An impeller

Abstract

An impeller 10 for a turbomachine. The impeller 10 comprises a rotatable hub 11 and a plurality of blades 12 extending from the hub 11. Each blade 12 comprises a leading edge 15 and a trailing edge 16. Each leading edge 15 comprises a plurality of serrations 21 and a plurality of elongate slots 22, and each slot 22 extends from a root 26 of an adjacent serration 21 towards the trailing edge 16 of the respective blade 12.

Description

AN IMPELLERBACKGROUND

[0001] Turbomachines, such as compressors fans and pumps, transfer energy between an impeller that is driven to rotate and a fluid (such as air) passing through the turbomachine. Turbomachines are used in a wide variety of applications, including, for example, appliances such as fans (e.g., air purifiers), vacuum cleaners and hair care devices (e.g., hair dryers).SUMMARY

[0002] In a first aspect there is disclosed an impeller for a turbomachine, the impeller comprising: a rotatable hub; and, a plurality of blades extending from the hub, each blade comprising a leading edge and a trailing edge, each leading edge comprising a plurality of serrations and a plurality of elongate slots, and each slot extending from a root of an adjacent serration towards the trailing edge of the respective blade.

[0003] Turbomachines generate noise in use, which is at least partly due to the interaction between fluid (e.g., air) and the blades of the rotating impeller of the turbomachine. It is desirable to reduce such noise, especially where such turbomachines are present in appliances (e.g., fans, vacuum cleaners and hair care devices), whereby noise can severely impact the experience of a user using such appliances.

[0004] The presence of serrations on the leading edges of the blades of the impeller can reduce the noise generated by the interaction between the blades and air flowing across the blades. The addition of elongate slots extending from the roots of the serrations (towards the trailing edge) can provide even further noise reduction.

[0005] During operation of the impeller, each serration may generate a small-scale vortex which can interact with one or more slots, thereby causing a reduction in acoustic energy and resulting sound power output. It is hypothesised that this noise reduction is provided because each slot creates two noise sources (at opposite leading / trailing ends of the slot) that radiate in antiphase (i.e., so as to at least partly cancel one another out).

[0006] Further noise reduction may arise when the small-scale vortices generated by the serrations interact with and disrupt the coherent structure of larger-scale vortices, such as those generated due to blade tip reinjection. Such interactions can cause the larger-scalevortices to break down into multiple smaller-scale vortices, thereby reducing the acoustic energy and resulting sound power output.

[0007] For the avoidance of doubt, the “root” of a serration is at the base of a serration, where the serration would otherwise be directly joined to the adjacent serration if not for the presence of a slot. In other words, the root of a serration is typically the nadir between two serrations in a row of serrations.

[0008] Optional features of the first aspect will now be set out. These are applicable singly or in any combination with any aspect.

[0009] Each slot may extend from the respective root towards the trailing edge of the respective blade, thereby defining a slot length. That is, each slot may have a leading end at the root of the serration and may extend towards the trailing edge to a trailing end. The length of the slot may be defined between the leading and trailing ends of the slot.

[0010] The slots of at least one of the leading edges may vary in length. That is, at least one of the leading edges may have slots of varying length.

[0011] A first slot of at least one of the leading edges may have a first length. A second slot of the at least one leading edge may have a second length that is greater than the first length. The first slot may be radially inward of the second slot. The first slot may be a radially innermost slot of the at least one leading edge. The second slot may be a radially outermost slot of the at least one leading edge. Thus, at least one leading edge may have a longer radially outermost slot than radially innermost slot.

[0012] The slots of at least one of the leading edges may increase in length in a direction away from the hub. Thus, each successive slot (moving in a radially outward direction along a leading edge) may be greater in length than the preceding slot.

[0013] As may be appreciated, upon rotation of the hub of the impeller, the speed at a radially outermost portion of each blade (i.e., blade tip speed) will be higher than the speed at a radially innermost portion of the blade. As a result, the speed of air flowing over the leading edge (relative to the leading edge) is greater at the radially outermost portion of the leading edge than the radially innermost portion of the leading edge.

