Induction heater

Abstract

An induction heater 100 comprising an induction coil 110, an inner heat exchanger 120 and an outer heat exchanger 130. The inner heat exchanger is positioned inwardly of the induction coil, and the ou

Description

BACKGROUND Various configurations and arrangements of heater for heating an airflow are known. One example of a known heater configuration is a resistive heating element attached to a heat exchanger. SUMMARY In a general sense, an arrangement for an induction heater is described herein. The arrangement involves positioning an induction coil between at least two heat exchangers. Positioning the induction coil between the at least two heat exchangers may facilitate an increase in heating efficiency for a given size of induction heater. For example, an induction heater having the arrangements and / or configurations described herein may be able to achieve similar heating efficiencies and / or heating temperatures as known induction heaters, but with a relatively smaller size. This reduction in size, amongst other effects achieved by the induction heaters described herein, may contribute to the induction heaters described herein being suitable for use in heating airflows flowing through domestic appliances - for example, in haircare appliances such as hairdryers. There is provided herein an induction heater comprising: an induction coil; an inner heat exchanger positioned inwardly of the induction coil; and an outer heat exchanger positioned outwardly of the induction coil. The inner heat exchanger and the outer heat exchanger are inductively heatable by a magnetic field generated in response to an application of current through the induction coil, and the inner heat exchanger and the outer heat exchanger exchange heat with an airflow moving through the induction heater. The induction heater comprises a longitudinal axis about which the induction coil is coiled, and the inner heat exchanger and the outer heat exchanger each comprise a plurality of fins that extend in radial directions relative to the longitudinal axis. The inner heat exchanger and / or the outer heat exchanger comprises a support that extends circumferentially around the longitudinal axis, and the plurality of fins comprises a first set of fins that extend from the support in a direction towards the induction coil, and a second set of fins that extend from the support in a direction away from the induction coil. The application of current through the induction coil induces the generation of a magnetic field about the induction coil. The generated magnetic field penetrates both the inner heat exchanger and the outer heat exchanger and consequently induces the generation of currents (which may be referred to as ‘eddy currents’) in the respective surfaces of both the inner heat exchanger and the outer heat exchanger. As air flows over the respective surfaces of the inner heat exchanger and the outer heat exchanger, each of the inner heat exchanger and the outer heat exchanger heat the air by conduction. The induction heater may define an airflow path for air to flow therethrough. The heat exchangers of the induction heater may be positioned to be in direct contact with the air as it flows through the induction heater to improve the efficiency with which the heat exchangers heat the air. The inner heat exchanger is a thermally and electrically conductive object positioned inwardly of the induction coil. For example, the inner heat exchanger may be positioned inside a volume bounded by the induction coil. Correspondingly, the outer heat exchanger is a thermally and electrically conductive object positioned outwardly of the induction coil. For example, the outer heat exchanger may be positioned outside the volume bounded by the induction coil. In use, the various components of the induction heater may be capable of withstanding different temperature limits. For example, the induction coil may be damaged at temperatures over 200°C. Meanwhile, the inner heat exchanger and the outer heat exchanger may each be damaged only at higher temperatures, for example over 500°C. The arrangements described herein may provide an induction heater with relatively higher temperatures induced in the inner and outer heat exchangers than in the induction coil, thus improving the operational efficiency of the induction heaters described herein. By providing both an inner heat exchanger positioned inwardly of the induction coil and an outer heat exchanger positioned outwardly of the induction coil, a number of benefits may be realised. For example, by providing both an inner heat exchanger positioned inwardly of the induction coil and an outer heat exchanger positioned outwardly of the induction coil, the total surface area in which eddy currents may be induced may be increased. This corresponds to an increase in the total surface area that is heated by induction heating and, therefore, corresponds to an increase in the effective heat exchanger area of the induction heater. Additionally, by providing two heating elements that are, for example, magnetically permeable, either side of the induction coil the reluctance of the magnetic circuit (i.e., the ‘circuit’ in which magnetic fields are induced) may be reduced. Consequently, a stronger magnetic field may be generated for the same input current and, as such, a greater degree of inductive heating may be achieved than for a similar induction heater having only one heat exchanger positioned either inwardly or outwardly of the induction coil. The effective heat exchange area of the induction heater may be considered to be the total surface area of both the inner heat exchanger and the outer heat exchanger (collectively referred to herein as “the heat exchangers”) over which air may be flowed, in use, so as to heat the air flowing through the induction heater. Increasing the effective heat exchanger area may increase the efficiency with which the heat exchangers conductively heat air flowing over the respective surface areas of the heat exchangers. As such, the induction heaters described herein provide a heater that is capable of efficiently heating air flowing therethrough. Providing both an inner heat exchanger positioned inwardly of the induction coil and an outer heat exchanger positioned outwardly of the induction coil may reduce the size of induction heater needed to heat the air to a predetermined temperature. By increasing the effective heat exchange area of the induction heater (as described above), the total volume of air that can be heated by the induction heater, for a given length of induction heater, is increased. As such, the induction heaters described herein, having both an inner heat exchanger and an outer heat exchanger, may be able to heat the air to a predetermined temperature in a relatively smaller volume than induction heaters not having this arrangement. This may result in a more compact induction heater that may, therefore, be easier and / or more convenient to install in a variety of settings - such as in a domestic appliance configured to heat air flowing therethrough, e.g., a haircare appliance such as a hairdryer, or similar. An induction coil is a coil of wire through which current may be flowed to induce the generation of a magnetic field. The induction coil may comprise a coiled wire having a single strand. Alternatively, the induction coil may comprise a multistrand wire arranged in a coil. The multistrand wire may, for example, be a Litz wire. Litz wires may be used, for example, in cases where the magnetic field is inductively generated by the application of a radiofrequency alternating current. The induction coil may be driven by an alternating current having a radiofrequency, for example the frequency may be between 100 kHz and 2 MHz. The alternating current frequency may be chosen to ensure that the eddy current skin depths are fully formed in the surfaces of the heat exchangers. This may improve the efficiency with which heat is inductively generated in the heat exchangers. Meanwhile, an alternating current frequency of less than 2 MHz may avoid unnecessary alternating current loss in the induction coil, thereby reducing the resistive heating of the induction coil. The induction coil of the induction heaters described herein may comprise a wire coiled in a helical, or otherwise spiral, shape around and along a longitudinal axis of the induction heater. The inner heat exchanger may extend in a direction parallel to the longitudinal axis about which the induction coil is coiled. The outer heat exchanger may extend in a direction parallel to the longitudinal axis about which the induction coil is coiled. The inner heat exchanger may extend around an interior of the induction coil, and the outer heat exchanger may extend around an exterior of the induction coil. For example, the inner heat exchanger may comprise a body that extends around an interior of the induction coil. The body may, for example, define a cylinder that is surrounded by the induction coil. The body may, for example, be a support for other components of the inner heat exchanger. The support may extend circumferentially around the longitudinal axis about which the induction coil is coiled. In this way, the inductive generation of heat in the inner heat exchanger may be distributed evenly across the surface of the inner heat exchanger. The inner heat exchanger may define a closed electrical circuit. The closed electrical circuit may be defined by the cylindrical-shaped body of the inner heat exchanger. In such cases, eddy currents may only be induced in the surface of the inner heat exchanger that is facing the induction coil. In cases where the body of the inner heat exchanger defines a cylinder, the cylinder of the body of the inner heat exchanger may be