Patent Publication Number: US-7901180-B2

Title: Enhanced turbine airfoil cooling

Description:
BACKGROUND OF INVENTION 
     1. Technical Field 
     This invention relates to the internal cooling of gas turbine engine, turbine airfoils and particularly the end portions thereof. 
     2. Background Art 
     Modern gas turbine engines operate at temperatures approaching 3000° F. Accordingly, it is a common practice to cool various components employed in such engines with air provided by the engine&#39;s compressor. Perhaps the most critical components to cool with compressor air are the first stage turbine blades and vanes which are exposed to products of combustion exiting the engine&#39;s combustor. 
     It is well known to provide such compressor discharge cooling air to first stage turbine blades and vanes by routing such air through passages internally of the airfoil portions thereof. Such passages may be cast into the airfoil portions or drilled into the blades or vanes by mechanical or electrochemical machining processes. 
     In the case of turbine blades and vanes for large industrial gas turbine engines, it is a common practice to employ shaped tube electrochemical machining to form cooling air passages which extend radially from the inner end of the airfoil to the outer end thereof. For enhanced convective cooling, the cooling air passages often include discontinuities in the walls thereof to enhance the turbulence of the flow of cooling air through the passages by eliminating the boundary layer of airflow along the passage walls. Such discontinuities, often referred to as turbulence promoters or turbulators, may take the form of grooves or ridges in the cooling passage walls. 
     While such turbulators enhance the convective cooling of the interiors of turbine blades and vanes, they necessarily increase the losses associated with the flow of cooling air through the passages and thus adversely affect the overall efficiency of the engine. Therefore, it has been the conventional wisdom to use such turbulators only where they are most necessary from the standpoint of thermal loading. It is generally accepted in the prior art that the locations where internal cooling of turbine blades and vanes is most critical (where thermal loads are greatest) are those locations in the blade or vane airfoils intermediate the root and tip portions thereof. Accordingly, as a result of qualitative analyses of the operating characteristics of blades and vanes, it has been the practice to provide such turbulators only in the intermediate portions of the internal cooling passages of turbine blades and vanes, the root and tip ends of the passages being smooth to minimize the inefficiencies associated with the creation of turbulent flow therein. 
     However, inspections of modern industrial gas turbine engines, as part of the routine overhaul and maintenance thereof, has revealed that the blades and vanes of such engines experience significant and often unanticipated thermal stress at the ends thereof as evidenced by, for example, cracking in the blade shrouds, such as, in the fillet where the shroud joins the blade. Several solutions to such thermal stress and damage to the blade have been proposed and typically involve a rather complex distribution of additional cooling passages and chambers in the shroud. While such cooling schemes have met with limited success, they greatly increase the complexity of the internal cooling passage configuration and thus greatly increase the complexity and manufacturing costs of the blade. These increased costs may more than offset the savings in operating costs associated with having smooth bores at the radially inner and outer ends of the airfoil cooling passages. 
     DISCLOSURE OF INVENTION 
     The present invention is predicated on the recognition that the qualitative analyses which led to the implementation of turbulators only in the intermediate portions of blade and vane radial cooling passages may have failed to take into account factors which would cause destructive thermal loading at the end portions of the blades and vanes, for example, at blade shrouds through which the unturbulated portions of the cooling passages extend. 
     One factor which would give rise to destructive thermal loading of the blade and vane end portions is a reduced total airflow through the cooling passages due to anomalies in the cooling air flow circuit beginning with the gas turbine engine&#39;s compressor and terminating with the blade or vane itself. Such anomalies include, for example, partial blockage of the flow passages with foreign matter, anomalies in the operation of the engine&#39;s compressor, wear of rotating seal components etc. 
