Abstract:
A fan impeller includes rotating ring attached to the tip of dual-swept fan blades. Besides shrouding the impeller; the blades are dual-swept forward, and sweep increases in magnitude towards the tip. The shrouded dual-swept impeller resides inside classical fan housing. The integrated effects of shrouding the impeller, forward sweep into the direction of incoming flow, and forward sweep into the direction of rotation (circumferential forward sweep) render the fan quiet; the magnitude of noise reduction is between 7 and 12 dB.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to fans and in particular to impeller designs. 
         [0002]    Axial fans comprise an impeller driven by a rotating force. Typically, the rotating force is a motor embedded in the hub of the impeller. Impeller designs abound with various design features to reduce noise and/or increase efficiency. 
         [0003]    For example, one such design focuses on the shape of the blade. More specifically, the blade tip is swept to reduce noise. “Sweep” refers to the displacement of the centers of mass of successive airfoils of the blade. Many designs sweep only the tip forward (i.e., in the direction of incoming flow) to reduce noise by few (2 to 4) dB. A less tried approach is a circumferential sweep which claims to reduce noise by few dB. U.S. Pat. No. 5,064,345 teaches an example of a blade tip sweep design. 
       BRIEF SUMMARY OF THE INVENTION 
       [0004]    The present invention provides an impeller design that integrates three mechanisms into a single impeller design, namely a rotating tip shroud, forward blade sweep, and forward circumferential blade sweep. Also in accordance with the invention, the sweep is gradual as we move from the blade hub-section to the blade tip-section. 
         [0005]    The term “shrouded” impeller is used to describe the rotating ring that covers or cloaks the fan blades. The term “dual-swept” is used because blades of the present invention are swept into two different directions: one along the axis of rotation, the other along the circumferential direction. The shrouded and dual-swept impeller can be housed inside a classical stationary housing. 
         [0006]    The present invention integrates three elements to significantly reduce fan noise: (1) the use of a shroud; (2) the provision of forward axial sweep in the fan blades; and (3) the further provision of forward circumferential sweep in the fan blades. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  shows three views (top, perspective bottom, and profile) of a component of a fan in accordance with the present invention. 
           [0008]      FIGS. 2A-2C  illustrate axial details of a fan blade according to the present invention. 
           [0009]      FIGS. 3A and 3B  illustrate circumferential details of a fan blade according to the present invention. 
           [0010]      FIG. 4  is a top view of the fan blades of an impeller according to the present invention. 
           [0011]      FIG. 5  shows an illustrative embodiment of a fan according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]      FIG. 1  shows a top view of the present invention as embodied in an axial fan impeller  100 .  FIG. 1  also shows a bottom perspective view of the fan impeller  100 . The impeller  100  includes a hub  106  to which a set of blades  104  are attached. Each blade  104  is attached to the hub  106  at its blade root  104   a  (or simply root) and terminates at its blade tip  104   b  (or simply tip). A tip ring  102  (impeller shroud, or simply shroud) can be attached to tips  104   b  of the blades  104 . 
         [0013]    The particular embodiment of the present invention provides for a space  106   a  in the hub  106 . The space  106   a  can be used to contain a motor for driving the impeller. Referring to  FIGS. 1 and 5 , an illustrative example of a fan unit  500  embodied in accordance with the present invention is shown. The fan unit  500  comprises a motor  502  connected to the impeller  100 . A cutaway view of hub  106  shows the motor  502  housed within hub. The motor  502  is encased in a suitable housing  522 . The motor is shown mounted to a suitable base  524 . Drive electronics for operating the motor  502  can be provided on the base  524 , and connected with appropriate wiring. 
         [0014]    In the particular embodiment shown, the motor  502  is a brushless DC (direct current) motor. The motor  502  includes stator windings  512  which can be affixed to the housing  522 . The motor  502  further includes a permanent magnet rotor  514  comprising a shaft  516  and annular permanent magnet(s)  518 . The rotor  514  is rotatably supported by shaft  516  on a portion of the housing  522  for rotation about the shaft. 
         [0015]    Operation of the fan unit  500  results in an inflow of air toward the inlet face of the impeller  100  and a corresponding outflow of air (not illustrated) exiting the outlet face of the impeller. The direction of rotation is indicated in the figures. This direction of flow of air from the inlet face toward the outlet face is referred to as the downstream direction. The upstream direction is the opposite direction, namely the direction from the outlet face of the impeller  100  toward the inlet face of the impeller. 
         [0016]    The fan unit  500  can be incorporated in a conventional stationary fan housing consisting of sidewalls  524 ′ (shown in phantom lines) mounted to the base  524 . Common examples are PC fans comprising a housing within which is a fan unit. 
         [0017]      FIGS. 2A-2C  illustrate features of the present invention. The figures show a side view of a portion of the impeller  100  illustrated in  FIG. 1 . The hub  106  is shown with one fan blade  104 . The axis of rotation  202  is shown for reference. Other points of reference include the direction of the incoming air during operation of the fan  500  as the impeller  100  is rotated in the direction shown about axis  202  by motor  502 . The direction of the incoming airflow is referred to as the downstream direction, while the direction against the incoming airflow is referred to as the upstream direction. 
