Apparatus for drying and / or styling hair

Abstract

An apparatus for drying and / or styling a length of hair comprises a main body (10, fig.1a) with a head portion for drying and / or styling the length of hair and a mixed-flow fan assembly (100, fig.1a)

Description

Field of the Invention The present invention relates to an apparatus for drying and / or styling the hair of a person (or conceivably an animal), for example after washing the hair or as part of a styling process. The present invention also relates to a fan assembly for incorporation in an apparatus for drying and / or styling hair. The applicant has previously described apparatus for drying and / or styling hair in WO2021 / 019239 and in WO2023 / 118836, the disclosures of which are incorporated herein in its entirety by reference. The applicant has made developments relating to the fan assembly for incorporation in such apparatus. Some of these developments are specific to the apparatus comprising such a fan assembly, whilst some are more generally applicable to a fan assembly for incorporation in any hair drying / styling apparatus. Summary of the Invention The present invention provides an apparatus for drying and / or styling a length of hair, the apparatus comprising a main body comprising a head portion for drying and / or styling the length of hair; and a mixed-flow fan assembly arranged in the main body and configured to deliver an airflow to the head portion, wherein the mixed-flow fan assembly comprises: a rotor comprising a plurality of rotor blades, and a stator comprising a plurality of stator blades, wherein the rotor and the stator are arranged along a common axis and the rotor is rotatable relative to the stator about the common axis, and wherein the rotor is configured to output the airflow with an axial velocity component along the common axis, a radial velocity component that is radially away from the common axis and a tangential velocity component that is tangential to a circle around the common axis. In some embodiments, the head portion is configured to engage the length of hair so as to be capable of holding the length of hair in tension. The head portion may comprise one or more outlets for directing the airflow to the length of hair, and the fan assembly may be arranged to provide the airflow to the one or more outlets. Alternatively, the head portion may comprise one or more inlets for receiving the airflow. The fan assembly may be arranged to receive the airflow from the one or more inlets. The inlets may further be arranged for pulling the length of hair into the head portion. In some embodiments, the head portion is configured, in use, to provide a confined space and to guide the length of hair within the confined space. The one or more outlets may direct the airflow to the confined space. In some embodiments, the apparatus is a hand-held apparatus comprising a handle. In such embodiment, the mixed-flow fan assembly may be arranged within the handle. In some embodiments, the main body comprises two mutually-opposing arms adapted for movement between an open configuration for receiving the length of hair therebetween and a closed configuration adjacent the length of hair. In such embodiments, contacting surfaces may be disposed on each of the two mutually-opposing arms, the contacting surfaces being arranged to come together when the two mutually-opposing arms are in the closed configuration In some embodiments, the main body is elongate and comprises air inlets, wherein the mixed-flow fan assembly is arranged to draw air from the air inlets. In such embodiments, the air inlets may comprise one or more side inlets arranged along the length of the elongate main body. The side inlets may be arranged on the handle. The side inlets may be arranged within a central 80%, preferably a central 60% of the length of the handle. The apparatus may comprise control electronics for controlling operation of the apparatus. The fan assembly may be arranged between the control electronics and the one or more outlets. The side inlets may be arranged on the handle between the control electronics and the fan assembly. The air inlets may comprise a main inlet. The main inlet may be provided at the end of the handle that is distal to the head portion. Some embodiments comprise a hinge between the two mutually-opposing arms for guiding the movement of the arms. In such embodiments, the mixed-flow fan assembly may be arranged downstream of the hinge, wherein the hinge provides an air inlet. Some embodiments comprise heating means arranged within the main body and configured to heat the airflow. The heating means may be arranged within the handle. The heating means may be arranged downstream of the mixed-flow fan assembly. The heating means may be arranged between the mixed-flow fan assembly and the one or more outlets. Some embodiments comprise a motor for driving rotation of the rotor. In such embodiments, the motor may comprise a motor housing that overlaps with the stator when viewed perpendicularly to the common axis. The present invention also provides a mixed-flow fan assembly for incorporation in a hand-held apparatus for drying and / or styling hair, the mixed-flow fan assembly comprising: a rotor comprising a plurality of rotor blades, a stator comprising a plurality of stator blades, wherein the rotor and the stator are arranged along a common axis and the rotor is rotatable relative to the stator about the common axis, and wherein the rotor is configured to output an airflow with an axial velocity component along the common axis, a radial velocity component that is radially away from the common axis and a tangential velocity component that is tangential to a circle around the common axis. In some embodiments, the mixed-flow fan assembly is configured to operate at a design operating point within a target operating range combining a static pressure increase in the range from 300 litres per minute to 700 litres per minute with an airflow rate in the range from 1500 pascals to 3500 pascals at a rotational speed of the rotor in the range from 50,000 rotations per minute to 100,000 rotations per minute. In some embodiments, each of the rotor blades comprises a respective leading edge and a respective trailing edge, and each of the stator blades comprises a respective leading edge and a respective trailing edge, and when viewed in the meridional view, the leading edges of the stator blades are parallel to the trailing edges of the rotor blades. In some embodiments, the mixed-flow fan assembly has an outer diameter in a plane perpendicular to the common axis that is in the range from 20 millimetres to 26 millimetres. The mixed-flow fan assembly may have a length along the common axis that is in the range from 30 millimetres to 50 millimetres. The rotor blades may be spaced from the stator blades, when viewed in the meridional view, by a spacing in the range from 0.4 millimetres to 1 millimetre. In some embodiments, the rotor comprises a hub portion comprising an outer surface facing away from the common axis, wherein the plurality of rotor blades protrudes from the outer surface. The outer surface may be inclined relative to the common axis at a hub angle. In such embodiments, the hub angle may be in the range from 35° to 60°. In some embodiments, the mixed-flow fan assembly comprises a shroud portion having an inner surface facing towards the common axis, wherein the shroud portion surrounds the plurality of rotor blades. The inner surface may be inclined relative to the common axis at a shroud angle. In such embodiments, the shroud angle may be in the range from 10° to 30°. The shroud angle may be smaller than the hub angle. In some embodiments, the rotor inlet area at the leading edges of the rotor blades is greater than or equal to the rotor outlet area at the trailing edges of the rotor blades. A leading edge of each stator blade may be, when viewed in the meridional view, inclined relative to the common axis at a stator blade lean angle that is equal to the hub angle. A leading edge of each rotor blade may be, when viewed in the meridional view, inclined relative to the common axis at a rotor blade lean angle that is equal to the difference of 90 degrees and the hub angle. In some embodiments, each rotor blade is angled from a rotor trailing edge relative to the common axis at a rotor trailing edge blade angle in the range from 10° to 40°. In some embodiments, the plurality of rotor blades comprises an odd number of rotor blades. Additionally or alternatively, the plurality of stator blades may comprise an odd number of stator blades. In such embodiments, the number of rotor blades may be different to the number of stator blades. In some embodiments, the number of stator blades is equal to or greater than 1.5 times the number of rotor blades or the number of rotor blades is equal to or greater than 1.5 times the number of stator blades. The number of rotor blades may be in the range from 5 to 11. The number of stator blades may be in the range from 3 to 13. The height of a rotor leading edge of each rotor blade, when viewed in the meridional view, may be in the range from 4 millimetres to 9 millimetres. The height of a rotor trailing edge of each rotor blade, when viewed in the meridional view, may be in the range from 3 millimetres to 7 millimetres. The height of the rotor trailing edge may be less than the height of the rotor leading edge. The length of the rotor blades along the hub portion, when viewed in the meridional view, may be in the range from 4 millimetres to 8 millimetres. The length of the rotor blades along an edge facing away from the hub portion, when viewed in the meridional view, may be in the range from 5 millimetres to 9 millimetres. The height of the stator blades in a direction perpendicular to the common axis may be greater than 3 millimetres. The length of the stator blades in a direction along the common axis may be in the range from 10 millimetres to 30 millimetres. In some embodiments, the mixed-flow fan assembly further comprises an inlet portion comprising an inner surface facing the common axis, wherein the inner surface is parallel to the common axis. The inlet portion may have an inlet diameter in a direction perpendicular to the common axis in the range from 15 millimetres to 19 millimetres. The inlet portion may have a length in a direction parallel to the common axis in the range from 4 millimetres to 7 millimetres. The inlet portion may comprise an inlet round at an upstream edge facing the airflow, wherein the inlet round ha a radius of curvature in the range from 1 millimetre to 3 millimetres. In some embodiments, the mixed-flow fan assembly further comprises an inlet diffuser arranged upstream of the rotor along the common axis. In such embodiments, the inlet diffuser may comprise a plurality of inlet diffuser blades arranged to receive the airflow in a direction parallel to the common axis and to output the airflow to the rotor with an axial velocity component along the common axis and a tangential velocity component that is tangential to a circle around the common axis. In some embodiments, the stator comprises a mixed flow diffuser comprising a plurality of mixed-flow diffuser blades and an axial stator portion comprising a plurality of axial stator blades. In such embodiments, the mixed-flow diffuser may be arranged to receive the airflow from the rotor and is configured to gradually remove the radial velocity component from the airflow so as to output the airflow to the axial stator portion without the radial velocity component. Each of the mixed flow diffuser blades may comprises a leading edge and each of the rotor blades may comprise a trailing edge. When viewed in the meridional view, the leading edges of the mixed flow diffuser blades may be parallel to the trailing edges of the rotor blades. Each of the mixed flow diffuser blades may comprise a trailing edge and each of the axial stator blades may comprise a leading edge. When viewed in the meridional view, the leading edges of the stator blades may be parallel to the trailing edges of the mixed flow diffuser blades. In some embodiments, the mixed-flow fan assembly further comprises one or more Helmholtz resonators, each comprising a cavity and a neck, wherein the cavity is in fluid communication with the airflow via the neck. The one or more Helmholtz resonators may overlap the stator when viewed along the common axis. The one or more Helmholtz resonators may at least partially overlap the inlet portion when viewed perpendicularly to the common axis. The one or more Helmholtz resonators comprise 4 to 8 Helmholtz resonators. In some embodiments, the mixed-flow fan assembly comprises a first stage and a second stage, wherein the first stage comprises the rotor and the stator, and wherein the second stage comprise a second rotor arranged along the common axis downstream of the stator and a second stator arranged along the common axis downstream of the second rotor, wherein the second rotor is an axial-flow rotor. The rotor and the second rotor may be arranged to be driven by the same motor. Further aspects of the present invention are set out in the detailed description. Brief Description of the Drawings Embodiments of the invention will now be described, by way of example only, and with reference to the drawings in which: Figure 1a is a perspective overview of a combined hair dryer / styler device comprising mutually-opposing drying / styling arms in an open configuration, with each arm incorporating a pair of heater plates and an airflow guide structure; Figure 1b is a perspective longitudinal cross-sectional view of the lower arm of the device shown in Figure 1a, illustrating the main internal components of the device and the airflow within the device during use; Figure 1c illustrates the device of Figure 1a in use when the mutually opposing arms are in a closed configuration with hair sandwiched between the arms being dried / styled; Figure 2a shows a cross-sectional view of a mixed-flow fan assembly used in the device shown in Figure 1; Figure 2b shows a cross sectional exploded view of the mixed-flow fan assembly shown in Figure 2a; Figure 2c shows a perspective exploded view of the mixed-flow fan assembly shown in Figure 2a; Figure 2d shows a frontal view of the rotor of the mixed-flow fan assembly shown in Figure 2a; Figure 3a shows a 2D blade diagram of the rotor and the stator of the mixed-flow fan assembly shown in Figure 2a; Figure 3b shows a meridional view of the mixed-flow fan assembly shown in Figure 2a; Figure 4a shows experimental data illustrating the performance of a prototype of the mixed-flow fan assembly shown in Figure 2a; Figure 4b shows experimental data contrasting the performance of a prototype of the mixed-flow fan assembly shown in Figure 2a to a similar prototype of an axial-flow fan assembly; Figure 5a shows a perspective view of an inlet diffuser that may be used with the mixed-flow fan assembly shown in Figure 2a; Figure 5b shows an exploded perspective view of the inlet diffuser combined with the other components of the mixed-flow fan assembly shown in Figure 2a; Figure 5c shows a 2D blade diagram of the inlet diffuser and the rotor of the mixed-flow fan assembly shown in Figure 5b; Figure 6 shows a meridional view of the mixed-flow fan assembly including a mixed-flow diffuser that may be used with the mixed-flow fan assembly shown in Figure 2a; Figure 7 shows a cross-sectional view of the mixed-flow fan assembly shown in Figure 2a including Helmholtz resonators; Figure 8 shows a meridional view of a dual stage mixed-flow fan assembly; Figures 9a to 9f show various different apparatus that may incorporate the mixed-flow fan assembly shown in Figure 2a; and Figures 10a shows a schematic cross-sectional views of an apparatus for drying and / or styling hair, and Figure 10b shows a schematic cross-sectional view of the head portion of the apparatus of Figure 10a. In the figures, like elements are indicated by like reference numerals throughout. Detailed Description of Preferred Embodiments The present embodiments represent the best ways known to the applicant of putting the invention into practice. However, they are not the only ways in which this can be achieved. Overview of Device Figure 1a is a perspective overview of a combined hair dryer / styler device 10, with arms 14, 16 in an open configuration. The arms 14, 16 may be closed, for example when in use, such as shown in Figure 1 c. Figure 1 b shows a further view of the lower arm 14 of the device 10. Referring initially to Figure 1a, the device 10 is an all-in-one handheld device that can be used to dry hair in a quick and easy manner, whilst also enabling styling of the hair (e.g. to straighten the hair, or to add “body and volume” to it). The device 10 comprises first and second mutually-opposing arms 14, 16 arranged in a broadly similar manner to the arms of a handheld hair styler. The first arm 14 has a main body part 12 at a proximal end 92 of the arm 14 and a hair treating distal end 93. The second arm 16 is coupled at its proximal end 94 to the first arm 14 and has a hair treating distal end 95. The first and second arms 14, 16 are