Abstract:
A case assembly for a gas turbine engine comprising annular case components each having a central axis. Radial struts each have a radial axis and intersect the annular case components. A stress dissipation mass projecting from a continuous surface of at least one of the struts at the intersection with a corresponding annular case component, the stress absorption mass being on either side of a plane passing through the radial axis of the strut and the central axis of the corresponding annular case component. A method for dissipating thermal and mechanical stresses on a strut in a case assembly for a gas turbine engine is also provided.

Description:
TECHNICAL FIELD 
       [0001]    The present application relates to a gas turbine engine and to a case therefore. 
       BACKGROUND OF THE ART 
       [0002]    Turbofan engines typically have a case assembly with a fan case, an intermediate case, a compressor case, a gas generator case, a turbine case and a turbine exhaust case about a centreline. The hot section of the engine, including the gas generator case, the turbine case and turbine exhaust case, are typically made of steel or nickel alloys. The cooler sections such as the intermediate case and the compressor case may be made of lighter materials such as aluminium or magnesium. However, steel is conventionally used for the fan case because of its strength. 
         [0003]    In the intermediate case, there is a compressor air passage, and a bypass air passage, defined by an annular splitter. The splitter extends forward of radial struts integrated between the intermediate case and the fan case. The intermediate case with the splitter and the struts is generally integrally cast and then machined. The parts are of uniform material thickness resulting in above limit stresses in certain locations. 
         [0004]    Low cycle fatigue is a persistent problem which can reduce the useful life of various structural components due to cycling between idling and operating conditions and thus subjected to stresses generated by thermal expansion effects and mechanical loads. Due to the high loads experienced on intermediate case designs, low cycle fatigue locations are especially noted on all struts. Increasing the mass to compensate increases the thermal stresses, while decreasing mass increases the g loading and thrust contribution. 
         [0005]    Improvement in case design is desired. 
       SUMMARY 
       [0006]    In one aspect, the present disclosure provides a case assembly for a gas turbine engine comprising: annular case components each having a central axis; radial struts each having a radial axis, the radial struts intersecting the annular case components; and a stress dissipation mass projecting from a continuous surface of at least one of the struts at the intersection with a corresponding annular case component, the stress absorption mass being on either side of a plane passing through the radial axis of the strut and the central axis of the corresponding annular case component. 
         [0007]    In another aspect, the present disclosure provides a method for dissipating thermal and mechanical stresses on a strut in a case assembly for a gas turbine engine comprising: determining a location of substantial thermal and mechanical stresses, determining a volume of mass required to dissipate the thermal and mechanical stresses at the location, and forming a discrete mass on the strut protruding from a continuous surface of the strut, at the determined location on either side of the strut. 
         [0008]    In yet another aspect, the present disclosure provides A gas turbine engine comprising: a case assembly of an annular case components each having a central axis; radial struts in the case assembly, each having a radial axis, the radial struts intersecting the annular case components; and a stress dissipation mass projecting from a continuous surface of at least one of the struts at the intersection with a corresponding annular case component, the stress dissipation mass being on either side of a plane passing through the radial axis of the strut and the central axis of the corresponding annular case component. 
         [0009]    Further details of these and other aspects of the present invention will be apparent from the detailed description and figures included below. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0010]    Reference is now made to the accompanying figures depicting embodiments of the present invention, in which: 
           [0011]      FIG. 1  is a schematic cross-sectional view of a turbofan gas turbine engine; 
           [0012]      FIG. 2  is a fragmentary perspective view of a strut and splitter assembly with a detail in accordance with an embodiment; and 
           [0013]      FIG. 3  is a fragmentary, enlarged perspective view of the detail shown in  FIG. 2 ; 
           [0014]      FIG. 4  is a schematic view showing a bulge size in the presence of a weldline for the detail of  FIG. 2 ; and 
           [0015]      FIG. 5  is a schematic view showing a bulge size in the absence of a weldline for the detail of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Referring to  FIG. 1 , a turbofan gas turbine engine which is an exemplary application of the described subject matter includes an engine outer case  10 , a core case  11 , a low pressure spool assembly (not indicated) which includes a fan assembly  12 , a low pressure compressor assembly  13  and a low pressure turbine assembly  14  connected by a shaft  15 , and a high pressure spool assembly (not indicated) which includes a high pressure compressor assembly  16  and a high pressure turbine assembly  17  connected by a turbine shaft  18 . The core case  13  surrounds the low and high pressure spool assemblies to define a main fluid path (not numbered) therethrough. The high and low pressure spool assemblies co-axially define a rotational engine axis X of the engine  10 . 
         [0017]    It should be noted that the terms “radial”, “axial” and “circumferential” used throughout this specification and appended claims, unless otherwise specified, are with respect to the engine axis X. 
