Abstract:
Disclosed is a hub of an axial fan. The hub includes primary fan blades and splitter blades disposed between pairs of the primary fan blades. The resulting hub has been observed to reduce tonal noise during fan operation.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]     The present application claims priority from U.S. Provisional Application No. 60/755,474, filed Dec. 29, 2005, and is fully incorporated herein by reference for all purposes. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates generally to axial fans and in particular to a configuration of fan blades to reduce noise.  
         [0003]      FIG. 4  shows an exploded cross-sectional view of components comprising a conventional axial fan. The figure shows a base  402  that is part of the cooling fan housing (not shown) onto which a stator is mounted. Typically, the base  402  includes a small printed circuit board for the electronics which control motor operation. Power and control wires (not shown) run from the printed circuit board for connection to an external power source and to a computer. The stator assembly comprises a coil subassembly  404  comprising some number of individually activated coils wound about a bearing liner  406 . A rotor assembly is positioned around the stator coil  404 . The rotor assembly includes a yoke  408  which is shaped like a cup that fits around the stator coil  404 . An axle  410  is axially connected to the interior of the yoke  408 . A number of permanent magnets  412  are fixedly mounted about the interior periphery of the yoke  408 . When the yoke  408  is assembled with the stator assembly, the axle  408  is received within the bearing liner  406  and the permanent magnets  412  are disposed around the coil subassembly  404 . The axle  410  rests on a bearing surface neat the bottom of the bearing liner  406 . An impeller  414 , comprising a hub  416  and some number of fan blades  418  attached to the hub, fits over the yoke  408  and is connected to the yoke.  
         [0004]     A common problem with fans is the noise they generate during operation. A particularly displeasing noise component is tonal noise. Tonal noise is a result of the rotation of the fan blades. The frequency spectrum of tonal noise comprises largely of components of the blade passing frequency (fundamental and harmonics), which is the number of fan blades times the shaft speed (revolutions per second). Broadband noise is another noise component, but is less noticeable as compared to tonal noise since its frequency spectrum is generally much broader that the frequency spectrum of tonal noise and the amplitudes of its frequency components are lower.  
       BRIEF SUMMARY OF THE INVENTION  
       [0005]     One embodiment according to the invention alternates the chord length of each blade in order to break up any tonal noise related to the blade passing frequency. For example, on an 8-bladed impeller, four blades are of one chord length and four blades are of another chord length. Varying the length of the chord of the blades with respect to the other blades is a key aspect of the invention. This reduces the tonal noise of the blade passing frequency by changing one strong blade passing frequency into two smaller blade passing frequencies. Other possibilities include an increased number of chord lengths within a fan design.  
         [0006]     A result of cooling fans having fan blade configurations according to the present invention is significant reduction of tonal noise due to blade passing frequency. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is an illustration of the hub of an axial fan according to the present invention.  
         [0008]      FIG. 1A  is an image of a prototype of the hub illustrated in  FIG. 1 .  
         [0009]      FIG. 2  is a schematic view a full blade and splitter blade arrangement according to the present invention.  
         [0010]      FIG. 3  is schematic view of a simple embodiment of the present invention.  
         [0011]      FIG. 4  is an exploded view of a conventional fan.  
         [0012]      FIG. 5  is a diagram of an airfoil, showing various parameters of an airfoil. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]      FIG. 1  illustrates an axial impeller  100  made in accordance with the teachings of the present invention.  FIG. 1A  is a photograph of a prototype of the impeller shown in  FIG. 1 . The impeller comprises a hub  102 . Disposed about the hub  102  is a plurality of fan blades  104 ,  106 . The figure shows what are commonly referred to as “full blades”  104 . Disposed between a pair of full blades  104  is what is would be referred to as a “splitter blade”  106 . The blades  104 ,  106  are connected to the hub  102  at the roots of the blades. When the impeller  100  rotates about its axis of rotation, an axial air flow is created, as illustrated by the arrows. In accordance with the present invention, the splitter blades  106  in  FIG. 1  are connected to the hub  102  such that their axial position relative to the full blades fall between the leading edges  112  and the trailing edges  114  of the full blades  104 . This will be discussed in more detail in  FIG. 2 .  
         [0014]     Referring for a moment to  FIG. 5 , a discussion of the cross-sectional view of a fan blade is given. The figure shows various parameters for fan blades which define, in part, the cross-sectional shape  514  of the fan blade. Each cross-section of the blade (referred to as an airfoil section) has a leading edge  516 , a trailing edge  518 , an upper surface  522 , and a lower surface  524 . The cross-section  514  may be further defined by the stagger angle  526 , the camber angle  528 , a chord line  532 , its chord length (denoted by “c”)  534 , a mean camber line  536 , and a thickness  538  measurement. In prior art fans, the chord length  534  typically is substantially the same for each fan blade comprising the fan.  
