Patent Publication Number: US-9890663-B2

Title: Turbine exhaust case multi-piece frame

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 disposed through and around the fairing to support a bearing load. The multi-piece frame comprises an inner ring, an outer ring, and a plurality of strut bosses. The outer ring is disposed concentrically outward of the inner ring, and has open bosses at strut locations. The plurality of radial struts pass through the vane fairing, are secured to the inner ring via radial fasteners, and are secured via non-radial fasteners to the open boss. 
    
    
     
       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 first turbine exhaust case of the gas turbine generator of  FIG. 1 . 
         FIG. 3  is a simplified cross-sectional view of an alternative turbine exhaust case to the turbine exhaust case of  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 compressor  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 one embodiment of turbine exhaust case  28 , labeled turbine exhaust case  28   a .  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   a  (with frame  100   a , outer ring  102   a , inner ring  104 , strut  106   a , inner radial strut fasteners  108 , outer cover  110   a , chordwise expandable diameter fastener  112 , circumferentially-oriented expandable diameter fasteners  114   a , fairing  116 , outer platform  118 , inner platform  120 , fairing vane  122 , and frame boss  126   a ). 
     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  116 , which defines an aerodynamic flow path from low pressure turbine  26  to power turbine  30 . Fairing  116  can be formed in a unitary, monolithic piece, while frame  100  is assembled about fairing  116 . 
     Outer platform  118  and inner platform  120  of fairing  116  define the inner and outer boundaries of an annular gas flow path from low pressure turbine  26  to power turbine  30 . Fairing vane  122  is an aerodynamic vane surface surrounding strut  106   a . Fairing  116  can have any number of fairing vanes  122  at least equal to the number of struts  106   a . In one embodiment, fairing  116  has one vane fairing  122  for each strut  106   a  of frame  100 . In other embodiments, fairing  116  may include additional vane fairings  122  through which no strut  106   a  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 comprising three distinct types of structural components, plus connecting fasteners. The outer diameter of frame  100  is formed by outer ring  100   a , a substantially frustoconical annulus with strut boss  126   a , a radially outward-extending hollow boss that carries chordwise expandable diameter fasteners  112  and circumferentially-oriented expandable diameter fasteners  114   a  for securing strut  106   a . Chordwise expandable diameter fasteners  112  and circumferentially-oriented expandable diameter fasteners  114   a  may, for instance, be expandable diameter bolts, shafts, or pins capable of extending entirely through both strut  106   a  and strut boss  126   a , and expanding to take in corresponding tolerances and account for thermal drift. Chordwise expandable diameter fasteners  112  extend substantially axially through strut boss  126   a  and strut  106   a , while circumferentially-extending expandable diameter fasteners  114   a  extend circumferentially through strut boss  126   a  and strut  106   a , and are secured on either angular side of strut boss  126   a . As depicted in  FIG. 1 , circumferentially-extending expandable diameter fasteners  114   a  may be situated at more than one radial location with respect to axis A. Strut bosses  126   a  have strut apertures SA at their radially outer extents to receive struts  106   a . Strut apertures S A  can be sealed by covers  110   a . As depicted in  FIG. 2 , cover  110   a  is a flat lid secured over strut aperture S A . 
     The inner diameter of frame  100  is defined by inner ring  104 , a substantially cylindrical structure with inner radial strut fasteners  108 . Inner radial strut fasteners  108  may, for instance, be screws, pins, or bolts extending radially inward through inner ring  104  and into strut  106   a  to secure strut  106   a  at its radially inner extent to inner ring  104 . In other embodiments, inner radial strut fasteners  108  may be radial posts extending radially inward from inner ring  106   a , and mating with corresponding post holes at the inner diameter of strut  106   a . Struts  106   a  are rigid posts extending substantially radially from inner ring  104 , through fairing vanes  122 , into strut bosses  126   a . Struts  106   a  are anchored in all dimensions by the combination of chordwise expandable diameter fasteners  112  and circumferentially-oriented expandable diameter fasteners  114   a . 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. 
       FIG. 3  is a simplified cross-sectional view of an alternative embodiment of turbine exhaust case  28 , labeled turbine exhaust case  28   b .  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   b  (with frame  100   b , outer ring  102   b , inner ring  104 , strut  106   b , inner radial strut fasteners  108 , outer cover  110   b , circumferentially-oriented expandable diameter fasteners  114   b , fairing  116 , outer platform  118 , inner platform  120 , fairing vane  122 , and cover fasteners  124 , and strut boss  126   b ). Turbine exhaust case  28   b  differs from turbine exhaust case  28   a  only in frame  100   b , outer ring  102   b , cover  110   b , circumferentially-oriented expandable diameter fasteners  114   b , and cover fasteners  124 ; in every other way the embodiments depicted in  FIGS. 2 and 3  are identical. Frame  100   b  differs from frame  100   a  in that strut boss  126   b  includes no apertures for chordwise expandable diameter fasteners. Strut  114   b  is secured solely by circumferentially-extending expandable diameter fasteners  114   b  in strut boss  126   b , and need extend as far radially as strut  106   a . Cover  110   b  is a sealing plate secured in an airtight seal over strut aperture S A  by cover fasteners  124 , which may for instance be bolts, pins, rivets, or screws. 
     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 onto inner radial strut fasteners  108 . In some embodiments (e.g. where inner radial strut fasteners are screws or bolts) inner radial strut fasteners  108  can then be secured to the inner diameter of strut  106 . Circumferentially-oriented expandable diameter fasteners  114  (and chordwise expandable diameter fasteners  112 , in the embodiment of  FIG. 2 ) are next slotted through corresponding holes in strut  114   a  and strut boss  126 , tightened, and expanded to lock strut  106  to outer ring  102 . 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 comprises a turbine exhaust case comprising a fairing defining and airflow path through the turbine exhaust case, and a multi-piece frame disposed through and around the fairing to support a bearing load. The multi-piece frame comprises an inner ring, an outer ring, and a plurality of strut bosses. The outer ring is disposed concentrically outward of the inner ring, and has open bosses at strut locations. The plurality of radial struts pass through the vane fairing, are secured to the inner ring via radial fasteners, and are secured via non-radial fasteners to the open boss. 
     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.   further comprising airtight sealing plates covering each open boss.   wherein the non-radial fasteners comprise a circumferentially-oriented expandable diameter fastener.   wherein the non-radial fasteners further comprise at least one chordwise-oriented expandable diameter fastener.   wherein the radial fasteners comprise radial bolts extending through the inner ring and into the radial struts.       

     A turbine exhaust case comprising an inner cylindrical ring; an outer frustoconical ring with a plurality of angularly distributed hollow strut bosses; and a plurality of radial struts secured to the inner cylindrical ring via radial fasteners, and to the angularly distributed hollow strut bosses via non-radial expandable diameter 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 the radial fasteners are bolts, pins, or screws extending radially through the inner cylindrical ring and into the radial struts.   wherein the inner non-radial expandable diameter fasteners comprise a circumferentially-oriented expandable diameter fastener.   wherein the inner non-radial expandable diameter fasteners comprise a chordwise-oriented expandable diameter fastener.   further comprising a sealing plate providing an air seal over the outer radial extent of the hollow strut bosses.       

     A method of assembling a turbine exhaust case, the method comprising: aligning fairing vanes of a flow path defining fairing, radial fasteners on an inner frame ring, and strut apertures in a strut boss 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 strut to the inner frame ring via the radial fasteners; and securing the radial strut to the strut boss via non-radial expandable diameter fasteners. 
     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:
         further comprising covering the sealing aperture with an airtight sealing plate.       

     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. 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.