ATTACHMENT FOR A HAIR CARE APPLIANCE Technical Field The present invention relates to an attachment for a hair care appliance, and a hair care appliance comprising such an attachment. Background Haircare appliances are generally used to treat or style hair, and some haircare appliances may treat or style hair using airflow and/or heat. It may be desirable to provide sufficient airflow and/or heat to hair without providing discomfort to a user. Summary According to a first aspect of the present invention, there is provided an attachment for a hair care appliance, the attachment comprising an air inlet for receiving an airflow from the hair care appliance; a main body arranged to receive the airflow from the air inlet; an air outlet for emitting the airflow; and a projection spaced from the main body to define a hair-receiving channel between the projection and the main body, the air outlet arranged to emit airflow into the hair-receiving channel, the projection comprising a hair-receiving surface; wherein, when in use, hair located in the hair- receiving channel is blown by the airflow away from the main body into contact with the hair-receiving surface, such that hair is collected at the hair-receiving surface. During use, hair is located in the hair-receiving channel and airflow from the air outlet of the main body pushes the hair into contact with the hair-receiving surface of the projection. For example, a majority of the hair in the hair-receiving channel may be blown away from the main body and collected at the hair-receiving surface. This allows air to be applied to the hair in order to style and/or dry the hair, for example where the airflow is. The hair-receiving surface engages the hair collected in the hair-receiving channel and allows the user to manipulate the hair in order to style the hair whilst additionally applying airflow to the hair which may provide additional style control. In this way the attachment enables a user to both engage the hair and heat or dry the hair using a single attachment. The air outlet may be placed near the scalp, near the roots of the hair, with the projection spaced away from the scalp. Hair located in the hair-receiving channel is thereby blown away from the scalp to thereby achieve a root-lift effect, where the root of the hair is directly styled to improve the volume of the user’s hair. The hair-receiving surface may comprise an aperture through which at least some of the airflow received from the air outlet can pass. In other words, in use, airflow blows hair into contact with the hair-receiving surface, and at least a portion of this airflow continues to pass through the air outlet. This can reduce airflow turbulence around the hair-receiving surface, which may reduce disruption to both hair engaged by the hair- receiving surface and to hair located near to the attachment. This may improve the style control available to the user. The air outlet may be arranged to emit airflow in a direction non-orthogonal to the plane of the air inlet. The air outlet may be arranged to emit airflow in a direction parallel to the plane of the air inlet. This can allow the hair to be collected by the hair-receiving channel, and airflow applied to the hair in such a way that air is not directly incident, or incident at all, on the user’s scalp, which may reduce thermal discomfort to the user. Furthermore, emitting the airflow in a direction non-orthogonal to the plane of the air inlet, or parallel to the plane of the air inlet, can reduce the required manipulation of the hair-care appliance to provide airflow to the hair in a certain direction, for example, in a direction away from the scalp. The hair-receiving surface may have an arcuate cross-sectional profile. When hair is engaged with the hair-receiving surface, the arcuate cross-sectional profile can provide style control which can result in improved style and/or volume of the user’s hair as the hair is engaged against a curved plane. The hair-receiving surface may have a first and a second arcuate cross-sectional profile, the first cross-sectional profile in a plane orthogonal to a plane of the second cross- sectional profile. This can provide style control which can result in improved style and/or volume of the user’s hair as the hair is engaged against a plane which is curved in two directions. The engaged hair can thereby styled with respect to a first axis and with respect to a second orthogonal axis. The hair-receiving surface may comprise a plurality of apertures, each aperture permitting at least some of the incident airflow to pass through. Each aperture of the plurality of apertures may have a substantially similar geometry. The plurality of apertures may be regularly distributed across the hair-receiving surface. This can reduce turbulence to the airflow, which reduces disruption to the hair. A plurality of apertures may permit the same portion of airflow to pass through the hair-receiving surface as a single larger aperture, but reduce the likelihood of hair engaged by the hair- receiving surface from passing through the aperture, therefore improving the style control afforded to the user and preventing tangling of the hair with the projection. The hair-receiving surface may be opposite to and at least as large as the air outlet. In this way, the hair-receiving surface may collect substantially all the hair blown by the airflow in the hair-receiving channel, which in turn may reduce disruption to