1. Field of the Invention
The present invention relates generally to fluid reaction surfaces, and more specifically to a large turbine airfoil with a cooling circuits.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In a gas turbine engine such as an industrial gas turbine engine, a turbine section includes a plurality of rotor blades that react with the hot gas flow passing through the turbine to produce mechanical work by rotating the rotor shaft. In an industrial gas turbine, four stages of rotor blades and stator vanes are used to extract the energy from the flow. As the inlet temperature to the turbine increases, the size of the fourth stage rotor blade also increases because the flow into the fourth stage has higher energy than previous lower temperature engines. These fourth stage rotor blades can be over 30 inches from platform to blade tip, and also have very large taper and twist in order to react with the flow.
With the higher gas flow temperature exposed to the fourth stage blade, internal air cooling is required in order to increase the life of the rotor blade. However, prior art methods of casting turbine blades having internal cooling circuits are not practical with these larger blades. Radial holes cannot be drilled into the large highly twisted and tapered blade because of the large amount of twist from the blade attachment to the tip. A straight hole cannot be placed within the blade. Reduction of available airfoil cross section area for drilling radial holes is a function of the blade twist. Higher airfoil twist yields a lower available cross sectional area for drilling radial cooling holes. Cooling of the large, highly twisted blade by this manufacturing process will not achieve the optimum blade cooling effectiveness. FIG. 1 shows a profile view of a Prior Art large rotor blade with radial cooling holes drilled into the blade.
It is therefore an object of the present invention to provide for a large turbine blade that is highly tapered and twisted with an internal cooling circuit that can be cast into the blade.
Another object of the present invention is to provide for a large turbine blade that is highly tapered and twisted with an internal cooling circuit that will give the blade a very high airfoil chordwise sectional strength to prevent airfoil un-twisting.
Another object of the present invention is to provide for a large turbine blade that is highly tapered and twisted with an internal cooling circuit that will yield a lower and more uniform blade sectional mass average temperature at lower blade span height to improve blade creep life capability.
Another object of the present invention is to provide for a large turbine blade that is highly tapered and twisted with an internal cooling circuit that will provide cooler blade leading and trailing edge corners to enhance the blade high cycle fatigue (HCF) capability.
Another object of the present invention is to provide for a large turbine blade that is highly tapered and twisted with an internal cooling circuit that will allow for the rotation of the blade to provide a centrifugal pumping effect so that a lower cooling air supply pressure is required, resulting in lower leakage flow around the blade attachment and cooler cooling air supply temperature.