During operation, gas turbine engines, whether used for flight or stationary power generation, develop extremely high temperature and high velocity gases in a combustor portion of the engine. These gases are ducted on blades of a turbine rotor to cause rotation of the rotor and are redirected by the stator vanes onto additional rotor blades to produce more work. Because of the high heat of the gases, it is desirable to cool the blades and vanes to prevent damage and, to extend the useful life of, these engine components. It is known in the art that a turbine component such as that shown in FIG. 16 can be cooled by film cooling that is provided by a plurality of fabricated features, for example, cooling holes.
A commonly used method of cooling a turbine component 20 is to duct cooling air through internal cavities or passages and then vent the cooling air through a plurality of cooling holes 22. This air cools internal surfaces of the component by convection and cools the components outer surfaces by film cooling. The cooling holes 22 are typically formed along a line generally parallel to, and a selected distance from, a trailing edge 24 of the component to provide a film of cooling air over a surface of the component when the cooling holes discharge air during engine operation. Other rows or arrays of cooling holes or vents may be formed in the blade and vane components of a rotor or stator of a turbine depending upon design constraints.
To facilitate the distribution of the cooling air substantially completely over the convex and concave surfaces of the blade airfoil or platform, as shown in FIG. 17, the upstream end of each cooling hole 22 has a generally cylindrical, inlet portion 26 that extends from a location 28 inside of a wall of the component 20. At the location 28, the cooling hole 22 then flares or diverges to provide a discharge portion 30 that terminates on an exterior surface 32 of the component 20 to be cooled by the air flow. The shape of the discharge end functions as a diffuser to reduce the velocity of the cooling airstreams being discharged from the cooling holes 22. The lower velocity cooling airstreams are more inclined to cling to the surface 32 for improved cooling. High quality cooling holes 22 with diffusers 30 provide superior performance but are costly and difficult to manufacture.
After the cooling holes have been manufactured, it is necessary to inspect each of the holes to determine whether it exists and is properly formed as a complex hole. One method of inspection is a manual method in which an inspector is provided with a drawing of the desired hole pattern and a pin. The inspector first confirms that a hole exists at each location identified by the pattern; and then, the inspector inserts the pin through each of the holes to determine whether the hole is properly drilled as a through-hole. As can be appreciated, such an inspection process is highly repetitive, tedious and stressful for the inspector and, in addition, is expensive and inefficient for the manufacturer of the turbine component.
Other known hole inspection processes are automated and utilize a laser or a flow of fluid through the holes. The flowing fluid used most commonly is either air or water. In the case of air, the mass of air flowing through a feature can be measured. With water, a visual signal of a flow pattern is possible. These methods need a human visual check or physical measurement of a single feature to characterize its flow condition. All of these known methods are time-consuming and rely on human intervention to perform the characterization which leads to errors.
Thus, there is a need for an inspection apparatus and process that can automatically inspect and identify qualitative characteristics of complex cooling features in gas turbine components faster, more precisely and less expensively than known inspection apparatus and processes.