         [0018]      FIG. 2B  identifies representative airfoils  214   a - d  of the blade  104 . An airfoil is the shape of a wing or blade (of a propeller, rotor or turbine) as seen in cross-section. In practice, a blade is described in terms of its airfoils (also referred to as blade sections). Depending on the size of the blade and the desired resolution, a blade can be defined by a few as five airfoils to hundreds of airfoils. For the sake of clarity, the figures illustrate only four such airfoils including the airfoil at the blade tip  104   b.  As is conventionally known, a center of mass is associated with each airfoil  214   a - d.  In  FIG. 2A , the centers of mass associated with the airfoils  214   a - d  are shown projected onto an axial plane (explained below) and are represented as heavy dots  212 . 
         [0019]    As can be seen in  FIGS. 2A-2C , a blade  104  in accordance with the present invention is characterized by a forward axial sweep, namely a sweep along the direction of the axis (axial direction) and in the upstream direction. Stated differently, the present invention teaches blades  104  having a sweep in the axial direction and heading into the incoming flow of air when the blades are rotated. The sweep is “axial” in the sense that the direction of the sweep is along the axis of rotation  202 . The sweep is “forward” in the sense that the direction of the sweep is in the upstream direction.  FIG. 2B  identifies the leading edge (LE) and trailing edge (TE) of blade  104 . Thus, a blade  104  of according to the present invention has an axial sweep toward the leading edge of the blade. 
         [0020]      FIG. 2B  further illustrates a blade dimension referred to as the blade section axial length (axial blade length) which measures the length of an airfoil in the axial direction. Two representative axial blade lengths B 1  and B 2  are shown in the figure. In accordance with the present invention, the blade section axial length increases with each successive airfoil  214   a - d  progressing in the direction from the root  104   a  toward the tip  104   b.  Thus, the blade section axial length of airfoil  214   b  is B 1 , which is shorter than the blade section axial length B 2  of airfoil  214   d  at the blade tip  104   b.  Forward axial sweep can be produced by increasing the blade section axial length gradually from the hub  106  to the tip  104   b  and ending the trailing edges of all sections (airfoils) at the same axial location. 
         [0021]    An “axial plane” can be defined by an axis (call it the Z-axis) parallel to the axis of rotation  202  serving as one axis of the plane and by an axis (call it the R-axis for the radial direction) that is perpendicular to the axis of rotation. Referring to  FIG. 2C , the centers of mass  212  are shown projected onto the axial plane defined by the Z-axis and the R-axis. A forward axially swept blade  104  in accordance with the present invention can be defined by the locus of centers of mass  212  of the constituent airfoils  214   a - d  projected on the axial plane. More specifically, for each airfoil  214   a - d  of blade  104  along the R-axis and in the direction from the root  104   a  toward the tip  104   b,  the location of its center of mass  212  on the axial plane (Z-R plane) is forward (along the Z-axis) of the center of mass of the previous airfoil in the upstream direction. 
         [0022]    Consider, for example, the innermost airfoil  214   a  (blade section) illustrated in  FIG. 2C . The distance on the axial plane of its associated center of mass  212  from the R-axis is d 1 , in the upstream direction. Likewise, the center of mass  212  of the next airfoil  214   b  has a distance d 2  (d 2 &gt;d 1 ) from the R-axis. Thus on axial plane, the center of mass  212  of airfoil  214   b  is axially forward (in the upstream direction) of the center of mass of the previous airfoil, namely airfoil  214   a.  Likewise, the airfoil  214   c  has a center of mass  212  having a distance d 3  (d 3 &gt;d 2 &gt;d 1 ) which is forward of the center of masses of airfoils  214   a  and  214   b  in the upstream direction. Finally, the airfoil  214   d  at the tip  104   b  has its center of mass  212  at d 4  (d 4 &gt;d 3 &gt;d 2 &gt;d 1 ) which is forward of the center of masses of airfoils  214   a - c  in the upstream direction. 
         [0023]    In the particular embodiment of the blade  104  shown in  FIG. 2C , the locus of centers of mass  212  of airfoils  214   a - d  defines a straight line, referred to herein as an “axial line”  216 . It is noted that axial line  216  need not be straight and can be arcuate. An example of a line having an arcuate characteristic is shown in  FIG. 3B , identified by the reference numeral  316 . Thus, in accordance with the present invention, the locus of centers of mass  212  in the axial plane can define an arcuate axial line. 