coaxial with the longitudinal axis about which the induction coil is coiled. For example, the inner heat exchanger may be concentric with the induction coil. This may improve the evenness of the generated heat distribution in the inner heat exchanger. Similarly, the outer heat exchanger may comprise a body that extends around an exterior of the induction coil. The body may, for example, define a cylinder surrounds the induction coil. The body may, for example, be a support for other components of the outer heat exchanger. The support may extend circumferentially around the longitudinal axis about which the induction coil is coiled. In this way, the inductive generation of heat in the outer heat exchanger may be distributed evenly across the surface of the outer heat exchanger. The outer heat exchanger may define a closed electrical circuit. The closed electrical circuit may be defined by the cylindrical-shaped body of the outer heat exchanger. In such cases, eddy currents may only be induced in the surface of the inner heat exchanger that is facing the induction coil. As discussed below, providing the outer heat exchanger in an arrangement that defines a closed electrical circuit may enhance an electromagnetic shielding effect providable by the outer heat exchanger. In cases where the body of the outer heat exchanger defines a cylinder, the cylinder of the body of the outer heat exchanger may be coaxial with the longitudinal axis about which the induction coil is coiled. For example, the outer heat exchanger may be concentric with the induction coil. This may improve the evenness of the generated heat distribution in the outer heat exchanger. In cases where both the inner heat exchanger and the outer heat exchanger comprise respective bodies defining corresponding cylinders, the induction coil may be concentric with the bodies of both heat exchangers, the inner heat exchanger, the outer heat exchanger, and the induction coil may all be concentric relative to one another. The outer heat exchanger may, in addition to being inductively heated during use of the induction heater, provide shielding that constrains or blocks the expansion of the generated magnetic field outside the induction coil. In this way, electromagnetic interference between the generated magnetic field and other nearby electrical circuits may be reduced. This electromagnetic shielding may, for example, be provided by arranging the outer heat exchanger to define a closed electrical circuit in the circumferential direction (i.e., a direction that extends circumferentially around the induction coil). Further, the outer heat exchanger may, by constraining the magnetic field, reduce the risk of accidentally or inadvertently inductively heating objects outside of the induction heater. For example, in cases where the induction heat is provided as part of a personal grooming appliance such as a haircare appliance (e.g., a hairdryer, or similar) the outer heat exchanger may provide shielding that presents the inductive heating of an object (e.g., jewellery such as a ring) worn or held by the user. In this way, the safety of operating the induction heater may be improved without requiring the provision of dedicated shielding. As such, the induction heaters described herein may be able to provide electromagnetic shielding in a relatively smaller volume than induction heaters that require dedicated shielding. This may result in a more compact induction heater that may, therefore, be easier and more convenient to install in some settings, such as in a domestic appliance. The inner heat exchanger and the outer heat exchanger comprise a plurality of fins. The fins of the inner heat exchanger may be referred to herein as ‘inner fins’, while the fins of the outer heat exchanger may be referred to herein as ‘outer fins’. The fins of both the inner heat exchanger and the outer heat exchanger may be referred to herein collectively as ‘the fins’. The fins of the induction heater may function as heat exchange fins of their respective heat exchangers. The fins of the induction heater may increase the surface area of their respective heat exchanger. In this way, the effective heat exchanger area of the respective heat exchanger may be increased. This may increase the efficiency with which heat is transferred from the respective heat exchanger to air flowing over the surface of the respective heat exchanger. Each of the fins may be a planar fin extending out from a body of the respective heat exchanger, and extending in a direction parallel to a longitudinal axis about which the induction coil is coiled. Each of the fins may extend perpendicularly out from the body of the respective heat exchanger. Each of the inner fins may be spaced apart equidistantly across a surface of the inner heat exchanger. This may ensure a consistent distribution of heat generation across the surfaces of the inner fins. Similarly, each of the outer fins may be spaced apart equidistantly across a surface of the outer heat exchanger. This may ensure a consistent distribution of heat generation across the surfaces of the outer fins. The fins may, for example, form an integral body with their respective heat exchanger. For example, the inner fins may be integral with other parts (e.g., a body or support) of the inner heat exchanger. Similarly, the outer fins may be integral with other parts (e.g., a body or support) of the outer heat exchanger. This may simplify the manufacturing of the heat exchangers of the induction heaters described herein. In some cases, instead of - or in addition to - one or more of the fins of the induction heater, the inner heat exchanger and / or the outer heat exchanger may comprise one or more respective surface disruption features in the surface of the corresponding heat exchanger. The surface disruption features may, for example, include any one or more of (and any combination of any one or more of): grooves, notches, channels, bumps, undulations, and / or roughened portions in the surface of the corresponding heat exchanger. The induction heater comprises a longitudinal axis about which the induction coil is coiled, and the fins extend in radial directions relative to the longitudinal axis. The inner fins may be spaced apart azimuthally around the longitudinal axis. In cases where the inner heat exchanger comprises a cylindrical body, the inner fins may extend radially from a cylindrical surface of the body of the inner heat exchanger. Each of the inner fins may be spaced apart azimuthally around the cylindrical surface. The outer fins may be spaced apart azimuthally around the longitudinal axis. In cases where the outer heat exchanger comprises a cylindrical body, the inner fins may extend radially from a cylindrical surface of the body of the outer heat exchanger. Each of the outer fins may be spaced apart azimuthally around the cylindrical surface. The inner heat exchanger and / or the outer heat exchanger comprises a support that extends circumferentially around the longitudinal axis. The fins of the inner and / or outer heat exchanger may comprise a first set of fins that extend from the support in a direction towards the induction coil, and a second set of fins that extend from the support in a direction away from the induction coil. For example, the inner heat exchanger may comprise a respective support (referred to herein as ‘the inner support’). In cases where the inner heat exchanger comprises an inner support and a plurality of inner fins, the plurality of inner fins may comprise a first set of inner fins that extend from the inner support in a direction towards the induction coil. The inner heat exchanger may additionally or alternatively comprise a second set of inner fins that extend from the inner support in a direction away from the induction coil. The first set of inner fins may extend from an outer surface of the inner support (e.g., an outer cylindrical surface of the inner support) in a radially outward direction towards the induction coil. Each fin of the first set of inner fins may be spaced apart azimuthally around the outer surface of the inner support. The first set of inner fins may increase the surface area of the inner heat exchanger that is proximal to the induction coil. In this way, the inductive volume of the inner heat exchanger may be increased. This may increase the efficiency with which heat is inductively generated in the inner heat exchanger. The second set of inner fins may extend from an inner surface of the inner support (e.g., an inner cylindrical surface of the inner support) in a radially inward direction away from the induction coil. Each fin of the second set of inner fins may be spaced apart azimuthally around the inner surface of the inner support. Heat may be transferred to the second set of inner fins from the inner support and / or from the first set of inner fins by conduction. In cases where the inner heat exchanger comprises both the first set of inner fins and the second set of inner fins, one or more of the first set of inner fins may be aligned with one or more of the second set of inner fins. This may improve the efficiency of conductive heat transfer between the first set of inner fins and the second set of inner fins. The second set of inner fins may comprise a different number of fins than the first set of inner fins. For example, the respective number of fins in the first set of inner fins and the second set of inner fins may be selected such that the effective heat transfer area of the first set of inner fins is balanced with (e.g., is similar or equal to) the effective heat transfer area of the second set of inner fins. The number and / or size of fins in the first set of inner fins and the number and / or size of fins in the second set of inner fins may be selected