     Another factor which theoretically can cause destructive thermal loading of blade and vane end portions is a deviation from a normal (uniform) temperature profile at the exit of the engine&#39;s combustor. Typically, gas turbine engine combustors are designed to provide combustion gases at a generally uniform temperature profile across the flow path of the engine&#39;s products of combustion. Foreign matter or pollutants in the engine&#39;s fuel system can cause blockage of some of the full nozzles in the combustor, resulting in asymmetries in the temperature profile across the combustor exhaust, thereby resulting in hot spots in the vanes and nozzles. Moreover, when replacement vanes and blades are employed in engines with unknown nominal operating parameters such as combustion exhaust temperature profiles, it would most efficacious to provide such blades with sufficient turbulation at the ends of the cooling passages to accommodate any anomalies in engine operation such as unevenness in the temperature profile at the combustor exhaust. 
     Recognizing that the heretofore common practice of providing turbulation only at the intermediate or medial portion of blade and vane cooling passages may not provide adequate convective cooling of gas turbine engine blades and vanes, in accordance with the present invention, turbulence promoters are provided in such blades and vanes at the radial extremities thereof. In a preferred embodiment of the present invention, in a turbine blade having radial cooling holes substantially along the entire length thereof, turbulence promoters are provided all the way to the tip of the blade including through any outer shroud thereof. The turbulence promoters may take on any of various known shapes such as annular or partially annular ribs or grooves. 
     In accordance with another aspect of the present invention, the thermal performance of prior art blades and vanes may be improved upon by adding turbulation promoters to the smooth walled portions of radial cooling channels, thereby restructuring such channels to increase the turbulent flow and thus the convective cooling provided in such smooth walled portions to accommodate the unanticipated destructive thermal loading outlined above. 
     It has been determined that perhaps counterintuitively, adding such turbulation promoters to such smooth walled portions of the cooling channels does not unacceptably lower the operating efficiency of the associated engine nor does it appreciably increase the manufacturing costs of the blades and vanes since fully turbulated holes may be formed without undue attention to the depth of placement of the tooling which forms the turbulators at the beginning and conclusion of the turbulator forming process. 
     Finally, it is believed that at least in the case of the provision of turbulators in the radially outer ends of shrouded turbine airfoils, the enhanced convective cooling of the shroud by a resultant turbulent cooling may reduce the need for stress reducing structures such as fillets and the like, thereby minimizing the size and weight of such structures as well as reducing the need for added cooling holes, passages and other fluid handling structural intricacies in the shroud and, in general, increase the overall mechanical and thermal capacity of such blades. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is an isometric view of a turbine blade in accordance with the present invention; 
         FIG. 2  is an enlargement of a tip of the blade of  FIG. 1 , including a tip shroud thereon; 
         FIG. 3  is an enlarged sectional view of one of the cooling passages in the blade&#39;s shroud, taken in the direction of line  3 - 3  in  FIG. 2 ; 
         FIG. 4  is a sectional view of the cooling passage of  FIG. 3  taken in the direction of line  4 - 4  thereof; 
         FIG. 5  is an enlarged sectional view of a first alternate embodiment of the cooling passage shown in  FIG. 3 ; 
         FIG. 6  is an enlarged sectional view of a second alternate embodiment of the cooling passage shown in  FIG. 3 ; 
         FIG. 7  is an enlarged sectional view of a third alternate embodiment of the cooling passage shown in  FIG. 3 ; and 
         FIG. 8  is an enlarged sectional view of a fourth alternate embodiment of the cooling passage shown in  FIG. 3 . 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Referring to the drawings,  FIG. 1  and  FIG. 2  illustrate a turbine blade  10  for use in a gas turbine engine. The turbine blade  10  has an airfoil portion  15  which typically contains a plurality of radially extending internal cooling passages  20 . The airfoil portion  15  has a tip end  25  to which an outer shroud  30  is integrally formed typically by casting or attached as a separate component. The shroud  30  is shaped to mate with like shrouds on adjacent turbine blades so as to lend rigidity to the radially outer portion of a circumferential array of such blades and prevent combustion gases from leaking around the turbine blade  10 . Similarly, blade  10  has a root end  32  including inner shroud or platform  34  which typically mates with platforms on adjacent blades for mechanical integrity of the blade array and to prevent products of combustion from leaking around airfoil portion  15 . 
