Patent Publication Number: US-8979484-B2

Title: Casing for an aircraft turbofan bypass engine

Description:
TECHNICAL FIELD 
     The described subject matter relates generally to turbofan gas turbine engines, and more particularly to an intermediate case of a turbofan gas turbine engine. 
     BACKGROUND OF THE ART 
     Aircraft turbofan engines typically have a segmented case assembly including, for example a fan case, an intermediate case, a compressor case, a gas generator case, a turbine case and a turbine exhaust case, all positioned about an engine central axis. A splitter structure may extend forwardly of struts in the intermediate case. The intermediate case is conventionally cast with struts and the splitter structure integrally cast therein. However, casting is a process which is difficult to control and which requires minimum weight thicknesses to achieve acceptable quality because the structure not only performs aerodynamic functions but must also bear thrust loads. There is also a need for using interior spaces of a splitter and strut structures for services for air/oil systems, instrumentation and maintenance activities such as borescope inspections. 
     Accordingly, there is a need to provide an improved intermediate case of an aircraft turbofan engine. 
     SUMMARY 
     In one aspect, the described subject matter provides a casing for an aircraft turbofan bypass engine comprising: an outer ring and an inner hub defining an annular space therebetween, the inner hub configured for connection to at least one spool bearing, the outer ring configured for connection to at least one engine mount; a plurality of hollow radial struts arranged in a circumferential array mounting the inner hub to the outer ring; and an annular splitter box disposed between the inner hub and outer ring and configured to be connected with an upstream annular splitter tip structure to divide an air flow through the annular space into a core air flow and a bypass air flow, the splitter box defined by an inner wall and an outer wall, the splitter box further having an intermediate wall extending downstream conically inward from said outer wall, the splitter box having openings in each of said inner, outer and intermediate walls for receiving said struts passing therethrough, each of said splitter box walls terminating at a downstream end configured for connection to a downstream engine case, the struts being mounted to said splitter box with a respective peripheral weld or braze between the intermediate wall and the struts at the openings of the intermediate wall. 
     In another aspect, the described subject matter provides an aircraft turbofan bypass engine comprising: a fan assembly, a compressor assembly, a combustion gas generator assembly and a turbine assembly; and a fabricated case having an annular splitter box supporting an upstream annular splitter tip structure, the annular splitter tip structure dividing a fan driven inlet air flow into a bypass air flow and a core air flow, the fabricated case including: an outer ring and an inner hub defining an annular space therebetween, the inner hub configured for connection with at least one spool bearing, the outer ring configured for connection with at least one engine mount; a plurality of load-bearing hollow radial struts arranged in a circumferential array to mount the inner hub to the outer ring, an annular splitter box disposed within the annular space and including an annular outer wall and an annular inner wall, the annular inner wall being disposed within the annular outer wall, the annular splitter box being connected to the upstream annular splitter tip structure and a downstream engine case, the annular outer wall in combination with the outer ring defining a section of a bypass air duct for directing said bypass air flow, the annular inner wall in combination with the inner hub defining a section of a core fluid path of the engine for directing said core air flow, the annular outer and annular inner walls defining a plurality of respective circumferentially spaced openings for allowing the individual struts to radially extend therethrough, and an annular intermediate wall extending downstream conically inward from the outer wall and connected to the downstream engine case, the intermediate wall having a plurality of circumferentially spaced openings receiving the individual struts to radially extend therethrough, the annular intermediate wall being affixed to the struts by welding or brazing along a periphery of a respective one of said openings in the annular immediate wall, an upstream end of the annular intermediate wall being welded or brazed to the annular outer wall and a downstream end of the annular intermediate wall being welded or brazed to a plurality of circumferentially spaced brackets, each bracket being welded or brazed to a corresponding one of the respective struts. 