such that the effective heat transfer area of the inner heat exchanger is balanced (e.g., is similar or equal) on each side of the inner support (e.g., on the outside and inside of a cylinder defined by the inner support). The number, size and / or positioning of the second set of inner fins may be adjusted to provide space to accommodate one or more additional elements of the induction heater, for example a thermal breaker switch as described in more detail below. In some cases, the number of fins in the first set of inner fins may be the same as the number of fins in the second set of inner fins. In such cases, each of the first set of inner fins may be aligned (e.g., azimuthally aligned) with a corresponding fin of the second set of inner fins. The first set of inner fins and / or the second set of inner fins may be integral with the inner support. This may simplify the manufacturing of the inner heat exchanger and may increase the efficiency of conductive heat transfer between the inner support, and the first and / or second sets of inner fins. In addition to, or as an alternative to, the inner heat exchanger comprising the inner support, the outer heat exchanger may comprise a respective support (referred to herein as ‘the outer support’). In cases where the outer heat exchanger comprises an outer support and a plurality of outer fins, the plurality of outer fins may comprise a first set of outer fins that extend from the outer support in a direction towards the induction coil. The outer heat exchanger may additionally or alternatively comprise a second set of outer fins that extend from the outer support in a direction away from the induction coil. The first set of outer fins may extend from an inner surface of the outer support (e.g., an inner cylindrical surface of the outer support) in a radially inward direction towards the induction coil. Each fin of the first set of outer fins may be spaced apart azimuthally around the inner surface of the outer support. The first set of outer fins may increase the surface area of the outer heat exchanger that is proximal to the induction coil. In this way, the inductive volume of the outer heat exchanger may be increased. This may increase the efficiency with which heat is inductively generated in the outer heat exchanger. The second set of outer fins may extend from an outer surface of the outer support (e.g., an outer cylindrical surface of the outer support) in a radially outward direction away from the induction coil. Each fin of the second set of outer fins may be spaced apart azimuthally around the outer surface of the outer support. Heat may be transferred to the second set of outer fins from the outer support and / or from the first set of outer fins by conduction. In cases where the outer heat exchanger comprises both the first set of outer fins and the second set of outer fins, one or more of the first set of outer fins may be aligned with one or more of the second set of outer fins. This may improve the efficiency of conductive heat transfer between the first set of outer fins and the second set of outer fins. Additionally, or alternatively, the second set of outer fins may comprise a different number of fins than the first set of outer fins. For example, the respective number of fins in the first set of outer fins and the second set of outer fins may be selected such that the effective heat transfer area of the first set of outer fins is balanced with (e.g., is similar or equal to) the effective heat transfer area of the second set of outer fins. The number and / or size of fins in the first set of outer fins and the number and / or size of fins in the second set of outer fins may be selected such that the effective heat transfer area of the outer heat exchanger is balanced (e.g., is similar or qual) on each side of the outer support (e.g., on the outside and inside of a cylinder defined by the outer support). In some cases, the number of fins in the first set of outer fins may be the same as the number of fins in the second set of outer fins. In such cases, each of the first set of outer fins may be aligned (e.g., azimuthally aligned) with a corresponding fin of the second set of outer fins. The first set of outer fins and / or the second set of outer fins may be integral with the outer support. This may simplify the manufacturing of the inner heat exchanger and may increase the efficiency of conductive heat transfer between the outer support, and the first and / or second set of outer fins. Each of the first set of fins may have a radial height greater than that of the second set of fins. As an example, the fins of the first set of inner fins may extend in a radial direction away from the inner support to a greater extent than the fins of the second set of inner fins. That is, the fins of the first set of inner fins may be taller (i.e., extend further from the inner support) than the fins of the second set of inner fins. Similarly, the fins of the first set of outer fins may extend in a radial direction away from the outer support to a greater extent than the fins of the second set of outer fins. That is, the fins of the first set of outer fins may be taller (i.e., extend further from the outer support) than the fins of the second set of outer fins. As set out above, the dimensions of the first and second sets of inner fins and the first and second sets of outer fins may be selected to balance the effective heat transfer areas either side of the inner / outer support respectively. The inner heat exchanger may comprise a plurality of inner fins, the outer heat exchanger may comprise a plurality of outer fins, and the inner fins may be fewer and / or taller than the outer fins. For example, the number of fins in the plurality of inner fins may be less than the number of fins in the plurality of outer fins. Additionally, or alternatively, the height (e.g., the radial height - that is, the extent to which a fin extends in the radial direction) of each of the inner fins may be greater than the height of each of the outer fins. In cases where the number of fins in the plurality of inner fins is less than the number of fins in the plurality of outer fins, providing the inner fins with a greater height than the outer fins may help to balance (e.g., make similar or equal) the effective heat transfer areas of the inner and outer heat exchangers. This may provide a more consistent temperature distribution in an airflow heated by the induction heaters described herein. The induction heater may comprise one or more connecting members for connecting the inner heat exchanger to the outer heat exchanger. The one or more connecting members may physically connect the inner heat exchanger to the outer heat exchanger. In this way, the inner heat exchanger may be suspended in position inside the outer heat exchanger, with the induction coil wound / coiled therebetween. The induction coil may be coiled such that the one or more connecting members pass between turns of the induction coil. In this way, the induction coil may be uninterrupted by the one or more connecting members. The one or more connecting members may be integral with the inner heat exchanger and / or integral with the outer heat exchanger. For example, the one or more connecting members, the inner heat exchanger, and the outer heat exchanger may be defined by an integral body. This may simplify the manufacturing of the induction heater. Limiting the number of connecting members may reduce the amount of conductive heat transfer between the inner heat exchanger and the outer heat exchanger and may reduce the extent to which the induction coil is heated by the inner heat exchanger and the outer heat exchanger. The one or more connecting members may be distributed evenly around the longitudinal axis about which the induction coil is coiled. The one or more connecting members may be a plurality of connecting members positioned around the longitudinal axis in one or more pairs of diametrically opposed connecting members. In cases where the inner heat exchanger comprises a plurality of inner fins, the one or more connecting members may be defined by a subset of the plurality of inner fins (e.g., a subset of the first set of inner fins). In cases where the outer heat exchanger comprises a plurality of outer fins, the one or more connecting members may be defined by a subset of the plurality of outer fins (e.g., a subset of the first set of outer fins). In cases where the inner heat exchanger comprises a plurality of inner fins, and the outer heat exchanger comprises a plurality of outer fins, each of the one or more connecting members may be defined by a respective common fin extending between the heat exchangers. The one or more common fins may define a subset of the plurality of inner fins (e.g., a subset of the first set of inner fins) as well as a subset of the plurality of outer fins (e.g., a subset of the first set of outer fins). The number of connecting members may be less than the number of inner fins and / or less than the number of outer fins. The inner heat exchanger and / or the outer heat exchanger may comprise a plurality of sections arranged along the longitudinal axis, and each of the sections may comprise a different arrangement of fins. By employing a different arrangement of fins (e.g., different numbers, shapes or sizes), sections having different heat flux densities may be achieved. As a result, different levels of heating may be achieved along the length of the induction heater. The induction heater may comprise a first axial end and a second axial end. The first axial end may be an upstream or inlet end of the induction heater, and the second axial end may be a downstream or outlet end of the induction heater. The inner heat exchanger and / or the outer