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 generally to gas turbines, for example, for electrical power generation, and more particularly to cooling the stage one nozzles of such turbines. The invention relates in particular to an insert design for a gas turbine nozzle cavity that provides for both convection and impingement cooling. 
     The traditional approach for cooling turbine blades and nozzles was to extract high pressure cooling air from a source, for example, from the intermediate and final stages of the turbine compressor. In such a system, a series of internal flow passages are typically used to achieve the desired mass flow objectives for cooling the turbine blades. In contrast, external piping is used to supply air to the nozzles, with air film cooling typically being used and the air exiting into the hot gas stream of the turbine. In advanced gas turbine designs, it has been recognized that the temperature of the hot gas flowing past the turbine components could be higher than the melting temperature of the metal. It was therefore necessary to establish a cooling scheme to protect the hot gas path components during operation. Steam has been demonstrated to be a preferred cooling media for cooling gas turbine nozzles (stator vanes), particularly for combined-cycle plants. See, for example, U.S. Pat. No. 5,253,976, the disclosure of which is incorporated herein by this reference. For a complete description of the steam-cooled buckets, reference is made to U.S. Pat. No. 5,536,143, the disclosure of which is incorporated herein by reference. For a complete description of the steam (or air) cooling circuit for supplying cooling medium to the first and second stage buckets through the rotor, reference is made to U.S. Pat. No. 5,593,274, the disclosure of which is incorporated herein by reference. 
     Because steam has a higher heat capacity than the combustion gas, however, it is inefficient to allow the coolant steam to mix with the hot gas stream. Consequently, in conventional steam cooled buckets it has been considered desirable to maintain cooling steam inside the hot gas path components in a closed circuit. Nevertheless, certain areas of the components in the hot gas path cannot practically be cooled with steam in a closed circuit. For example, the relatively thin structure of the trailing edge of the nozzle vane effectively precludes steam cooling of that edge. Accordingly, air cooling is used to cool those portions of the nozzle vanes. For a complete description of the steam cooled nozzles with air cooling along the trailing edge, reference is made to U.S. Pat. No. 5,634,766, the disclosure of which is incorporated herein by reference. The flow of cooling air in a trailing edge cavity per se is the subject of a U.S. Pat. No. 5,611,662, the disclosure of which is incorporated herein by reference. 
     In the closed circuit system, a plurality of nozzle vane segments are provided, each of which comprises one or more nozzle vanes extending between inner and outer side walls. The vanes have a plurality of cavities in communication with compartments in the outer and inner side walls for flowing cooling media in a closed circuit for cooling the outer and inner walls and the vanes per se. Thus, cooling media may be provided to a plenum in the outer wall of the segment for distribution to chambers therein and passage through impingement openings in a plate for impingement cooling of the outer wall surface of the segment. The spent impingement cooling media flows into leading edge and aft cavities extending radially through the vane. At least one cooling fluid return/intermediate cooling cavity extends radially and lies between the leading edge and aft cavities. A separate trailing edge cavity may also provided. 
     Conventionally, in each of the leading edge, intermediate and aft cavities, inserts are provided, having impingement flow holes. Thus, impingement cooling is typically provided in the leading and aft cavities of the vane, as well as in the return cavities of the first stage nozzle vane. The inserts in the leading and aft cavities comprise sleeves having a collar at their inlet ends for connection with integrally cast flanges in the outer wall and extend through the cavities spaced from the walls thereof. The inserts have impingement holes in opposition to the walls of the cavity whereby steam or air flowing into the inserts flows outwardly through the impingement holes for impingement cooling of the vane walls. Similarly, inserts in the return intermediate cavities have impingement openings for flowing impingement cooling medium against the side walls of the vane. 
     BRIEF SUMMARY OF THE INVENTION 
     A problem encountered in conventional closed circuit cooled turbine nozzles, whether air or steam is used as the coolant, is that the post impingement coolant can become cross flow and reduce the effectiveness of more downstream impingement cooling. This also causes uncertainty in the calculations used to determine the cross flow effect upon heat transfer coefficient along the cavity. 
     Another problem encountered in conventional nozzle cavity impingement cooling systems is that due to the significant post impingement cross flow in a small cavity, a large pressure drop is needed to achieve adequate heat transfer coefficients. This large pressure drop results in a more complex design of other parts of the nozzle cooling circuit, to balance the pressure drop from other branches of the closed circuit. In most cases, excessive pressure drop from the cooling flow may not be possible due to other restrictions in the design. Reducing this pressure drop would allow for more simplified designs elsewhere in the flow circuit. It may also be required for the system to operate efficiently. 
