a heat dump for the electronics unit. However, if the temperature of the outer skin rises above 80 °C, as can occur in high-speed flight, the temperature of magnet 120 will also rise above 80 °C as it is heated by ferrofluid conducting heat from the skin, as well as by the heat from the electronics unit. Magnet 120 is therefore demagnetised, and the ferrofluid is no longer pumped around the fluid circuits. In these circumstances ferrofluid motion stops and thermal transfer through device 10 is limited. Device 10 instead helps to insulate the electronics unit from any heat generated at the outer skin. It will be noted that, in some missile systems, the electronics unit may be fastened to an airframe which in turn is fastened to the outer skin. Typically this may be the case in larger missile systems. In such cases, the device can be positioned as part of a fastening between the outer skin and the airframe, and function in the same manner as described above, but with the airframe acting as an intermediate part in the thermal link between the outer skin and the electronics unit. Figure 6 is a schematic illustration of a device 60 that operates to transfer heat between opposing surfaces 62 and 64. Device 60 operates in a similar manner to device 10, comprising a fluid circuit for a ferrofluid, with a magnet 620 positioned to influence a part of the fluid circuit, but has a different geometry to device 10. Device 60 has the form of a thermal gap pad. Moreover, in device 60, only one fluid circuit 630 is present, with a first section (seen in Figure 6) proximate to surface 62, and a second section (not shown in Figure 6) proximate to surface 64. First section 632 and magnet are shown in cut-away form in Figure 6. First section 632 of the fluid circuit follows a serpentine path proximate to the surface 62. A serpentine path provides for an extended length of the fluid path in contact with the surface 62, which in turn enables more effective heat transfer between the ferrofluid in the fluid circuit and the surface 62. As with device 10, when the magnet 620 is magnetised, it operates to pump ferrofluid around the fluid circuit. In contrast to device 10, magnet 620 is an electromagnet. Device 60 further comprises a temperature sensor 640 positioned proximate to surface 64. Temperature sensor 640 and electromagnet 620 communicate with an external controller (not shown in Figure 6) operable to switch the current to the electromagnet 620 on and off in dependence on the temperature sensed by temperature sensor 630. In this way, a threshold temperature can be set, and the current to the electromagnet 620 switched off when the threshold temperature is reached. The thermal conductivity of device 60 can therefore be switched between a relatively high thermal conductivity mode, in which the electromagnet is magnetised, and a relatively low thermal conductivity mode, in which the electromagnet is demagnetised. A device having a form such as device may be suitable for fitting to circuit cards, either as a surface mount component, for fitting between a circuit card and an external body that can act as a heat sink in limited circumstances, or for fitting to individual circuit card components. Such devices may for example function as a thermal gap pad. Such devices will be of use where excess heat can be removed to an external heat dump that is effective under certain circumstances, but can become a heat source under other circumstances. Whilst a number of specific embodiments have been described above, it will be noted that variations and modifications to the above described embodiments will be possible and apparent to those skilled in the art. It will also be appreciated that, whilst particular dimensions have been provided for the device 10, the device can be scaled in size as appropriate for a particular application. For example, the device can be scaled to function as a washer for a bolt of 3 mm diameter (M3), a bolt of 5 mm diameter (M5), or a various other sizes of bolt. The thickness of the device may also vary. Thus, whilst in the above it has been described that the device 10 is 4.3 mm thick, it will be possible to fabricate the device to have a wide variety of thicknesses, for example in the range between 3 mm and 10 mm. The device may be fabricated to have a larger thickness, for example up to 100 mm. It will be appreciated that different numbers of fluid circuits can be used in such devices. For example in larger devices, such as a device scaled to function as a washer for an M5 bolt, it may be preferred to define four fluid circuits, or a greater number of fluid circuits in still larger devices. In smaller devices, it may be preferred to define only two fluid circuits, or to only use one fluid circuit. It will also be appreciated that, rather than defining individual fluid circuits, it will be possible to define only one fluid circuit in a device such as device 10, but to provide for the fluid circuit to follow a serpentine path as has been described with respect to device 60. Moreover, it will be possible to fabricate the device to have many different geometries, in addition to those described above. It will also be noted that, whilst for device 10 it has been described to form a part of the fluid circuit within a permanent magnet, it would be possible to 5 form the fluid circuit in a single component and provide a magnet in proximity to that component, such that the magnet generates a field affecting the a section of the fluid circuit when magnetised. Furthermore, it should be understood that, whilst it has been described in the above to place the device with the magnet close to the surface that is to be cooled (because the magnet will preferentially 10 attract colder ferrofluid), it is likely that the pumping effect will occur whenever there is a temperature gradient present in the ferrofluid. As a result the precise position of the magnet in the device is not considered critical to the operation of the device to transfer heat. Finally, it should be clearly understood that any feature described above 15 in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments.

Claims

1. A heat transfer device for transferring heat from a first surface to a second surface, the device comprising:a. a body defining a fluid circuit, the fluid circuit containing a ferrofluid and being arranged to pass in proximity to the first and second surfaces; andb. a magnet arranged, when magnetised, to produce a magnetic field in a part of the fluid circuit;such that, when the magnet is magnetised, the ferrofluid is transported around the fluid circuit so as to transfer heat from the first surface to the second surface.

2. The device of claim 1, wherein the magnet is arranged to switch between a magnetised state and a non-magnetised state at a predetermined temperature.

3. The device of claim 1 or claim 2, wherein the magnet is an electromagnet.

4. The device of claim 3 when dependent on claim 2, further comprising a temperature sensor and a controller operable to switch the magnet from the magnetised state to the non-magnetised state when the temperature sensor senses that the temperature of the device has risen above the predetermined temperature.

5. The device of claim 1 or claim 2, wherein the magnet is a permanent magnet.

6. The device of claim 5, wherein the magnet is a neodymium magnet.

7. The device of claim 5 or claim 6 when dependent on claim 2, wherein the predetermined temperature is the Curie temperature of the magnet.

8. The device of any one of the preceding claims, wherein the first and second surfaces are opposed surfaces of the body.

9. The device of claim 8, wherein the body further comprises an evacuated cavity between the first and second surfaces.

10. The device of any one of the preceding claims, wherein the fluid circuit comprises a first section following an extended path proximate to the first 5 surface, a second section following an extended path proximate to the second surface, and a link between the first and second sections.11.The device of any preceding claim, comprising a plurality of fluid circuits, each of the fluid circuits containing a ferrofluid and being arranged to pass in proximity to the first and second surfaces, and wherein the magnet is 10 arranged to produce a magnetic field in a part of each of the fluid circuits.

12. The device of claim 11, wherein a first of the fluid circuits is generally parallel to a second of the fluid circuits.s

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