[0014] Providing longer slots at the outer portions of a leading edge may thus compensate for the increased (relative) air flow speed across the leading edge. This may help to ensure, for example, that noise sources at the leading / trailing ends of the slot are substantially in antiphase (which may aid in reduction of noise).

[0015] Each slot may have a slot width that is equal to or greater than 1.0 mm. Each slot may have a slot width that is equal to or greater than 1.3 mm, e.g. greater than 1.5 mm. Providing slots of such width can facilitate formation of the slots via a moulding process (e.g., injection moulding). The width of the slot may be taken as the dimension extending perpendicular to the length of the slot.

[0016] Each slot may have a slot width that is from 1 mm to 2.5 mm.

[0017] At least one (e.g., each) slot may have a substantially constant width for substantially the entire length of the slot. In some embodiments, at least one (e.g., each) slot may taper in a direction from the leading edge towards the trailing edge.

[0018] Each slot may have a substantially rectangular shape. The slots may, however, have other shapes (e.g., may be tapered so as to be triangular).

[0019] One or more (e.g., each) of the plurality of serrations may be tapered so as to narrow to a point. A tip of one or more of the serrations may be pointed. In other embodiments the tip may be curved or truncated.

[0020] Each serration may comprise first and second edge portions that join at the tip of the serration. One or both of the first and second edge portions may be linear (i.e., straight). In other embodiments, one or both of the first and second edge portions may be curved.

[0021] A radially outermost serration of at least one of the leading edges (e.g., each leading edge) may comprise a wider tip than one or more (e.g., all) of the other serrations of the at least one leading edge. Alternatively, or additionally, in some embodiments, a radially innermost serration of at least one of the leading edges may comprise a wider tip than one or more of the other serrations of the at least one leading edge.

[0022] The radially outermost serration of at least one of the leading edges may comprise a truncated tip. Alternatively, or additionally, in some embodiments, the radially innermost serration of at least one of the leading edges may comprise a truncated tip.

[0023] In such embodiments (i.e., in which the outermost tip is wider / truncated) the impeller may be more readily mounted within a shroud, for example, by ultrasonic welding. Likewise, in embodiments in which the innermost tip is wider / truncated, the impeller blades may be more readily mounted to the hub, for example, by ultrasonic welding. This is because thin regions of material can fail under vibrations generated in ultrasonic welding processes (and truncating and / or providing a wider tip can provide sufficient strength to avoid such failure).

[0024] Each blade may be integrally formed with the hub (e.g., the blades and hub may be formed together by an injection moulding process). Alternatively, the hub may be formed separately from the blades. Each blade may then be connected to the hub, for example by ultrasonic welding.

[0025] Each leading and trailing edge may have a distal end that is distal from the hub. Each blade may have a chord length, which may be defined as the distance between the distal ends of the trailing and leading edges of the blade. The chord length of at least one blade may be e.g. from 50 mm to 250 mm.

[0026] At least one slot (e.g., each slot) may have a length such that the ratio of the length of the at least one slot to the chord length of the respective blade is from 0.03 to 0.15, or e.g. from 0.04 to 0.10.

[0027] Each serration may have a height defined as the distance between the root and tip of the serration (i.e., in the direction of extension of the serration as opposed to e.g. a direction along an edge of the serration).

[0028] At least one serration may have a height such that the ratio of the height of the serration to the chord length is from 0.05 to 0.40 or e.g. from 0.05 to 0.20.

[0029] The serrations of each leading edge may be arranged in a row along the leading edge. The serrations of each leading edge may be arranged so as to have a wavelength defined as the distance between the tips of two adjacent serrations (of the serrations of the leading edge). At least one slot (e.g., each slot) may have a width such that a ratio of the slot width to the wavelength of the serrations of the at least one leading edge is from 0.03 to 0.40, e.g. from 0.03 to 0.25.

[0030] The serrations of the at least one leading edge may have a wavelength such that a ratio of the wavelength to the chord length of the respective blade is from 0.050 to 0.2, or e.g. from 0.075 to 0.10.