     One way in which this cross flow problem has been partially addressed is to define ribs oriented generally transverse to the radial extent of the nozzle cavities so that post impingement coolant flows in a chord-wise direction to a post impingement cooling flow channel for passage to the radially inner wall of the vane segment. However, it would be desirable to address the foregoing problems associated with current nozzle insert design in a manner that would simplify the design of the vane cavity and the insert, reduce or eliminate the cross flow effect and reduce the uncertainty associated with the design. 
     The inventors have recognized that reducing the amount of impingement, or changing it from impingement cooling to convective cooling, will reduce or eliminate the cross flow effect and reduce the uncertainty associated with the design. More specifically, the present invention provides a novel cavity insert design wherein the amount of impingement flow is reduced so that the cooling provided along a portion of the length of the nozzle cavity is changed from impingement cooling to convective cooling. This reduces or eliminates the cross-flow effect and reduces the uncertainty associated with the design. 
     Accordingly, in an embodiment of the present invention, there is provided a closed circuit stator vane segment comprising radially inner and outer walls spaced from one another, a vane extending between the inner and outer walls and having leading and trailing edges and pressure and suction sides, the vane including discrete cavities between the leading and trailing edges and extending lengthwise of the vane, and an insert sleeve in at least one of those cavities, the insert sleeve having impingement holes for directing the cooling media against interior wall surfaces of the cavity. The impingement holes are defined in first and second walls of the insert sleeve facing respectively the pressure and suction sides of the vane. However, the impingement holes of at least one of those first and second walls are defined along substantially only a first, upstream portion thereof whereby the cooling flow is predominantly impingement cooling along the first, upstream portion and the cooling flow is predominantly convective cooling along a second, downstream portion thereof. 
     In a currently preferred embodiment, the impingement holes of both the first and second walls of the insert sleeve extend along substantially only respective first, upstream portions thereof so that there is a transition to convective cooling along both those walls. Even more preferably, the impingement holes in the second wall, facing the suction side of the vane extend along a lesser extent of that wall than the impingement holes in the first wall. 
    
    
     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, cross-sectional view of an exemplary first stage nozzle vane embodying the invention; 
     FIG. 2 is a schematic, broken away perspective view of a first stage nozzle vane with an impingement cooling insert sleeve embodying the invention disposed in a vane cavity thereof; 
     FIG. 3 is a perspective view of another insert sleeve embodying the invention; and 
     FIG. 4 is a schematic vertical cross-section of yet another insert sleeve embodying the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As discussed previously, the present invention relates in particular to cooling circuits for the first stage nozzles of a turbine, reference being made to the previously identified patents for disclosures of various other aspects of the turbine, its construction and methods of operation. Referring now to FIG. 1, there is schematically illustrated in cross-section a vane  10  comprising one of the plurality of circumferentially arranged segments of the first stage nozzle. It will be appreciated that the segments are connected one to the other to form an annular array of segments defining the hot gas path through the first stage nozzle of the turbine. Each segment includes radially spaced outer and inner walls  12  and  14 , respectively, with one or more of the nozzle vanes  10  extending between the outer and inner walls. The segments are supported about the inner shell of the turbine (not shown) with adjoining segments being sealed one to the other. It will therefore be appreciated that the outer and inner walls and the vanes extending therebetween are wholly supported by the inner shell of the turbine and are removable with the inner shell halves of the turbine upon removal of the outer shell as set forth in U.S. Pat. No. 5,685,693. For purposes of this description, the vane  10  will be described as forming the sole vane of a segment. 
     As shown in the schematic illustration of FIG. 1, the vane has a leading edge  18 , a trailing edge  20 , and a cooling steam inlet  22  to the outer wall  12 . A return steam outlet  24  also lies in communication with the nozzle segment. The outer wall  12  includes outer side railings  26 , a leading railing  28 , and a trailing railing  30  defining a plenum  32  with the upper wall surface  34  and an impingement plate  36  disposed in the outer wall  12 . (The terms outwardly and inwardly or outer and inner refer to a generally radial direction). Disposed between the impingement plate  36  and the inner wall  38  of outer wall  12  are a plurality of structural ribs  40  extending between the side railings  26 , leading railings  28  and trailing railing  30 . The impingement plate  36  overlies the ribs  40  throughout the full extent of the plenum  32 . Consequently, steam entering through inlet port  22  into plenum  32  passes through the openings in the impingement plate  36  for impingement cooling of the inner surface  38  of the outer wall  12 . 
     In this exemplary embodiment, the first stage nozzle vane  10  has a plurality of cavities, for example, a leading edge cavity  42 , two aft cavities  52 ,  54 , four intermediate return cavities  44 ,  46 ,  48  and  50 , and also a trailing edge cavity  56 . 
