Abstract:
A turbine blade tip plenum has means for preferentially directing or diverting cooling air from a cooling passageway network to cool areas subject to extreme heat.

Description:
BACKGROUND OF THE INVENTION 
   (1) Field of the Invention 
   This invention relates to turbomachinery, and more particularly to cooled turbine blades. 
   (2) Description of the Related Art 
   Heat management is an important consideration in the engineering and manufacture of turbine blades. Blades are commonly formed with a cooling passageway network. A typical network receives cooling air through the blade platform. The cooling air is passed through convoluted paths through the airfoil, with at least a portion exiting the blade through apertures in the airfoil. These apertures may include holes (e.g., “film holes” distributed along the pressure and suction side surfaces of the airfoil and holes at junctions of those surfaces at leading and trailing edges. Additional apertures may be located at the blade tip. In common manufacturing techniques, a principal portion of the blade is formed by,a casting and machining process. During the casting process a sacrificial core is utilized to form at least main portions of the cooling passageway network. Proper support of the core at the blade tip is associated with portions of the core protruding through tip portions of the casting and leaving associated holes when the core is removed. Accordingly, it is known to form the casting with a tip pocket into which a plate may be inserted to at least partially obstruct the holes left by the core. This permits a tailoring of the volume and distribution of flow through the tip to achieve desired performance. Examples of such constructions are seen in U.S. Pat. Nos. 3,533,712, 3,885,886, 3,982,851, 4,010,531, 4,073,599 and 5,564,902. In a number of such blades, the plate is subflush within the casting tip pocket to leave a blade tip pocket or plenum. 
   BRIEF SUMMARY OF THE INVENTION 
   One aspect of the invention involves providing the plenum with means for preferentially directing or diverting cooling air from a leading edge branch of the network along the pressure side of the rim forming the plenum. This may be achieved by a tip plate only partially blocking a leading port in the casting. The plate may have a leading edge positioned to direct flow through the port preferentially along the compression (pressure) side. The plate leading edge may be angled forwardly to a local meanline of the airfoil section. The plate may block more area of the port on the suction side of the mean line than on the pressure side. A length along a pressure side of the blade tip pocket ahead of the plate may be longer than a length along the suction side. 
   In another aspect of the invention, the blade is provided with means for preferentially directing flow from a trailing passageway to the pressure side. This may be achieved by having a plate trailing portion extending along a suction side of a trailing port but not along an adjacent pressure side. The trailing portion along the suction side may protrude relative to a portion thereahead. The trailing portion along the pressure side may be recessed relative to the portion thereahead. 
   In another aspect, a wall of the tip plenum may have a side trailing edge gap on one side (e.g., the pressure side) with means for reducing stress concentration at the gap. This may be achieved by having a radius of curvature at a leading inboard corner of the gap effective to relieve thermal and mechanical stresses. 
   The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a view of a turbine blade according to principles of the invention. 
       FIG. 2  is a view of a tip of a casting of the blade of FIG.  1 . 
       FIG. 3  is a view of a cover plate for a tip compartment of the blade of FIG.  1 . 
       FIG. 4  is a partial view of a trailing edge tip portion of a pressure side of the blade of FIG.  1 . 
       FIG. 5  is a view of the tip of the blade of FIG.  1 . 
       FIG. 6  is a view of a leading portion of the tip of the blade of FIG.  1 . 
       FIG. 7  is a partial view of a trailing portion of a tip compartment of the blade of FIG.  1 . 
   

   Like reference numbers and designations in the various drawings indicate like elements. 
   DETAILED DESCRIPTION 
     FIG. 1  shows a turbine blade  40  having an airfoil  42  extending along a length from a proximal root  44  at an inboard platform  46  to a distal end tip  48 . A number of such blades may be assembly side by side with their respective inboard platforms forming a ring bounding an inboard portion of a flow path. In an exemplary embodiment, a principal portion of the blade is unitarily formed of a metal alloy (e.g., as a casting). The casting is formed with a tip compartment in which a separate cover plate  50  is secured. 