adapted for movement between an open configuration (as shown in Figure 1a) for receiving a length of wet hair therebetween, and a closed configuration (as shown in Figure 1c) adjacent the hair, to create tension in the hair, such that, in use, when the arms 14, 16 are in the closed configuration they form an inter-arm plenum chamber across which the hair passes. Figure 1 b is a perspective view of the body part 12 of the lower arm 14 with half of the housing removed to expose the main components mounted within the body part 12. As can be seen in Figure 1b, the body part 12 houses a fan assembly 100 at the proximal end 92 of the body part 12, for drawing air (represented by the arrows 73) into the body part 12 through an air inlet 60 and for forcing the air along the body part 12 towards the distal hair treating end of the lower arm 14. The fan assembly 100 has an impeller (also referred to as a rotor) and is typically also provided with a filter 65 for preventing dust and debris from reaching the fan assembly 100. The airflow from the fan assembly 100 is heated by a heater 30 before being directed into the distal end 93 of the lower arm 14 where an air-flow guide structure 24 guides the air towards the user’s hair. More specifically, the heated air from the heater 30 passes along the distal end 93 of the lower arm 14 in a direction D1 that is substantially parallel to the length of the arm 14 and the air-flow guide structure 24 causes the air to change direction to a second direction D2, that is from the arm 14 towards the opposing arm 16, i.e. inwards into an inter-arm plenum chamber 13 formed by the arms 14, 16 in the closed position. The heater typically takes the form of an electrically-powered heating coil (or other electrical heating elements), that is operable to heat the air drawn in by the fan assembly 100. Control electronics may be mounted within the body part 12, typically between the fan assembly 100 and the heater 30. This allows the incoming air to keep the control electronics cool during use. Electrical power is provided to the device 10 via a power cord 64 which typically connects to an AC mains power supply. However, in an alternative embodiment the device 10 may be powered by one or more DC batteries or cells (which may be rechargeable, e.g. from the mains or a DC supply via a charging lead), thereby enabling the device 10 to be a cordless product. The control electronics control the operation of the device 10. As shown in Figure 1a, a control button or switch 23 may be provided on the device 10, to enable it to be turned on or off, and the control electronics can control the switching on and off of an indicator light to show whether the power is on. The control electronics can also cause a sound generator (not illustrated) to play out a sound when the device 10 is switched on and ready to use. As illustrated for example in Figure 1a, the second arm 16 is coupled to the body part 12 of the first arm 14 by means of a hinge 18, by virtue of which the first and second arms 14, 16 are movable relative to one other (in the illustrated embodiment, by moving the second arm 16 towards and away from the first arm 14). Thus, the first and second arms 14, 16 can be brought together, into the closed configuration (as shown in Figure 1c), or moved apart, into the open configuration (as shown in Figure 1a), by a user in use. In the illustrated embodiment, each of the arms 14, 16 widens relative to the body part 12 to form a “head” of the device 10, distal from the body part 12, although other embodiments are possible in which the head does not widen in the illustrated manner. The hinge 18 can incorporate any suitable means for allowing the first and second arms 14, 16 to be moved relative to one other. Preferably the hinge 18 also incorporates spring means configured to bias the first and second arms 14, 16 into the open configuration, such that the user is required to apply pressure to the arms 14, 16 to close them together (overcoming the effect of the spring means), and such that the arms 14, 16 automatically open, under the effect of the spring means, once the pressure is removed. For example, the hinge 18 may incorporate a leaf spring or a coiled spring. The hinge 18 and the spring means can be one and the same. For example, the spring means itself can be used to couple the second arm 16 to the body part 12, thereby avoiding the need to provide a separate mechanical hinge and simplifying the overall construction of the device 10. As shown in Figure 1a in the illustrated embodiment the inner surface of the first arm 14 incorporates first and second elongate heater plates 20a, 20b, extending along the length of the arm 14 either side of the airflow guide structure 24. The second arm 16 also incorporates first and second elongate heater plates 22a, 22b (not visible in Figure 1a) in corresponding positions to heater plates 20a and 20b. Each of the heater plates 20a, 20b, 22a, 22b is provided with a respective electrical heating element, operable to cause the respective heater plate to heat up. In the illustrated embodiment the target operating temperature of the heater plates 20a, 20b, 22a, 22b is typically around 120-130°C when operating on wet hair. The target operating temperature of the heater plates 20a, 20b, 22a, 22b may be around 185-200 °C when operating on dry hair. The first and second arms 14, 16 and the first and second heater plates on each arm 20a, 20b, 22a, 22b are arranged such that, when the arms 14, 16 are in the closed configuration, the first and second heater plates 20a, 20b of the first arm 14 come into contact with the first and second heater plates 22a, 22b of the second arm 16. Preferably the heater plates 20a, 20b, 22a, 22b are made of a material having relatively high thermal conductivity, and are preferably provided with one or more temperature sensors (e.g. a temperature sensor for each plate, or one or more temperature sensors that each serves a plurality of heater plates) for sensing the temperature of the heater plate. The sensed temperatures are then fed back to the control electronics so that the control electronics can control the power delivered to the heater plates 20a, 20b, 22a, 22b to maintain them at a desired operating temperature. The heater plates 20a, 20b, 22a, 22b serve a number of purposes during use of the device 10. Firstly, with the user having sandwiched a length of wet, damp or dry hair (dry hair for styling only applications) between opposing plates 20a and 22a, and between opposing plates 20b and 22b (i.e. transversely across the plenum chamber 13 formed by the first and second arms 14, 16 in the closed configuration), and by drawing the device 10 along the length of wet hair, the heater plates 20a, 20b, 22a, 22b subject the wet hair to a squeegeeing effect, removing excess unbound water, and also heat the hair to promote subsequent evaporation of the water. Secondly, the heating provided by the heater plates 20a, 20b, 22a, 22b causes the walls of the plenum chamber 13 to be heated (via thermal conduction), and also helps maintain the temperature of the airflow delivered through the plenum chamber 13 by the fan assembly 100. Thirdly, the heater plates 20a, 20b, 22a, 22b can be used to style the hair, as an integral part of the drying process. The heater plates 20a, 20b, 22a, 22b are preferably configured as ceramic float plates with springs having a low spring rate or stiffness, thereby giving good control of hair tension. Allowing for the airflow heating coil (or other heater elements) and the heater plates 20a, 20b, 22a, 22b, as well as the fan assembly 100, the overall power consumption of the device 10 is around 600-800 W, which is significantly less than a 2000 W conventional hairdryer. Of course, higher powered motors could be used in the device 10 which would increase the power consumption but would allow the device 10 to dry the user’s hair more quickly. The device 10 described above has been found to provide efficient drying and styling of the user’s hair. The combination of conductive heating of the hair by the heater plates 20a, 20b, 22a and 22b and the convective heating of the hair in the enclosed chamber 13 by the heated airflow significantly increases the drying efficiency of the device 10 over conventional hot air devices. The inventors have made a number of improvements to the fan assembly 100 that may be incorporated into the device 10 described above and these will now be described below. Improvements to fan assembly Conventionally, the fan assembly 100 in hair styling devices 10 and / or hair drying devices 10 is an axial fan assembly. In an axial fan assembly, airflow enters the rotor of the axial fan assembly in an axial direction, so along the axis of rotation of the rotor and exits the rotor of the axial fan assembly in the axial direction. Axial fan assemblies are conventionally used due to the relatively high achievable airflow rate. The inventors have found that a relatively high thrust airflow is beneficial to the effective functioning of a hair style device 10. The thrust airflow is equal to the product of air outlet velocity and air mass flow rate from the device 10. An increase in the thrust airflow may, depending on the application, result in enhanced efficiency of drying of a length of hair, an enhanced transfer of thermal energy to and from the hair, and / or an improved distribution of atomized liquid into the hair. In a drying application, for example, a higher thrust airflow increases the evaporation rate of unbound water in the hair by imparting a relatively larger force on the hair to overcome water surface tension. As more unbound water is removed, the surface area for thermal energy transfer is increased and thus thermal energy can more readily be transferred into the length of hair held by the device 10. In addition, because the hair is typically held under tension by the device 10, a higher thrust airflow creates a larger pressure differential across the length of hair that is held in tension, thereby further increasing airflow through the hair so as to improve the transfer of thermal energy. A relatively high thrust airflow at an outlet of the device 10 may be achieved by a fan assembly 100 delivering both a relatively high flow rate and a relatively high pressure. In particular, the inventors have found that a fan assembly 100 operating, under standard operating conditions, at a design operating point within a target operating range that combines an airflow rate (i.e. the volumetric flow rate of the airflow) in the range from 300 litres per minute (LPM) to 700 LPM with a pressure in the range from 1500 pascals (Pa) to 3500 Pa achieves a high thrust airflow enhancing the effectiveness of the hair style device 10. Conventional axial fan assemblies may only achieve this design operating point, in particular the desired pressure range, when run at extreme speeds (e.g. of more than 100,000 rotations per minute (100 kRPM)). However, such extreme speeds disproportionately increase the power consumption of the fan assembly (requiring large power supplies and low energy efficiency), and further lead to a significant noise level of the device 10. An axial fan assembly may alternatively use a large diameter impeller to achieve the desired pressure range. However, a fan assembly with such increased dimensions would be impractical for incorporation into the handle or elsewhere in a hand-held hair-styling device 10. The inventors have thus found that axial fan assemblies are not particularly suitable for achieving the design operating point. The present invention relates to a mixed-flow fan assembly 100 for incorporation in a hand-held hair styling device 10. The inventors have found that such a mixed-flow fan assembly 100 can achieve the design operating point for improved effectiveness of a hair styling device while maintaining a compact form factor and a relatively high energy efficiency. The mixed-flow fan assembly 100 may be designed to have an outer diameter of up to 26mm. Preferably, the outer diameter of the fan assembly 100 is in the range from 20mm to 26mm, for example around 25mm. The length of the fan assembly 100 (excluding a motor for driving the fan assembly 100) may be in the range from 30 to 50 mm. The fan assembly 100 may thus be suitable for incorporation in the device 10 without significant alteration of the form factor of the device 10. The mixed-flow fan assembly 100 achieves the design operating point at significantly lower rotational speeds (e.g. around 73 to 79 kRPM) than a comparable axial fan assembly. For example, experiments have shown that the mixed-flow fan assembly 100 may achieve a flow rate of 400 LPM and a pressure of 2000 Pa at a fan speed of 73 kRPM, requiring an input power of 28W for a total efficiency of 47%. The mixed-flow fan assembly 100 may further achieve a flow rate of 400 LPM and a pressure of 2600 Pa at a fan speed of 79 kRPM, requiring an input power of 35W for a total efficiency of 49%. The mixed-flow fan assembly 100 is more energy efficient and / or quieter at the design operating point than a comparable axial fan assembly. In general, the mixed-flow fan assembly 100 may be designed to achieve the design operating point at rotational speeds in the range from 50 kRPM to 100 kRPM. Mixed-flow fan assembly Figures 2a to 2c show various schematic views of the mixed-flow fan assembly 100. Figure 2a shows a cross-sectional view of the mixed-flow fan assembly 100, Figure 2b shows a cross sectional exploded view of the mixed-flow fan assembly 100 and Figure 2c shows a perspective exploded view of the mixed-flow fan assembly 100. The mixed-flow fan assembly 100 comprises a rotor 120 and a stator 130. The mixed-flow fan assembly 100 further comprises an inlet part 110. The inlet part 110 may also be referred to as an inlet shroud 110. The inlet part 110, the rotor 120 and the stator 130 are arranged along a common axis A. The rotor 120 is arranged downstream of the inlet part 110. The stator 130 is arranged downstream of the rotor 120. In use, air flows downstream through the mixed-flow fan assembly 100. So, in use, airflow enters the inlet part 110 and passes via the rotor 120 to the stator 130. The terms “downstream” and “upstream” are used herein to indicate directions along the common axis A, where downstream is a first direction along the common axis A and upstream is a second, opposite direction along the common axis A. Unless stated otherwise, angles and directions described herein are relative to the common axis A. An axial direction is a direction along the common axis A, a radial direction is a direction that is radial to the common axis A, and a tangential direction is a direction that is tangential to a circle around the common axis A. The rotor 120 is rotatable around the common axis A relative to the stator 130. The inlet part 110 is static relative to the stator 130. When incorporated in a device 10, the stator 130 is typically static relative to the device 10. Unless stated otherwise, any velocities and other flow parameters described herein are relative to the stator 130. Figure 2a shows arrows indicative of the airflow through the mixed-flow fan assembly 100. Airflow entering the inlet part 110 has a velocity that is in a direction D3 that is substantially parallel to the common axis A. The rotor 120 redirects the airflow in a direction D4 that is angled relative to the common axis A. So, the rotor 120 adds a component to the velocity of the airflow that is radially away from the common axis A. On exiting the rotor 120, the airflow has a velocity with both a velocity component that is radial (and so perpendicular) to the common axis A and a velocity component that is along (and so parallel to) the common axis A. The rotor 120 also adds angular momentum to the airflow. So, on exiting the rotor 120, the airflow has a velocity comprising further a velocity component that is tangential to a circle around the common axis A. The stator 130 redirects the airflow in a direction D1 that is substantially parallel to the common axis A. The stator 130 also removes (or at least reduces) the angular momentum from the airflow. On exiting the stator 130, the airflow has a velocity with a velocity component along the common axis A. The change in direction and removal of angular momentum of the airflow is recovered as an increase in static pressure by the stator 130. So, the stator 130 recovers at least part of the radial velocity component and of the tangential velocity component as an increase in static pressure. The mixed-flow fan assembly 100 differs from a conventional axial-flow fan assembly used in hair styling devices 10 in that the rotor of an axial-flow fan assembly does not add a component to the velocity of the airflow that is radially away from a common axis A. The addition of the radial velocity component by the rotor 120 of the mixed-flow fan assembly 100 allows a greater amount of momentum to be induced in the airflow for a given airflow rate compared to an axial-flow fan assembly with comparable airflow rate and dimension. So, more static pressure can be recovered by the mixed-flow fan assembly 100 than by a comparable axial fan assembly 20. The mixed-flow