Abstract:
A turbine shroud cooling cavity is partitioned to define a plurality of cooling chambers for sequentially receiving cooling steam and impingement cooling of the radially inner wall of the shoud. An impingement baffle is provided in each cooling chamber for receiving the cooling media from a cooling media inlet in the case of the first chamber or from the immediately upstream chamber in the case of the second through fourth chambers and includes a plurality of impingement holes for effecting the impingement cooling of the shroud inner wall.

Description:
This invention was made with Government support under Government Contract No. DE-FC21-95-MC31176 awarded by the Department of Energy. The Government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to the cooling of turbine shrouds and, more particularly, to an apparatus for the impingement cooling of turbine shrouds as well as a system for flowing a cooling medium, in series, through several cooling cavities of a turbine shroud in a single, closed circuit. 
     Shrouds in an industrial gas turbine engine are located over the tips of the bucket. The shrouds assist in creating the annulus that contains the hot gas path air used by the buckets to produce rotational motion and, therefore, power. Thus, the shrouds are used to form the gas path of the turbine section of the engine. In advanced gas turbine designs, it has been recognized that the temperature of the hot gas flowing past the turbine components could be higher then the melting temperature of the metal. It is therefore necessary to establish a cooling scheme to protect the hot gas path components during operation. 
     Typical turbine shrouds are cooled by conduction, impingement cooling, film cooling or combinations of the above. More specifically, one method for cooling turbine shrouds employs an air impingement plate which has a multiplicity of holes for flowing air through the impingement plate at relatively high velocity due to a pressure difference across the plate. The high velocity air flow through the holes strikes and impinges on the component to be cooled. After striking and cooling the component, the post-impingement air finds its way to the lowest pressure sink. 
     Cooling air usage in a gas turbine is very costly for performance and emissions. However, as noted above, high technology engines produce high firing temperatures and the hot gas path components need to be actively cooled to be able to withstand the high gas path temperatures encountered under these circumstances. 
     Steam has been demonstrated to be a desired alternative cooling media for cooling gas turbine parts, particularly for combined-cycle plants. However, because steam has a higher heat capacity than the combustion gas, it is inefficient to allow coolant steam to mix with the hot gas stream. Consequently, it is desirable to maintain cooling steam inside the hot gas path components in a closed circuit. Using a closed circuit cooling system achieves the objectives of greater performance with less emissions. 
     U.S. Pat. No. 5,391,052, the disclosure of which is incorporated herein by this reference, describes apparatuses and methods for impingement cooling of turbine components, particularly turbine shrouds using steam as a cooling medium. U.S. Pat. No. 5,480,281, the disclosure of which is incorporated herein by this reference, provides an apparatus for impingement cooling turbine shrouds in a manner to reduce cross flow effects as well as a system for flowing a cooling medium, in series, through a pair of cooling cavities of the turbine shroud in a single flow circuit. While the apparatuses and methods disclosed in these patents afford effective steam cooling of turbine shrouds, there remains a continuing need for improving turbine shroud cooling while minimizing the amount of cooling media required and reducing cross flow effects. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides an improved closed cooling flow circuit for cooling turbine shrouds which provides for flowing a cool medium through a plurality of cooling chambers defined in the cooling cavity of the shroud so as to achieve a series of impingement cooling operations to maximize the cooling of the wall of the shroud exposed to the hot gas path and to minimize detrimental cross flow effects without reducing the area that is subject to impingement cooling. 
     The closed circuit cooling configuration described hereinbelow may be used with any cooling medium. However, in the presently preferred embodiment, the cooling medium is steam and thus steam will generally be referred to hereinbelow in a non-limiting manner as the cooling medium. 
     The invention is embodied, therefore, in an apparatus in which steam is brought on board into the outer shroud and spilt so as to be directed to the respective inner shrouds. Within each inner shroud, the steam or other cooling medium is impinged on the shroud inner surface opposite the hot gas path surface of the inner shroud. The post impingement steam flows into a second chamber of the inner shroud to again be impinged on the shroud inner surface for impingement cooling of that portion of the inner shroud. In the presently preferred, exemplary embodiment, the flow of post impingement steam and re-impingement of the inner shroud surface is then repeated through third and fourth chambers of the inner shroud. The spent steam is then returned to the system for being reused in the cycle. The system described hereinbelow is particularly adapted for a combined cycle system installation. 
     The present invention improves engine performance and reduces engine emissions while still maintaining the program requirements of part life and cost effectiveness. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These, as well as other objects and advantages of this invention, will be more completely understood and appreciated by careful study of the following more detailed description of the presently preferred exemplary embodiments of the invention taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic elevational view of a stage  1  shroud as disposed in a gas turbine; 
     FIG. 2 is a perspective view of a steam cooled shroud assembly embodying the invention; 
     FIG. 3 is an exploded perspective view of the assembly of FIG. 2; and 
     FIG. 4 is an exploded perspective view of the stage  1  inner shroud assembly. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The shroud system which surrounds the buckets forming the gas path is composed of a number of outer shrouds which are the carriers of at least one inner shroud. In the illustrated example, one outer shroud and two inner shrouds make up one shroud assembly and forty-two ( 42 ) such shroud assemblies make up one shroud set. FIG. 1 illustrates a shroud assembly  10  disposed radially outside the stage  1  buckets  12 , only one of which is shown in FIG.  1 . Also shown in FIG. 1 is the turbine shell interface  14 , nozzle hook interface  16  and the inflow of cooling media shown by dash dot line S. As noted above, the closed circuit cooling configuration described hereinbelow may be used with any cooling medium. However, in the presently preferred embodiment the cooling medium is steam and thus steam will generally be referred to hereinbelow in a non-limiting manner as the cooling medium. 
