Abstract:
A cooling fan is disclosed. The cooling fan comprises fan blades having a primary blade component and a secondary blade component. The primary blade component produces an axial airflow. The secondary blades produce a radial airflow. The axial airflow and the radial airflow, respectively created by the primary blades and the secondary blades, combine to provide an increase axial outflow. The primary and secondary blades combine the axial and radial flows respectively created by the blades to produce an increased outflow. The design has been observed to reduce tonal noise as well.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]     The present application claims priority from U.S. Provisional Application No. 60/755,303, filed December  29 ,  2005 , and is fully incorporated herein by reference for all purposes. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates generally to cooling fans and in particular to an impeller design to improve airflow and noise performance in axial fan designs.  
         [0003]     Prior art fans (e.g.,  FIG. 6 ) comprise a separate housing and fan assembly. The fan assembly fits into an air passage region provided in the housing. The circumference of the air passage must be larger than the circumference of volume of space defined by the rotating blades. Consequently, there is a gap between the tips of the fan blades and the housing wall which defines the air passage region. This gap can be large and accordingly reduce the performance of the air mover device.  
       BRIEF SUMMARY OF THE INVENTION  
       [0004]     A cooling fan according to the present invention comprises fan blades having a primary blade component and a secondary blade (may also be referred to as the tip blade) component. The primary blade component produces an axial airflow. The secondary blades produce a radial airflow. The axial airflow and the radial airflow, respectively created by the primary blades and the secondary blades, combine to provide an increase axial outflow, thus enabling a greater movement of air to increase the cooling effect of the fan. The resulting device improves aerodynamic efficiency. Tests have also shown reductiond in tonal noise as compared to conventional blade designs which do not incorporate a tip blade.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]      FIG. 1  shows a side view and a top view of a cooling fan according to an embodiment of the present invention.  
         [0006]      FIGS. 2A and 2B  are schematic illustrations of fan blades according to the present invention.  
         [0007]      FIG. 3A  illustrates axis of rotation.  
         [0008]      FIG. 3B  illustrates radial direction.  
         [0009]      FIG. 4  illustrates the air flows produced by a fan in accordance with the present invention.  
         [0010]      FIGS. 5A and 5B  illustrate variations of wire-frame housings in accordance with the present invention.  
         [0011]      FIG. 6  shows a conventional fan with a housing. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]      FIG. 1  shows side view and top view images of a prototype for a cooling fan  100  according to an embodiment of the present invention. The images show an impeller unit  104  comprising a motor  124  and fan blades  126 . The fan blades  126  are connected to a hub  122 , which in turn is fixedly coupled to a rotor portion  128  of the motor  124 . The impeller unit  104  is mounted to a fan housing  102 . In the embodiment shown in  FIG. 1  the fan housing consists only of a base portion  112  onto which the motor is mounted. The images of  FIG. 1  also show wires for electrical connection to the motor  124 .  
         [0013]     In accordance with the present invention, the illustrative embodiment of  FIG. 1  shows that the fan blades  126  comprise a primary blade  132  and a secondary blade  136 . In the illustrative embodiment of  FIG. 1 , the secondary blade  134  is disposed at the tip of the primary blade  132 .  
         [0014]     Further in accordance with an aspect of the present invention as shown in the embodiment of  FIG. 1 , the fan housing  102  comprises only the base portion  112 , and is absent the conventional casing enclosure that encloses the fan blades in the radial direction. See, for example,  FIG. 6  which shows a conventional cooling fan design. The conventional fan housing includes a base portion and a fan blade enclosure (casing) that encloses the fan blades along a radial direction, but is open in the axial direction to provide an axial air passage.  
         [0015]      FIGS. 2A and 2B  are illustrations showing configurations of a fan blade  226  in accordance with the present invention.  FIG. 2A  shows a primary blade  232  having a root end  242  that connects to the hub  222 . The other end of the primary blade is referred to as the blade tip  244 , or simply tip. In accordance with the present invention, a secondary blade  234  is disposed proximate the blade tip  244 . As shown in  FIG. 1  and illustrated in  FIG. 2A , the secondary blade  234  is formed substantially at the tip  244  of the primary blade  232 .  FIG. 2B  illustrates that the secondary blade  234  does not have to be formed at the tip  244 , and that some portion of the tip of the primary blade  232  may extend beyond the outer surface of the secondary blade. It is only necessary that the secondary blade  234  be disposed near the tip of the primary blade  232 .  
         [0016]      FIG. 2A  further illustrates that the secondary blade  234  is disposed roughly in perpendicular relation to the primary blade  232 . It will be appreciated that the secondary blade  234  can be provided at an angle θ other than 90°. It will be further appreciated that the secondary blade  234 , not unlike the primary blade  232 , can be designed to have any suitable combination of parameters used in blade design, e.g., chord length, camber, and so on. In fact, the secondary blades  134  are characterized much in the same way as the primary blades  132 . This is illustrated in  FIG. 1 , where the top view image assumes a direction of fan rotation as shown. For example, the primary blades  132  each has a leading edge  152  and a trailing edge  154 . Similarly, the secondary blades  134  each has a leading edge  142  and a trailing edge  144 .  