the hair and improve style control afforded to the user. The projection may be positioned such that the hair-receiving channel is no wider than 3cm, for example no more than 2cm, or no more than 1cm. This can ensure that the airflow within the hair-receiving channel remains uniform as the airflow traverses the hair-receiving channel as the airflow does not have the travel distance to break up or become turbulent. The hair-receiving channel being no wider than 3cm, for example no more than 2cm, or no more than 1cm can also ensure that the heat in the airflow does not dissipate before reaching the hair engaged by the hair-receiving surface, thereby improving the heating and drying provided by the airflow. The attachment may comprise a plurality of air outlets. A plurality of air outlets can allow the airflow across the hair-receiving channel to be arranged to achieve a particular style control. The geometry of each air outlet may be dependent on its position on the main body. The geometry of each air outlet being dependent on its position in the distribution element can allow for the airflow across the hair-receiving channel to be arranged to achieve a particular style control, and may allow for disruption at the edge of the hair- receiving channel to be reduced, for example, by preferentially placing larger air outlets such that they emit into the centre of the hair-receiving channel. Each air outlet may be configured to emit airflow in substantially the same direction. This may reduce disruption to hair in the hair-receiving channel and engaged by the hair-receiving surface. The main body may comprise a plurality of channels, each channel arranged to receive a respective portion of the airflow from the air inlet and guide the respective portion to a respective air outlet. This allows the airflow in the main body to be divided and provided to each air outlet according to a desired distribution. Each channel may be configured to receive a substantially equal portion of the airflow. Each channel may be configured to receive a respective portion of the airflow dependent on the position of the respective air outlet. In this way, the airflow in the hair-receiving channel can further be controlled, with the airflow being made uniform across the hair-receiving channel and at the hair-receiving surface, or the airflow being concentrated in some regions of the hair-receiving channel and at the hair-receiving surface. At least a portion of the main body may have a triangular profile or unguiculate profile in a plane perpendicular to the air inlet plane. This may allow the main body to guide hair more effectively into the hair-receiving channel, as the main body is shaped to penetrate the mass of hair, reducing physical resistance to movement through the hair. The attachment may comprise a thermal shield element positioned behind the air outlet. The position of the thermal shield element, behind the air outlet, may be on the opposite surface of the main body to the location of the air outlet. The position of the thermal shield element may be located outside of the direction of the airflow from the air outlet. The thermal shield element may be located outside of the hair-receiving channel. In use this surface may be a surface close to the user’s scalp. This can shield the user’s scalp from thermal damage or discomfort which may result from the attachment becoming hot through use. The main body may comprise a further air outlet arranged to emit airflow in a direction which is not towards the projection. The airflow emitted by the further air outlet is not contained within the hair-receiving channel. The airflow can act to dry hair outside the hair-receiving channel and can cool the user’s scalp or surrounding hair to reduce thermal discomfort to the user. The further air outlet may be arranged to emit airflow in a direction generally orthogonal to the direction in which airflow is emitted from the air outlet (or plurality of air outlets). The main body may be divided into first and second chambers, the first and second chambers each being configured to receive a portion of the airflow from the air inlet. The first and second chambers may be configured to receive a substantially equal portion of the airflow from the air inlet. One or more air outlets may be in fluid communication with the first chamber. For example, one or more air outlets may be arranged in the first chamber. One or more further air outlets may be in fluid communication with the second chamber. For example, one or more further air outlets may be arranged in the second chamber. In embodiments in which the main body comprises a plurality of channels, each channel may be divided into first and second chambers, the first and second chambers each being configured to receive a portion of the airflow from the air inlet. The first and second chambers may be configured to receive a substantially equal portion of the airflow received by each channel. Each of the first chambers may be in fluid communication with one or more air outlets. For example, one or more air outlets may be arranged in each of the first chambers. Each of the second chambers may be in fluid communication with one or more further air outlets. For example, one or more further air outlets may be arranged in each of the second chambers. The attachment may comprise a valve. The valve may be arranged to be driven by a motor. The valve may comprise a blocking member moveable between a