     As can be seen in  FIG. 1 , the shroud  30  has a major outer surface  35  on which a knife edge  40  is attached. The knife edge  40  is substantially linear in shape and intersects the chord line of the airfoil portion  15  at an angle. The knife edge  40  may have any desired width and/or height and terminates in ends  50  and  55 , and in a manner well known in the art, mates with a groove in radially adjacent honeycomb stator (not shown) material to provide a rotating seal which helps in preventing working fluid from leaking around the blade tips. 
     Referring to  FIGS. 2 and 3 , the knife edge  40  has a central region  60  which is spaced from the ends  50  and  55 . In this central region  60 , a pair of cutter blades  65  and  70  are formed by machining out portions of the knife edge  40 . Cutter blades  68  and  70  cut the above-mentioned groove in the stator honeycomb as the knife edge rubs thereagainst upon engine start-up. As can be seen in  FIGS. 1 and 2 , machining of the cutter blades  65  and  70  results in the knife edge  40  having a base portion  75  which is wider than the radially outer edge of the knife edge  40 . 
     Still referring to  FIGS. 1 and 2 , each of the internal cooling passages  20  extends through the blade  10  over its entire length, including from root end including platform  34  to the tip end  25  including outer shroud  30 . Typically, the turbine blade  10  has a plurality of such cooling passages  20 . Each of the cooling passages exits at the outer surface of shroud  30  either at the major portion  35  of the outer surface thereof or the base portion  75  of knife edge  40 . Each of the cooling passages  20  conducts a cooling fluid, i.e., air, from a radially inner inlet in communication with a source the air, such as compressor bleed air, throughout its entire length for purposes of cooling the blade. 
     Turbine blade  10  may be formed from any suitable material known in the art such as a nickel based superalloy. To improve the cooling characteristics of the turbine blade  10 , each of the cooling passages  20  has a plurality of turbulation promoters (turbulators) disposed therealong, not only within airfoil portion  15 , but also along the radially inner and outer portion thereof, within shrouds  30  and  34 . 
     Referring now to  FIGS. 3 and 4 , there is shown a first embodiment of a cooling passage  20  which has a circular cross section. The cooling passage  20  extends along an axis  80  from the root end to the tip end of the blade and comprises a wall  85 . The wall  85  defines a passage (having a diameter D) for the cooling fluid. 
     A plurality of turbulation promoters (turbulators)  90  are incorporated into the passage  20 . The turbulation promoters may comprise arcuately shaped trip strips which have a height e and which circumscribe an arc of less than 180 degrees. The ratio of e/D is preferably in the range of from 0.05 to 0.30. Trip strips  95  may be annular or take the form of spaced arcuate members (see  FIG. 4 ) having an angular span of less than 180 degrees with end portions  100  and  105  spaced apart by a gap g. The gaps g may be in the range of e to 4e or from 0.015 inches to 0.050 inches. The gaps g are preferably oriented away from the maximum heat load. 
     As can also be seen from  FIG. 3 , a plurality of pairs of trip strips  95  are positioned along the axis  80 . The pairs of trip strips  95  are separated by a pitch P, the distance between mid-points of adjacent trip strips  95 . In a preferred embodiment of the present invention, the ratio of P/e is in the range of from 5 to 30. 
     The pairs of trip strips  95  are preferably aligned so that the gaps g of one pair of trip strips  95  is aligned with the gaps g of adjacent pairs of trip strips  95 . It has been found that such an arrangement is desirable from the standpoint of creating turbulence in the flow in the passageway  20  and minimizing the pressure drop of the flow. 
