Abstract:
An air-cooled generator may be provided with a centrifugal blower configured for rotation on a shaft of the generator. The centrifugal blower may have an axially oriented outlet. An impeller of the blower may have blades with trailing edges oriented at an angle between 80° to about 90° relative to an axis of the shaft of the generator.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to air-cooled high speed electrical generators with integral blowers. 
     Compact high-power generators are desirable for many aircraft applications. Proper cooling of such generators is made challenging as evolving design requirements for such machines have resulted in increasingly smaller and more compact generators. High-power-density generators may require high pressure head to push cooling air through narrow cooling paths. Integrated centrifugal blowers attached directly to a generator shafts are often employed for cooling such generators. While centrifugal blowers may provide high pressure rise with high efficiency, they typically have radial exits. Thus output from these centrifugal blowers may not be aligned with cooling paths in the generator, many of which may be oriented axially. Output flow may be changed to an axial direction with a radial-to-axial diffuser. However such diffusers take up space in a generator and may produce significant pressure loss when employed in small spaces of a compact generator. 
     As can be seen, there is a need for a compact high-speed generator in which pressurized cooling air may be directed axially into cooling paths of the generator without use of a radial-to-axial diffuser. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, an air-cooled generator may comprise a centrifugal blower with an impeller configured for rotation on a shaft of the generator, the centrifugal blower having an axially oriented outlet. 
     In another aspect of the present invention, a centrifugal blower may comprise: an impeller rotatable about an axis; a plurality of blades attached to a body of the impeller, wherein the blades have trailing edges oriented at an angle between about 80° to about 90° relative to the axis. 
     In still another aspect of the invention, a method for cooling a generator may comprise the steps of: rotating an impeller attached to a shaft of the generator to compress air against a shroud attached to a housing of the generator; and discharging the compressed air directly from trailing edges of blades of the impeller in a direction parallel to an axis of the generator. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial cross-sectional view of an air-cooled generator in accordance with an embodiment of the invention; 
         FIG. 2  is a perspective view of an impeller for a centrifugal blower in accordance with an embodiment of the invention; 
         FIG. 3  is a meridional profile diagram of an impeller blade in accordance with an embodiment of the invention; and 
         FIG. 4  is a flow chart of a method for cooling a generator in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description is of the best currently contemplated modes of carrying out exemplary embodiments of the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
     Various inventive features are described below that can each be used independently of one another or in combination with other features. 
     Broadly, embodiments of the present invention generally provide a compact high-speed generator with an integral centrifugal blower having an axially oriented outlet for pressurized cooling air. 
     Referring now to  FIG. 1 , an exemplary embodiment of a generator  10  may comprise a centrifugal blower  11 , a housing  12 , a rotor  14  and a stator  16 . A blower impeller  18  may be attached to rotate with a shaft  20  of the rotor  14 . 
     In an exemplary embodiment of the generator  10 , rotational speed may be about 12,000 rpm. Consequently, the impeller  18  may be designed to produce a desired mass flow at a desired pressure when rotated at the rotational speed of the generator  10 . An outlet  22  of the impeller  18  may be positioned at one end of the generator  10  so that pressurized air emerging from the impeller  18  may flow axially along cooling paths  24  (indicated by arrows  24 ) through the generator  10 . 
     Referring now to  FIG. 2 , it may be seen that an exemplary embodiment of the impeller  18  may comprise a body  26  and a plurality of blades  28 . Referring back to  FIG. 1 , it may be seen that leading edges  32  of the blades  28  may be oriented at an angle of about 80° to about 90° relative to an axis  30  of the generator  10 . Trailing edges  34  of the blades  28  may be oriented at an angle of about 80° to about 90° relative to the axis  30 . Because the trailing edges  34  may be substantially normal to the axis  30 , pressurized air may emerge from the impeller  18  as axially oriented cooling-air flow. 
     It may be noted that a shroud region  36  (hereinafter shroud  36 ) of the housing  12  may be shaped to provide a uniform clearance from the blades  28 . The clearance may be 0.010 inch to about 0.030 inch. Clearances in this range may be small enough to provide minimal pressure loss while still being large enough to accommodate normal variations in dimensions of components that arise during manufacturing and assembly. It may also be noted that the shroud  36  may be produced as a separate component and bolted to the housing  12  during assembly of the generator  10 . When the impeller  18  and the shroud  36  are assembled together, the combination of these elements may comprise a centrifugal blower  38 . 
     Referring now to  FIG. 3 , a meridional profile diagram  40  may illustrate various features of an exemplary embodiment of the blades  28  of the impeller  18 . In the embodiment of  FIG. 3 , the blades  28  may be shaped in accordance with a relationship:
 
ε=( R   2m   −R   1m )/ Lx,  
 
where:
 
     R 2m  is blade mean radius (relative to the axis  30 ) at the trailing edge  34 ; 
     R 1m  is blade mean radius (relative to the axis  30 ) at leading edge  32 ; 
     Lx is the blade axial length; and 
     ε may be greater than 0.5. 
     Along a mean streamline  41  from blade leading edge  32  to trailing edge  34 , the blade  28  may comprise three sections. An inducer portion  44  which may contain the blade leading edge  32  and a small length of blade with slightly growing mean streamline radius. The inducer portion  44  may suck in ambient air to the impeller  18  from an axial direction and may turn the air it into a slightly radial direction. A mid portion  45  may be constructed with a rapid radial dimension increase within a relatively short axial distance. The mid portion  45  may provide energy transfer by imparting energy from the impeller  18  to the air stream by accelerating the flow via centrifugal effect associated with radial displacement change. An exducer portion  46  at an exit end of the blade  28  may contain the blade trailing edge  34  and a mean streamline with slightly growing radius. The exducer portion  46  may further compress the air while redirecting the flow to an axial direction to match the orientation of the generator cooling path entrance. Shape of the blade  28  may also be defined by a tip contour  42  and a hub contour  43 . Both contours  42  and  43  may be defined by smooth spline curves with their tangents at the blade leading edge  32  and trailing edge  34  parallel to or nearly parallel to the generator rotating axis  30 . The blade tip contour  42  may conform to the contour of the stationary shroud  36  of the housing  12  to form a preset uniform tip clearance. 
     Referring now to  FIG. 4 , a flow chart  400  may illustrate an exemplary method which may be employed for cooling a generator. In a step  402 , an impeller, attached to a shaft of the generator, may be rotated to compress air against a shroud attached to a housing of the generator (e.g., the impeller  18  may be rotated on the shaft  20  of the generator  10  to compress air against the shroud  36 ). In a step  404 , the compressed air may be discharged directly from a trailing edge of the impeller in a direction parallel to an axis of the generator (e.g., the air may be discharged into the cooling paths  24  directly from trailing edges  34  of the impeller  18 ). 
     It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.