first position in which it blocks airflow to the air outlet (and does not block airflow to the further air outlet) and a second position in which it blocks airflow to the further air outlet (and does not block airflow to the air outlet). The blocking member may be moveable to a third position in which it allows airflow to both the air outlet and further air outlet (i.e. in which it does not block airflow to the air outlet and further air outlet). For example, the valve may comprise a blocking member moveable between a first position in which it blocks airflow into the first chamber (and does not block airflow into the second chamber) and a second position in which it blocks airflow into the second chamber (and does not block airflow into the first chamber). The blocking member may be moveable to a third position in which it allows airflow into both the first and second chambers (i.e. in which it does not block airflow into the first and second chambers). In some embodiments, the valve may comprise a blocking member moveable between a first position in which it blocks airflow to the air outlet and a second position in which it does not block airflow to the air outlet. For example, the valve may comprise a blocking member moveable between a first position in which it blocks airflow into the first chamber and a second position in which it does not block airflow into the first chamber. In some embodiments, the valve may comprise a blocking member moveable between a first position in which it blocks airflow to the further air outlet and a second position in which it does not block airflow to the further air outlet. For example, the valve may comprise a blocking member moveable between a first position in which it blocks airflow into the second chamber and a second position in which it does not block airflow into the second chamber. According to a second aspect of the present invention there is provided a haircare appliance comprising an attachment according to the first aspect of the present invention. The attachment may be releasably attachable to the haircare appliance. According to a third aspect of the present invention there is provided a haircare appliance comprising an air inlet; a handle unit in which an airflow generator is housed, the airflow generator for generating an airflow form the air inlet through the handle unit; an air outlet for emitting the airflow; and a projection spaced from the main body to define a hair-receiving channel between the projection and the main body, the air outlet arranged to emit airflow into the hair-receiving channel, the projection comprising a hair-receiving surface; wherein, when in use, hair located in the hair- receiving channel is blown by the airflow away from the main body into contact with the hair-receiving surface, such that hair is collected at the hair-receiving surface. The haircare appliance may comprise a heater for heating the airflow. This may provide increased styling flexibility, and may, for example, enable the airflow to provide a drying function. Optional features of aspects of the present invention may be equally applied to other aspects of the present invention, where appropriate. Further features and advantages of the invention will become apparent from the following description of preferred embodiments of the invention, given by way of example only, which is made with reference to the accompanying drawings. of the Drawings Figure 1 is a schematic perspective view of a haircare appliance according to the present invention; Figure 2 is a schematic cross-sectional view of a handle unit of the haircare appliance of Figure 1; Figure 3a is a first perspective view of an attachment of the haircare appliance according to Figure 1; Figure 3b is a second perspective view of the attachment according to Figure 3a; Figure 3c is a schematic view of the protrusion of the attachment according to Figure 3a in isolation; Figure 4 is a schematic cross-sectional view of the attachment according to Figure 3a; Figure 5 is a schematic view of the attachment according to Figure 3a receiving a user’s hair; Figures 6a and 6b are schematic perspective views of an attachment according to a second embodiment of the present invention; Figure 7a is perspective view of an attachment according to a third embodiment of the present invention; Figure 7b is a schematic cross-sectional view of the attachment according to Figure 7a; Figure 7c is a schematic view of the attachment according to Figure 7a receiving a user’s hair; Figure 8a is a schematic perspective view of three haircare appliances with different orientations of attachments according to the present invention; and Figure 8b is a schematic cross-section of a handle unit of the hair care appliances of Figure 8a. Detailed Description A haircare appliance according to the present invention, generally designated 10, is shown schematically in Figure 1. The haircare appliance 10 comprises a handle unit 12, and an attachment 100 removably attachable to the handle unit 12. The handle unit 12 is shown schematically in isolation in Figure 2, and comprises a handle portion 16, a head portion 18, an airflow generator 20 and a heater 22. The handle portion 16 is generally cylindrical and hollow in form, and houses the airflow generator 20. The handle portion 16 has an air inlet 24 in the form of a plurality of perforations at a first end 26 of the handle portion 16. The head portion 18 is generally cylindrical and hollow