[0031] The impeller may have a maximum diameter that is from 50 mm to 400 mm (e.g., at the trailing edge of at least one of the blades). That is, the impeller may have a maximum radius that is from 25 mm to 200 mm (e.g., about 40 mm in one example). The shroud, when present, may have a corresponding maximum radius of from 25 mm to 200 mm (e.g., about 40 mm in one example).

[0032] The hub may have a radius of from 1 mm to 50 mm (e.g., about 5 mm in one example) at the leading edge of at least one of the blades.

[0033] At least one blade may have a twisted shape (e.g., may twist about a radially extending axis). A leading portion (i.e., at the leading edge) of at least one blade may have a shape such that an outer region of the leading portion is angled with respect to an inner region of the leading portion. Thus, for example, the outer region may be tilted by a first angle relative to the axial direction (i.e., a direction parallel to the rotational axis) and the inner region may be tilted by a second angle (relative to the axial direction) that is smaller than the first angle. The tilt of the leading portion away from the axial direction may, for example, increase gradually in an outward direction along the leading portion.

[0034] At least one blade may be shaped such that an outermost serration projects in a first direction and the innermost serration projects in a second direction. The first direction may be at a first angle to the axial direction and the second direction may be at a second angle to the axial direction which may be smaller than the first angle. In general, the leading edge of the at least one blade may be shaped such that each successive serration (moving outward along the edge) is angled further from the axial direction than the preceding serration. In some cases, the innermost serration may project in the axial direction (i.e., the second angle may be zero degrees). The axial direction is a direction that is parallel to the rotational axis of the impeller.

[0035] The impeller may be a mixed flow impeller or a radial flow impeller. Thus, for example, the impeller may be configured to receive an axial airflow (flowing substantially parallel to a rotational axis of the impeller) and turn the airflow so as to discharge the airflow in a direction that is oblique or perpendicular to the rotational axis.

[0036] The hub may comprise a concave outer surface (i.e., concave in an axially extending plane).

[0037] In a second aspect, there is disclosed a compressor comprising the impeller according to the first aspect.

[0038] The compressor may further comprise a shroud. An outer edge of each blade, distal from the hub, may be fixed to the shroud (e.g., by way of ultrasonic welding).

[0039] The compressor may be configured to rotate, in use, at a speed from around 500 rpm to 10,000 rpm (or e.g., from around 1,000 rpm to 6,000 rpm).

[0040] The compressor may be configured for fluid flow rates (through the compressor) of from 0.01 m3 / s to 0.50 m3 / s.

[0041] As may be appreciated, noise reduction may vary across different frequencies. To determine the frequency at which the highest noise reduction may be experienced for a given impeller geometry, flow rate, and compressor operating speed, the following relationship may be used:NU f“ (H + L)Where: f is the frequency of highest noise reduction;N is a non-dimensional frequency preferably having a value of about 1.5;U is the velocity of the fluid over the leading edge (which may be determined using the rotational speed of the impeller and the flow rate of the fluid);H is serration height; and,L is slot length.

[0042] In a third aspect there is disclosed an appliance comprising the compressor of the second aspect. The appliances may be any one of e.g. air treatment appliance (e.g., fan, air purifier, heater, etc.), vacuum cleaner, or hair care appliance (e.g., hair dryer).

[0043] In a fourth aspect there is disclosed a method of forming the impeller according to the first aspect, the method comprising forming the impeller by injection moulding, wherein the serrations and / or the elongate slots are formed by a tool moved in a direction along a central (i.e., rotational) axis of the hub (i.e., an axial tool action).

[0044] The tool may comprise a plurality of concentric annular projections, each for forming a respective slot in a leading edge of a blade of the impeller. The annular projections may be arranged such that there is a substantially constant radial spacing between adjacent projections.

[0045] The annular projections may be of substantially equal height. As may be appreciated, however, when the blades have a twisted shape as described above (i.e., where outer regions of the leading edge of the blade are tilted further from the axial direction), the resulting slots (formed by the projections) will vary in length. In particular, slots formed in regions having greater tilt will be longer (in a direction along the blade surface) and slots formed in regions tilted to a lesser extent will be shorter.

