Abstract:
A ring segment for a turbine in an industrial gas turbine engine, the ring segment having an inner side with a number of pedestals extending radially inward, each pedestal having an inlet metering hole connected to a diffusion chamber having an opening flush with a TBC covering the pedestals. The pedestals form a larger surface area to secure the TBC to the ring segment so that the TBC can be formed thicker than the prior art without spalling.

Description:
GOVERNMENT LICENSE RIGHTS 
     None. 
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     None. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to gas turbine engine, and more specifically to a ring segment for a turbine in an industrial gas turbine engine. 
     2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98 
     In a gas turbine engine, such as a large frame heavy-duty industrial gas turbine (IGT) engine, a hot gas stream generated in a combustor is passed through a turbine to produce mechanical work. The turbine includes one or more rows or stages of stator vanes and rotor blades that react with the hot gas stream in a progressively decreasing temperature. The efficiency of the turbine—and therefore the engine—can be increased by passing a higher temperature gas stream into the turbine. However, the turbine inlet temperature is limited to the material properties of the turbine, especially the first stage vanes and blades, and an amount of cooling capability for these first stage airfoils. 
     The first stage rotor blade and stator vanes are exposed to the highest gas stream temperatures, with the temperature gradually decreasing as the gas stream passes through the turbine stages. The first and second stage airfoils (blades and vanes) must be cooled by passing cooling air through internal cooling passages and discharging the cooling air through film cooling holes to provide a blanket layer of cooling air to protect the hot metal surface from the hot gas stream. 
     A row or stage of turbine rotor blades rotate within an annular arrangement of ring segments in which blade tips form a small gap with an inner or hot surface of each ring segment. The size of the gap changes due to different thermal properties of the blade and the ring segments from a cold sate to a hot state of the turbine. The smaller the gap, the less hot gas leakage will flow between the blade tips and the ring segments. 
     An IGT engine operates for long periods of time at steady state conditions, as opposed to an aero gas turbine engine that operates for only a few hours before shutting down. Thus, the parts in the IGT engine must be designed for normal operation for these long periods, such as up to 40,000 hours of operation at steady state conditions. A thin TBC (Thermal Barrier Coating) is applied to the inner or hot surface of each ring segment in order to insulate the ring segment from the hot gas flow and reduce the metal temperature of the ring segment. A reduced metal temperature requires less cooling air flow and thus improves the turbine efficiency. As the turbine inlet temperature increases, the cooling flow demand for cooling the ring segments will also increase and therefore reduce the turbine efficiency. One method of reducing the cooling air consumption while allowing for higher turbine inlet temperatures is to use a thicker TBC and film cooling for the ring segments. Thus, the design of the cooling circuit for the ring segments relies more on the endurance of the TBC. Therefore, the TBC becomes the main factor in the design of the ring segment cooling circuit. A problem is that the thicker the TBC the higher the chance of spallation (when pieces of the TBC break away). 
     BRIEF SUMMARY OF THE INVENTION 
     An improvement in TBC durability on a cooled ring segment is achieved with the ring segment of the present invention in which the ring segment includes an array of pedestals each with a metering inlet section and a diffusion outlet section that are embedded within the ring segment and open onto the inner or hot surface. These multiple metering and diffusion holes in the pedestals are formed at a normal direction or at a small angle to the inner or hot surface of the ring segment. A TBC applied onto the cooled ring segment inner or hot surface will fill into the pedestals and therefore form an attachment mechanism for the TBC. During engine operation, expansion of the pedestal metal due to an increase of the ring segment metal temperature will function to compress the TBC within the ring segment and better secure the TBC to the ring segment and prevent spallation which increases the useful life of the TBC on the cooled ring segment. 
     Metering and diffusion of the cooling air flow through the ring segment pedestals will produce convection cooling as well as film cooling for the ring segment. Individual metering and diffusion holes can be sized based on the ring segment gas side pressure distribution in both the streamwise and circumferential directions. Also, each individual metering and diffusion hole can be designed based on the ring segment local external heat load to achieve a desired local metal temperature. The individual metering and diffusion holes are arranged in a staggered array along the ring segment against the mainstream hot gas flow. The ring segment cooling hole design will maximize the use of cooling air for a given ring segment inlet gas temperature and pressure profile. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  shows a cross section view of a ring segment mounted in a ring carrier of the present invention. 
         FIG. 2  shows a cross section side view of a ring segment of the present invention. 
         FIG. 3  shows a view of a section of the ring segment of the present invention from the inner or hot surface side. 
         FIG. 4  shows a cross section side view of a ring segment of the present invention with the TBC attachment construction. 
         FIG. 5  shows a detailed cross section view of a section of the ring segment with two of the pedestals of the present invention. 