in form, and is disposed at a second end 27 of the handle portion 16, with a central axis of the head portion 18 orthogonal to a central axis of the handle portion 16 such that the handle unit 12 is generally T-shaped in form. The head portion 18 houses the heater 22. The head portion 18 comprises a bore 28 through which air is entrained, and an air outlet 30. The air outlet 30 is generally annular in form about a periphery of the bore 28. The head portion 18 further comprises an annular magnet (not shown) for releasably connecting the handle unit 12 to the attachment 100. The annular magnet extends annularly about the air outlet 30. The attachment 100 is shown schematically in Figures 3a-5, along with an indicative set of reference axes H-W-L, with respect to which heights (H), widths (W) and lengths (L), as well as height-width (H-W), width-length (W-L) and height-length (H-L) planes of the components of attachment 100 are described. In Figure 3b the attachment is flipped rotated 180 degrees about its length (i.e. about an axis parallel to L). The attachment 100 comprises an air inlet 101, a main body 103 arranged to receive airflow from the air inlet 101, a plurality of air outlets 110 for emitting the airflow (only a subset are labelled), and a projection 130 which is spaced from the main body 103 to define a hair-receiving channel 150. The air inlet 101 is annular to receive air from the annular outlet 30 of the haircare appliance 10. A plurality of magnets (not shown) are disposed annularly about the air inlet 101 for attachment with the handle unit 12. The main body 103 is hollow and defines a plenum through which air received at the air inlet 101 can flow. The main body 103 comprises five identical protrusions 107a-e in a row which extend by a length (parallel to axis L) in a direction perpendicular to the plane of the air inlet 101. Each protrusion 107a-e tapers in height (parallel to axis H) and width (parallel to axis W) along its length, decreasing in height and width from the proximal end (nearest the air inlet 101) to the distal end (furthest from the air inlet 101) The projection 130 is spaced from, and connected to, the main body 103 by a spacing element 131 which extends from the main body 103 at the proximal end of the protrusions 107a-e in a direction parallel to axis H. The projection 130 comprises a hair-receiving surface 135, the hair-receiving surface 135 comprising a plurality of apertures 137. Figure 3c illustrates a schematic view of the projection 130 in isolation which illustrates the arrangement of apertures 137 within the hair-receiving surface 135. The hair-receiving surface 135 extends in the width-length (L-W) plane and faces the row of protrusions 107a-e and the hair-receiving channel 150. The hair-receiving surface 135 has a first cross-sectional profile in the height-width (H-W) plane which is arcuate, and a second cross-sectional profile in the height-length (H-L) plane which is arcuate. The first cross-sectional profile is substantially concave relative to the hair- receiving channel 150 and the second cross-sectional profile is substantially convex relative to the hair-receiving channel 150, which provides the hair-receiving surface 135 with a saddle shape. The hair-receiving surface 135 sits opposite to, and spans a larger area than, the plurality of air outlets 110. The hair-receiving surface 135 comprises a plurality of apertures 137. The apertures 137 are regularly spaced along the width of the hair-receiving surface 135 to form a grating structure across the hair receiving surface 135, as seen in Figure 3c. The apertures 137 are long and narrow and substantially similar to one another, each aperture 137 extending along the entire length of the hair-receiving surface 135 and having a width much smaller than the total width of the hair-receiving surface 135. The spacing between neighbouring apertures 137 is roughly the width of each aperture 137. A hair-receiving channel 150 is formed between the hair-receiving surface 135 of the projection 130 and the main body 103. The height (parallel to axis H) of the channel 150 is determined by the height of the spacing element 131 as well as the specific geometry of the main body 103 and hair-receiving surface 135. In the attachment 100, the first and second arcuate cross-sectional profiles of the hair-receiving surface 135 mean that the hair-receiving channel 150 does not have a constant height throughout. The plurality of air outlets 110 are distributed along the length of each protrusion 107a- e of the main body 103. The air outlets 110 face the hair-receiving surface 135, and are thereby arranged to emit airflow received from the main body 103 into the hair receiving channel 150 in a direction which is parallel to the plane of the air inlet 101. The air outlets 110 are configured to emit airflow in substantially the same direction into the hair-receiving channel 150. The air outlets 110 vary in size and shape based on their position. Air outlets 110 nearer the proximate end of their respective protrusion 107a-e are wider than those at the distal end of their respective protrusion 107a-e, for example, due to the tapering geometry of the protrusion 107a-e. Figure 4 illustrates a cut-through of the attachment 100 viewed in the height-length plane. The main body 103 comprises a plurality of channels 112 which provide airflow to the air outlets 110, each channel being defined by vanes 113 which span the width