     Leading edge cavity  42  and aft cavities  52 ,  54  each have an insert sleeve,  58 ,  60 , and  62 , respectively, while each of the intermediate cavities  44 ,  46 ,  48  and  50  have similar insert sleeves  64 ,  66 ,  68 , and  70 , respectively, all such insert sleeves being in the general form of hollow sleeves, having perforations as described in greater detail herein below. The insert sleeves are preferably shaped to correspond to the shape of the particular cavity in which the insert sleeve is to be provided and sides of the sleeves are provided with a plurality of impingement cooling openings, along portions of the insert sleeve which lie in opposition to the walls of the cavity to be impingement cooled. For example, as shown in FIG. 2, in the leading edge cavity  42 , the forward edge of the insert sleeve  58  would be arcuate and the side walls would generally correspond in shape to the side walls of the cavity  42 , with such walls of the insert sleeve having impingement openings along a portion of the length thereof as described herein below. The back side of the sleeve or insert sleeve  58 , disposed in opposition to the rib  72  separating cavity  42  from cavity  44 , however, would not have impingement openings. Similarly, in the aft cavities  52 ,  54 , the side walls of the insert sleeves  60  and  62  have impingement openings along a portion of the length thereof, as also described in more detail herein below, whereas the forward and aft walls of insert sleeves  60  and  62  are of a solid non-perforated material. 
     It will be appreciated that the insert sleeves received in cavities  42 ,  44 ,  46 ,  48 ,  50 ,  52 , and  54  are spaced from the walls of the cavities to enable cooling media, e.g., steam, to flow through the impingement openings to impact against the interior wall surfaces of the cavities, hence cooling the wall surfaces. 
     The conventional insert sleeve design has impingement cooling holes defined along the entire length of the insert sleeve although the holes are generally confined to the sides of the insert sleeve facing exterior walls of the vane, as noted above. While heat transfer in the cavity in which such insert sleeves are disposed has been increased by the impingement generated by such insert sleeves, as noted above, there is a large pressure drop over the cavity which leads to more complicated designs elsewhere in the nozzle configuration. In addition, as the accumulated post impingement coolant progresses downstream from the upstream end of the cavity, the cross-flow degradation increases. This causes both low heat transfer coefficient and high uncertainty in calculating the coefficient. 
     The present invention was developed to decrease the pressure drop over the length of the cavity, allowing for more simplified designs elsewhere in the nozzle. The invention was further developed to decrease the uncertainty involved in estimating the heat transfer coefficients. The invention was also developed to increase the Low Cycle Fatigue (LCF) life along the cavity to meet design requirements. 
     The insert sleeve provided as an embodiment of the invention has impingement cooling holes located on an upstream part of the insert. The other, downstream part of the insert sleeve is substantially imperforate in that it does not contain impingement holes, but rather acts as a blocking mechanism to increase the heat transfer coefficient by reducing the coolant flow area in the cavity to the gap between the insert sleeve and the cavity interior wall. This design reduces unintended post impingement coolant cross-flow, allows for heat transfer coefficients to be more accurately estimated and allows for a reduction in pressure drop from the inlet of the cavity to the outlet. 
     The general form of exemplary insert sleeves embodying the invention is illustrated in FIGS. 2-4. FIG. 2 illustrates an exemplary insert sleeve for the leading edge cavity, whereas FIG. 3 illustrates an exemplary insert sleeve for one of the return cavities and FIG. 4 illustrates an exemplary impingement hole distribution for an aft cavity. 
     The insert sleeve illustrated in FIGS. 2-3, for example, insert sleeve  64 , comprises an elongated sleeve  78  having an open lower or radially inner end with a marginal flange  80  for connection with a marginal flange (not shown) about the opening to the corresponding cavity, e.g., cavity  44 . The side walls  82 ,  84  of the sleeve  78  are provided with a plurality of impingement cooling openings  86 ,  88 , respectively. As illustrated, impingement cooling holes or openings  86 ,  88  are defined along first, upstream portions  87 ,  89  of this sleeve for flowing the cooling medium into the spaces between the sleeve and the interior vane wall surfaces to be impingement cooled. Second, downstream portions  90 ,  92  of the sleeve  78  do not have impingement holes. Instead, the downstream portions reduce the coolant flow area in the cavity  42  by defining channels that receive post impingement cooling flow from the spaces defined adjacent the first, impingement hole portions of the sleeve, thereby to increase the heat transfer coefficient. This design reduces the undesirable post impingement coolant (air or steam) cross-flow, allows for the heat transfer coefficient to be more accurately estimated, and allows for a reduction in pressure drop from the inlet of the cavity to the outlet. 
     As is further shown in FIG. 3, the extent of the portions of the sleeve on which the impingement holes  86 ,  88  are respectively provided is further dependent, in the presently preferred embodiment of the invention, upon whether the insert sleeve side wall faces the pressure side or suction side of the airfoil. While the extent of the impingement holes on each side can be varied as deemed necessary or desirable to achieve the objectives of the invention, it can be seen that the extent of the impingement is preferably greater on the pressure side  82  of the sleeve  78  than on the suction side  84 . 