         [0015]     Continuing with  FIG. 2 , in accordance with the present invention, two or more splitter blades can be disposed between a pair of full blades. While the embodiment of  FIG. 1  shows one splitter blade between a pair of full blades,  FIG. 2  shows an example where two splitter blades are provided between a pair of full blades. Of course, additional numbers of such splitter blades may be provided. The chord lengths of the full blades, denoted respectively by c 1  and c 4 , are greater than the chord lengths of the splitter blades, denoted respectively by c 2  and c 3 .  
         [0016]     It is noted that the stagger angle and the camber angle of the splitter blades need not be the same as those of the full blades. In general, the splitter blades can have different stagger angles, camber angles, and chord lengths.  
         [0017]     It is further noted that chord lengths c 1 , c 4  can be equal or different values. Similarly, the chord lengths c 2 , c 3  of the splitter blades can be equal or different values. It is further noted that in the case where full blades have different chord lengths, the full blades should be arranged symmetrically about the hub to which the full blades attach so that their chord lengths are symmetrically distributed about the hub. Similarly, the splitter blades should be arranged about the hub such that their chord lengths are symmetrically distributed about the hub. This symmetrical distribution about the hub ensures that the impeller is balanced so as to avoid wobble during operation of the fan.  
         [0018]      FIG. 3  shows a simple embodiment of the present invention. A single splitter blade  302  is positioned so that the leading edge of the splitter blade is downstream of the leading edges of the corresponding pair of full blades  304   a,    304   b  (collectively  304 ), and likewise the trailing edge of the splitter blade  302  is upstream of the trailing edges of the full blades  304 . As commonly understood, the “upstream” direction refers to a direction pointing into the airflow (shown by the arrows in  FIG. 3 ). Conversely, the “downstream” direction refers to the direction of the airflow. Thus, the splitter blade  302  is disposed between the leading edge of the full blades and the trailing edges of the full blades.  
         [0019]     Similarly for the case where there are two or more splitter blades between their associated full blades, such as shown in  FIG. 2 , the leading edge of each splitter blade is downstream of the leading edges of the associated pair of full blades and the trailing edge of each splitter blade is upstream of the trailing edges of the associated full blades. Stated differently, each splitter blade is disposed between the leading edged of its corresponding full blades and the trailing edges of the corresponding full blades.  
         [0020]     Thus in general, the chord length can be the same for each splitter blade, while the other end of the spectrum, the chord length can be different for each splitter blade. In other embodiments, the chord length varies among some of the splitter blades. As noted above, the other parameters (e.g., stagger angle, camber angle) can be fixed or variable among the splitter blades. In some embodiments, the number of splitter blades between each pair of full blades is the same. In other embodiments, the number of splitter blades between a pair of full blades varies from pair to pair. It is noted that the splitter blades should be arranged about the hub in symmetric fashion. For example, if the number of splitter blades between pairs of full blades varies, that number should vary in a symmetric manner about the hub.  
         [0021]     In accordance with the present invention, the splitter blades create area compression zones and area expansion zones between a pair of full blades. These compression and expansion zones serve to reduce blade passing noise of the airflow (acoustic wave). Referring to  FIG. 3 , an axially directed airflow is shown by the arrows. It will be understood that as the airflow passes between the pair of full blades  304 , the airflow splits into two flows when it encounters the splitter blade  302 . The acoustic wave of the lower component of the airflow (as shown in  FIG. 3 ) which passes between the splitter blade  302  and the full blade  304   b  is subject to area compression in a compression zone C (i.e., the cross-sectional area is reduced). As the airflow continues in the downstream direction, the spacing between the splitter blade  302  and the full blade  304   b  increases, thus creating an area expansion zone (i.e., the cross-sectional area expands). The acoustic wave expands into this area expansion zone El and as a result of the expansion, the energy in the acoustic wave is reduced and consequently the noise is reduced. As can be seen in  FIG. 3 , the a second expansion zone E 2  is the area expansion zone created by the pair of full blades  304   a,    304   b.    
         [0022]     As indicated above, the chord length can the same for each splitter blade, while the other end of the spectrum, the chord length can be different for each splitter blade. In other embodiments, the chord length varies among some of the splitter blades. In some embodiments, the number of splitter blades between each pair of full blades is the same. In other embodiments, the number of splitter blades between a pair of full blades varies from pair to pair. It is noted that the splitter blades should be arranged about the hub in symmetric fashion. For example, if the number of splitter blades between pairs of full blades varies, that number should vary in a symmetric manner about the hub.  
         [0023]     A fan embodiment according to the present invention can be obtained by replacing the hub  416  shown in  FIG. 4  with the hub  102  shown in  FIG. 1 . An alternate hub configuration is illustrated in  FIG. 2  where two splitter blades are disposed between a pair of full blades.  
         [0024]     It is 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.