Blades for Low Speed Propeller Fan

This application is related to U.S. Patent application 07/319,216 entitled "Blades for Propeller Fan" and U.S. Patent application 07-219,390 entitled "Blades for High Speed Propeller Fan", both of which were filed on the same day as the subject application.

1. Field of the Invention

This invention relates to blades for a propeller fan which is driven at a relatively low speed for supplying air to automobile engines or other kinds of apparatus for cooling or other purposes, and which is made of, for example, a synthetic resin or metal.

2. Description of the Prior Art

A known propeller fan is partly shown in FIG. 6 or 7 by way of example. Both of the fans comprise a plurality of blades 12 projecting radially outwardly from the outer peripheral surface of an annular boss 11. Each blade 12 of the fan shown in FIG. 6 has a multiplicity of small apertures 14 formed therethrough between its longitudinally central portion and its rear edge 13. The apertures 14 allow air to flow therethrough and thereby reduce a difference of air pressure between the suction and delivery sides of each blade 12 in order to decrease the separation of air in a boundary layer from the surface of the blade on its suction side and thereby the noise of the fan. Each blade 12 of the fan shown in FIG. 7 has along its rear edge 13 a plurality of notches 15 which are provided for preventing the appearance of a large swirling flow of air in the vicinity of the rear edge 15 to reduce the noise of the fan.

The propeller fan produces a noise when it is driven. The noise is mainly caused by the sound of the separating air, the sound of the flowing air and the sound of the pitching air. The sound of the separating air is due to the appearance of a swirling flow of air as a result of the separation of air in the boundary layer from the surface of each blade on its suction side. There exist two flows of air along the surface of each blade on its suction side, i.e., a flow of air in an outer layer having a substantially constantly high velocity and a large amount of kinetic energy and a flow of air in an inner or boundary layer contacting the blade surface and having a low velocity and a small amount of kinetic energy which is due to the viscosity of air. The boundary layer has a thickness which increases with an increase in air velocity. The boundary layer grows and has an increased thickness toward the rear edge of the blade until it is eventually separated from the blade surface to form a swirling flow of air.

Neither of the blade constructions shown in FIGS. 6 and 7 is very effective for reducing the noise of the fan including the sound of the separating air. Moreover, the presence of the apertures or notches lowers the air supplying capacity of the fan, as well as the mechanical strength of the blades per se.

The inventor of this invention has experimentally discovered that if each blade has a finely coarsened surface on its suction side, it can agitate the air in the boundary layer having only a small amount of kinetic energy and thereby form a finely divided swirling flow of air which is mixed with the air in the outer layer having a large amount of kinetic energy to impart a large amount of kinetic energy to the boundary layer and reduce its thickness, so that the separation of the boundary layer from the blade surface may be prevented or at least delayed.

It is, therefore, an object of this invention to provide blades for a low speed propeller fan which can effectively overcome the drawbacks of the prior art as hereinbefore pointed out.

In a propeller fan having a relatively low rotating speed, the boundary layer of air which extends along the surface of each blade on its suction side has so small a thickness that the amount of its kinetic energy is not greatly different from that of the air in the outer layer. As a result, the boundary layer is not separated from the blade surface in the vicinity of its front edge when the fan is driven, but is separated therefrom in the vicinity of its rear edge. According to this invention, therefore, each blade has on its suction side an at least partly coarsened surface having a coarsened density or depth which gradually increases from the front edge of the blade to its rear edge. The term "front edge" as herein used means the leading edge which the blade presents when it is rotated and the "rear edge" is, therefore, the opposite edge. The coarsened surface can minimize the appearance of a large swirling flow of air otherwise resulting from the separation of the boundary layer and thereby reduce the noise of the fan without lowering appreciably its air supplying capacity or the mechanical strength of the blade per se.

This invention essentially consists in a low speed propeller fan which includes a member connected to a rotating body and a plurality of blades extended radially outwardly from the outer peripheral surface of the member, each of the blades characterized by having a coarsened surface at least part of its suction side, the coarseness of the surface gradually increasing from the front edge of the blade to its rear edge. The coarsened surface is a grooved surface or a roughened surface or a combination thereof.

