Abstract:
A fan assembly ( 10 ) includes a shrouded fan rotor ( 24 ) including a plurality of fan blades ( 28 ) extending from a rotor hub ( 30 ) and rotatable about a central axis ( 26 ) of the fan assembly and a fan shroud ( 32 ) extending circumferentially around the fan rotor ( 24 ) and secured to the plurality of fan blades ( 28 ). The shroud ( 32 ) has a first axially extending annular portion ( 38 ) secured to the plurality of fan blades ( 28 ), a second axially extending annular portion ( 40 ) radially outwardly spaced from the first axially extending annular portion ( 38 ), and a third portion ( 44 )connecting the first ( 38 ) and second ( 40 ) axially extending annular portions. A casing ( 22 ) is positioned circumferentially around the fan shroud ( 32 ) defining a radial clearance between the casing and the fan shroud. The casing ( 22 ) includes a plurality of casing elements ( 48 ) extending from a radially inboard surface ( 46 ) of the casing toward the shroud ( 32 ) and defining a radial element gap and an axial element gap.

Description:
BACKGROUND 
       [0001]    The subject matter disclosed herein relates to shrouded axial flow fans. More specifically, the subject matter disclosed herein relates to structure to reduce aerodynamic noise and increase stall margin of shrouded axial flow fans. 
         [0002]    Axial flow fans are widely used in many industries ranging from automotive to aerospace to HVAC but are typically limited in their application by operating range restrictions and noise considerations. While vane-axial fans can achieve high static efficiencies, noise generation from fluid interaction between the rotating fan and the stationary stator vanes often limits their use considerably. Further restrictions imposed by limited operating range due to blade stall typically make the vane-axial fan impractical for use in systems requiring appreciable static pressures without resorting to high rotational speeds, thereby compounding existing noise problems. Of particular importance to the stability and operating range of the axial fan is the nature of the tip clearance or shroud recirculation flow. In this case, a rotating shrouded fan is considered in which a circumferential band unitarily connects the outboard tips of the blades. 
       BRIEF DESCRIPTION 
       [0003]    In one embodiment, a fan assembly includes a shrouded fan rotor including a plurality of fan blades extending from a rotor hub and rotatable about a central axis of the fan assembly and a fan shroud extending circumferentially around the fan rotor and secured to the plurality of fan blades. The shroud has a first axially extending annular portion secured to the plurality of fan blades, a second axially extending annular portion radially outwardly spaced from the first axially extending annular portion, and a third portion connecting the first and second axially extending annular portions. A casing is positioned circumferentially around the fan shroud defining a radial clearance between the casing and the fan shroud. The casing includes a plurality of casing elements extending from a radially inboard surface of the casing toward the shroud and defining a radial element gap between a first element surface and a maximum radius point of the shroud and an axial element gap between a second element surface and an upstream end of the fan shroud. 
         [0004]    In another embodiment, a casing assembly for an axial flow fan includes a casing inner surface extending circumferentially around a central axis of the fan. A plurality of casing elements extend radially inwardly from the casing inner surface. Each casing element includes a first element surface defining a radial element gap between the first element surface and a fan rotor, and a second element surface defining an axial element gap between the second element surface and an upstream end of the fan rotor. 
         [0005]    These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0007]      FIG. 1  is a perspective view of an embodiment of a fan assembly; 
           [0008]      FIG. 2  is a partial cross-sectional view of an embodiment of a fan assembly illustrating a fan shroud and casing interface; 
           [0009]      FIG. 2A  is a partial cross-sectional view of another embodiment of a fan assembly illustrating a fan shroud and casing interface; 
           [0010]      FIG. 2B  is a partial cross-sectional view of yet another embodiment of a fan assembly illustrating a fan shroud and casing interface; 
           [0011]      FIG. 3  is an isometric view of an embodiment of a casing for a fan assembly; 
           [0012]      FIG. 3A  is a partial cross-sectional view of another embodiment of a casing for a fan assembly; 
           [0013]      FIG. 4  is another partial cross-sectional view of an embodiment of a fan assembly illustrating a fan shroud and casing interface; 
           [0014]      FIG. 4   a  is a partial cross-sectional view of another embodiment fan assembly illustrating a fan shroud and casing interface; 
           [0015]      FIG. 5  is another upstream-facing cross-sectional view of an embodiment of a rotor casing illustrating angles formed between casing wedge sides and tangents to the casing; 
           [0016]      FIG. 6  is a plan view of an interior of an embodiment of a casing; 
           [0017]      FIG. 7  is a perspective view illustrating an embodiment of circumferentially swept stator vanes; 
           [0018]      FIG. 8  is a cross-sectional view illustrating an embodiment of axially swept stator vanes; and 
           [0019]      FIG. 9  is a perspective view illustrating an embodiment of circumferentially swept fan blades. 