     Further details of these and other aspects of the described subject matter will be apparent from the detailed description and drawings included below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference is now made to the accompanying drawings depicting aspects of the described subject matter, in which: 
         FIG. 1  is a schematic partial cross-sectional view of a turbofan bypass gas turbine engine as an exemplary application of the described subject matter; 
         FIG. 2  is a partial perspective view of an annular splitter box structure of an intermediate case, as shown in a circled area  2  in  FIG. 1 , with a front portion cut away to show the inside of the annular splitter box structure; and 
         FIG. 3  is a partial rear perspective view of the annular splitter box structure of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a turbofan bypass gas turbine engine includes a housing or nacelle  10 , a core casing  13 , a low pressure spool assembly (not numbered) which includes a fan assembly  14 , a low pressure compressor assembly  16  and a low pressure turbine assembly  18  connected by a shaft  12 , and a high pressure spool assembly (not numbered) which includes a high pressure compressor assembly  22  and a high pressure turbine assembly  24  connected by a turbine shaft  20 . The housing or nacelle  10  surrounds the core casing  13  and in combination the housing  10  and core casing  13  define an annular bypass air duct  28  for directing a bypass air flow (indicated by arrows  32 ) which is driven by the fan assembly  14 , to be discharged, thereby providing thrust to the engine. The core casing  13  surrounds the low and high pressure spool assemblies to define a core fluid path  30  therethrough. In the core fluid path  30  there is provided a combustor  26  to form a combustion gas generator assembly which generates combustion gases to power the high pressure turbine assembly  24  and the low pressure turbine assembly  18 . A core air flow (indicated by arrow  34 ) driven by the fan assembly  14 , is directed through the core fluid path  30  to the combustor  26  for combustion. 
     The terms “axial”, “radial” and “circumferential” used for various components below are defined with respect to the main engine axis shown but not numbered in  FIG. 1 . The terms “upstream” and “downstream” mentioned in the description below generally refer to the air flow direction indicated such as by arrows  32  and  34 . 
     Referring to  FIGS. 1-3 , a fabricated intermediate case  36  includes an outer ring  38  which is a portion of the housing or nacelle  10  of the engine and is configured for connection with at least one engine mount, and an inner hub  40 , in combination defining an annular space (not numbered) radially therebetween. The inner hub  40  may be connected to one or more bearing assemblies (not numbered) to support either one or both shafts  12  and  20 . A plurality of load-bearing hollow struts  42  are arranged in a circumferential array and extend from the inner hub  40  radially outwardly to the outer ring  38 , thereby mounting the inner hub  40  to the outer ring  38 . As used herein, the term “fabricated” indicates that the case is made from individually formed sheet metal and other components and then joined together to provide a fabricated assembly, rather than integrally cast as a complete unit as in typical prior art engine cases. 
     An annular portion of the engine core casing  13 , as shown in the circled area  2  in  FIG. 1 , is disposed within the annular space between the outer ring  38  and the inner hub  40  and includes an annular outer wall portion  44  and an annular inner wall portion  46  of the engine core casing  13 . The annular inner wall portion  46  is disposed within the annular outer wall portion  44 . The annular portion of the engine core casing  13  formed with the annular outer wall portion  44  and the annular inner wall portion  46  is connected to an annular splitter tip structure  48  located upstream of the annular portion in the circled area  2  (of  FIG. 1 ), of the engine core casing  13 . The annular splitter tip structure  48  forms an annular upstream edge of the core casing  13 , to divide the fan driven air flow into the bypass air flow  32  and the core air flow  34 . Therefore, the annular outer wall portion  44  and annular inner wall portion  46  in combination form an annular splitter box (not numbered) which supports the annular splitter tip structure  48  to bear loads during engine operation. 
     The annular outer wall portion  44  which is a connected section of an inner annular boundary of the bypass air duct  28  and the annular inner wall portion  46  which is a connected section of an annular outer boundary of the core fluid path  30 , define a plurality of circumferentially spaced openings  50 ,  52 , respectively, for allowing the individual struts  42  to radially extend therethrough. Welding or brazing may be applied along the periphery of the respective openings  50 ,  52  to connect the struts  42  to the respective annular outer and inner wall portions  44 ,  46 . 
