Trailing edge cooling for gas turbine airfoils

Abstract: A hollow airfoil has a pressure side wall 12, a suction side wall 14, a cavity 16 formed between the pressure and suction side walls, a plurality of cooling ports 18 disposed within the pressure side wall 12, and a plurality of passages 42, each extending between the cavity 16 and one of the cooling ports 18. Each passage 42 has a first wall 54 adjacent the suction side wall 14, a pair of passage 58 side walls extending substantially toward the pressure side wall 12, and a second wall 56 adjacent the pressure side wall 12. Each passage further includes a pair of fillets 60,62 extending between the passage side walls 58 and the second wall 56. Alternatively, or in addition, each passage 42 includes a jog adjacent each cooling port 18 so as to extend parallel to the pressure side wall 12.

Description: This invention relates to hollow airfoils in general, and to geometries of trailing edge cooling holes within hollow airfoils in particular. In modem axial gas turbine engines, turbine rotor blades and stator vanes require extensive cooling. A typical rotor blade or stator vane airfoil includes a serpentine arrangement of passages connected to a cooling air source, such as the compressor. Air bled from a compressor stage provides a favorable cooling medium because its pressure is higher and temperature lower than the core gas traveling through the turbine; the higher pressure forces the compressor air through the passages within the component and the lower temperature transfers heat away from the component. Cooling air ultimately exits the airfoil via cooling holes in the airfoil walls or cooling ports distributed along the trailing edge. Cooling is particularly critical along the trailing edge, where the airfoil narrows considerably. Most airfoil designs include a line of closely packed cooling ports in the exterior surface of the pressure side wall, distributed along the entire span of the airfoil. A relatively small pressure drop across each of the closely packed ports encourages the formation of a boundary layer of cooling air (film cooling) aft of the ports that helps cool and protect the aerodynamically desirable narrow trailing edge. In addition to cooling, turbine rotor blade and stator vane airfoils must also accommodate high cycle fatigue (HCF) resulting from vibratory loadings. This is particularly true along the narrow trailing edge, where each of the closely packed cooling ports represents a significant stress concentration. Left unchecked, HCF can create stress fractures which can eventually compromise the mechanical integrity of the airfoil. FIG.1 shows a sectional view of a conventional trailing edge with a cooling port in the pressure side wall, connected to an internal cavity via a passage. The width of the pressure side wall narrows considerably adjacent the cooling port, making that portion of the pressure side wall particularly susceptible to HCF. Moving the port forward to increase the wall thickness minimizes susceptibility to HCF, but also adversely effects film cooling aft of the port (film cooling effectiveness generally degrades with distance). Hence, what is needed is an airfoil with trailing edge cooling apparatus that inhibits HCF, one that enhances downstream film cooling, and one that can be readily manufactured. Various trailing edge cooling arrangements are disclosed in US-A-5,368,441, US-A-5,503,529, US-A-4,601,638 (which forms the basis for the preamble of claim 1), and US-A-5,405,242. According to the present invention, there is provided a hollow airfoil as claimed in claim 1. An advantage of the present invention is that HCF is minimized. In a conventional airfoil, the taper of the pressure side wall and suction side walls toward one another causes the pressure side wall to become undesirably thin, and therefore susceptible to HCF, particularly adjacent the forward and side edges of the cooling ports. In contrast, the present invention passages provide enough wall material around the cooling port to substantially minimize HCF in that region. A further advantage of the present invention is that the geometry of the passages and cooling ports can be cast within an airfoil, thereby making the present invention airfoil readily manufacturable. Some preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: FIGS. 1A to 1C show diagrammatic partial sectional views of a prior art gas turbine airfoil having a cooling port adjacent the trailing edge of the airfoil. FIG.2 is an example of an gas turbine airfoil having cooling ports distributed spanwise, adjacent the trailing edge. FIG.3 is a diagrammatic cross-section of an gas turbine airfoil having a plurality of internal cavities disposed between pressure and suction side walls. FIG.4A is a diagrammatic view of a gas turbine airfoil having a cooling port adjacent the trailing edge of the airfoil. FIGS. 4B-4E and 5 are sections of the gas turbine airfoil shown in FIG.4A FIG.6 is a section of the gas turbine airfoil shown in FIG.4A, taken at the section of FIG.4B, showing an alternative passage cross-section. Referring to FIGS. 2 and 3, a hollow