     FIG. 2 shows in greater detail the assembly of the outer shroud  18  and first and second inner shrouds  20  in this exemplary embodiment. The steam inlet port is shown at  22  whereas the outlet or exit port is designated  24 . The inlet and exit ports are formed in the outer cover to the outer shroud  18 . 
     FIG. 3 shows this exemplary embodiment of the invention in greater detail. As noted above, the steam inlet port  22  and steam outlet port  24  are defined in outer cover  26 . This particular system has steam tubes or piping  28  internal to the outer shroud that interfaces between the inlet and exit ports and the inner shroud interfaces for flowing the steam to respective inner shrouds, and returning spent cooling media, as described in greater detail below. This piping is enclosed in the outer shroud during shroud assembly. 
     Only one of the inner shrouds  20  is shown in FIG. 3 although, as noted above, in this exemplary embodiment, two inner shrouds are associated with each outer shroud  18 . The inner shroud is engaged with the outer shroud in a conventional manner and in this example an inner shroud anti rotation pin  30  extends therebetween. The inner shroud is partitioned by ribs or partition walls  32 ,  34 ,  36 ,  38  as shown in greater detail in FIG. 4 to define four cooling chambers  40 ,  42 ,  44 ,  46 . An impingement baffle inserts  48 ,  50 ,  52 ,  54  is disposed in each of these four chambers, as described in greater detail below, and an inner shroud cover plate  56  is provided to over lie the impingement baffles and to communicate with the respective cooling media tubes  28 ,  90  which extend through a compartment  58  therefor defined in the outer shroud  18 . The cover plate  56  thus closes the chambers  40 ,  42 ,  44 ,  46  of the inner shroud  20  and controls/limits the cooling media inflow to and outflow from the inner shroud chambers. 
     Each impingement baffle divides its respective cooling chamber into a first, upstream compartment, and a second, downstream compartment. In the illustrated embodiment the impingement baffle insert defines an interior space that comprises the upstream chamber. Furthermore, in the illustrated embodiment, the second, downstream compartment is the volume of the respective chamber that surrounds the impingement baffle insert, but is predominantly defined between the impingement baffle insert and the radially inner wall of the respective chamber. Each impingement baffle insert has a plurality of flow openings defined therethrough for communicating cooling medium from the first compartment through those openings into the second compartment for impingement cooling of radially inner wall of the chamber; which is also the radially inner wall of the shroud assembly  10 . 
     Thus, as illustrated, steam is brought on board through an interface at the forward end of the outer shroud  18 . The steam is then carried through the steam piping  28  and split between the two inner shrouds  20  associated with the respective outer shroud  18 . In the inner shroud  20 , the steam enters the first chamber  40  of the four illustrated chambers, more specifically a first,upstream compartment  60  thereof defined by the impingement baffle  48  received therewithin. The cooling steam is impinged through the impingement holes  62  on the bottom surface, and in this example also on the side wall, of the impingement baffle  48  and is impinged upon the inner surface of the inner shroud radially inner wall  64 . 
     The post impingement steam then flows from the first chamber  40  to the second chamber  42 . As shown, the impingement baffle  48  of the first chamber is spaced from the rearward wall  32  that separates the first and second chambers  40 ;,  42  so as to allow post impingement cooling media to flow therebetween. One or more apertures, such as a cooling media aperture  66  is defined in wall  32  so as to allow the flow of that post impingement cooling media into the second chamber  42 . 
     As shown in FIG. 4, a cooling media inlet  68  is defined in the impingement baffle  50  of the second chamber  42  to receive the flow of cooling media from the first chamber  40  into the first, upstream compartment  70  of the second chamber that is defined therewithin. The cooling media then flows through holes  72  to be again impinged onto the inner surface of the inner shroud radially inner wall  64 . 
     The impingement baffle  50  of the second chamber  42  is spaced from the rib or wall  34  separating the second and third chambers  42 ,  44  so as to allow the post impingement cooling media to flow therebetween and then through the cutout or aperture(s)  74  defined in wall  34 . An aperture (not shown) is defined in the impingement baffle  52  of the third chamber  44  so that the cooling media will flow into the upstream compartment of the third chamber, defined within the impingement baffle  52 . The cooling media flows through holes  76  to again impinge on the inner shroud inner surface for further cooling thereof. 
     The flow of the cooling media through the inner shroud continues as the cooling steam flows through an aperture or cutout  78  in the wall  36  disposed between the third and fourth chambers  44 ,  46  into the impingement baffle  54  of the fourth, and in this embodiment final, cooling chamber  46 . The cooling media is once again impinged by flowing through holes  80 , to impinge against the inner surface of the inner shroud radially inner wall. The spent cooling steam thereafter flows to the steam exit  82  through a gap  84  defined between the exit plate  86  and the upper wall  88  of the impingement baffle  54 , as shown. The steam flows through the exhaust passage defined by exit tube  90  to be combined with the spent cooling media from the second inner shroud (not shown in FIG. 4) and exits through the steam piping  28  to an interface at the forward end of the outer shroud where it is returned to the combined cycle system. 
     As mentioned above, the illustrated system has piping  28  internal to the outer shroud  18  that interfaces between the inlet and exit ports  22 ,  24  and the inner shroud cover plate  56 . This piping is enclosed in the outer shroud during the assembly of the shroud fabrication. An access hole  92  is provided in the outer shroud to access the piping connection to the inner shroud to inspect the connection to ensure that the connection is satisfactory. This access has been covered by a plate  94 , as shown in FIG. 3, to complete the shroud cooling system. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.