A fan with convex blades

Abstract: The fan is of the type generating an axial flow, and is equipped with convex or backwards-curved blades (2), curving in the opposite direction to the rotation direction of the fan, and has a constant maximum transversal dimension along its entire longitudinal development: the said fan blades (2) have their anterior or entry edges (5) and posterior or exit edges (6), with respect to the the rotation direction of the fan, defined by two circumference arcs when observed parallel to the rotation axis.

Description: The invention relates to a fan generating an axial flow and equipped with convex or backward-curved blades, that is, blades curved in the opposite direction to the rotation direction of the fan. Fans in general comprise a plurality of blades radiating outwards from a hub, said blades moving in a perpendicular plane to the said hub which supports them. Each blade has a transversal dimension which is practically flat, and is inclined in such a way as to form a certain angle with respect to the rotation plane of the said fan. The inclination is such that, when the fan rotates, the blades generate an air flow which is parallel to the rotation axis of the hub. The "simplest" fans have straight blades, that is with the central longitudinal line of each blade being straight in the sense that is develops radially with respect to the hub to which it is keyed. Fans structured in this way have shown themselves to be extremely efficient and capable of reaching elevated performance levels. One drawback they have, however, is that they are extremely noisy since the blades strike the air to be fanned with all their operative surface at once and the resulting impact is violent. With the aim of eliminating this drawback of noisiness, the prior art teaches fans with curved blades having their median line curved in the rotation direction of the fan or in the opposite direction, realising in this way a gradual penetration of the fanning surface of the blades in the air. Patent US-PS 4.358.245, for example, teaches an axial fan whose blades are curved in the rotation direction of the fan, and have an angle which grows according to the diameter of the blade at a certain point with respect to the maximum diameter of the same blade. The blade angle increases in the final 30% of the blade itself. Patent US-A 4,569,632 teaches a fan of the type described above, whose blades curve backwards, that is in the opposite direction to the rotation direction of the fan itself, and are oriented in an inclination ratio with respect to the rotation plane which ratio diminishes constantly as the radius of the blade increases in the final 30% of its length. The German utility patent DE-U-89 03 903 teaches another fan of the type described above, whose blades curve forwardly, and the projections, on a plane perpendicular to the axis of rotation of the fan, of the front edge and the rear edge are defined, starting from the hub of the fan, by a first straight radial section until the 25% of the radial length of the blade and then by a second section following two arcs. Fans thus made have shown themselves to be very silent, but have the drawback of having only a modest efficiency in comparison with that of the straight-bladed fans. The aim of the present invention is to structure an axial-flow fan of the convex-blade type or backward-curving type in which an efficacy-silence compromise is reached. The invention as characterized in the claim solves the above-described problem by proposing an axial fan with convex blades or backwards-curved blades in which the maximum transversal dimension of the blades is constant along their entire longitudinal development that is considering the section of a blade as an airfoil profile, the chord of such profile is constant along the longitudinal axis of the blade, i.e. in substantially radial direction, and their anterior or entry edges and posterior or exit edges are defined with respect to their rotation direction and observing parallel to the fan's rotation axis, by two circular arcs which exend from the hub to the external stiffening ring, further characteristcs and advantages of the present invention will better emerge from the detailed description which follows, made with reference to the accompanying drawings which represent a preferred but non-limiting embodiment included by way of example, in which: Figure 1 shows, in axial view, the axial fan object of the present invention; Figure 2 shows, in schematic axial view, some construction lines of one blade of the fan of figure 1, with the aim of better evidencing the said blade's characteristics; Figures 3 and 4 show section III-IIII and respectively section IV-IV of figure 2. With reference to the drawings, the axial flow fan according to the invention is denoted by 1 and comprises a central hub 3 from which a plurality of blades 2 radiate out in angularly equidistanced radial directions. The hub 3 is keyable on to a motor shaft (not illustrated)which places it in rotation about a common axis. The blades 2 are of the convex type, curved backwards, that is, with their median lines curved in the opposite sense to the rotation direction of reciprocally connected by means of an external stiffening ring 4. The blades 2 are also inclined with respect to the rotation direction of the fan 1 in such a way as to generate, during the said fan's 1 rotation, an air flow parallel to the common rotation axis. The invention envisages that the maximum transversal dimension of the blades 2 is constant along their entire longitudinal development (see figures 3 and 4). Furthermore, observing the fan 1 along its axial direction, the anterior or entrance edges 5 and the posterior or exit edges 6 of the blades 2, with respect to the rotation direction of the fan 1, develop along circle arcs as evidenced by radii Ri and Re in figure 2. The twisting of the blades 2, that is, the progressinve variation of the keying angle on the hub 3 starting from the said hub 3 towards the external ring 4, remains constant along the entire development of the blades 2 themselves, in the sense that the derivative calculated in successive points along the longitudinal development of the blade 2 defines a constantly varying angular coefficient at point after point. From figure 2 it can be seen that each blade 2 is structured as if it had been cut out of a straight blade in which each of its transversal sections had been made to slide on to the immediately contiguous transversal section. Still with reference to figure 2, in can be seen that these ideal slidings of the transversal sections can happen along concentric circumferences whose centre lies on the rotation axis of the fan 1, so that the maximum transversal dimension is measured along the said circumferences. The present fan thus reaches the proposed aims of silence, thanks to the backwards curvature of the blades 2, and of high performance, thanks to the substantial conformation of the same blades 2 which is comparable to the performance of straight blades.

Claims: An axial fan with the blades curved in the opposite direction to the rotation direction of the fan, constituted by a hub (3) from which blades (2) radiate out wards to an external stiffening ring (4), the said blades (2) having a section which is an airfoil profile the chord (L) of each blade being constant along the radial direction of the fan, characterised in that the projections, on a plane perpendicular to the axis of rotation of the fan, of the blade (2) anterior or entry edges (5) and the blade (2) posterior or exit edge (6) are defined by two circular arcs which extend from the hub to the external siffening ring (4).

Inventors: SPAGGIARI ALESSANDRO, SPAGGIARI, ALESSANDRO
Applicants: SPAL SRL, SPAL S.R.L.
Application year: 1992
Main classification: F04D29
IPC codes: F04D29