Fan with improved ducting

Abstract

The invention relates to an axial fan with a duct, which is shown below in full at 20. The fan comprises a duct suitable for defining a channel of circular cross-section extending in the axial direction around the rotor. In the fan according to the invention, the duct comprises an annular seat extending circumferentially around the rotor; and the tips of the blades are received at least partially in the annular seat of the duct.

Description

Description Fan with improved duct The present invention relates to an axial fan with a duct. This term here and hereinafter refers to an axial fan with a diameter Dr greater than 0.5 m, preferably greater than 1 m. In the industrial context, the use of axial fans is commonly known to ensure adequate airflow around specific radiating surfaces, in implants that require significant amounts of heat dissipation. Axial fans, for example for industrial use, usually consist of a central hub defining an axis of rotation on which a large number of blades are mounted. The rotation of the hub rotates the blades and, as the skilled person will understand, imposes different tangential velocities on different sections of each blade. In fact, the tangential velocity of each blade section is the product of the angular velocity (which is the same for all sections) and the radial distance from the axis of rotation (which increases with distance from the axis of rotation). For this reason, as is well known to those skilled in the art, axial fan blades are not capable of operating effectively over their entire radial span. The tangential velocity of the innermost radial section of the blade is often too low to achieve effective relative motion with respect to the air flow. The result is that the actual performance of the fan is largely delegated to the outer radial sections, which provide almost the entire air flow produced by the axial fan. As the skilled person can understand, such a flow distribution makes the axial fan generally inefficient. While some technical solutions have been proposed to better exploit the inner radial sections of the blades, there is also a need to improve the efficiency of the outer radial sections. In the known method, in fact, the outer sections are subject to tip effects that limit their efficiency. As previously mentioned, since most of the flow is generated precisely by the outer radial sections, even a small inefficiency in percentage terms in this area leads to a large inefficiency in absolute terms for the entire fan. Along the middle sections of an aerodynamic surface, whether it is a vane or a fan blade, the high-pressure and low-pressure air regions are physically separated by the presence of the blade. At the tip of the blade, this separation does not exist and therefore a spontaneous airflow is created that tends to move from the high-pressure region to the low-pressure region. This creates a tip vortex that creates significant resistance to the blade's progress through the air. The first solution proposed for this type of problem was to duct the fan, thus enclosing it inside a casing with a diameter slightly larger than the outer diameter of the fan itself. This casing is referred to below as the duct. By adding a duct, the dimensions of the tip vortices are significantly reduced, and consequently the amount of air displaced by these vortices and, consequently, the induced resistance is reduced. However, as the person skilled in the art will well understand, not only can the distance between the tips of the blades and the inner diameter of the duct not be made zero, but such a distance cannot even be reduced below a certain limit. In fact, any contact between the duct and the tips of the blades must be avoided in the most absolute way, and for this purpose a safety distance is considered. Therefore, the blades cannot be manufactured with precise tolerances due to their size and cost of maintenance. Furthermore, the blades may be subject to vibration phenomena and may deform during operation. Therefore, even in the presence of an optimal duct, tip vortices cannot be eliminated. Another solution, borrowed from aeronautics, is to provide a lateral surface called a wingtip device or a small wing at the tip of each blade. First of all, the small wing has the function of forming a baffle that opposes the movement of the air, thus counteracting the formation of the tip vortex. Furthermore, depending on the shape adopted, the small wing can also influence the residual tip vortex, optimizing it and thus limiting the formation of noise. These solutions, although widely welcomed, are not without their drawbacks. In fact, despite the arrangement of the duct and the blades, probably in addition to each other, the formation of tip vortices is to some extent inevitable. Therefore, the efficiency of axial fans remains limited. Therefore, the aim of the present invention is to overcome the drawbacks mentioned above with respect to the prior art. In particular, the object of the present invention is to provide an axial duct fan having improved efficiency. Furthermore, the