heat exchanger may have a first number of fins at the first axial end and a second smaller number of fins at the second axial end. As a result, the heat flux of the inner and / or outer heat exchanger may be higher at the first axial end, and lower at the second axial end. This can increase the power density of the induction heater and / or reduce thermal strain, particularly when the first axial end is an upstream or inlet end of the induction heater. In examples, the first number of fins at the first axial end of the heat exchanger may be less than 50% of the second number of fins at the second axial end of the heat exchanger. Additionally, or alternatively, the height (e.g. radial height) of fins in an upstream section of the inner heat exchanger and / or the outer heat exchanger may be less than those in a downstream section of the respective heat exchanger. This arrangement of different fin heights may be provided by the upstream section comprising separate fins to the downstream section, or by having continuous fins across said sections that reduce in height along the axial length of the induction heater (e.g. in a tapered or stepwise manner). These fin configurations (i.e. varying in number, shape, and / or size) can provide the fins proximate an upstream end of the induction heater with a temperature closer to that of the fins proximate a downstream end of the induction heater during operation of the induction heater, compared to that if the fins had a uniform arrangement along the entire axial length of the induction heater. The upstream section may form greater than or equal to 10% of the axial length of the induction heater and / or less than or equal to 20% of the axial length of the induction heater. The induction coil may have an magnetomotive force density that varies along the longitudinal axis of the induction heater. Energy losses are proportional to the square of the magnetomotive force (MMF). Accordingly, different energy losses (and thus different levels of heating) may be achieved along the length of the induction heater by varying the MMF density of the induction coil. The induction coil may be non-uniform along the longitudinal axis of the induction heater. The induction coil may have a turn density (i.e. a number of turns per unit length) and / or a profile having a thickness that varies along the longitudinal axis of the induction heater. As a result, the MMF density and thus the energy losses of the induction coil may vary along the length of the induction heater. In some examples, the induction coil has a varying cross-sectional area in a plane perpendicular to the longitudinal axis of the heater along the longitudinal axis of the heater. In particular, the cross-sectional area of the induction coil in a plane perpendicular to the longitudinal axis of the heater may be greater at a first position along the longitudinal axis than a second position along the longitudinal axis, the first position upstream of the second position. This may be achieved by varying the profile of one or more wires forming the induction coil along its length. In this way, the radial thickness of the induction coil (e.g. the difference in radial position between the radially innermost portion of the induction coil and the radially outermost portion of the induction coil) may vary along the longitudinal axis of the heater. In some examples, the turn density is greater at a first position along the longitudinal axis than a second position along the longitudinal axis, the first position upstream of the second position. The inner heat exchanger and the outer heat exchanger may have surfaces facing the induction coil. A ratio of total surface areas of the surfaces of the inner heat exchanger and the surfaces of the outer heat exchanger may be between 0.8 and 1.2. The surfaces of the inner heat exchanger and outer heat exchanger that are facing the induction coil may define respective inductive volumes of the heat exchangers. The inductive volume of a heat exchanger may be understood as a volume defined by the product of skin depth of the heat exchanger with the surface area of the heat exchanger in which eddy currents may be inductively generated (e.g., the surfaces of the heat exchanger facing the induction coil). The inductive volume defines the effective volume of the heat exchanger that is resistively heated by the flow of induced eddy currents therethrough. The skin depth of a material is a known parameter that may be determined for a given induction frequency (i.e., the frequency of the current used to inductively generate the magnetic field from the induction coil) as: 5 = (2p / cop)1 / 2 where 5 is the skin depth of the heat exchanger, p is the resistivity of the material form which the heat exchanger is formed, co is the frequency of the alternating current (in rad / s), and p is the relative magnetic permeability of the material form which the heat exchanger is formed. By balancing the surface areas of the inner heat exchanger and the outer heat exchanger that are facing the induction coil - that is, the inductively heatable areas of the inner heat exchanger and the outer heat exchanger (e.g., by providing the ratio of total surface areas of the inner heat exchanger to the total surface areas of the outer heat exchanger between 0.8 and 1.2) - the inductive volumes of the heat exchangers may be balanced. By balancing the inductive volumes of the heat exchangers, the inductive generation of heat in the inner heat exchanger may be balanced with the inductive generation of heat in the outer heat exchanger. For example, the inductive generation of heat may be distributed evenly between the heat exchangers. This may ensure a consistent temperature distribution across an airflow heated by the induction heaters described herein. The inductive volume of the inner heat exchanger and / or the outer heat exchanger may be adjusted by adjusting a size and / or number of fins in the pluralities of inner fins and outer fins. Additionally, or alternatively, the inductive volume of the inner heat exchanger and / or the outer heat exchanger may be adjusted by changing the material from which said heat exchanger is formed. A ratio of a total surface area of the inner heat exchanger and a total surface area of the outer heat exchanger may be between 0.8 and 1.2. For example, the ratio of the effective heat transfer area of the inner heat exchanger and the effective heat transfer area of the outer heat exchanger may be between 0.8 and 1.2. That is, the effective heat transfer areas of the heat exchangers may be balanced. This may ensure that the transfer of heat into the air flowing over the surfaces of the inner heat exchanger and the outer heat exchanger is evenly distributed between the heat exchangers to ensure a consistent temperature distribution across the airflow heated by the induction heaters described herein. The effective heat exchange areas of the inner heat exchanger and / or the outer heat exchanger may be adjusted by adjusting a size and / or number of fins in the pluralities of inner fins and / or outer fins. The induction heater may comprise an inner airflow channel positioned inwardly of the induction coil and an outer airflow channel positioned outwardly of the induction coil. The inner heat exchanger may be positioned within the inner airflow channel. The outer heat exchanger may be positioned within the outer airflow channel. In this way, two distinct airflow channels may be provided by the induction heaters described herein. By selective adjustment of the geometries of the inner and outer heat exchangers, the temperatures to which the airflows through the inner and outer airflow channels may be adjusted relative to one another. For example, the geometries of the inner and outer heat exchanger may be adjusted such that an airflow through the inner airflow channel is heated to a higher temperature than an airflow through the outer airflow channel. This may reduce the risk of heat being transferred from the outer airflow channel out of the induction heater to a product in which the induction heater has been installed. A ratio of cross-sectional areas of the inner airflow channel and the outer airflow channel may be between 0.8 and 1.2. In this way, the flow rates through the inner and outer airflow channels may be balanced to provide a consistent overall airflow through the induction heater. The induction heater may comprise a thermal circuit breaker configured to break an electrical connection between the induction coil and a current supply configured to apply current through the induction coil if the temperature of the thermal circuit breaker exceeds a predetermined temperature threshold. The inner heat exchanger may surround the thermal circuit breaker. The thermal circuit breaker may function as a switch that opens in response to the temperature of the thermal circuit breaker exceeding the predetermined temperature threshold. In this way, the induction coil may be disconnected from the current supply to prevent the induction coil from being overheated and damaged. By surrounding the thermal circuit breaker with the inner heat exchanger, the thermal circuit breaker may be positioned such that the temperature of the thermal circuit breaker is representative of a temperature inside the induction heater, and consequently indicative of a temperature of the induction coil. The predetermined temperature threshold may, for example, be between 180°C and 300 °C, for example between 200°C and 240°C. The thermal circuit breaker may comprise a bimetallic strip arranged to operate as a thermomechanical switch. The bimetallic strip may comprise two strips of different metals laid one atop another. The first strip is formed from a metal having a different coefficient of thermal