   The airfoil extends from a leading edge  60  to a trailing edge  62 . The leading and trailing edges separate pressure and suction sides or surfaces  64  and  66 . For cooling the blade, the blade is provided with a cooling passageway network coupled to ports (not shown) in the platform. The exemplary passageway network includes a series of cavities extending generally lengthwise along the airfoil. A foremost cavity is identified as a leading edge cavity extending generally parallel to the leading edge. An aftmost cavity is identified as a trailing edge cavity extending generally parallel to the trailing edge. These cavities may be joined at one or both ends and/or locations along their lengths. The network may further include holes extending to the pressure and suction surfaces  64  and  66  for further cooling and insulating the surfaces from high external temperatures. Among these holes may be an array of trailing edge holes  80  extending between the trailing edge cavity and a location proximate the trailing edge. 
   In an exemplary embodiment, the principal portion of the blade is formed by casting and machining. The casting occurs using a sacrificial core to form the passageway network. An exemplary casting process forms the resulting casting with the aforementioned casting tip compartment  100  (FIG.  2 ). The compartment has a web  102  having an outboard surface  103  forming a base of the casting tip compartment. The outboard surface  103  is below a rim  104  of a wall structure having portions  105  and  106  on pressure and suction sides of the resulting airfoil. The web  102  is formed with a series of apertures  110 ,  112 ,  114 ,  116 ,  118 , and  120  from leading to trailing edge. These apertures may be formed by portions of the sacrificial core mounted to an outboard mold for support. The apertures are in communication with the passageway network. The apertures may represent an undesired pathway for loss of cooling air from the blade. Accordingly it is advantageous to fully or partially block some or all of the apertures with the cover plate  50  (FIG.  3 ). The cover plate has inboard and outboard surfaces  130  and  132  (FIG.  4 ). The inboard surface  130  lies flat against the web surface  103  and the outboard surface  132  lies recessed (subflush) below the rim  104  to leave a blade tip pocket or compartment. In operation, the rim (subject to recessing described below) is substantially in close proximity to the interior of the adjacent shroud (e.g., with a gap of about 0.1 inch). 
   The cover plate  50  is initially formed including a perimeter having a first portion  140  generally associated with the contour of the airfoil pressure side and a second portion  142  generally associated with the airfoil suction side. Exemplary cover plate material is nickel-based superalloy (e.g., UNS N06625 0.03 inch thick). The portions  140  and  142  are (subject to departures describe below) dimensioned to closely fit within the tip compartment adjacent the interior surface of the wall structure portions  105  and  106 . In the exemplary embodiment, the perimeter portions  140  and  142  do not extend all the way to the leading edge. They terminate at a linking portion  144  which in the exemplary embodiment is recessed from the leading edge along both pressure and suction sides. Toward the trailing edge, the portions are joined by a trailing perimeter portion  146 . As is described in further detail below, a trailing part  148  of the perimeter portion  140  is slightly recessed from a remainder thereof and a trailing part  150  of the perimeter portion  142  is slightly protruding relative to a remainder. The cover plate further includes apertures  160 ,  162 , and  164 . 
   The cover plate  50  is initially formed including a perimeter having a first portion  140  generally associated with the contour of the airfoil pressure side and a second portion  142  generally associated with the airfoil suction side. Exemplary cover plate material is nickel-based superalloy (e.g., UNS N06625 0.03 inch thick). The portions  140  and  142  are (subject to departures describe below) dimensioned to closely fit within the tip compartment adjacent the interior surface of the wall structure portions  105  and  106 . In the exemplary embodiment, the perimeter portions  140  and  142  do not extend all the way to the leading edge. They terminate at a cover plate perimeter linking portion  144  which in the exemplary embodiment is recessed from the leading edge along both pressure and suction sides. Toward the trailing edge, the portions are joined by a trailing perimeter portion  146 . As is described in further detail below, a trailing part  148  of the perimeter portion  140  is slightly recessed from a remainder thereof and a trailing part  150  of the perimeter portion  142  is slightly protruding relative to a remainder. The cover plate further includes apertures  160 ,  162 , and  164 . 
