Abstract:
An airfoil has first and second ends, leading and trailing edges, and an internal cooling passageway network. A plurality of trailing edge holes extend from the trailing edge to a trailing edge cavity of the network. The trailing edge holes are arrayed at a spacing which progressively changes from the first end toward the second end.

Description:
U.S. GOVERNMENT RIGHTS 
     The invention was made with U.S. Government support under contract F33657-94-d-2001 awarded by the U.S. Air Force. The U.S. Government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     This invention relates to turbomachinery, and more particularly to cooled turbine blades and vanes. 
     (2) Description of the Related Art 
     Trailing edge cooling is a common feature of turbine blades and vanes. In one common method of manufacture, the main passageways of a cooling network within the blade/vane airfoil are formed utilizing a sacrificial core during the blade/vane casting process. The airfoil surface may be provided with holes communicating with the network. Some or all of these holes may be drilled. In one method of manufacture, an array of trailing edge holes may be drilled parallel to each other and at an even pitch. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly one aspect of the invention involves an airfoil having first and second ends, leading and trailing edges, and an internal cooling passageway network. A plurality of trailing edge holes extend from the trailing edge to a trailing edge cavity of the network. The trailing edge holes are arrayed at a spacing which progressively changes from the first end toward the second end. The network may be adapted to direct cooling gas within the trailing edge cavity to increase in temperature in a first direction parallel to the trailing edge. The spacing may substantially progressively decrease in that first direction. The trailing edge cavity may be an impingement cavity. Other aspects of the invention relate to methods of manufacture of a turbine element. 
     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 vane. 
         FIG. 2  is a partial sectional view of the vane of  FIG. 1 , taken along line  2 — 2 . 
         FIG. 3  is a view of the vane of  FIG. 2 , taken along line  3 — 3 . 
         FIG. 4  is a view of a vane/blade manufacturing apparatus. 
         FIG. 5  is an x-ray view of a turbine blade. 
       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  48  at an outboard platform  50 . A number of such vanes may be assembly side-by-side with their respective inboard and outboard platforms forming inboard and outboard rings bounding inboard and outboard portions of a flow path. In an exemplary embodiment, the vane is unitarily formed of a metal alloy. 
     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  (FIG.  2 ). For cooling the airfoil, the airfoil is provided with a cooling passageway network coupled to ports in one or both platforms. The exemplary passageway network includes a series of cavities extending generally lengthwise along the airfoil. An aftmost cavity is identified as a trailing edge cavity  70  extending generally parallel to the trailing edge. A penultimate cavity  72  is located ahead of the trailing edge cavity  70 . The cavities may be joined at one or both ends and/or locations along their lengths so as to permit flows from the penultimate cavity to the trailing edge cavity. In the illustrated embodiment, the cavities  70  and  72  are impingement cavities. The penultimate cavity  72  receives air from a supply cavity  73  through an array of apertures  75  ( FIG. 3 ) in the wall separating the two. The supply cavity  73  receives air from a port  74  in the platform  50 . Likewise, the trailing edge cavity  70  receives air from the penultimate cavity  72  via apertures  77  in the wall between the two. To the extent that the cooling air in the supply cavity  73  is heated as it progresses radially inward, the cooling air temperatures in the impingement cavities  70  and  72  will similarly increase in the radially inward direction. 
     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 a location proximate the trailing edge and an aft extremity of the trailing edge impingement cavity  70 .  FIG. 2  shows one such hole having a surface  82  centered about an axis  500  and extending along a length  502 . The exemplary hole has a circular section with a diameter  504 . 
       FIG. 3  shows a portion of the array of trailing edge holes  80 . The holes, or more precisely their axes  500 , are shown as at an angle θ 1  relative to the trailing edge and at an angle θ 2  relative to the local aft extremity  90  of the trailing edge impingement cavity  70 . To the extent that the trailing edge  62  and aft extremity  90  are parallel, the illustrated angles θ 1  and θ 2  will be identical to each other for a given hole  80 . A pitch  510  of the holes is measured as the hole centerline spacing along the trailing edge. 
     As is explored in further detail below, the axes  500  of every hole  80  need not be parallel to each other. Similarly, the angles θ 1  and/or θ 2  of each hole  80  need not be the same, nor need be their diameters  504  and lengths  502 . Structural integrity and manufacturing considerations may influence or dictate the separation of the trailing edge  62  from the aft extremity  90  of the cavity  70 . It is advantageous that the holes  80  be short and narrow so as to maximize possible cooling close to the trailing edge. The narrowness (e.g., the diameter) is largely limited by ease of drilling. Subject to additional manufacturing and terminal considerations (discussed below) this minimization would be achieved by having the axes  500  as close as possible to mutually perpendicular to the trailing edge  62  and aft extremity  90 . 
