BACKGROUND OF THE INVENTION
The present invention relates generally to an axial flow fan and in particular to a fan impeller having improved noise characteristics.
Conventional fans typically comprise a motor and an impeller coupled to the motor. The impeller includes a hub to which a set of fan blades are attached. The hub is connected to the rotor of the motor. Existing fan designs typically include a stationary housing to house the fan. The housing is a common source of fan noise in such designs.
Every rotating fan radiates noise. Noise is function of a number of factors which include fan speed, size, and blade shape. Most of the acoustic energy reveals itself as frequencies or tones (referred to as “tonal noise”) which contain most of the acoustic power, and are thus annoying to the listener. These tonal modes are the harmonics; the first being the fundamental frequency (f, or the blade passing frequency), the second being 2f, the third is 3f, and so on. In other words, acoustic power is distributed among the harmonics, which show up as peaks in the noise spectrum.
BRIEF SUMMARY OF THE INVENTION
A fan impeller according to the present invention includes a hub having attached to it a plurality of fan blades. A ring is provided at the tips of the fan blades. The ring, which has a curved interior surface cross-sectional profile, reduces tonal noise.
An axial fan according to the present invention includes a fan impeller having fan blades radially attached to a hub and having a ring attached to tips of the fan blades. The interior wall surface of the ring is a shaped profile, where the inside diameter of the ring varies along a height dimension of the ring.
In accordance with the present invention, the fan housing is replaced by a rotating tip ring. The ring location (at the tip), its shape (circular arc extended upstream and downstream of the blade), and its function (silencer) are unique to the present invention.
An advantage of the present invention is the elimination, minimization, or otherwise reduction of noise during operation of the fan, particularly tonal noise. An axial fan according to the present invention exhibits increased performance. An axial fan according to the present invention is cheaper to produce due to a reduced parts count; i.e., there is no housing. The tip ring prevents or otherwise reduces the occurrence of fan stall.
The invention provides a rotating tip ring attached to the tip of the blades of an axial fan impeller which replaces classical housing structures, and which acts as a silencer by eliminating noise created by rotation of the impeller, particularly tonal noise. To enhance performance, an embodiment of the tip ring is constructed as a circular-arc which starts upstream of the leading edge and which ends downstream of the trailing edge.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates an embodiment of a fan impeller for an axial flow fan in accordance with the present invention in a perspective rendering.
FIG. 2 is a cross-sectional view of the impeller shown in FIG. 1.
FIGS. 3 and 3A are cross-sectional plan views of the impeller shown in FIG. 1.
FIG. 4 shows a compression zone and a expansion zone in the tip ring in accordance with the present invention.
FIGS. 5A-5D illustrate various cross-sectional profiles of a tip ring of the present invention.
FIG. 6 is a schematic representation of an axial flow fan in accordance with the present invention.
FIGS. 7 and 8 show the acoustic profiles for a conventional production fan and a fan according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The figures described below demonstrate the basic concept and an illustrative specific instantiation of the tip-ring structure of an impeller in accordance with the present invention.
FIG. 1 is a perspective view of an embodiment of an impeller 100 for an axial flow fan according to the present invention. FIG. 2 shows a cross-sectional view of the impeller 100. The impeller 100 includes a hub 102 and a plurality of fan blades 104. The fan blades 104 are connected to the hub 102 at respective roots 104b of the fan blades. The figures illustrate an annular member 106 is disposed at respective tips 104a of the fan blades104. The annular member 106 is referred to herein variously as the ring, tip ring, band, etc.
The figures show that the tips 104a of the fan blades 104 are connected to the annular member 106. In other words, the annular member 106 is attached to the tips of the fan blades 104. This connection provides support for the fan blades 104 in addition to other benefits discussed below. The figures show that the tips 104a do not extend beyond the exterior surface of the annular member 106.
The direction of an airflow resulting during operation of the impeller 100 is shown in the figures, where the airflow enters an inlet side of the impeller and exits an outlet side. In FIG. 2, the direction of airflow is indicated by arrows 202a and arrows 202b. The inflow of air into the impeller 100 is identified as 202a and the outflow of air exiting the impeller is identified as 202b. Consistent with the conventional meaning of the terms “upstream” and “downstream,” the “upstream direction” is the direction pointing into the airflow direction (i.e., the direction opposite the airflow 202a, 202b). Conversely, the “downstream direction” is in the direction of the airflow 202a, 202b. FIG. 2 also identifies for each fan blade 104 a leading edge 104c of the fan blade and trailing edge 104d of the fan blade.
