Patent Publication Number: US-9885368-B2

Title: Stall margin enhancement of axial fan with rotating shroud

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. provisional application, 61/651,277, filed May 24, 2012, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The subject matter disclosed herein relates to shrouded axial fans. More specifically, the subject matter disclosed herein relates to structure to enhance stall margin of shrouded axial fans. 
     Axial flow fans are susceptible to leakage flow from the high pressure side to low pressure side of the fan blades, typically a flow from the downstream side of the fan to the upstream side of the fan. The leakage flow occurs at either the fan blade tip, specifically between the tip and the casing in an unshrouded fan, or between the shroud and the casing in the case of a shrouded fan. This leakage flow is reingested into the fan at, for example, a front clearance gap between the shroud and the casing, at a leading edge of the shroud. As the leakage flow reenters the fan, it gives rise to rotating swirl flow and instabilities at the blade tip, often causing the flow at the blade tip to separate and stall prematurely. The result is a generally limited stable operating range for a typical axial flow fan that is limited in its range of applications. Many configurations of “casing treatments” have been developed to address the leakage flow issue, most of which are specifically applicable to unshrouded axial fans or impellers used in high-speed compressor applications, while only a limited number are suitable for use with shrouded fans. In one such case, a number of vanes extend from the interior of the casing toward the shroud to reduce swirl in the recirculating flow. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one embodiment, a fan assembly includes a shrouded fan rotor including a plurality of fan blades 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 substantially S-shaped cross-section along an axial direction. A casing is located circumferentially around the fan shroud defining a radial clearance between the casing and the fan shroud. The casing includes a plurality of casing wedges extending from a radially inboard surface of the casing toward the shroud and defining a radial wedge gap between a first wedge surface and a maximum radius point of the shroud and an axial wedge gap between a second wedge surface and an upstream end of the fan shroud. 
     In another embodiment, a casing for an axial flow fan includes a casing inner surface extending circumferentially around a central axis of the fan. A plurality of casing wedges extends radially inwardly from the casing inner surface. Each casing wedge includes a first wedge surface defining a radial wedge gap between the first wedge surface and a fan rotor and a second wedge surface defining an axial wedge gap between the second wedge surface and an upstream end of the fan rotor. 
     In yet another embodiment, a fan assembly includes a shrouded fan rotor having a plurality of fan blades extending from a rotor hub and rotatable about a central axis of the fan assembly. A fan shroud extends circumferentially around the fan rotor and secured to the plurality of fan blades. The shroud includes 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 located circumferentially around the fan shroud defining a radial clearance between the casing and the fan shroud. The casing includes a plurality of casing wedges extending from a radially inboard surface of the casing toward the shroud and defining a radial wedge gap between a first wedge surface and a maximum radius point of the shroud and an axial wedge gap between a second wedge surface and an upstream end of the fan shroud. 
     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 
       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: 
         FIG. 1  is a perspective view of an embodiment of a fan assembly; 
         FIG. 2  is a partial cross-sectional view of an embodiment of a fan assembly illustrating a fan shroud and casing interface; 
         FIG. 2A  is a partial cross-sectional view of another embodiment of a fan assembly illustrating a fan shroud and casing interface; 
         FIG. 2B  is a partial cross-sectional view of yet another embodiment of a fan assembly illustrating a fan shroud and casing interface; 
         FIG. 3  is a partial cross-sectional view of an embodiment of a casing for a fan assembly; 
         FIG. 4  is another partial cross-sectional view of an embodiment of a fan assembly illustrating a fan shroud and casing interface; 
         FIG. 4 a    is a partial cross-sectional view of another embodiment fan assembly illustrating a fan shroud and casing interface; 
         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; and 
         FIG. 6  is a plan view of an interior of an embodiment of a casing. 
     
    
    
     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 
     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. The fan  10  may be driven by an electric motor (not shown) connected to the fan  10  by a shaft, or alternatively a belt or other arrangement. In operation, the motor drives rotation of the fan  10  about a fan axis  26  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 . 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 (not shown) located either upstream or downstream of the fan rotor  24 . 
     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 fan shroud  32  then includes an outwardly flaring second portion  40 , which extends from the first portion  38  and transitions from an outwardly concave to an outwardly convex shape at a maximum radius location  42 . From the maximum radius location a tapering third portion  44  extends to the upstream end  36 . In some embodiments, 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. 
     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 wedges  48  extending radially inboard from the casing inner surface  46  toward the fan shroud 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. 
     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 0.05 and 2, 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 0.002 to 0.2. 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 . 
     Referring again to  FIG. 2 , the casing wedges  48  in some embodiments are shaped to conform to and wrap around the S-shaped second portion  40  and third portion  44  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. 
     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 S 1  is zero. The forward casing surface  58  may be a constant radial surface or may be a curvilinear surface. 
     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. 
     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. 
     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. 
     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.