expansion than the second strip. For example, the first strip may be formed from steel, and the second strip from copper or brass, or vice versa. Upon heating, the two strips expand at different rates due to the different coefficients of thermal expansion of the first strip and the second strip, thereby causing the bimetallic strip to bend and consequently break the circuit between the current supply and the induction coil. Alternatively, the thermal circuit breaker may be a thermal fuse or a wax melt, or any other suitable form of thermally responsive circuit breaker. The induction heater may further comprise a casing that surrounds the outer heat exchanger. The casing may surround all the components of the induction heater. The casing may be an open casing to facilitate the provision of an airflow path through the induction heater. For example, the casing may be a cylindrical casing (or similar), with both ends of the cylindrical casing open (or partially open) to provide an airflow inlet and airflow outlet either side of the induction heater. The casing may protect the other components of the induction heater from physical damage (e.g., as a result of being dropped or knocked). In some examples, the outer heat exchanger may not define a closed electrical circuit. In such cases, magnetic fields (and consequently heat) may be generated in the surfaces of the outer heat exchanger that are facing towards the induction coil and in the surfaces of the outer heat exchanger that are facing away from the induction coil. As a counterbalance, however, by not defining a closed electrical circuit, the shielding provided by the outer heat exchanger (as discussed above) may be reduced relative to a closed-circuit arrangement. In such cases, the casing may provide the electromagnetic shielding effect discussed above in relation to the outer heat exchanger. In cases where the casing provides electromagnetic shielding, the casing may be provided with a sufficient thickness to ensure effective shielding. The inner heat exchanger and / or the outer heat exchanger may be formed from a magnetically permeable material. In the context of the induction heaters described herein, forming one or both of the heat exchangers form a magnetically permeable material may increase the efficiency of the inductive heating of said heat exchanger(s) by a factor of 5 to 10 relative to an arrangement wherein the inner heat exchanger and / or the outer heat exchanger are formed from a non-magnetically permeable material. One or both of the heat exchangers may be formed from a material that is resistant to corrosion, e.g., resistant to corrosion at high temperatures. In some cases, the induction heaters described herein may be provided as part of a domestic appliance. The airflow through such induction heaters may include air with one or more contaminant entrained in the airflow. For example, water may be entrained in the airflow, which may cause certain materials to rust. Rusting may reduce the efficiency with which the heat exchangers are inductively heated by the induction coil and may also reduce the efficiency with which the heat exchangers heat the air flowing over the surfaces of the heat exchangers. Meanwhile, chemical contaminants may risk corroding the heat exchangers. Forming one or both of the heat exchangers from a corrosion-resistant material may therefore increase the lifetime of the induction heater and reduce performance degradation over time. The inner heat exchanger and the outer heat exchanger may be formed form the same material or from different materials. One or both of the heat exchangers may be formed from any one or more of: titanium or an alloy of titanium (e.g., grade 5 titanium), stainless steel, austenitic steel, martensitic steel (e.g., 400-series stainless steel), orKanthal ®. There is also provided herein a domestic appliance comprising: an induction heater as described herein; a drive circuit for supplying a current through the induction coil of the induction heater; and an airflow generator for generating an airflow. The induction heater is configured to heat at least a part of the airflow. The domestic appliance may comprise an airflow inlet and an airflow outlet. The airflow may follow an airflow path between the airflow inlet and the airflow outlet. The induction heater may be disposed in the airflow path. The induction heaters described herein may be provided in a compact arrangement that is suitable for installation in a domestic appliance without unduly increasing the size and / or weight of the domestic appliance. The current may be an alternating current having a frequency of between 100 kHz and 2 MHz. For example, the induction coil may be driven by an alternating current having a frequency between 100 kHz and 2 MHz. An alternating current frequency of at least 100 kHz may ensure that eddy currents are fully formed in the surfaces of the heat exchangers. This may improve the efficiency with which heat is inductively generated in the heat exchangers. Meanwhile, an alternating current frequency of less than 2 MHz may avoid unnecessary alternating current loss in the induction coil, thereby reducing the resistive heating of the induction coil. The domestic appliance may be an appliance configured to generate a heated airflow in a domestic setting. For example, the domestic appliance may be useable in the home to heat a room - e.g., a heater fan, or may be useable to provide a heated airflow for the purpose of drying - e.g., a clothes dryer. As another example, the domestic appliance may be a kitchen appliance useable to generate a heated airflow to cook food - e.g., an air fryer. The domestic appliance may be a personal grooming appliance, such as a haircare appliance. For example, the domestic appliance may be a haircare appliance configured to generate a heated airflow for application to hair for grooming and / or styling purposes. For example, the domestic appliance may be a hairdryer, or similar. Any of the features set out above in relation to any of the examples described above may be combined in any suitable combination, except where expressly prohibited, or where such a combination is clearly impossible or a combination of incompatible features. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1A shows a perspective view of an induction heater. Figure IB shows a cross-sectional view of the induction heater of Figure 1A. Figure 2 shows a schematic of the operation of a portion of the induction heater shown in Figures 1A and IB. Figure 3 shows a perspective view of the induction heater of Figures 1A and IB, wherein an induction coil of the induction heater has been omitted in order to better depict connecting members connecting inner and outer heat exchangers of the induction heater. Figures 4A and 4B illustrate the bending of a bimetallic strip that is useable as the thermal circuit breaker of the induction heater shown in Figures 1A and IB. Figure 5 shows an example domestic appliance into which the induction heater of Figures 1A and IB may be installed to heat an airflow flowing through the domestic appliance. DETAILED DESCRIPTION Figures 1A and IB show perspective and cross-sectional views respectively of an induction heater 100. The induction heater 100 comprises an induction coil 110, an inner heat exchanger 120 positioned inwardly of the induction coil 110, an outer heat exchanger 130 positioned outwardly of the induction coil 110, a plurality of connecting members 140 connecting the inner heat exchanger 120 to the outer heat exchanger 130, a thermal circuit breaker 150 positioned inwardly of the inner heat exchanger 120, and a casing 160 positioned inwardly outwardly of the outer heat exchanger 130. The induction heater 100 includes an airflow path therethrough. The airflow path facilitates the flow of air over the surfaces of the inner and outer heat exchangers 120, 130. In use, as described in greater detail below in relation to Figure 3, the inner and outer heat exchangers 120, 130 are inductively heated and exchange heat with the airflow flowing over the surfaces of the heat exchangers 120, 130 to heat the airflow. The airflow path through the induction heater 100 may be defined by an inner airflow channel positioned inwardly of the induction coil 110 and an outer airflow channel positioned outwardly of the induction coil 110. In such cases, the inner heat exchanger 120 may be positioned within the inner airflow channel and the outer heat exchanger 130 may be positioned within the outer airflow channel. A ratio of the cross-sectional areas of the inner airflow channel and the outer airflow channel may be between 0.8 and 1.2 This may serve to balance the volumetric flow rate through each of the inner and outer airflow channels so that the overall airflow through the induction heater 100 is consistent across the crosssection of the induction heater 100. As is discussed herein, the geometries and materials used to form the inner and outer heat exchangers 120, 130 may be adjusted to adjust the temperature to which air flowing through the inner airflow channel and the outer airflow channel may be respectively heated. For example, air flowing through the inner airflow channel may be heated to a different (e.g., a higher) temperature than air flowing through the outer airflow channel. The induction coil 110 may be formed from a multistrand wire. The multistrand wire may, for example, be a Litz wire comprising 10 or more strands of wire. Litz wire is suitable for flowing radiofrequency alternating currents therethrough, thereby making Litz wire a suitable structure for an induction coil. The induction coil 110 is coiled around a longitudinal axis of the induction heater 100. The induction coil 110 may define a helical path that comprises 10 or fewer turns along the