Fan with noise reduction

A fan for moving a gaseous fluid, e.g., air, is described in which the high audible frequency noise resulting from the phenomenon occurring at the trailing edges of the blades, known as vortex shedding, is reduced. This is accomplished by notching an edge of each of the blades so that the pattern of vortices leaving the blade, which causes the noise, is disturbed and a turbulence condition engendered. The turbulence distributes the pressure fluctuations resulting from movement of the blades through the fluid over a relatively broad band of frequencies and reduces the annoying noise frequencies. Various notch configurations are disclosed.

The present invention relates to the reduction of noise in devices for 
moving air or other gaseous fluids, and more particularly, to means for 
reducing noise caused by the passage of fan blades through a gaseous 
medium. 
In fans or similar devices, the rotation of one or more blades through a 
gaseous medium, e.g., air, to create a flow of the medium, sometimes gives 
rise to audible noises that are objectionable if not intolerable. For 
example, in large electronic installations such as electronic computer 
systems, many small fans are employed in the computer cabinets to cool the 
electronic components housed within them. If such fans generate 
appreciable amounts of noise, personnel using the computer are disturbed 
and their efficiency impaired. It is apparent that in this, as well as 
other environments, the reduction of unwanted and irritating noise is a 
salutary achievement. 
The noises associated with fans or blowers may be grouped into two broad 
categories. The first of these encompasses those due to the components 
responsible for causing rotational movement of the blades or impeller. In 
these components, such as the motor and bearing assemblies, noise is 
generated primarily by the impact or relative movement of mechanical 
elements. The second category encompasses those noises created by the 
movement of air or other fluid caused by rotation of the fan blades. 
The latter category includes what is known as "blade passing noise", which 
arises in those fans in which the blades rotate in proximity to mechanical 
elements, such as struts which support the fan structure relative to some 
housing or shroud. As each blade passes a strut, a pressure variation 
occurs which, when multiplied by the number of blades, the number of 
struts, and the speed of rotation of the fan blades, may create an audible 
noise of appreciable and annoying volume. Several factors contribute to 
the generation of blade passing noise, such as the geometric relationship 
of the blade edges with respect to the struts, the number of blades and 
struts, the shape of the struts themselves, etc. and this noise is 
controllable to a tolerable degree by manipulation of those factors. 
In some types of fans, the aerodynamic relationships present give rise to 
another noise output, often at a pitch relatively high in the audible 
spectrum, which can be of sufficient intensity to become particularly 
annoying. This noise is referred to as vortex noise and occurs as a result 
of what is termed "vortex shedding", i.e., a phenomenon resulting from air 
leaving the trailing edges of the blades during operation in the form of 
vortices or swirls. The vortex pattern may cause pressure fluctuations at 
frequencies and intensities within the audible range. 
While the existence of this phenomenon and the resulting noise are known in 
the art, relatively little attention has been paid to them in the past. In 
many fans, because of the characteristics of the fan itself, the vortex 
noise may be at a frequency or of an intensity such that it does not 
warrant special attention or is masked by other noises of the fan. Where 
vortex noise is a factor, it heretofore has been suggested to minimize the 
noise by making the blade surfaces, in particular, the surfaces adjacent 
the trailing edges of the blades, of a porous material, the purported 
effect of which was to prevent or reduce the creation of vortex flow of 
the gas leaving the blade edge. 
The primary solution to the problem thus far, however, has been to avoid 
operating conditions which could cause such noise. Since vortex noise is a 
function of the operating condition of the fan and occurs only when the 
fan is operated at certain portions of its pressure vs. flow 
characteristics, fan manufacturers specify the suggested operating ranges 
of a fan to exclude those portions of its characteristic that give rise to 
vortex noise. Operating within these ranges will avoid the noise problem, 
but at the same time, will limit the utility of the fan. 
The object of the present invention is to provide for reduction of vortex 
noise by simple, reliable means and thereby enable a given fan to be used 
at any part of its characteristic without giving rise to annoying vortex 
noises. 
Briefly, in accordance with the invention, the vortex flow normally 
engendered at the trailing edge of a moving fan blade and which, under 
normal operating conditions of the fan, would give rise to pressure 
fluctuations at an audible frequency, is broken up by providing 
irregularities in the edges of the blades. These irregularities may take 
the form of notches arranged longitudinally along the blade edges from tip 
to root and may be V-shaped, U-shaped, rectangular, etc. The effect of 
these notches is to create a mixture of vortices leaving the blade edge at 
different points and in different directions. This engenders a turbulence 
condition at the blade trailing edge, the pressure fluctuations of which 
occur over a relatively wide range of frequencies. Consequently, the 
particularly annoying high pitched vortex noise is significantly reduced 
in intensity, making use of the fan at the conditions that ordinarily gave 
rise to such noise, quite satisfactory.