FIG. 3 shows a cross-sectional plan view of the impeller 100 shown in FIG. 1. In accordance with the present invention, the inside diameter of the ring 106 near the inlet side generally increases in the direction upstream of the leading edge 104c of the fan blades 104. Likewise, the inside diameter of the ring 106 generally increases in the downstream direction of the trailing edge 104d of the fan blades 104. See FIG. 3A for an illustration of this discussion.
FIG. 3A shows examples of various measurements of the inside diameter of the ring 106. In accordance with the present invention, the inside diameter of the ring generally increases in the upstream direction from the leading edge 104c of the fan blades 104, which is shown in FIG. 3A as the direction from position p1 to position p2. The inside diameter of the ring 106 at position p1 (roughly the position of the leading edge 104c of the fan blades 104) increases from a measurement of D1 to a measurement of D2 at a position p2 at the inlet face of the ring.
Similarly, the inside diameter of the ring generally increases in the downstream direction from the trailing edge 104d of the fan blades 104, which is shown in FIG. 3A as the direction from position p3 to position p4. The inside diameter of the ring 106 at position p3 (roughly the position of the trailing edge 104d of the fan blades 104) increases from a measurement of D3 to a measurement of D4 at a position p4 at the outlet face of the ring.
In the particular embodiment of the present invention shown in FIG. 3, the ring 106 has an arcuate cross-sectional profile. More particularly, the inside surface of the ring 106 has an arcuate profile. As can be seen in FIG. 3, the inside diameter of the ring 106 at the inlet face is represented by DA. The inside diameter of the ring 106 at the outlet face is represented by DB. In accordance with the present invention, a minimum diameter measurement DC of the ring 106 occurs between the ring's inlet face and the ring's outlet face. In FIG. 3, the minimum diametric measurement DC occurs roughly in the middle of the ring 106, although this is not necessary; the minimum diametric measurement can occur near the middle, either to the upstream side of the middle of the ring or to the downstream side of the middle of the ring.
This construction creates a flow area that converges (a compression zone) downstream of the blade leading edge 104c and a flow area that diverges (a expansion zone) downstream of the blade trailing edge 104d. As a result of the compression of the air inflow followed by an expansion of the air outflow, the ring 106 eliminates (or otherwise reduces) tonal noise associated with the fundamental frequency and its harmonics. In other words, the ring 106 acts as a silencer, either eliminating or otherwise reducing noise, particularly tonal noise.
FIG. 4 illustrates diagrammatically the compression zone at the inlet (upstream) side of the impeller 100 and the expansion zone at the outlet (downstream) side of the impeller. As the impeller 100 rotates during operation of the fan, the air drawn into the inlet side of the impeller is compressed by virtue of the reduced diameter of the ring 106, and subsequently expands on approach to the outlet side.
Returning to FIG. 3, the term “thickness” will be used herein to refer to the dimension of the ring 106 between the ring's inlet side and the outlet side as illustrated in the figure. In accordance with the present invention, the ring 106 has a thickness measurement T that exceeds the axial chord measurement CZ of the fan blades 104 (i.e., T>CZ). The inset in FIG. 3 shows a fragment of the ring 106 with a cutaway view of one of the fan blades 104 illustrating this aspect of the present invention more clearly. The axial chord CZ is the length of the projection of the fan blade 104, as attached to the ring 106, onto a line parallel to the axis of rotation.
As can be seen in the figure, the tip ring 106 extends upstream of the fan leading edge 104c by a non-zero amount d1 (i.e., d1>0). The tip ring 106 extends downstream of the fan trailing edge 104d by a non-zero amount d2 (i.e., d2>0). In the particular embodiment of the invention shown in FIG. 3, d1=d2; however, the equality relationship is not necessary. Thus, the d1 measurement can be different from the d2 measurement.
The particular embodiment of the present invention illustrated in FIG. 3 shows a cross-sectional profile of the tip ring 106 having an arcuate shape. This shape is reproduced in FIG. 5A as a simplified cross-sectional view of the tip ting 106, absent the hub 102 and fan blades 104. FIGS. 5B-5D show a non-exhaustive list of representative alternate profiles. The alternate profile shown in FIG. 5B illustrates an angular profile. The alternate profile shown in FIG. 5C shows a tip ring 106 comprising a segmented profile.