fan assembly 100 differs from a radial fan assembly or centrifugal fan assembly, which is another type of fan assembly, in that the rotor of a radial fan assembly redirects the airflow in a radial direction and substantially removes a component of the velocity of the airflow in a direction along a common axis A. A radial fan assembly is capable of recovering increased static pressure compared to an axial fan assembly. However, the inventors have found that a radial fan assembly is impractical for incorporation in a hand-held hair-styling device 10. The fan assembly 100 desirably outputs airflow in an axial direction. Achieving airflow output in an axial direction with a radial fan assembly requires more space than is typically available in a hand-held hair-styling device 10. The inventors have found that a mixed-flow fan assembly 100 is capable of achieving the target operating range to provide the improved specifications for enhancing operation of a hand-held hair styling device 10. The mixed-flow fan assembly 100 may be designed to operate at a design operating point within the target operating range under standard operating conditions. The actual operating point of the fan assembly 100 may differ from the design operating point under non-standard operating conditions, for example when the output of airflow from the device 10 is blocked or constrained by adjacent hair. An exemplary detailed embodiment of the fan assembly 100 is described with reference to Figures 2a to 2c. Dimensions and other parameters defining an exact design of the mixed-flow fan assembly 100 to achieve the target operating range while maintaining a compact form factor are also set out in the following description. The rotor 120 comprises a hub portion 122 and a plurality of rotor blades 124. The rotor 120 may be a rigid body. Each rotor blade 124 is fixedly coupled to the hub portion 122. The rotor blades 124 protrude from the hub portion 122 away from the common axis A. The plurality of rotor blades 124 is rotationally symmetric about the common axis A, in particular n-fold rotationally symmetric, where n is the number of rotor blades 124. The hub portion 122 comprises an outer surface to which the plurality of rotor blades 124 are coupled. The outer surface of the hub portion 122 faces away from the common axis A. The outer surface of the hub portion 122 is exposed to the airflow. The outer surface of the hub portion 122 is angled relative to the common axis A. The outer surface of the hub portion 122 is frustoconical in the exemplary embodiment, so comprises of portions of the outer surface of a truncated cone. The hub portion 122 further comprises a bore 128 for receiving an output shaft of a motor 140, although in general any other feature allowing coupling to the motor 140 may be provided on the hub portion 122. The stator 130 comprises an outward-facing surface 132, an inward-facing surface 136, and a plurality of stator blades 134. The outward-facing surface 132 faces away from the common axis A. The inwards-facing surface 136 faces towards the common axis A. Both of the outward-facing surface 132 and the inward-facing surface 136 are exposed to the airflow. The plurality of stator blades 134 is rotationally symmetric about the common axis A, in particular m-fold rotationally symmetric, where m is the number of stator blades 134. The outward-facing surface 132 is cylindrical in the exemplary embodiment, in particular parallel to the common axis A. The inwards-facing surface 136 is cylindrical, in particular parallel to the common axis A. The outward-facing surface 132 and the inwards-facing surface 136 face each other. The outward-facing surface 132 and the inwards-facing surface 136 are spaced from each other so as to form a space therebetween. The plurality of stator blades 134 is provided in the space. The stator blades 134 are arranged between the inwards-facing surface 136 and the outwardfacing surface 132 when viewed along the common axis A. The stator 130 may be a rigid body. The stator blades 134 protrude from and are fixedly connected to both the inwards-facing surface 136 and the outward-facing surface 132. The inlet part 110 comprises a first portion 112 and a second portion 114. The first portion 112 is located upstream of the rotor 120 along the common axis A. The first portion 112 may also be referred to as an inlet portion 112. The second portion 114 overlaps with the rotor 120 along the common axis A. The second portion 114 may also be referred to as a shroud portion 114. The inlet part 110 may be a rigid body. The first portion 112 comprises an inner surface that faces towards the common axis A. The inner surface is exposed to the airflow. A portion of the inner surface of the first portion 112 is rounded, such that the portion tapers towards the common axis A. The cross-section that is orthogonal to the common axis A of the space within the first portion 112 is gradually reduced in the downstream direction. Another portion of the inner surface of the first portion 112 is parallel to the common axis A. The inner surface of the first portion 112 comprises a cylindrical surface. The second portion 114 comprises an inner surface that faces towards the common axis A. The inner surface is exposed to the airflow. The inner surface of the second portion 114 is angled relative to the common axis A. In the depicted exemplary embodiment, the inner surface of the second portion 114 is frustoconical, although in general the inner surface of the second portion 114 may be shaped differently and may be, for example, curved. The inner surface of the second portion 114 faces the rotor 120. The inner surface of the second portion 114 is spaced apart from the outer surface of the hub portion 122 so as to form a space therebetween. The rotor blades 124 are arranged in the space. Figure 2a further shows a motor 140. The motor 140 is arranged to drive rotation of the rotor 120 relative to the stator 130. The motor 140 comprises a motor housing 142 and an output shaft 144. The output shaft 144 is coupled to the rotor 120, for example to the bore 128 of the rotor 120. The motor 140, in use, drives rotation of the output shaft 142. The motor 140 may be any type of motor suitable for driving rotation of the rotor 120, for example a conventionally available compact AC or DC motor. As shown in Figure 2a, the motor housing 142 of the motor 140 may overlap with the stator 130 along the common axis A, so may overlap when viewed perpendicularly to the common axis A. The length of the fan assembly 100 along the common axis A may thus be reduced compared to a design in which such overlap is not enabled. At least part of the motor housing 142 of the motor 140 may be arranged within the stator 130, so at least part of the stator 130 may surround or envelop at least part of the motor housing 142. Relation of blade angles Figure 3a shows a 2D blade diagram of a rotor blade 124 and a stator blade 134. Figure 3a also shows the velocities of the airflow at various points on the rotor blades 124 and stator blades 134. V0, V1 and V2 are absolute velocities of the airflow relative to the stator 130. W0, W1 and W2 are relative velocities of the airflow relative to the rotor 120. The rotor blade 124 comprises a rotor leading edge 124a and a rotor trailing edge 124b. The stator blade 134 comprises a stator leading edge 134a and a stator trailing edge 134b. The leading edges 124a, 134a face upstream and the trailing edges 124b, 134b face downstream. The rotor blade 124 is angled from the rotor leading edge 124a at a rotor leading edge blade angle a1 relative to the common axis A. The rotor blade 124 is angled from the rotor trailing edge 124b at a rotor trailing edge blade angle pi relative to the common axis A. The stator blade 134 is angled from the stator leading edge 134a at a stator leading edge blade angle a2 relative to the common axis A. The stator blade 136 is angled from the stator trailing edge 134b at a stator trailing edge blade angle p2 relative to the common axis A. The blade angles cd, pi, a2, p2 are the respective angles of the camber line (an imaginary line halfway between opposing surfaces of the blade) relative to the common axis A when projected onto the hub portion 122, so relative to the common axis A rotated in the meridional plane by the hub angle ¢1. The blade angles cd, pi, a2, p2 are defined herein relative to the common axis A, and not relative to a plane that is perpendicular to the common axis A. The blade angles cd, pi, a2, p2 are dependent on each other to achieve optimal performance at the design operating point. In general, the blade angles cd, pi, a2, P2 are set such that the blade edges 124a, 124b, 134a, 134b align with the velocity of the absolute or relative velocity of the airflow at the respective blade edges. Figure 3b shows the meridional view of the rotor blades 124 and stator blades 134. The span of the rotor blades 124 (from the hub portion 122 towards the shroud portion 114) and the span of the stator blades 134 (from the outward-facing surface 132 to the inward-facing surface 136) is shown in Figure 3b by the span lines numbered from 0.00 to 1.00. The blade angles cd, pi, a2, p2 may vary across the span of the blades. For each blade edge of the rotor 120 and of the stator 130, five blade angles cd, pi, a2, p2 are typically defined to specify the blade design. The analysis below is done for a single blade angle cd, pi, a2, p2 for each edge of the rotor 120 and stator 130, but it will be appreciated that multiple blade angles cd, pi, a2, p2 (e.g. five blade angles cd, pi, a2, p2) may be defined for each edge of the rotor 120 and stator 130. The fan assembly 100 may be configured such that: 1) The absolute velocity V0 of the airflow at the rotor inlet, and so at the rotor leading edge, is parallel to the common axis A (at the design operating point). So, V0 in Figure 3a parallel to the common axis A. In alternative embodiments comprising an inlet diffuser, as described in relation to Figures 5a to 5c, the absolute velocity V0 of the airflow at the rotor inlet may have a tangential velocity component and not be parallel to the common axis A. 2) The absolute velocity V1 of the airflow at the rotor trailing edge 124b is equal to the absolute velocity V1 of the airflow at the stator leading edge 134a. So, the stator blade 134 is angled from the stator leading edge 134a so as to be parallel to the absolute velocity V1 of the airflow at the rotor trailing edge 124b. 3) The absolute velocity V2 of the airflow at the stator outlet (and so at the stator trailing edge 134b) is parallel to the common axis A. Swirl (angular momentum) of the airflow has been completely removed by the stator. The stator trailing edge angle p2 aligns with the absolute velocity V2, and so is 0: 02 =0° In view of the three points above and the velocity triangles shown in Figure 3a, the optimal blade angles cd, pi, a2, p2 may be determined. These blade angles cd, pi, a2, p2 are dependent on each other for optimal performance at the design operating point, and so considered as one entity herein. Each rotor blade 124 is angled from the rotor leading edge 124a so as to align with the relative velocity W0 of the airflow relative to the rotor blade 124 at the rotor leading edge 124. The relative velocity W0 is equal to the vector sum of the absolute velocity V0 at the rotor leading edge 124a and the tangential velocity uO of the rotor leading edge 124a. The tangential velocity uO is equal to the product of rotor rotational velocity co (in rad / s) and radius rO of the rotor leading edge 124a, noting that the radius rO increases along the span of the rotor blade 134 because the rotor blade 134 protrudes away from the common axis A. The absolute velocity V0 is the quotient of flow rate Q and inlet area A0 at the rotor leading edge 124a. The inlet area A0 is the total area of a surface between the hub portion 122 and the shroud portion 114 that is perpendicular to the direction of the airflow at the leading edges of the rotating rotor blades 124. The rotor leading edge blade angle a1 may be defined as a function of the blade radius rO at the rotor leading edge 124a: / ur0A0\ a, = tan 1 ------ \ Q J Q is the volumetric airflow rate and co is the rotational velocity of the rotor about the common axis A. Q and co may be set by the design operating point of the fan assembly 100. A0 is the inlet area at the rotor leading edge 124a. In embodiments in which the stator 130 is arranged directly downstream of the rotor 120, as depicted in Figure 3a, the absolute velocity V1 of airflow at the rotor trailing edge 124b is equal to the absolute velocity of airflow at the stator leading edge 134a. The relation between rotor trailing edge blade angle pi and the stator leading edge blade angle a2 may thus be defined as: 6>A1r1 tan a2 = ——--tan pr where A1 is the outlet area at the rotor trailing edge, and r1 is the blade radius at the rotor trailing edge. The outlet area A1 at the rotor trailing edge is assumed to be the same as the inlet area at the stator leading edge, due to the short distance dr-s between these two areas. V1 is equal to the quotient of the airflow rate Q and the outlet area A1, where the outlet area A1 is the total area of a surface between the hub portion 122 and the shroud portion 114 that is perpendicular to the direction of the airflow at the trailing edges of the rotating rotor blades 124. The outlet area A1 at the rotor trailing edge 124b is assumed to be substantially equal to the inlet area at the stator leading edge 134a. The spacing between the rotor 120 and the stator 130, in particular the spacing dr-s between the respective blades, may be minimized for improved efficiency. The inventors have found that the spacing dr-s is preferably in the range from 0.4mm to 1mm, for improved performance while allowing for manufacturing tolerances. Similarly, the radius r1 at any point on the rotor trailing edge 124b is assumed to be substantially equal to the radius r1 at a corresponding point at the stator leading edge 134a. As shown in Figure 3b, the stator leading edge 134a is parallel to the rotor trailing edge 124b, in particular when viewed in the meridional view. It will be appreciated that, while the above expressions for the blade angles may be used to optimize performance at a design operating point, in practice the actual blade angles cd, pi, a2, p2 may diverge from the values of blade angles calculable by these expressions. The actual blade angles cd, pi, a2, p2 may, for example, be within 5 degrees of the theoretically calculable values above. Theoretical estimation of pressure rise A theoretical estimate of the pressure rise at the design operating point may be expressed as: AP = puijuj — 2uxV tanpr + V^^tan2 px + sin2 0X) Where: AP is the theoretical pressure rise of the fan assembly 100; p is the density of the fluid (e.g. air) flowing through the fan assembly 100; ui is the tangential velocity of the rotor trailing edge 124b; Vnl is the velocity component in a direction parallel to the common axis A when projected onto the hub portion 122 (so parallel to the common axis A rotated in the meridional plane by the hub angle ¢1) of the velocity of the airflow at the rotor trailing edge 124; Pi is the rotor trailing edge blade angle; and ¢1 is the mixed-flow hub angle. This theoretical estimate does not account for aerodynamic losses or other inefficiencies, and so in practice the pressure rise of the fan assembly 100 is expected to differ from the theoretical pressure rise AP. However, the theoretical estimate provides key insights into parameters affecting the pressure rise. The pressure rise is proportional to the square of the tangential speed at the rotor trailing edge 124b. The tangential speed may be increased by increasing the angular velocity of the rotor 120. So, a faster spinning rotor will achieve a greater pressure rise. In practice, the maximum angular velocity of the rotor 120 will be limited by power and noise considerations. The pressure rise is inversely proportional to the rotor trailing edge blade angle Pi. So, reducing the blade angle Pi may lead to an increase in the pressure rise. The pressure rise is proportional to the mixed-flow hub angle ¢1. So, increasing the mixed-flow hub angle ¢1 may lead to an increase in the pressure rise. Stall considerations A lower bound for the rotor trailing edge blade angle Pi may be determined by considering conditions under which the rotor blade 124 undergoes stall. The flow rate at which stall occurs can be estimated using Lieblein’s Diffusion Factor D. Lieblein’s Diffusion Factor D is an experimentally derived coefficient that relates the relative boundary layer thickness to the inlet and outlet velocities of the airflow. A diffusion factor D that is lower than 0.6 may consistently avoid rotor blade stall. Lab testing has shown that blade stalls occurs approximately when the Diffusion Factor D at mid-span (so at span 0.50) reaches 0.6 +- 10%. The model for the Diffusion Factor D shows that stall typically starts at the blade tip (span 1.00) and proceeds down the blade to the root (span 0.00). The Diffusion Factor D can be determined by: i Wo+ 2gW <0.6 where: W1 is the velocity