Leading edge cooling for a gas turbine blade

Abstract: A hollow airfoil 10 is provided with a trench 18 which is aligned with a stagnation line 42 extending along the leading edge 26. Cooling air apertures 38 extend between an inner cavity of the airfoil and the trench 18 to supply cooling air thereto.

Description: This invention relates to cooled rotor blades and/or stator vanes for gas turbines in general, and to apparatus and methods for cooling the leading edge and establishing film cooling along the surface of the rotor blade or stator vane in particular. In the turbine section of a gas turbine engine, core gas travels through a plurality of stator vane and rotor blade stages. Each stator vane or rotor blade has an airfoil with one or more internal cavities surrounded by an external wall. The suction and pressure sides of the external wall extend between the leading and trailing edges of the airfoil. Stator vane airfoils extend spanwise between inner and outer platforms and the rotor blade airfoils extend spanwise between a platform and a blade tip. High temperature core gas (which includes air and combustion products) encountering the leading edge of an airfoil will diverge around the suction and pressure sides of the airfoil, or impinge on the leading edge. The point along the leading edge where the velocity of the core gas flow goes to zero (i.e., the impingement point) is referred to as the stagnation point. There is a stagnation point at every spanwise position along the leading edge of the airfoil, and collectively those points are referred to as the stagnation line. Air impinging on the leading edge of the airfoil is subsequently diverted around either side of the airfoil. The precise location of each stagnation point along the length of the leading edge is a function of the angle of incidence of the core gas relative to the chordline of the airfoil, for both rotor and stator airfoils. In addition to the angle of incidence, the stagnation point of a rotor airfoil is also a function of the rotational velocity of the airfoil and the velocity of the core gas. Given the curvature of the leading edge, the approaching core gas direction and velocity, and the rotational speed of the airfoil (if any), the location of the stagnation points along the leading edge can be readily determined by means well-known in the art. In actual practice, rotor speeds and core gas velocities vary depending upon engine operating conditions as a function of time and position along the span of the airfoil. As a result, the stagnation points (or collectively the stagnation line) along the leading edge of an airfoil will move relative to the leading edge. Cooling air, typically bled off of a compressor stage at a temperature lower and pressure higher than the core gas passing through the turbine section, is used to cool the airfoils. The cooler compressor air provides the medium for heat transfer and the difference in pressure provides the energy required to pass the cooling air through the stator or rotor stage. In many cases, it is desirable to establish film cooling along the surface of the stator or rotor airfoil. A film of cooling air traveling along the surface of the airfoil transfers thermal energy away from the airfoil, increases the uniformity of the cooling, and insulates the airfoil from the passing hot core gas. A person of skill in the art will recognize, however, that film cooling is difficult to establish and maintain in the turbulent environment of a gas turbine. In most cases, film cooling air is bled out of cooling apertures extending through the external wall of the airfoil. The term "bled" reflects the small difference in pressure motivating the cooling air out of the internal cavity of the airfoil. One of the problems associated with using apertures to establish a cooling air film is the films sensitivity to pressure difference across the apertures. Too great a pressure difference across an aperture will cause the air to jet out into the passing core gas rather than aid in the formation of a film of cooling air. Too small a pressure difference will result in negligible cooling air flow through the aperture, or an in-flow of hot core gas. Both cases adversely affect film cooling effectiveness. Another problem associated with using apertures to establish film cooling is that cooling air is dispensed from discrete points along the span of the airfoil, rather than along a continuous line. The gaps between the apertures, and areas immediately downstream of those gaps, are exposed to less cooling air than are the apertures and the spaces immediately downstream of the apertures, and are therefore more susceptible to thermal degradation. Another problem associated with using apertures to establish film cooling is the stress concentrations that accompany the apertures. Film cooling effectiveness generally increases when the apertures are closely packed and skewed at a shallow angle relative to the external surface of the airfoil. Skewed, closely packed apertures, however, create stress concentrations. What is needed is an apparatus that provides adequate cooling along the leading edge of an airfoil, one that accommodates a variable position stagnation line, one that creates a uniform and durable cooling air film downstream of the leading edge on both sides of the airfoil, and one that creates minimal stress concentrations in the airfoil wall. GB-A-2127105, upon which the preamble to the independent claims is based, discloses an airfoil having a trench formed in its leading edge, with cooling holes opening into the trench. GB-A-435906 discloses a blade having multiple slots in its leading edge. According to a first aspect of the present invention, a hollow airfoil is provide as claimed in claim 1. According to a second aspect of the present invention, a method for cooling an airfoil is provided as claimed in claim 7. An advantage of the present invention is that uniform and durable film cooling downstream of the leading edge is provided on both sides of the airfoil. The cooling air bleeds out of the trench on both sides and creates continuous film cooling downstream of the leading edge. The trench minimizes cooling losses characteristic of cooling apertures, and thereby provides more cooling air for film development and maintenance. z Another advantage of the present invention is that stress is minimized along the leading edge and areas immediately downstream of the leading edge. The trench of cooling air that extends continuously along the leading edge minimizes thermally induced stress by eliminating the discrete cooling points separated by uncooled areas characteristic of conventional cooling schemes. The uniform film of cooling air that exits from both sides of the trench also minimizes thermally induced stress by eliminating uncooled zones between and downstream of cooling