Patent Publication Number: US-10329957-B2

Title: Turbine exhaust case multi-piece framed

Description:
BACKGROUND 
     The present disclosure relates generally to gas turbine engines, and more particularly to heat management in a turbine exhaust case of a gas turbine engine. 
     A turbine exhaust case is a structural frame that supports engine bearing loads while providing a gas path at or near the aft end of a gas turbine engine. Some aeroengines utilize a turbine exhaust case to help mount the gas turbine engine to an aircraft airframe. In industrial applications, a turbine exhaust case is more commonly used to couple gas turbine engines to a power turbine that powers an electrical generator. Industrial turbine exhaust cases may, for instance, be situated between a low pressure engine turbine and a generator power turbine. A turbine exhaust case must bear shaft loads from interior bearings, and must be capable of sustained operation at high temperatures. 
     Turbine exhaust cases serve two primary purposes: airflow channeling and structural support. Turbine exhaust cases typically comprise structures with inner and outer rings connected by radial struts. The struts and rings often define a core flow path from fore to aft, while simultaneously mechanically supporting shaft bearings situated axially inward of the inner ring. The components of a turbine exhaust case are exposed to very high temperatures along the core flow path. Various approaches and architectures have been employed to handle these high temperatures. Some turbine exhaust case frames utilize high-temperature, high-stress capable materials to both define the core flow path and bear mechanical loads. Other turbine exhaust case architectures separate these two functions, pairing a structural frame for mechanical loads with a high-temperature capable fairing to define the core flow path. Turbine exhaust cases with separate structural frames and flow path fairings pose the technical challenge of installing vane fairings within the structural frame. Fairings are typically constructed as a “ship in a bottle,” built piece-by-piece within a unitary frame. Some fairing embodiments, for instance, comprise suction and pressure side pieces of fairing vanes for each frame strut. These pieces are inserted individually inside the structural frame, and joined together (e.g. by welding) to surround frame struts. 
     SUMMARY 
     The present disclosure is directed toward a turbine exhaust case comprising a fairing defining an airflow path through the turbine exhaust case, and a multi-piece frame. The multi-piece frame is disposed through and around the fairing to support a bearing load, and comprises an inner ring, an outer ring disposed concentrically outward of the inner ring, a plurality of bossed covers, and a plurality of radial struts. The plurality of bossed covers are bolted to the outer ring at locations circumferentially distributed about the outer diameter of the outer ring. The plurality of radial struts pass through the fairing and are secured via non-radial connectors to the inner ring and the bossed covers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a gas turbine generator. 
         FIG. 2  is a simplified cross-sectional view of a turbine exhaust case of the gas turbine generator of  FIG. 1 . 
         FIG. 3  is a perspective view of multi-piece frame depicted in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a simplified partial cross-sectional view of gas turbine engine  10 , comprising inlet  12 , compressor  14  (with low pressure compressor  16  and high pressure compressor  18 ), combustor  20 , engine turbine  22  (with high pressure turbine  24  and low pressure turbine  26 ), turbine exhaust case  28 , power turbine  30 , low pressure shaft  32 , high pressure shaft  34 , and power shaft  36 . Gas turbine engine  10  can, for instance, be an industrial power turbine. 
     Low pressure shaft  32 , high pressure shaft  34 , and power shaft  36  are situated along rotational axis A. In the depicted embodiment, low pressure shaft  32  and high pressure shaft  34  are arranged concentrically, while power shaft  36  is disposed axially aft of low pressure shaft  32  and high pressure shaft  34 . Low pressure shaft  32  defines a low pressure spool including low pressure compressor  16  and low pressure turbine  26 . High pressure shaft  34  analogously defines a high pressure spool including high pressure compressor  18  and high pressure turbine  24 . As is well known in the art of gas turbines, airflow F is received at inlet  12 , then pressurized by low pressure compressor  16  and high pressure compressor  18 . Fuel is injected at combustor  20 , where the resulting fuel-air mixture is ignited. Expanding combustion gasses rotate high pressure turbine  24  and low pressure turbine  26 , thereby driving high and low pressure compressors  18  and  16  through high pressure shaft  34  and low pressure shaft  32 , respectively. Although compressor  14  and engine turbine  22  are depicted as two-spool components with high and low sections on separate shafts, single spool or three or more spool embodiments of compressor  14  and engine turbine  22  are also possible. Turbine exhaust case  28  carries airflow from low pressure turbine  26  to power turbine  30 , where this airflow drives power shaft  36 . Power shaft  36  can, for instance, drive an electrical generator, pump, mechanical gearbox, or other accessory (not shown). 
