Abstract:
A vane frame for a turbomachine and a method for minimizing the weight of a vane frame. The vane frame includes an inner shroud made up of a plurality of inner shroud segments, an outer shroud circumscribing the inner shroud and made up of a plurality of outer shroud segments, and guide vanes structurally interconnecting the inner and outer shrouds. The guide vanes include a plurality of first guide vanes between and connecting first pairs of the inner and outer shroud segments, and a plurality of second guide vanes between and connecting second pairs of the inner and outer shroud segments. The first and second guide vanes are formed of different first and second materials, respectively, with the first material having lower strength, modulus, and/or density than the second material. The structural interconnection between the inner and outer shrouds is dominated by the second guide vanes.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to turbomachinery and vanes for guiding air flow through a turbomachine. More particularly, this invention relates to a fan outlet guide vane frame comprising sectors of outlet guide vanes formed of different materials, in which some of the sectors are load-bearing and others are not. 
         [0002]    High-bypass turbofan engines are widely used for high performance aircraft that operate at subsonic speeds. As schematically represented in  FIG. 1 , a high-bypass turbofan engine  10  includes a large fan  12  placed at the front of the engine  10  to produce greater thrust and reduce specific fuel consumption. The fan  12  serves to compress incoming air  14 , a portion of which flows into a core engine (gas turbine)  16  that includes a compressor section  18  containing low and high pressure compressor stages  18 A and  18 B to further compress the air, a combustion chamber  20  where fuel is mixed with the compressed air and combusted, and a turbine section  22  where a high pressure turbine  22 A extracts energy from the combustion gases to drive the high pressure stages  18 A of the compressor section  18  and a low pressure turbine  22 B extracts energy from the combustion gases to drive the fan  12  and the low pressure stages  18 B of the compressor section  18 . A larger portion of the air that enters the fan  12  is bypassed to the rear of the engine  10  to generate additional engine thrust. The bypassed air passes through an annular-shaped bypass duct  24  that contains one or more rows of stator vanes, also called outlet guide vanes  28  (OGVs), located immediately aft of the fan  12  and its fan blades  26 . The fan blades  26  are circumscribed by a fan casing  32 , which in turn is surrounded by the fan cowling or nacelle  34  that defines the inlet duct  36  to the turbofan engine  10  as well as a fan nozzle  38  for the bypassed air exiting the bypass duct  24 . 
         [0003]    The outlet guide vanes  28  form part of a vane frame  40  that further includes inner and outer shrouds  42  and  44  at the radially inward and outward extents, respectively, of the guide vanes  28 . A common construction is to form the vane frame  40  of segments, each comprising one or more vanes  28  connecting a pair of inner and outer shroud segments. The outer shroud  44  (formed by the assembly of the outer shroud segments) is secured to the fan casing  32 , while the inner shroud  42  (formed by the assembly of the inner shroud segments) is secured to the core engine  16 , and more particularly to an inner frame (not shown) of the core engine  16 . The fan nacelle  34  is shown in  FIG. 1  as attached to and supported by the core engine  16  through the outlet guide vanes  28 . The guide vanes  28  have cambered airfoil shapes to modify the air flow through the bypass duct  24  to promote deswirling of the fan air, thus improving efficiency and reducing engine noise. 
         [0004]    Because of its dual functions, the vane frame  40  is an important structural component whose design considerations include aerodynamic criteria as well as the ability to provide sufficient structural support and stiffness to the fan nacelle  34  for maintaining the shape of the inlet duct  36  and adequately transitioning static and dynamic loads to the engine core  16 . For these reasons, it is important to select appropriate constructions, materials and assembly methods when manufacturing the vane frame  40  and its individual components, including the guide vanes  28  and inner and outer shrouds  42  and  44  and their connections to the fan casing  32  and core engine  16 . Various materials and configurations for outlet guide vanes have been considered. Metallic materials, and particularly aluminum alloys, have been widely used. Composite materials have also been considered, as they offer the advantage of significant weight reduction. However, in order to be reliably capable of supporting the fan nacelle and transitioning fan nacelle loads, outlet guide vanes formed of composite materials have generally required complex attachment geometries and hardware, which increases weight and manufacturing and material costs. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0005]    The present invention provides a vane frame for a turbomachine and a method for minimizing the weight of a vane frame while maintaining a suitable level of structural strength. The vane frame and method are particularly applicable to fan outlet guide vane frames, in which the guide vanes are formed of different materials and some of the vanes are load-bearing while others are not. 