of the airflow at the rotor trailing edge 124b relative to the rotor trailing edge 124b, where W. = = —-—; a a 1 cosp1 A1cosp1 W0 is the velocity of the airflow at the rotor leading edge 124a relative to the rotor leading edge 124a, where Wo = + u„ = (—) + (<yr0)2 J Wei is the tangential component of the velocity of the airflow at the rotor trailing edge 124b relative to the rotor 120 and equal to Wgi = = —; sin pi sin pi Weo is the tangential component of the velocity of the airflow at the rotor leading edge 124a relative to the rotor 120 and equal to Wgo = = —-—; cosa yt0cosa o is the blade solidity, equal to the blade chord length L to the rotor arc per blade, so a = where nh is the number of rotor blades. The blade chord length L is the distance between the leading edge and the trailing edge of a blade. So, the onset of rotor stall may be delayed (i.e. the airflow rate at which stall occurs may be reduced) by increasing W1, decreasing WO (thereby also decreasing W0O) and increasing the blade solidity o. As such, there are a number of ways to delay the onset of rotor stall by design of the dimensions and configuration of the rotor. For example, the rotor inlet area AO and the rotor outlet area A1 may be decreased to delay the onset of stall. Such a reduction in rotor inlet area AO and / or rotor outlet area A1 comes, however, at the expense of a decrease of the airflow rate. The chord length L may be increased to delay the onset of stall. Increasing the chord length L may, however, increase the dimensions of the rotor 120. The rotor trailing edge blade angle pi may be increased to delay the onset of stall. As such, there is effectively a lower limit to the rotor trailing edge blade angle pi for achieving a desired rotor stall performance. Changing the blade angles at various points throughout the rotor blade 124 may only delay the onset of stall to a limited extent for a given design and may decrease the achievable pressure rise. The onset of rotor stall may also be delayed by adding inlet guide vanes so as to decrease the values of WO and WO. This is explained further in relation to the inlet diffuser of Figures 5a to 5c. Similar considerations are relevant to the stator blade angle p2 in relation to stator stall. In particular: 0.6 where: V2 is the velocity of the airflow at the stator trailing edge 134b relative to the stator trailing edge 134b; V1 is the velocity of the airflow at the stator leading edge 134a relative to the stator leading edge 134a; Ve2is equal to the tangential component of the velocity of the airflow at the stator trailing edge 134b relative to the stator trailing edge 134b, so Q tan B2 = vt2 = Vn2 tan^2 = Vei is equal to the tangential component of the velocity of the airflow at the stator trailing edge 134b relative to the stator trailing edge 134b, so ^0i = = «i - Ku tan & o is the blade solidity, equal to the stator chord length L to the stator arc per blade, so a = where nh is the number of stator blades So, an increase of the stator trailing edge blade angle / ?2 results in a delay in the onset of stall, at the expense of a decrease in pressure rise. Initial estimations show that the stall flow rate of the stator 130 cannot be significantly shifted through blade angles alone but is an inherent characteristic of axial and mixed-flow fan assemblies. The inventors have found that, on balance, rotor stall is more detrimental to the performance of the fan assembly 100 than stator stall. The stator trailing edge blade angle may thus be set to 0° so as to improve the pressure rise in the stator 130. Blade angles In view of the above, the inventors have determined values for the blade angles a1, pi, a2, p2 that make the fan assembly 100 particularly suitable for achieving the design operating point. The rotor trailing edge blade angle pi, and also the stator leading edge blade angle a2 by virtue of its relation to the rotor trailing edge blade angle pi, has a significant impact on the efficiency and stall performance of the fan assembly 100. Reducing the rotor trailing edge blade angle pi leads to an increase in the pressure rise achievable by the fan assembly 100 but may lead to a higher stall airflow rate. Increasing the rotor trailing edge blade angle pi leads to a delay in the onset of stall (i.e. a reduction in the stall airflow rate) but may lead to a reduced pressure rise. The rotor trailing edge blade angle pi may be in the range from 10° to 40°. The rotor trailing edge blade angle pi may be about 30°, for example. The stator leading edge blade angle a2 may be calculated at the design operating point by its relation to the rotor trailing edge blade angle pi. The stator leading edge blade angle a2 may be in the range from 50° to 80°, for example in the range from 60° to 75°. The rotor leading edge blade angle a1 may be in the range from 45° to 80°, for example in the range from 55° to 75°. The stator trailing edge blade angle p2 is in the range from 0° to 10°, preferably about 0°. Hub angle, shroud angle and blade lean angles Figure 3b shows the hub angle ¢1 and the shroud angle ¢2. The hub angle ¢1 is the angle between the outer surface of the hub portion 122 and the common axis A, for example when viewed in the meridional view. The shroud angle ¢2 is the angle between the inner surface of the shroud portion 114 and the common axis A, for example when viewed in the meridional view. The shroud angle ¢2 is for illustrative purposes shown on an edge of the rotor blade 124 that is facing away from the hub portion 122 in the meridional view of the rotor blade 124 in Figure 3b. The edge of the rotor blade 124 that is facing away from the hub portion 122 is generally parallel to the inner surface of the shroud portion 114 in the meridional view. The rotor tip gap between the rotor blades 124 and the shroud portion 114 may be relatively small so as to improve aerodynamic efficiency and reduce noise due to vortex generation at the rotor tips. The rotor tip gap may be in the range from 0.1mm to 0.4mm, for example. The hub angle ¢1 may be in the range from 35° to 55 °, for example about 45°. The shroud angle ¢2 may be in the range from 10° to 30°, for example about 20°. The shroud angle ¢2 may be smaller than the hub angle ¢1. As apparent from the theoretical estimate of the pressure rise, described above, the pressure rise increases with an increase in the hub angle ¢1. However, an increase in the hub angle ¢1 results in a decrease of the airflow rate that can be achieved by the fan assembly 100, because the normal velocity at the rotor trailing edge 124b is reduced. In general, a relatively larger hub angle ¢1 may be used to approach a radial or centrifugal fan assembly design to achieve a larger pressure rise at the expense of a smaller flow rate. A relatively smaller hub angle ¢1 may be used to approach an axial fan assembly design to achieve a larger airflow rate at the expense of a smaller flow rate. The shroud angle ¢2 may be defined, relative to the hub angle ¢1, by the open area AO at the rotor leading edge 124a and the open area A1 at the rotor trailing edge 124b. The airflow rate Q is proportional to the open area AO at the at the rotor leading edge 124a, so Q oc Ao. As such, it is desirable to maximise AO so as to increase the airflow rate Q. From the theoretical stall considerations, and considering that 1111 Wn oc —, W, oc —, W^n oc —, oc —, V / 1 X / 4 A A n0 nl n0 nl it is further desirable to minimize A1 in an effort to delay the onset of stall of the rotor 120. The inventors have found that the performance of the fan assembly 100 is improved when the open area A0 at the rotor leading edge 124a is equal to or greater than the open area A1 at the rotor trailing edge 124b, so Ao >Ar. Expansion of the airflow volume, which would lead to an adverse pressure gradient and risks unstable airflow and airflow separation, may thus be avoided. As such, the shroud angle ¢2 is preferably smaller than the hub angle ¢1, so >< / >2- Figure 3b also shows the stator blade lean angle 01*. The stator blade lean angle <t>1* is the angle, when viewed in the meridional view, between the stator leading edge 134a and the common axis A. The stator blade lean angle 01* may be in the range from 35° to 55°, for example about 45°. The inventors have found that the stator blade lean angle 01* is preferably equal to the hub angle ¢1. Figure 3b also shows the rotor blade lean angle ¢3. The rotor blade lean angle ¢3 is the angle, when viewed in the meridional view, between the rotor leading edge 124a and the common axis A. The inventors have found that the rotor blade lean angle ¢3 is preferably equal to the difference between 90 degrees and the hub angle ¢1, so ¢3 = 90°- ¢1. So, the rotor leading edge 124a and / or the rotor trailing edge 124b may, when viewed in the meridional view, be perpendicular to the outer surface of the hub portion 122. Number of blades The number of rotor blades 124 and the number of stator blades 134 has an effect on the efficiency and reliability of the fan assembly 100. The number of rotor blades 124 may be in the range from 5 to 11 (including 5 and 11). The number of stator blades 134 may be in the range from 3 to 13 (including 3 and 13). The inventors have found that these numbers of blades achieve a balance of efficiency and reliability, while maintaining relatively low noise, that makes the fan assembly 100 particularly suitable for incorporation in the device 10. A larger number of rotor blades 124 and stator blades 134 may help improve the efficiency of the fan assembly 100, up to a limit. Such an improvement in efficiency is dependent on the blade angles a1, pi, a2, p2 and the blade chord of the rotor blades 124 and stator blades 134. Figure 2d shows a view of the rotor 120 in a downstream direction along the common axis A. Figure 2d shows the blade turn angle 6 of each blade around the common axis Aina plane that is perpendicular to the common axis A. The blade turn angle 6 is the angle, when viewed in a plane perpendicular to the common axis A, between i) a line connecting the common axis A and connection point of the blade to the hub portion 122 or inner surface 132 that is closest to the common axis A and ii) a line connecting the common axis A and a connection point of the blade to the hub portion 122 or inner surface 132 that is furthest away from the common axis A. If the number of blades N is relatively small for a given blade turn angle 6 (so N <^), then the blades do not interact with the air efficiently such that the magnitude of airflow and increase in pressure that can be produced is limited. If the number of blades N is relatively large for a given blade turn angle 6 (so N >^), then the airflow may be 3 blocked by the rotor 120 and / or stator 130 such that the volume of air that is movable by the fan assembly 100 is limited. There may also be manufacturing limitations on how close the blades can be arranged on the rotor 120 and / or stator 130, dependent on the blade angles a1, pi, a2, p2. The number of blades may be the quotient of 360 degrees and the blade turn angle 6, rounded up or down to the nearest integer. Furthermore, as described in relation to stall limitations, the number of blades has an effect on the blade solidity o. An increase in the number of blades results in an increased blade solidity, such that stall of the rotor 120 and / or stator 130 can be delayed by increasing the number of rotor blades 124 and / or stator blades 134. Preferably, the number of rotor blades 124 and / or the number of stator blades 134 is odd. Further preferably, the number of rotor blades 124 is different from the number of stator blades 134. Providing an odd number of rotor blades 124 and / or stator blades 134 and different numbers of rotor blades 124 and / or stator blades 134 improves the sound quality of the fan assembly 100. The significant frequency of the fan assembly 100 is the blade pass frequency, so the frequency at which the rotor blades 124 pass the stator blades 135. The blade pass frequency f is the product of the number of rotor blades nb and the rotational speed a) of the rotor 120, so f=nb m. Providing a different odd number of rotor blades 124 and stator blades 134, such as 7 rotor blades 124 and 11 stator blades 134, the interaction between rotor 120 and stator 130 is reduced so as to keep the blade pass frequency amplitude to a minimum. The ratio of the number of rotor blades 124 to the number of stator blades 134 may be greater or equal to 1.5, or less or equal to 1 / 1.5. So, keeping in mind that the number of rotor blades 124 and the number of stator blades 134 may be an odd number: stator 3nh . Drotor 2 OR Testator 2nh . °rotor odd odd For example, the number of rotor blades 124 may be 5 and the number of stator blades 134 may be 7 or 3. Alternatively, the number of rotor blades 124 may be 7 and the number of stator blades 134 may be 11 or 5. Alternatively, the number of rotor blades 124 may be 9 and the number of stator blades 134 may be 13, or 5 or 7. Alternatively, the number of rotor blades 124 may be 11 and the number of stator blades 134 may 7. Rotor blade dimensions Figure 3b shows the height HR1 of the rotor leading edge 124a and the height HR2 of the rotor trailing edge 124b. The heights HR1 and HR2 are the distances, when viewed in the meridional view, between the hub portion 122 and an outermost point of the rotor leading edge 124a and rotor trailing edge 124b respectively, where the outermost point is furthest away from the common axis A. The length LR1 of the rotor blade 124 along the hub portion 122 and the length LR2 of the rotor blade 124 along an edge away from the hub portion 122 are also shown. The length LR1 is the distances, when viewed in the meridional view, of the rotor blade 124 along the hub portion 122. The length LR2 is the distances, when viewed in the meridional view, of an outermost edge of the rotor blade 124, so the distance between the outermost points of the rotor leading edge 124a and the rotor trailing edge 124b. The height HR1 of the rotor leading edge 124a may be in the range from 5mm to 9mm. The height HR2 of the rotor trailing edge 124b may be in the range from 4mm to 7mm. The height HR1 of the rotor leading edge 124 may be greater than the height HR2 of the rotor trailing edge 124b. The length LR1 of the rotor blade 124 along the hub portion 122 may be in the range from 4mm to 8mm. The length LR2 of the rotor blade 124 along an edge facing away from the hub portion 122 may be in the range from 5mm to 9mm. The inventors have found that these values for the heights HR1 and HR2 and lengths LR1 and LR2 achieve a balance between achievable performance and compactness of the fan assembly 100 that makes it particularly suitable for incorporation in the device 10. An increase in the heights HR1 and HR2 leads to an increase in the cross-sectional area of the rotor 124, such that a greater airflow rate can be achieved due to an increase in the lift force applied by the rotor blades 124 on the airflow. It is thus desirable to increase the heights HR1 and HR2. The height HR1 may be increased by decreasing the value of ro in Figure 3b, so by decreasing the shortest distance between the rotor blades 124 and the common axis A. It is thus desirable to minimise the minimum radius of the rotor leading edge 124. The compactness of the fan assembly 100 presents a conflicting criterium for the heights HR1 and HR2. The height HR2 is desirably minimized to reduce the outer diameter of the fan assembly 100. In axial fan assemblies, the lengths LR1 and LR2 do not have a significant effect on fan assembly 100 performance, provided that the desired blade angles and velocities at the rotor trailing blade edge can be achieved. However, if the difference between rotor leading edge blade angle a1 and rotor trailing edge blade angle pi is relatively large, a short rotor blade 124 may lead to a reduction in efficiency of the fan assembly 100 due to the relatively rapid change in direction of the airflow along the rotor blade 124. The aspect ratio (so ratio of height HR1 to LR1) of the rotor of an axial flow assembly is typically approximately 1. In a mixed-flow fan assembly 100, the lengths LR1 and LR2 have a more significant effect on fan assembly 100 performance due to the varying radius of the rotor blade 124. The theoretical pressure rise of the mixed-flow fan assembly 100, described above, suggests that an increase in the value of uA = results in an increase in the pressure rise. So, it is desirable to maximise the radius of the rotor trailing edge 124b. This may be achieved by increasing the blade lengths LR1 and LR2. The blade lengths LR1 and LR2 also have an effect on the onset of stall of the rotor 120. In particular, the blade chord length L and the blade turn angle 0 are dependent on the length LR1 and the rotor blade angles a1 and pi. From Lieblein’s Stall Criterion, described above, an increase in the blade solidity a delays the onset of stall. The blade solidity a is proportional to the blade chord length L, so an increase in the blade lengths LR1 and LR2 may delay the onset of stall. However, as discussed in relation to the number of blades, a decrease in the blade chord length L may be balanced with an