object of the present invention is to provide a ducted axial fan that limits the formation of tip vortices more than fans of the known type. Furthermore, the object of the present invention is to provide a ducted axial fan that, in addition to introducing further advantages, also maintains the advantages obtained from fans of the known type. According to claim 1, such purpose and tasks are accomplished using a ducted axial fan. For a better understanding of the invention and to appreciate its advantages, some exemplary and non-limiting embodiments thereof are described below with reference to the accompanying drawings, in which: - Figure 1 schematically shows a view of the design of a fan according to the present invention; - Figure 2 schematically shows an enlarged view of the details mentioned as II in Figure 1; - Figure 3 schematically shows a cross-sectional view taken along line III-Ill of Figure 2; - Figure 4 schematically shows a cross-sectional view taken along line IV-IV of Figure 3; - Figures 4.b to 4.n schematically show views of some alternative parts similar to Figure 4.a; - Figure 5 shows a perspective view of a part of the bottom of a fan according to the invention; - Figure 6 shows a perspective view of a fan according to the invention, in which the duct has been partially removed for greater clarity; - Figure 7 shows a surface view of another fan according to the invention; - Figure 8 shows a sectional view taken along line VIII-VIII of Figure 7; - Figure 9 shows a sectional view taken along line IX-IX of Figure 7; - Figure 10 represents a perspective view of a portion of a fan duct according to the invention; - Figure 11 shows an enlarged view of the detail indicated as XI in Figure 10. - Figure 12 shows an aircraft comprising a ducted rotor according to the invention. - Figure 13 shows an enlarged view of the detail indicated as XIII in Figure 12. - Figure 14 schematically shows an enlarged view of the details indicated as XIV in Figure 13; and - Figure 15 shows a cross-sectional view taken along line XV-XV of Figure 14, in three different configurations. In the context of the present discussion, some terminological conventions have been adopted to facilitate and streamline the study. These terminological conventions are explained below with reference to the attached figures. The term "duct" hereinafter refers to the side wall or casing, usually cylindrical, that surrounds a ducted fan and creates a channel in which the airflow is restricted. The fan according to the invention is intended to create a flow of air from an inlet region (below in the accompanying drawings) to an outlet region (above in the accompanying drawings). It is therefore understood that in relation to the direction of flow (indicated by a in the drawings) the terms "above", "previous" and the like are expressly defined, with respect to the terms "below", "next" and the like. The terms "convergent" and "divergent" must also be interpreted in relation to the direction of flow a. Since the fan according to the invention uniformly defines an axis of rotation X, the terms "axial", "radial", "tangential" and "circumferential" are defined in relation to this axis. The different "quantity" values ​​are explained below. The adverb "quantity" is intended to indicate a difference within 10% of the higher quantity between the two, preferably within 5% of the higher quantity between the two. This invention relates to a ducted axial fan, generally indicated below at 20. Fan 20 includes: - a rotor 22 rotatable about the X axis and consisting of a plurality of blades 24; and - a duct 26 suitable for defining a circular cross-section channel 28 extending in the axial direction around the rotor 22. According to the invention, in fan 20, duct 26 includes an annular-like seat 30 extending circumferentially around rotor 22; and the tips of blades 24 are received at least partially in annular-like seat 30 of duct 26. That is, in the annular housing 30, the outer diameter Dr of the rotor 22 is greater than the inner diameter Ds of the annular housing 30 (see, for example, FIG. 9). For example, the outer diameter Dr of the rotor 22 is greater than 0.5 m and preferably greater than 1 m. Preferably, the rotor 22 of the fan 20 includes a hub 23 defining the axis of rotation X. A number of blades 24 are mounted on the hub 23. Preferably, the blades 24 are constructed independently of the hub 23 and are subsequently mounted on the hub 23 so that the angle can be changed to suit specific design requirements. Preferably, the blades 24 are mounted on the hub 23 by means of bolts (see, for example, FIG. 6). Preferably, at least one blade 24 of the fan 20 includes a wing tip 32, also referred to simply as a wing 32. The wing 32 is itself a known device arranged at the tip of the blades 24 to reduce their noise and to reduce the drag caused by the formation of tip vortices. Preferably, the wing 32 has a baffle 34 that extends at least partially in the axial direction. Advantageously, the main development of the baffle 34 of the wing 32 follows a surface defined by the axial direction and the circumferential