of the respective protrusion 107a-e of the main body 103. The structure of the channels 112, defined by the vanes 113, determines the respective portion of the airflow in the main body 103 emitted by the air outlets 110, and also determines the direction of airflow emitted by the air outlets 110. The vanes 113 vary in length based on their position in the protrusions 107a-e, with vanes 113 nearer the main body being longer than vanes 113 further away from the main body. Through selection of the vane 113 geometries, air channels 112 can be arranged such that each air channel 112 receives an equal portion of the airflow from the main body 103 and each air outlet 110 emits an equal portion of the airflow into the hair-receiving channel 150. Alternatively, the air channels 112 can be arranged such that the air outlets 110 emit an airflow which is not uniform across the hair-receiving channel. In use, air is received from the hair care appliance 10 at the air inlet 101, and flows through the main body 103 to the plurality of apertures 110. Hair is received between the projection 130 and the protrusions 107a-e of the main body 103 in the hair-receiving channel 150. Air is emitted from the apertures 110 into the hair-receiving channel 150 in a direction towards the hair-receiving surface 135, whereupon it blows the hair received in the channel 150 into contact with the hair-receiving surface 135 such that hair is collected at the surface 135. The hair-receiving surface 135 enables the hair to be shaped and styled, and in the attachment 100 the arcuate profile of the hair-receiving surface 135 can further enhance this shaping and styling. The apertures 137 of the hair-receiving surface 135 permit at least some of the incident airflow to pass through the hair-receiving surface 135. Permeability of the hair- receiving surface 135 can reduce turbulence of airflow within the hair-receiving channel 150 as the apertures 137 provide an exit route for the airflow from the hair- receiving channel 150. This can also reduce redirection of airflow from the hair- receiving channel 150 into surrounding hair not currently located within the hair- receiving channel 150, which reduces disruption to the surrounding hair and improves the style control afforded by the attachment 100. The hair-receiving surface 135 being larger than the air outlets 110 means that substantially all the airflow emitted by the air outlets 110 is incident upon the hair- receiving surface 135 which means hair blown by the airflow emitted by the air outlets 110 is more likely to be collected by the hair-receiving surface 135, further reducing disruption to surrounding hair and improving style control afforded by the attachment 100. The air channels 112 receiving different portions of airflow from the main body 103 allows the air outlets 110, for example, to preferentially or more strongly blow hair into contact with a specific section of the hair-receiving surface 135 to improve style or shaping control, or more weakly blow hair which is near the edges of the hair-receiving channel 150 to reduce disruptive airflow into surrounding hair, or to blow hair uniformly across the hair-receiving channel 150 to provide uniform style control, depending on the portion of airflow received by each air channel 112. The air outlets 110 emit air parallel to the plane of the air inlet 101. More generally, the air outlets 110 are arranged to emit airflow in a direction non-orthogonal to the plane of the air inlet 101. This allows the attachment 100, in use, to be oriented such that the air emitted by the air outlets 110 is less incident upon the scalp of the user whilst hair is in the hair-receiving channel 150, which can reduce thermal discomfort to the user when the air is heated by the hair care appliance 10. This, in turn, allows the air to be hotter, which can improve drying and styling efficacy. The vanes 113 of the air channels 112 may be structured to determine the direction of air emitted by the air outlet. In the attachment 100 all the air outlets 110 emit air in substantially the same direction, but in other examples each air outlet 110 may emit in a respective direction non-orthogonal to the plane of the air inlet 101. This allows for the airflow across the hair-receiving channel 150 to be arranged to, for example, preferentially push hair onto a particular section of the hair-receiving surface 135 to improve style control afforded by the device. The main body 103 comprises a thermal shield 105. The thermal shield 105 is a surface located behind and on the opposite side to the air outlets 110 and hair-receiving channel. The thermal shield 105 is separated from the protrusions 107a-e by struts and the space between the surface and the protrusions 107a-e is filled with air. The thermal shield 105 can provide, in use, a barrier between the protrusions 107 and the user’s scalp which further reduces thermal discomfort if the protrusions 107a-e heats up due to receiving heated air from the hair care appliance 10. In other examples, the thermal shield may instead be provided by an insulating material. The triangular cross-sectional profile of the protrusions 107a-e of the main body 103 helps the main body 103 penetrate under the hair to the root which can ease the delivery of hair into the hair-receiving channel 150. Figure 5 shows the attachment 100 in intended use, with hair 160 received by the hair receiving channel 150, and airflow 155 emitted by the