airfoil 10 for gas turbine engine includes a pressure side wall 12, a suction side wall 14, a plurality of internal cavities 16 disposed between the pressure 12 and suction 14 side walls, and a plurality of cooling ports 18. The internal cavities 16 are connected to a source of cooling air 19. The pressure 12 and suction 14 side walls extend widthwise 20 between a leading edge 22 and a trailing edge 24, and spanwise 26 between the inner radial platform 28 and an outer radial surface 30. The thickness 32 of the airfoil 10 is defined as the distance between pressure side wall exterior surface 34 and the suction side wall exterior surface 36. The thickness of an airfoil wall 12, 14 may be measured in a similar direction, between the wall's interior and exterior surfaces. The exemplary airfoil 10 shown in FIG.2 is a rotor blade having a root 38 with cooling air inlets 40. An airfoil 10 acting as a stator vane may also embody the present invention. FIG.3 shows a cross-section of an airfoil (stator vane or rotor blade) embodying the present invention, having a plurality of internal cavities 16, connected to one another in a serpentine manner. "N" number of passages 42 connect the aft most cavity 16 to "N" number of cooling ports 18, where "N" is an integer. Referring to FIGS. 2, 3, and 4A, the cooling ports 18 are disposed within the pressure side wall 12, and distributed spanwise adjacent the trailing edge 24. Each cooling port 18 includes an aft edge 44, a forward edge 46, a pair of side edges 48, and a pair of fillets 50 (see FIG.4A). The side edges 48 intersect with the aft edge 44, and extend substantially toward the forward edge 46. Each fillet 50 extends between one of the side edges 48 and the forward edge 46. The length 52 of each fillet 50 is defined as the widthwise distance between its intersection with the side edge 48 and its intersection with the forward edge 46. Referring to FIGS. 4B-4E, 5, and 6, each passage 42 connecting a cooling port 18 to the aft most cavity 16 (see FIG.5) has a cross-sectional geometry that includes a first wall 54, a second wall 56, and a pair of side walls 58 (see FIGS. 4B-4E and 6). The first wall 54 is adjacent the suction side wall 14 and the second wall 56 is adjacent the pressure side wall 12. The side walls 58 extend outwardly from the first wall 54, substantially toward the pressure side wall 12. The cross-sectional geometry of the passage 42 further includes a first fillet 60 extending between one of the side walls 58 and the second wall 56, and a second fillet 62 extending between the other of the side walls 58 and the second wall 56. The geometry of the first and second fillets 60,62 and/or the second wall 56 can be varied to suit the application at hand. FIG.6, for example, shows the first and second fillets 60,62 and second wall 58 as arcuately shaped. FIG. 4B, on the other hand, shows a passage 42 cross-section where the fillets 60,62 nearly meet one another at the center of the second wall 56. FIG.4B also shows the pressure side wall 12 at the forward edge 46 of the cooling port 18 having a thickness equal to "x". The thickness of the first and second fillets 60,62 is equal to or greater than "x" (FIGS. 4C and 4D show the fillets 60,62 equal to thickness "x"). Referring to FIG.5, downstream of the cooling port forward edge 46, each passage 42 skews an amount (illustrated by angle ), thereafter extending substantially parallel to the pressure side wall exterior surface 34 for at least the length 52 of the cooling port fillets 50. As a result, the thickness 63 of the pressure side wall 12 remains substantially constant for the length 52 of the cooling port fillets 50. Aft of the cooling port fillets 50, the passage preferably skews again, this time extending substantially parallel to the exterior surface 36 of the suction side wall 14. The dotted lines in FIG.5 represent a conventional trailing edge cooling port and passage geometry. To better understand the present invention, compare the conventional trailing edge cooling apparatus shown in FIG.1 to the present invention trailing edge cooling embodiments shown in FIG.5. In the conventional trailing edge cross-section (FIG.1), a passage 64 connects each cooling port 66 to the internal cavity 68, and each cooling port 66 includes a pair of fillets 70. The width of the pressure side wall 78 narrows considerably in the fillets 70, making that portion of the pressure side wall 78 particularly susceptible to HCF. The present invention, in contrast, avoids the narrow wall characteristic of conventional design by: skewing the passage 42 aft of the forward edge 46 of the cooling port, such that the passage 42 extends substantially parallel to the exterior surface 34 of the pressure side wall 12 (see FIG.5) and preferably providing a filleted 60,62 passage geometry (see FIGS. 4B-4E, and 6). From the above, it will be seen that there is provided an airfoil having trailing edge cooling apparatus that inhibits HCF; an airfoil having trailing edge cooling apparatus that enhances downstream film cooling; and an airfoil having trailing edge cooling apparatus that can be readily manufactured.

