US20070243064A1

US20070243064A1 - Fan blade assembly for electric fan

Info

Publication number: US20070243064A1
Application number: US11/403,045
Authority: US
Inventors: Tsuguji Nakano; Daniel Hinch; Tsukasa Yoshinaka
Original assignee: JCS THG LLC
Current assignee: Sunbeam Products Inc
Priority date: 2006-04-12
Filing date: 2006-04-12
Publication date: 2007-10-18
Also published as: CA2578514A1

Abstract

Fan blade assemblies are provided for quiet and efficient operation in portable electric axial flow fans intended for home or office use. The blades of the fan blade assemblies include a forward sweep to provide relatively quiet operation. The camber lines of the blades are curved to provide generally concave pressure surfaces and generally convex suction surfaces.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The field of the invention relates to electric fans and blade assemblies for such fans.
2. Brief Description of the Related Art
Electric fans have a variety of uses and are designed in various ways to provide desired air flows. Axial flow fans move air substantially parallel to the axis of rotation of the fan blade assembly. The operating characteristics of axial flow fans can vary significantly depending on parameters such as blade width and shape and the number of blades in the assembly.
A number of terms are employed to described the physical characteristics of axial flow fans. The leading edge is the edge of the blade that first meets the air as the blade is rotated. Blades also have trailing edges. The camber line is a line extending through the center or midpoint of a blade between the leading edge and the trailing edge. An axial flow fan commonly has a curved camber line. The camber angle is an angle created by the intersection of two straight lines tangent to the camber line: one straight line passing through the leading edge and the other passing through the trailing edge. The camber angle distribution may be substantially uniform between the root and the tip of a blade or it may be variable. The mean camber angle is defined as the average of the camber angles measured between the root and the tip of the blade.
The chord line of a fan blade is a straight line extending between the leading and trailing airfoil edges. The space between adjacent blades, or pitch, is the distance in the direction of rotation between corresponding points on adjacent blades. The solidity is the ratio of blade chord to the spacing or pitch. Solidity can be increased by adding blades and/or by using blades with a wider chord. Blade load increases with decreases in solidity. If the solidity is too low, the airflow from the fan may separate. While the pressure capability of a fan increases with increasing solidity, skin friction losses will also increase. Solidity should accordingly be considered in designing an effective fan blade assembly.
Loading of a fan as described herein refers to aerodynamic loading. Loading can be defined as the difference in air velocity on the suction and pressure sides of a blade. A larger difference in velocity on each side of the blade is associated with a larger difference in static pressure on each side, leading to additional load. Loading distribution relates to how the total work provided by a fan blade to the air flow is distributed by the blade from the leading edge to the trailing edge. Work performed by a fan is related to the number of blades, the blade chord, blade contour, and fan velocity.
Portable electric fans used in homes and offices include a support, an electric motor mounted to the support and a fan blade assembly operatively associated with the motor. The type of support determines whether the fan is categorized as a table fan, a stand fan or other type of fan. Most fans of these types are axial flow fans. As they are used where people may be working, listening to music, watching television or engaged in conversation, it is desirable that such fans do not cause excessive noise. Noise is proportional to fan speed. A fan having a relatively high solidity can produce the same airflow at a lower speed than an otherwise similar fan having lower solidity. As discussed above, efficiency may, however, be reduced with increasing solidity. There is accordingly a range of optimal solidity where it is not so high that skin friction losses render the fan inefficient nor so low that loading is increased to the extent that airflow stalls and separates.