         [0017]     In operation, a cooling fan configured in accordance with the invention will create a radial inflow of air in addition to the normal axial inflow. The resulting axially-directed outflow of air is thus increased because of the additional radial inflow. Referring to  FIGS. 1 and 4 , a cooling fan according to the present invention is absent the conventional enclosure that houses the fan. The absence of the housing opens up the sides, allowing the secondary blades  134  to capture air from the radially-directed periphery of the fan blades  126 .  
         [0018]      FIG. 4  shows various inflows that are produced by a cooling fan of the present invention. First, there is the conventional axial inflow of air that is produced by the turning of the primary blades  132 . The axial airflow is the flow of air that is substantially parallel to the axis of rotation ( FIG. 3A ) of the blades.  
         [0019]      FIG. 4  also shows a radial inflow of air entering along the radial direction. As understood by one of ordinary skill, the radial direction is the direction along which the primary blades extend from the hub toward the tip, as illustrated in  FIG. 3B . The radial inflow results from a scooping action of air in the radial direction when the air is captured by the secondary blades  134  as the fan blades  126  turn. This scooping of air along the radial direction is facilitated by not enclosing the impeller  104  in an enclosure, thus eliminating the side walls. The additional radial inflow air stream contributes to the resulting axially-directed outflow stream of air, thus significantly increasing the outflow.  
         [0020]     The air that is captured by the secondary blades  134  in the radial direction is forced in the axial direction and combines with the axial inflow to produce the axial outflow as illustrated in  FIG. 4 . As will be appreciated, the capture of air and subsequent redirection and combination with the axial inflow are controlled by design of the primary blades  132  and secondary blades  134 . Parameters including camber angle, stagger angle, chord length, and number of blades are principal design parameters that control the capture effect of the secondary blades. For example, the secondary blades  134  can be designed to produce an amount of radial air flow that is 10% of the axial air flow created by the primary blades  132 . Such a design would result in roughly an increase in outgoing airflow by 10%.  
         [0021]     The embodiment illustrated in  FIG. 1  shows that a secondary blade  134  is provided for each primary blade  132 . However, alternate possible embodiments include providing secondary blades for only some of the primary blades. For example, every n th  primary blade can be configured with a secondary blade proximate its tip. The number for n will depend on the total number of primary blades. The secondary blades should be evenly distributed among the primary blades to ensure the impeller  104  is balanced in order to avoid wobble during operation. Also, symmetrical distribution of the secondary blades in the circumferential direction (or azimuthal direction) ensures a proper scooping action. For example, in one configuration, the impeller can comprise a secondary blade for every other primary blade. In another configuration, the impeller can comprise every third primary blade provided with a secondary blade.  
         [0022]     The embodiment illustrated in  FIG. 1  shows that the fan housing  102 comprises only a base portion  112 . There is no casing or enclosure within which the impeller unit  104  is housed. The embodiment of  FIG. 1  is simple and cost-effective to build. However, it may be desirable to provide some kind of open-spaced enclosure to house the impeller  104  unit. This aspect of the present invention will now be discussed in connection with  FIGS. 5A and 5B .  
         [0023]      FIGS. 5A and 5B  show alternative embodiments to illustrate a further aspect of the present invention.  FIG. 5A  shows an embodiment in which a wire-frame cage  502  (represented by dashed lines) can be provided as an enclosure. For example, the wire-frame members  512  can be solid members, tubular, have a circular cross-section, and so on. Note that the cage structure does not contain side walls in order to reduce obstructions to the flow of air in the radial directed during fan operation.  
         [0024]      FIG. 5B  shows a variation where the cage  502  includes strut members  514  to add mechanical strength to the cage. Though the strut members  514  are provided on the sides of the cage, it is noted that they do not substantially occlude radially directed airflow. Based on the examples of cage construction shown in  FIGS. 5A and 5B , it can be appreciated that any suitable enclosure can be provided so long as the sides of such enclosure are sufficiently open to allow for adequate radially directed airflow. In this way, when an impeller of the present invention is operated within such an open-sided enclosure design, the secondary blades  136  can create a radially-directed inflow of air.  
         [0025]     The foregoing embodiment exemplars of the present invention show a fan housing that is substantially absent the enclosure. As noted above, the reason for this is to reduce obstructions to the flow air in the of radial direction to facilitate the capture of air by the secondary blades  134 . However, while a fan housing that is absent the conventional enclosure may be a suitable embodiment for some uses, the present invention does not require the complete absence of an enclosure. The wire-frames of  FIGS. 5A and 5B  are an example of such an enclosure. In another example, an enclosure (not shown) that is more that the wire cages of  FIGS. 5A and 5B  can be provided with openings on its sides to allow for a radially-directed inflow of air ( FIG. 6  shows what is meant by sides of the enclosure, in the context of this discussion). An enclosure having openings on its sides may be desirable in order to protect a user from the spinning fan blades, while at the same time improving the axial outflow provided by the present invention. The openings can be appropriately designed so that a sufficient amount of air can pass through, allowing the secondary blades  136  to create an adequate radial inflow and direct that flow into the axial outflow stream. The openings can be varied in terms of numbers, sizing, shapes (e.g., slotted, circular, etc), orientation (e.g., diagonal, horizontal, etc), and so on.  
         [0026]     It is also understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims.