Patent Publication Number: US-6902371-B2

Title: Internal low pressure turbine case cooling

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to cooling of casing of low pressure turbine case of a gas turbine engine and, more particularly, to such cooling by flowing cooling air between shrouds and the case. 
   2. Description of Related Art 
   A gas turbine engine of the turbofan type generally includes a forward fan and booster compressor, a middle core engine, and an aft low pressure power turbine (LPT). The core engine includes a high pressure compressor, a combustor, and a high pressure turbine in a serial flow relationship. The high pressure compressor and high pressure turbine of the core engine are interconnected by a high pressure shaft to the high pressure rotor. The high pressure compressor is rotatably driven to compress air entering the core engine to a relatively high pressure. This high pressure air is then mixed with fuel in the combustor and ignited to form a high energy gas stream. The gas stream flows aftwardly and passes through the high pressure turbine, rotatably driving it and the high pressure shaft which, in turn, rotatably drives the compressor. 
   The gas stream leaving the high pressure turbine is expanded through a low pressure turbine. The low pressure turbine rotatably drives the fan and booster compressor via a low pressure shaft, all of which form the low pressure rotor. The low pressure shaft extends through the high pressure rotor. Most of the thrust produced is generated by the fan. Engine frames are used to support and carry the bearings which, in turn, rotatably support the rotors. Conventional turbofan engines have a fan frame, a turbine center frame, and an aft turbine frame. 
   The turbine center frame typically has an external casing and an internal hub which are attached to each other through a plurality of multiple radially extending struts. A flowpath frame liner provides a flowpath that guides and directs hot engine gases through the frame and is not intended to carry any structural loads. Cooling air may be introduced into an annular chamber between the external casing and a radially outer flowpath liner of the flowpath frame liner, such as in the GE90. The flowpath frame liner protects the struts and rest of the frame from the hot gases passing through the frame. 
   Downstream of the turbine center frame is the low pressure turbine. Hot flowpath gases ingested into cavities between the casing and outer flowpath components could transfer heat into the casing by convection. The heat increases the metal temperatures of the casing and in turn reduces the useful life of the casing materials due to low cycle fatigue. The time-dependent properties of the casing material become limiting and unacceptable permanent casing deformations occur that adversely affect interstage turbine clearances, thereby reducing component service life of the casing. 
   Cooling by way of purge air is provided to annular cavities between the low pressure turbine casing, which for the GE90 is a single piece ring extending across six low pressure stages, and alternating blade shroud segments and low pressure turbine nozzle band segments from which are radially inwardly suspended turbine vane airfoils. Purge air  98  from a turbine center frame  100  of the GE90 engine illustrated in  FIGS. 1 and 2  travels through a flow circuit into a small first stage stator cavity  112  and is bounded by an aft  100  rail of a turbine center frame case, a low pressure turbine flange  110  of a low pressure turbine casing  111 , and a trailing edge  114  of a first stage stator flowpath outer band  116 . Flow passages  118  at a forward lip  120  of a first stage low pressure turbine shroud  122  permits purge air flow to enter a first cavity  124  between the low pressure turbine casing  111  and above the first stage shroud  122 . Leakage paths  128  at an aft end  130  of the first cavity  124  and shroud allow the purge air to exit the first cavity. The purge air circuit produces a small reduction in the low pressure turbine casing  111  and low pressure turbine stage one shroud  122  metal temperatures. The ability to purge cooling air from the first cavity  124  above the shroud controls the amount of flowpath gas that can enter the first cavity. The purge or cooling air flow reduces the convection heating of the LPT casing shell. The exiting of this cooling air reduces the heat transfer from the shroud to the LPT Casing by convection and conduction. 
   Therefore, it would be very beneficial to be able to improve the amount and control of purge air flow in the cavities above shrouds and turbine nozzle bands in the low pressure turbine. It has been found to be particularly useful to cool the first two of these cavities in order to cool the shell of the low pressure casing. 
   BRIEF DESCRIPTION OF THE INVENTION 
   A low pressure turbine casing has a conical annular shell circumscribed about a centerline, a forward flange radially inwardly depending from a forward end of the annular shell, and a forward hook extending axially aftwardly from the forward flange. Axially spaced apart annular first and second rails having first and second hooks, respectively, extend axially aftwardly from the annular shell and are located axially aft of the forward hook. First and second pluralities of first and second cooling holes extend through the first and second rails, respectively. In the exemplary embodiment, a plurality of cooling air feed holes extend through the forward flange. The plurality of cooling air feed holes may be substantially parallel to the centerline. The first and second pluralities of first and second cooling holes may be radially disposed through the first and second rails, respectively, with respect to the centerline or disposed through the first and second rails at an oblique angle with respect to the centerline. 