SUMMARY OF THE INVENTION

The invention relates to fans and fan blade assemblies designed for quiet and efficient operation in homes, offices and similar settings.
In accordance with a first embodiment of the invention, a fan blade assembly is provided for an electric axial flow fan. The assembly includes a hub and plurality of fan blades mounted to the hub for providing axial flow as the hub is rotated about an axis of rotation. Each blade includes a blade body having an airfoil portion including radially inner, outer and mid-span portions. The blade body is preferably substantially uniform in thickness. It includes a leading edge, a trailing edge, and an outer edge connecting the leading and trailing edges. The outer edge is shorter in length than the lengths of the leading and trailing edges. The blade body further includes a pressure surface and a suction surface. The camber line is curved such that the pressure surface is generally concave and the suction surface is generally convex between the leading and trailing edges. The leading and trailing edges are curved such that the radially inner portion of the airfoil has a rearward sweep with respect to the direct of rotation of the hub and the radially outer portion thereof has a forward sweep. The pressure surface is generally concave and the suction surface is generally convex between the hub and the outer edge. The forward swept angle of the leading edge is preferably greater than 60 degrees and the solidity is preferably about 0.4 or greater.
A portable fan is provided by the invention, and includes a support, an electric motor mounted to the support, a fan blade assembly operatively associated with the electric motor and a grill at least partially enclosing the fan blade assembly. The fan blade assembly includes a hub and a plurality of fan blades mounted to the hub. Each blade includes an airfoil portion of substantially uniform thickness and including inner, outer and mid-span portions. The blade body further includes a leading edge, a trailing edge, an outer edge connecting the leading and trailing edges, a pressure surface, and a suction surface. The pressure surface is generally concave and the suction surface is generally convex between the leading and trailing edges. The leading and trailing edges are curved such that the airfoil radially inner portion is rearward swept with respect to the direction of rotation of the hub and the radially outer portion is forward swept. The pressure surface is generally concave and the suction surface is generally convex between the hub and the outer edge.
In accordance with a further embodiment of the invention, a fan blade assembly for an axial flow electric fan is provided that includes a hub and a plurality of fan blades mounted to the hub. The blades are designed to provide axial air flow as the hub is rotated about an axis of rotation. Each blade includes a blade body having an airfoil portion, a leading edge, a trailing edge, a pressure surface, and a suction surface. The blade body further includes a curved camber line wherein the pressure surface is generally concave and the suction surface is generally convex between the leading and trailing edges. The leading and trailing edges are curved such that the airfoil portion includes a radially inner portion having a rearward sweep and a radially outer portion having a forward sweep with respect to the direction of rotation of the hub. The solidity is preferably about 0.6 or greater.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a fan in accordance with a first embodiment of the invention;
FIG. 2 is a front elevation view of a fan blade assembly in accordance with the first embodiment of the invention;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a side elevation view thereof;
FIG. 5 is a rear elevation view of a blade for the blade assembly shown in FIGS. 2-4;
FIG. 6 is an end view thereof;
FIG. 7 is a partially sectional view thereof taken along line 7-7 of FIG. 5;
FIG. 8 is a graph showing the blade angle (β) distribution of the blade shown in FIGS. 1-7;
FIG. 9 is a perspective view of a fan blade assembly in accordance with a second embodiment of the invention;
FIG. 10 is a front elevation view thereof;
FIG. 11 is a rear elevation view thereof;
FIG. 12 is a side elevation thereof;
FIG. 13 is a front elevation view of a blade for the blade assembly of FIGS. 8-11;
FIG. 14 is a perspective view thereof;
FIG. 15 is an end view thereof; and
FIG. 16 is a graph showing the blade angle (β) distribution of the blade shown in FIGS. 9-15.