         FIG. 6  shows a view of a section of the ring segment of the present invention from the inner or hot surface side with the metering holes and the diffusion section in the pedestals. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The ring segment of the present invention is shown in  FIG. 1  secured within a blade ring carrier  11 . a forward hook  12  and an aft hook  13  extend from the ring carrier  11  and form attachment points for the ring segments  31 . A cooling air supply cavity  14  is formed within the ring carrier  11  that is supplied through one or more cooling air feed holes  15 . An impingement ring or plate  21  with impingement holes  22  is secured to either the ring carrier  11  or the ring segment upper surface. The ring segment  31  includes leading edge purge air holes  35  and mate face purge air holes  36  on the forward sides and the aft sides of each ring segment  31 . The ring segment  31  includes an arrangement of pedestals  32  surrounded by a TBC  41 . The inner side of the ring segment includes four sides that form a depression in which the pedestals extend from a bottom of this depression. An opening of the diffusion chambers of the pedestals is flush with the outer ends of the four sides. The TBC fills the space within the depression and around the sides of the pedestals so that the finished inner surface of the ring segment that forms the flowpath for the hot gas stream is flush. Because of the pedestal design for the ring segments, the TBC  41  can be thicker than in the prior art. A rotor blade rotates within the inner or hot surface of the ring segment  31  covered by the TBC  41 . 
       FIG. 2  shows a side view of the ring segment  31  with the impingement plate  21  over the backside or top surface. The pedestals  32  are arranged on the inner or bottom side and open onto the surface of the TBC  41 .  FIG. 3  shows the bottom or hot side surface of a section of the ring segment with an arrangement of pedestals  32  with the TBC  41  filled in-between the pedestals  32 . 
       FIG. 4  shows a ring segment  31  with each pedestal  32  having a metering inlet hole opening onto the upper or top side of the ring segment and a diffusion section connected to the metering hole and opening onto an inner or hot surface of the ring segment. The TBC  41  covers over the sides of the pedestals so that the opening of the diffusion section is flush with the inner surface of the TBC. 
       FIG. 5  shows a detailed view of the pedestals  32  formed within the TBC  41  on the ring segment  31 . Each pedestal  32  extends from an underside of the ring segment  31 . The pedestals  32  can be formed as a separate piece from the ring segment  31  and secured individually in position to the ring segment  31 , or formed as one piece with the ring segment  31 . Each pedestal  31  includes a metering hole  33  that opens onto the inner side of the ring segment  31  and a diffusion section connected to the metering hole  33  and opens onto the surface of the TBC  41 . With the pedestals  32  in place on the ring segment  31 , the TBC  41  is applied to fill in the areas around the pedestals  32  and form a TBC surface flush with the diffusion section openings. The metering holes  33  extend through the ring segment surface so that the impingement cooling air supplied through the impingement plate  21  can be used to flow through the metering holes  33  and then the diffusion sections  34 .  FIG. 6  shows the inner or hot side surface of a section of the ring segment  31  with a number of pedestals  32  surrounded by the TBC  41 . Each pedestal includes the metering hole  33  opening into the diffusion section  34 . 
     For film cooling, cooling air is metered through each individual pedestal metering hole  33  and then diffused in the semi-circular shaped diffusion cavity  34 . This allows for the cooling air to be diffused uniformly into the diffusion cavity prior to being discharged into the hot gas flow path at a reduced cooling air exit momentum in order to maximize the film coverage on the ring segment hot surface. Coolant penetration into the gas path is therefore minimized; yielding a good build-up of the coolant sub-boundary layer next to the ring segment surface. Thus, a better film coverage in the streamwise and circumferential directions for the ring segment is achieved. The combination of multiple hole convection cooling plus diffusion hole film cooling with very high film coverage yields a very high cooling effectiveness and a uniform wall temperature for the ring segment. Also, the diffusion chamber  34  reduces the chance for the metering hole  33  to be plugged as the blade tips rub into the ring segment  31 . 
     Cooling air supplied through the blade ring carrier  11  through cooling air feed holes  15  flows into the cooling air cavity  14  and then through the impingement holes  22  in the impingement plate  21  to produce impingement cooling of the backside surface of the ring segment  31 . The spent impingement cooling air is then collected in the impingement cavity (formed between the impingement plate  21  and the ring segment  31 ) and then flows through the metering holes  33  and the diffusion chambers  34  formed within the pedestals  31 . 
     The amount of cooling air for each individual circumferential and streamwise pedestal  32  is sized based on the local gas side heat load and pressure, which therefore regulates the local cooling performance and the metal temperature of the ring segment. The spent cooling air is metered through the metering holes  33  prior to being discharged through the diffusion chambers  34 . With the design of the present invention, the usage of cooling air for a given ring segment inlet gas temperature and pressure profile is maximized. Also, cooling air is metered twice prior to being diffused into the diffusion chambers  34  which allows for the cooling air to generate a very high level of backside convection cooling achieving a uniform cooling for the ring segment. This design also allows for the amount of cooling air discharged at various locations on the ring segment to be controlled. The spent cooling air is discharged from the ring segment as a layer of film cooling air onto the hot surface of the ring segment and the TBC surface. 
     Major design advantages of the ring segment construction and cooling circuit over the prior art ring segments are described below. The TBC attachment construction increases the TBC effective thickness that results in a higher reduction of ring segment metal temperature or a higher reduction of cooling flow. The series of diffusion chambers on the ring segment surface reduces the ring segment hot side convection surface and thus reduces the heat load on the ring segment. The series of pedestals increases the total bonding surface area for the TBC. During engine operation, the TBC in-between each pedestal is compressed and therefore increases the life and endurance of the TBC. A thicker layer of TBC can be used with less chance of spallation occurring. Multiple metering and diffusion holes are used in the ring segment cooling design. The diffusion chambers located at the exit of the metering holes reduces the film hole plugging issues associated with prior art film cooling holes.