FIG. 1 is a fragmentary front elevational view of a low speed propeller fan embodying this invention;

FIG. 2 is a view similar to FIG. 1, but showing another embodiment of this invention;

FIG. 3 is a view similar to FIG. 1, but showing still another embodiment;

FIG. 3A is a sectional view taken along the line III--III of FIG. 3;

FIG. 4 is a view similar to FIG. 1, but showing a further embodiment of this invention;

FIG. 4A is a sectional view taken along the line IV--IV of FIG. 4;

FIG. 5 is a view similar to FIG. 1, but showing a still further embodiment;

FIG. 6 is a fragmentary front elevational view of a known propeller fan; and

FIG. 7 is a view similar to FIG. 6, but showing another known propeller fan .

Referring to FIGS. 1 to 4, each fan embodying this invention comprises an annular boss 1 having a wall 1' at which it is connected to a rotating body (not shown), and a plurality of blades 2 projecting radially outwardly from the outer peripheral surface of the boss 1. The fan can, for example, be made of a synthetic resin or soft metal such as aluminum or aluminum alloy. Each blade 2 has on its suction side a surface 3 which is at least partly coarsened to form a coarsened surface 4. The coarsened surface 4 has a coarsened density which gradually increases from the front edge of the blade to its rear edge as shown in FIG. 1 or 2, or a coarsened depth which likewise increases as shown in FIGS. 3 and 3A or 4 and 4A. The coarsened surface may be formed by a multiplicity of fine grooves 5 as shown in FIG. 1 or 3, or is a roughened surface 6 as shown in FIG. 2 or 4. The grooves 5 may extend in parallel to one another in a substantially regular pattern as shown in FIG. 1 or 3, or may alternatively extend in an irregular pattern or cross one another to form a network pattern. The coarsened surface may alternatively be a combined grooved and roughened surface.

The arrow in each of FIGS. 1 to 4 shows the direction in which the fan is rotated. Therefore, the righthand longitudinal edge of the blade is its front edge.

The foregoings are the embodiments relating to the propeller fan comprising an annular boss 1 having a mounting wall 1' for connecting the annular boss to a rotating body and a plurality of blades 2 formed integrally with the annular boss.

The invention is not restricted to such the embodiments as above mentioned. The propeller fan may be constructed with, for example, as shown in FIG. 5, a spider 7 having a mounting wall 8 for connecting the spider to a rotating body and a plurality of blades 2 formed independently of the spider, wherein the blades are fixed to the spider by rivets 9 or likes. In case that the blades are made of a synthetic resin or soft metal, they may be mould to have a blade insert which is to be fixed to the outer periphery of the spider by the rivets or likes. In case that the blades are made of a hard metal such as iron, they may be provided with a coarsened surface which can be formed by rubbing sandpaper or wire brush against the surface coated previously with a paint or coating the surface with a paint mixed with fine particle material or spraying the fine particle material onto the surface which was previously coated with an adhesive agent.

The coarsened surface 4 serves to agitate the air in the boundary layer to form a finely divided swirling flow of air which can effectively be mixed with the air in the outer layer having a larger amount of kinetic energy to give the boundary layer a larger amount of kinetic energy and reduce its thickness so that its separation from the blade surface 3 may be prevented or at least delayed. Since the separation of the boundary layer is likely to occur in the vicinity of the rear edge of the blade as hereinbefore stated, the coarsened surface 4 has a greater coarsened density or depth in the vicinity of the rear edge of the blade than in the vicinity of its front edge. Therefore, it can effectively prevent the appearance of a large swirling flow of air or divide it finely to thereby reduce the noise of the fan effectively.

Moreover, this invention makes it possible to provide blades for a propeller fan which is easy to manufacture without calling for any essential change in construction of the blades and without lowering the air supplying capacity of the fan or the mechanical strength of the blades per se.