FIGS. 1 and 2 illustrate a fan produced commercially by the inventor's 
assignee and sold under the name "Feather Fan." The overall diameter of 
this fan is 7 inches and its depth is approximately 21/2 inches. The fan 
is capable of delivering up to 270 cu.ft/min and is employed for cooling 
electronic equipment such as computer consoles, relay racks, power 
supplies and the like. The present invention will be described as applied 
to the specific structure of the illustrated fan, but it will be 
understood that the principles of the invention are applicable to other 
types of gas or air moving devices in which vortex noise occurs, of which 
the Feather Fan is representative. 
As shown in FIGS. 1 and 2, the fan assembly includes a venturi ring or 
shroud 10 within which the fan assembly, indicated generally at 12, is 
supported by a plurality of radially extending struts 14. The struts 14 
support at their inner ends a stator hub 16, which carries the stator of 
an electric motor to drive the fan blades. 
A rotor hub 18 carries a plurality of fan blades 20 (three in the fan 
illustrated) spaced at equal angular distances from each other about the 
hub. The entire rotor structure of the motor, including the rotor hub and 
blades, is journaled in suitable bearings and secured for rotation 
relative to the stator structure. Thus, the struts 14 support the entire 
motor and fan blade assembly within the venturi ring 10. The fan assembly 
of FIGS. 1 and 2 produces a flow of air in an axial direction, indicated 
by the horizontal arrows in FIG. 2 in response to rotation of the blades 
20 in the counterclockwise direction as indicated in FIG. 1. The legend 
20a denotes the leading edge of the blade 20 while 20b denotes the 
trailing edge. 
As briefly referred to above, fan blades of the type illustrated in FIGS. 1 
and 2, but without the notched edges illustrated therein, give rise under 
certain operating conditions to noise caused by vortex shedding. This 
noise results from the aerodynamic characteristics of the blade and is a 
function of the velocity of flow leaving the blade, the blade thickness 
and the pressure differences between the suction and high pressure 
surfaces of the blade. These characteristics produce vortices at the 
trailing edges of the blade which are manifested by fluctuating pressures, 
the fluctuations of which occur at frequencies in the audible range. If 
the trailing edge of the blade is continuous, i.e., a smooth, unbroken 
line, these pressure fluctuations occur within a narrow range of 
frequencies, centered about a relatively high audible peak frequency 
(e.g., 2000 Hz), and are of sufficient intensity to become annoying. 
According to the present invention, this vortex noise is reduced by 
physically altering the shape of the blade to disturb the air patterns at 
discharge such that the fluctuations resulting from vortex flow occur at 
relatively similar, and low, intensities over a wide range of frequencies, 
rather than essentially at a single peak frequency. Referring to FIG. 1, 
the physical deformation takes the form of a series of notches 21, in this 
case V-shaped, formed in the trailing edge 20b of the blade. Because of 
these notches, the vortex shedding does not start in regular fashion along 
the trailing edge of the blade, as would occur in the conventional blade, 
but rather irregularly, all along the edges of the notches. This increases 
the shedding length and results in changes in the vortex intensities 
leaving the blade, the strongest vortices occurring at the roots of the 
notches and the weakest vortices at the crests. 
The result of this mixing of vortex intensities is to establish a 
turbulence condition along the trailing edges of a blade. The irregular 
pressure fluctuations occurring in such a turbulence condition are at 
frequencies spread over a relatively broad frequency band, without 
significant amplitude peaks at any single frequency. This broad banding 
then, has the effect of lessening the intensity of noise at the undesired 
relatively high frequency to a tolerable level. 
The notching of the invention may be applied to blades having a constant 
thickness, such as that shown in FIG. 2, or to blades having an airfoil 
cross section. On a blade of the latter type, the effect of the notches is 
to produce a variation in the thickness in the trailing edge and, since 
the peak frequency of vortex shedding noise is a function of this 
thickness, the unevenness helps to reduce noise at the annoying frequency. 
It has been found that the noise reduction effect of the notching is 
optimized if the notches are of equal size and constant pitch along the 
blade edge. As applied to the Feather Fan illustrated in FIGS. 1 and 2, it 
has been found that notches having a depth of approximately 0.25 inch, a 
width at the blade edge of 0.3 inch and a crest width of 0.1 inch results 
in optimum noise reduction. The precise dimensions will of course vary 
with the characteristics of the fans and blades to which the invention is 
applied. 
The present inventor has also found that the noise reduction resulting from 
the blade edge notching may be enhanced if the series of notches in 
respective blades are off-set or staggered relative to one another so as 
to create different flow patterns on the respective blades. This is 
illustrated in FIG. 1 wherein the series of notches in the trailing edges 
of the blades 20, while all of the same pitch and dimensions, are 
longitudinally displaced along the edge of the blades from each other by 
an amount less than the pitch of the notches. By so staggering the 
starting point of the series of notches on the respective blades, 
repetitive flow patterns, which could reinforce and give rise to an 
undesirable noise component, are avoided. 
In the specific example illustrated, i.e., the Rotron Feather Fan, it was 
found that noise centered at approximately 2000 Hz occurred at 
objectionable intensities under certain operating conditions. By provision 
of the notching as illustrated in FIG. 1, i.e., V-shaped notches of the 
dimensions given above, with the series of notches longitudinally 
displaced from each other on the respective blades, the noise reduction at 
this frequency was on the order of 10 db. Although some reduction in fan 
performance resulted from the turbulence caused by the notching, it was 
relatively small and well worth the noise reduction for which it was 
traded. 
In FIGS. 3 and 4 are illustrated other form of notching which may be used 
in accordance with the invention. In FIG. 3, the roots of the notches are 
essentially U-shaped and the crests rounded in the same manner to provide 
rounded notching. In FIG. 4, the notches are rectangular cut-outs formed 
in the trailing edge. In the cases of both types of notching, the 
dimensions are essentially those given hereinabove with respect to the 
V-shaped notches shown in FIGS. 1 and 2. 
In the embodiments of FIGS. 1 to 4, the notching of the trailing edges of 
the blades extends over the entire length of the edge, i.e., from root to 
tip. The inventor has found that substantial vortex noise reduction can be 
obtained with the notching extending only about half way from the tip to 
the root, as shown in FIG. 5. While slightly less effective in noise 
reduction than full length notching, the half length notching results in 
less performance loss, the loss being neglible compared to an unnotched 
standard blade.