         [0018]    As shown concurrently in  FIGS. 1 and 2 , an intermediate case  22  is illustrated having an inner hub  24  and an outer ring  26 . The inner hub  24  may be mounted onto the turbine shaft  18  to support the turbine shaft  18  when it rotates. The intermediate case  22  may be immediately downstream of the fan case surrounding fan  12  as shown in  FIG. 1 . A plurality of struts  28  extend from the inner hub  24  to the outer ring  26 . 
         [0019]    Splitter ring  30  separates the bypass air flow from the flow entering the compressor section ( FIG. 1 ), with the flow entering the compressor section being radially inward of the bypass air flow. The splitter ring  30  is supported by the struts  28 , and may have a gaspath baffle  31 . A support ring  32  may also be connected to the struts  28  (e.g., welded) between the inner hub  24  and the splitter ring  30 , and may be used to support a bleed-off valve, among other possibilities. 
         [0020]    Referring now to  FIGS. 2 and 3 , a stress dissipating mass  36  (i.e., stress distribution mass) is positioned at the joint  34  between one of the struts  28  and the splitter ring  30 , which joint  34  typically comprises a fillet. The stress dissipating mass  36  is formed by a pair of bulges  36   a ,  36   b  (a.k.a., lobes), placed symmetrically, one on either side of a plane extending in the radial axis of the strut  28  and the longitudinal axis of the inner hub  24  (i.e., the engine axis X). The bulges  36   a ,  36   b  mirror geometries, although they may not be mirror images of one another as well. According to an embodiment, the stress dissipating mass  36  may be machined from the stock forming the strut  28 , or may have other constructions as well. As shown in  FIG. 3 , the strut  28  may comprise a flange-like portion  38  to contact the splitter ring  30 , with the bulges  36   a  and  36   b  at the junction between the main radial portion of the strut  28  and the flange-like portion  38 . The flange-like portion  38  of the strut  28  may be welded to the splitter ring  30  along weld lines  40 , among other possibilities. The distance between the bulges  36   a ,  36   b  and the weld line  40  is established to avoid the weld bead being close to the bulge radius. 
         [0021]    The bulges geometry may be proportional to the strut leading edge fillet radius, to spread the load in front of the strut  28 . The minimum width (in the tangential direction, also referred to as length) may be equivalent to the strut leading edge fillet radius. The lobe width should not exceed 2 times the strut fillet radius. Larger lobes will add weight to the part without any further stress reduction. 
         [0022]    In the embodiment in which there is no welded joint in front of the strut  28  (e.g., weld line  40 ), the bulges  36   a ,  36   b  may be longer. A suitable maximum length may be one time the strut leading edge fillet radius. 
         [0023]    In an embodiment, the bulges  36   a ,  36   b  are not in the gas path, as they are underneath the gaspath baffle  31  to avoid disturbing the gas flow. Hence, the height of the bulges  36   a ,  36   b  may be smaller than a height of the baffle  31 . Stated differently, the bulges  36   a ,  36   b  are used to spread the load in front of the strut  28 . The load and thus the stress was concentrated in the strut leading edge area. The stress dissipating mass  36  redistributes the load without adding extra thickness all over the splitter ring  30  and thus without adding excessive weight. 
         [0024]    Referring to  FIG. 4 , one of the bulges  36   a  is shown being about 0.150 in away from the weld line  40  to avoid having double stress concentration (the distance being given as an example). The bulge  36   a  has a height h that may be about 3 times the ring thickness to have significant stiffness change to transfer the stress away from the leading edge of the strut  28 . 
         [0025]    Referring to  FIG. 5 , an exemplary embodiment is shown in which there is no weld line at the junction between strut  28  and splitter ring  30 . In such a case, the length L of bulge  36   a  may be increased, for instance up to about 3 times the strut leading edge fillet radius RL. Also, the radius RG of the bulge  36   a  may be increased to reduce the stress concentration 
         [0026]    The discretely selected, increased mass from the bulges  36   a ,  36   b  dissipates the thermal and mechanical stresses at the joint of the strut  28  and the splitter ring  30 , without adding significant weight to the assembly. The location of the stress dissipating mass  36  at the junction between the strut  28  and the splitter ring  30  may stiffen the overall carcass from bending. Moreover, the junction between the strut  28  and the splitter ring  30  may be a critical location in terms of fatigue, whereby the stress dissipating mass  36  strengthens the junction. It is contemplated that the stress dissipating mass  36  be applied in other case sections, for instance the exhaust case  20 . The stress dissipation mass  36  may be defined as a protuberance on the surface of the strut  28 , which would otherwise be a generally continuous and arcuate junction between two generally planar surface. The stress dissipating mass  36  is radially inward oriented relative to the splitter ring  30 . Due to its location and relatively low profile, the stress dissipating mass  36  does not have a significant on gas flow. 
         [0027]    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 departing from the scope of the invention disclosed. Other modifications which fall within the scope of the present invention 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.