   In the exemplary embodiment, when so installed, a leading portion  180  ( FIG. 6 ) of the cover plate partially covers the leading aperture  110  and thus partially blocks the leading edge cavity from communication with the blade tip compartment or plenum. In the exemplary embodiment, the trailing extremity of the aperture  110  is nearly perpendicular to a local mean line  520 . Most of the leading portion  180  covering the aperture  110  covers that portion of the aperture on the suction side of the mean line and covers a greater proportion of the aperture area on the suction side than on the pressure side. The nature of the blocking will be influenced by port geometry and airfoil section. In exemplary embodiments, area of the leading port blocked by the plate on the suction side of the mean line is 2-6 times (or, more narrowly 4-5 times) the area blocked on the pressure side. 
   The shape of the leading portion  180  may vary. In the exemplary embodiment, the cover plate perimeter linking portion  144  is nearly straight and makes an angle θ of lees than 90° with the chordline on the pressure side in the leading direction. Due to this incline, the suction side perimeter portion  142  extends closer to the leading edge than does the pressure side portion  140 . The result of this arrangement is that the leading portion  180  preferentially directs airflow toward the pressure side for enhanced cooling on the pressure side. This produces a more efficient use of airflow as the pressure side may require greater cooling. 
   In the exemplary embodiment, the second web aperture  112  and first cover plate aperture  160  are substantially coextensive whereas the cover plate may substantially or more significantly obstruct the remaining web apertures. In the exemplary embodiment, the cover plate apertures  162  and  164  are aligned with the web apertures  114  and  116  but are substantially smaller and therefore substantially reduce airflow through such apertures. In the exemplary embodiment, the cover plate substantially seals the web aperture  118  and, as described in further detail below, extends partially over the trailing web aperture  120 . Relatively low restriction of flow through the aperture  112  provide for efficient use of cooling air as such air can be expected to pass along the greater portion of the tip compartment than would air introduced more toward the trailing edge. 
     FIG. 7  shows the trailing portion  190  of the cover plate partially covering the trailing aperture  120  of the casting. Specifically, the trailing portion  190  covers a leading suction side portion of the aperture, the recessed part  148  being spaced apart from a suction side perimeter of such aperture. This configuration again preferentially directs the air from the trailing edge cavity through the aperture  120  along the pressure side. 
     FIG. 4  further shows the suction side tip wall portion  106  extending substantially all the way to the trailing edge  62 . The pressure side wall portion  105  does not so extend intact. The wall portion  105  extends intact to a location  200 , to the trailing edge of which it is recessed relative to the adjacent area of the wall  106 . In the exemplary embodiment, the location  200  is a distance  540  ahead of the trailing edge. In the exemplary embodiment, the wall portion  105  vanishes to the rear of a trailing edge extremity of the trailing edge cavity. The wall portion  105  merges with a base surface  202  recessed relative to the rim  104  along the surface portion  106  by a distance  542 . The exemplary distance  542  may be approximately the same as the, recess of the web surface  103  relative to the rim surface  104 . A trailing portion of the exemplary wall portion  105  has a continuously curving concave transition  204  to the surface  202 . This transition has a radius or radi of curvature and is sufficiently large to reduce thermal/mechanical stress concentrations contrasted with a right angle transition and reduce the chances of resulting cracking. Exemplary radii are between 0.4 and 1.0 times (more narrowly 0.6 and 0.8 times) the distance  542 . An exemplary numerical range is between 0.100 inch and 0.300 inch. 
   One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, many details will be application-specific. To the extent that the principles are applied to existing applications or, more particularly, as modifications of existing blades, the features of those applications or existing blades may influence the implementation. Accordingly, other embodiments are within the scope of the following claims.