or tangential direction. A duct of the known type has a circular cylindrical shape, at least in the axial part forming the rotor. Furthermore, in a known manner, the internal diameter of the duct is slightly greater than the external diameter of the corresponding rotor. The conduit 26 according to the invention, and in particular its annular seat 30, may assume various configurations depending on the embodiment. According to some embodiments, the duct 26 has a circular cylindrical shape in the axial section comprising the rotor 22 and an inner diameter Dd slightly greater than the outer diameter Dr of the rotor 22. According to other embodiments, the duct 26 has a circular cylindrical shape and in the section directly above the rotor 22 has an inner diameter slightly smaller than the outer diameter Dr of the rotor 22. In these embodiments, the duct 26 then terminates near the rotor 22, where the annular seat of the shape 30 is arranged. In this case, at the top of the rotor 22, the inner diameter of the duct 26 coincides with the inner diameter Ds of the annular seat of the shape 30. At the bottom of the rotor 22, in some cases the duct 26 assumes an inner diameter Dd slightly greater than the outer diameter of the rotor 22, while in other embodiments, the duct 26 again assumes an inner diameter Ds slightly smaller than the outer diameter of the rotor 22. According to some embodiments, the duct 26 has a circular cylindrical shape and, in the portion directly above the rotor 22 and corresponding to the rotor 22 (i.e., where the annular seat of Figure 30 is located), has an inner diameter Dd slightly greater than the outer diameter Dr of the rotor 22. In such a particular embodiment, the duct 26 continues at the bottom of the rotor 22 with an inner diameter slightly smaller than the outer diameter of the rotor 22. In this case, at the bottom of the rotor 22, the inner diameter of the duct 26 coincides with the inner diameter Ds of the annular seat of Figure 30. According to some embodiments, the annular housing 30 includes a flat aerodynamic surface 36. For example, the annular housing 30 may include a flat aerodynamic surface 36c, preferably disposed directly above the rotor 22. Alternatively or additionally, the annular housing 30 may include a divergent flat aerodynamic surface 36d, preferably disposed directly below the rotor 22. According to some embodiments, the aerodynamically smooth surface 36 (converging 36c and / or diverging 36d) determines the narrowing of the channel 28 defined by the duct 26 . According to some embodiments, the annular seat of Figure 30 is open in an axial direction. For example, the annular seat of Figure 30 may be open axially at the top (i.e., toward the inlet region) or at the bottom (i.e., toward the outlet region). According to some embodiments, the annular seat 30 is open radially inwardly of the conduit 26. Preferably, the annular seat 30 extends in an upward and / or downward axial direction. According to some embodiments, the annularly shaped seat 30 extends generally outside the conduit 26 , while in other embodiments, the annularly shaped seat 30 extends generally inside the conduit 26 . According to some embodiments, at least one blade 24 of the fan 20 includes a blade tip 32 having a baffle 34 extending in an axial direction. For example, the baffle 34 of the blade 32 may extend axially at the top, bottom, or both sides. Preferably, each blade 24 includes a baffle 32. The vane 32 may have various shapes. FIG. 6 shows, for example, a rotor 22 which includes a known type of vane 32 in which the baffle 34 has a relatively small extension in the axial direction. Other designs of vane 32 are shown in FIGS. 3 and 8. In these cases, it is noted that the axial extension is greater at the top of the vane 32 and is wider in the tangential direction (i.e. along the waterline of the airfoil blade 24). In the fan 20 according to the invention, this type of vane 32 with a greater axial extension allows for greater engagement of the annular space of FIG. 30. In certain embodiments, the duct 26 of the fan 20 according to the invention includes a converging opening 38. In a known manner, the converging opening 38 is defined at the upper end of the duct 26 and serves to receive air. It delivers the flow to the inlet region and gently conveys it to the rotor 22. In the embodiments of FIGS. 5, 6 and 8-11, the converging opening 38 is defined in a known manner by the wall of the duct 26. With respect to other schematic embodiments, for example in FIGS. 4.k and 4.1, the converging opening 38 is defined by a protrusion above the aerodynamically flat surface 36, in particular the aerodynamically flat surface 36c. In the embodiment shown schematically in Fig. 4. a, the duct 26 has a circular cylindrical shape in the axial section containing the rotor 22 and has an inner diameter Dd slightly greater than the outer diameter Dr of the rotor 22. In such an embodiment, the annular space 30 is thus obtained from a conventional duct 26 by the addition of a smooth aerodynamic surface 36. In particular, an aerodynamic smooth surface 