     Referring to FIG. 4, a similar type of insert sleeve  60  is provided in vane cavity  52 . As illustrated, e.g. in FIG. 2, the peripheral outline of insert sleeve  60  follows the contour of the shape of cavity  52 . The insert sleeve has impingement openings or holes  94 ,  96  on the side walls  98 ,  100  thereof whereby the coolant, whether it be steam or air, directed into the insert sleeve  60  from the plenum  32  (FIG. 1) flows outwardly through the impingement openings  94 ,  96  for impingement cooling of the outer walls of the vane on opposite sides of the cavity  52 . 
     The extent of the portion of the insert sleeve  60  on which the impingement holes  94 ,  96  are respectively provided is further dependent, in the presently preferred embodiment of the invention, upon whether the insert sleeve side wall faces the pressure side or suction side of the airfoil. In that regard, while the extent of the impingement holes on each side can be varied as deemed necessary or desirable to achieve the objectives of the invention, it can be seen that the extent of the impingement holes is preferably greater on the pressure side  98  of the insert sleeve  60  than on the suction side  100 . 
     The impingement cooling holes or openings  94 ,  96  are again located in upstream portions  102 ,  104  of the insert sleeve whereas the other, downstream portions  106 ,  108  of the insert sleeve  60  do not have impingement holes. Instead, the downstream portions reduce the coolant flow area in the cavity  52 , thereby to increase the heat transfer coefficient. As with the insert sleeve in the leading edge cavity, and the return cavities, the design of this insert sleeve reduces the undesirable post impingement coolant cross-flow, allows for the heat transfer coefficient to be more accurately estimated, and allows for a reduction in pressure drop from the inlet of the cavity to the outlet. 
     Flow analysis software was used to determine the heat transfer coefficients, and pressure drop along both the impingement and convectively cooled regions of the cavity. The analysis showed a decrease in pressure drop along with an increase in the heat transfer coefficient with the above described design. For example, for the sixth cavity  52  of the stage one nozzle of an exemplary turbine system having a vane  10  with a length of about 6.32 inches, impingement holes  94  extending along about 5.05 inches (80%) and impingement holes  96  extending along about 2.88 inches (45%) was determined to provide adequate heat transfer coefficients on both pressure and suction sides and a minimum pressure drop across the cavity. 
     As illustrated in FIG. 1, the post-impingement cooling steam flows into a plenum  73  defined by the inner wall  14  and a lower cover plate  76 . Structural strengthening ribs  75  are integrally cast with the inner wall  14 . Radially inwardly of the ribs  75  is an impingement plate  74 . As a consequence, it will be appreciated that the spent impingement cooling steam flowing from cavities  42 ,  52 , and  54  flows into the plenum  73  for flow through the impingement openings of impingement plate  74  for impingement cooling of the inner wall  14 . The spent cooling steam flows by direction of the ribs  75  towards the openings (not shown in detail) for return flow through the cavities  44 ,  46 ,  48 , and  50 , respectively, to the steam outlet  24 . Insert sleeves  64 ,  66 ,  68 , and  70  are disposed in the cavities  44 ,  46 ,  48 , and  50  in spaced relation from the side walls and ribs defining the respective cavities. The impingement openings lie on opposite sides of the sleeves for flowing the cooling media, e.g., steam, from within the insert sleeves through the impingement openings for impingement cooling of the side walls of the vane, as generally discussed above. The spent cooling steam then flows from the gaps between the insert sleeves and the walls of the intermediate cavities to outlet  24  for return to the coolant, e.g., steam, supply. 
     The air cooling circuit of the trailing edge cavity  56  of the combined steam and air cooling circuit of the vane illustrated in FIG. 1 generally corresponds to that of the &#39;766 patent and, therefore, a detailed discussion herein is omitted. 
     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.
A turbine vane segment is provided that has inner and outer walls spaced from one another, a vane extending between the inner and outer walls and having leading and trailing edges and pressure and suction sides, the vane including discrete leading edge, intermediate, aft and trailing edge cavities between the leading and trailing edges and extending lengthwise of the vane for flowing a cooling medium; and an insert sleeve within at least one of the cavities and spaced from interior wall surfaces thereof. The insert sleeve has an inlet for flowing the cooling medium into the insert sleeve and has impingement holes defined in first and second walls thereof that respectively face the pressure and suction sides of the vane. The impingement holes of at least one of those first and second walls are defined along substantially only a first, upstream portion thereof, whereby the cooling flow is predominantly impingement cooling along a first region of the insert wall corresponding to the first, upstream portion and the cooling flow is predominantly convective cooling along a second region corresponding to a second, downstream portion of the at least one wall of the insert sleeve.