increase in the number of blades to maintain a desired blade solidity a. So, the blade lengths LR1 and LR2 may be kept relatively short to achieve a compact fan assembly 100 design by increasing the number of blades to achieve a desired blade solidity and stall performance. Stator blade dimensions Figure 3b also shows the height HS1 of the stator blade 134 and the length LS1 of the stator blade 134. The height HS1 is the height of the stator blades 134 in a direction radially outward from the common axis A. The height HS1 is the distance between the outward-facing surface 132 and the inward-facing surface 136. The height HS1 is the distance, when viewed in the meridional view, between an edge of the stator blade 134 that is closest to the common axis A and an edge of the stator blade 134 that is furthest from the common axis A. The length LS1 is the distance, when viewed in the meridional view, of the edge of the stator blade 134 that is coupled to the outwardfacing surface 132. The height HS1 of the stator blades 134 may be greater than 3mm. The length LS1 of the stator blades 134 may be in the range from 10mm to 30mm. The inventors have found that these values for the height HS1 and length LS1 achieve a balance between achievable performance and compactness of the fan assembly 100 that makes it particularly suitable for incorporation in the device 10. The height HS1 of the stator blades 134 is dependent on the height HR2 of the rotor trailing edge 124b and the dimensions of the motor 140. The height HS1 may be constant along the length of the stator blade 134. The inventors have found a height HS1 of at least 3mm ensures that the risk of choke in the stator 130 is reduced. The maximum value for the height HS1 is set by the overall desired diameter of the fan assembly 100 and the diameter of the motor 140 that is surrounded by the stator 130. The length LS1 of the stator blades 134 has an effect on the onset of stall and on the airflow rate through the stator. A relatively longer stator 130 allows the direction of the airflow to change more gradually, thus reducing the likelihood of airflow separation at the stator 130. A relatively longer stator 130 achieves a relatively higher stall airflow rate. Overall, the length LS1 is mostly determined by the length of the motor 150 or other size requirements of the fan assembly 100. There is also a mechanical and manufacturing limit to the possible values for the length LS1. Inlet dimensions Figure 2b shows dimensions of the inlet portion 112, in particular the inlet diameter d, the inlet length I and the inlet round ri. The inlet diameter d is the narrowest diameter of the inlet portion 112 in a plane that is perpendicular to the common axis A. A relative larger inlet diameter d leads to a larger rotor leading edge height HR1, and so allows a higher airflow rate to be achieved. The inlet diameter d is dependent on the leading edge rotor height HR1 and the mixed-flow hub and shroud angles 01 and 02. The inlet diameter may be in the range from 15mm to 19mm. The inventors have found that an inlet diameter d within that range achieves a practical balance of a sufficient airflow rate and a compact design of the fan assembly 100. The inlet length I is the length of the inlet portion 112. A relatively greater inlet length I leads to an improvement in performance of the fan assembly 100, due to the alignment of the inlet flow to the axial direction along the common axis A. The inlet length I may be in the range from 4mm to 7mm. The inventors have found that an inlet length I within that range achieves a practical balance of performance of the fan assembly 100 and a compact design of the fan assembly 100. The inlet round ri is the upstream edge of the inlet portion 112. The inventors have found that a relatively greater radius of curvature of the inlet round ri results in an improvement of the efficiency of the fan assembly 100. The inventors have found that the inlet round may have a radius of curvature in the range from 1mm to 3mm. Performance of mixed-flow fan assembly Figures 4a and 4b show experimental data showcasing the performance of the mixed-flow fan assembly 100, in Figure 4b compared to the performance of an axial-flow fan assembly. Figures 4a shows the performance of a prototype fan assembly 100 having an outer diameter of 25mm. The inlet length L is 5mm, with a round ri of r2mm. The hub angle 01 is 45° and the shroud angle 02 is 20°. The height HR1 of the rotor leading edge 124a is 7.5mm. The length LR1 of the rotor blade 124 along the hub portion 122 is 5mm. The height HR2 of the rotor trailing edge is 5.2mm. The length LS1 of the stator blade 134 along the outward-facing surface 132 is 15mm. The spacing d(r-s) between rotor 120 and stator 130 is 0.8mm. The design rotational speed is 75 kPRM. The table below summarizes the detailed design of the blades of the prototype, for a design airflow rate (AFR) of 450 LPM, showing (at five positions along the span of the rotor 120) the rotor leading edge blade radius r(LE), the rotor trailing edge blade radius r(TE), the rotor leading edge blade height h(LE), the rotor trailing edge blade height h(TE), as well as values for the velocities uO, u1, Vn1, Vn2 and the blade angles cd, pi, a2, p2. The blade angle pi is set to 30° and the remaining blade angles cd, a2, P2 are calculated as per the blade relations described above. AFR r(LE) r(TE) h(LE) h(TE) uO u1 Vn1 Vn2 Span (LPM) (mm) (mm) (mm) (mm) (m / s)(m / s)(m / s) (m / s) cd p1 a2 p2 (°) (°) (°) (°) 0 450 4.06 7.60 7.47 5.17 31.9 59.7 22.08 24.18 55.3 30 62.1 0 0.25 450 5.65 8.57 7.47 5.17 44.4 67.3 22.08 24.18 63.6 30 65.6 0 0.5 450 7.24 9.55 7.47 5.17 56.9 75.0 22.08 24.18 68.8 30 68.4 0 0.75 450 8.83 10.52 7.47 5.17 69.3 82.7 22.08 24.18 72.3 30 70.6 0 1 450 10.41 11.50 7.47 5.17 81.8 90.3 22.08 24.18 74.9 30 72.4 0 Figure 4a shows a chart of the performance of the prototype of the mixed-flow fan assembly 100. The x-axis represents the airflow rate (in LPM) and the y-axis represents the static pressure rise (in Pa). The dotted line 201 is a lower target for the resistance to the airflow expected to be present in a device 10. The dotted line 202 is an upper target for the resistance to the airflow expected to be present in a device 10. The line 211 represents the performance of the prototype at a rotational speed of 73kRPM. The line 212 represents the performance of the prototype at a rotational speed of 79kRPM. The intersections of lines 211, 212 with the dotted lines 201, 202 represent the operating point achievable by the fan assembly 100 in respect of the respective resistance to the airflow. As apparent from Figure 4a, the prototype can achieve a design operating point within the desired operating range. The target airflow rate is set to 400 LPM. The prototype (having compact dimensions) can achieve the target airflow rate at a pressure of about 2000 Pa for the lower target of the resistance of airflow with a rotational speed of 73 kRPM. This requires an approximate continuous power for driving the motor 140 of 28W, achieving a total efficiency at the design operating point of about 47%. The prototype (having compact dimensions) can achieve the target airflow rate at a pressure of about 2600 Pa for the higher target of the resistance of airflow with a rotational speed of 79 kRPM. This requires an approximate continuous power for driving the motor 140 of 35W, achieving a total efficiency at the design operating point of about 49%. The mixed-flow fan assembly 100 is thus well-suited for achieving the design operating point within the desired operating range, within a compact form factor and moderate rotational speeds. Figure 4a also shows the points 231 of maximum efficiency of the mixed-flow fan assembly 100. The region 233 corresponds to an operating range at which blade stall occurs. So as to reduce the risk of blade stall, the mixed-flow fan assembly 100 is desirably operated in region 232, so just below the point of maximum efficiency. Figure 4b shows the performance of a mixed-flow fan assembly 100 compared to that of a comparable axial-flow fan assembly (with similar overall diameter), driven at the same input power of 32W. The x-axis represents the airflow rate (in LPM) and the y-axis represents the static pressure rise (in Pa). The dotted line 201 is a lower target for the resistance to the airflow expected to be present in a device 10 (also referred to as the back pressure of the device 10). The line 211 represents the performance of a mixed-flow fan assembly at a rotational speed of 86.5kRPM. The line 221 represents the performance of an axial-flow fan assembly at a rotational speed of 80kRPM. The intersection of lines 211, 221 with the dotted line 201 represent the operating point achievable by the fan assemblies 100 in respect of the resistance to the airflow. As apparent from Figure 4b, the mixed-flow fan assembly 100 can achieve a design operating point within the desired operating range, in particular an airflow rate of 400 LPM at a pressure of about 2000 Pa. By contrast, the axial-flow fan assembly (being driven by the same input power of 32W) achieves a lower flow rate of about 320 LPM at a pressure of about 1300 Pa, thus not achieving a design operating point within the desired operating range. This is despite being able to achieve a higher maximum airflow rate (at low pressure) than the mixed-flow fan assembly 100. The axial-flow fan assembly 100 would need to be operated at much higher rotational speeds, requiring significantly more input power, to match the performance of the mixed-flow fan assembly 100. Also apparent from Figure 4b is that the airflow-rate / pressure curve of the mixed-flow fan assembly 100 is significantly steeper than that of the axial-flow fan assembly. Inlet diffuser Figure 5a shows a perspective view of an inlet diffuser 160. The fan assembly 100 may comprise the inlet diffuser 160, as shown in Figure 5b. Figure 5c shows a 2D blade diagram of an inlet diffuser blade 164 and a rotor blade 124. The inlet diffuser 160 is arranged along the common axis A. The inlet diffuser 160 is arranged upstream of the rotor 120. The inlet diffuser 160 is static relative to the stator 130. The inlet diffuser 160 may be part of the inlet part 110, or be a component separate from the inlet part 110. The inlet diffuser 160 overlaps the inlet part 110, in particular the inlet portion 112 and optionally additionally the shroud portion 114, when viewed orthogonally to the common axis A. The inlet diffuser 160 comprises a plurality of inlet diffuser blades 164. The inlet diffuser 160 further comprises a centre portion 162. Each inlet diffuser blade 164 is fixedly coupled to the centre portion 162. The inlet diffuser 160 is a rigid body. The inlet diffuser blades 164 protrude from the centre portion 162 away from the common axis A. The plurality inlet diffuser blades 164 is rotationally symmetric about the common axis A, in particular o-fold rotationally symmetric, where o is the number of inlet diffuser blades 164. The inlet diffuser 160 may delay the onset of rotor stall by reducing both WO and W0O. As shown in Figure 5c, the inlet diffuser 160 may add a velocity component to the airflow that is in a tangential to a circle about the common axis A. As apparent from a comparison of the blade diagram of Figure 5c to that of Figure 3a (without inlet diffuser), WO may thus be reduced. The rotor leading edge blade angle a1 may be reduced accordingly. The number of inlet diffuser blades 164 may be as described in relation to the stator blades 134. The number of inlet diffuser blades 164 may be odd. The number of inlet diffuser blades 164 may be different to the number of rotor blades 134, for example differ by a factor of at least 1.5. The number of inlet diffuser blades 164 may be in the range from 3 to 13 (including 3 and 13). The table below shows experimental data collected by testing prototypes of fan assemblies 100, allowing a comparison of the Diffusion Factor D at the rotor 120 between a fan assembly 100 without inlet diffuser 160 (see top half of the table) and a fan assembly 100 with inlet diffuser 160 (see bottom half of the table). As shown, the values for W0 and W0O are reduced, in particular from mid-span of the rotor blades 124, in the fan assembly 100 with the inlet diffuser 160 compared to the fan assembly 100 without the inlet diffuser 160. Stall incurs when the diffusion factor at mid-span (so at a span equal to 0.5) reaches the limit of 0.6. The addition of the inlet diffuser 160 leads to a reduction of the diffusion factor for spans 0.50 to 1.00, thereby delaying the onset of stall and reducing the airflow rate at which stall occurs in the rotor 120. The operating efficiency of the fan assembly 100 is thus improved. Without Inlet diffuser wo W1 W0O W01 Diffusion Stall FlowaO Span Rate (LPM) (deg) (m / s) (m / s) (m / s) (m / s) sigma Factor 0.00 340 - 25.3 21.9 25.3 37.9 1.10 0.360 0.25 340 - 34.9 21.9 34.9 37.9 1.08 0.413 0.50 340 - 45.9 21.9 45.9 37.9 1.08 0.603 0.75 340 - 57.5 21.9 57.5 37.9 1.09 0.774 1.00 340 - 69.4 21.9 69.4 37.9 1.12 0.886 With Inlet diffuser Stall FlowaO Span Rate (LPM) (deg) W0 W1 W0O W01 Diffusion (m / s) (m / s) (m / s) (m / s) sigma Factor 25.3 21.9 16.9 37.9 1.106070 0.511 33.5 21.9 27.7 37.9 1.078730 0.488 42.9 21.9 38.5 37.9 1.078650 0.495 47.3 21.9 43.4 37.9 1.094770 0.590 48.1 21.9 44.3 37.9 1.124320 0.604 0.00 340 0 0.25 340 5 0.50 340 10 0.75 340 30 1.00 340 50 One downside of adding the inlet diffuser 160 may be additional resistance to the airflow upstream of the rotor, leading to a reduction in the airflow rate through the fan assembly 100. Mixed-flow diffuser As described above, the mixed-flow fan assembly 100 achieves a larger pressure rise compared to axial fan assemblies by adding a radial component to the velocity of the airflow. The additional pressure rise obtainable by the radial component depends on the efficiency of returning the airflow to the axial direction in the stator. In the design of the fan assembly 100 of Figure 2a, the airflow undergoes a relatively sharp bend at the connection line between the shroud portion 114 and the inwardfacing surface 136 of the stator 130. The relatively sharp corner is a result of providing a shroud portion 114 with a steadily angled inner surface and a stator 130 with an inward-facing surface that is parallel to the common axis A. Providing such parts allows manufacturing of the inlet part 110 and the stator 130, as well as assembly of the fan assembly 100, to be relatively simple. A relatively sharp corner also allows the fan assembly to remain more compact. However, the efficiency of recovery of the radial velocity components as static pressure may be reduced by such a sharp corner, especially for relatively large hub angles 01 and shroud angles 02. The fan assembly 100 may comprise a mixed-flow diffuser 170. Figure 6 depicts a meridional view of part of the fan assembly 100, showing a mixed-flow diffuser blade 174 arranged between the rotor blade 124 and the stator blade 134. As shown, the mixed-flow diffuser 170 may gradually redirect the airflow to the axial direction, thereby more efficiently recovering static pressure from the radial velocity components of the airflow. The mixed-flow diffuser 170 may also reduce the tangential velocity components of the airflow so as to recover static pressure, by allowing the blade angles of the mixed-flow diffuser 170 to change from a leading blade edge to a trailing blade edge. The mixed-flow diffuser 170 comprises an outward-facing surface 172 and an inward facing surface 176. The outward-facing surface 172 faces away from the common axis A and the inward-facing surface 176 faces towards the common axis A. The outward-facing surface 172 and the inward facing surface 176 may be curved so as to gradually redirect the airflow to be parallel to the common axis A. The outward-facing surface 172 may be angled, at a portion arranged adjacent to the hub portion 122 of the rotor 120, at the hub angle 01 relative to the common axis A. The inward-facing surface 176 may be angled, at a portion arranged adjacent to the shroud portion 114 of the inlet part 110, at the shroud angle 02 relative to the common axis A. The angles relative to the common axis A of the outward-facing surface 172 and inward facing surface 176 may gradually reduce in a downstream direction. The outward-facing surface 172 and the inward-facing surface 176 may, at downstream-most portion, be parallel to the common axis A. The length of the outward-facing surface 172 and the inward-facing surface 176 along the common axis A may be in the order of millimetres, for example in the range from 1mm to 10mm. The mixed-flow diffuser 170 may be considered as being comprised by the stator, in that the mixed-flow diffuser 170 is arranged along the common axis A downstream of the rotor 120 and is static relative to the stator 130 described in relation to Figures 2a to 2c. The stator described in relation to Figures 2a to 2c may be referred to as an axial stator portion 130. So, the stator may comprise the mixed-flow diffuser 170 and the axial stator portion 130. The axial stator portion 130 is arranged downstream of the mixed-flow diffuser 170. The