   The low pressure turbine casing may be used in a low pressure turbine casing and shroud assembly having a forward flange radially inwardly depending from a forward end of the annular shell, a forward hook having a forward annular slot and extending axially aftwardly from the forward flange. An annular first shroud is spaced radially inwardly of the annular shell and has a forwardly extending first forward lip disposed in the forward annular slot. An aft flange of the first shroud is mounted to the first hook with an annular C-clip having an annular radially outer leg disposed in a first annular slot. 
   A first annular cavity is radially disposed between the annular shell and the first shroud, axially extends from the forward flange to the first hook, and is in fluid flow communication with the first plurality of first cooling holes. An annular nozzle retainer is axially trapped between a turbine flange and the forward flange. Cooling air flow first passageways extend from an annular cooling air plenum through the turbine flange, the annular nozzle retainer, and the forward flange, to the first annular cavity. The first passageways may include axially and radially open channels through the turbine flange, radially elongated holes extending axially through the annular nozzle retainer, and a plurality of cooling air feed holes extending through the forward flange to the first annular cavity. 
   A radially outer turbine vane band is suspended radially inwardly from the first and second hooks by first and second turbine vane flanges. An annular seal radially disposed between the annular shell and the outer turbine vane band and axially extends between the first and second turbine vane flanges. A second annular cavity is radially disposed between the annular shell and the annular seal, axially extends between the first and second rails, and is in fluid flow communication with the first and second pluralities of first and second cooling holes. 
   The low pressure turbine casing and low pressure turbine casing and shroud assembly can reduce the amount of hot flowpath gases ingested into cavities between the casing and LPT shrouds and nozzle bands and reduce the amount of heat transferred into the casing by convection. This lowers the operating metal temperatures of the casing and, in turn, increases the useful service life of the casing whose materials are subject to heat enhanced low cycle fatigue. 
   The low pressure turbine casing and low pressure turbine casing and shroud assembly can improve the amount and control of purge air flow in the cavities above shrouds and turbine nozzle bands in the low pressure turbine, particularly useful in the first two of these cavities, in order to cool the shell of the low pressure casing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and other features of the invention are explained in the following description, taken in connection with the accompanying drawings where: 
       FIG. 1  is a longitudinal cross-sectional view illustration of prior art first stage of a gas turbine engine low pressure turbine casing and shroud assembly. 
       FIG. 2  is an enlarged view of a prior art connection between a turbine nozzle and the casing and shroud assembly illustrated in FIG.  1 . 
       FIG. 3  is a longitudinal cross-sectional view illustration of an exemplary gas turbine engine low pressure turbine casing and shroud assembly in accordance with an exemplary embodiment of the invention. 
       FIG. 4  is an enlarged longitudinal cross-sectional view illustration of an area around a forward flange of the turbine casing illustrated in FIG.  3 . 
       FIG. 5  is a perspective view illustration of a nozzle retainer sheet metal and the forward flange of the turbine casing illustrated in  FIGS. 3 and 4 . 
       FIG. 6  is an enlarged longitudinal cross-sectional view illustration of a first hook with radial holes therethrough of the turbine casing and shroud assembly illustrated in FIG.  3 . 
       FIG. 7  is a longitudinal cross-sectional view illustration of a second hook with radial holes therethrough in the gas turbine engine low pressure turbine casing and shroud assembly illustrated in FIG.  3 . 
       FIG. 8  is a longitudinal cross-sectional view illustration of a first alternative embodiment of the exemplary gas turbine engine low pressure turbine casing and shroud assembly illustrated in FIG.  3 . 
       FIG. 9  is a longitudinal cross-sectional view illustration of a second alternative embodiment of the exemplary gas turbine engine low pressure turbine casing and shroud assembly illustrated in FIG.  3 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Illustrated in  FIG. 3  is a low pressure turbine casing and shroud assembly  40  having a low pressure turbine casing  10  with a conical annular shell  12  circumscribed about a centerline  14 . A forward flange  16  radially inwardly depends from a forward end  18  of the annular shell  12  and a forward hook  22  extends axially aftwardly from the forward flange  16 . Referring to  FIGS. 3 ,  6 , and  7 , axially spaced apart annular first and second rails  23  and  25  having first and second hooks  24  and  26 , respectively, extend axially aftwardly from the annular shell  12  and are located axially aft of the forward hook  22 . First and second hooks  24  and  26 , include first and second annular slots  34  and  36 , respectively. First and second pluralities of first and second cooling holes  27  and  29  extend through the first and second rails  23  and  25 , respectively allowing cooling air  58  to flow therethrough. 
