Abstract:
An axial fan assembly including a casing wall with a forward facing step formed therein and a fan rotor with blade tips, each having an aft facing step which radially overlaps the casing step so as to reduce the clearance backflow loss in the assembly. A vane is attached to the suction side of each of the blade tips with the vane having an aft facing step which radially overlaps the casing forward facing step to promote further reduction of clearance backflow. Variations on the invention include the option of an additional inlet bellmouth piece that further restricts the clearance flow and wedges integral to the casing step to improved flow stability.

Description:
BACKGROUND OF THE INVENTION  
       [0001]    This invention relates generally to axial flow fans and, more particularly, to a method and apparatus for reducing their clearance flow losses. 
         [0002]    Axial flow fans are used in a wide variety of applications, including HVAC, refrigeration, automotive, power systems and aerospace. In each of these applications, efficiency and space limitations are especially important considerations. 
         [0003]    Significant efficiency loss occurs in axial flow fans due to backflow in the clearance region between the fan rotor and the casing. The rotor may utilize conventional blades that extend outward with blade tips approaching the casing, or it may utilize blades that include a rotating shroud attached to the blade tips. In either case backflow is driven from the high pressure side of the rotor to the suction side across the clearance gap, leading to reduced performance, increased noise level and reduced stability and stall-margin. 
         [0004]    Various designs have been proposed for increasing fan efficiency by reducing or controlling clearance flows. The designs generally involve an interruption or decrease in the size of the gap. One approach is the use of a tip seal structure wherein a circumferentially extending groove in the casing circumscribes the tips of the blades as shown and described in U.S. Pat. No. 4,238,170. In another approach, an axial fan is provided with a casing having a bellmouth, and the shroud is so formed as to create a separation bubble between the bellmouth and the shroud in order to limit the circulation flow as shown in U.S. Pat. No. 7,086,825 assigned to the assignee of the present invention. 
         [0005]    Fan stability is affected by rotating flows within the clearance gap. These flows tend to develop into organized rotating cells which can lead to strong through-flow oscillations and excessive noise. 
         [0006]    Various designs have been proposed to improve fan stability by controlling these rotating flows. These designs are generally classified as casing treatment. 
       SUMMARY OF THE INVENTION  
       [0007]    Briefly, in accordance with one aspect of the invention, a sharp, forward facing step is provided in the fan casing which, when combined with an overlapping rearward facing step in the fan blade tips, tends to disrupt the backflow so as to thereby restrict clearance flow loss. 
         [0008]    In accordance with another aspect of the invention, each of the blades has an attached vane on its suction side, with the vanes having a rearward facing step that overlaps the casing forward facing step. 
         [0009]    In the drawings as hereinafter described, a preferred embodiment is depicted; however, various other modifications and alternate constructions can be made thereto without departing from the spirit and scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a perspective view of an axial fan assembly in accordance with the present invention. 
           [0011]      FIG. 2  is an enlarged view of a portion thereof. 
           [0012]      FIGS. 3A and 3B  are respective front and end views of a normal blade tip. 
           [0013]      FIG. 3C  is an axial cross sectional view thereof in relationship to the casing. 
           [0014]      FIGS. 4A and 4B  are respective front and end views of a blade tip with a step in accordance with the present invention. 
           [0015]      FIGS. 5A and 5B  are respective front and end views of a blade tip with a vane in accordance with the present invention. 
           [0016]      FIG. 6  is a suction side view of a blade tip and vane in accordance with the present invention. 
           [0017]      FIG. 7  is a pressure side view of a blade tip and vane in accordance with the present invention. 
           [0018]      FIG. 8  is a radially inward view of a blade tip and vane in accordance with the present invention. 
           [0019]      FIG. 9  is an axial cross sectional view of the  FIGS. 4A and 4B  embodiment of the blade tip in relationship to the casing. 
           [0020]      FIG. 10  is an axial cross sectional view as seen along lines  10 - 10  of  FIG. 2 . 
           [0021]      FIG. 11  is an axial cross sectional view as seen along lines  11 - 11  of  FIG. 2 . 
           [0022]      FIG. 12  is a partial view thereof showing the flow of air therein. 
           [0023]      FIG. 13  is an axial cross sectional view of the apparatus as shown in  FIG. 11  but with an added inlet bellmouth insert. 
           [0024]      FIG. 14  is a perspective view of an axial fan in accordance with an alternative embodiment of the invention. 
           [0025]      FIG. 15  is an enlarged view of a portion thereof. 
           [0026]      FIG. 16  is an axial end view thereof. 
           [0027]      FIGS. 17A and 17B  are other perspective views thereof. 
           [0028]      FIG. 18  is an axial end view of another alternative embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]    Referring to  FIGS. 1 and 2 , the invention is shown generally at  10  as applied to an axial fan assembly  11  that includes in serial airflow relationship an axial fan  12  and a stator  13 . The axial fan  12  includes a rotatable hub  14  and a plurality of fan blades  16 . The stator  13  includes a stationary hub and a plurality of radially extending stationary vanes  17  having their radially outer ends integrally connected to a cylindrical outer housing  18 . In operation, the fan  12  is rotated at relatively high speeds to induce the flow of air through the casing  18 , and in the process it creates a swirl in the direction of the fan rotation. The stator vanes  17  are so disposed and shaped as to substantially remove the swirl from the main airflow stream such that the flow at the downstream end is substantially axial in direction. 
         [0030]    As is well known in the art, the dimensions of the axial fan  12  are such that the radial clearance between the ends of the fan blades  16  and the inner diameter of the casing  18  are as small as possible but without engagement between the two elements. Because of this necessary radial clearance, there is a tendency for the air within the casing  18  to flow back through the radial gap to the forward side of the fan  12 . This results directly in reduced pressure rise and efficiency. The present invention is intended to significantly reduce the backflow. 