DETAILED DESCRIPTION OF THE INVENTION

The description which follows is directed to preferred embodiments of the invention. It will be appreciated that the invention is not to be considered limited to the preferred embodiments, and that the scope of the invention is to be determined in accordance with the appended claims.
A portable electric fan 10 according to a first embodiment of the invention is shown in FIG. 1. The fan includes a support 12, an electric motor assembly 13 mounted to the support, a fan blade assembly 14 operatively associated with the electric motor assembly 13, and a grill 16 enclosing the fan blade assembly. The grill includes two halves that are connected by wing clips 17. A grill nut 15 connects the rear half of the grill to the housing for the motor assembly 13. This type of fan 10 is known as a stand fan. The support includes a base 18, a long extension pole 19A coupled to the base, and a short extension pole 19B. The short extension pole 19B has a lower end of reduced diameter that extends within the top end of the long pole 19A. A set screw is employed to lock the poles together. The motor 13 is coupled to the top end of the short pole 19B by a bracket 21 A including a neck 21B. The neck 21B extends into the top end of the short pole 19B and is affixed thereto by a set screw. A weight 23 is mounted to the base 18 to provide stability.
The fan blade assembly 14 is comprised of four identical fan blades 20 and a hub 22, as shown in FIG. 2. The hub is comprised of a center portion 24 and four radially projecting arms 26, as best shown in FIG. 3. The center portion 24 includes a cylindrical projection 28 that is axially aligned with the axis of rotation of the fan blade assembly. A set screw 30 extends through the wall of the cylindrical projection 28 to allow the fan blade assembly to be coupled to the output shaft 31 of the electric motor assembly 13. The hub 22 is preferably of integral construction and substantially rigid. Each arm has substantially flat inner and outer surfaces. The outer end of each arm has an arcuate configuration.
Each arm 26 of the hub is coupled to a blade 20. The flat outer surfaces of the arms 26 adjoin substantially flat inner (suction) surfaces of the fan blades. Rivets 32 are employed for securely affixing the fan blades 20 to the arms. A pair of rivets 32 are preferably employed to affix a blade to each arm, one near the inner end of the arm and the other near the outer end thereof.
The blades 20 employed in the preferred embodiment are made of aluminum and have a substantially uniform thickness of about 1.0 millimeters. This thickness is not believed to be critical, and any thickness between about 0.8 mm and 1.5 mm should not materially affect performance of the fan blade assembly. The blades may be coated or painted for protection and/or aesthetic purposes. The radially inner portion 34 of each blade includes a substantially flat leading portion while the trailing portion is curved to form a generally concave outer pressure surface. Each blade inner portion 34 extends from a generally arcuate inner blade edge 36 to a generally arcuate boundary 38 near the outer edge of each hub arm 26. The four generally arcuate boundaries 38 of the blades as assembled to the hub 22 define arcs of a circle having a diameter of about 120 mm in a fan blade assembly 16 having an overall diameter OD of about 354 mm (about fourteen inches). The relatively flat leading portions 34A of the blade inner portions 34 are not coplanar with each other when mounted to the hub. They are instead slightly angled with respect to a vertical plane extending through the axis of rotation. When viewed along the axis of rotation of the fan blade assembly, there is a slight overlap of each blade inner portion 34 with the inner portions 34 of adjoining blades. As shown in FIG. 2, the curved part 34B of the inner portion 34 of each blade extends over a small relatively flat portion of an adjoining blade when the blade assembly is viewed from the front. A gap or passage between adjacent blades is defined in part by the suction side of the trailing portion of one blade and the pressure side of the leading portion of an adjacent blade. As the mean camber line of the blade 20 transitions from more tangential to more axial, the air passage width opens up and the average air velocity drops.
The portions of the blades extending radially outward from the inner portions 34 are curved, as shown in FIG. 4, such that the inner (suction) surfaces that face the motor are generally convex and the outer (pressure) surfaces are generally concave. The leading edge 40 of each blade is generally concave as viewed from the front or rear (FIG. 5) while the trailing edge 42 is generally convex. The leading edge is generally forward swept, which contributes to the quiet operation of the fan blade assembly 14. The leading and trailing edges both have radially inner portions that extend rearwardly and radially outer portions that extend forwardly (towards the direction of rotation), as shown in FIGS. 2 and 3. The radially inner portions of the blades accordingly have rearward sweep while the radially outer portions have a forward sweep. The ratio of average blade chord to pitch (solidity) is about 0.4, preferably greater, and most preferably 0.405. The chord line of each blade is fairly uniform, though it is slightly larger mid-span than near either end.
The camber angle θ, as shown in FIG. 6, exceeds about twenty-two degrees, and preferably is about 23 degrees. The camber angle does not vary significantly between the boundary 38 and the outer edge 46. FIG. 9 shows the blade angle β distribution for various points on the blade from the leading edge 40 (0% M) to the trailing edge 42 (100% M) at a distance of 16% span 41 (i.e., 16% of the distance from the boundary 38 to outer edge 46), 33% span 43 and at the outer edge 46. As shown in FIG. 6, the blade angle β for any particular point along the camber line of a blade is the angle created by the intersection of the axis of rotation 45 of the blade and the tangent to the camber line at that particular point.