longitudinal axis. For example, the induction coil 110 may have 8 or 9 turns along the longitudinal axis. In the context of the induction heater 100 described herein, a turn of an induction coil may correspond to an azimuthal revolution of 360° around the longitudinal axis. In use, the airflow through the induction heater flows in a direction generally parallel to the longitudinal axis. The induction heater 100, as can be seen from Figures 1A and IB has a generally cylindrical shape, with open ends - that is, the faces of the cylinder defined by the induction heater at opposite ends of the longitudinal axis are open to facilitate airflow therethrough. The heat exchanger structure and operation of the induction heater 100 will now be described in more detail with reference to Figure 2. Figure 2 shows a portion of the induction heater 100 shown in Figures 1A and IB. The inner heat exchanger 120 extends around an interior of the induction coil 110. In the example shown in Figures 1A and IB, the inner heat exchanger 120 comprises an inner support 122 and a plurality of inner fins 124, 126. The inner support 122 extends circumferentially around the longitudinal axis of the induction heater 100 within a volume circumscribed by the induction coil 110. The plurality of inner fins 124, 126 comprises a first set of inner fins 124 extending radially outwards from the inner support 122 towards the induction coil 110, and a second set of inner fins 126 extending radially inwards from the inner support 122 away from the induction coil 110. The fins of the first set of inner fins 124 are azimuthally spaced around an outer surface of the inner support 122. The fins of the second set of inner fins 126 are azimuthally spaced around an inner surface of the inner support 122. Each fin of the second set of inner fins 126 is azimuthally aligned with a corresponding fin of the first set of inner fins 124. As can be seen from Figures 1A and IB, in the example shown, the number of fins in the second set of inner fins 126 is fewer than the number of fins in the first set of inner fins 124. Each fin of the first set of inner fins 124 has a greater radial height than each fin of the second set of inner fins 126 - that is, the extent to which the fins of the first set of inner fins 124 extend away from the inner support 122 is greater than the extent to which the fins of the second set of inner fins 126 extend away from the inner support 122. The inner support 122 and the second set of inner fins 126 are sized and shaped to accommodate additional components of the induction heater 100, for example, the thermal circuit breaker 150 described in more detail below. The outer heat exchanger 130 extends around an exterior of the induction coil 110. In the example shown in Figures 1A and IB, the outer heat exchanger 130 comprises an outer support 132 and a plurality of outer fins 134, 136. The outer support 132 extends circumferentially around the longitudinal axis of the induction heater 100 within a volume circumscribing the induction coil 110. That is, the overall structure of the induction heater 100 comprises an inner heat exchanger 120 circumscribed by the induction coil 110, and the induction coil 110 is itself circumscribed by the outer heat exchanger 130. The plurality of outer fins 134, 136 comprises a first set of outer fins 134 extending radially inwards from the outer support 132 towards the induction coil 110, and a second set of outer fins 136 extending radially outwards from the outer support 132 away from the induction coil 110. The fins of the first set of outer fins 134 are azimuthally spaced around an inner surface of the outer support 132. The fins of the second set of outer fins 136 are azimuthally spaced around an outer surface of the outer support 132. Each fin of the second set of outer fins 136 is azimuthally aligned with a corresponding fin of the first set of outer fins 134. Each As can be seen from Figures 1A and IB, in the example, shown, the number of fins in the first and second sets of outer fins 134, 136 is the same. Each fin of the first set of outer fins 134 has a greater radial height than each fin of the second set of outer fins 136 - that is, the extent to which the fins of the fist set of outer fins 134 extend away from the outer support 132 is greater than the extent to which the fins of the second set of outer fins 136 extend away from the outer support 132. Each of the inner heat exchanger 120 and outer heat exchanger 130 may be formed from a material suitable for inductive heating. For example, the heat exchangers 120, 130 may be formed from a magnetically permeable, heat-resistant and / or corrosion-resistant material. As an example, one or both of the heat exchangers 120, 130 may be formed from any of: stainless steel, austenitic steel, martensitic steel (e.g., 400-series steel), Kanthal®, titanium and / or an alloy of titanium (e.g., grade 5 titanium). In use, a radiofrequency alternating current (e.g., an alternating current having a frequency between 100 kHz and 2 MHz) is applied through the induction coil 110. As described above, the induction coil 110 may define 10 or fewer turns along the longitudinal axis of the induction heater - for example, the induction coil 110 may define 8 or 9 turns along the longitudinal axis. As the alternating current flows through the induction coil 110, an alternating solenoidal magnetic field is inductively generated in the vicinity of the induction coil 110. This alternating magnetic field subsequently induces eddy currents in the proximal surfaces of the inner and outer heat exchangers 120, 130. The alternating magnetic field penetrates an outer surface of the inner support 122 and the first set of inner fins 124 to a depth proportional to the skin depth of the material from which the inner heat exchanger 120 is formed. The total surface area of the surfaces of the inner heat exchanger 120 facing the induction coil 110 (e.g., those surfaces that are proximal to the induction coil 110) and the skin depth of the inner heat exchanger 120 together define an inductive volume of the inner heat exchanger in which eddy currents are induced by the alternation of the penetrating alternating magnetic field. These eddy currents resistively heat the outer surface of the inner support 122 and the first set of inner fins 124. Heat is subsequently transferred conductively from the outer surface of the inner support 122 and the first set of inner fins 124 to the inner surface of the inner support 122 and the second set of inner fins 126. Subsequently, all the surfaces of the inner heat exchanger 120 (e.g., both the inner and outer surfaces of the inner support 122, and the surfaces of both the first and second sets of inner fins 124, 126) exchange heat with air flowing over those surfaces. In this way, the inner heat exchanger 120 conductively heats an airflow flowing over the surfaces of the inner heat exchanger 120. Similarly, the alternating magnetic field penetrates an inner surface of the outer support 132 and the first set of outer fins 134 to a depth proportional to the skin depth of the material from which the outer heat exchanger is formed. The total surface area of the surfaces of the outer heat exchanger 130 facing the induction coil 110 (e.g., those surfaces that are proximal to the induction coil 11) and the skin depth of the outer heat exchanger 130 together define an inductive volume of the outer heat exchanger in which eddy currents are induced by the alternation of the penetrating alternating magnetic field. These eddy currents resistively heat the inner surface of the outer support 132 and the first set of outer fins 134. Heat is subsequently transferred conductively from the inner surface of the outer support 132 and the first set of outer fins 134 to the outer surface of the outer support 132 and the second set of outer fins 136. Subsequently, all the surfaces of the outer heat exchanger 130 (e.g., both the inner and outer surfaces of the outer support 132, and the surfaces of both the first and second sets of outer fins 134, 136) exchange heat with air flowing over those surfaces. In this way, the outer heat exchanger 130 conductively heats an airflow flowing over the surfaces of the outer heat exchanger 130. In the context of the induction heater 100 shown in Figures 1A and IB, the induction coil 110 may exhibit only small losses when driven with a radiofrequency alternating current. For example, the losses in the induction coil may have a loss density of approximately 1.5 W cm'3. Meanwhile, the resistive heating of the inner and outer heat exchangers 120, 130 by the induced eddy currents may be characterised by a loss density of approximately 1.5 kW cm'3. In such a configuration, conductive heat losses transferred through the respective heat exchangers 120, 130 may result in different power outputs in different portions of the induction heater 100. The power outputs may be understood as the rate of heat transferred to air flowing over the surfaces of the heat exchangers 120, 130. For example, the power output of the outer side of the inner heat exchanger 120 (e.g., the portion of the inner heat exchanger 120 comprising the outer surface of the inner support 122 and the first set of inner fins 124) may be approximately 590 W. This may be achieved with a surface temperature of the outer side of the inner heat exchanger being up to 600°C. The power output of the inner side of the inner heat exchanger 120 (e.g., the portion of the inner heat exchanger 120 comprising the inner surface of the inner support 122 and the second set of inner fins 126) may be approximately 160 W. This may be achieved with a surface temperature of the inner side of the inner heat exchanger 120 being up to 350°C. The power output of the inner side of