         [0006]    According to a first aspect of the invention, the vane frame includes an inner shroud comprising a plurality of inner shroud segments, an outer shroud circumscribing the inner shroud and comprising a plurality of outer shroud segments, and guide vanes structurally interconnecting the inner and outer shrouds. The guide vanes comprise a plurality of first guide vanes between and connecting first pairs of the inner and outer shroud segments, and a plurality of second guide vanes between and connecting second pairs of the inner and outer shroud segments. The first and second guide vanes are formed of different first and second materials, respectively, with the first material having lower strength and modulus than the second material. The structural interconnection between the inner and outer shrouds is dominated by the second guide vanes. 
         [0007]    According to a second aspect of the invention, the above-described construction of a vane frame provides for a method capable of minimizing the weight of the vane frame. Namely, by forming the first and second guide vanes of different first and second materials, respectively, and selecting the first material to have a lower density than the second material, the weight of the vane frame can be minimized by constructing the vane frame to comprise more of the first guide vanes than of the second guide vanes, while the structural interconnection between the inner and outer shrouds is dominated by the second guide vanes. 
         [0008]    As evident from the above, a significant advantage of this invention is that, by constructing the vane frame so that most of the load applied to the vane frame is borne by the second guide vanes, the first guide vanes can be formed of lighter-weight and less costly materials, thereby reducing the weight of the vane frame. For example, the first guide vanes can be formed of a composite material, which offers the potential of additional benefits such as the ability to configure the first guide vanes for improved aerodynamic performance. Another advantage is that the first guide vanes can be configured in sectors, in which a relatively large number of guide vanes are between and interconnect a single pair of inner and outer shroud segments, thereby reducing the number of vane frame components, including attachment hardware and seals. 
         [0009]    Other aspects and advantages of this invention will be better appreciated from the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  schematically represents a cross-sectional view of a high-bypass turbofan engine. 
           [0011]      FIG. 2  is a view looking aft at a vane frame of the engine of  FIG. 1 , and represents the vane frame as comprising sectors of outlet guide vane having different constructions in accordance with particular aspects of this invention. 
           [0012]      FIG. 3  is an isolated partial side view representing the connection of a type of guide vane sector shown in  FIG. 2 . 
           [0013]      FIG. 4  is an isolated perspective view representing another type of guide vane sector shown in  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    The present invention provides vane frame constructions suitable for use in turbomachinery, and particularly within the bypass duct of a high-bypass turbofan engine, an example of which is the turbofan engine  10  represented in  FIG. 1 .  FIG. 2  is a view looking aft at the vane frame  40  of  FIG. 1 , and represents the vane frame  40  as comprising fan outlet guide vanes  28  of different constructions ( 28 A and  28 B) in accordance with particular aspects of this invention. While the vane frame  40  is represented as having forty-eight guide vanes  28 , lesser or greater numbers of vanes  28  are foreseeable. 
         [0015]    As discussed in reference to  FIG. 1 , in addition to the guide vanes  28 , the vane frame  40  comprises inner and outer shrouds  42  and  44 , respectively, which are adapted to secure the frame  40  to the inner frame of the core engine  16  and the fan casing  32  of the fan nacelle  34 . Each of the guide vanes  28 A is represented as part of a vane sector  30 A in which multiple vanes  28 A share a pair of inner and outer shroud segments  42 A and  44 A. As indicated in  FIG. 2 , several of the guide vanes  28 B located at the top of the frame  40  are also configured as part of a vane sector  30 B and share a pair of inner and outer shroud segments  42 B and  44 B. This sector  30 B preferably has multiple vanes  28 B to support the higher loads present at the top of the vane frame  40  due to its proximity to the engine forward mount to the wing pylon (not shown). In contrast, each remaining guide vane  28 B is shown as an individual airfoil between a pair of inner and outer shroud segments  42 B and  44 B, yielding a number of single-vane sectors  30 C. The vane sectors  30 B and  30 C are represented in  FIG. 2  as approximately equi-angularly interspersed among the guide vanes  28 A, with angular spacings of either about 52.5 or about 60 degrees. The top vane sector  30 B is represented as having three guide vanes  28 B, though it is within the scope of this invention that the sector  30 B could contain more or fewer vanes  28 B. Similarly, the vane sectors  30 A to either side of the top vane sector  30 B are represented as having six guide vanes  28 A, and the other vane sectors  30 A are represented as containing seven guide vanes  28 A, though it is within the scope of this invention that these sectors  30 A could contain more or fewer vanes  28 A. Additionally, though the remaining vane sectors  30 C contain a single guide vane  28 B, it is foreseeable that any one or more of these vane sectors  30 C could contain multiple guide vanes  28 B. Finally, it should be understood that the invention is not limited to the particular number, placement and shapes of the guide vanes  28 A and  28 B and vane sectors  30 A,  30 B and  30 C depicted in  FIG. 2 . 