Referring back to FIG. 5, the outer edge 46 of the blade has a radius of curvature of about 155 mm. Its length is substantially less than the length of either the leading edge 40 or the trailing edge 42. The forward swept angle α is greater than sixty degrees and is preferably about 61.2 degrees. In operation, it has been found that loading of the blade is nonuniform, and peaks near the leading edge in the area near the radially outer portion thereof and approximately mid-chord in the area near the hub.
The outer portions 44 of the blades have outer pressure surfaces that are generally concave between the leading and trailing edges 40, 42 as well as between the boundaries 38 and the outer edges 46. As viewed from the side of the blade assembly, the leading edge 40 of each blade angles rearwardly towards the electric motor and then forwardly. The camber line through the outer portion 44 of each blade is curved such that the leading portion thereof extends rearwardly with-respect to a plane extending perpendicularly through the axis of rotation. The trailing portion of the camber line extends forwardly with respect to this plane. The length L of each blade 20, as measured between inner and outer edges along a line extending through the two rivets 32, is about 157 mm. The width W of each blade 20, as measured between a first line tangent to the trailing edge 42 and a second parallel line running tangent to the outermost part of the leading edge 40 is about 132 mm. The maximum depth D of each blade (FIG. 7) is about twenty-one millimeters (21 mm). The lengths of the leading and trailing edges both substantially exceed the length of the outer edge 46.
A fan blade assembly 14 made in accordance with the preferred embodiment described above is capable of moving substantial volumes of air (e.g. about 450 cfm) quietly and efficiently. It will be appreciated that the fan blade assembly can be manufactured in different sizes and made from different materials. The assembly can be incorporated in fans other than stand fans. The fan may be operated at different speeds and include an oscillation mechanism.
A fan blade assembly 114 according to a second embodiment of the invention is shown in FIGS. 9 and 10. The assembly includes five identical blades 120 that are secured to a central hub 122. The hub 122 includes a generally hemispherical body. The rear side of the hub includes five pairs of threaded openings (not shown) used for securing the blades 120. It further includes a center opening 124, as shown in FIG. 11, for receiving the shaft of a motor (not shown).
Each blade 120 is preferably made from aluminum and has a substantially uniform thickness. In this embodiment, the thickness of the airfoil portion of the blade is about 1.5 mm. As discussed above with respect to the first embodiment, the blade thickness can vary over a range without materially affecting performance. The blade includes an inner or root portion 134 that may or may not have the same thickness as the airfoil portion of the blade. The inner portion includes a curved inner edge 136 and a generally arcuate boundary 138 that separates the inner portion 134 from the airfoil portion of the blade. The curvature of the boundary 138 substantially matches the curvature of the hub 122. When mounted to the hub, the inner portions 134 of the blades adjoin the rear surface of the hub. Openings 135 are aligned with the threaded openings in the hub. Screws 132 are employed to secure each blade to the hub. The boundaries 138 adjoin the peripheral surface of the hub. In this embodiment, the diameter of the rear surface of the hub is about 114 mm while the overall diameter OD of the fan blade assembly is about 394 mm. The depth DH of the hub is about 88.9 mm.
The blade 120 is generally “mid” loaded when in operation as opposed to front or rear loaded. While there is a small peak near the leading edge, loading is relatively uniform throughout the blade when compared to the blade 20 described above. The forward swept angle a of the blade, as shown in FIG. 13, is greater than seventy degrees, and preferably about 77.3 degrees. The blade has a length L of about 165 mm, a width W, as shown in FIG. 13, of about 144.3 mm, and a maximum depth D of about 46.6 mm.
The mean camber angle θ, as shown in FIG. 15, is greater than thirteen degrees, preferably about 13.65 to 13.7 degrees. The camber angle varies approximately from 7 degrees to 20 degrees. FIG. 16 shows the blade angle β distribution for various points on the blade from the leading edge 140 (0% M) to the trailing edge 142 (100% M) at the hub 138, at a distance of 10 % span 141, 20 % span 143, 45 % span 145, 62 % span 147, 75 % span 149, 87 % span 151, 90% span 153 and at the outer edge 146.
The leading edge 140 of the blade 120 is largely concave when viewed from the front or rear along the axis of rotation while the trailing edge 142 is generally convex. The leading edge 140 has a small convex portion 140 a near its inner (hub) end 138 while the trailing edge 142 has slightly concave portions 142 a near its inner and outer ends. The solidity is about 0.6 or greater, preferably about 0.68. As shown in FIGS. 11 and 13, each blade 120 is rearward swept near the hub and forward swept from about the mid-span area of the airfoil to the outer edge 146. The width of the blade is greater near the outer edge than near the hub. The blades overlap each other near the hub 122. The curvature of the camber line is greatest near the leading edge of the blade. The blade is also curved between the hub and outer edge such that the pressure surface is concave between these points.
The fan blade assembly 114 can be incorporated within a stand fan such as shown in FIG. 1 or other type of portable electric fan intended for home or office use.