     Along the trailing edge, less cooling may be required per linear dimension along one portion of the trailing edge than along another. In the exemplary embodiment, with cooling air flowing generally radially inward in the supply cavity  73 , the air passing through the impingement cavities  72  and  70  and through the holes  80  may be cooler near the end  48  than near the root  44  due to the shorter net flow path. As this cooler air is more effective for heat transfer, less volume of air per linear dimension need be passed through the holes  80  near the end  48  than near the root  44 . Accordingly, to efficiently utilize the cooling air passing through the cavity  70 , it may be advantageous that the hole spacing generally increase in an outboard direction along the trailing edge. Ignoring manufacturing considerations and terminal considerations, the change in spacing could well be continuous, with a slight change in spacing from each hole to the next in accordance with an appropriate cooling distribution. 
     If, for example, the terminal end of the cavity  70  did not extend as far as the outboard end of the trailing edge, it might be desirable to slightly fan the holes near the outboard edge or otherwise enhance cooling. The inboard end of the trailing edge may also pose manufacturability problems due to interference with a drilling apparatus. If it is desired to drill the holes perpendicular to the trailing edge, the inboard platform may interfere with drilling of the inboardmost hole or holes along the trailing edge, thus, for a given drilling apparatus, a restriction to perpendicular holes might place the inboardmost hole to far outboard. Accordingly, for this hole such considerations may cause a reduction in the angle θ 1  below 90° so as to permit the hole to be sufficiently inboard. Also, access to the trailing cavity at inboard or outboard ends of the trailing edge may alter the angle θ 1  from that which might otherwise be desirable. Additionally, it may be desired to gang drill several holes at a time with a single drilling apparatus  150  ( FIG. 4 ) having several bits  152  (10 bits shown). The use of such apparatus may restrict freedom in the spacing selection and in the hole orientation. One such apparatus might require the several gang-drilled holes to be parallel to each other, preventing independent selection of the angle θ 1  for each hole. Ease of constructing such apparatus might require that the several hole axes be evenly spaced from each other, thereby preventing independent selection of spacing for each hole. Alternatively, however, the spacing could change along a given apparatus with a characteristic spacing (e.g., a mean or median) of one apparatus differing from that of the next. With such apparatus, a continuous change in spacing may be achieved. 
     With this in mind, in one gang-drilling example, a single drilling apparatus is utilized to drill a given number N (e.g., 5-15) of holes at a time. The apparatus may be used to drill a number M (e.g., 5 or more) of sets of such N holes with an exemplary total number of holes being between 40 and 200. The axes of each set could be nonparallel to the axes of the other sets, thus permitting the sets of holes to be relatively close to perpendicular to the trailing edge (again subject to departures due to terminal considerations). In another example, different drilling apparatus  150  having different axis spacing may be utilized to drill the different sets of holes. 
     By way of example, the radial span of the trailing edge may be about 1.0-15 inches depending on the application. The hole diameters may be between about 0.01 inch and 0.15, more narrowly about 0.015-0.025. The hole length may be between 5-25 times the hole diameter. In an exemplary vane embodiment, the vane is dimensioned so that the, when the ring is assembled, the root at the trailing edge is at a radius of about ten inches relative to the engine centerline. The outboard end of the trailing edge is at a radius of about 12.5 inches. In the exemplary embodiment, the spacing starts at approximately 2.1 times the hole diameter near the inboard platform, remains generally the same until the middle of the length of the trailing edge and then increases to approximately 2.7 times the diameter toward the outboard platform. Thus one drilling apparatus with the smaller spacing may drill several groups of holes and then a second apparatus having the larger spacing may drill the remainder (which in the exemplary embodiment is a slightly smaller number of holes). In the exemplary embodiment, the hole length varies from approximately 14.5 times the hole diameter near the inboard platform to approximately 13.75 times the hole diameter near the outboard platform. In the exemplary embodiment, along about the inboardmost 10% of the trailing edge, the holes near the inboard platform are at a spacing larger than the 2.1 figure due to a reduced cooling need near the platform. Thus it can be seen that the progressive spacing may be over only a substantial portion of the trailing edge (e.g., 40-90% or, more narrowly, 50-80%). 
       FIG. 5  shows a turbine blade  200  having a platform  202  and an airfoil  204  extending from a proximal root  206  at the platform to a distal end tip  208 . The airfoil may have substantial similarities to the vane airfoil. In given turbine, and a number of such blades may be positioned with their platforms side-by-side to form a ring. Such blade rings may be interspersed with the vane rings, the inboard platforms of both forming a generally continuous inboard wall of the flow path through the turbine. The exemplary blade airfoil has leading and trailing edges  210  and  212  separating pressure and suction sides  214  and  216 . The blade airfoil may have an array of trailing edge holes  220  similar to that of the vane airfoil. 
     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, desired flow characteristics of the turbine may influence hole arrangement in view of available manufacturing techniques. This is particularly true in redesign or retrofit of existing turbines. Accordingly, other embodiments are within the scope of the following claims.