Claims: A hollow airfoil (10), comprising: a pressure side wall (12), having a first exterior surface (34); a suction side wall (14), having a second exterior surface (36); wherein said pressure and suction side walls (12,14) extend widthwise between a leading edge (22) and a trailing edge (24); a cooling air cavity (16), formed between said pressure and suction side walls (12,14); a plurality of cooling ports (18), disposed within said pressure side wall (12), distributed spanwise adjacent said trailing edge (24); and a plurality of passages (42), each extending between said cavity (16) and one of said cooling ports (18), and each having a first wall (54) adjacent said suction side wall (36), a pair of passage side walls (58) extending substantially toward said pressure side wall (12), and a second wall (56) adjacent said pressure side wall (12); characterised in that each said passage (42) skews adjacent said connected cooling port (18), such that said passage extends substantially parallel to said first exterior surface (34). A hollow airfoil according to claim 1, wherein each said cooling port comprises: an an edge (44); a pair of side edges (48) intersecting with said aft edge (44); a forward edge (46); a first fillet (50) extending between one of said side edges (48) and said forward edge (46); and a second fillet (50) extending between the other of said side edges (48) and said forward edge (46). A hollow airfoil according to claim 2, wherein downstream of said forward edge (46), each said passage (42) skews and extends substantially parallel to said first exterior surface (34). A hollow airfoil according to claim 2 or 3, wherein downstream of said first and second fillets, each said passage (42) skews and extends substantially parallel to said second exterior surface (36). A hollow airfoil according to claim 1 further comprising: a first fillet (60) extending between one of said passage side walls (58) and said second wall (56), and a second fillet (62) extending between the other of said passage side walls (58) and said second wall (56). A hollow airfoil according to claim 5, wherein each said cooling port (18) comprises: an aft edge (44); a pair of side edges (48) intersecting with said aft edge (44); a forward edge (46); a third fillet (50) extending between one of said side edges (48) and said forward edge (46); and a fourth fillet (50) extending between the other of said side edges (48) and said forward edge (46). A hollow airfoil according to claim 6, wherein said pressure side wall (12) has a first thickness adjacent said forward edge (46) of each said cooling port (18), and said first and second fillets (60,62) have a second thickness at least equal to said first thickness. A hollow airfoil according to claim 6 or 7, wherein downstream of said forward edge (46), each said passage (42) extends substantially parallel to said first exterior surface (34). A hollow airfoil according to any of claims 6 to 8, wherein. downstream of said third and fourth fillets (50), each said passage (42) extends substantially parallel to said second exterior surface (36). A hollow airfoil according to any preceding claim, wherein said passage side walls (58) and said second wall (56) are arcuate.

Inventors: PRZIREMBEL HANS R, SOECHTING FRIEDRICH O, PRZIREMBEL, HANS R., SOECHTING, FRIEDRICH O.
Applicants: UNITED TECHNOLOGIES CORP, UNITED TECHNOLOGIES CORPORATION
Application year: 1998
Main classification: F01D5
IPC codes: F01D9, F01D5