       
    
    
       [0020]    The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawing. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    Shown in  FIG. 1  is an embodiment of an axial-flow fan  10  utilized, for example in a heating, ventilation and air conditioning (HVAC) system as an air handling fan. The fan  10  may be driven by an electric motor  12  connected to the fan  10  by a shaft (not shown), or alternatively a belt or other arrangement. In operation, the motor  12  drives rotation of the fan  10  to urge airflow  16  across the fan  10  and along a flowpath  18 , for example, from a heat exchanger (not shown). The fan  10  includes a casing  22  with a fan rotor  24 , or impeller rotably located in the casing  22 . Operation of the motor  12  drives rotation of the fan rotor  24  about a fan axis  26 . The fan rotor  24  includes a plurality of fan blades  28  extending from a hub  30  and terminating at a fan shroud  32 . The fan shroud  32  is connected to one or more fan blades  28  of the plurality of fan blades  28  and rotates about the fan axis  26  therewith. In some embodiments, the fan  10  further includes a stator assembly  72  including a plurality of stator vanes  74 , located either upstream or downstream of the fan rotor  24 . In some embodiments, the fan  10  has a hub  30  diameter to fan blade  28  diameter ratio between about 0.45 and 0.65. Further the fan  10  nominally operates in a rotational speed between about 1500 RPM and about 2500 RPM with a fan blade  28  tip speed of about 0.1 Mach or less. 
         [0022]    Referring to  FIG. 2 , the fan shroud  32  defines a radial extent of the fan rotor  24 , and defines running clearances between the fan rotor  24 , in particular the fan shroud  32 , and the casing  22 . During operation of the fan  10 , a recirculation flow  70  is established from a downstream end  34  of the fan shroud  32  toward an upstream end  36  of the fan shroud  32 , where at least some of the recirculation flow  70  is reingested into the fan  10  along with airflow  16 . This reingestion may be at an undesired angle or mass flow, which can result in fan instability or stall. To alleviate this, the fan shroud  32  extends substantially axially from the downstream end  34  of the fan shroud  32  toward the upstream end  36  of the fan shroud  32  along a first portion  38  for a length L 1 , which may be a major portion (e.g. 80-90%) of a total shroud length L tot . The first portion  38  of the fan shroud  32  is connected to the fan blades  28 . A second portion  40  of the fan shroud  32  also may extend in an axial direction, but is offset radially outwardly from the first portion  38 , and defines a maximum radius  42  of the fan shroud  32 . A third portion  44  connects the first portion  38  and the second portion  40 . In some embodiments, as shown in  FIG. 2 , this results in a substantially s-shaped cross-section of the fan shroud  32 . In other embodiments, for example, as shown in  FIGS. 2   a - 2   b , the resulting cross-section is T-shaped and J-shaped, respectively. During operation, the fan shroud  32  forms a separation bubble  76  of flow between the upstream end  36  and the casing  22 . This separation bubble  76  is a small recirculation zone that creates an effectively smaller running clearance gap  78  between upstream end  36  and casing  22 , thereby limiting the amount of recirculation flow  70  through the running clearance gap  78 . 
         [0023]    The casing  22  includes a casing inner surface  46 , which in some embodiments is substantially cylindrical or alternatively a truncated conical shape, extending circumferentially around the fan shroud  32 . Further, the casing  22  includes a plurality of casing elements, or casing wedges  48  extending radially inboard from the casing inner surface  46  toward the fan shroud  32  and axially at least partially along a length of the fan shroud  32 . The casing wedges  48  may be separate from the casing  22 , may be secured to the inner surface  46 , or in some embodiments may be formed integral with the casing  22  by, for example, injection molding. While the description herein relates primarily to casing wedges  48 , in other embodiments other casing elements, such as casing fins  148  shown in  FIG. 3   a , may be utilized. 
         [0024]    Referring to  FIG. 3 , the casing wedges  48  are arrayed about a circumference of the casing  22 , and in some embodiments are at equally-spaced intervals about the circumference. The number of casing wedges  48  is variable and depends on a ratio of wedge width A of each wedge to opening width B between adjacent wedges expressed as A/B as well as a ratio of wedge width A to fan shroud  32  circumference, expressed as A/πD, where D is a maximum diameter of the fan shroud  32 . In some embodiments, ratio A/B is between 0.5 and 4, though may be greater or lesser depending on an amount of swirl reduction desired. In some embodiments, ratio A/πD is in the range of about 0.01 to 0.25. Further, the number of casing wedges  48  may be selected such as not to be a multiple of the number of fan blades  28  to avoid detrimental tonal noise generation between the recirculation flow  70  emanating from the casing wedges  48  and the rotating fan blades  28 . In some embodiments, the fan rotor  24  has 7, 9 or 11 fan blades  28 . 
         [0025]    Referring again to  FIG. 2 , the casing wedges  48  in some embodiments are shaped to conform to and wrap around the second portion  40  of the fan shroud  32 , leaving minimum acceptable running clearances between the casing wedges  48  and the fan shroud  32 . Thus, as shown in  FIG. 4 , the casing wedges  48  result in an axial step S 1  from a forward end  52  of the casing  22  and a radial step S 2  from the casing inner surface  46  at each casing wedge  48  around the circumference of the casing  22 . A magnitude of the step S 1  is between 1*G F  and 20*G F , where G F  is an axial offset from a forward flange  50  of the casing  22  to the second portion  40  of the fan shroud  32 . Similarly, a magnitude of S 2  is between 1*G S  and 20*G s , where G S  is a radial offset from the maximum radius location  42  to a radially inboard surface  52  of the casing wedge  48 . An axial wedge length  54  is between 25% and 100% of an axial casing length  56 . Further, the radially inboard surface  52 , while shown as a substantially radial surface, may be tapered along the axial direction such that S 2  decreases, or increases, along the axial wedge length  54  from an upstream casing end  58  to a downstream casing end  60 . A forward wedge surface  62 , which defines S 1 , while shown as a flat axial surface, may be similarly tapered such that S 1  decreases, or increases or both, with radial location along the forward wedge surface  62 . In other embodiments, forward wedge surface  62  may have a curvilinear cross-section. 