Claims

1. A fan blade assembly for an electric axial flow fan comprising:

a hub, and

a plurality of fan blades mounted to the hub for providing axial air flow as the hub is rotated in a direction about an axis of rotation, each blade including:

a blade body having an airfoil portion of substantially uniform thickness and including radially inner, outer and mid-span portions,

a leading edge,

a trailing edge,

an outer edge connecting the leading and trailing edges and having a length that is shorter than the lengths of the leading and trailing edges,

a pressure surface,

a suction surface,

a curved mean camber line wherein the pressure surface is generally concave and the suction surface is generally convex between the leading and trailing edges,

the leading and trailing edges being curved such that the airfoil radially inner portion is rearward swept with respect to the direction of rotation of the hub and the radially outer portion is forward swept with respect to the direction of rotation of the hub about the axis,

the pressure surface being generally concave and the suction surface being generally convex between the hub and the outer edge.

2. A fan blade assembly as described in claim 1 wherein the blade body includes a mean camber angle exceeding twent-two degrees.

3. A fan blade assembly as described in claim 2 wherein the leading edge defines a forward swept angle greater than sixty degrees.

4. A fan blade assembly is described in claim 3 wherein the ratio of blade chord to pitch is about 0.4 or greater.

5. A fan blade assembly as described in claim 4 wherein the ratio of blade chord to pitch is about 0.4.

6. A fan blade assembly as described in claim 4 wherein the hub includes a plurality of radially outwardly extending arms, a fan blade being secured to each arm.

7. A fan blade assembly as described in claim 1 wherein the mean camber line has a maximum curvature near the trailing edge.

8. A fan blade assembly as described in claim 7 wherein the blade body has a mean camber angle of the airfoil portion greater than twenty-two degrees.

9. A fan blade assembly as described in claim 7 wherein the ratio of blade chord to pitch is about 0.4 or greater.

10. A fan blade assembly as described in claim 7 wherein the leading edge defines a forward swept angle greater than sixty degrees.

11. A fan blade assembly as described in claim 7 wherein the blades are arranged on the hub and configured such that, when rotated about the axis of rotation, peak loading on the blade occurs near the leading edge of the outer and mid-span portions and away from the leading edge near the hub.

12. A portable fan comprising:

a support;

an electric motor mounted to the support;

a fan blade assembly operatively associated with the electric motor;

a grill at least partially enclosing the fan blade assembly; and

the fan blade assembly including a hub and a plurality of fan blades mounted to the hub, each blade including:

a leading edge,

a trailing edge,

an outer edge connecting the leading and trailing edges,

a pressure surface,

a suction surface,

13. A portable fan as described in claim 12 wherein the leading edge of each blade defines a forward swept angle greater than sixty degrees.

14. A portable fan as described in claim 12 wherein the ratio of blade chord to pitch is about 0.4 or greater.

15. A portable fan as described in claim 12 wherein the mean camber line has a maximum curvature near the trailing edge.

16. A portable fan as described in claim 15 wherein the blade body has a mean camber angle which exceeds twenty two degrees.

17. A portable fan as described in claim 15 wherein the hub includes four radially outward extending arms, a fan blade being secured to each arm.

18. A portable fan as described in claim 12 wherein the blades are arranged on the hub and configured such that, when rotated about the axis of rotation, peak loading on the blade occurs near the leading edge of the outer and mid-span portions and away from the leading edge at the inner portion near the hub.

19. A fan blade assembly for an axial flow electric fan, comprising:

a hub,

a blade body having an airfoil portion,

a leading edge,

a trailing edge,

a pressure surface,

a suction surface,

the leading edge and trailing edge being curved such that the airfoil portion includes a radially inner portion that is rearward swept and a radially outer portion that is forward swept with respect to the direction of rotation of the hub, and the ratio of blade chord to pitch being about 0.6 or greater.

20. A fan blade assembly as described in claim 19 wherein the airfoil portion of each blade has a substantially uniform thickness.

21. A fan blade assembly as described in claim 20 wherein five blades are mounted to the hub, each blade overlapping an adjoining blade near the hub.

22. A fan blade assembly as described in claim 21 wherein the hub includes a circular edge, each blade including a root portion adjoining the airfoil portion and an arcuate boundary separating the root and airfoil portions, the root portion being secured to the hub, the arcuate boundary adjoining the circular edge of the hub.

23. A fan blade assembly as described in claim 21 wherein each blade has a mean camber angle of about 13.65 to 13.7 degrees and the solidity is about 0.68.

24. A fan blade assembly as described in claim 20 wherein the camber line has a maximum curvature near the leading edge.