mixed-flow diffuser 170 and the axial stator portion 130 may be formed as separate parts, thereby allowing manufacture of the separate parts to be simpler compared to an embodiment in which the mixed-flow diffuser 170 and the axial stator portion 130 are integrally formed. The mixed-flow diffuser 170 and the axial stator portion 130 may be fixedly coupled to each other so as to form the stator 130. The mixed-flow diffuser 170 comprises a plurality of mixed-flow diffuser blades 174. Each mixed-flow diffuser blades 174 is fixedly connected between the outward-facing surface 172 and the inward-facing surface 176. The mixed-flow diffuser 170 is a rigid body. The plurality mixed-flow diffuser blades 174 is rotationally symmetric about the common axis A, in particular p-fold rotationally symmetric, where p is the number of mixed-flow diffuser blades 174. The number of mixed-flow diffuser blades 174 may be as described in relation to the stator blades 134. The number of mixed-flow diffuser blades 174 may be odd. The number of mixed-flow diffuser blades 174 may be different to the number of rotor blades 134, for example differ by a factor of at least 1.5. The number of mixed-flow diffuser blades 174 may be in the range from 3 to 13 (including 3 and 13). The number of mixed-flow diffuser blades 174 is the same as the number of stator blades 134 in an axial stator portion 130. The mixed-flow diffuser blades 174 may, in some embodiments, be integrally formed with the stator blades 134 of an axial stator portion 130. In general, the mixed-flow diffuser blades 174 align with the stator blades 134 of an axial stator portion 130, such that the airflow readily passes from the mixed-flow diffuser 170 to such an axial stator portion 130. The interface between the rotor 120 and the mixed-flow diffuser 170 may be as the interface between the rotor 120 and the stator 130 described in relation to Figures 2a to c. The leading edges of the mixed-flow diffuser blades 174 are parallel to the trailing edges of the rotor blades 134, in particular when viewed in the meridional view. The leading edge blade angle of the mixed-flow diffuser blades 174 is equal to the expression for a2 in section “Relation of blade angles” above. The mixed-flow diffuser 170 may, due to the relatively shorter length compared to the stator 130, remove some but not all of the tangential velocity components of the airflow. The trailing edge blade angle of the mixed-flow diffuser blades 174 may be in the range from 10° to 40°. The leading edge of the axial stator blades 134 is parallel to the trailing edge of the mixed-flow diffuser blades 174, and the leading edge blade angle of the axial stator blades 134 may be the same as the trailing edge blade angle of the mixed-flow diffuser blades 174, so be in the range from 10° to 40°. The trailing edge blade angle of the axial stator blades 134 is also 0°. The purpose of the axial stator portion 130 is to fully recover tangential velocity components from the airflow as static pressure. The spacing between the rotor blades 134 and the mixed-flow diffuser blades 174, as well as the spacing between the mixed-flow diffuser blades 174 and axial stator blades 134, may be in the range from 0.4mm to 1mm. Helmholtz resonators Figure 7 shows a cross-sectional view of an embodiment of the fan assembly 100. The fan assembly 100 comprises one or more Helmholtz resonator 180. Each Helmholtz resonator 180 comprises a cavity 182. The cavity 182 is in fluid communication with the airflow, in particular with the space occupied by the rotor blades 124 in Figure 7, via a neck 184. The neck 184 provides an opening to the airflow through the fan assembly 100. The resonant frequency of the Helmholtz resonator 180 may be tuned by appropriate configuration of the dimensions of the cavity 182 and of the neck 184. The Helmholtz resonator 180 may thus be configured to act as an acoustic dampener to the rotor 130, thereby improving the sound quality of the fan assembly 100 by reducing the peak frequencies generated thereby. The Helmholtz resonator 180 may further improve the efficiency of the fan assembly 100 by achieving a blowing effect. In particular, the air in the opening 184 may oscillate when the rotor 130 is driven, thereby effectively blowing air away from the gap between the rotor blades 130 and the shroud portion 114 so as to lead to improvements in aerodynamic efficiency. As shown in Figure 7, the Helmholtz resonators 180 may be integrated in the inlet part 110. The angled profile of the rotor 130 of the mixed-flow fan assembly 100 allows the Helmholtz resonators 180 to be included without adding to the outer diameter of the fan assembly 100. The Helmholtz resonators 180 may overlap with the stator 130 when viewed along the common axis A. The cavity 182 and the opening 184 may be provided in the inlet part 110. The cavity 182 may at least partially overlap the inlet portion 112 of the inlet part 110. The opening 184 may open up to the inner surface of the shroud portion 114. The cross-section of Figure 7 shows two Helmholtz resonators 180. In practice, at least one Helmholtz resonator 180 may be provided. The fan assembly 100 may comprise plural Helmholtz resonators 180, for example four to eight Helmholtz resonators 180. The Helmholtz resonators 180 may be distributed substantially equally around the common axis A when viewed along the common axis A. Dual stage fan assembly The fan assembly 100 may comprise multiple stages of rotors 120 and stators 130. The rotor 120 and stator 130 described in relation to Figures 2a-d may be a first stage of the fan assembly 100. The fan assembly 100 may comprise one or more additional stages of rotors 120’ and stators 130’. The rotor 120’ of any additional stage may be an axial-flow rotor 120’, and so may receive the airflow in an axial direction, and add a tangential velocity component (swirl) to the airflow so as to output the airflow with a velocity component in the axial direction and a velocity component in the tangential direction. Figure 8 shows a meridional view of a dual stage fan assembly 100. The first stage of the fan assembly 100 comprises the rotor 120 and the stator 130 described in relation to Figures 2a-d, and is a mixed-flow stage of the fan assembly 100. The second stage of the fan assembly 100 comprises a rotor 120’ and a stator 130’, and is an axial-flow stage of the fan assembly 100. The rotors 120, 120’ may be driven by the same motor 140, for example. Providing multiple stages of rotors 120, 120’ and stators 130, 130’ allows the fan assembly 100 to be designed to achieve a higher pressure rise at a lower rotational speed. The pressure ratios of the stages multiply so as to achieve a higher overall pressure rise for the same flow rate. For example, for a dual stage fan assembly 100, the increase in pressure AP can be determined by: Po Po Pi where pO is the ambient pressure, p1 is the pressure at the exit from the first stage and p2 is the pressure at the exit from the second stage. The performance of the stages of the fan assembly 100 is preferably matched so as to achieve high efficiency at the same operating point for all stages. For example, the stages may be designed to reach maximum efficiency at the same airflow rate (e.g. 400 LPM) and to stall at the same airflow rate. Providing two or more stages to the fan assembly 100 may reduce the complexity of the design of the motor 140 required to achieve the operating rotational speed. A lower rotational speed may be needed to operate at the design operating point. The sound quality of the fan assembly 100 may also be improved. Providing two or more stages to the fan assembly 100 may, however, lead to an increased length of the fan assembly 100, as well as increased cost and complexity of manufacture and assembly of the fan assembly, and more complex balance and vibration control of the fan assembly 100 within the device 10. Materials and manufacture The inlet part 110, the rotor 120 and the stator 130 may be made from a variety of materials providing rigidity to these components and allowing the fan assembly 100 to operate at the design operating point. The rotor 120 may be made, for example, from a high stiffness plastic or a lightweight metal such as aluminium. The rotor 120 may be made via either injection moulding, investment casting or CNC machining &turning. The rotor 120 requires a high level of balancing to minimise vibration and noise. The inlet part 110 may be made, for example, from a plastic such as Nylon, PBT or PPS, with high stiffness. The inlet part 110 may be injection moulded to reduce the overall cost of the assembly. Alternatively, the inlet part 110 may be machined or cast from aluminium. The inlet part 110 may be fixed to the stator 130 via a weld or adhesive. The stator 130 may, for example, be made from aluminium. The stator 130 may be made by the same processes as the rotor 120. The stator 130 may provide direct cooling to the motor 140. Integration of fan assembly in apparatus The fan assembly 100 may be integrated in an apparatus 10 for drying and / or styling hair. The apparatus may, for example, be the device 10 described in relation to Figures 1a to 1c. Figures 9a to 9f show various apparatus 10 incorporating the fan assembly 100. Figure 9a shows a schematic side view of the device 10 described in detail in relation to Figures 1a to 1c. The device 10 is a wet-to-style hair styling device. Providing a high thrust airflow using the mixed-flow fan assembly 100 may increase the drying rate of hair and improve the effectiveness of hair styling. The device 10 comprises a main body 11. The main body 11 is a housing or casing of the device 10. The main body 11 comprises the arms 14 and 16. The main body 11 is elongate. A head portion 17 is provided adjacent to one end of the elongate main body. The head portion 17 is for engaging the length of hair, and in the device of Figure 9a comprises the hair treating distal ends 93, 95 of the arms 14, 16 shown in Figure 1a. A handle 15 is provided adjacent to the other end of the elongate main body 11. The handle 15 is configured to be held by a user’s hand. The device 10 is thus a hand-held device. The fan assembly 100 is incorporated within the main body 11 of the device 10, in particular within the handle 15 thereof. The fan assembly 100 has dimensions allowing for incorporation within the handle 15 without requiring a change to the form factor of the handle 15 or affecting the ergonomics of the handle 15. The device 10 also comprises the heating means 30. The heating means 30 is arranged in the main body 10, in particular within the handle 15 thereof. The heating means 30 is arranged downstream of the fan assembly 10. The airflow delivered by the fan assembly 10 is heated by the heating means 30. The heated airflow is then guided to the head portion 17, in particular to an outlet 70. The outlet 70 of Figure 9a corresponds in essence to the air-flow guide structure 24 and channels 28 shown in Figure 1 b. The heated airflow from the outlet 70 heats the length of hair by convection. The airflow is provided to the inter-arm plenum chamber 13 formed by the arms 14, 16 in the closed configuration. The inter-arm plenum chamber 13 presents a confined space within which the hair is provided. Providing the airflow to the hair within such a confined space may reduce the airflow rate required to effectively dry and / or style the hair compared to a situation in which the airflow is provided to an open space (such as in a conventional hair dryer) to dry the hair. Similalry, providing the airflow to hair that is in direct engagement and / or held in tension may reduce the airflow rate required to effectively dry and / or style the hair. The mixed-flow fan assembly 100 may be particularly suitable (in comparison to an axial-flow fan assembly) for incorporation in devices 10 that directly engage the hair and / or provide a confined space within which the length of hair is guided. The device comprises the control electronics 37. The control electronics 37 is configured to control operation of the device 10, for example control operation of the fan assembly 100, of the heating means 30 and of any other electronically controllable components of the device 10. The control electronics 37 may comprise printed circuit boards, bulky components and wire routing. Conventionally, the control electronics 37 are arranged between the fan assembly 100 and the heating means 30. The inventors have found that, in order to improve the efficiency of the device 10, the control electronics 37 may be arranged away from the head portion 17. So, the fan assembly 100 may be arranged between the control electronics 37 and the head portion 17. The resistance to the airflow by the device, so the pressure drop of the airflow across the device, may then be reduced by providing additional inlets upstream of the fan assembly 100 and along the length of the elongate main body 11. The main body 11 comprises air inlets 60, 60a, 60b. The air inlets 60, 60a, 60b typically comprise a grille, an airflow filter and caseworks. A main air inlet 60 is provided at an end of the elongate main body 11, in particular at the end of the handle 15 distal to the head portion 17. The main airflow inlet 60 corresponds to the air inlet described in relation to Figure 1a and 1b. Additionally or alternatively, side air inlets 60a, 60b are arranged along the length of the main body 11. This is enabled by arranging the fan assembly 100 within a middle portion of the handle 15, instead of at the end of the handle 15 that is distal to the head portion 17. The side air inlets 60a, 60b are arranged on a middle portion along the length of the handle 15, for example within a central 70% of the length of the handle 15. The device 10 of Figure 9a comprises a hinge 18 allowing relative movement of the arms 14, 16, as described in relation to Figure 1a. Conventionally, the hinge 18 is arranged downstream of the fan assembly 100. The hinge 18 may undesirably leak airflow provided by the fan assembly 100, thereby reducing the airflow rate and / or pressure rise. Measurements show that about 30% of the airflow may leak through the hinge 18 if no seal is provided. This may be compensated for by increasing the rotational speed of the rotor 120, however at the expense of increased power consumption and noise level. A seal may be provided to reduce the leakage. Such a seal adds mechanical complexity and adds to the size of the device, reducing the ergonomics thereof. Because wires are typically routed through the hinge 18 to the arm 16, achieving a perfect seal is difficult and often not possible. As shown in Figure 9a, the hinge 18 may be provided upstream of the fan assembly 100. The hinge 18 thus provides a side air inlet 60b. The drawbacks of a hinge 18 downstream of the fan assembly 100 are avoided, while achieving the benefits of providing the side airflow inlets 60a, 60b. Providing the fan assembly 100 downstream of the hinge 18 may be enabled by the smaller diameter of the mixed-flow fan assembly 100 compared to conventional axial-flow fan assemblies. As apparent from Figure 1a, for example, the main body 11 may have a smaller cross-section downstream of the hinge 18 compared to upstream of the hinge 18. The effect of providing additional air inlets on the back pressure or resistance to airflow of the device 10 may be explained in analogy to an electronic circuit, where the pressure corresponds to voltage, the airflow rate corresponds to electrical current and the resistance to airflow corresponds to electrical resistance. Providing additional inlets effectively adds parallel paths for the airflow, thereby reducing the resistance to airflow in a manner similar to a reduction in electrical resistance achieved by connecting multiple electrical paths in parallel. The side air inlets 60a allow the path length of the airflow through the device 10 to be reduced, thereby reducing the resistance to the airflow. Fewer components may be arranged between the fan assembly 100 and the head portion 17, thereby avoiding the pressure drop across any components (such as the control electronics 37) otherwise arranged between the fan assembly 100 and the head portion 17. By arranging the control electronics 37 at a portion of the main body 11 that is not located between the fan assembly 100 and the head portion 17, the components of the control electronics 37 may be packed more closely without increasing the resistance to the airflow. So, more complex control electronics 37 may be included in the device 10 without negatively affecting the airflow performance of the device 10. Figure 9b shows a schematic side view of another device 10. The device 10 is a wet-to-style hair styling brush. Providing a high thrust airflow using the mixed-flow fan assembly 100 may improve the heating of spokes or bristles of the brush and provide an increased contact area with the air for improving drying and / or styling performance. The head portion 17 of the main body 11 of the device 10 may for example be shaped as a paddle or have a round or ovular cross-section when viewed perpendicularly to the length of the elongate main body 11. The head portion 17 may be fixed to the handle 