air outlets 110 (not shown) of the main body 103 blowing the hair 160 across the hair-receiving channel 150 towards the hair-receiving surface 130. Entrainment of surrounding air 158 through the channels between the protrusions 107a-e can improve thermal comfort to the user. Figures 6a and 6b shows a second embodiment of the invention, attachment 200, in side-on view (Figure 6a) and exploded perspective view (Figure 6b). The attachment 200 is substantially similar to the attachment 100, in that it comprises an air inlet 201, a main body 203 arranged to receive airflow from the air inlet 201, a plurality of air outlets 210 for emitting the airflow (only a subset are labelled), and a projection 230 which is spaced from the main body 203 to define a hair-receiving channel 250. The main body 203 comprises five identical protrusions 207a-e in a row by a length in a direction perpendicular to the plane of the air inlet 201. The projection 230 comprises a hair-receiving surface 235 and plurality of apertures 237 spaced regularly across the surface. In attachment 200, the projection 230 is mounted to the main body 203 by a collar 255, which provides the height of hair receiving channel 250. The collar 255 and projection 230 are detachable from the main body 203, which can allow for, for example, the projection 230 to be interchangeable with alternative projections according to the invention. In attachment 200, the air outlets 210 are located on the sides of the protrusions 207a- e, whereas in attachment 100 the air outlets 110 are located on the top of the protrusions 107a-e. Air channels 212 (not pictured) are arranged internally such that the air outlets 210 emit air across the hair-receiving channel 250 and towards the hair-receiving surface 235, as indicated by the arrows A. This may allow the airflow to be directed more closely to the roots of the hair, whilst still ensuring the airflow pushes hair across the hair-receiving channel 250 to the hair-receiving surface 235, thereby improving style control. Figures 7a to 7c show an attachment 300 according to a third embodiment of the invention. The attachment 300 is substantially similar to the attachment 100 in accordance with the first embodiment of the invention, in that it comprises an air inlet 301, a main body 303 arranged to receive airflow from the air inlet 301, a plurality of (primary) air outlets 310 for emitting the airflow (only a subset are labelled), and a projection 330 which is spaced from, and connected to, the main body 303 by a spacing element 331 to define a hair-receiving channel 350. The main body 303 comprises five protrusions 307a-e arranged lengthways side by side in a row in a plane perpendicular to the plane of the air inlet 301. The projection 330 comprises a hair-receiving surface 335 and a plurality of apertures 337 spaced across the surface 335. The plurality of primary air outlets 310 are distributed along the length of each protrusion 307a-e of the main body 303. The primary air outlets 310 face the hair- receiving surface 335, and are thereby arranged to emit airflow received from the main body 303 into the hair receiving channel 350 in a direction which is parallel to the plane of the air inlet 301. The primary air outlets 310 are configured to emit airflow in substantially the same direction as each other into the hair-receiving channel 350. The primary air outlets 310 vary in size and shape based on their position. The primary air outlets 310 nearer the proximate end of their respective protrusion 307a-e are wider than those at the distal end of their respective protrusion 307a-e, for example, due to the tapering geometry of the protrusion 307a-e. The attachment 300 also comprises a plurality of secondary air outlets 311 (only a subset are labelled), The secondary air outlets 311 are distributed along the length of each protrusion 307a-e on an opposite side of each protrusion 307a-e to the plurality of primary air outlets 310. The secondary air outlets 311 are configured to emit airflow away from the hair-receiving channel 350, below the main body 303 of the attachment 300. In this example, the secondary air outlets 311 are configured to emit airflow away from the hair-receiving channel 350 in a direction that is generally orthogonal to the direction in which the primary air outlets 310 emit airflow into the hair-receiving channel 350. As shown in Figures 7a to 7c, the secondary air outlets 311 are provided on each protrusion 307a-e in a row of generally circular apertures of the same size. However, in some embodiments, the secondary air outlets 311 may be of different shapes and/or sizes, and multiple rows of secondary air outlets 311may be provided. Figure 7b illustrates a cut-through of the attachment 300 viewed in the height-length plane. Each of the protrusions 307a-e are split into an upper chamber 305 and a lower chamber 306 by a dividing wall 308. Each upper chamber 305 comprises a plurality of channels 312 which provide airflow to the primary air outlets 310, each channel being defined by vanes 313 which span the width of the respective protrusion 307a-e. Each lower chamber 306 comprises a plurality of secondary air outlets 311. In some embodiments, each protrusion 307a-e may define a single chamber comprising both the primary air outlets 310 and secondary air outlets 311 (i.e. the dividing wall 308 may be absent). The structure of the channels 312, defined by the vanes 313, determines the respective