     According to this embodiment, an annular intermediate wall  54  may be provided between the annular outer and inner wall portions  44 ,  46 . A plurality of circumferentially spaced openings  56  may also be defined in the annular intermediate wall  54  for allowing the individual struts  42  to radially extend therethrough. The annular intermediate wall  54  may be affixed to the struts  42  for example by welding or brazing along a periphery of the respective one of the openings  56 . 
     According to this embodiment, an upstream end (not numbered) of the annular intermediate wall  54  may be connected to the annular outer wall portion  44 , for example by being welded or brazed to the annular outer wall portion  44  at an axial location adjacent to leading edges  58  of the respective struts  42 . The annular intermediate wall  54  may extend from an upstream end (not numbered) to a downstream end (not numbered) thereof axially, inwardly away from the annular outer wall portion  44  and therefore the downstream end of the annular intermediate wall  54  may be radially spaced apart from both the annular outer and inner wall portions  44 ,  46 , thereby providing convenient access to the annular space between the annular outer and inner wall portions  44 ,  46 . 
     According to this embodiment, a plurality of circumferentially spaced brackets  60  may be provided, each connecting the annular intermediate wall  54  to a corresponding one of the respective struts  42 . Each of the brackets  60  may be formed with a plate (not numbered) having a substantially U-shaped slot  62  to receive a trailing edge portion  64  of the corresponding strut  42 . The brackets  62  may be affixed to the corresponding strut  42  by welding or brazing along an edge of the slot  62 . The annular intermediate wall  54  may further include an annular flange  66  extending radially inwardly from the downstream end of the annular intermediate wall  54 . The respective brackets  62  may be connected to the downstream end of the annular intermediate wall  54  by being welded directly to the annular flange  66 . 
     The upstream end of the respective annular outer and inner wall portions  44 ,  46  may be provided with connecting features, such as annular flanges  68 ,  70  for connection with the upstream annular splitter tip structure  48 . The downstream end of the respective annular outer and inner wall portions  44 ,  46  (the downstream end of the annular inner wall portion  46  is only schematically shown in  FIG. 1  but is not numbered) and the annular flange  66  at the downstream end of the annular intermediate wall  54 , may also be provided with mounting features, such as mounting holes, such that the annular splitter box structure as shown in the circled area  2  (see  FIG. 1 ) can be mounted to other components in a downstream section of the annular core casing  13  of the engine. 
     The annular intermediate wall  54  may have a web (not numbered) which is thicker than the annular skin of the respective annular outer and inner wall portions  44 ,  46 . The annular intermediate wall  54  extends substantially in the axial direction and is integrated by welding or brazing to the annular outer wall portion  44  and all struts  42 . Therefore, the annular intermediate wall  54  functions as a single stringer within the annular splitter box, shown in the circled area  2  in  FIG. 1 , to evenly distribute torque and axial loads applied to the annular splitter box tip structure  48  and the splitter box during engine operation, to all the struts  42 . The struts  42  then transfer the torque and axial loads to an engine mount (not shown) through the outer ring  38 . The optional brackets  60  integrated by welding or brazing to both the annular intermediate wall  54  and respective struts  42 , may function as tertiary braces to enhance integration of the annular intermediate wall  54  with all the struts  42 , thereby helping to even distribution of loads from the splitter box to all the struts  42 . 
     The substantially axial orientation of the annular intermediate wall  54  with the downstream end thereof radially spaced apart from both annular outer and inner wall portions  44 ,  46 , provides axial access to the annular space defined between the annular outer and inner wall portions  44 ,  46 . This axial access makes it convenient to provide services within the annular splitter box for air/oil systems, instrumentation and maintenance activities of the engine. For example, a service port  72  may be provided on the trailing edge portion  64  of one hollow strut  42  which may allow air/oil service lines to be inserted into the hollow strut  42  or may allow borescope inspection therethrough. 
     The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departure from the scope of the described subject matter. For example, the turbofan gas turbine engine as illustrated, is an example taken to illustrate the application of the described subject matter and does not limit the various features and structures of the engines to which the described subject matter may be applicable. Furthermore, the intermediate case may include various other components which are not described. Still other modifications which fall within the scope of the described subject matter will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.