     In addition to defining an airflow path from low pressure turbine  26  to power turbine  30 , turbine exhaust case  28  can support one or more shaft loads. Turbine exhaust case  28  can, for instance, support low pressure shaft  32  via bearing compartments (not shown) disposed to communicate load from low pressure shaft  32  to a structural frame of turbine exhaust case  28 . 
       FIG. 2  is a simplified cross-sectional view of turbine exhaust case  26  and adjacent components of gas turbine engine  10 .  FIG. 2  illustrates low pressure turbine  26  (with low pressure turbine casing  42 , low pressure vane  36 , low pressure rotor blade  38 , and low pressure rotor disk  40 ) and power turbine  30  (with power turbine case  52 , power turbine vanes  46 , power turbine rotor blades  48 , and power turbine rotor disks  50 ), and turbine exhaust case  28  (with frame  100 , outer ring  102 , inner ring  104 , strut  106 , inner ring flange  108 , cover  110 , expandable diameter fasteners  112 , inner diameter fasteners  114 , and cover fasteners  116  with corresponding nuts  118 .  FIG. 3  is a perspective view of turbine exhaust case  28  illustrating frame  100  with outer ring  102 , inner ring  104 , strut  106 , inner ring flange  108 , cover  110 , expandable diameter fasteners  112 , inner diameter fasteners  114 , and cover fasteners  116 , with fairing  120  removed. 
     As noted above with respect to  FIG. 1 , low pressure turbine  26  is an engine turbine connected to low pressure compressor  16  via low pressure shaft  32 . Low pressure turbine rotor blades  38  are axially stacked collections of circumferentially distributed airfoils anchored to low pressure turbine rotor disk  40 . Although only one low pressure turbine rotor disk  40  and a single representative low pressure turbine rotor blade  38  are shown, low pressure turbine  26  may comprise any number of rotor stages interspersed with low pressure rotor vanes  36 . Low pressure rotor vanes  36  are airfoil surfaces that channel flow F to impart aerodynamic loads on low pressure rotor blades  38 , thereby driving low pressure shaft  32  (see  FIG. 1 ). Low pressure turbine case  42  is a rigid outer surface of low pressure turbine  26  that carries radial and axial load from low pressure turbine components, e.g. to turbine exhaust case  28 . 
     Power turbine  30  parallels low pressure turbine  26 , but extracts energy from airflow F to drive a generator, pump, mechanical gearbox, or similar device, rather than to power compressor  14 . Like low pressure turbine  26 , power turbine  30  operates by channeling airflow through alternating stages of airfoil vanes and blades. Power turbine vanes  46  channel airflow F to rotate power turbine rotor blades  48  on power turbine rotor disks  50 . 
     Turbine exhaust case  28  is an intermediate structure connecting low pressure turbine  26  to power turbine  30 . Turbine exhaust case  28  may for instance be anchored to low pressure turbine  26  and power turbine  30  via bolts, pins, rivets, or screws. In some embodiments, turbine exhaust case  28  may serve as an attachment point for installation mounting hardware (e.g. trusses, posts) that supports not only turbine exhaust case  28 , but also low pressure turbine  26 , power turbine  30 , and/or other components of gas turbine engine  10 . 
     Turbine exhaust case  28  comprises two primary components: frame  100 , which supports structural loads including shaft loads e.g. from low pressure shaft  32 , and fairing  120 , which defines an aerodynamic flow path from low pressure turbine  26  to power turbine  30 . Fairing  120  can be formed in a unitary, monolithic piece, while frame  100  is assembled about fairing  120 . 
     Outer platform  122  and inner platform  124  of fairing  120  define the inner and outer boundaries of an annular gas flow path from low pressure turbine  26  to power turbine  30 . Fairing vane  126  is an aerodynamic vane surface surrounding strut  106 . Fairing  120  can have any number of fairing vanes  126  at least equal to the number of struts  106 . In one embodiment, fairing  120  has one vane fairing  126  for each strut  106  of frame  100 . In other embodiments, fairing  120  may include additional vane fairings  126  through which no strut  106  passes. Fairing  120  can be formed of a high temperature capable material such as Inconel or another nickel-based superalloy. 
     Frame  100  is a multi-piece frame comprised of four distinct structural elements, plus connecting fasteners. The outer diameter of frame  100  is formed by the combination of outer ring  102  and a plurality of covers  110 . Outer ring  102  is a rigid, substantially frustoconical annulus with strut apertures S A  at angular locations corresponding to locations of struts  106 . Covers  110  are bossed caps that seal strut apertures S A , and interface with struts  106  via expandable diameter fasteners  112 . Expandable diameter fasteners  112  may, for instance, be expandable diameter bolts, shafts, or pins capable of extending entirely through both cover  110  and strut  106 , and expanding to take in corresponding tolerances and account for thermal drift. Expandable diameter fasteners  112  extend in a circumferential direction through strut  106  and cover  110 , and are secured to either angular side of cover  110  (see  FIG. 3 ). Cover  110  is secured to outer ring  102  of frame  100  by cover fasteners  116 , which may for instance be screws, pins, rivets, or bolts (with corresponding nuts  118 ). 