apertures characteristic of conventional cooling schemes. Another advantage of the present invention is that the leading edge cooling apparatus accommodates a plurality of stagnation lines. In the most preferable embodiment, the trench is preferably centered on the stagnation line which coincides with the largest heat load operating condition for a given application, and the width of the trench is large enough such that the stagnation line will not travel outside of the side walls of the trench under all operating conditions. As a result, the present invention provides improved leading edge cooling and cooling air film formation relative to conventional cooling schemes. A preferred embodiment of the invention will now be described by way of example only with reference to the accompanying drawings in which: FIG. 1 is a diagrammatic perspective view of a turbine rotor blade for a gas turbine engine. FIG.2 is a partial sectional view of the airfoil portion of the rotor blade shown in FIG. 1, including core gas flow lines to illustrate the relative position of the trench and the stagnation point of the airfoil. The partial sectional view of the airfoil shown in this drawing also represents the airfoil of a stator vane. FIG.3 is a diagrammatic sectional view of a trench disposed in the leading edge of an airfoil. Referring to FIG.1, a gas turbine engine turbine rotor blade 10 includes a root portion 12, a platform 14, an airfoil 16, a trench 18 disposed in the airfoil 16, and a blade tip 20. The airfoil 16 comprises one or more internal cavities 22 (see FIG.2) surrounded by an external wall 24, at least one of which is proximate the leading edge 26 of the airfoil 16. The suction side 28 and the pressure side 30 of the external wall 24 extend chordwise between the leading edge 26 and the trailing edge 32 of the airfoil 16, and spanwise between the platform 14 and the blade tip 20. The leading edge 26 has a smoothly curved contour which blends with the suction side 28 and pressure side 30 of the airfoil 16. Referring to FIG.2, the trench 18 includes a base 34 and a pair of side walls 36 disposed in the external wall 24 along the leading edge 26, preferably extending substantially the entire span 36 (see FIG. 1) of the airfoil 16. A plurality of cooling apertures 38 provide passages between the trench 18 and the forward most internal cavity 22 for cooling air. The shape of the cooling apertures 38 and their position within the trench 18 will vary depending upon the application. FIG.2 includes streamlines 40 representing core gas within the core gas path to illustrate the direction of core gas relative to the airfoil 16. As stated earlier, the stagnation point 42 (or in collective terms, the stagnation line) at any particular position along the span will move depending upon the engine operating condition at hand. The trench 18 is preferably centered on those stagnation points 42 which coincide with the largest heat load operating condition for a given application, and the width 44 of the trench 18 is large enough such that the stagnation line 42 will not travel outside of the side walls 36 of the trench 18 under all operating conditions. If, however, it is not possible to provide a trench 18 wide enough to accommodate all possible stagnation line 42 positions, then the width 44 and the position of the trench 18 are chosen to accommodate the greatest number of stagnation lines 42 that coincide with the highest heat load operating conditions. The most appropriate trench width 44 and depth 46 for a given application can be determined by empirical study. Referring to FIG.3 for example, empirical studies indicate that a trench 18 for a rotor airfoil 16 having a depth 46 substantially equal to one (1) cooling aperture 38 diameter ("D") and a width 44 substantially equal to three (3) cooling aperture 38 diameters ("3D"), where the cooling aperture 38 is that which is disposed within the trench 18, provides favorable leading edge 26 cooling and downstream cooling air film formation. In the operation of the invention, cooling air typically bled off of a compressor stage (not shown) is routed into the airfoil 16 of the rotor blade 10 (or stator vane) by means well known in the art. Cooling air disposed within the internal cavity 22 proximate the leading edge 26 of the airfoil 16 is at a lower temperature and higher pressure than the core gas flowing past the external wall 24 of the airfoil 16. The pressure difference across the airfoil external wall 24 forces the internal cooling air to enter the cooling apertures 38 and subsequently pass into the trench 18 located in the external wall 24 along the leading edge 26. The cooling air exiting the cooling apertures 38 diffuses into the air already in the trench 18 and distributes within the trench 18. The cooling air subsequently exits the trench 18 in a substantially uniform manner over the side walls 36 of the trench 18. The exiting flow forms a film of cooling air on both sides of the trench 18 that extends downstream. One of the advantages of distributing cooling air within the trench 18 is that the pressure difference problems characteristic of conventional cooling apertures (not shown) are minimized. For example, the difference in pressure across a cooling aperture 38 is a function of the local internal cavity 22 pressure and the local core gas pressure adjacent the aperture 38. Both of these pressures vary as a function of time. If the core gas pressure is high and the internal cavity pressure is low adjacent a particular cooling aperture in a conventional scheme (not shown), undesirable hot core gas in-flow can occur. The present invention minimizes the opportunity for the undesirable in-flow because the cooling air from all apertures 38 distributes and increases in uniformity within the trench 18, thereby decreasing the opportunity for any low pressure zones to occur. Likewise, the distribution of cooling air within the trench 18 also avoids cooling air pressure spikes which, in a conventional scheme, would jet the cooling air into the core gas rather than add it to the film of cooling air downstream. From the above it will be seen that there is disclosed herein an airfoil having improved cooling along the leading edge, with leading edge cooling apparatus that accommodates a plurality of stagnation lines, that establishes uniform and durable film cooling downstream of the leading edge on both sides of the airfoil, and that creates minimal stress concentrations within the airfoil wall. Although this invention has been shown and described with respect to the detailed embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail thereof may be made without departing from the scope of the invention as defined by the claims. For example, FIG.2 shows a partial sectional view of an airfoil 16. The airfoil 16 may be that of a stator vane or a rotor blade.