     Referring now to  FIG. 5 , instead of trip strips formed on the wall  80 , the turbulation promoters  95  may comprise notches  115  cut into the wall  80  by any suitable process such as electrochemical machining as noted above. As is the case with respect to the embodiment of  FIGS. 3 and 4 , each of the notches  115  may be arcuate in shape and may circumscribe an arc of less than 180 degrees. Still further, the notches may have a ratio of e/D which is in the range of from 0.05 to 0.30 and may have a surface  120  which is normal to the axis  85  and the flow of the cooling fluid through the passageway  14 . The ratio of P/e is preferably in the range of from 5 to 30. 
     Referring now to  FIG. 6 , there is shown an alternative embodiment of a cooling passageway  14  having turbulation promoters  125  which have a surface  130  which is at an angle a in the range of 30 degrees to 70 degrees, such as 45 degrees, with respect to the axis  85  and the flow of the cooling fluid through the passage  20 . The turbulation promoters may be either trip strips on the wall  80  or notches in the wall. As before, the turbulation promoters  125  are preferably arcuate in shape and circumscribe an arc less than 180 degrees. The turbulation promoters  125  may be aligned pairs of which have end portions spaced apart by a gap and each pair may be offset along the axis  85  as shown in  FIG. 4 . This has the benefit of a reduced pressure drop for an equivalent heat transfer level. Here again, the ratio P/e may be in the range of from 5 to 30. Alternately, the turbulation promoters may comprise a continuous helix. 
     Referring now to  FIG. 7 , another embodiment of a cooling passage  20  is illustrated. In this embodiment, the turbulation promoters include a first set of trip strips  130  and a second set of trip strips  135  offset from the first set of trip strips. The trip strips  130  and  135  are both arcuate in shape and circumscribe an arc of less than 180 degrees. As before, the trip strips  130  and  135  have a ratio of e/D in the range of from 0.05 to 0.30. The ratio P/e for each of the sets is preferably in the range of from 5 to 30. 
     Referring now to  FIG. 8 , there is shown still another embodiment of a cooling passage  20  having offset turbulation promotion devices  140 . The offset turbulation devices  80  take the form of a first set of notches  145  and a second set of offset notches  150 . Each of the notches  145  and  150  is arcuate in shape and circumscribes an arc less than 180 degrees and may have a ratio of e/D in the range of from 0.05 to 0.30. In this embodiment, as in the others, the ratio P/e for each set of notches is in the range of 5 to 30. 
     As set forth hereinabove, the cooling passages shown in  FIGS. 3-8  may be formed using any suitable technique know in the art. In a preferred embodiment of the present invention, the cooling passages  14  with the various turbulation promoters are formed using an electrochemical drilling technique. 
     While the turbulence promoters are shown and described herein as acute in shape and circumscribing somewhat less than 180 degrees, it will be understood that fully annular turbulence promoters or turbulence promoters of any of various other known shapes such as full or partial helices may be employed with equal efficacy and may be formed by methods other than the aforementioned electrochemical machining operation, such as ordinary mechanical drilling and tapping methods. 
     Also, while the present invention as shown and described within the context of a blade or vane manufactured in accordance with the present invention, the present invention is equally applicable in the improvement of prior art blades or vanes wherein only the intermediate portions of the cooling air passages are turbulated. In such cases, the smooth bore portions of the cooling air passages may be machined by any of the methods mentioned hereinabove to add turbulence promoters thereto, resulting in the advantages and benefits discussed hereinabove. 
     Furthermore, while the invention herein has been described in connection with the outer shroud of a gas turbine engine turbine blade, it will be understood that this invention is equally applicable to inner turbine blade shrouds as well as inner or outer vane platforms and shrouds. 
     Therefore, it will be appreciated that various embodiments and applications of the present invention beyond those specifically discussed and illustrated herein are contemplated and it is intended by the appended claims to cover such embodiments and applications as full within the true spirit and scope of this invention.