portion of the airflow emitted by the primary air outlets 310, and also determines the direction of airflow emitted by the primary air outlets 310. The vanes 313 vary in length based on their position in the upper chamber 305 of each of the protrusions 307a-e, with vanes 313 nearer the main body 303 being longer than vanes 313 further away from the main body 303. Through selection of the vane 313 geometries, air channels 312 can be arranged such that each air channel 312 receives an equal portion of the airflow received by the upper chamber 305 from the main body 303 and each air outlet 310 emits an equal portion of the airflow into the hair-receiving channel 350. Alternatively, the air channels 312 can be arranged such that the primary air outlets 310 emit an airflow which is not uniform across the hair-receiving channel 350. In use, air is received from the hair care appliance 10 at the air inlet 301, and flows through the main body 303 to each of the protrusions 307a-e, whereby the airflow is split by the dividing wall 308 into a first stream flowing through the upper chamber 305 to the primary air outlets 310, and into a second stream flowing through the lower chamber 306 to the secondary air outlets 311. As described above with respect to the attachment 100 in accordance with the first embodiment of the invention, hair (not shown in Figure 7b) is received between the projection 330 and the protrusions 307a-e of the main body 303 in the hair-receiving channel 350. Air is emitted from the primary air outlets 310 into the hair-receiving channel 350 in a direction towards the hair-receiving surface 335, whereupon it blows the hair received in the channel 350 into contact with the hair-receiving surface 335 such that the hair is collected at the surface 335. The hair-receiving surface 335 enables the hair to be shaped and styled, and in the attachment 300 the arcuate profile of the hair-receiving surface 335 can further enhance this shaping and styling. At the same time, air is emitted from the secondary air outlets 311 below the protrusions 307a-e on a lower side of the main body 303 of the attachment 300. This air may be directed towards the scalp of the user whose hair is received in the hair-receiving channel 350, thereby enhancing drying of the roots of the hair. Figure 7c shows the attachment 300 in intended use, with hair 160 received by the hair- receiving channel 350. Airflow emitted by the primary air outlets 310 blows the hair 160 across the hair-receiving channel 350 towards the hair-receiving surface 330. Airflow emitted by the secondary air outlets 311 may be directed towards the user’s scalp whilst their hair 160 is received by the hair receiving channel 350. In some embodiments, the attachment 300 may comprise a first sensor 336 and a second sensor 338. The first sensor 336 may, for example, be a moisture sensor and the second sensor 338 may, for example, be a temperature sensor. The first sensor 336 may be positioned facing towards the projection 306 on an upper surface of one of the protrusions 307a-e. The second sensor 338 may be positioned on one of the protrusions 307a-e facing in a direction opposite to the first sensor 338, away from the projection 306, so that it faces towards a user’s scalp in use. The first and second sensors 336, 338 may feed information back to a controller in the haircare appliance 10, which may in turn be used to control parameters such as the rate and temperature of airflow generated by the haircare appliance 10. In some embodiments, the attachment 300 may comprise a valve arrangement (not shown in Figures 7a to 7c). The valve arrangement may be located within the main body 303 of the attachment 300. The valve arrangement may comprise a blocking member shaped and dimensioned to cover the entrances to the upper and/or lower chambers 305, 306 of the protrusions 307a-e. In this way, the blocking member may be moveable to cover the entrances to all of the upper chambers 305 simultaneously (leaving the entrances to the lower chamber 306 uncovered), or to cover the entrances to all of the lower chambers 306 simultaneously (leaving the entrances to the upper chambers 305 uncovered). The blocking member may also be movable to cover and/or uncover cover the entrances to all of the upper chambers 305 and lower chambers 306 simultaneously. The valve arrangement may suitably be arranged for connection to a drive motor (not shown) in the haircare appliance 10, such that the haircare appliance 10 may be able to control airflow through the plurality of primary air outlets 310 and plurality of secondary air outlets 311. For example, the haircare appliance 10 may suitably control airflow through the plurality of primary air outlets 310 and plurality of secondary air outlets 311 based on information received from the first sensor 336 and/or second sensor 338. Variations of the appliance according to the present invention, generally designated 10- a, 10a-b, 10a-c, are shown schematically in Figure 8a. The haircare appliance 10-a,b,c comprise a handle unit 12-a, and an attachment 100- a,b,c removably attachable to the handle unit 12-a. Attachments 100-a, 100-b and 100- c are according to the invention described herein, and are illustrated here to highlight various orientations of attachments relative to the handle unit 12-a of the haircare appliance the attachment 100-a,b,c is being used with. The handle unit 12-a is cylindrical and extends by a length in a direction