15 or be detachable from the handle 15. The head portion 17 comprises a plurality of spokes or bristles for brushing the length of hair. The plurality of spokes may be provided on one side of the head portion 17 (out of the page in Figure 9b) and need not be provided on the other side of the head portion 17 (into the page in Figure 9b). The fan assembly 100, the heating means 30 and the control electronics 37 are incorporated in the handle 15 of the device 10. A main inlet 60 and side inlets 60a are provided. The arrangement of air inlets 60, 60a and of the fan assembly 100, the heating means 30 and the control electronics 37 within the handle 15 achieves the benefits described in relation to the device of Figure 9a. The fan assembly 100 provides the airflow to the head portion 17, in particular to a plurality of airflow outlets 70 provided in the head portion 17. The airflow outlets 70 are a plurality of holes arranged to direct the airflow to a length of hair held by the head portion 17. The hair may, for example, be held in tension by the spokes such that the airflow leaving the airflow outlets 70 can be directed to the hair. The hair may be heated and / or dried by the heated airflow. Figure 9c shows a schematic side view of another device 10. The device 10 is a barrel curler. Providing a high thrust airflow using the mixed-flow fan assembly 100 may improve the cooling performance in such a device 10. The head portion 17 of the main body 11 of the device 10 may for example have a round or ovular cross-section when viewed perpendicularly to the length of the elongate main body 11. Hair may be wound in tension around the head portion 17 for curling the hair. The fan assembly 100 is incorporated in the handle 15 of the device 10. The fan assembly 100 provides the airflow to the head portion 17. A heating means is not required in the device of Figure 9c. The purpose of the airflow is to cool the head portion 17 and so the length of hair held by the head portion 17. The fan assembly 100 draws air in through the inlet 60 and delivers the airflow to a plurality of airflow outlets 70. The airflow outlets 70 are provided on the head portion 17 to direct the airflow to the length of hair held in tension by the head portion, thereby allowing the airflow to cool the hair by convection. Figure 9d shows a schematic side view of another device 10. The device 10 is a cool case hair styler. Providing a high thrust airflow using the mixed-flow fan assembly 100 may improve the cooling performance in such a device 10. The device 10 comprises a main body 11 with two arms 14, 16 that can be placed in an open or a closed configuration in a manner similar to the arms 14, 16 described in relation to the device of Figure 9a. The arms 14, 16 comprise, at the head portion 17, complementary contacting surfaces that come together in the closed configuration. The contacting surfaces are planar. In use, a length of hair is sandwiched between the planar contacting surfaces such that the length of hair is held in tension and may be straightened. The fan assembly 100 is incorporated in the handle 15 of the device 10. The fan assembly 100 provides the airflow to the head portion 17. Airflow outlets may be provided in one or both of the contacting surfaces so as to direct the airflow to hair sandwiches between the contacting surfaces or may be provided elsewhere in in the main body 17. The airflow may cool the hair convectively by being directed to the hair, or may cool the contacting surfaces so as to indirectly cool the hair via conductive heat transfer between the contacting surfaces and the hair. Figures 9e and 9f shows a perspective view and a schematic side view of another device 10. The device 10 is a one-stroke curler. Providing a high thrust airflow using the mixed-flow fan assembly 100 may improve the cooling performance in such a device 10. The device 10 is similar to the device of Figure 9d, except that the contacting surfaces are curved and not planar. In use, a length of hair is sandwiched between the curved contacting surfaces such that the length of hair is held in tension and guided along a curved path. Figures 10a and 10b show another apparatus 10 for drying and / or styling hair. The apparatus 10 comprises a handle 15 and a head portion 17. Figure 10a shows a schematic cross-sectional view of the apparatus 10 and Figure 10b shows a schematic cross-sectional view of the head portion 17 of the apparatus 10. The apparatus 10 may correspond to and comprise the features of the wet-to-style hair styling brush 10 described in relation to Figure 9b. The handle 15 is configured to be held by a user during use. In the depicted embodiment, the handle 15 houses the airflow inlet 60, the fan assembly 100 and the heating means 30. The heating means 30 are embodied by an airflow heater configured to heat the airflow generated by the fan assembly 100 and may generally correspond to the heater 30 described with reference to Figure 1. The heating means 30 typically takes the form of an electrically-powered heating coil (or other electrical heating elements), that is operable to heat the air drawn in by the fan assembly 100. The fan assembly 100 draws the airflow from the airflow inlet 60 and delivers the airflow, via the heating means 30, to the head portion 17. The fan assembly 100 may correspond to the mixed-flow fan assembly 100 described herein or may be a fan assembly 100 of another type, such as an axial fan assembly. The handle 15 may house any further components of the apparatus 10, for example the control electronics 37. The head portion 17 is configured to engage a user’s hair during use. The head portion 17 comprises a brush head that may be shaped as a paddle or have a round or ovular cross-section when viewed perpendicularly to the length of the head portion 17. The head portion 17 may be fixed to the handle 15 or be detachable from the handle 15. The head portion 17 comprises a plurality of spokes or bristles for brushing the length of hair. The plurality of spokes may generally be provided on one side of the head portion 17, as shown in Figure 10a, or may be provided on all sides of the head portion 17. The spokes may entirely surround the head portion 17 when viewed along the length of the apparatus, for example. The head portion 17 may engage the hair on one side, like a hair brush, or may engage the hair on all sides such that hair may be wound around the head portion 17. The head portion 17 further comprises a plurality of perforations 70 or holes 70, corresponding to airflow outlets 70. The perforations 70 may be generally evenly distributed in a part of the head portion 17 that is configured to engage the hair. The perforations 70 may, for example, be arranged between adjacent spokes of the brush formed by the head portion 17. The head portion 17 may comprise multiple such perforations, for example more than ten or more than fifty such perforations. The airflow may exit the head portion 17 through the perforations 70, thereby acting on a length of hair that engages the head portion 17. In the depicted embodiment, the head portion 17 further comprises a plurality of baffles 27. The baffles 27 are configured to re-direct the airflow, which flows in an axial direction along the length of the apparatus 10 (from right to left in Figures 10a and 10b) from the handle 15 to the head portion 17, to a radial direction that is radially outward from a centre line through the head portion 17. The airflow may exit the head portion 17 through the perforations 70 in a radial direction. In general, any other guide structure for guiding the airflow to the perforations 70 may be provided in the head portion 17. For example, the guide structure may be adapted to suit the shape of the brush head and / or the location(s) where the spokes or bristles are arranged. The head portion 17 further comprises one or more heaters 21, for example a pair of heaters 21. The heater 21 are separate from and may be controlled independently from the heating means 30. The heaters 21 may heat at least part of the head portion 17 conductively. The heaters 21 may be configured as described in relation to the heater plates 20a, 20b, 22a, 22b of Figures 1a-c, except that the heaters 21 may be either straight or may be curved so as to conform to the shape of the head portion 17. The heaters 21 are typically ceramic heaters 21 that comprise a ceramic layer 25 in which an electrically conductive heating element is embedded (e.g. a flat or curved ceramic layer 25) and a heater assembly 26. The ceramic layer 25 may be provided with the perforations 70, or the perforations may be arranged in the head portion 17 adjacent to the ceramic layer 25. The heater assembly 26 may comprise amongst other things a thermal fuse and temperature sensing function. The control electronics 37 may controllably apply electrical power to the electrically conductive heating element so as to heat up the ceramic layer 25, thereby heating up the heater 21 for providing heat to a length of hair that engages the head portion 17. The head portion 17 comprises a heater carrier 29. The heater carrier 29 is configured to receive and to fix the heater 21, in particular the heater assembly 26 and the ceramic layer 25. The heater 21 may be removably or fixedly coupled to the heater carrier 29. The combination of the heating means 30 for heating the airflow and the heaters 21 for heating the head portion 17 may improve the speed of drying of a length of hair and / or may improve styling of the length of hair. The apparatus 10 may be operable in different operating modes. The fan assembly 100, the heating means 30 and the one or more heaters 21 may be selectively turned on or off (i.e. be activated or deactivated) in the different operating modes. In a first operating mode, for example, the control electronics 27 may control all of the fan assembly 100, the heating means 30 and the one or more heaters 21 to operate, such that the length of hair is provided with heated airflow and engages a head portion 17 heated by the heaters 21. Such a first operating mode may be particularly useful for drying and / or styling wet hair. The wet hair may be dried faster and / or with a more consistent temperature of the head portion 17 compared to an apparatus 10 that does not provide both heated airflow and heaters 21. In a second operating mode, the control electronics 27 may control the fan assembly 100 and the heating means 30 not to operate (i.e. to be turned off) and the one or more heaters 21 to operate (i.e. to be turned on). Such a second operating mode may be particularly useful for styling dry hair. The apparatus 10 may operate as a hot brush, allowing styling of dry hair to be improved, for example allowing more voluminous or longer lasting hair styles to be achieved. Other operating modes are also possible, in general providing any possible combination of active and in-active fan assembly 100, heating means 30 and one or more heaters 21. For example, in some operating modes, only the fan assembly 100 may be operating and the heating means 30 and one or more heaters 21 may not be operating. Providing multiple operating modes, such as the first and second operating modes, allows the apparatus 10 to be used on both wet and dry hair and to improve the drying and / or styling thereof. The apparatus 10, for example the control electronics 37 thereof, may comprise an onboard accelerometer or gyroscope (not shown). The accelerometer or gyroscope may detect motion of the apparatus 10. The control electronics 37 may receive a signal indicative of motion of the apparatus 10 from the accelerometer or gyroscope and may automatically enter an ‘Idle Mode’ when it is detected that the apparatus 10 is placed down (for example on its integral stand) or when it is detected that the apparatus 10 is stationary for a set period of time. On entering the Idle Mode, the control electronics 37 may control the fan assembly 100, the heating means 30 and / or the heaters 21, as well as other electronically controllable components of the apparatus, not to operate (i.e. to be powered off or to remain deactivated). The control electronics 37 may resume operating the fan assembly 100, the heating means 30 and / or the heaters 21, as well as other electronically controllable components, when the control electronics 37 receives a signal from the accelerometer or gyroscope indicating that motion of the apparatus 10 has resumed, for example due to the apparatus 10 being picked up by a user. Providing the Idle Mode may achieve a reduction in energy consumption of the apparatus 10, as well as a reduction in undesirable heating of surroundings of the apparatus 10 and the noise of the apparatus 10. There is thus discloses an apparatus 10 for drying and / or styling hair according to the following numbered clauses: 1. An apparatus comprises a head portion for engaging a length of hair for drying and / or styling the length of hair, and wherein the head portion comprises one or more heaters for conductively heating at least part of the head portion, and wherein the head portion comprises at least one airflow outlet, the apparatus further comprising a fan assembly for providing an airflow to the at least one airflow outlet and a heating means for heating the airflow. 2. An apparatus according to clause 1, wherein the fan assembly is the mixed-flow fan assembly of any aspect or claim described herein. 3. An apparatus according to clause 1 or 2, further comprising a handle configured to be held by a user, wherein the fan assembly and the heating means are arranged in the handle. 4. An apparatus according to any preceding clause, further comprising control electronics configured to control operation of the fan assembly, the heating means and the one or more heaters. 5. An apparatus according to claim 4, wherein the apparatus is operable in a first operating mode in which the control electronics controls the fan assembly, the heating means and the one or more heaters to be powered on. 6. An apparatus according to claim 4 or 5, wherein the apparatus is operable in a second operating mode in which the control electronics controls the fan assembly and the heating means to be powered off and further controls the one or more heaters to be powered on. 7. An apparatus according to any of claims 4 to 6, further comprising an accelerometer or gyroscope for measuring acceleration or movement of the apparatus, wherein the control electronics are configured to enter an idle mode in response to the accelerometer or gyroscope detecting that the apparatus is stationary for a predetermined period of time, wherein the control electronics powers off the fan assembly, the heating means and / or the one or more heaters on entering the idle mode. Modifications and alternatives Detailed embodiments and some alternatives have been described above. As those skilled in the art will appreciate, a number of modifications and further alternatives can be made to the above embodiments whilst still benefiting from the inventions embodied therein. It will therefore be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the scope of the claims appended hereto. The fan assembly 100 has been described as comprising three parts, in particular the inlet part 110, the rotor 120 and the stator 130. In general, the fan assembly 100 may be formed of more than three parts. The inlet part 110 may, for example, be formed of two separate parts, such as a first part providing the inlet portion 112 and a second part providing the shroud portion 114. Similalry, the rotor 120 and / or stator 130 may be formed of multiple separate parts. The fan assembly 100 has been described in combination with an inlet portion 112 formed as part of the inlet part 110. In some embodiments, the inlet portion 112 is omitted and the airflow instead enters the rotor 120 directly on entering the fan assembly 100. Some of the components of the fan assembly 100 have been shown in cross-section as having parallel or steadily inclined surfaces, for example cylindrical or frustoconical surfaces. In general, these surfaces may be curved in cross-section and not be entirely cylindrical or frustoconical. A detailed design and parameters defining a preferred embodiment of the fan assembly 100 are presented in this application. In general, the parameters described herein may diverge from the disclosed ranges and still achieve the general advantages of providing a mixed-flow fan assembly 100 in a hair styling device 10. In the apparatus 10 described herein, the airflow inlets 60, 60a are generally described as being located in a handle 15 and the outlets 70 are generally described as being located in the head portion 16 so that the fan assembly 100 draws the airflow from the handle 15 to the head portion 17. Alternatively, the fan assembly 100 may draw the airflow from an airflow inlet 60 in the head portion 17 to an airflow outlet 70 in the handle 15, for example to draw hair into the head portion 17 and dry and / or style the hair thereby. Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “containing”, means “including but not limited to”, and is not intended to (and does not) exclude other components, integers or steps.