Referring again to FIG. 3, the outer surface of the tip ring 106 has an arcuate profile as well as an inner surface having an arcuate profile. In this respect, the ring 106 can be said to be a thin structure. However, such a thin is not necessary. As FIG. 5D illustrates, the outer surface of the tip ring 106 can have a non-arcuate profile, and in general is not limited to any particular profile. Thus, a tip ring 106 of the present invention is not limited to a thin-structure cross-sectional profile.
FIG. 6 shows an axial fan 600 according to the present invention. The fan 600 is shown mounted on a suitable base member 612. The fan 600 includes a motor 602 such as a brushless DC motor, although any of a number of motor types can be used. Details of the motor 602 are not provided as they are well known and otherwise not relevant to the present invention. The fan 600 incorporates the impeller 100 as disclosed in FIG. 1, including the tip ring member 106 (the fan blades cannot be seen in this side view). The hub 102 of impeller 100 is attached to the rotor of the motor 602 (shaft 604 is understood to be coupled to the rotor). As can be seen in this figure, the fan 600 does not include the stationary housing that is typical of conventional axial fans. The housing is replaced by the rotating tip ring element 106.
Though not important to the practice of the present invention, a brief discussion of the acoustic properties of the present invention will help with an appreciation of the novelty and advantages of the present invention.
Rotation of a conventional impeller enclosed in a stationary housing creates tonal noise associated with the blade passing frequency (BPF, f) and its higher harmonics (2f, 3f, 4f, etc). The fan acoustic profile reveals that acoustic power is concentrated at these discrete frequencies (f, 2f, 3f, etc).
Experiments revealed that removing the stationary housing of a conventional fan and replacing the impeller with a rotating impeller designed according to the present invention resulted in “taming” (removal or reduction in amplitude) of tonal noise. The fundamental frequency and higher harmonics are silenced due to sudden area expansion downstream of the ring as discussed above.
FIGS. 7 and 8 show the acoustic profiles for (1) a production fan having a stationary housing and (2) a houseless fan with a rotating tip-ring of the present invention. It can be seen that the houseless fan exhibits a weakened fundamental frequency (f) and weakened higher harmonics (i.e., reduced tonal noise).
FIG. 7 show acoustic profiles for a conventional fan and a fan according to the present invention, both operating at 5000 RPM (revolutions per minute). The BPF frequency (750 Hz) and a higher harmonic (1500 Hz) are shown for the conventional fan at A and C. The BPF frequency and a higher harmonic are shown for the tip-ring fan at B and D. It can be seen that the noise is reduced at the BPF and higher harmonics in the tip-ring fan more than in the conventional fan.
FIG. 8 show acoustic profiles for a conventional fan and a fan according to the present invention, both operating at 6350 RPM. The discrete tonal noise is near 550 Hz (BPF), and higher harmonics occur at 1100 HZ (first harmonic), 1650 Hz (second harmonic), and 2200 Hz (third harmonic). Points A, B, C, and D show the noise level at these frequencies for the conventional fan. The corresponding noise levels for the tip-ring fan are significantly lower at these frequencies and in fact appear to exhibit no tonal noise at all.
As best understood, the rotating tip-ring of the present invention creates other frequencies which interact with and eliminate tonal noise. In other words, the tip-ring acts like a silencer. By creating new frequencies, the tip ring weakens tonal modes by forcing the available acoustic power to be distributed over a broader frequency spectrum, those frequencies due to rotation PLUS those frequencies due to the rotating tip-ring. Put in another way, with a conventional impeller (absent the tip ring) the acoustic power ends up being concentrated about a discrete number of frequencies, namely the fundamental and the second and third harmonics. However, an impeller designed with the tip ring of the present invention, the generated acoustic power ends up being distributed over a large number of frequencies.
The tip ring of the present invention also enhances performance, enabling the fan to capture more flow and to produce more pressure (relative to a fan without tip ring but with stationary housing) more efficiently. The tip ring also hinders stall.
While the above provides a detailed description of various embodiments of the invention, many alternatives, modifications, and equivalents are possible. Therefore, the above description should not be taken as limiting the scope of the invention, which is defined by the appended claims.
Patent Application
20090148294