perpendicular to its circular cross- section. Attachment 100-a is orientated such that the height of the hair-receiving extends perpendicular to the length of the handle unit 12-a (see indicative axis going into the page). The axis of the row of protrusions – that is, the axis from which the protrusions and hair-receiving surface emanate to form the hair-receiving channel - is aligned parallel to the length of the handle unit 12-a (see indicative axis Attachment 100-b is aligned such that the axis of the row of protrusions of the attachment 100-b extends perpendicular to the length of the handle unit 12-a (see indicative axis and the height of the hair-receiving channel is aligned perpendicular to the length of the handle unit 12-a (see indicative axis ^ _^ି^ , going into the page). Attachment 100-c is aligned such that the axis of the row of protrusions of the attachment 100-c extends perpendicular to the length of the handle unit 12-a (see indicative axis ^ _^ି^ , going into the page), and the height of the hair-receiving channel is aligned parallel to the length of the handle unit 12-a (see indicative axis ^ _^ି^ ). The handle unit 12-a comprises a housing 14-a, an airflow generator 16-a, a heater 18- a, and a control unit 20-a, as can be seen schematically in Figure 8b. The housing 14-a is tubular in shape, and comprises an air inlet 22-a through which an airflow is drawn into the housing 14-a by the airflow generator 16-a, and an air outlet 24-a through which the airflow is discharged from the housing 14-a. The airflow generator 16-a is housed within the housing 14-a, and comprises an impeller 26-a driven by an electric motor 28-a. The airflow generator is configured to generate airflow at a flow rate in the region of 8 to 18 L/S, for example in the region of 10 to 16L/s. An appropriate airflow generator is the Dyson V9 Digital Motor, produced by Dyson Technology Limited. The heater 18-a is also housed within the housing 14-a, and comprises heating elements 30-a to optionally heat the airflow. The control unit 20-a comprises electronic circuitry for a user interface 32-a and a control module 34-a. The user interface 32-a is provided on an outer surface of the housing 14-a, and is used to power on and off the haircare appliance 10-a,b,c, to select a flow rate (for example high, medium and low), and to select an airflow temperature (for example hot, medium or cold). In the example of Figure 8b, the user interface comprises a plurality of sliding switches, but other forms of user interface 32-a, for example buttons, dials or touchscreens, are also envisaged. The control module 34-a is responsible for controlling the airflow generator 16-a, and the heater 18-a in response to inputs from the user interface 32-a. For example, in response to inputs from the user interface 32-a, the control module 34-a may control the power or the speed of the airflow generator 16 in order to adjust the airflow rate of the airflow, and the power of the heater 18-a in order to adjust the temperature of the airflow. The above embodiments are to be understood as illustrative examples of the invention. Further embodiments of the invention are envisaged. The main body may comprise a further air outlet arranged to emit airflow in a direction which is not towards the projection. The further air outlet may be arranged to direct air towards the roots of the hair to aid with drying, for example, whilst the tress of the hair is received within the hair-receiving channel. The main body may not comprise separate protrusions, and may not comprise a plurality of outlets which receive air from a plurality of respective air channels. Instead, air may emitted by a single air outlet which receives the airflow from the main body. The air may be emitted by a plurality of air outlets which are not disposed across separate protrusions, and the air may be emitted by a plurality of air outlets which are disposed across separate protrusions but do not feature internal channels. The hair-receiving surface may comprise apertures which are not distributed regularly across the hair-receiving surface and vary in shape and size. This may allow for the behaviour of airflow within the hair-receiving channel according to a particular styling or shaping profile desired from the attachment to be realised. The hair-receiving surface may be arcuate in only one cross-sectional profile, and straight in another profile. The hair-receiving surface may have other geometries which allows it to shape received hair in a desired manner. The hair-receiving surface may only comprise a single aperture, or may comprise no aperture at all, for example if the hair-receiving surface is suitably shaped to mitigate the impact of turbulence in the hair-receiving channel, or if the emitted airflow is diffuse and turbulence from the airflow does not significantly impact style control. The attachment may not comprise a thermal shield, as the thermal discomfort from contact with the main body may be minimal or non-existent. The height of the spacing element may be adjustable such that the height of the hair- receiving channel can be altered. Examples are also envisaged where, rather than the hair care appliance 10 comprising a handle unit 12 and an attachment 100, 200, the haircare appliance 10 is a single-piece unit, for example taking the form of the combined handle unit 12 and attachment 100, 200 previously described. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.