A combustion turbine typically comprises a compressor to draw in and compress a gas (usually air), a combustor to add energy to the compressed gas, and a turbine driven by the resulting expansion of heated gas. The turbine, in turn, can be used to power a wide range of equipment including ships, aircraft, and power generators.
The turbine typically comprises one or more stages of blade assemblies extending from a rotatable shaft and stationary guide vanes usually located adjacent the combustor. Both the blade assemblies and guide vanes typically comprise airfoils. An airfoil of a blade assembly usually extends outwardly from a platform connected to a root, which, in turn, is mounted to a turbine disk on the rotatable shaft. An airfoil of a guide vane is typically positioned between two stationary platforms. Hot gases from the combustor flow over the airfoils during operation of the combustion turbine causing the blade assembly to rotate.
Because of the high temperatures of the gases, it may be desirable to cool the airfoils. An approach to cooling an airfoil is to provide the airfoil with at least one interior passageway that carries a flow of cooling gas. Extending from the at least one interior passageway, may be a plurality of cooling holes that extend to the surface of the airfoil. As the gas exits the airfoil, it meets and is pushed by the flow of hot gas down and over the surface of the airfoil to form a cooling film.
U.S. Pat. No. 6,164,912 to Tabbita et al., for example, discloses providing a plurality of cooling holes or apertures spanwise along the leading edge of an airfoil. The cooling holes are curved and open at the surface of the leading edge to form oval shaped outlets whose major dimension is angled relative to the leading edge.
Forming non-linear cooling holes may be more difficult to manufacture as compared to linear ones thereby adding to the cost of manufacturing each airfoil. U.S. Pat. No. 6,379,118 to Lutum et al., for example, discloses linear or straight-hole cooling holes that extend from an interior passageway of an airfoil to its leading edge surface. The cooling holes are also angled in a radially upward direction toward the leading edge surface.
While such linear cooling holes may be more efficient in terms of manufacturing, the nature and shape of the airfoil typically constrains how large the outlet formed may be. The spanwise or radial dimension of the outlet (i.e., the breakout length), for example, is usually limited by the fact that cooling holes made by electro-discharge machining or by laser can be difficult to angle relative to the leading edge surface.
Accordingly, the breakout length may be less than desired since to increase the breakout length, the cooling hole would have to be formed at a shallow angle relative to the leading edge surface. If, however, the angle is too shallow relative to the leading edge surface, an electro-discharge device or other machining apparatus may skip across the leading edge surface. This, of course, could possibly damage the airfoil, but would not properly form the desired cooling hole. A laser beam at too shallow an angle, may reflect off the leading edge surface.