Claims

1. An apparatus for drying and / or styling a length of hair, the apparatus comprising:a main body comprising a head portion for drying and / or styling the length of hair; anda mixed-flow fan assembly arranged in the main body and configured to deliver an airflow to or from the head portion, wherein the mixed-flow fan assembly comprises:a rotor comprising a plurality of rotor blades, and a stator comprising a plurality of stator blades, wherein the rotor and the stator are arranged along a common axis and the rotor is rotatable relative to the stator about the common axis, andwherein the rotor is configured to output the airflow with an axial velocity component along the common axis, a radial velocity component that is radially away from the common axis and a tangential velocity component that is tangential to a circle around the common axis.

2. An apparatus according to claim 1, wherein the head portion is configured to engage the length of hair so as to be capable of holding the length of hair in tension, wherein the head portion comprises one or more outlets for directing the airflow to the length of hair or one or more inlets for receiving the airflow, wherein the fan assembly is arranged to provide the airflow to the one or more outlets or to receive the airflow from the one or more inlets.

3. An apparatus according to claim 2, wherein the head portion is configured, in use, to provide a confined space and to guide the length of hair within the confined space, wherein the one or more outlets direct the airflow to the confined space.

4. An apparatus according to any one of the preceding claims, wherein the apparatus is a hand-held apparatus comprising a handle, wherein the mixed-flow fan assembly is arranged within the handle.

5. An apparatus according to any one of the preceding claims, wherein the main body comprises two mutually-opposing arms adapted for movement between an openconfiguration for receiving the length of hair therebetween and a closed configuration adjacent the length of hair, wherein contacting surfaces are disposed on each of the two mutually-opposing arms, the contacting surfaces being arranged to come together when the two mutually-opposing arms are in the closed configuration6. An apparatus according any one of the preceding claims, wherein the main body is elongate and comprises air inlets, wherein the mixed-flow fan assembly is arranged to draw air from the air inlets, wherein the air inlets comprise one or more side inlets arranged along the length of the elongate main body.

7. An apparatus according to claim 5 and 6, comprising a hinge between the two mutually-opposing arms for guiding the movement of the arms, wherein the mixed-flow fan assembly is arranged downstream of the hinge and wherein the hinge provides an air inlet.

8. An apparatus according to any one of the preceding claims, further comprising heating means arranged within the main body and configured to heat the airflow.

9. An apparatus according to any one of the preceding claims, further comprising a motor for driving rotation of the rotor, wherein the motor comprises a motor housing that overlaps with the stator when viewed perpendicularly to the common axis.

10. A mixed-flow fan assembly for incorporation in a hand-held apparatus for drying and / or styling hair, the mixed-flow fan assembly comprising:a rotor comprising a plurality of rotor blades,a stator comprising a plurality of stator blades,wherein the rotor and the stator are arranged along a common axis and the rotor is rotatable relative to the stator about the common axis, andwherein the rotor is configured to output an airflow with an axial velocity component along the common axis, a radial velocity component that is radially away from the common axis and a tangential velocity component that is tangential to a circle around the common axis.

11. An apparatus according to any one of the preceding apparatus claims or a mixed-flow fan assembly according to claim 10, whereinthe mixed-flow fan assembly is configured to operate at a design operating point within a target operating range combining a static pressure increase in the range from 300 litres per minute to 700 litres per minute with an airflow rate in the range from 1500 pascals to 3500 pascals at a rotational speed of the rotor in the range from 50,000 rotations per minute to 100,000 rotations per minute.

12. An apparatus according to any one of the preceding apparatus claims or a mixed-flow fan assembly according one of the preceding mixed-flow fan assembly claims, whereineach of the rotor blades comprises a respective leading edge and a respective trailing edge, andeach of the stator blades comprises a respective leading edge and a respective trailing edge, andwhen viewed in the meridional view, the leading edges of the stator blades are parallel to the trailing edges of the rotor blades.

13. An apparatus according to any one of the preceding apparatus claims or a mixed-flow fan assembly according one of the preceding mixed-flow fan assembly claims, whereinthe mixed-flow fan assembly has an outer diameter in a plane perpendicular to the common axis that is in the range from 20 millimetres to 26 millimetres.

14. An apparatus according to any one of the preceding apparatus claims or a mixed-flow fan assembly according one of the preceding mixed-flow fan assembly claims, whereinthe rotor comprises a hub portion comprising an outer surface facing away from the common axis, wherein the plurality of rotor blades protrudes from the outer surface, and wherein the outer surface is inclined relative to the common axis at a hub angle that is in the range from 35° to 60°.

15. An apparatus according to any one of the preceding apparatus claims or a mixed-flow fan assembly according one of the preceding mixed-flow fan assembly claims, whereinthe mixed-flow fan assembly comprises a shroud portion having an inner surface facing towards the common axis, wherein the shroud portion surrounds the plurality of rotor blades, and wherein the inner surface is inclined relative to the common axis at a shroud angle that is in the range from 10° to 30°.

16. An apparatus according to any one of the preceding apparatus claims or a mixed-flow fan assembly according one of the preceding mixed-flow fan assembly claims, whereinthe rotor inlet area at the leading edges of the rotor blades is greater than or equal to the rotor outlet area at the trailing edges of the rotor blades.

17. An apparatus according to any one of the preceding apparatus claims or a mixed-flow fan assembly according one of the preceding mixed-flow fan assembly claims, whereineach rotor blade is angled from a rotor trailing edge relative to the common axis at a rotor trailing edge blade angle in the range from 20° to 40°.

18. An apparatus according to any one of the preceding apparatus claims or a mixed-flow fan assembly according one of the preceding mixed-flow fan assembly claims, whereinthe plurality of rotor blades comprises an odd number of rotor blades,the plurality of stator blades comprises an odd number of stator blades, and the number of rotor blades is different to the number of stator blades.

19. An apparatus or a mixed-flow fan assembly according to claim 18, wherein the number of stator blades is equal to or greater than 1.5 times the number of rotor blades or the number of rotor blades is equal to or greater than 1.5 times the number of stator blades.

20. An apparatus according to any one of the preceding apparatus claims or a mixed-flow fan assembly according one of the preceding mixed-flow fan assembly claims, whereinthe mixed-flow fan assembly further comprises an inlet portion comprising an inner surface facing the common axis, wherein the inner surface is parallel to the common axis.

21. An apparatus according to any one of the preceding apparatus claims or a mixed-flow fan assembly according one of the preceding mixed-flow fan assembly claims, whereinthe mixed-flow fan assembly further comprises an inlet diffuser arranged upstream of the rotor along the common axis, wherein the inlet diffuser comprises a plurality of inlet diffuser blades arranged to receive the airflow in a direction parallel to the common axis and to output the airflow to the rotor with an axial velocity component along the common axis and a tangential velocity component that is tangential to a circle around the common axis.

22. An apparatus according to any one of the preceding apparatus claims or a mixed-flow fan assembly according one of the preceding mixed-flow fan assembly claims, whereinthe stator comprises a mixed flow diffuser comprising a plurality of mixed-flow diffuser blades and an axial stator portion comprising a plurality of axial stator blades, wherein the mixed-flow diffuser is arranged to receive the airflow from the rotor and is configured to gradually remove the radial velocity component from the airflow so as to output the airflow to the axial stator portion without the radial velocity component.

23. An apparatus according to any one of the preceding apparatus claims or a mixed-flow fan assembly according one of the preceding mixed-flow fan assembly claims, whereinthe mixed-flow fan assembly further comprises one or more Helmholtz resonators, each comprising a cavity and a neck, wherein the cavity is in fluid communication with the airflow via the neck.

24. An apparatus according to any one of the preceding apparatus claims or a mixed-flow fan assembly according one of the preceding mixed-flow fan assembly claims, whereinthe mixed-flow fan assembly comprises a first stage and a second stage, 5 wherein the first stage comprises the rotor and the stator, and wherein the second stage comprise a second rotor arranged along the common axis downstream of the stator and a second stator arranged along the common axis downstream of the second rotor, wherein the second rotor is an axial-flow rotor.10IntellectualPropertyOfficeApplication GB2500098.5Search report under Section 17 of the Patents Act 1977Date search completed: 24 June 2025Claims searched: 1-24International classificationSubclass and subgroup Valid from A45D1 / 06 01 / 01 / 2006 A45D2 / 00 01 / 01 / 2006 A45D20 / 10 01 / 01 / 2006Field of searchWorldwide search of patent documents classified in the following areas of the IPC:A45D, F04DDatabases used in the preparation of this search report:SEARCH-PATENTDocuments considered to be relevantPatent literatureCategory Relevant Document of relevanceclaimsX, Y X-1,8-11, 13-15, 1719, 22, 24; Y-1-8 KR 102482413 B1 (LG ELECTRONICS INC), See figure 1 and paragraphs 0019, 0020 X, Y X-1,4, 813, 18-20, 24; Y-1-8 US 10729218 B2 (CHILDE et al.), See figures 4 and 5 X, Y X-1,8-11, 13, 16, 18, 20, 24; Y-1-8 EP 4119805 A1 (DREAME TECH SHANGHAI CO LTD), See figure 3 and paragraph 0033 X, Y X-1,8-11, 13, 14, 24; Y-1-8 CN 217565183 U (DREAME INNOVATION TECH SUZHOU CO LTD), See figures 4 and 7 and English translation Y 1-8 WO 2023118836 A2 (JEMELLA LTD), See whole document Non-patent Category iterature Relevant claims Document of relevance Categories Letter or symbol DescriptionXDocument indicating lack of novelty or inventive step.Letter or symbol Description Y Document indicating lack of inventive step, if combined with another document of the same category. & Member of the same patent family. A Document indicating technological background. P Document published on or after the priority date but before the fling date of the present application. E Earlier application published on or after the filing date of the present application.

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