[0046] In a fifth aspect there is disclosed a method of forming a compressor, the method comprising positioning an impeller according to the first aspect within a shroud, andconnecting the outer edges of the blades of the impeller to the shroud by way of ultrasonic welding.BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure l is a perspective view of an impeller with a shroud of the impeller omitted;

[0048] Figure 2 is a perspective section view of the impeller of Figure 1 with the shroud present;

[0049] Figure 3 is a detailed view of a leading edge of a blade of the impeller of Figure 1;

[0050] Figure 4 is a schematic view showing serrations of the leading edge of a blade of the impeller of Figure 1; and,

[0051] Figure 5 is a section view of a tool for forming the impeller of Figure 1.DETAILED DESCRIPTION

[0052] Figures 1 to 4 illustrate an impeller 10 for a turbomachine (in this case for the compressor of a bladeless-style tower fan).

[0053] The impeller 10 includes a rotatable hub 11 and a plurality of blades 12 extending generally radially outwardly from the hub 11. The hub 11 has a frustoconical shape, with a curved outer circumferential surface 13 (i.e., facing radially outward from the hub 11) from which the plurality of blades 12 extend. The outer surface 13 of the hub 11 is concave in an axial plane (i.e., extending parallel to a rotational axis 14 of the hub 11). In this way, air flowing over the outer surface 13 in use is turned from an axial direction (i.e., parallel to the rotational axis 14) to a partly radial direction (perpendicular to the rotational axis 14). Accordingly, the impeller 10 may be referred to as a mixed flow impeller.

[0054] Each blade 12 includes an inner edge 17 at which the blade 12 is joined to (in this case, integrally formed with) the hub 11, and an outer edge 18 that is spaced radially outward from the inner edge 17. As best seen in Figures 2 and 3, each outer edge 18 joins the respective blade 12 to a shroud 19 that extends circumferentially around the blades 12 (and circumferentially around the hub 11). In this way, the shroud 19 effectively forms an outer housing of the impeller 10. Although not apparent from the Figures, the shroud 19 is joined to each outer edge 18 by way of ultrasonic welding.

[0055] Each blade 12 includes opposite leading 15 and trailing 16 edges, which each extend between (so as to connect) the inner 17 and outer 18 edges of the respective blade 12.Accordingly, each leading edge 15 extends from a proximal end 32 (at the inner edge 17 of the respective blade 12) to a distal end 33 (at the outer edge 18 of the respective blade 12). Similarly, each trailing edge 16 extends from a proximal end 34 (at the inner edge 17 of the respective blade 12) to a distal end 35 (at the outer edge 18 of the respective blade 12).

[0056] As may be appreciated, in use, air first passes across the leading edge 15 of each blade 12, flows along the blade 12, and then passes across trailing edge 16 (so as to be discharged from the impeller 10). In particular, the air flows along passages 20 defined between the blades 12, hub 11 and shroud 19. The blades 12 are arranged such that these passages 20 extend in helical manner around the hub 11.

[0057] Each leading edge 15 includes a row of serrations 21 (in this case, five serrations 21) and a plurality of elongate slots 22 (in this case, four elongate slots 22). These are best seen in Figure 3 (showing a detailed view of a leading edge 15 of the impeller 10).

[0058] Each serration 21 comprises a first edge portion 23 and a second edge portion 24, both of which are substantially linear (i.e., straight). Each serration 21 is tapered so as to narrow to a point at the tip 25 of the serration 21 (except for an outermost serration 21’ and an innermost serration 21” as discussed further below). In other words, the first 23 and second 24 edge portions are angled so as to form an acute angle therebetween. In this way, each serration 21 has a generally triangular shape.

[0059] An outermost serration 21’ of the serrations 21 of each blade 12 includes a truncated tip 25 (i.e., so as not to extend to a point). Thus, the tip 25 of each outermost serration 21’ is wider than the tip 25 of one or more other serrations 21 of a respective blade 12. This can aid in connection of the shroud 19 to the blades 12 via ultrasonic welding. The additional width of the tip 25 each outermost serration 21’ allows each outermost serration 21’ to better withstand the vibrations involved in ultrasonic welding.