36c, which is located immediately above the rotor 22. The aerodynamic smooth surface 36c thus causes a narrowing in the channel 28 present in the duct 26, so that the inner diameter Ds of the annular space 30 is slightly smaller than the outer diameter Dr of the rotor 22. Due to the shape of the converging aerodynamic smooth surface 36c, the annular space 30 is axially open downwards. The blade 24 includes a wing tip 32 with a baffle 34 extending axially above and received in an annular housing 30. In the embodiment schematically shown in Figure 4.b, the duct 26 has a circular cylindrical shape and in the upper part of the rotor 22 has an inner diameter slightly smaller than the outer diameter Dr of the rotor 22. In this case, at the top of the rotor 22, the inner diameter of the duct 26 corresponds to the inner diameter Ds of the annular seat 30. The duct 26 then stops near the rotor 22, where the annular seat 30 is located, and continues at the bottom of the rotor 22 with an inner diameter Dd slightly larger than the outer diameter Dr of the rotor 22. Due to the shape of the duct 26, the annular seat 30 is axially open at the bottom. The blade 24 comprises a wing tip 32 with a baffle 34 extending axially at the top and received in the annular seat 30. The embodiment shown schematically in FIG. 4.c is very similar to that of FIG. 4.a, to which reference is made. In addition, in this embodiment, a diverging aerodynamically smooth surface 36d is located immediately below the rotor 22. The resulting annular space 30 generally extends in the channel 28 of the duct 26. Such annular space 30 is open radially inwardly and extends axially downwardly and upwardly. The blade 24 includes a wing tip 32 with a baffle 34. The blade tip 24 is received radially in the annular space 30, and the baffle 34 of the blade 32 extends axially downwardly and upwardly within the annular space 30. In the embodiment shown schematically in Figure 4.d, the duct 26 has a circular cylindrical shape in the axial section comprising the rotor 22 and has an inner diameter D slightly greater than the outer diameter Dr of the rotor 22. In such an embodiment, the annular space 30 is thus obtained from a conventional duct 26 by the addition of a smooth aerodynamic surface 36. In particular, a smooth aerodynamic surface diverging 36d is located immediately below the rotor 22. The smooth aerodynamic surface diverging 36d thus determines the narrowing of the channel 28 present in the duct 26, such that the inner diameter Ds of the annular space 30 is slightly smaller than the outer diameter Dr of the rotor 22. Due to the shape of the smooth aerodynamic surface diverging 36d, the annular space 30 is axially open at the top. The blade 24 includes a wing tip 32 with a baffle 34 extending axially downwardly and received in the annular seat 30 . In the embodiment schematically shown in Figure 4.e, the duct 26 has a circular cylindrical shape and in the upper and lower parts of the rotor 22 has an inner diameter slightly smaller than the outer diameter Dr of the rotor 22. In this case, the inner diameter of the duct 26 coincides with the inner diameter Ds of the annular seat 30. The duct 26 is then cut off near the rotor 22, where the annular seat 30 is located. The annular seat 30 extends generally outside the channel 28 of the duct 26. Such an annular seat 30 is open in the radially inward direction and extends in the axially downward and upward direction. The blade 24 includes a wing tip 32 with a baffle 34. The tip of the blade 24 is received radially in the annular seat 30 and the baffle 34 of the vane 32 extends axially at the bottom and top within the annular seat 30. In the embodiment shown schematically in Fig. 4.f, the duct 26 resembles the shape obtained by reversing the duct 26 of Fig. 4.b. The duct 26 has a circular cylindrical shape and in the part immediately above the rotor 22 and corresponding to the rotor 22, where the annular seat 30 is located, the inner diameter Dd is slightly larger than the outer diameter Dr of the rotor 22. The duct 26 continues at the bottom of the rotor 22 with an inner diameter slightly smaller than the outer diameter of the rotor 22. In this case, at the bottom of the rotor 22, the inner diameter of the duct 26 coincides with the inner diameter Ds. The annular seat 30 Due to the shape of the duct 26, the annular seat 30 is axially open at the top. The blade 24 includes a wing tip 32 with a baffle 34 extending axially downwardly and received in the annular seat 30 . The embodiment shown schematically in Figure 4.g is very similar to Figure 4.f referred to in the description. In addition, in this embodiment a smooth aerodynamic surface 36c is located immediately above the rotor 22. The resulting annular space 30 is open radially inwardly and extends axially downward and upwardly. The blade 24 includes a wing tip 32 with a baffle 34. The blade tip 24 is received radially in the annular space 30 and the baffle 34 of the wing 32 extends axially downward and upwardly within the annular space 30. The embodiment shown schematically in Figure 4.h is very similar to Figure 