     The inner diameter of frame  100  is defined by inner ring  104 , a substantially cylindrical structure with inner ring flanges  108  bracketing each strut  106 . Inner diameter fasteners  114  extend entirely through inner ring flanges  108  and strut  106 . Inner diameter fasteners  114  may be standard or expandable diameter fasteners, including bolts, pins, shafts, screws, or rivets. Struts  106  are rigid posts extending substantially radially from inner ring  104  through strut apertures S A  of outer ring  102 , and anchored via expandable diameter fasteners  112  to cover  110 . Frame  100  is not directly exposed to core flow F, and therefore can be formed of a material rated to significantly lower temperatures than fairing  120 . In some embodiments, frame  100  may be formed of sand-cast steel. 
     Turbine exhaust case  28  is assembled by axially and circumferentially aligning fairing  120  with inner ring  104  and outer ring  102 , and slotting each strut  106  through strut aperture S A  and fairing vane  126  from radially outside. Strut  106  can then be secured to inner ring  104  via inner diameter fasteners  114  through inner ring flanges  108 , e.g. by manual assembly from aft of turbine exhaust case  28 . Covers  110  are then installed over each strut aperture S A , and secured to struts  106  via variable diameter fasteners  112  to complete the assembly of turbine exhaust case  28 . The multi-piece construction of frame  100  allows turbine exhaust case  28  to be assembled around fairing  120 . Accordingly, fairing  120  can be a single, monolithically formed piece, e.g. a unitary die-cast body with no weak points corresponding to weld or other joint locations. 
     Discussion of Possible Embodiments 
     The following are non-exclusive descriptions of possible embodiments of the present invention. 
     A turbine exhaust case comprising a fairing defining an airflow path through the turbine exhaust case, and a multi-piece frame. The multi-piece frame is disposed through and around the fairing to support a bearing load, and comprises an inner ring, an outer ring disposed concentrically outward of the inner ring, a plurality of bossed covers, and a plurality of radial struts. The plurality of bossed covers are bolted to the outer ring at locations circumferentially distributed about the outer diameter of the outer ring. The plurality of radial struts pass through the fairing and are secured via non-radial connectors to the inner ring and the bossed covers. 
     The turbine exhaust case of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations, and/or additional components: 
     wherein the multi-piece frame is formed of steel. 
     wherein the multi-piece frame is formed of sand-cast steel. 
     wherein the fairing is monolithically formed. 
     wherein the fairing is formed of a material rated for a higher temperature than the multi-piece frame. 
     wherein the fairing is formed of a nickel-based superalloy. 
     wherein the radial struts are anchored via the non-radial connectors to a radial flange on the inner ring. 
     wherein each radial strut passes through a separate aperture in the outer ring covered by a separate bossed over. 
     wherein the non-radial connectors are circumferentially-oriented expandable diameter fasteners. 
     A turbine exhaust case frame comprising an inner cylindrical ring with a plurality of radially outward-extending flanges; an outer frustoconical ring with a plurality of angularly distributed strut apertures; a plurality of radial struts secured to the radially outward-extending flanges, and extending through the angularly distributed strut apertures; and a plurality of covers secured over each of the angularly distributed strut apertures, and secured to the radial struts via expandable diameter strut fasteners. 
     The turbine exhaust case frame of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations, and/or additional components: 
     wherein each of the radial struts is secured to two of the radially outward-extending flanges by inner diameter expandable diameter fasteners. 
     wherein the expandable diameter strut fasteners are oriented circumferentially. 
     wherein each expandable diameter strut fasteners extend fully through one of the struts and one of the plurality of covers. 
     A method of assembling a turbine exhaust case, the method comprising: aligning fairing vanes of a flow path defining fairing, flanges extending radially outward from an inner frame ring, and strut apertures of an outer frustoconical ring; inserting a radial strut from radially outside the outer frustoconical ring, through the strut aperture and the fairing vane; securing the radial struts to the flanges; and securing covers over the strut apertures to the outer frustoconical ring and the struts. 
     The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations, and/or additional components: 
     wherein securing the covers to the struts comprises inserting circumferentially oriented expandable diameter fasteners through each strut and cover. 
     While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. Although the present description describes turbine exhaust case  28  as abutting low pressure turbine  26 , gas turbine engine  10  may comprise any number of engine spools, of which turbine exhaust case  28  abuts the last. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.