[0060] In embodiments, each blade 12 may be integrally formed with the hub 11. In other embodiments, the hub 11 and blades 12 may be formed as separate components and then connected together, for example by ultrasonic welding. In such embodiments, such as that shown in Figure 3, an innermost serration 21” of the serrations 21 of each blade 12 may additionally include a truncated tip 25 (i.e., so as to not to extend to a point). Thus, the tip 25 of each innermost serration 21” is wider than the tip 25 of one or more other serrations 21 of a respective blade 12. This can aid in connection of the blades 12 to the hub 11 via ultrasonic welding. The additional width of the tip 25 each innermost serration 21” allowseach innermost serration 21” to better withstand the vibrations involved in ultrasonic welding.

[0061] A plurality of serration roots 26 are defined between adjacent serrations 21 in the row of serrations 21. Each slot 22 is elongate and extends from a leading end 27 at a respective serration root 26 to a trailing end 28 of the slot 22 (along the respective blade 12, in a direction towards the trailing edge 16 of the respective blade 12).

[0062] Thus, each slot 22 provides third 29, fourth 30, and fifth 31 edge portions. Each third edge portion 29 extends from a second edge portion 24 in a direction towards the trailing edge 16 of the blade 12. Each fifth edge portion 31 extends from a respective first edge portion 23, again in a direction towards the trailing edge 16 of the blade 12. Each fourth edge portion 30 extends transversely so as to join a respective pair of third 29 and fifth 31 edge portions at the trailing end 28.

[0063] As is shown schematically in Figure 4 (in which several reference numerals are omitted for clarity), each slot 22 has a width dimension W in a transverse direction across the slot 22, and a length dimension L in the elongate direction of the slot 22. Likewise, each serration 21 has a height H defined as the distance from the root 26 of the serration 21 to the tip 25 of the serration 21 in the direction of extension of the serration 21. Further, a serration wavelength is defined as the distance between the tips 25 of two adjacent serrations 21. For completeness, it is noted that each blade 12 has a thickness dimension extending between the two major faces of the blade 12 (into the page as illustrated).

[0064] Returning to Figure 3, each slot 22 has a substantially constant width W (for the entire length L of the slot 22). Also, all of the slots 22 of a respective blade 12 are of substantially equal width W.

[0065] On the other hand, the lengths L of the slots 22 on each blade 12 vary. In particular, the lengths L vary such that the slot length L increases in a direction from the inner edge 17 of a respective blade 12 to the outer edge 18 of the respective blade 12. Thus, slots 22 closer to the outer edge 18 have a greater length L than slots closer to the inner edge 17. As already discussed above, the relative airflow speed across the leading edge 15 is greater towards the outer edge 18 of a blade 12 than the inner edge 17 of the blade 12. The slot length L required for optimal noise reduction can differ with relative airflow speed (i.e., the speed of the airflow relative to the leading edge 15). In particular, the greater the relative airflow speed,the greater the slot length L required for optimal noise reduction. Accordingly, varying slot length L across each blade 12 can provide improved noise reduction.

[0066] The heights H of the serrations 21 on each blade 12 vary in a similar manner to the slots 22. In particular, serration height H increases in a direction from the inner edge 17 of a respective blade 12 to the outer edge 18 of the respective blade 12.

[0067] Each slot 22 is formed such that the ratio of the width W of the slot 22 to the wavelength of the serrations 21 is about 0.2. Further, the wavelength X of the serrations 21 of each blade 12 to the chord length Co (see Figure 1) of the respective blade is about 0.08.

[0068] Each slot 22 is further formed such that the ratio of the length L of the slot 22 to the chord length Co of the blade 12 (on which the slot 22 is provided) is at least 0.03, or at least 0.05.

[0069] Each serration 21 is formed such that the ratio of the height H of the serration to the chord length Co of the blade 12 (on which the serration 21 is provided) is at least 0.05.

[0070] Figure 5 illustrates a tool 36 that is used to form the leading edges 15 of the blades 12 of the impeller 10. In some examples, the impeller 10 (not including the shroud 19) is formed by way of an injection moulding process. The tool 36 is a tool that is moved axially in use (along the rotational axis 14) to form the serrations 21 and slots 22 of the impeller 10.