4.b, which is referred to in the description. In addition, in this embodiment, a diverging aerodynamic flat surface 36d is located immediately below the rotor 22. The resulting annular space 30 is open radially inwardly and extends axially downward and upwardly. The blade 24 includes a wing tip 32 with a baffle 34. The blade tip 24 is received radially in the annular space 30, and the baffle 34 of the blade 32 extends axially downward and upwardly within the annular space 30. The embodiment shown schematically in FIG. 4.i is very similar to that of FIG. 4.c, to which reference is made. However, in this embodiment, the blade 24 does not have any wing tip 32. The tip of the blade 24 is received radially in the annular seat 30. The embodiment shown schematically in FIG. 4.j is very similar to that of FIG. 4.e, to which reference is made. However, in this embodiment, the blade 24 does not have any wing tip 32. The tip of the blade 24 is received radially in the annular seat 30. In the embodiment shown schematically in Figure 4.k, the duct 26 is of circular cylindrical shape in the axial section including the rotor 22 and has an inner diameter Dd slightly greater than the outer diameter Dr of the rotor 22. In such an embodiment, the annular space 30 of a conventional duct 26 is thus obtained by adding an aerodynamically smooth surface 36. In particular, an aerodynamically smooth surface 36c is located immediately above the rotor 22. Furthermore, the aerodynamically smooth surface protrudes at the top to form a converging opening 38. In a manner similar to that described above in connection with FIG. 4. a, the smooth aerodynamic surface 36c causes the channel 28 to be narrowed, the inner diameter Ds of the annular seat 30 being slightly smaller than the outer diameter Dr of the rotor 22, the annular seat 30 being axially open at the bottom. The blade 24 comprises a wing tip 32 with a baffle 34 extending axially at the top and received in the annular seat 30. The embodiment shown schematically in Figure 4.1 is similar to Figure 4.e, to which reference is made. However, in this embodiment, the wall of the duct 26 is shaped such that a converging opening 38 is formed at the top of the rotor 22. The embodiment shown schematically in Figure 4.m is very similar to Figure 4.a, to which reference is made. However, in this embodiment, the aerodynamically smooth surface 36c is not formed to define a smooth, continuous taper in the channel 28, but is formed with a sharp corner profile that creates an abrupt, stepped taper. The embodiment schematically shown in FIG. 4.n is very similar to FIG. 4.c, to which reference is made. However, in this embodiment, the aerodynamically smooth surface 36c and the divergent aerodynamically smooth surface 36d are not formed to define smooth, continuous transitions in the channel 28, but are formed with sharp corner profiles that create abrupt step changes. These schematic embodiments in FIGS. 4.m and 4.n, although not aerodynamically optimal, may be useful under certain circumstances due to ease of implementation. The configuration of the conduit 26 and annular seat 30 described above with reference to Figure 4 is shown by way of example. As will be well understood by those skilled in the art, the annular seat 30 can take different forms than those detailed herein to meet specific needs. As can be seen by the skilled person from FIG. 4, the configuration according to the invention for the annular seat 30 and the blade tip 24 makes it possible to achieve a type of labyrinth seal. In a known manner, the labyrinth seal defines a spiral path that significantly reduces the spontaneous passage of a fluid from a high pressure region to a low pressure region. In the specific case, the configuration of the annular seat 30 and the blade tip 24 (with or without the vanes 32) defines a spiral path for air that flows spontaneously from the high pressure region (above the blade 24) to the low pressure region (below the blade 24). By reducing the amount of air passing from one region to another at the blade tip 24, the amount of tip vortex and consequently the induced resistance is reduced. The embodiments of Figures 5 to 11 are similar to that illustrated in Figure 4.a. Specifically, Figures 5 and 6 show one embodiment of the rotor 22, while Figures 7, 8, and 9 show a different embodiment of the rotor 22. The main difference between the two embodiments is in the shape and extension of the baffle 34 from the vane 32. Figure 6 shows a smaller vane 32 than that seen in Figure 8. The duct 26 and the annular seat 30 are common to both embodiments and are shown in greater detail in Figures 10 and 11. In the embodiments of FIGS. 5 to 11, the duct 26 has a circular cylindrical shape in the axial section including the rotor 22 and the inner diameter Dd is slightly larger than the outer diameter Dr of the rotor 22 (see FIG. 9). Thus, the annular space 30 is obtained from a conventional duct 26 by adding an aerodynamically flat surface 36. In particular, the aerodynamically flat surface 36c is located directly above the rotor 22. The aerodynamically flat