         [0016]    According to a particular aspect of the invention, while all of the guide vanes  28 A and  28 B preferably have cambered airfoil shapes to modify the air flow through the bypass duct  24  as discussed previously, the guide vanes  28 B are particularly adapted to provide the primary structural support role for the fan nacelle  34 , whereas the remaining guide vanes  28 A provide less structural support than the guide vanes  28 B as a result of being adapted to individually carry lower (if any) loads between the fan nacelle  34  and the engine core  16 . Furthermore, the structural interconnection between the inner and outer shrouds  42  and  44  is preferably dominated by the guide vanes  28 B, meaning that of the total structural load carried by the guide vanes  28 A and  28 B, and particularly loads transmitted from the fan nacelle  34  through the vane frame  40  to the core engine  16 , is primarily transmitted by the guide vanes  28 B. For example, more than 75% and preferably the entire total load imposed by the fan nacelle  34  is carried by the guide vanes  28 B, with any remaining load possibly though not necessarily being borne by the guide vanes  28 A. 
         [0017]    The “structural” guide vanes  28 B account for only eight of the forty-eight guide vanes  28  shown for the vane frame  40  of  FIG. 2 . Consequently, the structural guide vanes  28 B are formed of a material with higher strength and modulus than the material from which the “nonstructural” guide vanes  28 A are formed. In turn, the material for the guide vanes  28 A can have a lower density than the material for the structural guide vanes  28 B. Particular but nonlimiting examples include metallic materials for the guide vanes  28 B and their inner and outer shroud segments  42 B and  44 B, and composite materials for the guide vanes  28 A and their inner and outer shroud segments  42 A and  44 A. Titanium alloys are believed to be particularly suitable materials for the guide vanes  28 B and the inner and outer shroud segments  42 B and  44 B, which may be formed by conventional fabrication methods to yield one-piece sectors  30 B and  30 C. Suitable composite materials for the guide vanes  28 A and inner and outer shroud segments  42 A and  44 A include carbon and glass laminate or chopped fiber reinforcement materials in thermoset or thermoplastic matrix materials, and hollow, sandwich or syntactic foam-filled materials. The sectors  30 A formed by the guide vanes  28 A and inner and outer shroud segments  42 A and  44 A can be formed by conventional methods, including resin transfer molding (RTM), compression molding, and injection molding each sector  30 A as a unitary molded component. 
         [0018]    In view of the materials noted above, the vane sectors  30 A and particularly their guide vanes  28 A can be considerably lighter and less expensive to manufacture than the vane sectors  30 B and  30 C and their guide vanes  28 B. It is believed that the vane frame  40  of sufficient strength can be achieved in which the structural guide vanes  28 B account for not more than about twenty-five percent of the guide vanes  28  (not more than twelve of the forty-eight vanes  28  in  FIG. 2 ), suggesting that the use of composite materials for the nonstructural guide vanes  28 A offer a significant potential for weight and cost savings. A minimal count for the guide vanes  28 B, for example, six of the forty-eight vanes  28  in  FIG. 2 , is believed to be achieved by limiting the placement of the vanes  28 B to roughly 180 degrees apart, for example, at the twelve and six o&#39;clock positions if the turbofan engine  10  is mounted underwing. 