the outer heat exchanger 130 (e.g., the portion of the outer heat exchanger 130 comprising the inner surface of the outer support 132 and the first set of outer fins 134) may be approximately 470 W. This may be achieved with a surface temperature of the inner side of the outer heat exchanger 130 being up to 550°C. The power output of the outer side of the outer heat exchanger 130 (e.g., the portion of the outer heat exchanger 130 comprising the outer surface of the outer support 132 and the second set of outer fins 136) may be approximately 190 W. This may be achieved with a surface temperature of the outer side of the outer heat exchanger 130 being up to 300°C. In use, in addition to providing additional heating surfaces for the induction heater 100, the outer heat exchanger 130 may also provide shielding to prevent electromagnetic interference between the induction coil 110 and other components of a device or appliance into which the induction heater 100 has been installed. The outer heat exchanger 130 may also constrain the volume within which heat is inductively generated. In this way, the outer heat exchanger 130 may provide shielding that presents the inductive heating of an object (e.g., jewellery such as a ring) worn or held by the user of the device or appliance in which the induction heater 100 is installed. The inductive volumes of the inner heat exchanger 120 and the outer heat exchanger 130 may be balanced. For example, a ratio between the total surface area of surfaces of the inner heat exchanger 120 that are facing the induction coil 110 and the total surface area of surfaces of the outer heat exchanger 130 that are facing the induction coil 110 may be between 0.8 and 1.2 In this way, the inductive heating of the heat exchangers 120, 130 may be balanced. This may contribute to a more even temperature distribution across the heated airflow. The effective heat exchange areas of the inner heat exchanger 120 and the outer heat exchanger 130 may be balanced. For example, a ratio between the total surface area of all the surfaces of the inner heat exchanger 120 and the total surface area of all the surfaces of the outer heat exchanger 130 may be between 0.8 and 1.2 in this way, the heat transfer of heat from the heat exchangers 120, 130 to the airflow may be balanced across the heat exchangers 120, 130. This may contribute to a more even temperature distribution across the heated airflow. The induction heater of Figures 1A and IB further comprises a plurality of connecting member 140. Figure 3 shows a view of the induction heater 100 of Figures 1A and IB, depicting the plurality of connecting members 140 connecting the inner heat exchanger 120 to the outer heat exchanger 130. The inner heat exchanger 120 is suspended within an interior volume of the outer heat exchanger 130 by the plurality of connecting members 140. As can be seen from Figures IB and 3, each of the plurality of connecting members 140 may be defined by a common heat exchange fin shared between the inner heat exchanger 120 and the outer heat exchanger 130. Each connecting member 140 may effectively be defined by a fin of the first set of inner fins 124 that extends radially to join with a similarly / correspondingly radially extended fin of the first set of outer fins 134. In such cases, the inner heat exchanger 120, the outer heat exchanger 130 and the plurality of connecting members 140 may be integrally formed. In the induction heater 100 of Figures 1A and IB, the plurality of connecting members 140 are arranged in diametrically opposed pairs. In the example shown in Figures 1A and IB, the induction heater 100 comprises six connecting members 140, though other numbers of connecting members 140 are possible. The induction heater 100 further comprises a thermal circuit breaker 150. The thermal circuit breaker 150 in Figures 1A and IB comprises a bimetallic strip configured to operate as a thermomechanical switch. The arrangement of the bimetallic strip is shown in Figures 4A and 4B. Figures 4A and 4B illustrate the bending of a bimetallic strip in response to an increase in temperature. The bimetallic strip comprises a first bimetal layer 152 (e.g., formed from steel) laid atop a second bimetal layer 154 (e.g., formed from copper or brass). The first bimetal layer 152 has a coefficient of thermal expansion greater than that of the second bimetal layer 154. Figure 4A shows the bimetallic strip in an unheated state. Figure 4B shows the bimetallic strip in a heated state. In response to heating, the first bimetal layer 152 extends more than the second bimetal layer 154, this causes the bimetallic strip to bend away from the first bimetal layer. When used as a thermal circuit breaker 150, as shown in Figures 1A and IB, the thermal circuit breaker 150 is provided as an electrical connection between a current supply and the induction coil 110. If the temperature of the bimetallic strip exceeds a predetermined temperature threshold (e.g., 600°C), then the bimetallic strip thermo-mechanically deforms (e.g., bends) sufficiently far to disconnect the induction coil 110 from the current supply. This protects the induction coil from overheating and damage. Returning to Figures 1A and IB, the induction heater 100 further comprises a casing 160 surrounding the outer heat exchanger 130. The casing 160 surrounds all the other components of the induction heater 100. The casing 160 may be provided as a common casing shared between the induction heater 100 and a device or appliance in which the induction heater is installed. The casing 160 provides physical protection to the components of the induction heater 100. As shown in Figures 1A and IB, the casing is (at least partially) open at either end of the longitudinal axis of the induction heater 100 to facilitate an airflow therethrough. In some examples, during use, the temperature of the heater exchangers 120, 130 at the inlet end of the induction heater 100 may be lower than that at the outlet end. This disparity represents a loss of power density, since the heater exchangers 120, 130 have a higher heat transfer coefficient and temperature difference with the airflow at the inlet end than at the outlet end. Additionally, or alternatively, this disparity may result in premature ageing of the heater exchangers 120, 130 due to higher thermal strains. The disparity may be compensated by configuring the induction coil 110 such that energy losses in the heat exchangers 120, 130 are higher at the inlet end of the induction heater 100 and lower at the outlet end. Energy losses are proportional to the square of the magnetomotive force (MMF). Accordingly, different energy losses can be achieved along the axial length of the induction heater 100 by varying the MMF density of the induction coil 110 along the length of the induction heater 100 (i.e., from the inlet end to the outlet end). There are various ways in which the MMF density of the induction coil 110 might be varied. In one example, the induction coil 110 may have a higher turn density (i.e., a higher turns per unit length) at the inlet end, and a lower turn density at the outlet end. In another example, the profile of the induction coil 110 along the length of the induction heater 100 may be shaped such that the induction coil 110 is thicker at the inlet end and thinner at the outlet end. In particular, the thickness of the induction coil 110 may taper from the inlet end to the outlet end. This may be achieved using a different number of turns and / or wires of varying or different thicknesses. The disparity may additionally or alternatively be compensated by configuring the geometry of the heat exchangers 120, 130 over the axial length of the induction heater 100 such that a higher level of heat flux density is achieved at the inlet end of the induction heater 100. In examples, the inner heat exchanger 120 and / or the outer heat exchanger 130 may have an arrangement of fins 124, 126 134, 136 that differs along the length of the induction heater 100. For example, the number and / or the size of fins 124, 126 134, 136 at the inlet end of the induction heater 100 may be smaller than that at the outlet end. This then leads to an increase in the flux density carried axially in the surfaces of the fins 124, 126 134, 136. The reduced number and / or size of fins has a secondary effect of reducing the impedance to the eddy currents, which then increases their magnitude and thus energy losses. In one example, the inner heat exchanger 120 and / or the outer heat exchanger 130 may comprise a plurality of sections that are arranged along the axial length of the induction heater 100. Each section then has a different arrangement of fins (i.e., number, shape or size). Figure 5 shows an example domestic appliance 200 into which the induction heater 100 is installed. The domestic appliance shown in Figure 5 is a haircare appliance (e.g., a hairdryer), but other domestic appliances are also envisaged in the context of the induction heaters described herein. The domestic appliance comprises airflow generator for generating an airflow therethrough, an induction heater 100 for heating the generated airflow, and a drive circuit for supplying a current through the induction coil 110 of the induction heater 100. The drive circuit may be configured to supply a current that is an alternating current having a frequency of between 100 kHz and 2 MHz. The domestic appliance 200 comprises a casing 260 surrounding the airflow path through which air flows from the airflow generator to an airflow outlet 210 of the domestic appliance 200. The casing 260 may be a common casing shared between the induction heater 100 and the domestic appliance, or may be a separate casing 260 (i.e., the induction heater 100 may be provided with its own casing 160).