surface 36c thus defines a narrow channel 28 in the duct 26, such that the inner diameter Ds of the annular space 30 is slightly smaller than the outer diameter Dr of the rotor 22 (see FIG. 9 again). Due to the shape of the converging aerodynamically flat surface 36c, the annular space 30 is axially open at the bottom. The blades 24 include respective wing tips 32 which take different shapes, but in each case have a baffle 34 which extends axially above and is received in the annular seat 30. Each of the variants described above allows for the achievement of certain advantages, some of which are described by way of example below. Embodiments consisting of a conventional duct 26 to which smooth aerodynamic surfaces 36 are added allow the existing fan 20 to be modified in accordance with the invention. Such embodiments are shown, for example, in Figures 4a., 4c., 4.d., 4.i., 4.k., 4. and 4.n. Embodiments including the narrowing of the channel 28 at the annular position 30 allow for local acceleration of the air flow. In this regard, it should be noted that the difference between the internal diameter Dd of the channel 26 and the internal diameter Ds of the position may in some cases reach up to 5% of the internal diameter Dd of the channel 26. In most cases, however, this difference is less than 2% of Dd. Since this reduction is located precisely at the radial edge, where the flow velocity is higher, the local effect of the narrowing on the flow velocity is even more pronounced. Such an embodiment is shown, for example, in Figures 4.a, 4.c, 4.d, 4.f, 4g, 4.i, 4.m and 4.n. Embodiments including the enlargement of the channel 28 at the position 30 allow for an optimal arrangement of the air flow for applications requiring a divergent outlet in the entire discharge of the duct 26. Such an embodiment is shown, for example, in Figures 4.b, 4h, and 4.k. Preferably, the fan 20 according to the invention also includes a motor (not shown) suitable for rotating the rotor 22 at the designed speed. In addition, the fan 20 according to the invention preferably has a structure (not shown) suitable for firmly supporting the duct 26, the rotor 22 and possibly the motor under all operating conditions. According to some embodiments, which are schematically shown in FIGS. 14 and 15, the rotor 22 is of the variable angle type. According to these embodiments, each individual blade 24 may rotate about an axis p that is radially oriented. The possibility of simultaneously rotating each blade 24 about the corresponding axis p allows you to change its radiation relative to the air (see FIG. 15), thus changing the flow rate of the ducted fan 20 itself. Thus, variable angle ducted fans 20 allow adaptation to different operating conditions and are therefore widely used in various fields. A field in which variable-angle duct fans 20 are of particular interest is the field of aviation. Various types of aircraft use variable-angle duct fans 20, for example for propulsion and / or aircraft control. A specific example of a variable-angle fan 20 is the helicopter ducted tail rotor 40 (see, for example, Figure 12). This solution, also commonly called a fenestra, although widely accepted, suffers from the same drawbacks previously identified above for ducted fans for industrial use. Even in this case, it is very advantageous to arrange an annular seat 30 extending circumferentially around the rotor 22 on the duct 26, in which the tips of the blades 24 are at least partially located in the annular seat 30. In this type of application, the embodiments schematically shown in Figures 4.c, 4.e, 4.i and 4.j are particularly suitable, although other embodiments may also be usefully used. The above description focuses on the technical features that distinguish the invention from prior art solutions. For all other features, which may be common to the prior art and the invention, reference may be made to the introduction in which the prior art is described. As the skilled person can easily understand, the invention allows to overcome drawbacks previously highlighted by reference to prior art. In particular, the present invention provides a ducted axial fan that has improved efficiency. Furthermore, the present invention provides a ducted axial fan that limits the formation of tip vortices more than fans of the known type. Furthermore, the present invention provides a ducted axial fan which, in addition to introducing further advantages, also maintains the advantages previously achieved by fans of the known type. It is understood that specific features have been described in connection with various embodiments of the invention by way of non-limiting examples. It is obvious that those skilled in the art will be able to make further changes and modifications to the present invention in order to meet possible and specific needs. For example, technical features described in connection with an embodiment of the invention may be extrapolated therefrom and applied to other embodiments of the invention. Such changes and modifications are also within the scope of the invention, as defined by the following claims.