         [0019]      FIG. 3  represents the radially outer end of one of the guide vanes  28 B, including its outer shroud segment  44 B, and a manner in which the outer shroud segment  44 B may be attached to the fan casing  32 . Embossments  48 B are shown that extend radially outward from the outer shroud segment  44 B to engage the fan casing  32 , and attachment can be made with any suitable fastening technique. Similar methods of attachment can be provided at the joint between the inner shroud segment  42 B and the inner frame of the engine core  16 . 
         [0020]      FIG. 4  represents a guide vane sector  30 A that contains five guide vanes  28 A sharing a pair of inner and outer shroud segments  42 A and  44 A. Similar to the outer shroud segment  44 B of  FIG. 3 , the outer shroud segment  44 A in  FIG. 4  is equipped with embossments  48 A for attachment of the sector  30 A to the fan casing  32 . Because the outer shroud segment  44 A may be formed of a material having lower strength and modulus than the outer shroud segment  44 B of  FIG. 3 , the embossments  48 A are shown to be further equipped with tapered ribs  50  for additional rigidity and support. As with the vane sector  30 B/ 30 C of  FIG. 3 , attachment of the outer shroud segment  44 A can be made with any suitable fastening technique. In contrast, the inner shroud segment  42 A in  FIG. 4  is not represented as being adapted for attachment to the inner frame of the engine core. This structural connection can be omitted or at least less robust than the connections for the vane sectors  30 B and  30 C because the primary structural connection between the fan nacelle  34  and engine core  16  is through the guide vanes  28 B instead of the guide vanes  28 A. Optionally, some form of attachment can be employed to connect the inner shroud segment  42 A to the engine core  16  to better secure and immobilize the vane sectors  30 A, and optionally carry part of the aerodynamic load of the sector  30 A. 
         [0021]    In addition to weight and cost benefits, another potential advantage of the invention is that, with higher numbers of guide vanes  28 A in each vane sector  30 A, the number of separate components required to construct the vane frame  40  is reduced, as are the number of attachment hardware and seals between adjacent shroud segments  42 A,  42 B,  44 A and  44 B, providing the potential for additional savings in weight and cost. Finally, aerodynamic benefits may be achieved as a result of fewer inner and outer shroud segments  42 A,  42 B,  44 A and  44 B being required to define the inner and outer flow paths of the vane frame  40  (as defined by the radially outward and inward faces of the inner and outer shrouds  42  and  44 , respectively). To further exploit this benefit, the outer shroud segments  44 A of the vane sectors  30 A can be configured to overlap the outer shroud segments  44 B of the vane sectors  30 B and  30 C to further reduce the number of locations where flow path seals are required. 
         [0022]    Because the configurations of the guide vanes  28 A are not restricted by structural considerations and the vanes  28 A can be produced of molded composite compounds, the shapes of the vanes  28 A can be readily contoured to promote desirable aerodynamic effects. For example, the guide vanes  28 A of any sector  30 A can be fabricated to have greater or lesser vane camber differences than other vanes  28 A of the same sector  30 A and/or of other sectors  30 A-C. For example, any one or more of the sectors  30 A can be fabricated to contain guide vanes  28 A having different cambers. As a nonlimiting example, any of the sectors  30 A can be produced with camber changes of about +1 to about +5 degrees between adjacent vanes  28 A within the sector  30 A. Furthermore, sectors  30 A can be produced to have vanes  28 A with increasing or decreasing cambers from one end of the sector  30 A to the other. This capability of fabricating sectors to have vanes  28 A of different cambers is especially desirable for improving the transition of air flow around the upper and lower bifurcations where, respectively, the engine pylon (not shown) enters the bypass duct  24  for attachment to the forward engine mount (not shown) of the engine core  16 , and drain lines (not shown) leaving the engine core  16  pass through the bypass duct  24 . 
         [0023]    While the invention has been described in terms of specific embodiments, other forms could be adopted by one skilled in the art. For example, the physical configuration of the turbofan engine  10 , vane frame  40 , and vane sectors  30 A,  30 B and  30 C could differ from those shown, and materials and processes other than those noted could be used. Therefore, the scope of the invention is to be limited only by the following claims.