         [0031]    Referring now to  FIGS. 3A and 3B , a normal blade is shown at  16 A, with a generally planar tip being shown in  FIG. 3B . That is, the blade tip is slightly curved to accommodate the curved inner diameter of the casing  18 A, but is of a substantially constant radius throughout the length of the blade tip. The blade tip of blade  16 A in combination with a standard casing  18 A is shown in  FIG. 3C . 
         [0032]    In  FIGS. 4A and 4B , the blade  16 B is shown to have a blade tip with a rearwardly facing (i.e. toward the downstream or pressure side of the blade  16 B) step as shown at  19 . That is, that portion  21  of the blade tip nearest the leading edge is of one fixed radius and that portion  22  thereof nearest the trailing edge is of a constant reduced radius. The face of the step  19  is generally planar in form and is aligned tangentially (i.e. normal to the fan axis). 
         [0033]    Referring now to  FIG. 9 , where the blade  16 B is shown with its blade tip profile that includes the rearwardly extending step  19  and the leading edge portion  21  and trailing edge portion  22 . Here it will be seen that the casing  18 B includes a matching forward facing step  23  which interconnects a larger radius portion  24  and a smaller radius portion  26  of the casing  18 . The forward facing step  23  is a generally planar surface and is aligned tangentially such that the rearwardly facing step  19  is generally parallel with and in close proximity to the forwardly facing step  23 . Similarly, the blade tip leading edge portion  21  is closely radially spaced from the larger radius portion  24 , and the trailing edge portion  22  of the blade tip is closely radially spaced from the smaller radius portion  26  of the casing  18 . This combination is provided for the purpose of reducing the backflow and its associated swirl that would otherwise result in a normal blade tip and casing relationship as shown in  FIGS. 3A and 3B . 
         [0034]    Referring now to  FIGS. 5A and 5B , a blade  16 C is shown with a rearwardly facing step  19 , leading edge portion  21  and trailing edge portion  22  as shown in  FIGS. 4A and 4B . However, the blade  16 C is further modified to include a vane  27  which is attached to the suction side of the blade as shown in  FIG. 2  and which forms part of the blade tip as shown in  FIGS. 5A and 5B . 
         [0035]    The vane  27  can best be seen in  FIGS. 6 ,  7  and  8  where it is shown as being attached to the blade  16 C.  FIG. 6  shows the blade  16  from the suction side,  FIG. 7  shows it from the pressure side and  FIG. 8  shows it from the radially inward direction as shown in  FIG. 8 . As will be seen, the vane  27  forms a part of the blade tip and is placed approximately in the middle of the suction side of the blade  16 C and extends approximately one-third of the way across. The size and shape of the vane  27  can be selectively varied to meet the particular axial fan assembly and operating requirements. 
         [0036]    An important feature of the vane  27  is that it too includes a rearwardly extending step  28  as will be seen in  FIG. 7 . This step  28  also interfaces with the forward facing step  23  of the casing  18 B in a manner similar to the rearwardly facing step  19  of the blade tip as discussed hereinabove to provide a further reduction of backflow that would otherwise occur around the blade tips. This can be seen in  FIG. 11  wherein the rearwardly facing step  28  of the vane  27  is closely aligned with the forward facing step  23  of the casing  18 B. In order to understand the structure of the blade tip of blade  16 C,  FIGS. 10 and 11  should be referred to in combination.  FIG. 10  is a sectional view through the stepped tip at a point forward of the vane  27 , whereas  FIG. 11  is a sectional view thereof at a point that includes both the stepped tip and the vane  27 . 
         [0037]    The design of both the casing and the fan rotor are such that they can be produced using straight-pull tooling (e.g. injection molding or die casting). 
         [0038]    In operation, as will be seen in  FIG. 12 , the relationship of the stepped blade tip and casing produces a convoluted path for the tip clearance leakage flow, which is highly restrictive. The effect is essentially similar to a labyrinth seal where the backflow and recirculation is forced to turn abruptly multiple times. Each flow turning produces a pressure drop which then enables the flow system to withstand a higher differential pressure and a lower leakage loss. 
         [0039]    The embodiment of  FIG. 11  can be used as shown without the use of inlet bellmouth insert. It will operate similarly but will benefit from the further use of an inlet bellmouth insert  29  as shown in  FIG. 13 . 
         [0040]    An alternative embodiment of the present invention is shown in  FIGS. 14-17  wherein the fan blades  16 D have a blade tip vane  31  which extends almost the full tangential span of the blade tip. That is, ends  32  and  33  extend to just short of the edges of the fan blade  16 D as shown. In such a case, the step feature is entirely within the tip vane and not in the blade tip, as shown in  FIG. 17A and 17B  wherein the tip vane  31  is located axially forward of the entire blade tip. 
         [0041]    In  FIG. 18 , there is shown an embodiment wherein the size of the tip vane  34  is lengthened along the tangential direction such that it extends at it two ends just beyond the edges of the blade  16 E. As discussed hereinabove, this variation is in keeping with the practice of selectively varying the size and shape of the vane to meet the particular axial fan assembly and operating requirements. 
         [0042]    It should be understood that the present invention can be used by itself for the reduction of backflow, or it may be used in combination with the wedges that are shown and described in the patent application being filed concurrently herewith and assigned to the assignee of the present invention. 
         [0043]    Although preferred and alternative embodiments of the invention have been disclosed and described, one of ordinary skill in the art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of the invention.