         [0026]    Referring to  FIG. 4   a , the forward wedge surface  62  of some embodiments may coincide with the forward casing surface  58 . In such cases, the forward axial step  51  is zero. The forward casing surface  58  may be a constant radial surface or may be a curvilinear surface. 
         [0027]    Referring to  FIG. 5 , wedge sides  64   a  and  64   b  of the casing wedges  48  form angles α and β, respectively at an intersection with a tangent of the casing inner surface  46 , where side  64   a  is a leading side relative to a rotation direction  66  of the fan rotor  24  and  64   b  is a trailing side relative to the rotation direction  66 . In some embodiments, α and β are in the range of 30° and 150° and may or may not be equivalent, complimentary or supplementary. The wedge sides  64   a  and  64   b  may be, for example, substantially planar as shown or may be curvilinear along a radial direction. 
         [0028]    Referring to  FIG. 6 , in the axial direction, wedge sides  64   a  and  64   b  form angles K and λ respectively with the upstream casing end  58 . In some embodiments, K and λ are between 90° and 150°, while in other embodiments, K and λ may be less than 90°. In embodiments where the casing wedges  48  are co-molded with the casing  22 , K and λ greater than 90° are desired to enable the use of straight pull tooling. With other manufacturing methods, however, K and λ of less than 90° may be desirable. Angles K and λ may or may not be equivalent, supplementary or complimentary. Further, while the wedge sides  64   a  and  64   b  are depicted as substantially planar, they may be curvilinear along the axial direction. 
         [0029]    Selecting angles α, β, K, and λ and axial and radial steps S 1  and S 2  as well as gaps G F  and G s  allows a reinjection angle of the recirculation flow  70  and a mass flow of the recirculation flow  70  to be selected and controlled. 
         [0030]    Referring now to  FIGS. 7 and 8 , in some embodiments, the stator vanes  74  are positioned to include lean or sweep in a circumferential and/or axial direction. The stator vanes  74  straighten flow  16  exiting from the fan rotor  24 , transforming swirl kinetic energy in the flow  16  into static pressure rise across the stator vanes  74 . As shown in  FIG. 7 , each vane  74  has a stacking axis  80  that extends from a vane base  82  at a stator hub  84  outwardly to a vane tip  86  at a stator shroud  88 . At the vane base  82 , the stacking axis  80  leans circumferentially from a radial direction at an angle r 1  of about 10 degrees to about 25 degrees toward a swirl direction  90  of the flow  16 . This degree of lean continues for about 75% of vane  74  span, where it changes direction to lean away from the swirl direction  90  at an angle r 2  of about 20 degrees to about 40 degrees. Further, as shown in  FIG. 8 , the vanes  74  include an axial sweep of the stacking axis  80 . This axial sweep results in a reduced level of rotor-stator interaction noise, while maintaining aerodynamic performance characteristics of the fan  10 . 
         [0031]    Referring now to  FIG. 9 , in some embodiments, the fan blades  28  include circumferential lean or sweep. Each fan blade  28  has a blade stacking axis  92  that leans circumferentially from a radial direction at an angle r 3  between −60 degrees and +60 degrees. Circumferential fan blade  28  sweep is used to selectively drive flow inboard or outboard along the blade span to provide the desired rotor outflow profile to be seen by the stator vanes  74 . Using this technique, multiple fan blade  28  designs can be produced in which the operating range of the rotor-stator combination is shifted to either lower or higher volume flow rates while using the same stator vane  74  design. Here, the circumferential fan blade  28  lean is tailored to produce the correct rotor outflow profile, thereby allowing the stator vanes  74  to still operate effectively. The fan blade  28  may be swept circumferentially forward into the incoming flow  16  to drive flow inboard to the rotor hub  30 , may be swept circumferentially rearward to drive flow outboard to the tip region of the fan blade  28 , or may be swept circumferentially in a combination of the two to migrate flow within the blade passage as desired, with the possibility of simultaneously driving flow inboard towards the hub  30  and outboard towards the tip. The amount of circumferential fan blade  28  sweep will depend on the amount of flow migration desired for the particular application and will be dictated largely by the stator vane  74  design and the desired operating envelope. Another significant result of the use of circumferentially swept fan blades  28  is to aid in the dephasing of the interaction between the fan blade  28  wakes and the stationary stator vanes  74 , thereby reducing the noise level of the fan  10  allowing for use of fan  10  in noise-limited environments such as residential environments. 
         [0032]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.