Claims

AMENDED CLAIMS 1. Ducted axial fan (20), comprising a rotor (22) rotatable around an axis X and comprising a hub (23) defining the rotation axis X and a plurality of blades (24), wherein the blades (24) are made structurally independent from the hub (23) and are mounted on the hub (23) so as to be able to vary the pitch; the fan (20) further comprising a duct (26) suitable for defining a circular section channel (28) developing in the axial direction around the rotor (22); wherein the duct (26) comprises an annular seat (30) circumferentially extending around the rotor (22) and wherein the tips of the blades (24) are at least partially received in the annular seat (30) of the duct (26), characterized in that at least one blade (24) comprises a tip winglet (32) and in that the tip winglet (32) has a baffle (34) extending in the axial direction and being received in the annular seat (30).

2. Fan (20) according to claim 1, wherein, in correspondence of the annular seat (30), the outer diameter of the rotor (22) Dr is larger than the inner diameter Ds of the annular seat (30).

3. Fan (20) according to one or more of the preceding claims, wherein the annular seat (30) comprises an aerodynamic smoothing surface (36).

4. Fan (20) according to one or more of the preceding claims, wherein the annular seat (30) comprises a converging aerodynamic smoothing surface (36c) placed immediately upstream of the rotor (22).

5. Fan (20) according to one or more of the preceding claims, wherein the annular seat (30) comprises a diverging aerodynamic smoothing surface (36d) placed immediately downstream of the rotor (22).

6. Fan (20) according to one or more of the claims 3 to 5, wherein the aerodynamic smoothing surface (36) determines a narrowing in the channel (28) defined by the duct (26).

7. Fan (20) according to the preceding claim, wherein the narrowing comprises a difference between the inner diameter Dd of the duct (26) and the inner diameter Ds of the annular seat (30), and wherein such difference is less than 5% of Dd , preferably less than 2% of Dd .

8. Fan (20) according to one or more of the preceding claims, wherein the annular seat (30) is axially open downstream.

9. Fan (20) according to the preceding claim, wherein the baffle (34) axially extends upstream and is received in the annular seat (30).

10. Fan (20) according to one or more of the preceding claims, further comprising a motor and / or a structure.

11. Fan (20) according to one or more of the preceding claims, wherein the rotor (22) is of the variable pitch type.

12. Fan (20) according to the preceding claim, wherein the fan (20) is the tail rotor of a helicopter (40).

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