   Referring to  FIGS. 4 and 5 , a plurality of cooling air feed holes  28  extend through the forward flange  16 . The plurality of cooling air feed holes  28  may be substantially parallel to the centerline  14 . The first and second pluralities of first and second cooling holes  27  and  29  may be radially disposed through the first and second rails  23  and  25 , respectively, with respect to the centerline  14  or disposed through the first and second rails  23  and  25  at an oblique angle  30  with respect to the centerline  14  as illustrated in FIG.  8 . 
   Referring to  FIGS. 3 ,  4 ,  5 , and  6 , the low pressure turbine casing  10  in the low pressure turbine casing and shroud assembly  40  includes the forward flange  16  radially inwardly depending from the forward end  18  of the annular shell  12 . The forward hook  22 , extending axially aftwardly from the forward flange  16 , includes a forward annular slot  32 . An annular first shroud  42  is spaced radially inwardly of the annular shell  12  and has a forwardly extending first forward lip  43  disposed in the forward annular slot  32 . An aft flange  46  of the first shroud  42  is mounted to the first hook  24  with an annular C-clip  47  having an annular radially outer leg  48  disposed in the first annular slot  34  of the first hook  24 . 
   A first annular cavity  50  is radially disposed between the annular shell  12  and the first shroud  42 , axially extends from the forward flange  16  to the first hook  24 , and is in fluid flow communication with the first plurality of first cooling holes  27 . An annular nozzle retainer  44  is axially trapped between a turbine flange  56  and the forward flange  16 . Cooling air flow first passageways  54  extend from an annular cooling air plenum  52  through the turbine flange  56 , the annular nozzle retainer  44 , and the forward flange  16 , to the first annular cavity  50 . The first passageways  54  may include axially and radially open channels  60  through the turbine flange  56 , radially elongated holes  62  extending axially through the annular nozzle retainer  44 , and the plurality of cooling air feed holes  28  extending through the forward flange  16  to the first annular cavity  50 . The radially open channels  60  are typically slots machined into the turbine flange  56 . 
   Referring to  FIGS. 3 ,  6 , and  7 , a radially outer turbine vane band  64  is suspended radially inwardly from the first and second hooks  24  and  26  by first and second turbine vane flanges  65  and  66 . An annular seal  68  radially disposed between the annular shell  12  and the outer turbine vane band  64  and axially extends between the first and second turbine vane flanges  65  and  66 . A second annular cavity  70  is radially disposed between the annular shell  12  and the annular seal  68 , axially extends between the first and second rails  23  and  25 , and is in fluid flow communication with the first and second pluralities of first and second cooling holes  27  and  29 . 
     FIG. 8  further illustrates how the second cooling holes  29  may pass through one part of the hook  26  into the second annular slot  36  to exhaust the cooling air  58  from the second annular cavity  70  through scalloped passages  88  in the second hook  26 . Illustrated in  FIG. 9  are second cooling holes  29  may in a combination of axially extending forward holes  80  in combination with radially extending holes  82  disposed through the second rail  25  to exhaust the cooling air  58  from the second annular cavity  70 . 
   The low pressure turbine casing  10  and low pressure turbine casing and shroud assembly  40  can reduce the amount of hot flowpath gases ingested into cavities between the casing and LPT shrouds and nozzle bands and reduce the amount of heat transferred into the casing by convection. This lowers the operating metal temperatures of the casing and, in turn, increases the useful service life of the casing whose materials are subject to heat enhanced low cycle fatigue. 
   While there have been described herein what are considered to be preferred embodiments of the present invention, other modifications of the invention shall be apparent to those skilled in the art from the teachings herein, and it is, therefore, desired to be secured in the appended claims all such modifications as fall within the true spirit and scope of the invention. 
   While the preferred embodiment of our invention has been described fully in order to explain its principles, it is understood that various modifications or alterations may be made to the preferred embodiment without departing from the scope of the invention as set forth in the appended claims.