Claims

1. An induction heater comprising:an induction coil;an inner heat exchanger positioned inwardly of the induction coil; andan outer heat exchanger positioned outwardly of the induction coil, wherein:the inner heat exchanger and the outer heat exchanger are inductively heatable by a magnetic field generated in response to an application of current through the induction coil, and the inner heat exchanger and the outer heat exchanger exchange heat with an airflow moving through the induction heater;the induction heater comprises a longitudinal axis about which the induction coil is coiled, and the inner heat exchanger and the outer heat exchanger each comprise a plurality of fins that extend in radial directions relative to the longitudinal axis; andthe inner heat exchanger and / or the outer heat exchanger comprises a support that extends circumferentially around the longitudinal axis, and the plurality of fins comprises a first set of fins that extend from the support in a direction towards the induction coil, and a second set of fins that extend from the support in a direction away from the induction coil.

2. The induction heater according to claim 1, wherein the inner heat exchanger extends around an interior of the induction coil, and the outer heat exchanger extends around an exterior of the induction coil.

3. The induction heater according to claim 1 or 2, wherein each of the first set of fins has a radial height greater than that of the second set of fins.

4. The induction heater according to any preceding claim, wherein the inner heat exchanger comprises a plurality of inner fins, and the outer heat exchanger comprises a plurality of outer fins, and the inner fins are fewer and / or taller than the outer fins.

5. The induction heater according to any preceding claim, wherein the inner heat exchanger and / or the outer heat exchanger comprises a plurality of sections arranged along the longitudinal axis, and each of the sections comprises a different arrangement of fins.

6. The induction heater according to any preceding claim, wherein the induction heater comprises a first axial end and a second axial end, and the inner heat exchanger and / or the outer heat exchanger have a first number of fins at the first axial end and a second smaller number of fins at the second axial end.

7. The induction heater according to any preceding claim, wherein the induction coil hasan magnetomotive force density that varies along the longitudinal axis.

8. The induction heater according to any preceding claim, wherein the induction coil has a turn density and / or a profile having a thickness that varies along the longitudinal axis.

9. The induction heater according to any preceding claim, wherein the induction heater comprises one or more connecting members for connecting the inner heat exchanger to the outer heat exchanger.

10. The induction heater according to any preceding claim, wherein the inner heat exchanger and the outer heat exchanger have surfaces facing the induction coil, and a ratio of total surface areas of the surfaces of the inner heat exchanger facing the induction coil and total surface areas of the surfaces of the outer heat exchanger facing the induction coil is between 0.8 and 1.2.

11. The induction heater according to any preceding claim, wherein a ratio of a total surface area of the inner heat exchanger and a total surface area of the outer heat exchanger is between 0.8 and 1.2.

12. The induction heater according to any preceding claim, wherein the induction heater comprises:an inner airflow channel positioned inwardly of the induction coil; and an outer airflow channel positioned outwardly of the induction coil, wherein the inner heat exchanger is positioned within the inner airflow channel, and the outer heat exchanger is positioned within the outer airflow channel.

13. The induction heater according to claim 12, wherein a ratio of cross-sectional areas of the inner airflow channel and the outer airflow channel is between 0.8 and 1.2.

14. The induction heater according to any preceding claim, wherein the induction heater comprises a thermal circuit breaker configured to break an electrical connection between the induction coil and a current supply configured to apply current through the induction coil if a temperature of the thermal circuit breaker exceeds a predetermined temperature threshold,wherein the inner heat exchanger surrounds the thermal circuit breaker.

15. The induction heater according to claim 14, wherein the thermal circuit breaker comprises a bimetallic strip arranged to operate as a thermomechanical switch.

16. The induction heater according to any preceding claim, wherein the induction heater comprises a casing that surrounds the outer heat exchanger.

17. The induction heater according to any preceding claim, wherein the inner heat exchanger and / or the outer heat exchanger are formed from a magnetically permeable material.

18. A domestic appliance comprising:an induction heater according to any preceding claim;a drive circuit for supplying a current through the induction coil of the induction heater; andan airflow generator for generating an airflow,wherein the induction heater is configured to heat at least a part of the airflow.

19. The domestic appliance according to claim 18, wherein the current is an alternating current having a frequency of between 100 kHz and 2 MHz.

20. The domestic appliance according to claim 18 or 19, wherein the domestic appliance is a haircare appliance.A

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