[0071] Thus, the tool 36 comprises a plurality of concentric (spaced) annular projections 37, which ultimately form the serrations 21 and slots 22 of the impeller 10. As is apparent from Figure 5, the projections 37 of the tool 36 are of equal height. However, due to the shape of each blade 12 at the leading edge 15 the projections 37 form slots 22 of varying height. This is because each blade 12 has a twisted form such that the outer edge 18 of each blade 12 is tilted further away from an axial direction (parallel to the rotational axis) 14 than the inner edge 17 of each blade 12. In particular, the tilt of the leading edge 15 of each blade 12 increases in a radially outward direction along the blade 12.

[0072] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

[0073] For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

[0074] Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0075] Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

[0076] It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / - 10%.

Claims

CLAIMS1. An impeller for a turbomachine, the impeller comprising: a rotatable hub; and, a plurality of blades extending from the hub, each blade comprising a leading edge and a trailing edge, each leading edge comprising a plurality of serrations and a plurality of elongate slots, and each slot extending from a root of an adjacent serration towards the trailing edge of the respective blade.

2. The impeller according to claim 1 , wherein each slot extends from the respective root towards the trailing edge of the respective blade, thereby defining a slot length, and wherein the slots of at least one of the leading edges vary in length.

3. The impeller according to claim 2, wherein a radially innermost slot of at least one of the leading edges has a first length and the radially outermost slot of the at least one leading edge has a second length that is greater than the first length.

4. The impeller according to claim 2 or 3, wherein the slots of at least one of the leading edges increase in length in a direction away from the hub.

5. The impeller according to any one of the preceding claims, wherein each slot has a slot width that is greater than 1.0 mm.

6. The impeller according to any one of the preceding claims, wherein each slot has a substantially constant width for substantially the entire length of the slot.

7. The impeller according to any one of the preceding claims, wherein one or more of the plurality of serrations are tapered so as to narrow to a point.

8. The impeller according to any one of the preceding claims, wherein a radially outermost serration and / or a radially innermost serration of at least one of the leading edgescomprises a wider tip than one or more of the other serrations of the at least one leading edge.

9. The impeller according to any one of the preceding claims, wherein a radially outermost serration and / or a radially innermost serration of at least one of the leading edges comprises a truncated tip.

10. The impeller according to any one of the preceding claims, wherein: each leading and trailing edge has a distal end that is distal from the hub, and each blade has a chord length defined as the distance between the distal ends of the trailing and leading edges of the blade; and, at least one slot has a length such that the ratio of the length of the at least one slot to the chord length of the respective blade is from 0.04 to 0.1.

11. The impeller according to any one of the preceding claims, wherein: each leading and trailing edge has a distal end that is distal from the hub, and each blade has a chord length defined as the distance between the distal ends of the trailing and leading edges of the blade; and, at least one serration has a height, defined as the distance between the root and tip of the serration, such that the ratio of the height of the serration to the chord length is from 0.05 to 0.2.

12. The impeller according to any one of the preceding claims, wherein the serrations of at least one of the leading edges has a wavelength defined as the distance between the tips of two adjacent serrations; and, wherein at least one slot has a width such that a ratio of the slot width to the wavelength of the serrations of the at least one leading edge is from 0.03 to 0.25.

13. The impeller according to any one of the preceding claims, wherein: each leading and trailing edge has a distal end that is distal from the hub, and each blade has a chord length defined as the distance between the distal ends of the trailing and leading edges of the blade;the serrations of at least one of the leading edges has a wavelength defined as the distance between the tips of two adjacent serrations; and, wherein the serrations of the at least one leading edge has a wavelength such that a ratio of the wavelength to the chord length of the respective blade is from 0.075 to 0.1.

14. A compressor comprising the impeller according to any one of the preceding claims.

15. The compressor according to claim 14, further comprising a shroud, and wherein an outer edge of each blade, distal from the hub, is fixed to the shroud.

16. A method of forming the impeller according to any one of claims 1 to 13, the method comprising forming the impeller by injection moulding, wherein the serrations and / or the elongate slots are formed by a tool moved in a direction along a central axis of the hub.

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