         [0024]      FIGS. 3A and 3B  illustrate a further aspect of the present invention. These figures show a top view of the partial impeller  100  shown in  FIGS. 2A-2C . An arrow indicates the direction of rotation of the impeller  100  in an operating fan (e.g., fan  500 ). In this case, the rotation is a counterclockwise rotation. Of course, it is understood that the blades can be designed for clockwise rotation.  FIG. 3A  shows the representative airfoils  214   a - d  illustrated in  FIG. 2B . Each airfoil  214   a - d  is also shown with its corresponding center of mass  212  shown projected on the “radial plane” (explained below). 
         [0025]      FIGS. 3A and 3B  show that a blade  104  in accordance with the present invention is further characterized by having a forward circumferential sweep, in addition to the forward axial sweep described above. The sweep is “circumferential” in the sense that the sweep of the blade  104  is in the plane of rotation of the impeller  100  during operation. The sweep is “forward” in the sense that the sweep of the blade is in the direction of rotation of the impeller  100  during operation, which for the embodiment shown in the illustrations is a counterclockwise direction. 
         [0026]    Referring to  FIG. 3B , a “radial plane” can be defined by two axes that are both perpendicular to the Z-axis. One axis of the radial plane is a line tangent to the rotation of the impeller  100  (call it the θ-axis). The other axis of the radial plane is the R-axis described above and is perpendicular to both the θ-axis and the Z-axis. 
         [0027]      FIG. 3A  shows the centers of mass  212  projected onto the radial plane. As can be seen in  FIG. 3B , a blade  104  according to the present invention is further defined by the locus of centers of mass  212  of the representative airfoils  214   a - d  projected on the radial plane. More particularly, for each airfoil  214   a - d  of blade  104  along the R-axis and in the direction from the root  104   a  toward the tip  104   b,  the location of its center of mass  212  in the radial plane (R-θ plane) is forward (along the θ-axis) of the center of mass of the previous airfoil in the direction of rotation of the impeller  100 . 
         [0028]    Consider for example, the locus of centers of mass  212  shown in  FIG. 3B . The center of mass associated with airfoil  214   a  has a measurement e 1  on the radial plane that represents its distance from the R-axis along the θ-axis. Moving away from the root, the next airfoil  214   b  has a center of mass that measures c 2  (c 2 &gt;c 1 ) from the R-axis in the radial plane. As can be seen, the center of mass of airfoil  214   b  is circumferentially forward (in the direction of rotation) of the center of mass of previous airfoil, namely airfoil  214   a.  Likewise, the next airfoil  214   c  has a center of mass that measures e 3  (e 3 &gt;e 2 &gt;e 1 ) from the R-axis in the radial plane. The center of mass of airfoil  214   c  is circumferentially forward of the centers of mass of previous airfoils, namely airfoils  214   a  and  214   b.  Finally, the airfoil  214   d  has a center of mass that measures c 4  (c 4 &gt;c 3 &gt;c 2 &gt;c 1 ) from the R-axis in the radial plane. The center of mass of airfoil  214   d  is circumferentially forward of the centers of mass of previous airfoils, namely airfoils  214   a - c.    
         [0029]    In the particular embodiment of the blade  104  shown in  FIG. 3B , the locus of centers of mass  212  of airfoils  214   a - d  defines an arcuate line, referred to herein as a “radial line”  316 . It is noted that radial line  316  need not be arcuate and, in fact, can be substantially straight. An example of a straight line is shown in  FIG. 2C  discussed above. Thus, in accordance with the present invention, the locus of centers of mass  212  as projected on the radial plane can define a straight radial line or an arcuate radial line. 
         [0030]      FIG. 4  shows a top view of the impeller  100  to illustrate a blade dimension referred to as the radial blade length. An outer circumference  402  of impeller  100  is delineated by the tips of blades  104 . A radial measurement Rh represents the radius of the hub  106  from its center  202  to its outer wall  106   b  ( FIG. 1 ) where the blade roots attach. A radial measurement R t  represents the radius of the impeller as measured from the center of the hub  106  to the blade tips. The radial length of the blade is (R t -R h ). 
         [0031]    Thus, an impeller in accordance with the present invention comprises blades  104  each characterized in having both a forward axial sweep and a forward circumferential sweep. Blades according to the present invention each is characterized by representative airfoils  214   a - d,  each airfoil having an associated center of mass  212 . For each successive airfoil  214   a - d  of a blade along its length from the hub  106  toward the outer circumference  402 , the airfoil&#39;s associated center of mass is axially forward and circumferentially forward of the center of mass associated with previous airfoils. 
         [0032]    Referring back to  FIG. 1 , an impeller according to the present invention further includes the shroud  102 . The shroud  102  in combination the blade forward-sweep and blade circumferential sweep (“dual swept”) into the direction of rotation results in significant noise reduction. Testing has shown that relative to conventional un-shrouded, un-swept fan blades the shrouded and dual-swept blades of the present invention reduce fan noise by 7 to 12 dB. 
         [0033]    The embodiment shown in  FIG. 3B  shows that the forward circumferential sweep of the blade  104  becomes stronger as we move from the hub  106  to the tip  104   b.  As can be seen in the figure, there is greater forward curvature in region B as compared to region A.