Claims: An airfoil (10), comprising: a body, having an external wall surrounding an internal cavity (22) and a spanwise extending leading edge (26); a trench (18), disposed in said external wall along said leading edge (26) and extending in a spanwise direction, said trench aligned with a stagnation line (42) extending along said leading edge (26); and a plurality of cooling apertures (38), disposed within said trench (18) and extending through said external wall, thereby providing a cooling air passage between said internal cavity (22) and said trench (18), characterised in that : said trench (18) is configured and positioned such that in use stagnation lines associated with all operating conditions of the airfoil are disposed within said trench. An airfoil according to claim 1, wherein said trench comprises: a first side wall (36); a second side wall (36); a base (34), extending between said first and second side walls; wherein said stagnation lines (42) are disposed between said first and second side walls (36). An airfoil as claimed in claim 1 or 2, wherein said trench (18) is centred on the stagnation line associated with the largest heat load operating condition for the airfoil. An airfoil according to claim 1, 2 or 3 wherein said hollow airfoil (10) is part of a stator vane. An airfoil according to any of claims 1 to 3, wherein said hollow airfoil (10) is part of a rotor blade. An airfoil according to any preceding claim, wherein each said cooling aperture (38) has a diameter (D) and said trench (18) has a depth (46) substantially equal to said diameter (D) and a width (44) substantially equal to three of said diameters (D). method for cooling an airfoil exposed to core gas within a gas turbine engine, wherein the airfoil (10) has a body that includes an external wall that surrounds an internal cavity (22), and a spanwise extending leading edge (26) comprising the steps of: providing a trench (18) in the external wall along the leading edge (26) aligned with a stagnation line (42); providing a plurality of cooling apertures (38), disposed within said trench (18) and extending through to the internal cavity (22); and providing cooling air at a temperature lower and a pressure higher than the core gas in the internal cavity (22); whereby said higher pressure cooling air exits the internal cavity (22) via said cooling apertures (38), passes into said trench (18), and subsequently exits said trench (18) to form a film of cooling air downstream of said trench (18); characterised by : providing said trench (18) with a width large enough such that said stagnation line (42) remains within said trench (18) under all airfoil operating conditions. A method for cooling an airfoil according to claim 7, wherein said trench comprises a first side wall (36), a second side wall (36), and a base (34) extending between said first and second side walls (36). A method for cooling an airfoil according to claim 7 or 8, further comprising the steps of: determining said stagnation line (42) that coincides with a largest heat load capacity for a given application; substantially centering said trench (18) on said stagnation line coinciding with said largest heat load capacity for a given application. A method for manufacturing a coolable gas turbine engine airfoil (10) having a body that includes an external wall that surrounds an internal cavity (22), and a spanwise extending leading edge (26), comprising the steps of: determining a stagnation line (42) for each of a plurality of select airfoil operating conditions; providing a trench (18), disposed in the external wall along the leading edge (26), having a laterally extending width and a depth; said trench (18) being aligned with said stagnation lines; and providing a plurality of cooling apertures (38), disposed within said trench (18) and extending through to the internal cavity (22), wherein said cooling apertures (38) provide a passage for cooling air travel between said internal cavity (22) and said trench (18); characterised by the steps of: determining a first lateral limit and a second lateral limit for said stagnation lines (42) for said plurality of select airfoil operating conditions, wherein said stagnation lines lie between said first and second lateral limits; providing said trench (18) with a pair of sidewalls (36), wherein said width extends between said sidewalls; and disposing said trench sidewalls (36) in said external wall laterally outside of or proximate said first and second lateral limits, thereby keeping all said stagnation lines between said trench side walls (36). A method for manufacturing a coolable gas turbine engine airfoil according to claim 10, further comprising the steps of: determining said stagnation line (42) that coincides with the largest heat load operating condition for a given airfoil application; and centering said trench (18) on said stagnation line that coincides with said largest heat load operating condition.

Inventors: AUXIER THOMAS A, DOWNS JAMES P, SOECHTING FRIEDRICH, TABBITA MARTIN G, AUXIER, THOMAS A., DOWNS, JAMES P., SOECHTING, FRIEDRICH, TABBITA, MARTIN G.
Applicants: UNITED TECHNOLOGIES CORP, UNITED TECHNOLOGIES CORPORATION
Application year: 1998
Main classification: F01D5
IPC codes: F01D5